Abstract:
An industrial process sensor having a sensor component exposed to process fluid detects when the sensor performance has been degraded by a sensor coating buildup from the process fluid. A baseline statistical metric, such as standard deviation of the process parameter sensed by the sensor, is determined during an initial operating period when the sensor component is clean. During continued operation of the sensor, the statistical metric is continually updated and monitored. An alarm output indicating that sensor coating has degraded sensor performance is produced when the current value of the statistical metric varies from the baseline value by an amount indicating degraded sensor performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present invention claims benefit to provisional application Ser. No. 60/728,201, filed Oct. 19, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to industrial process sensors and transmitters. In particular, the invention relates to automatic detection of material buildup on a sensor component exposed to process fluid.  
         [0003]     Industrial process sensors and transmitters are used to sense various characteristics of fluids flowing through a conduit, or contained within a vessel. The transmitters sense process parameters such as differential pressure, line pressure, temperature, and pH.  
         [0004]     Temperature sensors such as thermocouples, resistance temperature detectors or infrared sensors in process applications are usually protected by a metal or ceramic sheath. The sensor electrical leads are isolated from each other and from the metal sheath and metal parts through some kind of isolating material. The assembly consisting of the sensor, sensor electrical leads, sensor sheath, isolating material and installation fittings is called a sensor assembly.  
         [0005]     The sensor leads are connected to an electronic circuit that read the sensor signal and convert it to a temperature reading. This electronic circuit can reside in an input electronic card of a control, monitoring or safety system or in a transmitter. Transmitters are usually installed relatively close to the temperature sensor.  
         [0006]     The transmitter converts the sensor signal to a temperature measurement value and transmits the signal to a remote recipient such as a control, monitoring and/or safety system. The temperature value can be transmitted through different types of signals and media. It can be converted into an analog standard value such as 4 to 20 mA or through digital protocols such as HART, Fieldbus, Profibus, DeviceNet, Modbus, Ethernet, etc. The transmitting media can be via wires, fiber optic, infrared or RF.  
         [0007]     Temperature sensors used in industrial processes are typically fitted with a primary seal such as a thermowell. Thermowells are used to provide an additional protection to the temperature sensor. Thermowells are closed-end metal or ceramic tubes that protect temperature sensors from process pressure, erosion and corrosion. They also allow for the installing and removal of sensors without having to shut down the process. Many industrial processes involve fluids that cause sensor coating, a buildup of material on the thermowell (or on a temperature sensor that contacts the fluid directly). This sensor coating increases process temperature measurement response time, and affects control performance and plant safety. In some cases, the coating can become so extensive that it causes thermowell or sensor cracks or breakage.  
         [0008]     In many industrial plants, the process must be shut down from time-to-time to clean temperature sensors and thermowells. This maintenance must be done on a periodic basis, because it has been difficult to determine the extent of sensor coating without shutting down the process.  
         [0009]     Sensor coating problems produced by exposure of sensor components to industrial process fluids affect other types of process sensors as well. Examples of other components subject to sensor coating include pH probes, remote seals for pressure sensing, and vortex shedding flowmeter components.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     With the present invention, a degree of coating buildup on a process sensor component can be determined during process operation. The process parameter is sensed during an initial operating period when coating buildup has not yet been significant. Based on measured values of the process parameter during the initial operating period, a baseline statistical metric such as baseline standard deviation of the process parameter, is determined.  
         [0011]     By monitoring the statistical metric such as standard deviation of the process parameter during continued operation, and comparing it to the baseline value obtained while the sensor component was clean, an indication of the extent of material buildup on the sensor component can be determined. As material buildup changes sensor performance, there is a change in the statistical metric. An output based upon the change in statistical metric can provide an indication that buildup of the coating has reached a point that maintenance is required. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a process control system including a temperature sensor.  
         [0013]      FIGS. 2A, 2B , and  2 C are exploded views of embodiments of a temperature sensor/transmitter.  
         [0014]      FIG. 3  is a simplified block diagram of the temperature sensor/transmitter.  
         [0015]      FIG. 4  is a flow chart showing automatic detection of coating build-up. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  is a diagram illustrating process control system,  10 , which includes sensor/transmitter  12  and control room equipment  14  connected over a transmission loop  16  that can be a two or more wire cable, or a fiber optic cable, or a wireless link. In this embodiment, sensor/transmitter  12  measures temperature. Sensor/transmitter  12  is mounted on process piping  18 , and provides an output over loop  16  representing measured temperature of process fluid in piping  18 . Sensor/transmitter  12  may be a temperature transmitter, may be a sensing device that includes transmitter electronics located within a sensor housing, or may be a sensing device that communicates with control room equipment  14  directly or through a separate transmitter.  
         [0017]     Sensor/transmitter  12  transmits temperature information to control room equipment  14  in either analog or digital form. For example, sensor/transmitter  12  may transmit an analog signal representative of measured temperature by controlling the loop current flowing in loop  16  between 4 and 20 milliamps. In addition, sensor/transmitter  12  may transmit to control room  14  digital information related to measured temperature, to a measured secondary process parameter, or to diagnostic data. Transmission of digital information over loop  16  can, for example, be transmitted using the Highway Addressable Remote Transducer (HART) protocol. Alternatively, temperature information, as well as secondary measurements and diagnostic information can be transmitted by sensor/transmitter  12  to control room  14  using an all digital protocol such as Foundation Fieldbus, Profibus, Modbus, etc. Alternatively, the loop may employ various wireless techniques.  
         [0018]      FIGS. 2A-2C  show exploded views of three different sensor/transmitter configurations  12 A- 12 C, respectively.  
         [0019]     In  FIG. 2A , sensor/transmitter  12 A does not include transmitter circuitry, and communicates over loop  16  with control room equipment  14  either directly or through a separate transmitter. Sensor/transmitter  12 A includes sensor housing  20 , thermowell  22 , temperature sensor  24 , sensor assembly  26 , fitting  28 , and sensor leads  30  (which are connected to loop  16 ).  
         [0020]      FIG. 2B  shows sensor/transmitter  12 B, which is similar to sensor  12 A, but also includes internal transmitter  32 .  
         [0021]      FIG. 2C  shows sensor/transmitter  12 C, which is similar to sensor/transmitter  12 B. Instead of sensor housing  20 , sensor/transmitter  12 C includes transmitter housing  20 ′.  
         [0022]     Temperature sensor  24  may be, for example, a 2-wire, 3-wire, or 4-wire resistance temperature device (RTD) sensor or a thermocouple. An RTD sensor exhibits a change in resistance as a function of temperature, while a thermocouple exhibits a change in voltage as a function of sensor temperature.  
         [0023]     Fitting  28  is metal tube having threaded connections at each end to connect housing  20  or  20 ′and thermowell  22 . Fitting  28  surrounds the upper portions of sensor assembly  26  and provides a sealed passage from housing  20 ,  20 ′ to the upper end of thermowell  22 .  
         [0024]     Thermowell  22  provides a fluid tight seal tight that separates sensor assembly  26  and the interior of fitting  28  and housing  20  or  20 ′, from the process fluid. Thermowell  22  is directly exposed to process fluid. Over time, a material build-up (or sensor coating) caused by exposure to process fluids can cover the outer surface of thermowell  22 . This sensor coating can degrade sensor performance, and potentially cause damage to thermowell  22  and temperature sensor  24 .  
         [0025]     In other embodiments, temperature sensors are directly placed in contact with process fluid, rather then being positioned within a thermowell. Sensor coating is also a problem with sensors that are directly exposed to process fluid, which include not only temperature sensors, but also components of other industrial process sensors, such as pressure, flow, and pH sensors.  
         [0026]      FIG. 3  is a simplified electrical block diagram of sensor/transmitter  12 , and may be representative of any of the embodiments shown in  FIGS. 2A-2C . As shown in  FIG. 3 , sensor/transmitter  12  includes temperature sensor  24 , analog-to-digital (A/D) converter  40 , microprocessor  42 , clock  44 , memory  46 , input/output (I/O) interface  48 , power supply  50 , and terminals  52  and  54  (which are connected to loop  16 ).  
         [0027]     Signals from temperature sensor  24 , which are a function of the temperature to which sensor  24  is exposed, are converted to digital values by A/D converter  40 . The digital values are provided to microprocessor  42  for additional signal processing. Clock  44  provides clock signals necessary for operation of A/D converter  40 , as well as microprocessor  42 .  
         [0028]     Measured temperature values are used by microprocessor  42  to control I/O interface  48  in order to provide an output signal which is representative of the measured temperature. The output provided by I/O interface  48  can be an analog 4-20 mA loop current, or may be a digital signal representative of measured temperature. In addition, I/O interface  48  provides digital communications onto loop  16  based upon the information provided by microprocessor  42 . This information includes an indication of the status of sensor coating build-up.  
         [0029]     Power for all of the circuitry of sensor/transmitter  12  is derived from wire loop  16 . Power supply  50  is connected so that the loop current flows from terminal  52  through power supply  50  and I/O interface  48  to terminal  54 . It is appreciated the loop  16  may be wireless, and an alternative power source may be implemented to power the transmitter/sensor.  
         [0030]     Microprocessor  42  also stores measured temperature values on a periodic basis in memory  46 . These stored temperature measurement values are used by microprocessor  42  to perform statistical analysis in order to evaluate the extent of sensor coating build-up. Using stored configuration data, microprocessor  42  can also calculate an approximate coating thickness.  
         [0031]      FIG. 4  illustrates the automatic sensor coating detection feature, as performed by microprocessor  42 . During an initial operating period, when thermowell  22  is clean and any material build-up is minimal, microprocessor  42  periodically stores temperature measurement values in memory  46 . (Step  60 ).  
         [0032]     Using the stored values from the initial operating period, microprocessor  42  performs a statistical analysis of the measurement data. (Step  62 ). From this statistical analysis, at least one baseline statistical metric is derived, and is stored for later use (Step  64 ). The statistical metric must be one that changes with sensor coating build-up, so that periodic comparison of the metric derived from later-gathered measurement data can be used to determine how sensor performance has changed with respect to the baseline metric value.  
         [0033]     One example of a statistical metric that can be used for detecting material build-up is standard deviation of the measured parameter. A baseline standard deviation of measured temperature when the sensing component (for example, thermowell  22 ) is clean, can be compared to standard deviation during subsequent operation, to give very good indication of the extent of sensor coating material build-up. As material build-up increases, the temperature measurement time constant increases, and as a result there is a change in the standard deviation for a given process condition.  
         [0034]     During the period subsequent to the initial operating period, microprocessor  42  continues to store measurement values (Step  60 ) and perform the statistical analysis (Step  62 ). Microprocessor  42  compares the results to the baseline value. (Step  66 ). When the current standard deviation (or other statistical metric of the process parameter) has changed from the baseline value to an extent that indicates unacceptable material build-up, microprocessor  42  provides an alarm output through I/O interface  48  to control room equipment  14 . (Step  68 ).  
         [0035]     In addition, the standard deviation can also be used to modify process gain, in order to compensate for the effects of sensor coating. (Step  70 ). When sensor coating is present, the control loop becomes more sluggish. When a change in standard deviation indicates an increase in sensor coating, the change can be used to increase gain in sensor/transmitter  12 . This may extend the time between required cleaning of the sensor component.  
         [0036]     With the automatic sensor coating detection feature, maintenance to clean up or replace sensor components due to sensor coating build-up can be performed as needed. Unnecessary shut downs of processes simply to check on the status of sensor coating build-up can be avoided.  
         [0037]     Although the sensor coating detection feature has been described in the context of a temperature sensor or transmitter, it is also applicable to other types of sensors and transmitters, including pressure, flow, and pH sensors and transmitters. Similarly, although communication has been described over a two-wire or three-wire loop, other configurations using additional wires, or using wireless communication, also can take advantage of the automatic sensor coating detection feature.  
         [0038]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.