Diagnosing Final Control Elements

Greg: I have known James Beall way back to his days at Eastman Chemical when he generated some eye opening data on the response of valve positioners. James started out as instrument engineer. When James moved to the field he had to make the instrumentation and valves that he had selected actually work! This lead to a career in process control improvement at Eastman and his present position as principal consultant at Emerson. I have always been impressed with James field experience, practical approach, and open mind. In a way, I followed a similar path. When I first started at Monsanto, I was immediately transferred to Electrical and Instrument construction to install and commission what other engineers had selected. I learned what worked and didn't work. After my career at Monsanto and Solutia (now Eastman), I ended up as principal consultant as well but on a contract basis. At the end of December most of my contract work ends freeing me to relieve a knowledge overload in my head via overdue books. I have 4 books in the queue.

Stan:  Since automation systems generally affect the process by manipulating a flow, the control valve is the key final control element. What was the biggest step forward that helped you improve valve performance?

James: The biggest revelation was getting the actual valve or more accurately the "actuator" position into the data historian and my process control data analysis system. Before the advent of the high performance digital positioner with readback, we had to install temporary or permanent position transmitters.

Greg: I installed position transmitters on surge valves because it was essential for making sure the pres-stroke dead time less than a second and the stroking time less than 1 second. It would haven been helpful to have this indication for all control valves but the cost of the separate transmitter and wiring relegated the installation to special cases. Not knowing what the valve was actually doing led to creative explanations of process variability. The Control feature I wrote at an ISA conference in the early 1990s "Valve Position: The Missing Link" revealed the absurdity of the situation. The feature ended up as a chapter in the ISA book How to Become an Instrument Engineer - Part 1.523 coauthored with Monsanto engineers with a similar sense of humor.

James: Without actual valve position (AVP) in the data historian, it is particularly difficult to diagnose when a valve moves despite a steady signal. If something was strange, the first thing to do was plot the AVP. In one case the valve would move on its own at a certain spot in the piston due to a worn O-ring. In another case the valve cycle was following the instrument air compressor pressure cycle. We found there was no air pressure regulator for the valves but even so, after checking the positioner specifications, the double acting pistons should not have been moving.  The real problem turned out to be that the cross-over pressure was set to zero so there was no stiffness except for the spring that assisted fail action. Since the pressure could not go below zero, the actuator was acting like a single acting rather than a double acting piston.

Stan: Can you figure out everything from the control room?

James: You always need to look at what is out in the field or it will bite you.  Once on a level loop on a decanter boot, a relatively fast integrating process, showed a limit cycle indicating a deadband of about 1%. When we checked the valve in the field, the valve would respond to a 0.1% change in signal if you waited long enough, so technically the supplier could say the valve had a resolution of 0.1%. The positioner had a 2 stage relay. For changes larger than 1%, the high capacity relay would kick in and the valve would respond within a second or two. For smaller changes in signal the valve would take 40 seconds or more respond. The fast integrating loop effectively revealed the real resolution limit because the level was ramping while waiting for the valve to respond. We put on a digital valve controller (DVC) that is a high performance digital positioner and the problem went away. Since then I have recommended putting this type of positioners on all valves.

Greg: It seems a lot of poor performance can be related to suppliers and users not realizing that response time, deadband, and resolution are important. The ISA Standards ISA-75-25 sheds light on how to measure these metrics but stops at revealing the implications and providing guidelines for various applications. I got burnt big time in 1975 as lead engineer for the world's largest acrylonitrile plant when the contract design and construction firm said we could save big bucks by omitting positioners on several hundred loops. The lead engineer pulled out a Nyquist plot study from a supplier that showed positioners would hurt performance and if speed was needed a booster should be used instead of a positioner. During startup, I ended up putting positioners on all the valves because many did not move unless the signal changed by 25%.  I got burnt again about ten years later when I tried to replace positioners with boosters on surge valves. The huge butterfly valves slammed shut when the compressor started. The instrument tech showed me how he could manually easily move the 24 inch butterfly valve by simply grasping the shaft. When the positioner was put on he could not budge the valve as you might expect. The high outlet port sensitivity of the booster led to positive feedback with the diaphragm actuator. A couple of years later at a factory acceptance test, the valve supplier made the same mistake. I went up to the 30 inch valves and showed how I could stroke them with my hand on the shaft.