Since early in my career, I have always been glad to see a diaphragm actuator rather than a piston actuator on a control valve, so I was surprised to read a two-part article in the 1990s by an experienced consultant who maintained that control valves should use piston actuators. The article had excellent information and shared my view that the use of positioners did not cause problems on fast loops. Why, then, did we have dramatically different opinions as to whether to use a diaphragm or piston actuator?
The author of the article was focusing on the advantages of piston actuators in terms of being smaller, faster, more powerful, and stiffer. He did not consider diaphragm actuators have a threshold sensitivity that is one-fifth to one-tenth that of a piston actuator, have no O-ring seals to wear out, and boosters for diaphragm actuators don’t need a deadband. The author of the article may not have been so adamant about piston actuators being best if he had recognized that “flow to close” valves are rarely used, balanced trim reduces the actuator force needed at shutoff, on-off valves should be used instead of control valves for tight shutoff (isolation), the actuator and spring should be sized to provide at least 150% of the worst-case thrust or torque requirement, a booster can easily be added to speed up valve response, and all tests should be done with small step changes.
Until recently, step tests predominantly used a step change of 20% or larger. For large step changes, the piston actuator’s poorer threshold sensitivity is not apparent. In addition, differences in speed of response are more accentuated for large steps because of the differences in slewing rate. A control valve in service is normally responding to step changes of less than 0.5% with each execution of the PID; larger step changes are only made for large setpoint changes and for very fast disturbances.
Until the 1990s, there was little testing of the ability of a valve to respond to small signal changes and to prevent limit cycles. The result was a proliferation of cheap spool type positioners (Tip #82) and on-off valves (Tip #84) that looked fine for 20% changes but were completely inadequate for 0.5% changes. Because positioners did not offer readback of actual position, the problem remained largely unknown. High historian data compression due to high data storage costs in the 1980s and 1990s helped to hide the problem.
The ISA technical report on testing the response of control valves, issued in 2000 and updated in 2006, was a huge step forward. ANSI/ISA-TR75.25.02 showed lost motion for step sizes less than 2% and an order of magnitude increase in response time when the step size became less than 0.5%. Valve manufacturers and model numbers were not identified but it is safe to assume that the worst ones were tight shutoff valves with piston actuators and spool type positioners. The valves with the best response were most likely low friction valves with diaphragm positioners and high gain relay positioners.
Today, the maximum air pressure of diaphragm actuators has increased from 35 psig to 100 psig, which provides more thrust and torque, and digital positioners now offer better sensitivity and more flexibility in tuning. External-reset feedback in the PID can be used with fast position readback to prevent a PID output from changing faster than the valve can respond to large upsets.
Concept: A diaphragm actuator offers the best threshold sensitivity. 100 psig air pressure models are now offered, overcoming an earlier disadvantage of diaphragm actuators. A double acting or spring return piston style actuator is the next best choice. Marginally sized actuators of either type will cause erratic behavior, deadband, and stickslip.
Details: The threshold sensitivity is about 0.1% for diaphragm actuators, 0.5% for double acting piston actuators, and 1% for link arm, rack and pinion, and scotch yoke piston actuators. The link arm and scotch yoke type have excessive deadband. The slip of a rack and pinion actuator increases as the teeth wear. Piston seal wear causes the performance of piston actuators to decrease over time. Use diaphragm actuators and, if necessary, double acting or spring return piston actuators. Marginally sized actuators may cause greater limit cycling and erratic behavior near the seat, including more overshoot on breakaway. Size an actuator to provide at least 150% of maximum thrust or torque requirement.
Watch-outs: The complete valve assembly, including actuator, shaft connections, links, shaft length and diameters, packing friction, and seating and sealing friction, must be right for the valve to be able to respond to the small signal changes needed for tight control. Putting a diaphragm actuator and/or digital positioner on an on-off valve does not eliminate the stiction and backlash inherent in these valve designs (Tip #83).
Exceptions: For liquid or polymer pressure control, furnace pressure control, and other extremely fast loops, a variable speed drive (VSD) may be needed instead of a control valve. The VSD and pump must have sufficient discharge head at minimum speed for highest static head, a high resolution input card, and no speed rate limit or signal deadband in setup. For very fast and very high process pressures, a hydraulic actuator may be needed. These actuators are a last resort because of the special maintenance requirements and the propensity for leaks. Small electronic stepper motor valves are capable of modulating extremely small changes (0.0005 inch) at fast speeds (100 millisecond time constant). These valves are limited to small sizes (e.g., < 1 inch) and stainless steel or aluminum construction.
Insight: Tight control requires the complete valve assembly to be designed to respond with adequate precision to changes in signal smaller than 0.5%.
Rule of Thumb: Where possible, use a diaphragm actuator sized to meet at least 150% of the maximum thrust or torque requirement.