What position should a control valve be in?

09 Mar.,2024

 

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The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #68, and was written by Greg.

 

When I am in an instrument and valve repair shop, I see many more control valves than instruments, particularly with the advances in sensor technology, transmitter intelligence, and asset management systems. Valves are mechanical devices and as such require more maintenance. Packings, seals, seats, and o-rings wear out. To ease maintenance, a control valve must be located so it can be readily and safely removed from the pipeline. However, there is much more to consider in locating a control valve.

With liquid streams, a change in control valve position causes an immediate change in liquid pressure and, in a full and pressurized pipeline, a pressure wave traveling at the speed of sound. In less than a second, the pressure imbalance from the wave provides a driving force that overcomes the liquid’s inertia and accelerates the liquid to a new velocity. Consequently, within a plant the location of a control valve does not appreciably affect the response of a process variable (PV) within the pipeline. However, if you are measuring PV response in a pipeline and the control valve is in a different pipeline, throttling a flow that is being added to the pipeline that is being measured, the distance of the valve from the measurement may cause a transportation delay. This piping and valve arrangement commonly occurs in the dilution and blending of streams.

 

There are some really bad control valve locations that show no understanding of the negative impact of deadtime by process and mechanical design engineers (Tip #70). One of the worst is where there are several vessels between the control valve and the measurement. The residence time of the smaller vessels in series with the larger vessel becomes deadtime. Also bad is gravity flow. Now a change in control valve position starts a wave traveling slowly down the partially filled pipeline. For small flows in vertical runs, the flow is a falling film causing a large and unpredictable deadtime. The worst case of deadtime resulting from valve location is encountered in pH control. A control valve often throttles the reagent flow to a dip tube. The dip tube has a minimum size for structural integrity and normal mixing rules put the dip tube down near the impeller. Unfortunately, this creates a dip tube volume of 2 gallons. If the reagent flow is 1 gallon per hour, when the control valve opens, it takes 2 hours for the reagent to flush the process fluid out of the dip tube. When the control valve closes, the reagent will continue to migrate into the process for several hours. The solution is to have the control valve add the reagent to a high flow recirculation line, thereby reducing the injection delay to seconds.

To prevent flashing, control valves should be located so the fluid pressure in the vena contracta (that is, the narrowest opening in the flow path) does not drop below the vapor pressure of the fluid. If flashing cannot be avoided, the valve type and trim design should be selected to prevent cavitation in the valve or downstream equipment. Stan Weiner, my coauthor of the Control Talk column, recommended the flashing control valve be installed directly on an inlet nozzle near the top of a vessel so the collapsing of bubbles would occur in the vessel vapor space when cavitation could not be prevented.

Concept: Control valves do not require much in the way of straight runs and do not introduce appreciable delays within plants when they are in the same pipeline as the measurement. (Long-distance oil and gas pipelines are another story.) Valves adding flows to process equipment can cause large injection delays or bypass mixing in the equipment. Valve location on streams to equipment should not introduce excessive deadtime or noise. Valve location, type, and trim should minimize flashing and cavitation and provide safe and easy access for removal and repair. On-off valve locations should minimize the volume to the destination when flow is stopped to prevent totalization errors in charges.

Details: Control valves should be at floor level or accessible from platforms. Block, flush, and drain valves should be installed to enable them to be safely removed. Control valves should be located on the same equipment or pipeline as the measurement and downstream of flow measurements. Reagent control valves should be moved from dip tube to recirculation line injection to eliminate injection delays for pH control. On-off valves close to the point of injection should be added to provide isolation and to shut off the flow. For pH and reactor control, the volume between the on-off valve and the nozzle should be minimized by flanging the on-off valve to the nozzle or nozzle block valve for low reagent and reactant flows and high process sensitivity. Gravity flow piping is not recommended because of variable head and velocities, but if it is used, the control valve should be as close to the nozzle of the destination as possible.

 

Watch-outs: The location of the nozzle and dip tube entry points into a vessel must not result in the flow being injected close to an exit nozzle; thereby short circuiting inlet flow to outlet flow and bypassing the mixing in the vessel. Throttle valves should not be used as isolation valves, and isolation valves should not be used as throttle valves (see Tip #83). If an on-off valve for batch control is not close to the flowmeter, the pipeline inventory between the on-off valve and flowmeter can cause the charge to be significantly different than the batch setpoint. To minimize excess charge, the on-off valve should stroke as fast as necessary when the “close” command is given.

Exceptions: For Coriolis meters in liquid service with no possibility of flashing, the control valve can be located upstream of the flowmeter because a Coriolis meter is not sensitive to velocity profile.

Insight: Control valves can cause damage to piping from cavitation and poor control from injection delay and short circuiting.

Rule of thumb: Locate control valves to be maintainable, provide fast injection into mixing zones, and prevent flashing and cavitation.

 

 

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

 

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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.

 

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This Control Talk column appeared in the August 2019 print edition of Control. To read more Control Talk columns click here or read the Control Talk blog here. 

Greg: One of our primary objectives in this column is to show straightforward solutions developed by practitioners that address the hearts of the challenges to get the best out of the control system and consequently, your process. Here, we show how to first greatly increase the rangeability and sensitivity of control valves, then optimize the process by simple addition of one or more PID controllers, without any additional hardware or software. Often, it’s a configuration change that can be done in a matter of days once you understand what’s truly needed, which is our goal here. We start with George Buckbee, who has considerable experience in process control and is currently an outstanding source of getting the most out of your PID, as seen in his many presentations and papers. George is an ISA Fellow and author of the focused book, “Mastering Split Range Control.” He heads up the Performance Solutions group at Metso.

Valve position control and override control can greatly improve your valve and process performance. You can achieve these results without creating oscillations, upsetting other loops or interfering with the primary process PID controller’s ability to do its job. The challenges addressed are valve stiction and backlash, installed flow characteristic, interactions, disturbances and improper tuning of optimized and related PID controllers.

George, how do you implement valve position control?

George: Addition of valve position control (VPC) is an excellent method to obtain accurate and precise control, even over a wide range of operation. With regular PID and a single valve, it can be difficult to control precisely. Control valves can exhibit stiction, backlash and other nonlinearities that can vary over the full range of operation.

One common scenario involves two controllers, one manipulating the large valve, and one manipulating the small valve. The small valve is configured as a normal PID control loop, to control the measured process variable to its setpoint. A second controller, with a large valve, becomes the valve position controller, or VPC. In this second controller, the signal to the small valve is supplied as the process variable (controlled variable). The setpoint is generally set to its most desired throttle position. In this way, the second controller is adjusting the process flow to keep the small, precise valve near the middle of its best range of operation. For sliding stem valves, the setpoint is about 50%, but for rotary valves, it may be 30% due to flattening of the installed flow characteristic above 60 degrees of rotation.

This arrangement has several advantages over standard PID control. First, the effective range of operation under precise control can be increased dramatically by moving the small valve for small disturbances and only moving the large valve to keep the small valve in a good throttle range. This solution is much better than split-range control, where you end up trying to move the big valve most of the time. Since backlash and stiction are a percent of valve capacity, moving the small valve is much more precise. Also, the small valve can more likely be a sliding stem valve with a diaphragm positioner that has a much smaller percent backlash and stiction than a rotary valve with a piston actuator, which is often used for the large valve. Plus, you avoid the discontinuity and nonlinearity at the split-range point. Most processes tend to oscillate at the split-range point.

Greg: What PID structure and tuning rules do you use for valve position control?

George: The primary controller, with the small valve, does most of the work, moving quickly to keep the process stable and in-control. The VPC moves much more slowly. Its goal is to keep the primary controller in a good control range. The primary controller should be tuned to be fast responding. Aggressive tuning, with PI or PID methods, should be used. The VPC should be tuned more loosely, with P-only, PI or I-only control, to avoid interaction. The VPC should be at least five times slower than the primary controller. This can be done many ways, using Lambda tuning, by matching frequency responses, or using methods such as Relative Response Time.

There may be other considerations, such as a desire to limit movement of a large valve by the VPC. In one case, we were trying to extend valve life by limiting the movement of a 48-inch valve that was remotely located on an island. You can reduce travel using P-only control, or, if the process allows, implementing a gap action VPC.

Greg: There are several VPC challenges to be addressed. Valve backlash and stiction is particularly an issue for the big valve being manipulated by the VPC. Big valve movement can be suppressed by adaptive tuning where the gain is nearly zero for a tolerable offset from VPC setpoint (e.g., gap action). Backlash can be compensated by an increment and decrement equal to the deadband for a change in signal direction that is positive and negative, respectively. You are stuck with stiction. However, in addressing another challenge—large disturbances (potentially causing the primary controller manipulating the small valve to run out of valve)—by adding a feedforward signal to the VPC output to preemptively position the large valve, you can increase the gain for small changes in feedforward signal to get it through the resolution limit. The positioners for both valves should not use integral action and should be tuned with aggressive proportional action to minimize deadband and the resolution limit seen by the VPC. External-reset feedback (e.g., dynamic reset limit) can be turned on in the VPC to stop limit cycles if there is a good readback of actual final closure element position (e.g., actual plug, ball or disk position). Note that shaft position feedback is often not a good indicator of actual ball or disk position for rotary valves designed for tight shutoff due to high seal friction and backlash in ball- or disk-to-stem and in stem-to-actuator-shaft. Balls or disks with integrally cast stems and splined stem-to-shaft connections can minimize the backlash from shaft windup. See the Control Talk feature article “How to specify valves and positioners that don’t compromise control” to avoid this increasing problem due to the emphasis on capacity, leakage and cost and not valve response.

Signal characterization applied to each PID output should be used to linearize the installed flow characteristic, which tends to be quite nonlinear for rotary valves. This helps feedback and feedforward control. Tuning of the VPC is still a challenge even after linearization of valves because any change initiated by the VPC has to work through the primary PID closed-loop response before it is seen as the corresponding change in the small valve position that is the controlled variable of the VPC. Thus, the VPC tuning depends upon the primary PID tuning besides the open-loop response of the process. This means the primary PID should be tuned first to provide a fast response. The VPC is then tuned to provide a closed-loop response at least five times slower than the primary PID. This can correspond to the VPC lambda being five times the primary PID lambda. For near-integrating and true integrating processes, the lambda is an arrest time (time to stop a process excursion from a load disturbance on process input). In a way, these rules about tuning the lower loop first for a fast response and then tuning the upper loop for a five times slower smooth response is similar to that for cascade control. However, in the VPC case, tight control in the upper VPC loop is not important. If a positioner or primary PID is retuned, the VPC must be retuned.

In general, the VPC goal is a smooth, gradual optimization with a fast getaway for abnormal conditions. Feedforward and nonlinear adaptive tuning (e.g., higher VPC gain as the small valve approaches the end of the desired throttle range) can be used to help prevent running out of small valve. Directional velocity limits—by putting different up and down setpoint rate limits on what the VPC is manipulating, and turning on external-reset feedback (e.g., dynamic reset limit)—is a powerful tool.

There are many applications where VPC can optimize a process by pushing a primary PID control valve to a maximum or minimum throttle position with room to maneuver. In nearly all cases, the signal to the valve being pushed to limit can be used instead of the actual valve position as the controlled variable of the VPC. Note that if signal characterization is applied to the VPC output, the valve signal is the characterizer output. The article “Don’t Overlook PID in APC” (Control, Nov. ’11, p. 39) provides significant guidance on how prime mover, chiller and cooling tower energy, purchased fuel and reagent costs can be minimized, and how reactor and column production rates can be maximized.

The ability to handle abnormal conditions by directional velocity limits, feedforward and adaptive tuning is greater for these applications because, in most cases, valves are being pushed to be as far open or closed as possible without interfering with the ability of the primary PID to provide tight control.

When there is more than one valve position to be optimized, such as several downstream feed valves or pressure and temperature valves, override control is used by having the multiple VPC outputs as inputs to a signal selector whose output is the upstream primary PID setpoint that is being optimized. If the setpoint is being maximized and minimized, low and high signal selectors are used, respectively. The override controllers should use external-reset feedback so integral action is suspended until the VPC is selected. Positional algorithms for override control are recommended by Harold Wade so that point of takeover is determined by the position of the override PID PV relative to its effective proportional band set by PID gain. For much more on override control and the PID algorithm functionality and capability, including reset windup, see Harold Wade’s ISA book, Basic and Advanced Regulatory Control - System Design and Application, Third Edition and Karl Astrom’s and Torre Hagglund’s ISA book, Advanced PID Control.

Override control is sequential in that only one override controller is selected at a time. For simultaneous optimization, more complex process performance goals, and better handling of interactions and disturbances, model predictive control (MPC), with its linear program optimizer, should be considered. However, MPC requires special software and expertise. The quicker and less expensive solution is generally VPC and override control, which can be the starting point to justify a more extensive and intelligent solution by MPC. For more on the practical considerations with PID control, see my Momentum Press book, Tuning and Control Loop Performance, Fourth Edition, and for a comprehensive look at what makes an application successful, see the McGraw-Hill 2019 Process/Industrial Instruments and Controls Handbook, Sixth Edition, capturing the expertise of 50 leaders in industry.

What position should a control valve be in?

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