Methods and apparatus for operating and performing diagnostics in a control loop of a control valve

A control loop of a control valve is operated using outlet pressure from a pneumatic amplifier as the control parameter. The control loop may be operated continuously in pressure control mode, or may be switched from another mode, such as travel control mode, to pressure control mode in response to certain operating conditions such as operation in the cutoff range, operation with the throttling element engaging a travel stop, or as a backup in the event of primary control parameter sensor failure. Operating the control loop in pressure control mode further allows diagnostics to be performed on the control loop components, even when the system is operating in cutoff range or has engaged a travel stop. The diagnostics may be performed using pressure and displacement sensors normally provided with a positioner. A processor may be programmed to receive data from the sensors and generate fault signals according to a logic sub-routine. The logic sub-routine may include calculating mass flow of control fluid through pneumatic amplifier outlet ports and comparing other operating parameters to detect leaks and blockages in the control loop components. Once a fault is detected, the location of the root cause of the fault may be identified by characterizing operating parameters of the control loop at the time of the fault.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to control valves and, more particularly, to methods and apparatus for operating a control loop for controlling the control valve and for performing diagnostics on the control loop components.

BACKGROUND OF THE DISCLOSURE

Control valves are used to regulate process fluid flow through a pipe or conduit. Such valves typically include a throttling element disposed in the process fluid flow path and connected to an actuator. While various types of actuators are known, many control valves use a pneumatic actuator which uses air, natural gas, or other fluid under pressure to adjust the position of the actuator. In a spring and diaphragm actuator, for example, a spring applies a force to one side of the actuator while fluid pressure is controlled on an opposite side of the actuator, thereby adjusting the position of the throttling element. Alternatively, a piston actuator may be used in which the piston divides the actuator housing into upper and lower chambers and the fluid pressures of both chambers are controlled to drive the actuator to a desired position. In any type of pneumatic actuator there may be a nominal bleed-off of the control fluid to atmosphere.

A positioner, through internal servo control, manages the fluid pressure supplied to one or both chambers of a pneumatic actuator. The positioner typically includes a processor and interface circuitry, a current to pressure (I/P) converter, a second stage pneumatic amplifier (i.e., a spool valve or pneumatic relay), and a valve travel feedback sensor. The processor generally monitors input or command signals and feedback signals through the interface circuitry. The servo action programmed within the processor creates an electronic corrective signal that is supplied to the I/P converter. The I/P converter is connected to a supply pressure and delivers a desired control fluid pressure or pneumatic control signal to the second stage pneumatic amplifier. Subsequently, the pneumatic control signal directs the control fluid through the second stage pneumatic amplifier toward a chamber of the actuator creating a movement in the actuator. Movement of the actuator causes a corresponding movement of the throttling element, thereby to control flow of its process fluid. Thus, the positioner responds to the command signal, typically from a process controller, and compares the reference signal to valve travel feedback, thereby to drive the I/P converter (and second stage pneumatics) to move the valve toward a position corresponding to the reference signal.

With the growing use of processor-based control, the spool valves used in positioners have become heavily instrumented. When used with a piston actuator, for example, the spool valve will include an inlet port for receiving supply pressure, a first outlet port fluidly communicating with a first chamber of the actuator, and a second outlet port fluidly communicating with a second actuator chamber. Spool valves are known in which a pressure sensor is positioned at the inlet port, first outlet port, and second outlet port for providing feedback to the processor. In addition, conventional spool valves include a displacement sensor for detecting the position of the spool valve and providing a feedback signal to the processor.

Conventional positioners have components that are susceptible to various control fluid leaks or blockages that may degrade or disable operation of the control valve. The I/P converter, for example, includes an inlet having a sealed connection with the supply pressure. The I/P converter includes a restriction defining a primary orifice and a nozzle for directing control fluid toward a flapper. The I/P converter further includes a sealed outlet for directing control fluid to a spool valve. The I/P converter is often located at an industrial site where the surrounding air may be contaminated with oil, dissolved minerals, grit, and the like. Consequently, when such air is used as the control fluid, the contaminants may partially or completely plug the primary orifice or nozzle. In addition, the seals provided at the inlet and outlet of the I/P converter may fail. Such blockages or leaks may slowly degrade the performance of the control valve, resulting in inefficiencies, or may cause complete failure of the control valve. In either event, it is difficult to determine that the positioner is the cause of the fault, let alone to determine the specific location of the fault within the positioner.

Similarly, leaks may develop in the actuator housing or blockages may form in the connections between the spool valve and the actuator that may degrade control valve performance or cause failure. For example, a leak may form between the upper or lower actuator chamber and atmosphere, or a piston ring may fail causing leakage from one chamber to the other. In any of these circumstances, the processor must adjust its control signal for a given position of the throttling element. Leak detection is particularly important when the control medium is natural gas. Such leaks may develop over time and, in a noisy plant environment, may go unnoticed until the valve no longer operates.

DETAILED DESCRIPTION

A positioner14is schematically illustrated inFIG. 1connected to an actuator12. The actuator12is mechanically coupled to a valve body10, which controls the flow of a process fluid through a conduit, such as a pipe (not shown). The positioner14includes a processor18having a memory20, an I/P converter24, second stage pneumatics (such as spool valve26or pneumatic relay200), a control fluid valve assembly displacement sensor84, and a valve travel sensor68, collectively referred to herein as a control loop. A reference signal, such as a command signal from a process controller, is provided to the positioner14and represents a desired actuator position. The positioner14compares the reference signal to the actual actuator position provided by the travel sensor68and forwards an error signal to the processor18. The processor then generates an electronic I/P drive signal based on the error signal and feedback from the displacement sensor84.

As shown in greater detail inFIG. 2, the actuator12includes a piston60which divides the actuator housing62into the upper and lower chambers56,58. The upper chamber56includes a spring64for applying a force to the piston. A stem66extends from the piston62to the valve body10. A travel sensor68may be provided for detecting the position of the stem66and providing feedback to the processor18.

According to the illustrated embodiment, the I/P converter24provides a signal amplification stage and the spool valve26provides a pneumatic amplification stage. The I/P converter24includes an inlet28in fluid communication with a supply of control fluid under pressure30. A connection between the inlet28and control fluid supply30may be sealed with an O-ring32. A restriction34disposed in the I/P connector24defines a primary orifice36. A nozzle38is provided downstream from the primary orifice36for directing control fluid toward a flexible flapper40. In the illustrated embodiment, a solenoid coil42is provided for positioning the flapper40with respect to the nozzle38. Alternatively, the solenoid coil-based I/P converter24may be removed and the flapper40function may be formed from a piezoelectric material, or any other known flapper construction may be used. An outlet44fluidly communicates with a diaphragm45. The connection between the outlet44and the diaphragm45may be sealed by an O-ring46. A sensor85may be provided for detecting a supply pressure of the control fluid entering the I/P converter24.

The spool valve26includes an inlet port50for receiving control fluid from the control fluid supply30. First and second outlet ports52,54may be provided in fluid communication with upper and lower chambers56,58of the actuator12. A valve member70is disposed inside the spool valve housing for controlling fluid communication between the inlet port50and the first and second outlet ports52,54. In the illustrated embodiment, the valve member70includes a rod72carrying first and second lands74,76. An annular valve chamber77is formed in the spool valve housing and sized to closely fit the first and second lands74,76. The diaphragm45, which receives a pressure signal from the I/P converter24, engages a first end of the valve member70. A spring82engages an opposite end of the valve member70to apply a bias load to the valve member70.

In operation, a control fluid pressure regulated by the I/P converter24is output to the diaphragm45which applies a load to the valve member70in a direction opposite the bias load of the spring82. Movement of the first and second disks,74,76will partially or completely block fluid flow from the inlet port50to either of the first and second outlet ports52,54. Accordingly, the position of the valve member70determines an area of restriction for each outlet port52,54through which control fluid may flow. A displacement sensor84is located to detect a position of the valve member70and provide feedback to the processor18. In addition, first and second outlet pressure sensors86,88are provided for detecting control fluid pressure levels at the first and second outlet ports52,54, respectively.

WhileFIG. 2illustrates a double-acting piston actuator with fail-closed spring action, it will be appreciated that other types of pneumatic actuators may be used. Examples of alternative actuators include a double-acting piston actuator with fail-open spring action, a double-acting piston actuator with no spring, a single-acting spring-and-diaphragm actuator with fail-open or fail-closed spring action, or any known substitute. If the actuator is single-acting, the spool valve26includes a single outlet port in fluid communication with the actuator chamber opposite the spring.

Still further, the positioner14may use alternative means for the second stage pneumatics. Instead of the spool valve26, the positioner may include, for example, a pneumatic relay. A double-acting pneumatic relay200is illustrated inFIG. 6attached to the I/P converter24, valve body12, and source of pressurized supply fluid30. The relay200includes supply pressure plenums202a,202b. Plenum202aincludes a first outlet port204in fluid communication with the actuator lower chamber58, while plenum202bhas a second outlet port206in fluid communication with the actuator upper chamber56. A first poppet valve208has an end210positioned to removably engage the first aperture204, while a second poppet valve212has an end214positioned to removably engage the second aperture206. A beam216is supported for rotation about fulcrum218, and includes a first orifice220positioned to engage a second end222of the first poppet valve208and a second orifice224positioned to engage a second end226of the second poppet valve212. Output from the I/P converter24is provided to chamber228to rotate the beam216in a first direction (i.e., clockwise inFIG. 6) while a reference chamber230is provided with a reference pressure to counterbalance the force of the chamber228. The first poppet valve208controls flow of control fluid to the actuator lower chamber58while the second poppet valve212controls flow to the actuator upper chamber56.

In operation, when the I/P nozzle pressure increases, the beam216will rotate clockwise forcing the first poppet valve208to the right. The second end222of the first poppet valve208closes off the first orifice220to prevent flow to atmosphere, while the first end210of the first poppet valve208opens the first outlet port204to allow control fluid at the supply pressure to flow to the lower chamber58. At the same time, the second poppet valve212opens the second orifice224and closes the second outlet port206to allow control fluid to exhaust from the upper chamber56to atmosphere. The opposite occurs when the I/P nozzle pressure decreases. It will be appreciated that as the first and second poppet valves208,212move into and out of the first and second outlet ports204,206, the area of restriction of the outlet ports204,206are varied. Accordingly, the position of the beam216may be used to infer the position of the poppet valves208,212and, therefore, the area of restriction through the first and second outlet ports204,206.

The positioner with pneumatic relay200may include the same sensors as described above. Accordingly, the first and second outlet pressure sensors86,88are positioned near the first and second outlet ports52,54to detect control fluid pressure to the upper and lower actuator chambers56,58, respectively. The inlet pressure sensor85is positioned at the inlet port50to detect control fluid supply pressure, while the actuator travel sensor68is positioned to detect the position of the stem66. In addition, the displacement sensor84is positioned to detect the position of the beam216.

The positioners described above are generally known in the art. Up to now, however, the displacement sensor84has been used strictly to provide feedback. In accordance with the teachings of the present disclosure, the displacement sensor84may also be used for diagnostic purposes. In addition, the various sensors may be used to discriminate between the various fault conditions possible in the positioner. The sensors may also be used to calculate mass flow of control fluid, which may be used to help identify root causes of the faults. The diagnostic calculations and analysis may be performed by a diagnostics unit provided with the positioner14, such as where the processor18and memory20function as the diagnostics unit, or in a remote host19communicatively coupled to the positioner14.

With respect to the actuator12, the diagnostics unit may be programmed with a diagnostics routine that uses feedback from the sensors to estimate mass flow of control fluid to the actuator chambers. The diagnostics routine may further use the calculated mass flows, with or without additional feedback parameters, to identify leaks or other faults in the actuator. More specifically, the mass flow of control fluid through the first and second outlet ports may be approximated using the following equation:
dm/dt=KYAgc(2ρ(p1−p2))½

To calculate mass flow through the first outlet port52, for example, the appropriate coefficients and variables are inserted into the above equation. Upstream pressure p1is the inlet pressure sensed by pressure sensor85and p2is the pressure detected by sensor86at the first outlet port52. The equation may be used to estimate both supplying and exhausting mass flows. For example, when spool valve displacement is positive (i.e., to the right inFIG. 2), port54will supply control fluid to the lower actuator chamber while port52exhausts control fluid from the upper actuator chamber. For port54, spool valve displacement may be used to calculate the exposed port area and sensors85,88may provide the upstream and downstream pressures. For port52, spool valve displacement may be used to calculate the exposed port area and sensor85may provide upstream pressure. A sensor on the exhaust port is not required since the spool exhausts to atmosphere, which is at a known pressure. In addition, supply pressure to the control valve is often regulated, and therefore the supply pressure sensor85may be eliminated and a fixed value that approximates the supply pressure may be substituted into the air mass flow equation.

When the control fluid is air, the above equation may be reduced to:
dm/dt=0.048KYA(p1(p1−p2))1/2
The mass flow equation may be similarly reduced for other fluids, such as natural gas. In addition to the above-noted equations for estimating mass flows through an orifice, standard flow equations, such as those noted in ISA-575.01-1985: Flow Equations For Sizing Control Valves, may be used. The mass flow estimates obtained by the above equations have been found to closely match measurements made with an external air mass flow sensor, especially when using a low-pass digital filter to attenuate bit noise. Accordingly, the diagnostics unit18may be programmed to receive feedback from the pressure sensors85,86,88and the displacement sensor84and calculate mass flow through the first and second outlet ports52,54using the above equation. The above equations may be modified to correct for leakage flow across the lands, and may also be used to calculate mass flow through alternative second stage pneumatics, such as the pneumatic relay200ofFIG. 6.

The diagnostics routine may use the mass flow calculations to identify leaks or blockages between the spool valve and the actuator12. For example, in a spring-and-diaphragm actuator, control fluid is provided to a single actuator chamber opposite the spring. During normal operation, the processor18controls output of control fluid from the spool valve26to drive the actuator12and connected throttling element to a desired set point. During steady state operation, a small amount of fluid may bleed to atmosphere, and therefore a small amount of control fluid will flow through the spool valve outlet port. If a leak develops in the actuator chamber or in the connection between the spool valve outlet port and the actuator, the pressure level inside the actuator chamber will drop and the spring will cause the actuator to move from its desired position. Feedback regarding process fluid pressure and/or actuator travel is provided to the processor18, and the processor18will alter the drive signal to the I/P converter24to increase control fluid flow to the actuator. Consequently, mass flow to the actuator will increase as illustrated in the graph provided atFIG. 3A. By estimating mass flow of control fluid over time, the diagnostics unit may be programmed to detect increases in control fluid flow to the actuator. The diagnostics unit may further be programmed with a maximum control fluid flow rate above which the diagnostics routine will generate a fault signal. A low pass filter may be used to minimize the chance of normal transients generating a false signal.

Conversely, blockage in the air line between the spool valve and the actuator12may be identified when control fluid flow is constant as spool valve displacement increases.FIG. 3Billustrates a blockage situation, where the solid line represents mass flow and the dashed line represents spool displacement. Similarly, a partial blockage may be identified if spool displacement is large but mass flow is relatively small.

Detecting leaks in a piston actuator is slightly more complicated. The leak may occur in the actuator chamber with the spring, the actuator chamber without the spring, or between the actuator chambers, such as when there is a leak in a piston ring or when a bypass valve on the actuator has been left open. As with the spring-and-diaphragm actuator, however, deviation in air mass flow can be used to locate and quantify leaks or obstructions.

To help identify faults, deviations from normal operating parameters may be identified. One such parameter is the pressure inside the actuator chambers, which is typically maintained at roughly 60–80% of the supply pressure. An average or “crossover” pressure may be determined by averaging the pressures in the actuator chambers.

If there is a leak to atmosphere in the chamber opposing the spring, the processor18will move the spool valve26to provide make-up air to that chamber. This will also depressurize the chamber with the spring, so that the piston actuator behaves effectively like a spring and diaphragm actuator. The mass flow profile through the first and second outlet ports52,54for such a leak are shown inFIG. 4A. Initially, there is a nominal mass flow through both outlet ports52,54due to normal leakage in the system. When a leak develops at point A, mass flow to the chamber with the leak will increase to equal the amount of air exhausted to atmosphere, as shown by the solid line inFIG. 4A. For the chamber with the spring, mass flow will be temporarily out of the chamber as the actuator moves to a new position, but will eventually return to near zero since the chamber is depressurized, as shown in the broken line inFIG. 4A. Furthermore, the crossover pressure in the actuator will be approximately one-half of the pressure in the chamber opposite the spring.

If a leak develops in the spring-side chamber of the actuator, the positioner14does not provide make-up air since that would require the positioner to exhaust air (and reduce the force) from the chamber opposing the spring. Accordingly, the processor18allows the chamber with the spring to become depressurized and will control the valve by adjusting the pressure in the opposite chamber. At steady state, air mass flow to the spring-side chamber will be near zero, air mass flow from the chamber opposing the spring will be near zero, and the crossover pressure will be one-half of the pressure in the chamber without the spring. Accordingly, by detecting the decreased crossover pressure in the mass flow profiles through each port, the presence and location of a leak may be determined.

The mass flow calculations may further be used by the diagnostics unit to detect leaks which result in control fluid flowing from one actuator chamber to the other, such as leaks in the piston ring. Such a leak may be difficult to detect using traditional measurement techniques since each chamber may remain pressurized. If the leak causes control fluid flow from the lower chamber58to the upper chamber56, for example, the positioner14will move the spool to provide make-up control fluid to the lower chamber58. At the same time, however, control fluid will flow from the lower chamber58to the upper chamber56and back to the spool valve26.

A graph illustrating fluid flow profiles through each outlet port52,54for a piston ring leak is provided atFIG. 4C, wherein fluid flow through the first outlet port52is shown in a dashed line while fluid flow through the second outlet port54is shown in a solid line. Initially, each port has a nominal flow rate that discharges to atmosphere. When the leak in the piston ring develops, mass flow through the second outlet port54increases while mass flow through the first outlet port52decreases by a proportional amount. Unlike conventional mass flow sensors which do not indicate the direction of fluid flow, the mass flow approximation equation indicates direction of flow, wherein a positive number represents fluid flow into the actuator while a negative number represents fluid flow out of the actuator. Accordingly, by monitoring control fluid flow through the first and second outlet ports52,54, the processor18may detect a sustained situation where fluid flow through one port is positive while fluid flow through the other port is negative, and generate a fault signal.

In addition to detecting control fluid leaks and blockages to the actuator, the pressure and displacement sensors of the spool valve may also be used to detect faults in the I/P converter24located upstream of the spool valve26. Various types of faults may occur in the I/P converter24that will disrupt or stop control fluid flow to the spool valve26, thereby degrading or disabling control valve operation. Because specific components of the I/P converter, such as the flapper40, are not directly applicable to servo control, these components are not typically instrumented. It has been found, however, that the sensors provided with the spool valve26may be used to infer the internal states of the I/P converter components.

Before addressing the specific faults that may occur in the I/P converter24, it should be noted that the control fluid supply30that provides pressurized control fluid to the I/P converter may fail, and therefore this fault should be addressed before considering other failures in the I/P converter24itself. Accordingly, the signal provided by the inlet pressure sensor85may be used to detect whether the control fluid supply30has lost pressure.

One fault that may occur within the I/P converter24is the complete plugging of the primary orifice36. When the primary orifice36is plugged, pressure to the diaphragm45will decrease so that the spring82moves the spool valve70to a zero pressure (or negative) state, causing the actuator to move accordingly. The processor18will increase the drive signal to the solenoid coil42in an attempt to close or cap off the nozzle38, which normally would increase control fluid pressure exiting the outlet44. Instead, the plugged primary orifice36prevents any flow of control fluid.

A fault may also arise when mineral deposits or other contaminants build up on the flapper40so that the nozzle38is completely plugged. In this case, control fluid pressure out of the outlet44increases to the supply pressure and causes the spool valve to move away from a null position to a positive position, thereby moving the actuator. In response, the processor18will decrease the drive signal to the I/P converter24in an attempt to open or uncap the nozzle38.

Alternatively, the primary orifice may become partially plugged. As with a completely plugged primary orifice, a partial plugging will move the drive signal higher as the processor18attempts to compensate for the reduced air to the nozzle38. A partially plugged primary orifice will slow down movement of the spool valve in response to changes in the I/P signal. Increased time constant may, however, result from low ambient temperature, which stiffens the diaphragm. In any event, when the I/P drive signal is high and all other states are operating properly, then it may be inferred that the primary orifice is partially plugged.

Similarly, the nozzle38may become partially plugged. Partial plugging of the nozzle38also affects the time constant of the I/P converter which, as noted above, may also be caused by the effect of changes in ambient temperature on the diaphragm. Accordingly, a low I/P drive signal with all other states nominal may indicate a partially plugged nozzle.

A further fault may arise from failure of the outlet O-ring46. To compensate for a leak through the outlet-ring46, the processor18will increase the drive signal, but the time constant of the I/P converter will not be altered significantly. Accordingly, failure of the outlet O-ring46will affect operation of the control loop in a manner similar to a plugged primary orifice36.

Further faults in addition to those specifically noted above may also occur in the I/P converter. For example, the solenoid coil42may fail or the flapper40may break. While it may not be possible to discern the specific failure, each fault may be detected by monitoring for significant deviations in the drive signal to the I/P converter. This may be accomplished by putting a linear or nonlinear digital filter on the drive signal to remove high frequency content and looking for deviations from normal operating conditions.

To help identify and characterize various faults in the I/P converter24, the diagnostics unit, such as the processor18and memory20of the positioner14or the remote host19having a processor and memory, may be programmed to perform a diagnostics routine based on the parameters measured by various sensors of the positioner14. The diagnostics routine may include one or more logic sub-routines in which the measured parameters are characterized to develop a fault template, which may be used to identify one or more root causes for a fault.

A fault must first be detected before it may be characterized. The diagnostics routine may be programmed to detect sustained deviations in the I/P drive signal. The I/P drive signal may be set at approximately 70% to center the control fluid valve assembly of the pneumatic amplifier at its null position. A normal operating range for the drive signal may be 60–80%. Accordingly, the diagnostics routine may generate a fault signal when the I/P drive signal moves outside of the normal operating range (i.e., less than 60% or more than 80%). An order statistics filter may be used to remove normal transients, so that a fault signal is generated only when the I/P drive signal is outside of the normal range for a sustained period of time. Alternatively, the diagnostics unit may be programmed to monitor for large shifts in the nominal position of the control fluid valve assembly, or to monitor an error signal (i.e., deviation of reference from valve stem travel, outlet port pressure, or other control parameter), to trigger a fault analysis. In either event, once a fault has been detected, control fluid pressure at the supply30should first be checked so that it may be ruled out as a cause of the fault.

Once a fault has been detected, it may be characterized to determine its general or specific location within the control loop. After the deviation has been detected in the I/P drive signal, the fault can be located by tracing the deviation back through the control loop. For a blocked primary orifice36in a positioner system using a stem travel reference signal, for example, the control loop will be affected as follows: flow through the primary orifice36will stop, causing the spool valve to move to its zero pressure (negative) state, which in turn decreases pressure in the actuator chamber, which causes the throttling element to move, which generates an error signal back to the processor. The processor will increase the I/P drive signal to compensate for the fault.

To identify the specific location of the fault, one must proceed backwards through this chain of events. For the completely plugged primary orifice example, the analysis begins with detection of an I/P drive signal above the upper limit of the normal operating range (i.e., a positive I/P drive signal deviation). Next, the error signal generated by movement of the throttling element is characterized as largely positive, which means the actual actuator travel is less than desired. A differential pressure between outlet port pressures, where the pressure at the first inlet port52is subtracted from the pressure at the second outlet port54, may then be characterized as being negative. Next, the displacement sensor84provides feedback regarding the spool valve position, which would be characterized as largely negative with respect to its null position due to the control fluid pressure reduction caused by the blockage. By characterizing the measured parameters in this fashion, certain root causes for the fault may be eliminated. Several root causes may have the foregoing characteristics, of which a blocked primary orifice is one.

In a similar fashion, all faults may be mapped out using a decision tree, as illustrated inFIG. 5. InFIG. 5, measured variables are denoted by circles, the characterized values of those parameters are labeled on the lines emanating from the circles, and component failures are denoted by squares. Triangles denote invalid regions such as, for example, the combination of a large drive signal and a large negative error signal, which is not possible. The diagnostics routine illustrated inFIG. 5is based on existing sensors commonly provided with positioners, and therefore certain component failures that are indistinguishable have been grouped together inFIG. 5. Additional sensors may be used to further distinguish the grouped component failures. Component faults cascade down through the tree until the I/P drive signal deviates. The root cause of the deviation may then be identified by moving backwards through the tree.FIG. 5and its related description assume that the reference signal is generated with respect to valve stem travel. It will be appreciated that similar diagnostics may be performed in systems having a reference signal tied to other control parameters, such as the fluid pressure delivered to the actuator by the pneumatic amplifier.

Returning toFIG. 5, at measurement100the diagnostics routine may detect an I/P drive signal that deviates from the normal operating range. The drive signal may be characterized as high if it is above the range and low if it is below the range. If the I/P drive signal is high, the stored diagnostics routine will proceed up in the tree to characterize a reference signal used in the control loop. The reference signal may be the command signal sent to the positioner from a process controller. In this embodiment, the I/P drive signal is a function of the difference between the reference and travel feedback.

There are three scenarios where the I/P drive signal may be above or below its normal operating point, two of which are not the result of an equipment fault. The first is when the positioner is in “cutoff.” Cutoff occurs when the reference signal exceeds a user-defined threshold. When in high-cutoff, the servo controller is bypassed altogether and a 100% drive signal is sent to the I/P. When in low-cutoff, the servo controller is bypassed and a 0% drive signal is sent to the I/P. An example of how cutoffs may be implemented is depicted in the schematic block diagram ofFIG. 8. Both high- and low-cutoff are valid operating regions, and do not indicate an equipment fault. High- and low-cutoff are indicated inFIG. 5at boxes103,131, respectively.

The second scenario is when the valve body engages a travel stop. When the valve body hits a stop, travel feedback is no longer active and the process controller essentially operates open-loop. Again, this is normal control valve behavior and does not indicate an equipment fault. High and low travel stops are indicated inFIG. 5at boxes104,132, respectively.

The third scenario is where an equipment fault has caused a large error signal. In order to compensate for a large error signal, the I/P drive signal is adjusted accordingly. Once cutoffs and travel stops are ruled out, the analysis may proceed along the decision tree set forth inFIG. 5. For a high I/P drive signal, the analysis proceeds up the tree, while for a low I/P drive signal, the analysis proceeds down the tree.

A high I/P drive signal is first analyzed by characterizing the error signal at105. The error signal may be classified as largely positive, null, or largely negative. When the I/P drive signal is high, it is not possible to have a large negative error signal, and therefore the upper right branch ofFIG. 5indicates that all outcomes are not valid. Accordingly, the only possible outcomes from the error signal characterization105are largely positive (i.e., the reference signal is greater than the actual travel feedback signal) or null. In either event, the diagnostics routine will next proceed to characterize a pressure differential between the first and second outlet ports52,54by subtracting the pressure at the first outlet port52from the pressure at the second outlet port54, as indicated at106,107. The pressure differential may be characterized as being negative near the supply pressure, nominal, or positive near the supply pressure. A negative pressure differential indicates that pressure at the first outlet port52is greater than that at the second outlet port54. The converse is true for a positive pressure differential. A nominal pressure differential indicates that the actuator chambers are substantially balanced. For each pressure differential characterization, the diagnostics routine will proceed to characterize the position of the spool valve, as indicated at108–113. The spool valve position may be characterized as being largely positive, null, or largely negative. A large positive position indicates that the diaphragm45has pushed the spool valve too far, while a large negative means the opposite. The spool valve is at the null position when it remains within a normal operating range.

Once the spool valve position has been characterized, one or more potential root causes may be identified for the I/P drive signal deviation. If, for example, the spool valve is jammed114, the outlet O-ring46has failed115, the diaphragm45has failed116, or the primary orifice36is completely plugged117, the diagnostics routine will have characterized the fault as having the largely negative spool position, a negative pressure differential, and a largely positive error signal for a high I/P drive signal. If the fault is characterized as having a largely positive spool valve position, a nominal pressure differential, and a largely positive error signal for a high I/P drive signal, the root cause may be an external leak118, a worn spool valve119, or a low supply pressure120. For a fault having a largely negative spool valve position, nominal pressure differential, and a largely positive error signal for a high I/P drive signal, the root cause may be a low pressure supply121.

If, for a high I/P drive signal, the error signal is largely positive, the pressure differential is positive, and the spool valve position is largely positive, the root cause may be the valve body being stuck in a low position122, a blocked air line between the spool valve and the actuator123, or an active interlock124.

If a fault is characterized as having a largely positive spool valve position, a nominal pressure differential, and a null error signal for a high I/P drive signal, the root cause may be an external leak125. If the spool position is characterized as null, the pressure differential is nominal, and the error signal is null for a high I/P drive signal, the root cause for the fault may be a primary orifice36that is partially plugged126, the presence of grit in the I/P flapper or armature127, or an I/P calibration shift128.

Turning to the bottom half ofFIG. 5, the diagnostics routine may conduct a similar process for a low I/P drive signal. After ruling out low-cutoff131and low travel stop132, the analysis proceeds to characterize the error signal at133. Error signal characterization is similar to that at105described above, wherein the error signal may be largely negative, null, or largely positive. It is not possible to have both a low I/P drive signal and a largely positive error signal, and therefore the outcomes show at the bottom left portion ofFIG. 5are all indicated as being not valid. After error signal characterization, the diagnostics routine will characterize a pressure differential at134and135. Finally, the diagnostics routine will characterize the position of the spool valve at136–141.

As with a high drive signal deviation, analysis of a low drive signal deviation proceeds with identifying one or more possible root causes. If the error signal is largely negative, the pressure differential positive, and the spool valve position largely positive, the root cause for the fault may be a nozzle38that is blocked142, a pressed I/P flapper or armature143, a latched I/P144, or a jammed spool valve145. If the error signal is largely negative the pressure differential is negative, and the spool valve position is negative, the root cause for the fault may be the valve body stuck in a high position146or a blocked air line147. Finally, if the error signal is null, the pressure differential is nominal, and the spool valve position is null, the root cause for the fault may be an I/P calibration shift148, or a nozzle38that is partially plugged149.

The diagnostics routine may further classify component faults according to severity and provide predictive diagnostics. Certain root causes, such as a completely plugged primary orifice36or nozzle38, will bias the spool valve26in a manner that cannot be corrected by the processor18. Such causes may be characterized “red light” diagnostics and reported appropriately. Other root causes may result in a large deviation in the I/P signal, but all other variables in the feedback loop are operating normally. For example, the primary orifice36may become partially blocked so that the I/P signal will have to be driven harder in order to compensate for the degradation in flow to the nozzle. However, the error signal, the actuator pressure, and the spool valve position will all operate normally. By comparing the I/P signal deviation with other variables in the feedback loop, we can identify degradation and flag it before it becomes a catastrophic failure. These causes may be classified as “yellow light” diagnostics.

For situations in which a valve is on a travel stop, such as a valve seat, different diagnostic tests for the valve and the positioner may be required. For example, Emergency shutdown (ESD) valves are typically on-off devices that are used, for example, to shut off the flow of oil or natural gas in the event of a rupture in the pipe downstream.

As such, the ESD valve remains at the upper travel stop or in an opened condition most of the time and is only intermittently closed. A standard diagnostic test for a typical ESD device is a partial stroke test in which a reference signal to the servo moves along a predefined trajectory and sensor data are compared to a set of prediction data. However, there may be situations in which it is desirable to test a device such as a valve positioner without actually moving the valve off the travel stop. As previously described, cutoffs have been used to force full pressure on the actuator12and provide maximum seat load and, accordingly, maximum shutoff capability.

When the reference signal approaches 0% or 100%, the cutoffs become active, which means that the positioner may be operated in a saturated state. Thus, the control loop may be bypassed altogether and either a 100% or 0% signal may be applied directly to the I/P converter24. As a result, the output pressure to the actuator12is forced to supply or to atmosphere, depending upon the desired output state. As such, when cutoffs are active it may take additional time to get the valve10off of the valve seat, due to additional time required to bring the I/P converter24and the spool valve26from saturated states to null states.

Since the drive signal to the I/P converter24is no longer active (i.e., responding to changes in the difference between the command signal and the feedback signal), it becomes more difficult to determine if the electropneumatic and pneumatic stages within the positioner are operating properly.

Thus, for certain control situations and devices, it may be desirable to run the positioner14in a pressure control mode when the valve10is on a travel stop, such as a valve seat. In a pressure control mode, the positioner14may be operated in a closed loop control mode as depicted in the block diagram ofFIG. 7. In this mode, a pressure set point (rather than a position set point) may be selected near supply or near atmospheric pressure (i.e., near 100% or near 0%.) By doing this, near maximum seat load may be achieved, and the I/P converter24and the spool valve26or the relay200may be kept at their null states, allowing the valve10to come off its seat faster. With reference toFIG. 7, when operating in a pressure control mode, the I/P drive signal may be a function of the difference between the reference signal and a measured pressure feedback signal. For example, actuator pressure may be used as the pressure feedback signal for single-acting actuators, and the pressure differential across the piston may be used as the pressure feedback signal (error signal) for double-acting actuators. It will be appreciated a valve may be continuously operated in pressure control mode, or may be normally operated in another mode, such as travel control mode (i.e., where actuator travel is used as the control parameter), and switched to pressure control mode under certain operating conditions.

Another advantage of having the I/P converter24and the spool valve26or the relay200active and near null states when the valve is on the seat is that diagnostics for travel control can be used to assess the integrity of the I/P converter24and the spool valve26or the relay200, calculate air mass flow through the spool valve26or the relay200, and monitor the quality of the performance of the positioner14. Thus, the availability of the positioner14may be assessed when the valve10is on a travel stop. The air mass flow calculations may be the same as those described above with respect to single and dual-acting actuators, and diagnostics based on air mass flow calculations similar to those described in connection with FIGS.3A–B and4A–C may also be performed on a control loop operating in pressure control mode.

Such diagnostic capabilities are particularly important for applications such as compressor antisurge and turbine bypass, where the valve spends most of its time on the seat, and is only occasionally called into service. However, when such a valve is needed, it is important that the valve respond as designed. Travel stop diagnostics that may be performed when the valve is on the seat and the positioner14is operating in a pressure control mode allow operators to assess the functionality of the positioner14even though the valve10is not operational.

When using pressure control, faults may be mapped out using a decision tree, shown inFIG. 9, similar to that ofFIG. 5. InFIG. 9, as was the case in connection withFIG. 5, measured variables are denoted by circles, the characterized values of those parameters are labeled on the lines emanating from the circles, and component failures are denoted by squares. Triangles denote invalid regions such as, for example, the combination of a large drive signal and a large negative error signal, which is not possible. The diagnostics routine illustrated inFIG. 9is based on existing sensors commonly provided with positioners, and therefore certain component failures that are indistinguishable have been grouped together inFIG. 9. Additional sensors may be used to further distinguish the grouped component failures. Component faults cascade down through the tree until the I/P drive signal deviates. The root cause of the deviation may then be identified by moving backwards through the tree.

More specifically, at measurement300the diagnostics routine may be used in conjunction with a relay, such as the relay200shown inFIG. 6, and may detect an I/P drive signal that deviates from the normal 70% of the supply pressure. The drive signal may be characterized as high if it is above the 70% and low if it is below the 70%. If the I/P drive signal is high, the stored diagnostics routine will proceed up in the tree to characterize a reference signal used in the control loop. The reference signal may be the pressure command signal sent to the positioner from a process controller. The I/P drive signal is a function of the difference between the reference pressure signal and a pressure feedback signal.

A high I/P drive signal is first analyzed by characterizing the pressure error signal at305. The pressure error signal may be classified as high, nominal, or low. When the I/P drive signal is high, it is not possible to have a low pressure error signal, and therefore the upper right branch308ofFIG. 9indicates that all outcomes are not valid. Accordingly, the only possible outcomes from the error signal characterization305are high or nominal. In either event, the diagnostics routine will next proceed to characterize a relay position (e.g., the position of the pneumatic relay200ofFIG. 6), as indicated at306and307inFIG. 9. The relay position may be characterized as being largely positive, null, or largely negative.

Once the relay position has been characterized, one or more potential root causes may be identified for the I/P drive signal deviation. If, for example, the primary orifice36is blocked310, the pressure supply121is near atmospheric pressure312, the relay is jammed314, the I/P O-ring46has failed315, or a relay instrumentation diaphragm associated with the chamber228has failed316, the diagnostics routine will have characterized the fault as having a largely negative spool position, high pressure error signal for a high I/P drive signal. If the fault is characterized as having a largely positive relay position, a high pressure error signal for a high I/P drive signal, the root cause may be an external leak318, a diaphragm associated with the first orifice220has failed319, or a low supply pressure320.

If, for a high I/P drive signal, the pressure error signal is nominal, the pressure differential is positive, and the relay position is largely positive, the root cause may be an external leak322, or a diaphragm associated with the first orifice220has failed324.

If the relay position is characterized as null and the pressure error signal is nominal for a high I/P drive signal, the root cause for the fault may be a primary orifice36that is partially plugged326, the presence of grit in the I/P flapper or armature327, or an I/P calibration shift328.

Turning to the bottom half ofFIG. 9, the diagnostics routine may conduct a similar process for a low I/P drive signal. The analysis proceeds by characterizing the pressure error signal at333. Error signal characterization is similar to that at305described above, wherein the pressure error signal may be low, nominal, or high. It is not possible to have both a low I/P drive signal and a high pressure error signal, and therefore the outcomes show at340in the lower right portion ofFIG. 9are all indicated as being not valid. After pressure error signal characterization, the diagnostics routine will characterize the position of the relay at336and338.

As with a high drive signal deviation, analysis of a low drive signal deviation proceeds with identifying one or more possible root causes. If the pressure error signal is low and the relay position is largely positive, the root cause for the fault may be a nozzle38that is blocked342, a pressed I/P flapper or armature343, a latched I/P344, a jammed relay345, or a failure of the a diaphragm associated with the reference chamber230has failed346. Finally, if the pressure error signal is nominal and the relay position is null, the root cause for the fault may be an I/P calibration shift348, or a nozzle38that is partially plugged349.

If desired, further actuator performance diagnostics may be implemented when in a pressure control mode by using actuator position sensor feedback, as shown in the decision tree ofFIG. 10. If, for example in a fail-close device, pressure control is operating nominally, but the valve10has not moved to a commanded position, a positive or negative actuator travel error may be characterized at405. If the actuator travel error is positive, possible actuator faults are a valve stuck in a high position406, a blocked air line408, or an active interlock410. If the actuator travel error is negative, possible actuator faults are a valve stuck in a low position412, a blocked air line414, or an active interlock416.

In addition to using pressure control when the valve10is on the seat, one can use pressure control as a fallback should the travel sensor68fail. Throttling control of the valve10may not always be as precise with pressure control as it is with travel control, but it is usually good enough to keep the valve10operating and the process running. Even so, one can apply the diagnostics developed for travel control when the positioner is operating in pressure control.

In summary, two examples have been provided for the use of pressure control diagnostics in a positioner and both examples provide an extension of the range of operation over which on-line diagnostics can be used: pressure control while the valve is on the seat (or at an upper travel stop); and when pressure control is used as a backup for throttling control in the event of a travel sensor failure.

While the diagnostics unit has been described as preferably performing processing and diagnostics using software, it may use hardware, firmware, etc. using any type of processor, such as an ASIC, etc. In any event, the recitation of a routine stored in a memory and executed on a processor includes hardware and firmware devices as well as software devices. For example, the elements described herein may be implemented in a standard multi-purpose CPU or on specifically designed hardware or firmware such as an application-specific integrated circuit (ASIC) or other hard-wired devices as desired and still be a routine executed in a processor. When implemented in software, the software routine may be stored in any computer readable memory such as on a magnetic disk, a laser disk, an optical disk, or other storage medium, in a RAM or ROM of a computer or processor, in any database, etc. Likewise, this software may be delivered to a user or a process plant via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or over a communication channel such as a telephone line, the internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). Similarly, as used herein, a processor may include a programmable device, such as a microprocessor, or any hardwired or permanent memory device, such as an ASIC.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.