Patent Publication Number: US-2022235880-A1

Title: Partial stroke tests for shutdown valves

Description:
FIELD OF THE TECHNOLOGY 
     The present disclosure relates generally to valves and, more particularly, to running partial stroke tests of on/off valves. 
     BACKGROUND INFORMATION 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Safety instrumented systems (SIS) typically incorporate emergency shutdown valves which are normally in a fully opened or a fully closed position. An emergency shutdown controller or a Programmable Logic Controller (PLC), for example, controls an emergency shutdown valve to change its operational state (e.g., from fully opened to fully closed) in the event of emergency. Because the purpose of a shutdown valve is to either allow full flow of fluid through a pipeline or completely shut off the flow of fluid, a typical shutdown valve has high friction seals, loose linkage, a large volume (to close off a pipe that can be several feet in diameter), a high preload (e.g., a large bias to keep the valve in a closed position), and a shallow bench set (i.e., lower and upper air signal pressures used to set the initial preloading of the actuator biasing element). 
     In contrast to emergency shutdown valves, control valves generally are used for throttling control, e.g., to set the amount of fluid flow within a certain range between a fully opened and a fully closed position. Designs of control valves generally are meant to minimize the error signal between setpoint and travel feedback, which can include minimizing friction, characterizing trim, designing tight linkages, having springs with large rates and small preloads, setting pressures near the ends of the spring range, etc. 
     To ensure that the emergency shutdown valves in a system will function properly when needed, process control system operators and/or process control software can periodically run partial-stroke tests during which these valves partially open or partially close. These tests are typically performed when an emergency shutdown valve is online in a live process. On the other hand, because control valves are not used for shutdown service, control valves rarely undergo partial-stroke tests. 
     Because of these difference in design considerations, simply applying positioning technology developed for throttling (control) valves to on/off (shutdown) valves during partial stroke testing has certain drawbacks. For example, venting an actuator of a shutdown valve from a hard stop takes considerable time and introduces significant travel deviation. Further, larger actuators yield larger error signals, which effectively requires that a partial-stroke test be run slower (whereas it is important to perform a partial-stroke test of a valve that is online quickly and reliably). Still further, transitions to hard cut-offs at the end of a test can yield pressure readings that indicate stuck valve conditions when the shutdown valve operates properly. 
     For at least these reasons, approaches to partial-stroke testing of valves known today either fail to yield accurate results when applied to shutdown valves (or, more generally, to on-off valves), or produce results that are of little value to supervision and maintenance of shutdown valves, or take too long to produce useful results. 
     SUMMARY 
     A valve controller or another suitable instrument executes a partial-stroke test of a shutdown valve by generating a setpoint signal suitable specifically for valves that normally are fully open or fully closed. This setpoint signal does not require that valve travel catch up to the setpoint, nor does this setpoint signal go into a hard cutoff upon reaching a predefined threshold. Further, the valve controller applies acceptance criteria that ensure that the shutdown valve moves to a minimum amount from the hard stop at some point during the test, and to abort the partial-stroke test if the shutdown valve reaches a maximum travel displacement threshold. Still further, the valve controller applies a certain set of acceptance criteria to data indicative of the relationship between actuator pressure and valve travel, so as to accurately determine the stuck valve condition. In other words, in at least some of the implementations discussed below, system dynamics are identified by looking strictly at input pressure and resulting travel. 
     One embodiment of these techniques is a method for executing partial-stroke tests of valves. The method comprises generating a setpoint signal to stroke a valve during a partial-stroke test, applying the setpoint signal to the valve, and determining whether the valve passes the partial-stroke test using a response to the setpoint signal. Generating the setpoint signal includes determining a first target for the setpoint signal based at least on a travel displacement threshold, the travel displacement threshold corresponding to a desired extent of travel of the valve during the partial-stroke test, where the first target corresponds to a larger extent of valve travel than the travel displacement threshold. Generating the setpoint signal further includes ramping the setpoint signal from an initial value to the first target, during a first time interval; subsequently to the first time interval, maintaining the setpoint signal at the first target during a second time interval; determining a second target for the setpoint signal based at least on the initial value; and during a third time interval subsequent to the second interval, ramping the setpoint signal from the first target to the second target in a direction opposite to the ramping of the setpoint signal during the first time interval. 
     Another embodiment of these techniques is a method for detecting a stuck valve condition during a partial-stroke test of a shutdown valve. The method includes receiving a signal indicative of actuator pressure when the valve travels between an end point and a displaced position from the end point, and determining whether the actuator pressure is within a set of acceptance criteria during the partial-stroke test. The criteria include (i) a minimum actuator pressure when the valve travels between the end point and the displaced position, (ii) a maximum actuator pressure when the valve travels between the displaced position and the end point, and (iii) a breakout pressure when the valve travels between a stop threshold position and a valve stop position, the breakout pressure corresponding to a force required to break out of a hard stop. 
     Yet another embodiment of these techniques is a system including a shutdown valve configured to operate in a fully open position or in a fully closed position, a position sensor to generate a position signal indicative of a current position of the shutdown valve, and a digital valve controller coupled to the shutdown valve and configured to execute a partial-stroke test of the shutdown valve. To execute the partial-stroke test, the digital valve controller is configured to determine a first target for a setpoint signal based at least on a travel displacement threshold, the travel displacement threshold corresponding to a desired extent of travel of the valve during the partial-stroke test, where the first target corresponds to a larger extent of valve travel than the travel displacement threshold; ramp the setpoint signal from an initial value to the first target, during a first time interval; subsequently to the first time interval, maintain the setpoint signal at the first target during a second time interval; determine a second target for the setpoint signal based at least on the initial value; during a third time interval subsequent to the second interval, ramp the setpoint signal from the first target to the second target in a direction opposite to the ramping of the setpoint signal during the first time interval; and monitor positioning of the shutdown valve in response to the setpoint signal using the position signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example system in which a controller executes a partial-stroke test of a shutdown valve; 
         FIG. 2  is a block diagram illustrating an example pneumatic setup for an actuator of a shutdown valve that can operate in the system of  FIG. 1 ; 
         FIG. 3A  is a set of graphs that illustrate changes in parameter values during a partial-stroke test of an example shutdown valve, carried out in a conventional manner; 
         FIG. 3B  is a plot of actuator pressure versus actuator travel for an example shutdown valve configured to operate at multiple partially open positions; 
         FIG. 4A  is a plot of a nominal setpoint signal, which the controller of  FIG. 1  is configured to generate to execute a partial-stroke test of a shutdown valve; 
         FIG. 4B  is a plot of a setpoint signal with an early-return modification, which the controller of  FIG. 1  is configured to generate to execute a partial-stroke test of a shutdown valve; 
         FIG. 4C  is a plot of a setpoint signal with an early-return modification, where the valve position leads the setpoint signal, which the controller of  FIG. 1  is configured to generate to execute a partial-stroke test of a shutdown valve; 
         FIG. 5  is a plot that illustrates acceptance criteria applied to a pressure-versus-travel plot generated during a partial-stroke test, which the controller of  FIG. 1  can use to detect a stuck valve condition for a shutdown valve; 
         FIG. 6  is a flow diagram of an example method for generating a setpoint signal for a partial-stroke test of a shutdown valve, which can be implemented in the system of  FIG. 1 ; and 
         FIG. 7  is a flow diagram of an example method for detecting a stuck valve condition, which can be implemented in the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Generally speaking, the techniques of this disclosure allow an instrument to conduct an efficient and accurate partial-stroke test (PST) of an on-off valve such as an emergency shutdown valve. For clarity, an example system that includes a shutdown valve and a valve controller that tests the shutdown values in accordance with these techniques is briefly discussed with reference to  FIG. 1 , followed by a discussion of an example pneumatic setup for a shutdown valve as illustrated in  FIG. 2 . Example setpoint signals and parameter readings the are discussed with reference to  FIGS. 3A-5 , and example methods for executing a partial-stroke test and detecting a stuck valve condition are discussed with reference to  FIGS. 6 and 7 . 
     Example System and Pneumatic Actuator Setup 
     Referring first to  FIG. 1 , an example system  10  includes a shutdown valve  12  configured to stop fluid flow in a process  14  in an event of emergency. A pneumatic stage  16  can include a pneumatic positioner and actuator or a pneumatic relay and/or an actuator. The pneumatic stage  16  generates a signal to position a moveable valve member (e.g., valve plug) of the shutdown valve  12  to thereby restrict or increase the flow of process fluid. An example actuator which can be used in the pneumatic stage  16  is further discussed below with reference to  FIG. 2 . 
     A digital valve controller  18  (or simply “valve controller  18 ”) can control the shutdown valve  12  via the pneumatic stage  16 . The valve controller  18  can receive signals indicative of actuator pressure and valve travel from a pressure sensor  20  and a position sensor  22 , respectively. The sensors  20  and  22  can be implemented using any suitable components, including those currently known in the art. 
     As illustrated in  FIG. 1 , the valve controller  18  includes a processor  30 , a current-to-pressure (I/P) converter  32 , and a memory  34  storing a PST routine  36 . The memory  34  can include a non-transitory medium readable by the processor  30 , and the PST routine  36  can include instructions executable by the processor  30 , in any suitable programming language. The memory  34  also can store PST parameters  38 , such as ramp rate(s) for the setpoint signal, early turnaround selection, etc., as explained below. 
     A workstation  40  in this example configuration is coupled to the valve controller  34  to allow an operator to configure PST for the shutdown valve  12 , activate PST, monitor test progress, etc. The workstation  40  includes one or more processors, a memory readable by the one or more processors, a network interface (none shown to avoid clutter), and a user interface  42  such as a touchscreen, a conventional screen with a keyboard, etc. 
       FIG. 2  illustrates an example pneumatic setup for an actuator  100 , which can be used with the shutdown valve  12  of  FIG. 1 . More generally, however, the techniques of this disclosure are compatible with any type of an actuator to which a setpoint indicating a particular percentage of valve travel can be supplied, and in which at least a signal indicative of the current position of the throttling element can be measured. 
     In the actuator  100 , a rod  102  is coupled to a valve plug or another suitable throttling element via a yoke  104 . A rotatory position sensor  22  can be placed on the shaft of the actuator  100  to generate an electric signal indicative of valve travel. In other implementations, positions sensor can be coupled to actuators using other suitable techniques. 
     A spring  106  biases the actuator  100  toward a fully closed position. In other implementations, the spring  106  can be replaced with another biasing element. Pressure in a cylinder  108  prevents the spring  106  from driving the actuator  100  toward the fully closed position. Thus, the cylinder  108  is pressured during normal operation, when the shutdown valve on which the actuator  100  operates is inactive. Actuator pressure can be measured in the chamber  108 . To this end, any suitable pressure sensor can be used. 
     In an emergency, or in response to another event that requires that the valve shut down the flow, a solenoid  110  is de-energized, causing the chamber  108  to depressurize, which in turn allows the spring  102  to drive the valve toward a closed state. When the solenoid  110  is energized, pressure is supplied to the chamber  108  to thereby reposition the valve. During a PST, the solenoid  110  usually is powered and stationary. 
     Example Setpoint Signals 
     Prior to the discussion of example setpoint signals generated in accordance with the techniques of this disclosure, a setpoint signal along with travel and pressure changes during a conventional PST of a shutdown valve are briefly considered in connection with  FIG. 3A . The plots in  FIG. 3A  illustrate valve travel (plot  150 ), error percentage between setpoint and valve travel (plot  152 ), and actuator pressure (plot  154 ) as functions of time. These plots illustrate a typical response of a shutdown valve when the cylinder is depressurized. Generally speaking, the instrument in this scenario ramps a command (setpoint) signal to a given point, pauses so that valve travel can catch to the command signal, and then ramps the command signal back to the starting position. When the setpoint signal at a certain point levels off, the instrument lets valve travel to catch up. The instrument drives the pneumatics hard to full supply or full vent and, if travel lags setpoint, travel will snap into hard stop. This sudden change in pneumatics causes distortions in data at the stop, making interpretation of the results difficult. 
     In (approximate) region  160 , the valve begins to move after an initial decrease in pressure. As best illustrated in plot  150 , a setpoint signal  170  gradually changes while a valve travel signal  172  does not begin to change until a point in the region  160 . As a result, the error grows to almost 12%, as seen in plot  152 . Further, as seen in plot  154 , actuator pressure continues to decrease until reaching region  160 , where the pressure levels off. 
     If error is used as an indicator of whether the valve operates properly, the data illustrated in plot  152 , and especially the data points in region  160 , may be interpreted as a potential problem. However, the travel and pressure readings of  FIG. 3A  can correspond to normal operation of a shutdown valve assembly (i.e., a shutdown valve and an actuator) with a large volume, a shallow bench set, and high preload. In other words, whereas a 12% error may be a generally reliable indication that something is wrong with a conventional control valve, this error need not indicate failure for a shutdown value, particularly a large shutdown valve. Venting an actuator of a shutdown valve from a hard stop takes considerable time and introduces significant travel deviation, as indicated above. Thus, conducting a PST as illustrated in  FIG. 3A  can generate data that is misleading regarding the health of the shutdown valve. 
     A controller could minimize the error signal for large actuators by slowing down the test signal, so that the pneumatics have time to respond. However, this workaround only serves to prolong the test. These limitations are due to the error signal being the wrong variable for partial stroke testing, where typically it is desirable to determine whether friction is excessive and whether the valve fails to move. Error signal between reference and travel does not directly contain the necessary input variable to identify the system dynamics. Moreover, slowing down PSTs is inconsistent with another general objective, which is to complete a PST on a live shutdown valve quickly and with minimal disruption to the process. 
     Further, region  162  in  FIG. 3A  illustrates that at the end of the PST, the transition to a hard cutoff can introduce anomalies in the pressure data due to line restrictions near the pressure sensor increasing the pressure response locally. As can be seen in plot  154 , pressure appears to first climb from approximately 40 psig to 52 psig with no corresponding change in travel, and then rise gradually with the change in travel. Also, plot  180  in  FIG. 3B  illustrates changes in measured actuator pressure versus valve travel for two configurations of the same or similar shutdown valve. In particular, one of the configurations of the valve includes a rebreather (data points  184 ) , and the other configuration does not include a rebreather (data points  182 ). For both configurations, the valve is repositioned between 100% and approximately 68%. In  FIG. 3B , region  182  appears to show that there is no travel in response to a significant change in pressure between approximately 40 psig and 53 psig. 
     The discontinuities in regions  162  and  186  can be due to travel lagging the setpoint when a hard cutoff is engaged, causing flow to increase dramatically and pressure local to the sensor, rather the cylinder, to rise concomitantly. This data does not reflect what is happening in the cylinder and makes assessing stuck valve on the return stroke difficult. In particular, the data does not clearly convey whether the valve got stuck and then came loose, or whether the apparent discontinuities are an artifact of how the PST was executed. 
     Now referring to  FIG. 4A , plot  200  illustrates a nominal setpoint signal  202 , which an instrument such as the valve controller  18  can apply to a shutdown valve to efficiently and accurately test the shutdown valve  12 , without generating the ambiguities in data discussed with reference to  FIGS. 3A and 3B . More generally, the setpoint  202  can be generated by any suitable instrument and applied to any suitable on-off valve. 
     To initiate the PST of  FIG. 4A , an operator can specify the desired amount of travel for the shutdown valve  12  during a PST, as well as the desired rate(s) of change for the setpoint signal, via the user interface  42  of the workstation  40  (see  FIG. 1 ). For example, the user may wish the shutdown valve to travel 30%, from the 100% open to 70% open position, at the rate of 1% per second on the stroke (as parameter PST_RAMP_RATE) and 2% per second on return (as parameter PST_RAMP_RATE_RETURN). In some implementations, the user can specify the same rate for both directions; however it may be desirable to stroke relatively slowly and return relatively quickly. The workstation  40  in turn can supply these parameters to the valve controller  18 , to be used as PST parameters  38 . 
     The desired amount of travel is illustrated as the minimum required travel or minimum travel displacement threshold PST_STRK_TRAV illustrated in  FIG. 4A . Using the value of PST_STRK_TRAV, the valve controller  18  can calculate parameter PST_SP_CHANGE, which corresponds to the setpoint displacement from the hard stop. The PST_SP_CHANGE can be expressed as a percentage. In an example implementation, the valve controller  18  calculates PST_SP_CHANGE by multiplying PST_STRK_TRAV by a certain predetermined or preconfigured factor, such as 1.1, to define a 10% increase over PST_STRK_TRAV. More generally, the valve controller  18  can determine PST_SP_CHANGE by applying any suitable formula to PST_STRK_TRAV, but in any case PST_SP_CHANGE should define a larger value than PST_STRK_TRAV to compensate for calibration shifts, friction, or other offsets. It is noted that PST_SP_CHANGE does not define maximum travel displacement. Rather, this value defines one of the targets for the setpoint signal. 
     During the first interval INT 1 , the valve controller  18  ramps the signal  202  at the rate PST_RAMP_RATE from an initial position of the valve, e.g., the hard stop, to the first target PST_SP_CHANGE. In a typical situation, a travel signal  204  lags behind the setpoint  202 , as illustrated in  FIG. 4A . 
     After completing the ramp-up during the interval INT 1 , the nominal setpoint signal  202  remains constant during a next interval INT 2 . The duration of the interval INT 2  can be controlled by a parameter PST_PAUSE. Depending on the implementation, PST_PAUSE can be fixed at a certain value, such as twice the dead time off the valve stop (i.e., twice the time it takes the valve to initially respond to the setpoint signal and begin to move). In one example implementation, the PST_PAUSE is twice the dead time with a minimum value of 20 seconds. Further, in some implementations, an operator can override the default or suggested value for PST_PAUSE. 
     With continued reference to  FIG. 4A , the valve controller  18  can apply a maximum travel displacement threshold PST_STRK_MAX_TRAV as a secondary safety criterion. The value of PST_STRK_MAX TRAV can be calculated using PST_STRK_TRAV, for example. As a more specific example, PST_STRK_MAX_TRAV can be set to 1.3*PST_STRK_TRAV. In general, however, the value of PST_STRK_MAX_TRAV need not be larger than PST_STRK_TRAV. If the valve travel signal  204  reaches PST_STRK_MAX_TRAV, the valve controller  18  can abort the PST. This may occur when there are calibration errors (e.g., when the I/P bias is off) or due to other abnormal conditions. The valve controller  18  can compare the travel signal  204  to PST_STRK_MAX_TRAV during all of the intervals of the PST, INT 1  through INT 4 . 
     After the hold time between outgoing and return ramps during the interval INT 2 , the setpoint signal  202  begins to ramp in the return direction at the rate PST_RAMP_RATE RETURN during an interval INT 3 . The setpoint signal  202  in this example implementation ramps to a target that exceeds the hard stop position by PST_SP_OVER. The value of PST_SP_OVER can be preconfigured as a certain percentage, for example. 
     The valve controller  18  thus overdrives the servo and waits for the travel signal  204  to catch up during an interval INT 4  before engaging a hard cutoff. The value of the interval INT 4  can be set to PST_PAUSE or a different value, possibly including an operator-specified value, if desired. 
     In an example scenario, the valve controller  18  ramps the setpoint signal  202  from 100% to 80%, holds the setpoint signal  202  for 20 seconds, returns to 110%, holds the setpoint signal  202  for additional 20 seconds, and engages a hard cutoff. 
     Rather than using the error signal as an acceptance criterion in the manner discussed with reference to  FIG. 3A , the valve controller  18  uses PST_STRK_TRAV and PST_STRK_MAX_TRAV as acceptance criteria. On return, the valve controller  18  compares the travel signal  204  to the initial position value (in this case, the hard stop) to determine whether the valve returned to its initial position. In this manner, the valve controller  18  can conduct PSTs more efficiently and check conformance of the shutdown valve more accurately. 
     Now referring to  FIG. 4B , plot  250  illustrates application of a setpoint signal  252  to a shutdown valve, and the corresponding travel signal  254 . The setpoint signal  252  is generally similar to the setpoint signal  252 , except that here the operator has selected early-turnaround as one of the PST parameters. More particularly, in an example implementation, the operator specifies whether he or she wishes the setpoint signal to have the nominal profile illustrated in  FIG. 4A , where the duration of interval INT 2  is fixed at PST_PAUSE, or whether the duration of the interval INT 2  should be limited by the time it takes the travel signal to reach PST_STRK_TRAV. 
     Plot  250  illustrates the scenario where the operator has enabled the early-turnaround feature, and the valve controller  18  modifies the setpoint signal  252  at a turnaround point  260 , in response to receiving an indication from the travel sensor that the travel signal  254  reached PST_STRK_TRAV. Accordingly, the setpoint signal  252  begins to ramp at point  260  rather than staying at PST_SP_CHANGE. Similar to the setpoint signal  202 , the setpoint signal  252  can ramp beyond the hard stop to overdrive the servo, so that the travel signal  254  can catch up without slowing down near the end of the test. 
     Generally speaking, the early-turnaround capability allows the valve controller  18  to minimize the total test time and minimize process changes. For example, for a certain large shutdown valve, the total test time was reduced from approximately 240 seconds to approximately 150 seconds. The early-turnaround feature may be particularly useful when used with large actuators that tend to be slower. 
     When early turnaround is enabled, the valve controller  18  can redefine the initial conditions for the return setpoint as the actual travel or current setpoint, whichever is closer to the hard stop. Moreover, because the travel signal  254  often lags the setpoint signal  252 , the valve controller  18  can add a “lead” value, PST_RETURN_LEAD, to the setpoint signal  252  at the early-turnaround point  260 . This lead value causes valve travel to reverse immediately or almost immediately. 
     For example, if the nominal setpoint signal that runs 100% to 70% with a minimum travel threshold PST_STRK_TRAV set at 80%, the valve controller  18  can initialize the return setpoint at the greater of the current value of the setpoint signal or the current value of the travel signal. If the setpoint signal is below the travel signal when PST_STRK_TRAV is reached (as is the usual case), the valve controller  18  initializes the return setpoint to 80%, according to the nominal profile of the setpoint signal. However, because the travel signal lags the setpoint signal, setting the return setpoint at the level of the current travel signal will cause the shutdown valve to drift beyond this threshold. To solve this problem, the valve controller  18  can set the initial conditions for the return setpoint to 80% plus PST_RETURN_LEAD, such as 5%, so that the total initial condition for the return setpoint would be 85%. 
       FIG. 4B  illustrates the (more common) scenario where the travel signal  254  lags the travel setpoint  252  at the turnaround point  260 . Early turnaround is enabled, and the valve controller  18  immediately brings the setpoint up to PST RETURN LEAD and adds PST_STRK_TRAV to begin ramping the travel signal  254  in the return direction from this point. In other words, the valve controller  18  instantaneously modifies the setpoint by (PST_SP_CHANGE−PST_STRK_TRAV)+PST_RETURN_LEAD. On the other hand, in the plot of  FIG. 4C , a setpoint signal  302  lags a travel signal  304 . Accordingly, the valve controller  18  in this case adds PST_RETURN_LEAD to the current value of the setpoint signal  302 , thereby instantaneously modifying the setpoint by only PST_RETURN_LEAD. 
     Example Pressure Analysis 
     Next,  FIG. 5  illustrates an example pressure-versus-travel plot  350  generated during a PST of a shutdown valve in accordance with the techniques discussed above with reference to  FIGS. 4A-C . Similar to the pressure-versus-travel plot of  FIG. 3B , the points that make up the plot  350  correspond to measurements of the actuator pressure at different percentages of valve travel, in both directions. Accordingly, the plot  250  can contain two points with different actuator pressures for the same valve travel percentage: one point corresponding to the movement in the direction of the fully closed position, and the other point corresponding to the movement in the direction of the fully open position. 
     To detect a stuck valve condition or another abnormal condition, the valve controller  18  can apply the following acceptance criteria: for valve travel between a partially open position  352  (which normally corresponds to PST_STRK_TRAV) and a stop threshold  354 , the valve controller  18  determines whether the actuator pressure is between two fixed values, minimum actuator pressure and maximum actuator pressure. However, to account for forces required to break out of the hard stop, which may be larger than running force, the valve controller  18  determines the stop threshold  354  as a percentage of valve travel (e.g., 5%), and defines more permissive criteria for this region. 
     As illustrated in  FIG. 5 , the valve controller  18  determines whether actuator pressure is larger than breakout pressure  360 , and does not impose an upper limit on actuator pressure in the region between the stop threshold  354  and the hard stop. Thus, the valve controller  18  accounts for expected high actuator pressures in this region and does not signal failure when processing the data points illustrated as the plot  350 , as these data points actually describe normal behavior of a shutdown valve. In  FIG. 5 , regions corresponding to potential problems are shaded, and regions in which a pressure-travel data point can be located without triggering an alert are left un-shaded. 
     Referring back to  FIG. 1 , application of the acceptance criteria illustrated in  FIG. 5  also can be implemented in the workstation  40  as part of a post-processing stage. More generally, the valve controller  18  can record actuator pressure and valve travel measurements in any suitable storage, including cloud storage, and an authorized user can access this data locally or remotely, using a dedicated workstation or a general-purpose computer. 
     Example Methods 
     For further clarity, example methods that can be implemented in the valve controller  18 , the workstation  40 , or another suitable computing device are discussed next with reference to  FIGS. 6 and 7 . The methods of  FIGS. 6 and 7  can be implemented as sets of instructions in any one or several suitable programming languages and stored on a computer-readable medium. 
     Referring first to  FIG. 6 , a method  400  for generating a setpoint signal for a partial-stroke test of a shutdown valve begins at block  402 , where a minimum acceptable travel displacement threshold, PST_STRK_TRAV, is received along with the rate(s) of change of the setpoint signal in one or both directions (PST_RAMP_RATE, PST_RAMP_RATE_RETURN) and the early turnaround selection (YES/NO). Depending on the implementation, these parameters can be received from an operator, from a configuration file, or from an automated task. 
     At block  404 , the first setpoint target, PST_SP_CHANGE is determined based on the value of PST_STRK_TRAV, by multiplying this value by a certain factor, adding a predefined value, or in another suitable manner. The maximum travel displacement threshold, PST_STRK_MAX_TRAV, is determined in a generally similar manner at block  406 . 
     Next, the setpoint signal is ramped from the initial position toward the first target in accordance with the specified rate. If it is determined at block  410  that valve travel has reached PST_SP_CHANGE, the flow proceeds to block  412 , where the early turnaround selection is checked. If early turnaround has not been enabled, the setpoint remains at the first target value for PST_SP_PAUSE number of seconds. Otherwise, if early turnaround has been enabled, the flow proceeds to block  414 , where the setpoint signal is modified in view of the current travel signal. In particular, as discussed above with reference to  FIGS. 4B and 4C , the valve controller  18  can check whether the setpoint signal is ahead of the travel signal, or whether the travel signal got ahead of the setpoint signal, and make an instantaneous adjustment to the setpoint accordingly. 
     Next, at block  418 , the setpoint is ramped in the return direction in accordance with PST_RAMP_RATE_RETURN, toward a second target that can correspond to the initial value augmented by an overdrive value (e.g., PST_SP_OVER). Once the setpoint signal reaches the second target, hard cutoff is engaged at block  422 . Additionally or alternatively, valve position can be monitored so that when the travel signal reaches the hard stop, hard cutoff is applied to the setpoint signal. 
       FIG. 7  is a flow diagram of an example method  500  for detecting a stuck valve condition of a shutdown valve. The method  500  begins by collecting acceptance criteria for application to actuator pressure/valve travel data points. As used herein, the term “acceptance criterion” can refer to a factor applied to a data point to determine whether the data point is within an acceptable range. 
     More particularly, a stop threshold percentage is received from a preconfigured constant or an operator, for example, at block  502 . Next, maximum and minimum actuator pressures are received at blocks  504  and  506 , respectively. A breakout pressure limit is determined for an area near the stop position to define another acceptance criterion, as illustrated in  FIG. 5 . 
     Actuator pressure and valve travel are monitored during a PST at block  510  to collect a set of data points. Alternatively, a set of data points is received from a database or another storage device when the method  500  is executed as part of post-processing. The acceptance criteria collected at blocks  502 - 508  are applied to data points to determine whether any of the points exceed the acceptance criteria in terms of actuator pressure, valve travel, or both. Accordingly, the flow proceeds to block  514  when the data points indicate normal behavior or to block  516  when the data points indicate a potential problem. At block  516 , an alert can be generated or operator(s) may be otherwise notified. If the stuck-valve condition is detected at block  516 , the test is aborted, and the setpoint signal can ramp back or step back to the hard stop. 
     General Remarks 
     Unless specifically stated otherwise, discussions herein using words such as “processing,” “ computing,” “calculating,” “determining,” “identifying,” “presenting,” “displaying,” or the like may refer to actions or processes of a machine (e.g., a computer) that manipulates or transforms data represented as physical (e.g., electronic, magnetic, or optical) quantities within one or more memories (e.g., volatile memory, non-volatile memory, or a combination thereof), registers, or other machine components that receive, store, transmit, or display information. 
     When implemented in software, any of the applications, services, engines, routines, and modules described herein may be stored in any tangible, non-transitory computer readable memory such as on a magnetic disk, a laser disk, solid state memory device, molecular memory storage device, an optical disk, or other storage medium, in a RAM or ROM of a computer or processor, etc. Although the example systems disclosed herein are disclosed as including, among other components, software and/or firmware executed on hardware, it should be noted that such systems are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of these hardware, software, and firmware components could be embodied exclusively in hardware, exclusively in software, or in any combination of hardware and software. Accordingly, persons of ordinary skill in the art will readily appreciate that the examples provided are not the only way to implement such systems. 
     Thus, while the techniques of this disclosure have been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions may be made to the disclosed embodiments without departing from the spirit and scope of the invention.