A lot of time and money can be spent deciding which valves need positioners and which flows need measurement. We tend to look at short term costs such as hardware and not the cost of troubleshooting and the implications as to plant performance.
Most of my big mistakes were the result of trying to save upfront costs. Here we look at common misconceptions as to whether flow and valve position control should be used. Most of the controversy centers on the violation of the cascade rule where a lower loop is not at least 5 times faster than a higher loop. The rule “valve positioners should not be used on fast loops” still haunts us. While here we focus on the manipulation of flow which is the most common manipulated process input, the next blog will offer more general and concise guidance but with the same theme of not short changing yourself or the automation system.
In my book all feedback loops manipulating plant flows should have a valve positioner and a flow measurement. The use of external reset feedback and the positive feedback implementation of the integral mode will prevent an upper PID output from changing faster than a lower PID (e.g., secondary flow) can respond and will also prevent this lower PID output from changing faster than the control valve can respond (given fast readback). Thus, the violation of the cascade rule does not cause excessive oscillation.
Positioners must be used for piston actuators to function and to give the required transition between valves at a split range point. Less recognized are the many essential benefits for all types of actuators and applications.
I have never seen a case where a positioner should not be used. The high gain smart digital valve positioner reduces the amplitude of deadband from backlash, eliminates the confusing offset during manual operation, deals with the integrating response of actuator pressure, prevents positive feedback from a booster and diaphragm actuator (potentially dangerous situation), provides ability to tune for different types of actuators, and gives readback of actual valve position for visibility and external reset feedback.
The addition of a volume booster on the output of the positioner can reduce valve dead time to less than 0.1 second and increase slewing rate to 100% per sec. If this is not fast enough, then a pulse width modulated variable frequency drive with speed to torque cascade control in the drive. There are many design considerations with a VFD not commonly recognized. Most overlooked is the severe loss in turndown when the static pressure approaches the total head.
I am an advocate of flow loops as well. The flow loop compensates for pressure disturbances, provides better regulation of process stoichiometry leading to better composition control by the use of lower flow loops and coordinated flow ratio control, and more accurate feedforward control to preemptively correct for feed and utility disturbances (e.g. flow and temperature changes). For startup, the cascade control system can be operated with just the lower loop in service (e.g. flow ratio control) until operating conditions are reached. However, a secondary flow loop may cause the PID output response to be slower than desired for gas and liquid pressure control. Here the PID output should go directly to a fast valve (booster on the output of a positioner) or a VFD. I would still have a flow measurement for better process knowledge.
Flow measurements enable closure of material and energy balances, tracking down disturbances, better inputs for data analytics and verification of process simulations, online process metrics, and an accurate relative gain analysis (RGA) for interactions.
What I learned the Hard Way
In my 46 years of experience some of which was in E & I construction, I have never seen a case where a positioner should not have been installed. My recommendation for the last 40 years is any loop with a control valve should have a positioner. I have had several cases where the omission of a positioner has caused serious and potentially dangerous situations. However, not all positioners are created equal. Some valve designs can cause misleading position feedback and some positioner designs can have extremely poor threshold sensitivity (poor response to small changes). Here is my story.
I started out in E & I construction in 1969. In the four plant installations and startups over the next 2 years all the valves had positioners but the control valves from a piping valve manufacturer used spool type positioners with a pulley system, on-off piston actuators and quarter turn on-off plug valves. The control was horrible with these piping valves. The control valves supplied by a control valve manufacturer all were originally designed for throttling and had a high gain sensitive pneumatic relay positioner. The pneumatic positioner would lose its calibration but the valves responded to small changes and the calibration offset was compensated by the process PID with an output that could go below 0% to make sure the valve was shut despite the calibration offset. The valves from the control valve manufacturer designed for throttling had a smooth and precise response with minimal backlash and stiction. For more on the problem of piping valves posing as control valves and the whole issue of better valve response see my 2007 Chemical Processing article "Improve control loop performance" and the 2012 Control article "Is Your Control Valve an Imposter?"
About 5 years later as lead engineer for the world’s largest acrylonitrile plant with latest electronic analog controllers, the engineering contractor said he saved me a lot of money by not putting positioners on fast loops. Not ever having seen a valve try to do its job without a positioner, I agreed. During startup we found some controller outputs were 40% and the valve had still not opened. There was very little correlation between PID output and valve position. This was particularly confusing during startup and manual positioning of the valve. We ended up putting positioners on all valves during startup.
About 5 years later as I started on the path of improving compressor surge control systems I realized I needed to use a booster to make the valve faster. I read a paper by a prominent technologist that unequivocally concluded after a theoretical study using Nyquist plots that a booster instead of a positioner should be used on fast loops. The instrument technician objected but agreed to replace the surge valve positioner with a booster. That night the compressor shutdown and the cause was found to be the surge valve slamming shut despite the PID output asking for the valve to be opened. When I went up to the valve, the technician showed me how he could grab the stem of the 24 inch valve and move it at will. He then showed me how he could not do this to another 18 inch valve that still had a positioner. I had the positioner reinstalled and put the booster on the outlet of the positioner. I adjusted the booster bypass valve to eliminate high frequency oscillations from the positioner and booster in series. All of the surge valves ended up with this configuration and control was great.
When I ordered control valves for a phosphorous furnace pressure loop that could ramp off scale in a couple of seconds, I went with one of the best distributed control systems with a special option for a 0.03 second execution time making it about as fast as an electronic analog controller. I got a special transmitter with a minimum damping setting of 0.05 sec (0.05 sec time constant). I put a performance requirement that the valve stroking time was less than a second that would be witnessed by me by a test at the control valve suppliers factory. When I got there I saw boosters but no positioners on the 18 inch valves. I walked up to one valve and showed the guy how I could grab the shaft and move the valve. An old timer then arrived and gave out a 1958 article by C. Mamzic “Improving the Dynamics of Pneumatic Positioners” in ISA Journal 5, No.8 that said you should put the booster on the outlet of the positioner and slightly open the bypass valve. We did this and the valves worked fine and the combination of the fast controller fast transmitters and fast valves reduced the number of electrode seal blows by over 90%. Since then this has been my strategy on numerous fast loops.
The high threshold sensitivity of a diaphragm actuator and booster’s outlet port is desirable but creates a positive feedback situation enabling a person to stroke a valve or a pressure unbalance on a butterfly disc cause the valve to slam shut. Boosters have poor input port sensitivity and significant deadband, which is only an issue if not used in conjunction with a positioner.
My other main special area of expertise was pH control. Here I found the stick-slip and backlash from control valves was causing oscillations outside of the allowable control region in systems with relatively steep titration curves. In fact the main limit as how much you could reduce the number of stages of neutralization and the size of the volumes depending upon the precision of the reagent valves. These pH systems were extremely good at showing valve precision. I became sensitive to valve sensitivity.
For a huge grass roots facility in Asia, I was asked to provide guidance on what valves could have positioners omitted to save money. A Fellow and recognized expert on valves advocated only putting positioners on slow critical loops. I said all loops should have positioners.
I have never seen where the violation of the cascade rule (secondary loop not sufficiently faster than primary loop) by putting a positioner on a fast loop has caused a problem. I think the way we tune these PID loops with more integral than gain action and the speed of a proportional only analog positioner and the flexibility of tuning in a modern digital positioner eliminates the problem.
In subsequent years I was plagued over and over again with piping valves posing as throttling valves and attempts to use on-off valves needed for tight shutoff as throttling valves. The positioner feedback of actuator shaft of excellent positioners said everything was good in that the shaft responded to 0.5% changes in signal. Testing the valves in the shop with travel gauges added to the disc or ball showed that the disc or ball did not move until the changes in signal were greater than 8%. The positioner was fooled into thinking everything was OK by a misleading position feedback. The shaft moved but the disc or ball did not, a symptom of shaft windup aggravated by poor shaft to stem and stem to disc or ball connection design.
The high gain digital valve positioner reduces the amplitude of deadband from backlash, deals with the integrating response of actuator pressure, prevents positive feedback from a booster and diaphragm actuator combination, provides ability to tune for different types of actuators, provides readback of actual valve position for visibility and external reset feedback to prevent a PID output from changing faster than a valve can respond and enables a whole spectrum of valve performance diagnostics.
The misconception still exists. An Automation Hall of Fame member in the last 6 months publically made the recommendation not to use positioners on fast loops when asked.
If a positioner is a problem on a fast loop, you probably should not be using a pneumatic actuator or a digital controller execution time greater than 0.03 seconds. For such fast unusual applications you can go to speed control of the prime mover with a fan cooled motor, special cables, a pulse width modulated variable frequency drive with local speed to torque control, a high resolution input card, and zero rate limiting or deadband in drive setup. Even with all of these best practices, a VFD only has sufficient turndown in applications where the static head is small compared to pressure drop from system resistance, a fact not well recognized.
Benefits of Cascade Control
The process control benefits of cascade loops are numerous. Here is a list of the ones that come to mind.
- Lower loop isolates nonlinearities (e.g. valve and process) and stream and utility disturbances (e.g. pressure and temperature) from the upper loop.
- The lower loop encloses secondary time constant that converts the secondary time constant that would have been a detrimental term in a single loop to being beneficial term as the largest time constant in a lower loop.
- The cascade upper loop ultimate period is smaller than the original single loop enabling a faster upper loop and better rejection of disturbances in the upper loop.
- The peak error in the upper loop for a lower loop disturbance can be reduced to be as small as 12% for self-regulating, 2% for integrating, and 1% for runaway of the peak error for a single loop for a lower to upper dead time ratio of 0.6. For lower dead time ratios the improvement would be even more impressive. The best reduction in peak error for a given dead time ratio is achieved for time constant ratio approaching one where the secondary time constant was as large as the primary time constant. This latter relationship is often not recognized because it is counter intuitive and contradicts the cascade rule. The user must realize the cascade rule pertains to the closed loop response and not the open loop response. Since the ultimate period and lambda is a factor of the total loop dead time, the loop dead time sets the limits on the closed loop response.
- Better regulation of process stoichiometry leading to better composition control by the use of lower flow loops and coordinated flow ratio control.
- More accurate feedforward control to preemptively correct for feed and utility disturbances (e.g. flow and temperature changes).
- For startup, the cascade control system can be operated with just the lower loop in service (e.g. flow ratio control) until operating conditions are reached (e.g. distillation columns). The feedforward should be configured to be active with the upper loop in manual, which means the lower loop stays in cascade mode.
- For an upper loop measurement (e.g. analyzer) failure, the cascade control system can be operated with just the lower loop in service (e.g. flow ratio control) until the measurement is fixed.
Benefits of Flow Measurements
The process knowledge benefits of cascade control are not discussed much in the literature but can be just as important and more extensive. The improvement in the recognition and identification of relationships and the creation of models can translate to more intelligent setpoints and operator and process engineers understanding of confusing situations. An increase in process knowledge can be far reaching.
Since nearly all manipulated process inputs are flows, the addition of flow measurements for lower flow control loops offers many advantages. A control loop transfers variability in the process variable to the manipulated variable (e.g. flow). If the upper loop (e.g. level, composition, pH, or temperature) is tightly controlled, nearly all of the variability caused by changes in the process is seen in the manipulated flow rather than the process variable. The process variable in these loops stay right at setpoint. The process knowledge is in the size and pattern of changes in the manipulated flows.
All process simulations need to be compared to the plant and corrected. Process simulations have a difficult time getting the pressure drops and hence the installed flow characteristics right because of the incredible amount of detail on the geometry and characteristics of piping and valve systems needed besides the changes in interior surfaces (e.g. roughness and coatings). The control valve positions in a simulation (e.g. virtual plant) will not match up with the actual plant. The only way to improve simulations that are both doing a good job of control at setpoint is to match up the flows. The addition of flow measurements and flow control in the actual plant enables more accurate process simulations and hence process analysis and improvements. For more on these benefits see the Jan/Feb 2010 InTech article “Advances in flow and level measurements enhance process knowledge, control”
Here is a summary of the benefits of flow measurements beyond cascade control.
- More linear, accurate, and representative inputs for data analytics and neural networks particularly when loops are tightly controlled. Better analysis of whether correlations represent causes and effects or coincident occurrences.
- More accurate process simulations for better process analysis and improvements.
- Greater recognition of the source and path of disturbances and abnormal operation to reduce the consequences and frequency of disturbances and failures.
- More accurate online process metrics for process efficiency and capacity.
- Adaptation of parameters in a virtual plant synchronized with an actual plant.
- More effective interaction analysis by the use of manipulated flows instead of PID outputs in the computation of relative gains for a RGA
Much of what I have learned is shared in my Control Talk Blogs and Columns on the Control magazine website and in my ISA books 101 Tips for a Successful Automation Career and Advanced pH Measurement and Control and my Momentum Press books Axial and Centrifugal Compressor Control and Tuning and Control Loop Performance – 4th Edition.
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