# Patent

## Title
Application of abnormal event detection technology to olefins recovery trains

## Abstract

The present invention is a method for detecting an abnormal event for process units of an ethylene processing system. The method compares the operation of the process units to a model developed by principal components analysis of normal operation for these units. If the difference between the operation of a process unit and the normal operation indicates an abnormal condition, then the cause of the abnormal condition is determined and corrected.

## Background
This application claims the benefit of U.S. Provisional application 60/609,179 filed Sep. 10, 2004.


BACKGROUND OF THE INVENTION
The present invention relates to the operation of an ethylene plant. In particular, the present invention relates to determining when the ethylene plant is deviating from normal operation.
In the petrochemical industry, an Olefins Recovery Train (ORT) is a very important process for an ethylene plant. The ORT purifies ethylene and propylene and also recovers by-products of the olefins production plant. Due to the complicated dynamic nature and cryogenic portions of the ORT, abnormal process operations can easily result from many root problems that cause the ORT operation to deviate from the normal operating state. Abnormal operations of the ORT can have significant economic implications and, in many cases, can stop production. These abnormal situations can cause lost production, equipment damage, environmental emissions, injuries and fatalities. A primary job of the console operator is to identify the cause of the abnormal situation and execute compensatory or corrective actions in a timely and efficient manner.
The current commercial practice is to use advanced process control applications to automatically adjust the process in response to minor process disturbances. For moderate to severe abnormal operations, the current practice is to rely on human process intervention. And for very severe abnormal operations, the current practice is to use automatic emergency shutdown (ESD) systems that can have substantial economic consequences for the process equipment.
Currently, the console operator is notified of an abnormal condition via process alarms. These alarms are triggered when key process measurements (temperatures, pressures, flows, levels, compositions, valve positions, etc.) violate a pre-defined set of operating ranges. These operating ranges are often static values from initial designs or frequently adjusted values by console operators to envelop important operating regions. For highly integrated processes, such as ORT, the alarm system effectiveness is an often difficult balance of providing timely alarms during normal operation (early warning) and preventing “alarm floods” during abnormal operation (risk of high priority information being missed). Thus, the current notification technology is challenged to provide sufficient early notifications while generating an acceptable rate (near zero) of false notifications to ensure the alarm system remains effective during severe abnormal operation.
There can be many thousands of process measurements that cover the operation of a typical ORT, of which hundreds are considered key to normal operation. In addition, each process measurement can have as many as 15 different alarms being configured. Under a conventional Distributed Control System (DCS) like Honeywell's TDC3000, the operator must survey this list of sensors and its trends, compare them with mental knowledge of normal ORT operation, and discover the potential problems early enough to intervene before significant disruptions can occur. Due to the very large number of sensors in an operating ORT, abnormalities can be easily missed. With the current DCS based monitoring technology, the only automated detection assistance an operator has is the DCS alarm system which is based on the alarming of each sensor when it violates predetermined limits. Due to the complexity of an ORT, this type of notification often comes in too late to allow the operator to have sufficient time to take preventive action to mitigate a problem. Over-use of the alarm system for early detection can have even worse consequences. The alarms become an annoyance to the operator during normal operation and the operator ignores the alarm system. During abnormal conditions, the operator becomes flooded with alarms and misses critical information. The present invention provides a more effective notification to the operator of the ORT.
SUMMARY OF THE INVENTION
The present invention is a method for detecting an abnormal event for some process units of an ethylene processing system. The system includes a number of process units. The method compares the current operations of some of the process units to a model of normal operation for those units. If the difference between the current operation of the unit and the normal operation indicates an abnormal condition in a process unit, then the cause of the abnormal condition is determined and corrective action can be taken.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows how the information in the online system flows through the various transformations, model calculations, fuzzy Petri nets and consolidation to arrive at a summary trend which indicates the normality/abnormality of the process areas.
FIG. 2 shows a valve flow plot to the operator as a simple x-y plot.
FIG. 3 shows three-dimensional redundancy expressed as a PCA model.
FIG. 4 shows a schematic diagram of a fuzzy network setup.
FIG. 5 shows a schematic diagram of the overall process for developing an abnormal event application.
FIG. 6 shows a schematic diagram of the anatomy of a process control cascade.
FIG. 7 shows a schematic diagram of the anatomy of a multivariable constraint controller, MVCC.
FIG. 8 shows a schematic diagram of the on-line inferential estimate of current quality.
FIG. 9 shows the KPI analysis of historical data.
FIG. 10 shows a diagram of signal to noise ratio.
FIG. 11 shows how the process dynamics can disrupt the correlation between the current values of two measurements.
FIG. 12 shows the probability distribution of process data.
FIG. 13 shows illustration of the press statistic.
FIG. 14 shows the two-dimensional energy balance model.
FIG. 15 shows a typical stretch of Flow, Valve Position, and Delta Pressure data with the long period of constant operation.
FIG. 16 shows a type 4 fuzzy discriminator.
FIG. 17 shows a flow versus valve Pareto chart.
FIG. 18 shows a schematic diagram of operator suppression logic.
FIG. 19 shows a schematic diagram of event suppression logic.
FIG. 20 shows the setting of the duration of event suppression.
FIG. 21 shows the event suppression and the operator suppression disabling predefined sets of inputs in the PCA model.
FIG. 22 shows how design objectives are expressed in the primary interfaces used by the operator.
FIG. 23 shows the operator overview of the ORT operation decomposed into 12 individual monitors; 11 key operational sections and a flow versus valve monitor.
FIG. 24 shows the Methanator and H2O Driers area monitor has a warning alert.
FIG. 25 shows the result of clicking on triangle 1 in FIG. 24; a Pareto chart indicating the residual of sensor SP125 is outside its tolerance limit.
FIG. 26 shows that clicking on the Multi-Trend button brings up the trends of the value and model predictions of the sensors in the Pareto chart of FIG. 25.
FIG. 27 shows a Pareto ranking of the valve-flow models based on normalized-projection-deviation error.
FIG. 28 shows the details of the valve-flow model obtained by clicking on any bars from the bar chart of FIG. 27.
FIG. 29 shows the Fuzzy Logic networks for several procedural-induced abnormal conditions.
FIG. 30 shows a schematic diagram of a typical ethylene plant.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is a method to provide abnormal event detection (AED) to the operator that sections of an ethylene plant are not functioning properly. The present invention will be discussed with specific references to a successful application of the invention for demonstration only but not limitation.
The method uses fuzzy logic (see Appendix 1, Section Deploying PCA Models and Simple Engineering Models for AED) to combine multiple supportive evidence of abnormal situations that contribute to an operational problem and estimates its probability in realtime. The probability is presented in a continuous format to alert the operator using a single trend for a processing area of the plant. This method includes a set of tools that allow the operator to drill down to the root cause of a problem for focused action. This approach has been demonstrated to provide the operator with an advanced warning of the abnormal operation that can be minutes to hours sooner than the alarm system. This advanced warning allows the operator to take corrective action sooner and prevents escalation of the event. This method has been successfully applied to an olefins recovery train (ORT).
The ORT application uses specific operational knowledge of ORT operations to combine indications from Principal Component Analysis and engineering models, and relevant sensor readings into a fuzzy logic network as shown in FIG. 4. This fuzzy logic network aggregates the evidence and indicates the confidence level of a potential problem. Therefore, the network can detect a problem with higher confidence at its early stage and provide valuable time for the operator to make compensatory or corrective actions to avoid an ORT operational incident. This is a key advantage over the present commercial practice of monitoring ORT based on single sensor alarming from a DCS system because more often the alarm comes in too late for the operator to mitigate an operational problem due to the complicated dynamic and cryogenic nature of operating an ORT.
The ethylene recovery train is divided into equipment groups (referred to as key functional sections or operational sections). These equipment groups may be different for different ethylene recovery trains depending on its design. The procedure for choosing equipment groups which include specific process units of the ethylene recovery train is described in Appendix 1.
FIG. 30 shows a schematic diagram of a typical ethylene plant that was used to demonstrate the present invention. In the preferred embodiment for this plant, the present invention divides the ORT operation into key operational sections (see Appendix 1, section Developing PCA Models for AED). The example ethylene plant was divided into eleven sections as follows:
1. Charge Gas Compressor
2. Drier, Cold Box, and Methane Booster
3. Ethylene and Propylene Refrigeration
4. De-Methanizer
5. De-Ethanizer
6. Acetylene Converter
7. C2 Splitter
8. Methanator and H2O Drier
9. De-Propanizer
10. MAPD Hydrofiner
11. C3 Rerun and Propylene Fractionator
Besides monitoring the key operational areas, this invention also monitors the consistency between control valve position and flow for a set of key control valves (see Appendix 1, section Simple Engineering Models for AED). This invention also provides suppression of model calculations to eliminate false positives on special cause operations and enables the operator to selectively remove sensors from the models in the event that the sensor is out of service.
A. Operator Interface
The display is intended to give the operator a view of the probability that there is an abnormal event affecting the process unit.
FIG. 23 shows the operator display of the decomposition of the ORT operation into eleven key operational sections and a control valve position versus flow monitor. When the probability reaches 0.6, the problem indicator turns yellow (warning) and the indicator turns red (alert) when the probability reaches 0.9 (see Appendix 1, section Deploying PCA Models and Simple Engineering Models for AED).
The invention example includes eleven Principal Component Analysis (PCA) models to cover the entire operation of ORT (see Appendix 1, section Developing PCA Models for AED, subsection I). Based on process knowledge, we overlap key sensors that are affected by interacting sections in PCA models (see Appendix 1, Section Deploying PCA Models and Simple Engineering Models for AED, subsection III). For distillation columns, we transformed the rate dependent variables into ratios to minimize the effect of feedrate changes (see Appendix 1, section Developing PCA Models for AED, subsection IV).
ORT operation has a number of special cause operations, such as Steam Water Decoking at the cracking furnaces and Drier Regeneration for the Cold Box feed. These operations are less frequent routine operations comparing to the moves of advanced control applications and have significant temporary effects on the normality of operation. These special cause operations will give high residuals to some affected sensors in some PCA models. We use our operations knowledge of ORT and detect the onset of these special cause operations and developed suppression methodologies to remove their notifications (see Appendix 1, section Deploying PCA Models and Simple Engineering Models for AED, subsection I).
Often, in routine operations of an ORT, the operator makes setpoint changes to some key controllers in order to steer the ORT to a preferred state. Again, we developed suppression methodologies for the effect of these setpoint changes (see Appendix 1, section Deploying PCA Models and Simple Engineering Models for AED, subsection I).
Once the operator receives an indication of an abnormal condition, such as the warning alert indicated by the yellow triangle in FIG. 24, this novel method provides the operator with drill down capabilities to the leading indicator of the problems (see Appendix 1, section Deploying PCA Models and Simple Engineering Models for AED, subsection IV). FIG. 24 shows that the Methanator and H2O Driers have a warning alert. This assists the operator in isolating and diagnosing the root cause of the condition so that compensatory or corrective actions can be taken. FIG. 25 is the result of clicking on the yellow triangle in FIG. 24. FIG. 25 shows a Pareto chart indicating the residual of sensor SP125 is outside its tolerance limit and is the primary reason for the warning alert.
This drilldown tool isolates the problem area for the operator. Additional tools, such as calling up the trends of problem sensors, are also available for the operator in this application. For instance, FIG. 26 shows the trends of the value and model predictions of the sensors in the Pareto chart of FIG. 25.
This application also provides a Pareto chart for ranking the valve versus flow engineering models. FIG. 27 shows a Pareto ranking of the valve-flow models based on normalized-projection-deviation error. Clicking on a bar of the Pareto provides an operator drilldown to the details of the valve versus flow model as shown in FIG. 28.
Based on our operational knowledge of ORT, this application detects the onset of special cause operations such as steam-water decoking of the furnace and provides suppression to sensors that are affected by this operation. FIG. 29 shows the fuzzy logic network for steam-water decoking at the cracking furnaces.
In summary, the advantages of this invention include:

    
    
        1. Decomposing the entire ORT operation into key operational areas, which reduces thousands of individual measurements and alerts into a small number of easily monitored trends.
        2. The operational problems of the entire ORT are summarized into eleven single alerts for the example plant.
        3. The PCA models provide predictions of the 1300+ sensors in the example ORT.
        4. The abnormal deviations of these 1300+ sensors are summarized by the 11 alerts based on the Sum of Square Error of the 11 PCA models.
        5. Events resulting from special cause operation are suppressed to eliminate the false positives. It clearly indicates the dimensionality reduction is almost 2 order of magnitude, i.e. from 1300+ sensors to 11 indicators. Besides this tremendous dimensionality reduction, the high false positive rate of a single sensor alarm is resolved by the PCA modeling.

B. Development of Abnormal Event Detection Models for an ORT

    
    


The methodology for detecting abnormal events is described generally in Appendix 1. The steps for developing ethylene plant models are described below using the successful ORT application as an example.
The application at an ethylene plant is intended to monitor a broad portion of the process operations (about 80% of the console operator's area), provide an early warning of unexpected abnormal events and provide information on the measurements initially involved. The operator or the engineer would then rely on their process knowledge to diagnose the cause. Each event is expected to be unique or rare, and primarily the result of problems with the instruments and valves.
The following problem characteristics should be considered when selecting an abnormal event detection problem. Infrequent abnormalities (every 3-4 months) may not justify the effort to create an abnormal event detector. Also, when a particular abnormality occurs only every 3 or 4 months, an individual operator may go for years without seeing the event. As a consequence, he/she would not know what to do once the event finally occurs. Therefore the problem identification should be broad enough that the operator would be regularly interacting with the application.
When scoping the problem, it is common to get the wrong impression from site personnel that there would not be a sufficient number of abnormal events to justify an abnormal event detection application. In general, an overly low estimate of how frequently abnormal events affect the process occurs because:

    
    
        Abnormal events are often not recorded and analyzed. Only those that cause significant losses are tracked and analyzed.
        Abnormal events are often viewed as part of normal operations since operators deal with them daily.
    
    


Unless there is a regularly repeating abnormal event, the application should cover a large enough portion of the process to “see” abnormal events on a regular basis (e.g. more than 5 times each week). The abnormal event detectors are not meant to replace a solution that may be the preferred alternative (see Appendix 1, section Developing PCA Models for AED, subsection I.C).
AED Applications Do Not Replace the Alarm System
Whenever a process problem occurs quickly, the alarm system will identify the problem as quickly as an abnormal event detection application. The sequence of events (e.g. the order in which measurements become unusual) may be more useful than the order of the alarms for helping the operator diagnose the cause. This possibility should be investigated once the application is on-line.
However, abnormal event detection applications can give the operator advanced warning when abnormal events develop slowly (longer than 15 minutes). These applications are sensitive to a change in the pattern of the process data rather than requiring a large excursion by a single variable. Consequently alarms can be avoided. If the alarm system has been configured to alert the operator when the process moves away from a small operating region (not true safety alarms), this application may be able to replace these alarms.
C. Example of Development of AED Model for an Ethylene Plant
Preliminary Application Design
Application design requires two decisions: which process units should be covered by the application, and which measurements should be included in the model. In the present example, one model will be developed that incorporates several process units of the ethylene plant to maximize the dimensionality reduction of the data.
The specific process units included requires an understanding of the process integration/interaction. Similar to the design of a multivariable constraint control (MVCC such as DMC) application, the boundary of the application should encompass all significant process interactions and key upstream indications of process changes and disturbances. FIG. 30 shows the major process units included in this ethylene plant application. These units encompass a highly energy-integrated portion of a typical ORT. Often these same process units might be grouped together under one or two MVCC controllers.
Within these selected process units, there will be a substantial number of process measurements. For the preliminary design:

    
    
        Select all controller PVs, SPs and Outputs (including all intermediate cascade controllers) on these units
        Select key measurements used by the console operator to monitor the process
        Select any measurements used by the contact engineer to measure the performance of the process
        Select any upstream measurement of feedrate, feed temperature or feed quality changes
        Select extra redundant measurements for measurements that are felt to be important
        Select measurements that may be needed to calculate non-linear transformations
        Select any external measurement of a disturbance (e.g. ambient temperature)
        Select any other measurements which the process experts regard as important measures of the process condition

Exclude from this list any known erratic or problem measurements.

    
    


Following this list, about 1⁄2 of the available process measurements will be preliminarily considered for the application. These will be later trimmed down during the data analysis.
Initial Model Development
The model development strategy is to start with a very rough model (the consequence of a questionable training data set) then use the model to gather a high quality training data set. This data is then used to improve the model, which is then used to continue to gather better quality training data. This process is repeated until the model is satisfactory (see Appendix 1, section Developing PCA Models for AED).
Historical Data Collection
Developing a good model of normal operations requires a training data set of normal operations. This data set should:

    
    
        Span the normal operating range
        Only include normal operating data
    
    


Because it is very rare to have a complete record of the abnormal event history at a site, historical data can only be used as a starting point for creating the training data set. Operating records such as Operator logs, Operator Change Journals, Alarm Journals, Instrument Maintenance records provide a partial record of the abnormal process history.
The developer should gather several months of process data using the site's process historian, preferably getting one minute spot values. If this is not available, the highest resolution data, with the least amount of averaging should be used. The various operating journals for this time period should also be collected. Often setpoints and outputs are not historized in the plant historian, but that deficiency will be taken care of during later data gathering.
Data and Process Analysis
Initial Rough Data Analysis
Using the operating logs, the historical data is divided into periods with known abnormal operations and periods with no identified abnormal operations. The data with no identified abnormal operations will be the preliminary training data set.
Now each measurement needs to be examined over its history to see whether it is a candidate for the training data set. Measurements which should be excluded are:

    
    
        Those with many long periods of time as “BAD PV”
        Those with many long periods of time pegged to their EUHigh or EULow values
        Those which show very little variability (except those which are tightly controlled to their setpoints)
        Those which continuously show very large variability relative to their operating range
        Those which show little or no cross correlation with any other measurements in the data set
        Those with poor signal to noise ratios
    
    


While examining the data, those time periods where measurements are briefly indicating “BAD PV” or are briefly pegged to their EUHigh or EULow limits should also be excluded.
Once these exclusions have been made the first rough PCA model should be built as discussed in Appendix 1. Since this is going to be a very rough model the exact number of principal components to be retained is not important.
The training data set should now be run through this preliminary model to identify time periods where the data does not match the model. These time periods should be examined to see whether an abnormal event was occurring at the time. If this is judged to be the case, then these time periods should also be flagged as times with known abnormal events occurring. These time periods should be excluded from the training data set and the model rebuilt with the modified data.
Examine the Need for Dynamic Transformations
The developer should consider the need for this technique whenever there is a significant dynamic separation between variables used in the model. This will be especially true for those independent variables such as setpoints, which might be changed in large steps by the operator. Also the measurements which are upstream of the main process units being modeled may need to be dynamically reconciled.
Remove Operating Point Changes
There are continual operating point changes in process operations. These can be intentional, where the operator makes a change to a key setpoint or they can be due to process changes such as heat exchanger fouling or catalyst deactivation. To prevent these changes from appearing as abnormal events, those process measurements, which are affected by these changes, should be converted to deviation variables (see Appendix 1, section Developing PCA Models for AED, subsection IV,F).
Subtracting the exponentially filtered value of a measurement from its raw value and using this difference in the model accomplishes the operating point removal. The time constant for the exponential filter should be about the same size as the major time constant of the process. For the ethylene plant this was a time constant of about 45 minutes.
D. Creating the Initial PCA Model
Once the specific measurements have been selected and the training data set has been built, the PCA model can be built quickly using standard tools.
The engineering approach to selecting the correct number of principal components is to stop when the groups of variables, which are the primary contributors to the principal component no longer make engineering sense. This is discussed in Appendix 1, section Developing PCA Models for AED, subsection V,B. In the present case, Appendix 2 lists the named principal components for each PCA model and the number of principal components ranges from 4 to 15 for each PCA model.
Additional Engineering Models
The valve-flow consistency monitor was derived from a comparison of the measured flow (compensated for the pressure drop across the valve) with a model estimate of the flow. The model estimate of the flow is obtained from historical data by fitting coefficients to the valve curve equation (assumed to be either linear or parabolic). In the initial application, 27 valve-flow consistency models were developed. This type of model was developed to monitor the main process manipulation points. Several models were also developed for control loops that historically exhibited unreliable performance.
A time-varying drift term was added to the model estimate to compensate for long term sensor drift. The operator can also request a reset of the drift term after a sensor calibration or when a manual bypass valve has changed. A time-varying drift term was added to the model estimate to compensate for a long-term sensor drift. The operator can and has changed position. This modification to the flow estimator significantly improved the robustness for implementation within an online detection algorithm. The valve-flow consistency monitors also notify the operator in the event that a control valve is fully opened or closed.
Appendix 1 
Events and disturbances of various magnitudes are constantly affecting process operations. Most of the time these events and disturbances are handled by the process control system. However, the operator is required to make an unplanned intervention in the process operations whenever the process control system cannot adequately handle the process event. We define this situation as an abnormal operation and the cause defined as an abnormal event.
A methodology and system has been developed to create and to deploy on-line, sets of models, which are used to detect abnormal operations and help the operator isolate the location of the root cause. In a preferred embodiment, the models employ principle component analysis (PCA). These sets of models are composed of both simple models that represent known engineering relationships and principal component analysis (PCA) models that represent normal data patterns that exist within historical databases. The results from these many model calculations are combined into a small number of summary time trends that allow the process operator to easily monitor whether the process is entering an abnormal operation.
FIG. 1 shows how the information in the online system flows through the various transformations, model calculations, fuzzy Petri nets and consolidations to arrive at a summary trend which indicates the normality/abnormality of the process areas. The heart of this system is the various models used to monitor the normality of the process operations.
The PCA models described in this invention are intended to broadly monitor continuous refining and chemical processes and to rapidly detect developing equipment and process problems. The intent is to provide blanket monitoring of all the process equipment and process operations under the span of responsibility of a particular console operator post. This can involve many major refining or chemical process operating units (e.g. distillation towers, reactors, compressors, heat exchange trains, etc.) which have hundreds to thousands of process measurements. The monitoring is designed to detect problems which develop on a minutes to hours timescale, as opposed to long term performance degradation. The process and equipment problems do not need to be specified beforehand. This is in contrast to the use of PCA models cited in the literature which are structured to detect a specific important process problem and to cover a much smaller portion of the process operations.
To accomplish this objective, the method for PCA model development and deployment includes a number of novel extensions required for their application to continuous refining and chemical processes including:

    
    
        criteria for establishing the equipment scope of the PCA models criteria and methods for selecting, analyzing, and transforming measurement inputs
        developing of multivariate statistical models based on a variation of principle component models, PCA
        developing models based on simple engineering relationships restructuring the associated statistical indices
        preprocessing the on-line data to provide exception calculations and continuous on-line model updating
        using fuzzy Petri nets to interpret model indices as normal or abnormal
        using fuzzy Petri nets to combine multiple model outputs into a single continuous summary indication of normality/abnormality for a process area
        design of operator interactions with the models and fuzzy Petri nets to reflect operations and maintenance activities
    
    


These extensions are necessary to handle the characteristics of continuous refining and chemical plant operations and the corresponding data characteristics so that PCA and simple engineering models can be used effectively. These extensions provide the advantage of preventing many of the Type I and Type II errors and quicker indications of abnormal events.
This section will not provide a general background to PCA. For that, readers should refer to a standard textbook on PCA, see e.g. E. Jackson's “A User's Guide to Principal Component Analysis”, John Wiley & Sons, 1991.
The classical PCA technique makes the following statistical assumptions all of which are violated to some degree by the data generated from normal continuous refining and chemical plant process operations:

    
    
        1. The process is stationary—its mean and variance are constant over time.
        2. The cross correlation among variables is linear over the range of normal process operations
        3. Process noise random variables are mutually independent.
        4. The covariance matrix of the process variables is not degenerate (i.e. positive semi-definite).
        5. The data are scaled “appropriately” (the standard statistical approach being to scale to unit variance).
        6. There are no (uncompensated) process dynamics (a standard partial compensation for this being the inclusion of lag variables in the model)
        7. All variables have some degree of cross correlation.
        8. The data have a multivariate normal distribution
    
    


Consequently, in the selection, analysis and transformation of inputs and the subsequent in building the PCA model, various adjustments are made to evaluate and compensate for the degree of violation.
Once these PCA models are deployed on-line the model calculations require specific exception processing to remove the effect of known operation and maintenance activities, to disable failed or “bad acting” inputs, to allow the operator observe and acknowledge the propagation of an event through the process and to automatically restore the calculations once the process has returned to normal.
Use of PCA models is supplemented by simple redundancy checks that are based on known engineering relationships that must be true during normal operations. These can be as simple as checking physically redundant measurements, or as complex as material and engineering balances.
The simplest form of redundancy checks are simple 2×2 checks, e.g.

    
    
        temperature 1=temperature 2
        flow 1=valve characteristic curve 1 (valve 1 position)
        material flow into process unit 1=material flow out of process unit 1
    
    


These are shown to the operator as simple x-y plots, such as the valve flow plot in FIG. 2. Each plot has an area of normal operations, shown on this plot by the gray area. Operations outside this area are signaled as abnormal.
Multiple redundancy can also be checked through a single multidimensional model. Examples of multidimensional redundancy are:

    
    
        pressure 1=pressure 2= . . . =pressure n
        material flow into process unit 1=material flow out of process unit 1= . . . =material flow into process unit 2
    
    


Multidimensional checks are represented with “PCA like” models. In FIG. 3, there are three independent and redundant measures, X1, X2, and X3. Whenever X3 changes by one, X1 changes by a13 and X2 changes by a23. This set of relationships is expressed as a PCA model with a single principle component direction, P. This type of model is presented to the operator in a manner similar to the broad PCA models. As with the two dimensional redundancy checks the gray area shows the area of normal operations. The principle component loadings of P are directly calculated from the engineering equations, not in the traditional manner of determining P from the direction of greatest variability.
The characteristics of the process operation require exception operations to keep these relationships accurate over the normal range of process operations and normal field equipment changes and maintenance activities. Examples of exception operations are:

    
    
        opening of bypass valves around flow meters
        compensating for upstream/downstream pressure changes
        recalibration of field measurements
        redirecting process flows based on operating modes
    
    


The PCA models and the engineering redundancy checks are combined using fuzzy Petri nets to provide the process operator with a continuous summary indication of the normality of the process operations under his control (FIG. 4).
Multiple statistical indices are created from each PCA model so that the indices correspond to the configuration and hierarchy of the process equipment that the process operator handles. The sensitivity of the traditional sum of Squared Prediction Error, SPE, index is improved by creating subset indices, which only contain the contribution to the SPE index for the inputs which come from designated portions of the complete process area covered by the PCA model. Each statistical index from the PCA models is fed into a fuzzy Petri net to convert the index into a zero to one scale, which continuously indicates the range from normal operation (value of zero) to abnormal operation (value of one).
Each redundancy check is also converted to a continuous normal-abnormal indication using fuzzy nets. There are two different indices used for these models to indicate abnormality; deviation from the model and deviation outside the operating range (shown on FIG. 3). These deviations are equivalent to the sum of the square of the error and the Hotelling T square indices for PCA models. For checks with dimension greater than two, it is possible to identify which input has a problem. In FIG. 3, since the X3-X2 relationship is still within the normal envelope, the problem is with input X1. Each deviation measure is converted by the fuzzy Petri net into a zero to one scale that will continuously indicate the range from normal operation (value of zero) to abnormal operation (value of one).
For each process area under the authority of the operator, the applicable set of normal-abnormal indicators is combined into a single normal-abnormal indicator. This is done by using fuzzy Petri logic to select the worst case indication of abnormal operation. In this way the operator has a high level summary of all the checks within the process area. This section will not provide a general background to fuzzy Petri nets. For that, readers should refer to a standard reference on fuzzy Petri nets, see e.g. Cardoso, et al, Fuzzy Petri Nets: An Overview , 13th Word Congress of IFAC, Vol. 1: Identification II, Discrete Event Systems, San Francisco, Calif., USA, June 30-Jul. 5, 1996, pp 443-448.
The overall process for developing an abnormal event application is shown in FIG. 5. The basic development strategy is iterative where the developer starts with a rough model, then successively improves that model's capability based on observing how well the model represents the actual process operations during both normal operations and abnormal operations. The models are then restructured and retrained based on these observations.
Developing PCA Models for Abnormal Event Detection
I. Conceptual PCA Model Design
The overall design goals are to:

    
    
        provide the console operator with a continuous status (normal vs. abnormal) of process operations for all of the process units under his operating authority
        provide him with an early detection of a rapidly developing (minutes to hours) abnormal event within his operating authority
        provide him with only the key process information needed to diagnose the root cause of the abnormal event.
    
    


Actual root cause diagnosis is outside the scope of this invention. The console operator is expected to diagnosis the process problem based on his process knowledge and training.
Having a broad process scope is important to the overall success of abnormal operation monitoring. For the operator to learn the system and maintain his skills, he needs to regularly use the system. Since specific abnormal events occur infrequently, abnormal operations monitoring of a small portion of the process would be infrequently used by the operator, likely leading the operator to disregard the system when it finally detects an abnormal event. This broad scope is in contrast to the published modeling goal which is to design the model based on detecting a specific process problem of significant economic interest (see e.g., Kourti, “Process Analysis and Abnormal Situation Detection: From Theory to Practice”, IEEE Control Systems Magazine, October 2002pp. 10-25.)
There are thousands of process measurements within the process units under a single console operator's operating authority. Continuous refining and chemical processes exhibit significant time dynamics among these measurements, which break the cross correlation among the data. This requires dividing the process equipment into separate PCA models where the cross correlation can be maintained.
Conceptual model design is composed of four major decisions:

    
    
        Subdividing the process equipment into equipment groups with corresponding PCA models
        Subdividing process operating time periods into process operating modes requiring different PCA models
        Identifying which measurements within an equipment group should be designated as inputs to each PCA model
        Identifying which measurements within an equipment group should act as flags for suppressing known events or other exception operations

A. Process Unit Coverage

    
    


The initial decision is to create groups of equipment that will be covered by a single PCA model. The specific process units included requires an understanding of the process integration/interaction. Similar to the design of a multivariable constraint controller, the boundary of the PCA model should encompass all significant process interactions and key upstream and downstream indications of process changes and disturbances.
The following rules are used to determined these equipment groups:
Equipment groups are defined by including all the major material and energy integrations and quick recycles in the same equipment group. If the process uses a multivariable constraint controller, the controller model will explicitly identify the interaction points among the process units. Otherwise the interactions need to be identified through an engineering analysis of the process.
Process groups should be divided at a point where there is a minimal interaction between the process equipment groups. The most obvious dividing point occurs when the only interaction comes through a single pipe containing the feed to the next downstream unit. In this case the temperature, pressure, flow, and composition of the feed are the primary influences on the downstream equipment group and the pressure in the immediate downstream unit is the primary influence on the upstream equipment group. These primary influence measurements should be included in both the upstream and downstream equipment group PCA models.
Include the influence of the process control applications between upstream and downstream equipment groups. The process control applications provide additional influence paths between upstream and downstream equipment groups. Both feedforward and feedback paths can exist. Where such paths exist the measurements which drive these paths need to be included in both equipment groups. Analysis of the process control applications will indicate the major interactions among the process units.
Divide equipment groups wherever there are significant time dynamics (e.g. storage tanks, long pipelines etc.). The PCA models primarily handle quick process changes (e.g. those which occur over a period of minutes to hours). Influences, which take several hours, days or even weeks to have their effect on the process, are not suitable for PCA models. Where these influences are important to the normal data patterns, measurements of these effects need to be dynamically compensated to get their effect time synchronized with the other process measurements (see the discussion of dynamic compensation).
B. Process Operating Modes
Process operating modes are defined as specific time periods where the process behavior is significantly different. Examples of these are production of different grades of product (e.g. polymer production), significant process transitions (e.g. startups, shutdowns, feedstock switches), processing of dramatically different feedstock (e.g. cracking naphtha rather than ethane in olefins production), or different configurations of the process equipment (different sets of process units running).
Where these significant operating modes exist, it is likely that separate PCA models will need to be developed for each major operating mode. The fewer models needed the better. The developer should assume that a specific PCA model could cover similar operating modes. This assumption must be tested by running new data from each operating mode through the model to see if it behaves correctly.
C. Historical Process Problems
In order for there to be organizational interest in developing an abnormal event detection system, there should be an historical process problem of significant economic impact. However, these significant problems must be analyzed to identify the best approach for attacking these problems. In particular, the developer should make the following checks before trying to build an abnormal event detection application:

    1. Can the problem be permanently fixed? Often a problem exists because site personnel have not had sufficient time to investigate and permanently solve the problem. Once the attention of the organization is focused on the problem, a permanent solution is often found. This is the best approach.
    2. Can the problem be directly measured? A more reliable way to detect a problem is to install sensors that can directly measure the problem in the process. This can also be used to prevent the problem through a process control application. This is the second best approach.
    3. Can an inferential measurement be developed which will measure the approach to the abnormal operation? Inferential measurements are usually developed using partial least squares, PLS, models which are very close relatives to PCA abnormal event models. Other common alternatives for developing inferential measurements include Neural Nets and linear regression models. If the data exists which can be used to reliably measure the approach to the problem condition (e.g. tower flooding using delta pressure), this can then be used to not only detect when the condition exists but also as the base for a control application to prevent the condition from occurring. This is the third best approach.


Both direct measurements of problem conditions and inferential measurements of these conditions can be easily integrated into the overall network of abnormal detection models.
II. Input Data and Operating Range Selection
Within an equipment group, there will be thousands of process measurements. For the preliminary design:

    
    
        Select all cascade secondary controller measurements, and especially ultimate secondary outputs (signals to field control valves) on these units
        Select key measurements used by the console operator to monitor the process (e.g. those which appear on his operating schematics)
        Select any measurements used by the contact engineer to measure the performance of the process
        Select any upstream measurement of feedrate, feed temperature or feed quality
        Select measurements of downstream conditions which affect the process operating area, particularly pressures.
        Select extra redundant measurements for measurements that are important
        Select measurements that may be needed to calculate non-linear transformations.
        Select any external measurement of a disturbance (e.g. ambient temperature)
        Select any other measurements, which the process experts regard as important measures of the process condition
    
    


From this list only include measurements which have the following characteristics:

    
    
        The measurement does not have a history of erratic or problem performance
        The measurement has a satisfactory signal to noise ratio
        The measurement is cross-correlated with other measurements in the data set
        The measurement is not saturated for more than 10% of the time during normal operations.
        The measurement is not tightly controlled to a fixed setpoint, which rarely changes (the ultimate primary of a control hierarchy).
        The measurement does not have long stretches of “Bad Value” operation or saturated against transmitter limits.
        The measurement does not go across a range of values, which is known to be highly non-linear
        The measurement is not a redundant calculation from the raw measurements
        The signals to field control valves are not saturated for more than 10% of the time

A. Evaluations for Selecting Model Inputs

    
    


There are two statistical criteria for prioritizing potential inputs into the PCA Abnormal Detection Model, Signal to Noise Ratio and Cross-Correlation.
1) Signal to Noise Test
The signal to noise ratio is a measure of the information content in the input signal.
The signal to noise ratio is calculated as follows:

    1. The raw signal is filtered using an exponential filter with an approximate dynamic time constant equivalent to that of the process. For continuous refining and chemical processes this time constant is usually in the range of 30 minutes to 2 hours. Other low pass filters can be used as well. For the exponential filter the equations are:

Yn=P*Yn−1+(1−P)*Xn Exponential filter equation   Equation 1

P=Exp(−Ts/Tf) Filter constant calculation   Equation 2

where:



Yn the current filtered value
Yn−1 the previous filtered value
Xn the current raw value
P the exponential filter constant
Ts the sample time of the measurement
Tf the filter time constant

    2. A residual signal is created by subtracting the filtered signal from the raw signal

Rn=Xn−Yn   Equation 3

    3. The signal to noise ratio is the ratio of the standard deviation of the filtered signal divided by the standard deviation of the residual signal

S/N=σY/σR   Equation 4



It is preferable to have all inputs exhibit a S/N which is greater than a predetermined minimum, such as 4. Those inputs with S/N less than this minimum need individual examination to determine whether they should be included in the model
The data set used to calculate the S/N should exclude any long periods of steady-state operation since that will cause the estimate for the noise content to be excessively large.
2) Cross Correlation Test
The cross correlation is a measure of the information redundancy the input data set. The cross correlation between any two signals is calculated as:

    1. Calculate the co-variance, Sik, between each input pair, i and k






  
    
      
        
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    2. Calculate the correlation coefficient for each pair of inputs from the co-variance:

CCik=Sik/(Sii*Skk)1/2   Equation 6



There are two circumstances, which flag that an input should not be included in the model. The first circumstance occurs when there is no significant correlation between a particular input and the rest of the input data set. For each input, there must be at least one other input in the data set with a significant correlation coefficient, such as 0.4.
The second circumstance occurs when the same input information has been (accidentally) included twice, often through some calculation, which has a different identifier. Any input pairs that exhibit correlation coefficients near one (for example above 0.95) need individual examination to determine whether both inputs should be included in the model. If the inputs are physically independent but logically redundant (i.e., two independent thermocouples are independently measuring the same process temperature) then both these inputs should be included in the model.
If two inputs are transformations of each other (i.e., temperature and pressure compensated temperature) the preference is to include the measurement that the operator is familiar with, unless there is a significantly improved cross correlation between one of these measurements and the rest of the dataset. Then the one with the higher cross correlation should be included.
3) Identifying & Handling Saturated Variables
Refining and chemical processes often run against hard and soft constraints resulting in saturated values and “Bad Values” for the model inputs. Common constraints are: instrument transmitter high and low ranges, analyzer ranges, maximum and minimum control valve positions, and process control application output limits. Inputs can fall into several categories with regard to saturation which require special handling when pre-processing the inputs, both for model building and for the on-line use of these models.
Bad Values
For standard analog instruments (e.g., 4-20 milliarpp electronic transmitters), bad values can occur because of two separate reasons:

    
    
        The actual process condition is outside the range of the field transmitter
        The connection with the field has been broken
    
    


When either of these conditions occur, the process control system could be configured on an individual measurement basis to either assign a special code to the value for that measurement to indicate that the measurement is a Bad Value, or to maintain the last good value of the measurement. These values will then propagate throughout any calculations performed on the process control system. When the “last good value” option has been configured, this can lead to erroneous calculations that are difficult to detect and exclude. Typically when the “Bad Value” code is propagated through the system, all calculations which depend on the bad measurement will be flagged bad as well.
Regardless of the option configured on the process control system, those time periods, which include Bad Values should not be included in training or test data sets. The developer needs to identify which option has been configured in the process control system and then configure data filters for excluding samples, which are Bad Values. For the on-line implementation, inputs must be pre-processed so that Bad Values are flagged as missing values, regardless of which option had been selected on the process control system.
Those inputs, which are normally Bad Value for extensive time periods should be excluded from the model.
Constrained Variables
Constrained variables are ones where the measurement is at some limit, and this measurement matches an actual process condition (as opposed to where the value has defaulted to the maximum or minimum limit of the transmitter range—covered in the Bad Value section). This process situation can occur for several reasons:

    
    
        Portions of the process are normally inactive except under special override conditions, for example pressure relief flow to the flare system. Time periods where these override conditions are active should be excluded from the training and validation data set by setting up data filters. For the on-line implementation these override events are trigger events for automatic suppression of selected model statistics
        The process control system is designed to drive the process against process operating limits, for example product spec limits. These constraints typically fall into two categories:—those, which are occasionally saturated and those, which are normally saturated. Those inputs, which are normally saturated, should be excluded from the model. Those inputs, which are only occasionally saturated (for example less than 10% of the time) can be included in the model however, they should be scaled based on the time periods when they are not saturated.

B. Input from Process Control Applications

    
    


The process control applications have a very significant effect on the correlation structure of the process data. In particular:

    
    
        The variation of controlled variables is significantly reduced so that movement in the controlled variables is primarily noise except for those brief time periods when the process has been hit with a significant process disturbance or the operator has intentionally moved the operating point by changing key setpoints.
        The normal variation in the controlled variables is transferred by the control system to the manipulated variables (ultimately the signals sent to the control valves in the field).
    
    


The normal operations of refinery and chemical processes are usually controlled by two different types of control structures: the classical control cascades (shown in FIG. 6) and the more recent multivariable constraint controllers, MVCC (shown in FIG. 7).
1) Selecting Model Inputs From Cascade Structures
FIG. 6 shows a typical “cascade” process control application, which is a very common control structure for refining and chemical processes. Although there are many potential model inputs from such an application, the only ones that are candidates for the model are the raw process measurements (the “PVs” in this figure ) and the final output to the field valve.
Although it is a very important measurement, the PV of the ultimate primary of the cascade control structure is a poor candidate for inclusion in the model. This measurement usually has very limited movement since the objective of the control structure is to keep this measurement at the setpoint. There can be movement in the PV of the ultimate primary if its setpoint is changed but this usually is infrequent. The data patterns from occasional primary setpoint moves will usually not have sufficient power in the training dataset for the model to characterize the data pattern.
Because of this difficulty in characterizing the data pattern resulting from changes in the setpoint of the ultimate primary, when the operator makes this setpoint move, it is likely to cause a significant increase in the sum of squared prediction error, SPE, index of the model. Consequently, any change in the setpoint of the ultimate primary is a candidate trigger for a “known event suppression”. Whenever the operator changes an ultimate primary setpoint, the “known event suppression” logic will automatically remove its effect from the SPE calculation.
Should the developer include the PV of the ultimate primary into the model, this measurement should be scaled based on those brief time periods during which the operator has changed the setpoint and until the process has moved close to the vale of the new setpoint (for example within 95% of the new setpoint change thus if the setpoint change is from 10 to 11, when the PV reaches 10.95)
There may also be measurements that are very strongly correlated (for example greater than 0.95 correlation coefficient) with the PV of the Ultimate Primary, for example redundant thermocouples located near a temperature measurement used as a PV for an Ultimate Primary. These redundant measurements should be treated in the identical manner that is chosen for the PV of the Ultimate Primary.
Cascade structures can have setpoint limits on each secondary and can have output limits on the signal to the field control valve. It is important to check the status of these potentially constrained operations to see whether the measurement associated with a setpoint has been operated in a constrained manner or whether the signal to the field valve has been constrained. Date during these constrained operations should not be used.
2) Selecting/Calculating Model Inputs from Multivariable Constraint Controllers, MVCC
FIG. 7 shows a typical MVCC process control application, which is a very common control structure for refining and chemical processes. An MVCC uses a dynamic mathematical model to predict how changes in manipulated variables, MVs, (usually valve positions or setpoints of regulatory control loops) will change control variables, CVs (the dependent temperatures, pressures, compositions and flows which measure the process state). An MVCC attempts to push the process operation against operating limits. These limits can be either MV limits or CV limits and are determined by an external optimizer. The number of limits that the process operates against will be equal to the number of MVs the controller is allowed to manipulate minus the number of material balances controlled. So if an MVCC has 12 MVs, 30 CVs and 2 levels then the process will be operated against 10 limits. An MVCC will also predict the effect of measured load disturbances on the process and compensate for these load disturbances (known as feedforward variables, FF).
Whether or not a raw MV or CV is a good candidate for inclusion in the PCA model depends on the percentage of time that MV or CV is held against its operating limit by the MVCC. As discussed in the Constrained Variables section, raw variables that are constrained more than 10% of the time are poor candidates for inclusion in the PCA model. Normally unconstrained variables should be handled per the Constrained Variables section discussion.
If an unconstrained MV is a setpoint to a regulatory control loop, the setpoint should not be included, instead the measurement of that regulatory control loop should be included. The signal to the field valve from that regulatory control loop should also be included.
If an unconstrained MV is a signal to a field valve position, then it should be included in the model.
C. Redundant Measurements
The process control system databases can have a significant redundancy among the candidate inputs into the PCA model. One type of redundancy is “physical redundancy”, where there are multiple sensors (such as thermocouples) located in close physical proximity to each other within the process equipment. The other type of redundancy is “calculational redundancy”, where raw sensors are mathematically combined into new variables (e.g. pressure compensated temperatures or mass flows calculated from volumetric flow measurements).
As a general rule, both the raw measurement and an input which is calculated from that measurement should not be included in the model. The general preference is to include the version of the measurement that the process operator is most familiar with. The exception to this rule is when the raw inputs must be mathematically transformed in order to improve the correlation structure of the data for the model. In that case the transformed variable should be included in the model but not the raw measurement.
Physical redundancy is very important for providing cross validation information in the model. As a general rule, raw measurements, which are physically redundant should be included in the model. When there are a large number of physically redundant measurements, these measurements must be specially scaled so as to prevent them from overwhelming the selection of principle components (see the section on variable scaling). A common process example occurs from the large number of thermocouples that are placed in reactors to catch reactor runaways.
When mining a very large database, the developer can identify the redundant measurements by doing a cross-correlation calculation among all of the candidate inputs. Those measurement pairs with a very high cross-correlation (for example above 0.95) should be individually examined to classify each pair as either physically redundant or calculationally redundant.
III. Historical Data Collection
A significant effort in the development lies in creating a good training data set, which is known to contain all modes of normal process operations. This data set should:
Span the normal operating range: Datasets, which span small parts of the operating range, are composed mostly of noise. The range of the data compared to the range of the data during steady state operations is a good indication of the quality of the information in the dataset.
Include all normal operating modes (including seasonal mode variations). Each operating mode may have different correlation structures. Unless the patterns, which characterize the operating mode, are captured by the model, these unmodeled operating modes will appear as abnormal operations.
Only include normal operating data: If strong abnormal operating data is included in the training data, the model will mistakenly model these abnormal operations as normal operations. Consequently, when the model is later compared to an abnormal operation, it may not detect the abnormality operations.
History should be as similar as possible to the data used in the on-line system: The online system will be providing spot values at a frequency fast enough to detect the abnormal event. For continuous refining and chemical operations this sampling frequency will be around one minute. Within the limitations of the data historian, the training data should be as equivalent to one-minute spot values as possible.
The strategy for data collection is to start with a long operating history (usually in the range of 9 months to 18 months), then try to remove those time periods with obvious or documented abnormal events. By using such a long time period,

    
    
        the smaller abnormal events will not appear with sufficient strength in the training data set to significantly influence the model parameters
        most operating modes should have occurred and will be represented in the data.

A. Historical Data Collection Issues

1) Data Compression

    
    


Many historical databases use data compression to minimize the storage requirements for the data. Unfortunately, this practice can disrupt the correlation structure of the data. At the beginning of the project the data compression of the database should be turned off and the spot values of the data historized. Final models should be built using uncompressed data whenever possible. Averaged values should not be used unless they are the only data available, and then with the shortest data average available.
2) Length of Data History
For the model to properly represent the normal process patterns, the training data set needs to have examples of all the normal operating modes, normal operating changes and changes and normal minor disturbances that the process experiences. This is accomplished by using data from over a long period of process operations (e.g. 9-18 months). In particular, the differences among seasonal operations (spring, summer, fall and winter) can be very significant with refinery and chemical processes.
Sometimes these long stretches of data are not yet available (e.g. after a turnaround or other significant reconfiguration of the process equipment). In these cases the model would start with a short initial set of training data (e.g. 6 weeks) then the training dataset is expanded as further data is collected and the model updated monthly until the models are stabilized (e.g. the model coefficients don't change with the addition of new data)
3) Ancillary Historical Data
The various operating journals for this time period should also be collected. This will be used to designate operating time periods as abnormal, or operating in some special mode that needs to be excluded from the training dataset. In particular, important historical abnormal events can be selected from these logs to act as test cases for the models.
4) Lack of Specific Measurement History
Often setpoints and controller outputs are not historized in the plant process data historian. Historization of these values should immediately begin at the start of the project.
5) Operating Modes
Old data that no longer properly represents the current process operations should be removed from the training data set. After a major process modification, the training data and PCA model may need to be rebuilt from scratch. If a particular type of operation is no longer being done, all data from that operation should be removed from the training data set.
Operating logs should be used to identify when the process was run under different operating modes. These different modes may require separate models. Where the model is intended to cover several operating modes, the number of samples in the training dataset from each operating model should be approximately equivalent.
6) Sampling Rate
The developer should gather several months of process data using the site's process historian, preferably getting one minute spot values. If this is not available, the highest resolution data, with the least amount of averaging should be used.
7) Infrequently Sampled Measurements
Quality measurements (analyzers and lab samples) have a much slower sample frequency than other process measurements, ranging from tens of minutes to daily. In order to include these measurements in the model a continuous estimate of these quality measurements needs to be constructed. FIG. 8 shows the online calculation of a continuous quality estimate. This same model structure should be created and applied to the historical data. This quality estimate then becomes the input into the PCA model.
8) Model Triggered Data Annotation
Except for very obvious abnormalities, the quality of historical data is difficult to determine. The inclusion of abnormal operating data can bias the model. The strategy of using large quantities of historical data will compensate to some degree the model bias caused by abnormal operating in the training data set. The model built from historical data that predates the start of the project must be regarded with suspicion as to its quality. The initial training dataset should be replaced with a dataset, which contains high quality annotations of the process conditions, which occur during the project life.
The model development strategy is to start with an initial “rough” model (the consequence of a questionable training data set) then use the model to trigger the gathering of a high quality training data set. As the model is used to monitor the process, annotations and data will be gathered on normal operations, special operations, and abnormal operations. Anytime the model flags an abnormal operation or an abnormal event is missed by the model, the cause and duration of the event is annotated. In this way feedback on the model's ability to monitor the process operation can be incorporated in the training data. This data is then used to improve the model, which is then used to continue to gather better quality training data. This process is repeated until the model is satisfactory.
IV. Data & Process Analysis
A. Initial Rough Data Analysis
Using the operating logs and examining the process key performance indicators, the historical data is divided into periods with known abnormal operations and periods with no identified abnormal operations. The data with no identified abnormal operations will be the training data set.
Now each measurement needs to be examined over its history to see whether it is a candidate for the training data set. Measurements which should be excluded are:

    
    
        Those with many long periods of time as “Bad Value”
        Those with many long periods of time pegged to their transmitter high or low limits
        Those, which show very little variability (except those, which are tightly controlled to their setpoints)
        Those that continuously show very large variability relative to their operating range
        Those that show little or no cross correlation with any other measurements in the data set
        Those with poor signal to noise ratios
    
    


While examining the data, those time periods where measurements are briefly indicating “Bad Value” or are briefly pegged to their transmitter high or low limits should also be excluded.
Once these exclusions have been made the first rough PCA model should be built. Since this is going to be a very rough model the exact number of principal components to be retained is not important. This will typically be around 5% of the number measurements included in the model. The number of PCs should ultimately match the number of degrees of freedom in the process, however this is not usually known since this includes all the different sources of process disturbances. There are several standard methods for determining how many principal components to include. Also at this stage the statistical approach to variable scaling should be used: scale all variables to unit variance.

X′=(X−Xavg)/σ  Equation 7

The training data set should now be run through this preliminary model to identify time periods where the data does not match the model. These time periods should be examined to see whether an abnormal event was occurring at the time. If this is judged to be the case, then these time periods should also be flagged as times with known abnormal events occurring. These time periods should be excluded from the training data set and the model rebuilt with the modified data.
B. Removing Outliers and Periods of Abnormal Operations
Eliminating obvious abnormal events will be done through the following:

    Removing documented events. It is very rare to have a complete record of the abnormal event history at a site. However, significant operating problems should be documented in operating records such as operator logs, operator change journals, alarm journals, and instrument maintenance records. These are only providing a partial record of the abnormal event history.
    Removing time periods where key performance indicators, KPIs, are abnormal. Such measurements as feed rates, product rates, product quality are common key performance indicators. Each process operation may have additional KPIs that are specific to the unit. Careful examination of this limited set of measurements will usually give a clear indication of periods of abnormal operations. FIG. 9 shows a histogram of a KPI. Since the operating goal for this KPI is to maximize it, the operating periods where this KPI is low are likely abnormal operations. Process qualities are often the easiest KPIs to analyze since the optimum operation is against a specification limit and they are less sensitive to normal feed rate variations.

C. Compensating for Noise



By noise we are referring to the high frequency content of the measurement signal which does not contain useful information about the process. Noise can be caused by specific process conditions such as two-phase flow across an orifice plate or turbulence in the level. Noise can be caused by electrical inductance. However, significant process variability, perhaps caused by process disturbances is useful information and should not be filtered out.
There are two primary noise types encountered in refining and chemical process measurements: measurement spikes and exponentially correlated continuous noise. With measurement spikes, the signal jumps by an unreasonably large amount for a short number of samples before returning to a value near its previous value. Noise spikes are removed using a traditional spike rejection filter such as the Union filter.
The amount of noise in the signal can be quantified by a measure known as the signal to noise ratio (see FIG. 10). This is defined as the ratio of the amount of signal variability due to process variation to the amount of signal variability due to high frequency noise. A value below four is a typical value for indicating that the signal has substantial noise, and can harm the model's effectiveness.
Whenever the developer encounters a signal with significant noise, he needs to make one of three choices. In order of preference, these are:

    
    
        Fix the signal by removing the source of the noise (the best answer)
        Remove/minimize the noise through filtering techniques
        Exclude the signal from the model
    
    


Typically for signals with signal to noise ratios between 2 and 4, the exponentially correlated continuous noise can be removed with a traditional low pass filter such as an exponential filter. The equations for the exponential filter are:

Yn=P*Yn−1+(1−P)*Xn Exponential filter equation   Equation 8

P=Exp(−Ts/Tf) Filter constant calculation   Equation 9

Yn is the current filtered value
Yn−1 is the previous filtered value
Xn is the current raw value
P is the exponential filter constant
Ts is the sample time of the measurement
Tf is the filter time constant
Signals with very poor signal to noise ratios (for example less than 2) may not be sufficiently improved by filtering techniques to be directly included in the model. If the input is regarded as important, the scaling of the variable should be set to de-sensitize the model by significantly increasing the size of the scaling factor (typically by a factor in the range of 2-10).
D. Transformed Variables
Transformed variables should be included in the model for two different reasons.
First, based on an engineering analysis of the specific equipment and process chemistry, known non-linearities in the process should be transformed and included in the model. Since one of the assumptions of PCA is that the variables in the model are linearly correlated, significant process or equipment non-linearities will break down this correlation structure and show up as a deviation from the model. This will affect the usable range of the model.
Examples of well known non-linear transforms are:

    
    
        Reflux to feed ratio in distillation columns
        Log of composition in high purity distillation
        Pressure compensated temperature measurement
        Sidestream yield
        Flow to valve position (FIG. 2)
        Reaction rate to exponential temperature change
    
    


Second, the data from process problems, which have occurred historically, should also be examined to understand how these problems show up in the process measurements. For example, the relationship between tower delta pressure and feedrate is relatively linear until the flooding point is reached, when the delta pressure will increase exponentially. Since tower flooding is picked up by the break in this linear correlation, both delta pressure and feed rate should be included. As another example, catalyst flow problems can often be seen in the delta pressures in the transfer line. So instead of including the absolute pressure measurements in the model, the delta pressures should be calculated and included.
E. Dynamic Transformations
FIG. 11 shows how the process dynamics can disrupt the correlation between the current values of two measurements. During the transition time one value is constantly changing while the other is not, so there is no correlation between the current values during the transition. However these two measurements can be brought back into time synchronization by transforming the leading variable using a dynamic transfer function. Usually a first order with deadtime dynamic model (shown in Equation 9 in the Laplace transform format) is sufficient to time synchronize the data.




  
    
      
        
          
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Y—raw data
Y′—time synchronized data
T—time constant
Θ—deadtime
S—Laplace Transform parameter
This technique is only needed when there is a significant dynamic separation between variables used in the model. Usually only 1-2% of the variables requires this treatment. This will be true for those independent variables such as setpoints which are often changed in large steps by the operator and for the measurements which are significantly upstream of the main process units being modeled.
F. Removing Average Operating Point
Continuous refining and chemical processes are constantly being moved from one operating point to another. These can be intentional, where the operator or an optimization program makes changes to a key setpoints, or they can be due to slow process changes such as heat exchanger fouling or catalyst deactivation. Consequently, the raw data is not stationary. These operating point changes need to be removed to create a stationary dataset. Otherwise these changes erroneously appear as abnormal events.
The process measurements are transformed to deviation variables: deviation from a moving average operating point. This transformation to remove the average operating point is required when creating PCA models for abnormal event detection. This is done by subtracting the exponentially filtered value (see Equations 8 and 9 for exponential filter equations) of a measurement from its raw value and using this difference in the model.

X′=X−Xfiltered   Equation 10

X′—measurement transformed to remove operating point changes
X—original raw measurement
Xfiltered—exponentially filtered raw measurement
The time constant for the exponential filter should be about the same size as the major time constant of the process. Often a time constant of around 40 minutes will be adequate. The consequence of this transformation is that the inputs to the PCA model are a measurement of the recent change of the process from the moving average operating point.
In order to accurately perform this transform, the data should be gathered at the sample frequency that matches the on-line system, often every minute or faster. This will result in collecting 525,600 samples for each measurement to cover one year of operating data. Once this transformation has been calculated, the dataset is resampled to get down to a more manageable number of samples, typically in the range of 30,000 to 50,000 samples.
V. Model Creation
Once the specific measurements have been selected and the training data set has been built, the model can be built quickly using standard tools.
A. Scaling Model Inputs
The performance of PCA models is dependent on the scaling of the inputs. The traditional approach to scaling is to divide each input by its standard deviation, σ, within the training data set.

Xi′=Xi/σi   Equation 11

For input sets that contain a large number of nearly identical measurements (such as multiple temperature measurements of fixed catalyst reactor beds) this approach is modified to further divide the measurement by the square root of the number of nearly identical measurements.
For redundant data groups

Xi′=Xi/(σi*sqrt(N))   Equation 12

Where N=number of inputs in redundant data group
These traditional approaches can be inappropriate for measurements from continuous refining and chemical processes. Because the process is usually well controlled at specified operating points, the data distribution is a combination of data from steady state operations and data from “disturbed” and operating point change operations. These data will have overly small standard deviations from the preponderance of steady state operation data. The resulting PCA model will be excessively sensitive to small to moderate deviations in the process measurements.
For continuous refining and chemical processes, the scaling should be based on the degree of variability that occurs during normal process disturbances or during operating point changes not on the degree of variability that occurs during continuous steady state operations. For normally unconstrained variables, there are two different ways of determining the scaling factor.
First is to identify time periods where the process was not running at steady state, but was also not experiencing a significant abnormal event. A limited number of measurements act as the key indicators of steady state operations. These are typically the process key performance indicators and usually include the process feed rate, the product production rates and the product quality. These key measures are used to segment the operations into periods of normal steady state operations, normally disturbed operations, and abnormal operations. The standard deviation from the time periods of normally disturbed operations provides a good scaling factor for most of the measurements.
An alternative approach to explicitly calculating the scaling based on disturbed operations is to use the entire training data set as follows. The scaling factor can be approximated by looking at the data distribuion outside of 3 standard deviations from the mean. For example, 99.7% of the data should lie, within 3 standard deviations of the mean and that 99.99% of the data should lie, within 4 standard deviations of the mean. The span of data values between 99.7% and 99.99% from the mean can act as an approximation for the standard deviation of the “disturbed” data in the data set. See FIG. 12.
Finally, if a measurement is often constrained (see the discussion on saturated variables) only those time periods where the variable is unconstrained should be used for calculating the standard deviation used as the scaling factor.
B. Selecting the Number of Principal Components
PCA transforms the actual process variables into a set of independent variables called Principal Components, PC, which are linear combinations of the original variables (Equation 13).

PCi=Ai,1*X1+Ai,2*X2+Ai,3*X3+  Equation 13

The process will have a number of degrees of freedom, which represent the specific independent effects that influence the process. These different independent effects show up in the process data as process variation. Process variation can be due to intentional changes, such as feed rate changes, or unintentional disturbances, such as ambient temperature variation.
Each principal component models a part of the process variability caused by these different independent influences on the process. The principal components are extracted in the direction of decreasing variation in the data set, with each subsequent principal component modeling less and less of the process variability. Significant principal components represent a significant source of process variation, for example the first principal component usually represents the effect of feed rate changes since this is usually the source of the largest process changes. At some point, the developer must decide when the process variation modeled by the principal components no longer represents an independent source of process variation.
The engineering approach to selecting the correct number of principal components is to stop when the groups of variables, which are the primary contributors to the principal component no longer make engineering sense. The primary cause of the process variation modeled by a PC is identified by looking at the coefficients, Ai,n, of the original variables (which are called loadings). Those coefficients, which are relatively large in magnitude, are the major contributors to a particular PC. Someone with a good understanding of the process should be able to look at the group of variables, which are the major contributors to a PC and assign a name (e.g. feed rate effect) to that PC. As more and more PCs are extracted from the data, the coefficients become more equal in size. At this point the variation being modeled by a particular PC is primarily noise.
The traditional statistical method for determining when the PC is just modeling noise is to identify when the process variation being modeled with each new PC becomes constant. This is measured by the PRESS statistic, which plots the amount of variation modeled by each successive PC (FIG. 13). Unfortunately this test is often ambiguous for PCA models developed on refining and chemical processes.
VI. Model Testing & Tuning
The process data will not have a gaussian or normal distribution. Consequently, the standard statistical method of setting the trigger for detecting an abnormal event at 3 standard deviations of the error residual should not be used. Instead the trigger point needs to be set empirically based on experience with using the model.
Initially the trigger level should be set so that abnormal events would be signaled at a rate acceptable to the site engineer, typically 5 or 6 times each day. This can be determined by looking at the SPEx statistic for the training data set (this is also referred to as the Q statistic or the DMODx statistic). This level is set so that real abnormal events will not get missed but false alarms will not overwhelm the site engineer.
A. Enhancing the Model
Once the initial model has been created, it needs to be enhanced by creating a new training data set. This is done by using the model to monitor the process. Once the model indicates a potential abnormal situation, the engineer should investigate and classify the process situation. The engineer will find three different situations, either some special process operation is occurring, an actual abnormal situation is occurring, or the process is normal and it is a false indication.
The new training data set is made up of data from special operations and normal operations. The same analyses as were done to create the initial model need to be performed on the data, and the model re-calculated. With this new model the trigger lever will still be set empirically, but now with better annotated data, this trigger point can be tuned so as to only give an indication when a true abnormal event has occurred.
Simple Engineering Models for Abnormal Event Detection
The physics, chemistry, and mechanical design of the process equipment as well as the insertion of multiple similar measurements creates a substantial amount of redundancy in the data from continuous refining and chemical processes. This redundancy is called physical redundancy when identical measurements are present, and calculational redundancy when the physical, chemical, or mechanical relationships are used to perform independent but equivalent estimates of a process condition. This class of model is called an engineering redundancy model.
I. Two Dimensional Engineering Redundancy Models
This is the simplest form of the model and it has the generic form:

F(yi)=G(xi)+filtered biasi+operator bias+errori   Equation 14

raw biasi=F(yi)−{G(xi)+filtered biasi+operator bias}=errori   Equation 15

filtered biasi=filtered biasi−1+N*raw biasi−1   Equation 16

N—convergence factor (e.g. 0.0001 )
Normal operating range: xmin<x<xmax
Normal model deviation: −(max_error)<error<(max_error)
The “operator bias” term is updated whenever the operator determines that there has been some field event (e.g. opening a bypass flow) which requires the model to be shifted. On the operator's command, the operator bias term is updated so that Equation 14 is exactly satisfied (errori=0)
The “filtered bias” term updates continuously to account for persistent unmeasured process changes that bias the engineering redundancy model. The convergence factor, “N”, is set to eliminate any persistent change after a user specified time period, usually on the time scale of days.
The “normal operating range” and the “normal model deviation” are determined from the historical data for the engineering redundancy model. In most cases the max_error value is a single value, however this can also be a vector of values that is dependent on the x axis location.
Any two dimensional equation can be represented in this manner. Material balances, energy balances, estimated analyzer readings versus actual analyzer readings, compressor curves, etc. FIG. 14 shows a two dimensional energy balance.
As a case in point the flow versus valve position model is explained in greater detail.
A. The Flow Versus Valve Position Model
A particularly valuable engineering redundancy model is the flow versus valve position model. This model is graphically shown in FIG. 2. The particular form of this model is:




  
    
      
        
          
            Flow
            
              
                (
                
                  Delta_Pressure
                  /
                  
                    Delta_Pressure
                    reference
                  
                
                )
              
              a
            
          
          +
          
            filtered
            ⁢
            
              
            
            ⁢
            bias
          
          +
          
            operator
            ⁢
            
              
            
            ⁢
            bias
          
        
        =
        
          Cv
          ⁡
          
            (
            VP
            )
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        17
      
    
  




where:

Flow: measured flow through a control valve
Delta_Pressure=closest measured upstream pressure−closest measured downstream pressure
Delta_Pressurereference: average Delta_Pressure during normal operation
a: model parameter fitted to historical data
Cv: valve characteristic curve determined empirically from historical data
VP: signal to the control valve (not the actual control valve position)
The objectives of this model are to:


    
    
        Detecting sticking/stuck control valves
        Detecting frozen/failed flow measurements
        Detecting control valve operation where the control system loses control of the flow
    
    


This particular arrangement of the flow versus valve equation is chosen for human factors reasons: the x-y plot of the equation in this form is the one most easily understood by the operators. It is important for any of these models that they be arranged in the way which is most likely to be easily understood by the operators.
B. Developing the Flow Versus Valve Position Model
Because of the long periods of steady state operation experienced by continuous refining and chemical processes, a long historical record (1 to 2 years) may be required to get sufficient data to span the operation of the control valve. FIG. 15 shows a typical stretch of Flow, Valve Position, and Delta Pressure data with the long periods of constant operation. The first step is to isolate the brief time periods where there is some significant variation in the operation, as shown. This should be then mixed with periods of normal operation taken from various periods in history.
Often, either the Upstream_Pressure (often a pump discharge) or the Downstream_Pressure is not available. In those cases the missing measurement becomes a fixed model parameter in the model. If both pressures are missing then it is impossible to include the pressure effect in the model.
The valve characteristic curve can be either fit with a linear valve curve, with a quadratic valve curve or with a piecewise linear function. The piecewise linear function is the most flexible and will fit any form of valve characteristic curve.
The theoretical value for “a” is 1⁄2 if the measurements are taken directly across the valve. Rarely are the measurements positioned there. “a” becomes an empirically determined parameter to account for the actual positioning of the pressure measurements.
Often there will be very few periods of time with variations in the Delta_Pressure. The noise in the Delta_Pressure during the normal periods of operation can confuse the model-fitting program. To overcome this, the model is developed in two phases, first where a small dataset, which only contains periods of Delta_Pressure variation is used to fit the model. Then the pressure dependent parameters (“a” and perhaps the missing upstream or downstream pressure) are fixed at the values determined, and the model is re-developed with the larger dataset.
C. Fuzzy-net Processing of Flow Versus Valve Abnormality Indications
As with any two-dimensional engineering redundancy model, there are two measures of abnormality, the “normal operating range” and the “normal model deviation”. The “normal model deviation” is based on a normalized index: the error/max_error. This is fed into a type 4 fuzzy discriminator (FIG. 16). The developer can pick the transition from normal (value of zero) to abnormal (value of 1) in a standard way by using the normalized index.
The “normal operating range” index is the valve position distance from the normal region. It typically represents the operating region of the valve where a change in valve position will result in little or no change in the flow through the valve. Once again the developer can use the type 4 fuzzy discriminator to cover both the upper and lower ends of the normal operating range and the transition from normal to abnormal operation.
D. Grouping Multiple Flow/Valve Models
A common way of grouping Flow/Valve models which is favored by the operators is to put all of these models into a single fuzzy network so that the trend indicator will tell them that all of their critical flow controllers are working. In that case, the model indications into the fuzzy network (FIG. 4) will contain the “normal operating range” and the “normal model deviation” indication for each of the flow/valve models. The trend will contain the discriminator result from the worst model indication.
When a common equipment type is grouped together, another operator favored way to look at this group is through a Pareto chart of the flow/valves (FIG. 17). In this chart, the top 10 abnormal valves are dynamically arranged from the most abnormal on the left to the least abnormal on the right. Each Pareto bar also has a reference box indicating the degree of variation of the model abnormality indication that is within normal. The chart in FIG. 17 shows that “Valve 10” is substantially outside the normal box but that the others are all behaving normally. The operator would next investigate a plot for “Valve 10” similar to FIG. 2 to diagnose the problem with the flow control loop.
II. Multidimensional Engineering Redundancy Models
Once the dimensionality gets larger than 2, a single “PCA like” model is developed to handle a high dimension engineering redundancy check. Examples of multidimensional redundancy are:

    
    
        pressure 1=pressure 2= . . . =pressure n
        material flow into process unit 1=material flow out of process unit 1= . . . =material flow into process unit 2
    
    


Because of measurement calibration errors, these equations will each require coefficients to compensate. Consequently, the model set that must be first developed is:

F1(yi)=a1G1(xi)+filtered bias1,i+operator bias1+error1,i

F2(yi)=anG2(xi)+filtered bias2,i+operator bias2+error2,i

Fn(yi)=anGn(xi)+filtered biasn,i+operator biasn+errorn,i   Equation 18

These models are developed in the identical manner that the two dimensional engineering redundancy models were developed.
This set of multidimensional checks are now converted into “PCA like” models. This conversion relies on the interpretation of a principle component in a PCA model as a model of an independent effect on the process where the principle component coefficients (loadings) represent the proportional change in the measurements due to this independent effect. In FIG. 3, there are three independent and redundant measures, X1, X2, and X3. Whenever X3 changes by one, X1 changes by a1 and X2 changes by a2. This set of relationships is expressed as a single principle component model, P, with coefficients in unscaled engineering units as:

P=a1X1+a2X2+a3X3   Equation 19

Where a3=1
This engineering unit version of the model can be converted to a standard PCA model format as follows:
Drawing analogies to standard statistical concepts, the conversion factors for each dimension, X, can be based on the normal operating range. For example, using 3σ around the mean to define the normal operating range, the scaled variables are defined as:

Xscale=Xnormal operating range/6σ  Equation 20

(99.7% of normal operating data should fall within 3σ of the mean)

Xmid=Xmid point of operating range   Equation 21

(explicitly defining the “mean” as the mid point of the normal operating range)

X′=(X−Xmid)/Xscale   Equation 22

(standard PCA scaling once mean and σ are determined)
Then the P′ loadings for Xi are:




  
    
      
        
          b
          i
        
        =
        
          
            (
          
          ⁢
          
            a
            i
          
          ⁢
          
            /
          
          ⁢
          
            X
            
              i
              ⁢
              
                -
              
              ⁢
              scale
            
          
          ⁢
          
            
              )
            
            /
            
              
                (
                
                  
                    
                      ∑
                      N
                    
                    
                      k
                      =
                      1
                    
                  
                  ⁢
                  
                    
                      (
                    
                    ⁢
                    
                      a
                      k
                    
                    ⁢
                    
                      /
                    
                    ⁢
                    
                      X
                      
                        k
                        ⁢
                        
                          -
                        
                        ⁢
                        scale
                      
                    
                    ⁢
                    
                      
                        )
                      
                      2
                    
                  
                
                )
              
              
                1/2
              
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        23
      
    
  




(the requirement that the loading vector be normalized)
This transforms P to

P′=b1X1+b2*X2+ . . . +bn*XN   Equation 24

P′“standard deviation”=b1+b2+ . . . +bn   Equation 25

With this conversion, the multidimensional engineering redundancy model can now be handled using the standard PCA structure for calculation, exception handling, operator display and interaction.
Deploying PCA Models and Simple Engineering Models For Abnormal Event Detection
I. Operator and Known Event Suppression
Suppression logic is required for the following:

    
    
        Provide a way to eliminate false indications from measurable unusual events
        Provide a way to clear abnormal indications that the operator has investigated
        Provide a way to temporarily disable models or measurements for maintenance
        Provide a way to disable bad acting models until they can be retuned
        Provide a way to permanently disable bad acting instruments.
    
    


There are two types of suppression. Suppression which is automatically triggered by an external, measurable event and suppression which is initiated by the operator. The logic behind these two types of suppression is shown in FIGS. 18 and 19. Although these diagrams show the suppression occurring on a fuzzified model index, suppression can occur on a particular measurement, on a particular model index, on an entire model, or on a combination of models within the process area.
For operator initiated suppression, there are two timers, which determine when the suppression is over. One timer verifies that the suppressed information has returned to and remains in the normal state. Typical values for this timer are from 15-30 minutes. The second timer will reactivate the abnormal event check, regardless of whether it has returned to the normal state. Typical values for this timer are either equivalent to the length of the operator's work shift (8 to 12 hours) or a very large time for semi-permanent suppression.
For event based suppression, a measurable trigger is required. This can be an operator setpoint change, a sudden measurement change, or a digital signal. This signal is converted into a timing signal, shown in FIG. 20. This timing signal is created from the trigger signal using the following equations:

Yn=P*Yn−1+(1−P)*Xn Exponential filter equation   Equation 26

P=Exp(−Ts/Tf) Filter constant calculation   Equation 27

Zn=Xn−Yn Timing signal calculation   Equation 28

where:
Yn the current filtered value of the trigger signal
Yn−1 the previous filtered value of the trigger signal
Xn the current value of the trigger signal
Zn the timing signal shown in FIG. 20
P the exponential filter constant
Ts the sample time of the measurement
Tf the filter time constant
As long as the timing signal is above a threshold (shown as 0.05 in FIG. 20), the event remains suppressed. The developer sets the length of the suppression by changing the filter time constant, Tf. Although a simple timer could also be used for this function, this timing signal will account for trigger signals of different sizes, creating longer suppressions for large changes and shorter suppressions for smaller changes.
FIG. 21 shows the event suppression and the operator suppression disabling predefined sets of inputs in the PCA model. The set of inputs to be automatically suppressed is determined from the on-line model performance. Whenever the PCA model gives an indication that the operator does not want to see, this indication can be traced to a small number of individual contributions to the Sum of Error Square index. To suppress these individual contributions, the calculation of this index is modified as follows:




  
    
      
        
          E
          2
        
        =
        
          
            
              ∑
              n
            
            
              i
              =
              1
            
          
          ⁢
          
            
              w
              i
            
            ⁢
            
              e
              i
              2
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        29
      
    
  




wi—the contribution weight for input i (normally equal to 1)
ei—the contribution to the sum of error squared from input i
When a trigger event occurs, the contribution weights are set to zero for each of the inputs that are to be suppressed. When these inputs are to be reactivated, the contribution weight is gradually returned to a value of 1.
II. PCA Model Decomposition
Although the PCA model is built using a broad process equipment scope, the model indices can be segregated into groupings that better match the operators' view of the process and can improve the sensitivity of the index to an abnormal event.
Referring again to Equation 29, we can create several Sum of Error Square groupings:




  
    
      
        
          
            
              
                
                  
                    
                      
                        
                          
                            E
                            1
                            2
                          
                          =
                          
                            
                              ∑
                              
                                i
                                =
                                1
                              
                              l
                            
                            ⁢
                            
                              
                                w
                                i
                              
                              ⁢
                              
                                e
                                i
                                2
                              
                            
                          
                        
                      
                    
                    
                      
                        
                          
                            E
                            2
                            2
                          
                          =
                          
                            
                              ∑
                              
                                i
                                =
                                l
                              
                              k
                            
                            ⁢
                            
                              
                                w
                                i
                              
                              ⁢
                              
                                e
                                i
                                2
                              
                            
                          
                        
                      
                    
                  
                
              
              
                
                  ⋮
                
              
            
          
        
        
          
            
              
                E
                m
                2
              
              =
              
                
                  ∑
                  
                    i
                    =
                    k
                  
                  n
                
                ⁢
                
                  
                    w
                    i
                  
                  ⁢
                  
                    e
                    i
                    2
                  
                
              
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        30
      
    
  




Usually these groupings are based around smaller sub-units of equipment (e.g. reboiler section of a tower), or are sub-groupings, which are relevant to the function of the equipment (e.g. product quality).
Since each contributor, ei, is always adding to the sum of error square based on process noise, the size of the index due to noise increases linearly with the number of inputs contributing to the index. With fewer contributors to the sum of error square calculation, the signal to noise ratio for the index is improved, making the index more responsive to abnormal events.
In a similar manner, each principle component can be subdivided to match the equipment groupings and an index analogous to the Hotelling T2 index can be created for each subgroup.




  
    
      
        
          
            
              
                P
                
                  1
                  ,
                  a
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  l
                
                ⁢
                
                  
                    b
                    
                      1
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                P
                
                  1
                  ,
                  b
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    l
                  
                  k
                
                ⁢
                
                  
                    b
                    
                      1
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                P
                
                  1
                  ,
                  c
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    k
                  
                  n
                
                ⁢
                
                  
                    b
                    
                      1
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                P
                
                  2
                  ,
                  a
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  l
                
                ⁢
                
                  
                    b
                    
                      2
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                P
                
                  2
                  ,
                  b
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    l
                  
                  k
                
                ⁢
                
                  
                    b
                    
                      2
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                P
                
                  2
                  ,
                  c
                
              
              =
              
                
                  ∑
                  
                    i
                    =
                    k
                  
                  n
                
                ⁢
                
                  
                    b
                    
                      2
                      ,
                      i
                    
                  
                  ⁢
                  
                    x
                    i
                  
                
              
            
          
        
        
          
            
              
                T
                a
                2
              
              =
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  m
                
                ⁢
                
                  P
                  
                    i
                    ,
                    a
                  
                  2
                
              
            
          
        
        
          
            
              
                T
                b
                2
              
              =
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  m
                
                ⁢
                
                  P
                  
                    i
                    ,
                    b
                  
                  2
                
              
            
          
        
        
          
            
              
                T
                c
                2
              
              =
              
                
                  ∑
                  
                    i
                    =
                    1
                  
                  m
                
                ⁢
                
                  P
                  
                    i
                    ,
                    c
                  
                  2
                
              
            
          
        
      
    
    
      
        Equation
        ⁢
        
          
        
        ⁢
        31
      
    
  




The thresholds for these indices are calculated by running the testing data through the models and setting the sensitivity of the thresholds based on their performance on the test data.
These new indices are interpreted for the operator in the identical manner that a normal PCA model is handled. Pareto charts based on the original inputs are shown for the largest contributors to the sum of error square index, and the largest contributors to the largest P in the T2 calculation.
III. Overlapping PCA Models
Inputs will appear in several PCA models so that all interactions affecting the model are encompassed within the model. This can cause multiple indications to the operator when these inputs are the major contributors to the sum of error squared index.
To avoid this issue, any input, which appears in multiple PCA models, is assigned one of those PCA models as its primary model. The contribution weight in Equation 29 for the primary PCA model will remain at one while for the non-primary PCA models, it is set to zero.
IV. Operator Interaction & Interface Design
The primary objectives of the operator interface are to:

    
    
        Provide a continuous indication of the normality of the major process areas under the authority of the operator
        Provide rapid (1 or 2 mouse clicks) navigation to the underlying model information
        Provide the operator with control over which models are enabled. FIG. 22 shows how these design objectives are expressed in the primary interfaces used by the operator.
    
    


The final output from a fuzzy Petri net is a normality trend as is shown in FIG. 4. This trend represents the model index that indicates the greatest likelihood of abnormality as defined in the fuzzy discriminate function. The number of trends shown in the summary is flexible and decided in discussions with the operators. On this trend are two reference lines for the operator to help signal when they should take action, a yellow line typically set at a value of 0.6 and a red line typically set at a value of 0.9. These lines provide guidance to the operator as to when he is expected to take action. When the trend crosses the yellow line, the green triangle in FIG. 4 will turn yellow and when the trend crosses the red line, the green triangle will turn red. The triangle also has the function that it will take the operator to the display associated with the model giving the most abnormal indication.
If the model is a PCA model or it is part of an equipment group (e.g. all control valves), selecting the green triangle will create a Pareto chart. For a PCA model, of the dozen largest contributors to the model index, this will indicate the most abnormal (on the left) to the least abnormal (on the right) Usually the key abnormal event indicators will be among the first 2 or 3 measurements. The Pareto chart includes a red box around each bar to provide the operator with a reference as to how unusual the measurement can be before it is regarded as an indication of abnormality.
For PCA models, operators are provided with a trend Pareto, which matches the order in the bar chart Pareto. With the trend Pareto, each plot has two trends, the actual measurement (in cyan) and an estimate from the PCA model of what that measurements should have been if everything was normal (in tan).
For valve/flow models, the detail under the Pareto will be the two dimensional flow versus valve position model plot. From this plot the operator can apply the operator bias to the model.
If there is no equipment grouping, selecting the green triangle will take the operator right to the worst two-dimensional model under the summary trend.
Operator suppression is done at the Pareto chart level by selecting the on/off button beneath each bar.










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1
5,859,964
Jan. 12, 1999
Wang, et al, “System and method for performing real time data





acquisition, process modeling and fault detection of wafer





fabrication processes”


2
5,949,678
Sep. 7, 1999
Wold, et al, “Method for Monitoring Multivariable Processes”


3
6,522,978
Feb. 18, 2002
Chen, et al, “Paper web breakage prediction using principal





components analysis and classification and regression trees”


4
6,368,975
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Balasubramhanya, et al, “Method and apparatus for monitoring





a process by employing principal component analysis”


5
6,466,877
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components analysis and classification and regression trees”


6
6,521,080
Feb. 18, 2003
Balasubramhanya, et al, “Method and apparatus for monitoring





a process by employing principal component analysis”


7
6,564,119
May 13, 2003
Vaculik, et al, “Multivariate Statistical Model Based System





for Monitoring the Operation of a Continuous Caster and





Detecting the Onset of Impending Breakouts”


8
6,636,842
Oct. 21, 2003
Zambrano, et al, “System and method for controlling an





industrial process utilizing process trajectories”










II. Literature












1.
Cardoso, J. et al
“Fuzzy Petri Nets: An Overview”, 13th Word Congress of IFAC,




Vol. I: Identification II, Discrete Event Systems, San Francisco,




CA, USA, June 30-July 5, 1996, pp. 443-448.


2.
Jackson, E.
“A User's Guide to Principal Component Analysis”, John Wiley &




Sons, 1991


3.
Kourti, T.
“Process Analysis and Abnormal Situation Detection: From Theory to




Practice”, IEEE Control Systems Magazine, October 2002, pp. 10-25


4.
Ku, W.
“Disturbance Detection and Isolation for Statistical Process Control in




Chemical Processes”, PhD Thesis, Lehigh University, Aug. 17, 1994


5.
Martens, H., & Naes, T.,
“Multivariate Calibration”, John Wiley & Sons, 1989


6.
Piovoso, M. J., et al.
“Process Data Chemometrics”, IEEE Trans on Instrumentation




and Measurement, Vol. 41, No. 2, April 1992, pp. 262-268









Appendix 2







TABLE 1







Charge Gas Compressor and Caustic Wash Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Compressor Temperature Balance








WR FROM SURF COND
−0.173


CHG CMP2SUC COOLER
−0.172


WR FR SURFACE COND
−0.172


CGC CTW OUTLET
−0.171


CGC 1ST STAGE EXCHANGER 2 H2O OUT TEMP
−0.170


CGC 1ST STAGE EXCHANGER 1 H2O OUT TEMP
−0.167


CGC 1ST STAGE HC OUT TEMP
−0.165


CGC EXCHANGER INLET TEMP
−0.162


CHG CMP DIS COOLER
−0.160


CAUSTIC TOWER FEED
−0.159







2. Compressor Pressure Balance








CHG COMP 1ST DISC
0.224


C3R DRIER TUBE INLET PRESS
0.200


CHG COMP 2ND STG DISC
0.197


CGC 2ND STG DEL P
0.195


CHG CMP 2 STG DISC
0.187


NACC TRISEN SPEED SETPNT
0.180


MCGC TURBINE SPEED
0.175


CHG COMP 1ST STG SUCT Output
0.175


TOTAL FRESH FEED
0.175


CHARGE COMP RELIEF
0.157







3. Discharge Temperature Balance








C3R DRIER BACK PRESS
0.344


CGC COOLED OUTLET OVHD TO DRIERS
0.341


C3R DRIER CHARGE GAS OUTLET
0.338


MCGC COOLED OUTLET OH TO DRYERS
0.320


C3R DRIER BACK PRESS Output
−0.319


C3 = VAP FM C3R DRIER
0.315


CHG COMP AFT CHLR C3R DRIER Output
−0.204


CHG CMP 1 STG SUC
0.187


CHG COMP 1ST STG SUCT
0.182


CAUSTIC TWR DISCH
0.155







4. Fresh Feed








CHG CMP 1 STG SUC
0.322


CHG COMP 1ST STG SUCT
0.314


CAUSTIC TWR DISCH
0.259


CHARGE COMP RELIEF
0.244


TOTAL FRESH FEED
0.222


CHG CMP 2 STG SUC
0.211


COLD BOX FEED
−0.203


C3R DRIER BACK PRESS
−0.195


C3R DRIER CHARGE GAS OUTLET
−0.180


CGC COOLED OUTLET OVHD TO DRIERS
−0.174







5. Steam Balance








HP STM TO CHG CMP
−0.341


CGC EXT STM OVRRIDE SEL Output
0.339


CGC STEAM TURBINE 1ST STG NOZ
−0.337


CGC STEAM TURBINE INLET STM VAL POS
−0.325


STM COND FM ZCD01
0.265


CGC STEAM TURBINE EXTRACT STM PRESS
−0.255


CGC STEAM TURBINE EXTRT STM VAL POS
0.241


CGC STEAM TURBINE EXTRACT STM TEMP
−0.195


SURF COND INLET
0.170


CHG COMP SURF COND VAC
−0.164







6. Feed Quality








OLEFINS RECOVERY TRAIN FEED-H2
0.340


OLEFINS CRACKING TRAIN TOTAL PROD
0.335


MW CORRECTED STOH
−0.328


OLEFINS RECOVERY TRAIN FEED-C1
−0.317


CGC 2ND SUC MW
−0.240


OLEFINS RECOVERY TRAIN FEED-C3 =
−0.237


CH4 FEED RATE TO OLEFINS RECOVERY TRAIN
−0.219


CH4 FEED RATE TO OLEFINS RECOVERY TRAIN
−0.216


OLEFINS RECOVERY TRAIN FEED-C2 =
0.172


DRYERS FD MOLECULAR WT
−0.163







7. Recycle Feed








HP STM TO CHG CMP
−0.217


COMP DISCH. TO PP
0.204


FRESH FD CHGE CMPR
0.191


W CTW SUPPLY MAIN
0.189


CG 1ST STG SUC TEMP OVRD Output
0.188


RERUN OH CONDENSOR TO FEED
−0.188


CHG COMP 2ND STG SUCT
−0.183


RERUN OH CONDENSOR TO FEED Output
−0.172


CAUSTIC TOWER FEED COOLER QW OUT
0.171


CHG COMP 1ST STG SUCT
−0.166







8. Caustic Balance








TARGET NAOH BY ANALYZER
0.526


% SPENT - ANALYZERS
0.512


CO2 TO CAUSTIC TWR
0.461


H2S TO CAUSTIC TOWER
0.212


CHG COMP 2ND STG SUCT
0.176


CHG COMP 1ST STG SUCT
0.170


FEED EXCHANGER ACTUAL-PRED. DP
−0.103


CAUSTIC FEED FRM BL Output
−0.096


CO2 IN CGC DISCHARGE
0.077


CGC 2ND SUC MW
0.076







9. Feed Rate on Caustic








CH4 FEED RATE TO OLEFINS RECOVERY TRAIN
0.314


NEW SPENT CAUSTIC LINE
0.278


CHG COMP 1ST STG SUCT
0.238


CHG COMP 2ND STG SUCT
0.233


OLEFINS RECOVERY TRAIN FEED-C1
0.225


CAUSTIC TOWER 1ST STG
0.199


OLEFINS RECOVERY TRAIN FEED-C2O
−0.192


CAUSTIC FEED FRM BL
0.185


COLD BOX FEED
0.179


CONDENSATE TO TOP
0.170







10. Caustic Efficiency








CHG COMP 1ST STG SUCT
−0.362


CHG COMP 2ND STG SUCT
−0.358


CGC 2ND SUC MW
−0.233


FEED EXCHANGER ACTUAL-PRED. DP
0.209


CO2 TO CAUSTIC TWR
0.187


CHG COMP DISC DRUM DRW Output
−0.183


TARGET NAOH BY ANALYZER
0.178


CHG CMP 2 STG SUC
0.172


CH4 FEED RATE TO OLEFINS RECOVERY TRAIN
0.159


% SPENT - ANALYZERS
0.155
















TABLE 2







Ethylene and Propylene Refrigeration Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Ethylene Compressor Discharge








C2 = REFRIG DRUM
0.138


C2 = COMP DISCHARGE
0.135


C2R COMP OUTLET CONDENSOR A C2 = OUT
0.124


C2R COMP OUTLET CONDENSOR B C2 = OUT
0.124


C2 = COMPR 3RD SUCT
0.123


C2 = CMP 3RD SUCT DRUM
0.121


C2 = COMPR 2ND SUCT
0.121


C2R COMP OUTLET DRUM OUTLET
0.121


CHG CMP2SUC COOLER
0.120


C2R COMP OUTLET CONDENSOR A SHELL OUT
0.120







2. Propylene Compressor Discharge








PROP CMP 4 STG DIS
−0.142


RERUN TOWER CONDENSED OVHD
−0.135


H2O FM SURF COND 1
−0.133


CW OUT C3R OUTLET CONDENSOR A
−0.131


CW OUT C3R OUTLET CONDENSOR D
−0.130


H2O FM SURF COND 2
−0.130


C3R COMP 4TH STG DP
−0.129


CW OUT C3R OUTLET CONDENSOR C
−0.129


COND FM SURF COND 5
−0.127


PROP COMP TO R-FUEL
−0.126







3. Propylene Compressor Suction








C2 SPLITTER OVHD COND B LO OVERRIDE SEL Output
−0.218


C2 SPLITTER OVHD COND A LO OVERRIDE SEL Output
−0.218


C2 SPLITTER OVHD COND B VALVE DP
0.212


OUTLET BTM DRAW OFF
−0.177


C3R COMP MEDIAN SPEED
−0.173


PROP COMP 1ST STG SUCT Output
−0.163


ETY SPLT REFLUX DR
−0.159


C2 = SPLITTER TOTAL HEAT IN
−0.157


C3 = TO SPLTR REBOILER
−0.149


C2 = SPLITTER REBOILER BTU CONTROLLER
−0.144







Ethylene Compressor Load








C2 = COMPR DISCHARGE
0.201


C2R COMP MOTOR AMPS
0.197


DEMETHANIZER REBOIL PROP
−0.183


DEMETH REBOILER A/B SHELL INLET
−0.182


DEMETH FD CHILL EXCHANGER Output
−0.168


CHG GAS FROM C2 = SPLITTER BOTTOMS
−0.161


C2 = SPLITTER BOTTOMS C2 PRODUCT
−0.147


DEMETH RBLR A IN
−0.132


PROP PUMP OUT
0.129


DEMETH REBOILER A/B SHELL INLET Output
−0.128







5. Ethylene Compressor Suction








C2R FROM COLD BOX
0.224


C2 = CMP 1 SUCT DRUM
0.220


C2 = COMPR 1ST STAGE SUCT
0.215


PROP COMP 3RD STG DUMP
0.208


C2 = SPLITTER REBOILER SHELL OUT
0.206


DEPROP OH CONDENSR
0.202


ETHY SPLIT REB FD
0.201


DEMETH OVHD COND C2R LIQUID
0.193


FEED GAS FRM COLD BOX
0.187


DEMETH OVHD COND C2R VAPOR
0.185







6. Propylene Compressor Demand








C2 = SPLITTER TOTAL HEAT IN
−0.188


PROP COMP 4TH STG DISC Output
−0.183


PROP COMP TO R-FUEL
−0.183


DEPROP OH CONDENSR
0.181


DEMETH FEED DR 2
0.172


CG 1ST STG SUC TEMP OVRD Output
0.171


DEMETH FEED DR 3
0.165


C3 = TO SPLTR REBOILER Output
−0.163


PROP CMP 4 STG SUC
−0.161


C3 = TO COLDBOX FIRST CORE
−0.147







7. Ethylene-Propylene Compressors Interation








C3R COMP - C3R COMP OUTLET COND TEMP MARGIN
−0.193


C3R COMP 4TH STG DP
0.189


CW OUT C3R OUTLET CONDENSOR D
0.189


C2R COMP OUTLET DRUM ETHYLENE PURGE Output
0.181


COLD BOX C2 = VAPOR
0.176


DEMETH FD CHILL EXCHANGER Output
0.173


C2 = COMPR 3RD STG SUCT
0.170


C3 = PURGE TO FEED Output
−0.168


DEMETH FEED C2R COLDBOX CHILLER 1 LEVEL Output
−0.166


C2R COMP OUTLET EXCHANGER OUTLET TEMP
−0.162







8. Propylene Make-up








DEPROP OVERHEAD
−0.208


RERUN TOWER - RERUN OVHD COND VENT GAS
−0.207


PROP CMP 1 STG SUC
−0.182


ETHANE PRODUCT
−0.174


C2 PROD TO B LIMIT
−0.171


C3R COLDBOX VAPOR
0.168


PROP CMP 4 STG SUC
0.165


SPLTR REFLUX CND B
0.160


WR FR AFT COND
−0.158


PROP COMP 3RD STG DUMP
−0.138







9. Propylene Heat Sources








DEETH OVHD COND C3 = VAPOR Output
−0.316


DEMETH FEED CHILLER #1
−0.312


DEETH OVHD COND C3 = VAPOR
−0.309


PROP COMP SURF COND VAC
−0.309


PROP COMP 3 SUCT DRUM
−0.253


ETHANE PRODUCT Output
0.193


HP STEAM TO PROP CMP
0.135


PROP COMP 4TH STG SUCT
0.121


PROP CMP 4 STG DIS
0.119


DEMETH FEED COLD BOX LEVEL
0.111







10. Demethanizer Balance








DEMETH REBOILER A/B SHELL INLET Output
−0.202


C3 = FROM DEETH OVHD COND
−0.183


C2R COMP - DRUM LO SEL Output
−0.157


C2R COMP 2ND STAGE FEED DRUM LIQUID
−0.155


C2 SPLITTER OVHD COND B LO OVERRIDE SEL Output
−0.145


VAP FRM C2 = SPLITTER BOTTOMS
−0.144


DEMETH RBLR A IN
−0.142


ETY SPLT REFLUX DR
−0.135


DEMETH FEED DR 2
−0.134


DEMETH FEED DR 3
−0.134







11. Propylene Vapor Balance








DEMETH REBOILER A/B SHELL INLET Output
0.191


C3 = FROM DEETH OVHD COND
0.190


MAKEUP AND PURGE BTMS
0.184


WR FR AFT COND
−0.170


VAP FRM C2 = SPLITTER BOTTOMS
0.167


RERUN TOWER - RERUN OVHD COND VENT GAS
−0.167


C2R COMP 2ND STAGE FEED DRUM LIQUID
0.161


DEMETH RBLR A IN
0.159


C2R COMP - DRUM LO SEL Output
−0.155


ETY SPLT REFLUX DR
−0.153







12. De-Ethanizer Load








DEETH VAPOR PROD
0.199


CHG CMP2SUC COOLER
0.190


RERUN TOWER VENT COND RERUN OVHD COND Output
0.174


C3 = TO SPLTR REBOILER
0.169


C2R COMP - DRUM LO SEL Output
0.155


RERUN TOWER VENT COND RERUN OVHD COND Output
−0.152


C3R COMP 3RD STG SUCT
0.148


C2 = COMPR 1ST STAGE SUCT
0.137


ETY SPLT REFLUX DR
0.137


PROP PUMP OUT
0.136







13. Propylene Recycle








COOL KICKBACK TO C3R COMP FEED DRUM
0.198


MAPD REACTOR C3 RECYCLE TO FEED
−0.178


ETY SPLT REFLUX DR
0.177


PROP COMP 4TH STG DISC
0.175


C2 = COMP DISCHARGE
0.167


C3 = FROM DEETH OVHD COND
0.163


C2R COMP - DRUM LO SEL Output
0.158


C2 = DESUPERHEATER C2R OUTLET 2ND EXCHANGER
0.157


Output


DEMETH REBOILER A/B SHELL INLET Output
0.156


DEMETH FEED DR 3
0.150







14. Ethylene Splitter Load








RERUN TOWER - RERUN OVHD COND VENT GAS
−0.211


C2 = COMP DISCHARGE
0.184


SPLTR REFLUX CND B
0.181


PROP PUMP OUT
−0.179


MAPD REACTOR C3 RECYCLE TO FEED
−0.176


WR FR AFT COND
−0.167


ETHANE PRODUCT
−0.160


DEMETH REBOILER A/B SHELL INLET Output
−0.159


DEPROP OVERHEAD
−0.159


C2 PROD TO B LIMIT
−0.159







15. Coldbox and De-Methanizer Feed








DEMETH FEED COLD BOX LEVEL Output
0.208


DEMETHANIZER REFLUX DRUM
0.194


RERUN TOWER - RERUN OVHD COND VENT GAS
−0.191


SPLITTER BTM TEMP
−0.190


DEPROP OVERHEAD
−0.187


PROP PUMP OUT
0.186


PROP COMP 4TH STG DISC
0.178


HP ETHYLENE PROD
0.152


SPLTR REFLUX CND B
0.143


PROP COMP 4TH STG DISC
0.128
















TABLE 3







Dryers, Coldbox, and Methane Booster Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Hydrogen Purity








DIFF ACROSS COLDBOX CORE EXCHANGER 3
−0.200


H2 PRODUCT PURITY THCO
−0.178


LO PRES CH4 DP
−0.177


C3R COLDBOX VAPOR PRESSURE
−0.173


HYDROGEN WT FRACTION
0.171


HYDROGEN PURITY
0.170


HYDROGEN MOLECULAR WT
−0.169


FEED GAS FRM C2R COLDBOX CHILLER 1
−0.168


COLD BOX H2 MOL WT
−0.167


FEED GAS FRM C3R COLDBOX
−0.164







2. Methane Medium Pressure Temperature








MP METH TO OLEFINS CRACKING TRAIN-W
0.240


METH CMP 1 STG SUC
0.233


CH4 BOOSTER 1ST STAGE OUTLET GAS OUT
0.230


CH4 BOOSTER 2ND STAGE OUTLET GAS OUT
0.230


COLDBOX LP METHANE
0.230


H2 FROM COLDBOX
0.230


CB MPCH4 OUT TLCO
0.223


COLDBOX HP METHANE
0.217


METH CMP 2 STG SUC
0.217


CH4 BOOSTER 2ND STAGE OUTLET CTN RTM TEMP
0.212







3. Dryer Outlet Temperature








DRYER REGN FUR STK
−0.307


DRYER REG HTR OUT
−0.305


DRYER REGN OUTLET3
−0.304


DRYER REGN OUTLET4
−0.304


DRYER REGEN HEADER OUT
−0.303


DRYER REGN OUTLET1
−0.303


DRYER REGN OUTLET2
−0.303


FG TO REGEN HTR
−0.271


REGEN FURN CH4 TO COOLDOWN
−0.235


REG GAS TO DRYERS
−0.232







4. Coldbox Pressure Balance








DEMETH FEED DR 3
0.309


DEMETH FD DRUM
0.306


MIN DEMTH #4FD DRM PRESS
0.305


H2 SEPARATOR DRUM
0.300


DEMETH FEED DR 2
0.289


COLDBOX TO DEMETH DP
0.272


DEMETH FEED DR 1
0.269


H2 FROM COLDBOX
0.269


DEMETHANIZER FEED #3 Output
−0.193


DEMETH FEED DRM #4 LEVEL Output
−0.173







5. Methane Supply Balance








METHANE COMPR 2ND DISCHG
−0.290


MP METHANE TO OLEFINS CRACKING TRAIN-W
0.261


MED PRESS METHANE HEADER Output
−0.246


METHANE COMP 2ND STG SUC Output
0.233


MED PRESS METHANE HEADER
0.229


METH TO FUEL GAS
−0.223


OLEFINS RECOVERY TRAIN T.G. TO CMM
−0.216


METHANE COMPR MIN FLOW
0.210


MP METHANE FROM COLDBOX
0.201


MP METH TO OLEFINS CRACKING TRAIN-W
0.163







6. Coldbox Temperature








DEMETH FD CHILL EXCHANGER Output
−0.264


CHG GAS FROM C2 = SPLITTER BOTTOMS
−0.263


C2 = SPLITTER BOTTOMS C2 PRODUCT
−0.255


CHG GAS FROM C2 = SPLITTER HIP REBOILER
−0.226


C2 = SPLITTER BOTTOMS C20 (FEED)
0.218


VAP FRM C2 = SPLITTER BOTTOMS
−0.189


CHARGE GAS FROM COLDBOX FEED EXCHANGER
−0.188


DEMETH FD CHILL EXCHANGER
0.167


SPLTR HIP RBLR VAP
−0.162


DEMETH FEED DRUM #1
−0.156







7. Methane Header








NO1 FD TEMP CUTOUT
−0.202


FURNACE FG KO DRUM Output
−0.201


MED PRESS METHANE HEADER
0.195


MP METHANE FROM COLDBOX
0.188


DEMETHANIZER FEED1
−0.180


MP METHANE TO OLEFINS CRACKING TRAIN-W
0.179


MED PRESS METHANE HEADER Output
−0.164


C3R 2ND STAGE EXCHANGER C3 = VAPOR
−0.161


FEED GAS TO COLDBOX FEED DRUM
−0.160


C2R COLDBOX CORE EXCHANGER INLET
−0.160







8. Coldbox Methane Quality








DEMETHANIZER #4 FEED
−0.236


TOTAL MP METHANE FLOW
−0.213


FEED GAS FRM COLD BOX
−0.208


COLDBOX RECYCLE
−0.206


MED PRES CH4 DP
−0.196


COLDBOX INLET PRES PHCO
−0.194


COLD BOX C2 = R LIQUID
−0.161


HI PRESS C1 FROM COLDBOX
−0.153


METH TO FUEL GAS
−0.142


MP METHANE FROM COLDBOX
−0.140







9. Methane Compressor Load








METH CMP 1 DISCH
0.285


METH CMP 2 DISCH
0.268


METH COMP 1ST SUC
0.254


METHANE BOOSTER MOTOR AMPS
0.253


HIGH PRESS METHANE
0.213


METHANE COMP 1ST STG DIS
0.171


C2 = SPLITTER BOTTOMS SUPERHEAT CTRL OR Output
−0.171


METHANE COMPR MIN FLOW
0.166


METH CMP 2 SUCTION
0.154


SPLTR HIP RBLR IN
−0.149







10. Methane Letdown Balance








METH CMP 1 DISCH
0.231


C2 = SPLITTER BOTTOMS SUPERHEAT CTRL OR Output
0.227


METH CMP 2 DISCH
0.212


DEMETHANIZER FEED4
−0.183


METHANE BOOSTER MOTOR AMPS
0.180


SPLTR HIP RBLR IN
0.180


HIGH PRESS METHANE
0.159


METH COMP 1ST SUC
0.156


FEED GAS TO COLDBOX
−0.156


VAP FRM C2 = SPLITTER BOTTOMS
−0.153







11. Low Pressure Methane Quality








LOW PRESS CH4: ACETYLENE
0.276


LP METHANE C2 =
0.235


HIGH PRESS METHANE
0.210


DEMETHANIZER FEED #3
0.207


METH CMP 2 DISCH
0.204


METH BOOST DUMP TO FLARE
−0.178


METHANE COMP 1ST STG SUC
−0.176


MED PRES CH4: ACET
0.175


HYDROGEN SEPARATOR DRUM
−0.172


DEMETH FEED COLD BOX LEVEL
0.168







12. Dryer Temperature Balance








REGEN FURN BLOWDOWN CTW OUTLET TEMP
−0.338


REGEN FURN BLOWDOWN GAS INLET
−0.325


CHG DRYER REGN OUT
−0.309


REGEN FURN BLOWDOWN GAS OUTLET TEMP
−0.307


CH4 TO FG
−0.169


COLDBOX INLET PRES PHCO
0.159


COLDBOX RECYCLE
0.157


COLDBOX HP METHANE
0.144


COLDBOX HP METHANE
0.141


H2 FROM COLDBOX
0.132







13. Methane Recycle








REGEN FURN BLOWDOWN GAS INLET
−0.238


CHG DRYER REGN OUT
−0.225


REGEN FURN BLOWDOWN CTW OUTLET TEMP
−0.214


DEMETHANIZER FEED #3
−0.203


C3 = TO COLDBOX FIRST CORE
−0.175


CO TEMPERATURE
0.175


C3 = FROM COLDBOX
−0.168


REGEN FURN BLOWDOWN GAS OUTLET TEMP
−0.166


MED PRES CH4 DP
−0.145


OLEFINS CRACKING TRAIN-W FUEL GAS
−0.145







14. Methane Compressor Suction








METHANE COMP 1ST STG SUC
−0.275


METH BOOST DUMP TO FLARE
−0.267


C3R 2ND STAGE EXCHANGER C3 = VAPOR
−0.237


NO1 FD TEMP CUTOUT
−0.197


FEED GAS TO COLDBOX FEED DRUM
−0.189


C2R COLDBOX CORE EXCHANGER INLET
−0.177


DEMETHANIZER FEED1
−0.175


HYDROGEN SEPARATOR DRUM
−0.172


C3R COLDBOX VAPOR
0.171


C3R COLDBOX VAPOR PRESSURE
0.163







15. Methane Booster Discharge








C3 = FROM COLDBOX
0.236


C3 = TO COLDBOX FIRST CORE
0.219


METH BOOST DUMP TO FLARE
−0.218


METHANE COMP 1ST STG SUC
−0.205


METHANE COMP 1ST STG SUC Output
0.180


DEMETH FD CHL 1 C3 => COLDBOX FEED EXCHANGER
0.173


DEMETH FD CHL 1 C3 => COLDBOX FEED EXCHANGER
0.172


Output


LO PRES CH4 DP
0.160


FURNACE FG KO DRUM Output
−0.156


CO TEMPERATURE
−0.152
















TABLE 4







De-Methanizer Principal Components


With Sensor Title and Principal Component Loading










Sensor Description
Loading











1. Heat Balance










DEMETH RBLR A IN
0.222



DEMETH BTMS
0.215



DEMETH RBLR B IN
0.213



DEMETH OH COND HEAT
−0.203



DEMETH TOTAL REFLX HEAT
−0.199



DEMETH REBOIL B OUT TEM
0.184



DEMETHANIZER FEED #3
−0.181



DEMETHANIZER REBOIL PROP
0.179



DEMETH REBOILER A/B SHELL INLET
0.179



DEMETHANIZER REFLUX FLOW
−0.172







2. Mass Balance










DEMETHANIZER BTMS FLOW Output
0.220



DEMETH BOTTOMS LEVEL
0.220



DEMETH LEVEL TO DEETH
0.217



DEMETH TRAY 26
0.215



DEMETHANIZER REFLUX FLOW Output
0.208



C2 = COMPR 1ST STAGE SUCT
−0.191



LIQUID METHANE RECYCLE
−0.190



DEMETHANIZER #4 FEED
−0.187



DEMETHANIZER REBOIL PROP
0.176



DEMETH REBOILER A/B SHELL INLET
0.176







3. Overhead Temperature










DEMETH OH
0.259



DEMETH TRAY 5
0.237



DEMETH UPPER OH AVG TEMP
0.232



DEMETH OVHD COND C2R LIQUID
0.189



EXPANDER OUTLET PR
0.188



C2 = COMPR 1ST STAGE SUCT
0.182



EXPNDR DRUM
0.182



DEMETHANIZER REFLUX DRUM
0.169



DEMETH OVHD DRUM HL EXPND TRIP LHCO
0.168



DEMETH OVHD TO EXPANDER Output
0.167







4. Methane Compressor










EXPNDR SHAFT SPEED
−0.201



DEMETH REBOILER C3 = REBLR 4TH SUCT
−0.199



EXPNDR IN HI RNGE
−0.199



DEMETHANIZER FEED #3
−0.193



DEMETH OVHD TO EXPANDER Output
−0.188



DEMETH TRAY 34 TEMP CTL
0.185



DEMETH CUTPOINT CONTROL
−0.184



DEMETH TRAY 38
0.184



DEMETH TRAY 34
0.173



DEMETHANIZER FEED #1
0.172







5. Methane Turbo-Expander










EXPNDR RECMP INLET
−0.338



EXPNDR DRUM
−0.337



EXPANDER OUTLET PR
−0.332



EXPNDR RECMP OUT P
−0.281



DEMETH OFF GAS
−0.253



EXPNDR OUTLET TEMP
−0.217



EXPNDR SHAFT SPEED
0.210



EXPANDER RECOMPR DISCHG
0.171



DEMETH MIN BTM TEMP
0.168



DEMETH TRAY 16
0.162







6. Methane Quality










DEMETH TRAY 34 TEMP CTL
0.292



DEMETH TRAY 38
0.274



DEMETH TRAY 34
0.268



DEMETHANIZER REFLUX FLOW Output
0.227



DEMETH CUTPOINT CONTROL
−0.226



DEMETH REBOILER C3 = REBLR 4TH SUCT
−0.215



EXPANDER INLET PR
−0.185



DEMETHANIZER BTMS FLOW
−0.180



TURBOEXPANDER SEPR LEVEL Output
0.175



4TH STG SUCT C3 = TO DEMETH REBOILER
−0.164







7. Methane Recycle










EXPNDR RECMP INLET
0.430



EXPNDR RECMP OUT T
0.404



HI PRESS C1 FROM COLDBOX
−0.355



EXPANDER RECOMPR DISCHG Output
0.319



DEMETH TRAY 34
−0.202



CB MPCH4 OUT TLCO
0.193



DEMETH TRAY 38
−0.184



DEMETH TRAY 34 TEMP CTL
−0.164



DEMETH MIN BTM TEMP
−0.155



MIN DEMETH-DEETH DELTA-P
−0.140







8. Methane Pressure Balance










EXPNDR RECMP INLET
−0.283



EXPANDER INLET PR
−0.273



EXPNDR RECMP OUT T
−0.269



DEMETHANIZER OVHD PRESS
−0.252



DEMETH TRAY 38
−0.227



DEMETH TRAY 34
−0.226



EXPANDER RECOMPR DISCHG Output
−0.219



HI PRESS C1 FROM COLDBOX
0.202



DEMETH BOTTOMS LEVEL
0.186



DEMETH LEVEL TO DEETH
0.175







9. Feed Rate










LIQUID METHANE RECYCLE
0.021



HI PRESS C1 FROM COLDBOX
−0.094



DEMETHANIZER FEED #3
−0.051



DEMETHANIZER FEED #2
0.039



DEMETHANIZER FEED #1
0.006



DEMETHANIZER #4 FEED
0.088



#4 DEMETH FEED W/SF614
0.130



DEMETH TOTAL OH RATE
0.009



DEMETH TOTAL REFLX HEAT
−0.196



EXPNDR LIQ->MPMETH
0.154







10. Bottoms Balance










DEMETH REBLR KO POT KD21
0.283



DEMETHANIZER BTMS FLOW
−0.275



EXPNDR RECMP OUT P
−0.270



DEMETH REBLR KO POT KD21 Output
0.266



EXPNDR OUTLET TEMP
0.234



EXPNDR SHAFT SPEED
−0.201



MAKEUP AND PURGE BTMS
−0.200



EXPANDER RECOMPR DISCHG Output
0.198



TURBOEXPANDER SEPR LEVEL Output
0.183



DEMETHANIZER OVHD PRESS
0.181

















TABLE 5







De-Ethanizer Principal Components


With Sensor Title and Principal Component Loading










Sensor Description
Loading











1. Overhead Quality










DEETH TRAY 45
−0.214



DEETH B REBOILER INLET
−0.210



DEETH RBLR IN
−0.206



DEETH VAPOR PROD
−0.205



DEETH OH TO FLARE PRESS
−0.205



DEETH BTMS TO DEPR
−0.205



DEETH OH
−0.202



DEETH VAPOR PROD
−0.200



DEETH OVHD COND C3 = VAPOR
−0.199



DEETH REBOILER B OUTLET TEMP
−0.198







2. Bottoms Quality










DEETH BTMS Output
0.278



DEETH BTMS
0.275



REFLUX BTU CONTROL
0.273



DETH REFLUX FLOW
0.258



DEETH REFLUX DRUM
−0.236



DEETH OVHD COND C3 = VAPOR Output
0.207



AVG TEMP DEETH
0.193



DEETH REFLUX COND Output
0.186



DETH REFLUX FLOW Output
0.179



DEETH BTMS
0.177







3. Bottoms Heat Balance










DEMETH BTMS
−0.314



STM TO DEETH REBOILER
0.265



ACET CONVERT FEED VALVE POSITION
0.253



DEMETH BTMS C3 =
−0.240



REBOILER BTU CONTROL
0.240



DEETH TOT OH FLOW
0.232



DEETH FEED
−0.227



DEETH TRAY 25
−0.222



DEETH TRAY 20
−0.208



DEETH TRAY 31
−0.200







4. Overhead Heat Balance










DEETH TOT OH FLOW
−0.327



ACET CONVERT FEED VALVE POSITION
−0.315



DEETH CUTPOINT CONTROLLR
0.261



DEETH REFLUX COND Output
0.240



AVG TEMP DEETH
−0.228



DEETH OVHD COND C3 = VAPOR Output
0.225



DE-ETHANIZER MAT BALANCE
−0.220



STM TO DEETH REBOILER Output
0.212



DETH REFLUX FLOW
−0.211



DEMETH LEVEL TO DEETH
0.210







5. Bottoms Temperature










DEETH BTM TO DEPROP
−0.364



DEETH BOTTOMS TO DEPROP
−0.361



DEETH REBOILER B OUTLET TEMP
−0.241



DEETH BTMS TO DEPR
−0.239



DEETH REBOILER A OUTLET TEMP
−0.234



DEETH RBLR IN
−0.224



DEETH B REBOILER INLET
−0.213



C3 IN DEETH OH
0.195



DEMETH BTMS C2
0.174



DEETH STARTUP TEMP
−0.169







6. Product Quality










STM TO DEETH REBOILER A/B
−0.447



STM TO DEETH REBOILER
−0.437



DEETH CUTPOINT CONTROLLR
−0.388



REBOILER BTU CONTROL
−0.347



STM TO DEETH REBOILER
−0.204



DEETH TOP DP
0.160



DEETH REFLUX COND Output
0.151



DEETH OVHD COND C3 = VAPOR Output
0.138



STM TO DEETH REBOILER
0.136



ACET CONVERT FEED VALVE POSITION Output
0.135







7. Feed Quality










DEMETH BTMS C2 =
0.461



DEMETH BTMS C3 =
−0.417



DEMETH BTMS C3
−0.398



STM TO DEETH REBOILER
0.315



STM TO DEETH REBOILER A/B
0.314



DEMETH BTMS C2
0.176



DEMETH BTMS
0.154



STM TO DEETH REBOILER
−0.152



DEETH TRAY 31
0.137



AVG TEMP DEETH
0.134







8. Material Balance










STM TO DEETH REBOILER
−0.360



DE-ETHANIZER MAT BALANCE
0.298



DEETH STARTUP TEMP
−0.261



C3 = LIQ TO DEETH OVHD COND
−0.246



DEETH REFLUX COND Output
−0.229



AVG TEMP DEETH
−0.224



DEETH BTMS
0.223



DEETH BTMS Output
0.202



DEMETH BTMS C3
−0.190



DEETH REFLUX COND
0.183







9. Overhead Balance










DEETH STARTUP TEMP
0.352



AVG TEMP DEETH
0.280



DETH REFLUX FLOW
−0.260



REFLUX BTU CONTROL
−0.254



DE-ETHANIZER MAT BALANCE
0.252



STM TO DEETH REBOILER
−0.238



DEETH STM COND ACC
−0.235



ACET CONVERT FEED VALVE POSITION Output
0.231



DEETH PURGE TO C3R DRUM BTMS
0.194



STM TO DEETH REBOILER Output
0.193







10. Energy Balance










DEETH STM COND ACC
−0.386



DEETH REFLUX COND
0.318



STM TO DEETH REBOILER
−0.308



DE-ETHANIZER MAT BALANCE
−0.300



DEETH STM COND ACC Output
−0.235



C3 IN DEETH OH
−0.217



DEETH REFLUX DRUM
0.204



C3 = LIQ TO DEETH OVHD COND
0.199



STM TO DEETH REBOILER A/B
−0.192



STM TO DEETH REBOILER
−0.187

















TABLE 6







Acetylene Converter Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Feed Rate








CONVERTER FEED EXCHANGER FEED GAS IN
−0.225


ACET CONVERT FEED VALVE POSITION
−0.224


DEETH VAPOR PROD Output
−0.223


ACET CONV FD GAS
−0.223


CONVERTER A INLET PRESSURE
−0.219


ACT CNV FD EXH OUT
−0.219


CONVERTER A 1ST BED 44C TW7
0.217


C2 = SPLITTER FEED TO OLA-1X
−0.211


DEETH FEED PREHEAT EXCHANGER INLET
−0.210


CONVERTER A 1ST BED 44B TW5
0.210







2. Midbed Temperature








1ST BED CONVERTER H2 Output
0.289


CONVERTER A MIDBED EXCHANGER DELTA PRESS
0.237


H2/FEED RATIO 1ST BED
0.232


ACET CONV A MIDBED Output
0.228


CONVERTER A 1ST OUTLET
0.226


CONVERTER A 1ST OUTLET TC2
0.217


CONVERTER A 1ST OUTLET TC3
0.217


CONVERTER A 1ST OUTLET TC1
0.216


ACET CONV FEED
0.198


CO IN H2 PRODUCT
−0.192







3. Lead Bed Conversion








ACT CNV FD HTR OUT
−0.307


CONVERTER A 1ST BED INLET
−0.303


ACT CONV FD HTR OUT TEM
−0.263


CONVERTER A 1ST INLET
−0.260


CONVERTER A 1ST BED 44A TW3
−0.257


CONVERTER A 2ND BED 43A TW2
−0.224


ACET CONV A MIDBED
−0.203


CONVERTER A 2ND BED 43C TW5
−0.179


CONVERTER A 2ND BED 43B TW4
−0.172


CONVERTER A 2ND INLET
−0.165







4. Guard Bed Conversion








CONVERTER A 2ND BED 43C TW5
−0.280


CONVERTER FINAL FEED EXCHANGER INLET COOLER
−0.267


CONVERTER A 2ND OUTLET TC3
−0.264


CONVERTER A 2ND BED OUTLET
−0.263


CONVERTER A 2ND OUTLET TC1
−0.263


CONVERTER A 2ND OUTLET TC2
−0.263


CONVERTER A 2ND BED 43B TW4
−0.257


CONVERTER A 1ST BED INLET
0.237


ACT CNV FD HTR OUT
0.234


ACT CONV FD HTR OUT TEM
0.219
















TABLE 7







Ethylene Splitter Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Pressure Balance








SPLITTER FEED
−0.223


SPLTR HIP RBLR VAP
−0.222


C2 = TO BL_FEED_FL
−0.212


ETY SPLT REFLUX DR
−0.204


SPLITTER MID DP 1-113
−0.200


SPLITTER REFLUX FLCO Output
−0.197


SPLITTER MID DP 32-113
−0.197


SPLTR HIP RBLR IN
−0.185


OUTLET BTM DRAW OFF
−0.184


SPLTR TRAY 116
−0.183







2. Feed Rate








C2 = SPLITTER TOTAL HEAT IN
0.257


C3 = TO SPLTR REBOILER
0.256


C2 = SPLITTER REBOILER BTU CONTROLLER
0.255


INTER REFLUX/FEED RATIO
0.253


MSE23 DUTY
0.252


SPLITTER BOTTOMS
0.246


SPLITTER REFLUX FLCO
0.244


SPLITTER BTM DP 129-150
0.183


C2 = SPLITTER BOTTOMS C20 (FEED)
0.164


MATERIAL BALANCE C2 = SPLITTER
0.162







3. Product Quality








OLEFINS RECOVERY TRAIN C2 = PROD
−0.259


HP ETHYLENE PROD
−0.258


SPLITTER C2 = PROD
−0.243


C2 = VAPOR PRODUCT
−0.209


C2 = PRODUCT
−0.199


SPLITTER BOTTOMS
0.194


SPLITTER BOTTOMS
0.194


SPLIT LVL TO SP526
0.191


MATERIAL BALANCE C2 = SPLITTER
−0.188


SPLITTER C2 = PROD Output
−0.184







4. Overhead Energy Balance








C2 = VAPOR PRODUCT TLCO
0.245


C2 = VAPOR PRODUCT
0.239


HP ETHYLENE PROD
0.231


HP ETHYLENE PROD
0.222


PROP FROM ETHY HTR
0.217


ETHY PROD LOW TEMP OVRD Output
−0.185


C2 = PRODUCT
−0.174


OLEFINS RECOVERY TRAIN C2 = PROD
−0.154


HP ETHYLENE PROD
−0.154


SPLITTER C2 = PROD Output
−0.149







5. Bottoms Energy Balance








C2 = SPLITTER REBOILER SHELL OUT
−0.369


ETHY SPLIT REB FD
−0.368


SPLITTER REBOILER A SHELL LEVEL
−0.337


SPLITTER REBOILER B SHELL LEVEL
−0.334


C3 = TO SPLTR REBOILER Output
0.215


SPLITR HIP RBLR IN Output
−0.214


C2 = SPLITTER ENERGY BALANCE
0.187


SPLITR HIP RBLR IN
−0.158


C2 = SPLITTER HIP REBOILER
−0.156


SPLITTER HIP REBOILER HEAT DUTY
−0.144







6. Ethylene Refrigeration Balance








SPLITTER OVHD COND VAPOR
0.451


SPLITTER OVHD COND VAPOR PRESS
0.424


ETHY STLTR VNT CND Output
−0.417


SPLITTER OVHD COND VAPOR PRESS Output
−0.359


ETHY STLTR VNT CND Output
0.203


C2 = RECYC TO COLDBOX
0.192


C2 = RECYC TO COLDBOX
0.174


RECY ETHY FROM SPLITTER OVHD COND
0.165


ETHY SPLIT REB FD
0.131


C2 = SPLITTER REBOILER SHELL OUT
0.130







7. Temperature Profile








C2 = SPLITTER HIP REBOILER
−0.338


SPLTR HIP RBLR VAP
0.321


C2 = SPLITTER BOTTOMS C2 PRODUCT
0.294


PROP COMP 4TH STG DISC
0.248


SPLITR HIP RBLR IN Output
−0.214


C2 SPLITTER OVHD COND B VALVE DP
0.194


OVHD COND PURGE TO PGPU
0.186


MSPLITTER REFLUX PUMP SUCTION TEMP
0.161


C2 = SPLITTER BOTTOMS C20 (FEED)
−0.159


C2 SPLITTER OVHD COND A LO OVERRIDE SEL Output
−0.144







8. Bottoms Quality








SPLITTER BTMS SUPERHEAT CTRL OR Output
−0.384


SPLITTER BTMS C2 PRODUCT
0.353


C2 = SPLITTER BOTTOMS SUPERHEAT CTRL OR Output
0.239


EPW C2O AT SPLITTER BTMS OUTLET
−0.238


SPLITTER BOTTOMS Output
0.234


SPLITTER BTMS SATURATION TEMP
−0.233


ETHANE PRODUCT
0.181


C2 PROD TO B LIMIT
0.162


C2 = SPLITTER BOTTOMS C2 PRODUCT
−0.150


SPLTR REFLUX CND B
−0.137







9. Reflux Balance








SPLTR REFLUX CND B
0.467


SPLTR REFLUX CND A
0.466


C2 = PROD FROM MSD14
0.454


OVHD COND PURGE TO PGPU
0.267


C2 = SPLITTER BOTTOMS C2 PRODUCT
−0.169


C2 = SPLITTER HIP REBOILER
0.163


SPLTR HIP RBLR VAP
−0.142


ETHANE PRODUCT
0.132


MSPLITTER REFLUX PUMP SUCTION TEMP
0.132


SPLITR HIP RBLR IN Output
0.123







10. Overhead Propylene Refrigeration








C2 = PROD FROM MSD14
0.258


SPLTR REFLUX CND B
0.253


SPLTR REFLUX CND A
0.232


ETHY SPLIT REB FD
0.197


C2 = SPLITTER REBOILER SHELL OUT
0.196


SPLITR HIP RBLR IN Output
−0.187


SPLITR HIP RBLR IN
−0.185


C2 SPLITTER OVHD COND A LO OVERRIDE SEL Output
0.181


C2 SPLITTER OVHD COND B LO OVERRIDE SEL Output
0.173


C2 = SPLITTER HIP REBOILER
−0.168
















TABLE 8







Methanator and Hydrogen Dryers Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Temperature Profile








METHANATOR 42C TW4
−0.263


METHANATOR 42D TW5
−0.263


METHANATOR OUTLET
−0.260


METHANATOR 42C TW3
−0.260


METHANATOR 42D TW3
−0.260


METHANATOR 42A TW3
−0.259


METHANATOR 42B TW3
−0.259


METHANATOR 42B TW2
−0.254


METHANATOR OUTLET TC2
−0.252


UNMETH H2 MOL WT
0.244







2. Pressure Balance








BHLA H2 HEADR PRES
−0.301


METH COOLER K O DR
−0.299


H2 TO BPLA
−0.295


METHANATOR FEED
−0.288


H2 DRIER PRODUCT
−0.288


OLEFINS RECOVERY TRAIN H2->EPS/B5/RESINS
−0.285


H2 BYPASSING METHANATOR
−0.270


UNMETH H2 TO FUEL
−0.261


MIN DEMTH #4FD DRM PRESS
−0.212


METHANATOR OUTLET EXCHANGER DELTA PRESS
0.171







3. Feed Rate








METHANATED H2 PRODUCT
0.334


METHANATOR OUTLET EXCHANGER DELTA PRESS
0.323


H2 BYPASSING METHANATOR
−0.315


H2 FLOW THROUGH REFINERY HDR
0.305


MIN DEMTH #4FD DRM PRESS
0.285


MIN DP ACROSS SMALL CO FEED VLV
0.228


STEAM TO METHANATOR FEED EXCHANGER Output
0.216


H2 BYPASSING METHANATOR
0.208


H2 TO BHLA/SHLA
−0.201


METHANATOR COLDBOX TEMPERATURE
0.180







4. Product Energy Balance








MCO TEMPERATURE
0.384


H2 DRIER PRODUCT
0.372


H2 INLET TO FINAL CONVERTER FEED EXCHANGER
0.343


METH H2 COOLER OUT
0.297


MIDDLE METHANATOR OUTLET EXCHANGER WATER
0.277


OUT


METHANATOR DUMP TO FLARE
0.267


CHILLED METH H2
0.230


METH H2 FROM METHANATOR OUTLET EXCHANGER
0.225


C2 = FROM MC2R OUTLET 2ND EXCHANGER
0.191


METHANATED H2 MOL WT
−0.157







5. Feed Quality








CO IN H2 PRODUCT
−0.579


CONVERTER BED #1 CO FB
−0.571


1ST BED CONVERTER H2
0.241


H2 BYPASSING METHANATOR
−0.186


STEAM TO METHANATOR FEED EXCHANGER
−0.176


METHANATED H2 PRODUCT
0.158


CO IN METHANATOR FEED
−0.147


METHANATOR 42A TW1
−0.146


METHANATOR FEED TEMP
−0.138


METHANATOR 42B TW2
−0.127







6. Product Quality








MAPD REACTOR H2 FLOW TO 1ST BED
−0.393


H2 BYPASSING METHANATOR
0.377


H2 FLOW TO MAPD REACTOR 2ND BED
−0.351


METHANATED H2 PRODUCT
−0.315


METHANATOR COLDBOX TEMPERATURE
0.303


CONVERTER BED #1 CO FB
−0.275


CO IN H2 PRODUCT
−0.273


H2 TO BHLA/SHLA
−0.261


H2 FLOW THROUGH REFINERY HDR
−0.206


1ST BED CONVERTER H2
−0.193







7. Hydrogen Demand








H2 FLOW TO MAPD REACTOR 2ND BED
0.402


CHILLED METH H2
0.353


MAPD REACTOR H2 FLOW TO 1ST BED
0.323


METHANATOR DUMP TO FLARE
−0.270


METHANATED H2 MOL WT
0.254


C2 = FROM MC2R OUTLET 2ND EXCHANGER
0.253


STEAM TO METHANATOR FEED EXCHANGER
0.228


METHANATOR FEED TEMP
0.205


MIN DP ACROSS SMALL CO FEED VLV
0.196


H2 DRIER PRODUCT
−0.181







8. Feed Temperature








METHANATOR FEED TEMP
0.635


STEAM TO METHANATOR FEED EXCHANGER
0.564


CO IN H2 PRODUCT
−0.192


CONVERTER BED #1 CO FB
−0.155


H2 FLOW THROUGH REFINERY HDR
0.141


METHANATOR DUMP TO FLARE
0.122


H2 DRIER PRODUCT
0.122


CHILLED METH H2
−0.121


METHANATED H2 MOL WT
−0.114


MCO TEMPERATURE
0.108







9. Hydrogen Mass Balance








METHANATOR DUMP TO FLARE
0.396


H2 FLOW TO MAPD REACTOR 2ND BED
0.346


METH H2 COOLER OUT
−0.330


METH H2 FROM METHANATOR OUTLET EXCHANGER
−0.326


MAPD REACTOR H2 FLOW TO 1ST BED
0.250


H2 INLET TO FINAL CONVERTER FEED EXCHANGER
−0.235


H2 DRIER PRODUCT
0.218


1ST BED CONVERTER H2
−0.194


MIN DP ACROSS SMALL CO FEED VLV
0.190


H2 DRYER B INLET
0.178







10. Product Temperature Balance








C2 = FROM MC2R OUTLET 2ND EXCHANGER
−0.522


C2 = DESUPERHEATER C2R OUTLET 2ND EXCHANGER
0.417


CHILLED METH H2
−0.318


ETHY IN H2 PRODUCT
0.246


METHANATED H2 MOL WT
0.235


UNMETH H2 TO OXO
0.225


METHANATOR COLDBOX TEMPERATURE
0.215


C2 = DESUPERHEATER C2R OUTLET 2ND EXCHANGER
0.202


Output


METH H2 FROM METHANATOR OUTLET EXCHANGER
0.178


METHANATOR DUMP TO FLARE
0.141
















TABLE 9







Depropanizer Principal Components


With Sensor Title and Principal Component Loading










Sensor Description
Loading











1. Feed Rate










DEETH BOTTOMS TO DEPROP
0.270



DEPROP REFLUX Output
0.254



DEETH BTMS Output
0.254



DEETH BTMS
0.250



DEPROP TRAY 35
0.242



DEETH BTM TO DEPROP
0.241



DEPROP BOTTOMS Output
0.235



DEPROP TRAY 45
0.226



DEPROP BTM PRESS
0.223



DEPROP BOTTOMS
0.219







2. Bottoms Energy Balance










DEPROP REBOILER INLET
−0.351



DEPROP REBLR INLET
−0.334



DEPROP TRAY #47 TEMP
−0.302



DEPROP TRAY 45
−0.258



DEPROP REBOILER STM
0.251



MDEPROP OUTLET TEMP
−0.220



DEPROP TRAY 35
−0.209



DEPROP BTM PRESS
0.201



DEETH BTM TO DEPROP
−0.198



DEPROP TOTAL DELTA P
0.196







3. Overhead Energy Balance










MDEPROP OUTLET TEMP
0.385



DEPROP TRAY #7 TEMP
0.309



DEPROP REBOILER OUT
0.300



DEPROPANIZER OVHD TEMP
0.284



DEPROP REFLUX
0.278



DEPROP REBOILER STM
0.243



DEPROP REBOILER CONDENSATE
0.235



DEPROP BOTTOMS
−0.223



DEPROP TOTAL DELTA P
0.213



DEPROP REBOILER ST PHCO
0.185







4. Overhead Material Balance










MAPD REACTOR HYDROFINER FD FLCI
−0.414



DEPROP REFLUX DRUM
−0.343



DEPROP OH TO FLARE
−0.313



DEPROP OVERHEAD
−0.293



MAPD REACTOR HYDROFINER FD FLCI Output
−0.275



DEPROP TRAY 7
−0.251



DEETH BTMS Output
0.229



DEPROPANIZER OVHD TEMP
−0.202



DEPROP REBOILER CONDENSATE
0.184



DEPROP TRAY #7 TEMP
−0.184

















TABLE 10







MAPD Hydrofiner Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Guard Bed Temperature Profile








MAPD REACTORA/B/C BED TEMP TW18
−0.140


MAPD REACTOR GUARD OUTLET TEMP
−0.140


MAPD REACTORA/B/C BED TEMP TW14
−0.139


MAPD REACTORA/B/C BED TW16
−0.136


MAPD REACTORA/B/C BED TW17
−0.136


MAPD REACTORA/B/C BED TW14
−0.135


MAPD REACTORA/B/C BED TW15
−0.135


MAPD REACTORA/B/C BED TW18
−0.135


MAPD REACTOR GUARD OUTLET TC1
−0.135


MAPD REACTOR GUARD OUTLET TC2
−0.135







2. Lead Bed Conversion








HYDROGEN TO HF
−0.185


COS ADSORBER BED DP
−0.167


RERUN TOWER FEED STM VAPORIZER
−0.162


MAPD REACTOR LEAD BED 1ST SPREAD
−0.158


MAPD REACTOR LEAD OUTLET TEMP
0.156


HYDROFINER INLET TEMP
−0.155


MAPD REACTORA/B/C BED TEMP TW01
−0.154


MAPD REACTORA/B BED TEMP TW18
0.149


MAPD REACTORA/B/C BED TW02
−0.148


MAPD REACTORA/B/C INLET TEMP
−0.146







3. Midbed Quality








MAPD REACTORA/B/C INLET PRESSURE
−0.186


MAPD REACTOR OUTLET PRESS
−0.183


MA IN H/F MIDBED
−0.182


PD IN H/F MIDBED
−0.180


COS ADSORBER BED OUTLET
−0.175


MAPD REACTORA/B INLET PRESSURE
−0.168


H2 FLOW TO MAPD REACTOR 2ND BED Output
−0.164


MAPD REACTORA/B LEVEL5 SKIN TEMP
0.163


MAPD REACTOR HYDROFINER FD FLCI Output
−0.158


MAPD REACTORA/B LEVEL8 SKIN TEMP
0.157







4. Pressure Balance








MAPD REACTOR/C OUTLET PRESS
−0.260


HYDROFINER OUTLET
−0.258


MAPD REACTOR OUTLET PRESS
−0.234


MAPD REACTORA/B/C INLET PRESSURE
−0.231


RERUN TOWER FEED QW VAPORIZER
−0.221


COS ADSORBER BED OUTLET
−0.208


MAPD REACTORA/B INLET PRESSURE
−0.205


C3 HYDROFINER FEED
−0.177


MAPD REACTOR HYDROFINER FD FLCI Output
−0.165


MAPD REACTORA/B/C INLET TEMP
−0.165







5. Recycle Feed to Reactor








MAPD REACTORA/B BED TW11
−0.274


MAPD REACTORA/B BED TW02
−0.265


MAPD REACTOR C3 RECYCLE TO FEED
0.249


MAPD REACTOR C3 RECYCLE TO FEED Output
0.208


C3 HYDROFINER FEED
0.185


MAPD REACTOR H2 FLOW TO 1ST BED Output
0.181


MCOS ADSORBER BED BOTTOM
−0.171


MAPD REACTORA/B INLET TEMP
−0.167


DEPROP REFLUX
−0.167


MAPD REACTORA/B BED TW01
−0.159







6. Lead Bed Selectivity








MAPD REACTOR TOTAL SELECTIVITY
0.262


MAPD REACTORA/B BED TW01
0.218


MAPD REACTORA/B/C BED TW02
0.205


MAPD REACTORA/B/C BED TW11
0.197


MAPD REACTORA/B BED TW10
0.193


MAPD REACTOR LEAD BED 2ND SPREAD
0.189


MAPD REACTOR LEAD BED 6TH SPREAD
0.186


HYDROFINER OUTLET Output
−0.186


MAPD REACTORA/B INLET TEMPERATURE
0.180


MAPD REACTOR C3 RECYCLE TO FEED
0.166







7. Guard Bed Conversion








MAPD REACTOR LEAD BED 2ND SPREAD
−0.244


MAPD REACTORA/B BED TW10
−0.242


MAPD REACTOR TOTAL SELECTIVITY
0.235


MAPD REACTOR LEAD BED 6TH SPREAD
−0.231


MAPD REACTORA/B BED TW01
−0.229


MAPD REACTORA/B/C BED TW11
0.220


MAPD REACTORA/B/C BED TW02
0.217


MAPD REACTORA/B BED TW05
−0.176


MAPD REACTOR - QNCH H2O TO MAPD REACTOR
0.159


OUTLET COOLER


MAPD REACTOR LEAD SELECTIVITY
0.152







8. Energy Balance








MAPD REACTOR - QNCH H2O TO MAPD REACTOR
0.270


OUTLET COOLER


MAPD REACTOR LEAD BED 2ND SPREAD
0.218


RERUN FEED EXCHANGER INLET
0.218


C3 = VAP FROM MAPD REACTOR OUTLET COOLER
0.215


MAPD REACTOR OUTLET COOLER QUENCH
−0.210


WATER PHCO


MAPD REACTOR LEAD BED 6TH SPREAD
0.205


HYDROFINER OUTLET Output
0.195


QW EXCH HOT WATER TEMP
−0.184


MAPD REACTORA/B BED TW01
0.172


MAPD REACTORA/B BED TW10
0.171
















TABLE 11







Propylene Fractionator Principal Components


With Sensor Title and Principal Component Loading








Sensor Description
Loading










1. Temperature Profile








C3 = FRACT TRAY 15
−0.224


C3 = FRACT TRAY 37
−0.222


C3 = FRACT TRAY 57
−0.222


C3 = FRACT OVHD VAP
−0.221


C3 = FRACT - C3 = FRACT REBOILER B VAP OUT
−0.220


C3 = FRACT - C3 = FRACT REBOILER A VAPOR OUT
−0.220


C3 = FRACT REB IN
−0.218


C3 = FRACT REBOILER B VAP OUT TEMP
−0.213


RERUN TOWER CONDENSED OVHD
−0.207


C3 = FRACT RBLR IN TEMP
−0.207







2. Pressure Balance








RERUN TOWER - RERUN OVHD COND VENT GAS
0.295


RERUN TOWER FEED TO BURNER LINE
0.276


RERUN TOWER OVHDCONDENSOR TO C3R MAKEUP
0.270


C3 = RERUN TRAY 15
0.256


C3 = RERUN TRAY 10
0.256


C3 = FRACT REFLUX FLCI
−0.229


C3 = FRACT DP
−0.216


C3 = RERUN OH
0.210


RERUN TOWER C3 = VAP FROM RERUN OVHD COND
0.207


RERUN TOWER DP
−0.206







3. Product Rate








C3 = PRODUCT DRUM NET DISCHARGE Output
0.324


C3 = PRODUCT DRUM NET DISCHARGE
0.317


C3 = TO SURGE DRUM Output
0.314


C3 = FRACT OH PRODUCT
0.255


LIQ PROPYLENE PROD
−0.249


C3 = FRACT REFLUX DRUM
0.228


C3 = PRODUCT DRUM C3 = PROD SURGE DRUM
0.196


C3 = FRACT OH PRODUCT Output
0.191


STM TO C3 = FRACT REBOILER A BEST RANGE Output
−0.173


RERUN FEED EXCHANGER BYPS CTL-VALV Output
0.172







4. Feed Rate








RERUN TOWER DRAW TO C3 = FRACT Output
−0.291


STM TO C3 = FRACT REBOILER A BEST RANGE
−0.286


C3 = RERUN REFLUX FLCO
−0.280


C3 = RERUN REFLUX FLCO Output
−0.278


RERUN TOWER DRAW TO C3 = FRACT
−0.274


FRACTIONATOR COND ACCUM Output
−0.253


C3 = FRACT BOTTOMS LEVEL
−0.205


C3 = FRACT STM VALVE POS CNTL Output
−0.199


RERUN TOWER SIDE DRAW OFF LLCI
−0.184


RERUN TOWER CIRCULATE C3 = ′S Output
−0.167







5. Feed Quality








C3 = FRACT DP
−0.353


C3 = FRACT REFLUX FLCI
−0.346


RERUN TOWER DP
−0.288


RERUN TOWER VENT COND RERUN OVHD COND Output
0.234


C3 = FRACT BTM NET:BEST RANGE
−0.204


RERUN TOWER - RERUN OVHD COND VENT GAS
−0.200


RERUN TOWER C3 = VAP FROM RERUN OVHD COND
−0.198


RERUN FEED EXCHANGER BYPS CTL-VALV Output
−0.157


RERUN TOWER RERUN REFLUX DRUM
0.147


RERUN TOWER DRAW TO C3 = FRACT
−0.145







6. Energy Balance








C3 = RERUN REFLUX FLCO
−0.331


RERUN TOWER DRAW TO C3 = FRACT
−0.286


FRACTIONATOR COND ACCUM Output
0.284


C3 = FRACT STM VALVE POS CNTL Output
0.272


STM TO C3 = FRACT REBOILER A BEST RANGE
0.269


RERUN TOWER SIDE DRAW OFF LLCI
−0.261


RERUN TOWER DRAW TO C3 = FRACT Output
−0.206


MAPD REACTOR - QNCH H2O TO MAPD REACTOR
0.202


OUTLET COOLER


RERUN TOWER RERUN REFLUX DRUM
−0.199


C3 = FRACT REFLUX FLCI Output
0.188
















TABLE 12





Valve-Flow Models


With Model Title


Valve-Flow Models


Model Description

















Fresh Feed to CGC



Propylene Liquid to SCD02



Propylene Liquid to C3R DRIER



De-Methanizer Feed #1



De-Methanizer Feed #2



De-Methanizer Feed #3



De-Methanizer Feed #4



Hydrogen Separator Drum Flow



De-Methanizer Feed #5



De-Ethanizer Reflux Flow



Methane Compressor Minimum Flow



Methane Booster Second Stage Recycle



De-Methanizer Bottoms Flow



De-Propanizer Reboiler Out



De-Ethanizer Bottoms



Splitter Hip Reboiler



Splitter Reflux



Splitter Product



De-Methanizer Reflux Flow



Ethylene Vapor Product



Condensate Stripper Heated Feed



Steam to De-Ethanizer Reboiler



Liquid Ethylene to ND03



Fourth Stage Suction Propylene to DEMETH REBOILER



Hydrogen Flow to Acetylene Converter Lead Bed



Hydrogen Flow to Acetylene Converter Guard Bed



Acetylene Converter Flow

## Claims

1. A method for abnormal event detection (AED) for current operation for some process units of an ethylene processing system comprising:
(a) comparing online measurements from the process units to a set of models including principal component analysis models and/or engineering models for normal operation of the corresponding process units, wherein said ethylene processing system has been divided into equipment groups and process operating modes wherein there is minimal interaction between said equipment groups and principal component analysis models correspond to equipment groups and process operating modes,
(b) determining if the current operation differs from expected normal operations so as to indicate the presence of an abnormal condition in a process unit, and
(c) determining the underlying cause of an abnormal condition in the ethylene processing system.

2. The method of claim 1 wherein said set of models corresponds to equipment groups and process operating modes, one model for each group which may include one or more operating modes.

3. The method of claim 1 wherein said set of models correspond to equipment groups and process operating modes, one model for each group and each mode.

4. The method of claim 1 wherein said set of models of normal operations further includes engineering models.

5. The method of claim 1 wherein said ethylene processing system is divided into operational areas with a principal component analysis model for each area.

6. The method of claim 1 further comprising the steps of
(a) dividing the ethylene process into equipment groups with one or more operating modes,
(b) determining input variables and their operating range for said equipment groups and/or operating modes,
(c) determining historical training and testing data for said input variables,
(d) creating a set of principle component analysis models and/or engineering models representing normal operations, where each model corresponds to one of said equipment groups including one or more operating modes,
(e) dividing the ethylene process into operational areas where a set of one or more models corresponds an operational area, and
(f) comparing online measurements from the ethylene process units to a set of models for normal operation of the corresponding operational area.

7. The method of claim 1 wherein said equipment groups have minimal interactions among each other.

8. The method of claim 1 wherein said equipment groups include feedforward information from upstream equipment groups and feedback information from downstream equipment groups.

9. The method of claim 1 wherein inputs to the models are measurements of the performance and/or physical state of the ethylene process, and/or control signals sent to field actuators from the control system, and/or external factors affecting the ethylene process.

10. The method of claim 1 wherein the inputs to the models determine the physical state of the ethylene process.

11. The method of claim 1 wherein said indication of an abnormal event further comprises of suppressing model calculations to eliminate false positives on special cause operations.

12. The method of claim 1 wherein unwanted abnormal event indications are suppressed using override suppressors.

13. The method of claim 2 wherein said equipment groups include all major material and energy interactions in the same group.

14. The method of claim 2 wherein each said model further comprises suppressing model calculations to eliminate false positives on special cause operations.

15. The method of claim 2 further comprising the steps of(a)-determining said each model begins with a rough model based on questionable data, (b) using said rough model to gather high quality training data, and improve the model, and (e) repeating step (b) to further improve the model.

16. The method of claim 2 wherein said model includes transformed variables.

17. The method of claim 2 wherein principal components include process variables provided by online measurements.

18. The method of claim 4 wherein said engineering model further comprises determining consistency between some control valves and flow meters.

19. The method of claim 4 wherein the principle components for the principle component analysis models are selected based on named degrees of freedom in the equipment group.

20. The method of claim 4 wherein the number of principal components is chosen such that coefficients of the non-chosen principal components become about equal in size.

21. The method of claim 5 wherein there are eleven operational areas.

22. The method of claim 5 wherein said principal components analysis models include process variable values measured by sensors.

23. The method of claim 5 wherein said principal components analysis models for different process units include some process variable values measured by the same sensor.

24. The method of claim 5 wherein a number of principal components is selected by a magnitude of total process variation represented by successive components.

25. The method of claim 6 wherein operating modes are specific time periods wherein the ethylene process behavior is significantly different.

26. The method of claim 6 wherein historical data (raining sets arc segregated according to operating modes.

27. The method of claim 6 wherein said models includes transformed variables.

28. The method of claim 6 further comprising the steps
(a) determining said model begins with a rough model based on questionable data,
(b) using said rough model to gather high quality training data, and improve the model, and
(c) repeating step (b) to further improve the model.

29. The method of claim 6 wherein a set of models of normal operations includes one or more Principle Component Analysis Models and/or one or more engineering models.

30. The method of claim 6 wherein the number of principle components is selected by the magnitude of total process variation represented by successive principle components.

31. The method of claim 6 wherein said principal component analysis models for different equipment groups include as inputs some process variable values measured by the same sensor.

32. The method of claim 10 wherein said inputs to the models include temperatures, and/or pressures, and/or flows, and/or analyzer readings, and/or vibrations, and/or electrical current, and/or signals to field actuators.

33. The method of claim 12 wherein override suppressors act automatically whenever a defined condition exists.

34. The method of claim 12 wherein the operator initiates the action of an override suppressor.

35. The method of claim 13 wherein said equipment groups include quick recycles in the same group.

36. The method of claim 15 wherein said training data includes historical data for the model of the processing unit.

37. The method of claim 16 wherein said transformed variables include reflux to feed ratio in distillation columns, log of composition in high purity distillation, pressure compensated temperature measurement, sidestream yield, flow to valve position, and reaction rate to exponential temperature change.

38. The method of claim 16 wherein some pairs of measurements for two variables are brought into time synchronization by one of the variables using a dynamic transfer function.

39. The method of claim 16 wherein variables of process measurements that are affected by operating point changes in process operations are converted to deviation variables.

40. The method of claim 16 wherein said model inputs are corrected for noise.

41. The method of claim 16 wherein the measurements of a variable are scaled.

42. The method of claim 16 wherein measurements are transformed to bring tern into time synchronization, with one of the other variables using a dynamic transfer function.

43. The method of claim 17 wherein principal components include some measurement pairs that are time synchronized to one of the variables using a dynamic filter.

44. The method of claim 17 wherein process variables affected by operating point changes in the process operations are convened to deviation variables.

45. The method of claim 21 wherein the eleven operational areas of the ethylene processing system include; charge gas compressor; drier, cold box, and methane booster; ethylene and propylene refrigeration; demethanizer; deethanizer; acetylene converter; C2 splitter; methanator and H2 drier; depropanizer; MAPD hydrofiner; and propylene rerun and fractionator.

46. The method of claim 27 wherein said transformed variables include reflux to feed ratio in distillation columns, log of composition in high purity distillation, pressure compensated temperature measurement, sidestream yield, flow to valve position, and reaction rate to exponential temperature change.

47. The method of claim 28 wherein said training data includes historical data for the model of the processing unit.

48. The method of claim 36 wherein the number of principal components is chosen such that coefficients of each principal component become about equal m size.

49. The method of claim 36 wherein the historical data periods selected for training and testing data sets are selected to increase cross correlation among inputs and/or increase the signal to noise ratio of the inputs in the data.

50. The method of claim 39 wherein a rolling average of inputs from recent operations is subtracted from the current value of the measurements.

51. The method of claim 40 wherein said model inputs are corrected by filtering or eliminating noisy measurements of variables.

52. The method of claim 41 wherein the measurements are scaled to the expected normal range of that variable.

53. The method of claim 49 wherein data from time periods of steady operations are eliminated from the training data.

54. The method of claim 49 wherein data from severe abnormal operations are eliminated from the training data.

55. The method of claim 49 wherein potential model inputs are deleted from the inputs used in the model.

56. A system for abnormal event detection (AED) for some process units of an ethylene processing system comprising:
(a) on-line data acquisition of measurements and control signals,
(b) a set of models including principal component models and/or engineering models included in the set describing operations of the process units of said ethylene processing system, wherein said ethylene process system has been divided into equipment groups, wherein said set of models includes at least a principal component model corresponding to an equipment group,
(c) a set of displays which indicate if the current operation differs from expected normal operations so as to indicate the presence of an abnormal condition in the process unit, and
(d) a set of displays which indicates the underlying cause of an abnormal condition in the ethylene processing system.

57. The system of claim 56 wherein said model for each process unit is either a principal components model or an engineering model.

58. The system of claim 56 wherein inputs to the system are measurements of the performance and/or physical state of the ethylene process, and/or control signals sent to process field devices from the control system, and/or external factors affecting the ethylene process.

59. The system of claim 56 wherein said models further comprises suppression calculations to eliminate unwanted notifications of known conditions.

60. The system of claim 56 wherein an ethylene processing system is partitioned into operational sections and the models are grouped into sets according to their associated operational section.

61. The system of claim 56 wherein the operator interface includes a continuous indication of the likelihood of an abnormality within each operational section of the ethylene processing system.

62. The system of claim 56 wherein the operator interface allows the operator to navigate to a display associated with the highest likelihood of an abnormal condition with a single mouse click.

63. The system of claim 57 wherein an ethylene processing system is partitioned into operational sections with a principal components model for each section.

64. The system of claim 57 wherein said principal components include process variables provided by online measurements.

65. The system of claim 57 wherein said model further comprises calculations to eliminate operator induced notifications and false positives.

66. The system of claim 57 wherein said model includes transformed variables.

67. The system of claim 58 wherein said inputs to the system include temperatures, and/or pressures, and/or flows, and/or analyzer readings, and/or vibrations, and/or electrical current, and/or signals to field actuators.

68. The system of claim 58 wherein said inputs to the system become transformed inputs to the models.

69. A system of claim 59 wherein override suppressors act automatically whenever a defined condition exists.

70. A system of claim 59 wherein the operator initiates the action of an override suppressor.

71. The system of claim 61 wherein the likelihood of an abnormality within an operational section is indicated to the operator with a trend of the highest likelihood of an abnormality.

72. The system of claim 61 wherein the operator is alerted whenever the likelihood of an abnormality exceeds a defined threshold.

73. The system of claim 62 wherein said displays include Pareto charts.

74. The system of claim 62 wherein, for a set of models, the Pareto charts list models in the order of magnitude of the likelihood of an abnormal condition as indicated by each model.

75. The system of claim 62 wherein said displays include X-Y plots.

76. The system of claim 63 wherein there are eleven operational sections.

77. The system of claim 63 wherein fuzzy Petri nets are used to convert outputs from models to the likelihood of an abnormal process operation and/or equipment problem.

78. The system of claim 66 wherein said transformed variables include reflux to total product flow in distillation columns, log of composition and overhead pressure in distillation columns, pressure compensated temperature measurements, flow to valve position and bed differential temperature and pressure.

79. The system of claim 66 wherein some measurement pairs are time synchronized to one of the variables using a dynamic filter.

80. The system of claim 66 wherein process measurement variables affected by operating point changes in the process operations are converted to deviation variables.

81. The system of claim 66 wherein the measurements and transforms of a variable are scaled prior to model identification.

82. The system of claim 66 wherein a number of principal components is selected by a magnitude of total process variation represented by successive components.

83. The system of claim 66 wherein noise is removed from the system inputs and/or the transformed variables.

84. The system of claim 66 wherein the measurements of a variable are scaled prior to model calculation.

85. The system of claim 73 wherein, for principle component analysis models, the Pareto charts show in order of magnitude the specific contribution of each input to the estimate of the likelihood of an abnormal condition.

86. The system of claim 75 wherein, two dimensional models are shown as X-Y plots.

87. The system of claim 75 wherein, the X-Y plot shows the estimate of the normal operating relationship as well as the confidence region around that estimate.

88. The system of claim 75 wherein the operator can recalibrate the X-Y plot on demand.

89. The system of claim 75 wherein time adjacent data sample points are connected.

90. The system of claim 75 wherein the normal process operating range of the X axis variable is indicated.

91. The system of claim 77 wherein the likelihood of abnormality indicated by models within a set are combined using fuzzy Petri nets to estimate the likelihood of an abnormality within an operational section.

92. The system of claim 80 wherein the measurements are scaled by the expected normal range of that transformed and/or deviation variable.

93. The system of claim 83 wherein noise is removed using digital filters.

94. The system of claim 83 wherein noise is removed using logic.

95. The system of claim 85 wherein the operator can navigate to time plots of inputs whose order is determined by each input's position on the Pareto chart.