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 #80, and was written by Greg.
For pH control, where the requirements for precision are extraordinary, the reagent flow rates are so low that I was never tempted to use rotary valves other than as expendable on-off valves for pulsing. I once used small solenoid actuated ball valves for pulse duration control because the frequent stroking would wear out any other type of valve and the ball valves were cheap and relatively rugged, with little propensity to plug despite slime and solids in the reagents. For throttling control, some rotary valves can come close but none can achieve the precision of a sliding stem valve that was specifically designed to be a control valve. If I had used a rotary valve in the pH control application, the limit cycle would have been much larger than the allowable error around setpoint because of the extremely high pH process gain (e.g., steep titration curve around setpoint).
A sliding stem valve with a diaphragm actuator and digital positioner should have a precision (deadband, resolution, and threshold sensitivity) of less than 0.2%. Even the best rotary valves usually have a precision that is twice as large, which is pushing the 0.5% precision limit I deem as necessary for even less important loops. Some suppliers claim that rotary valves are better for control. I guess they are thinking about flow capacity and openness of flow path rather than precision of control. Precision sets the limit cycle amplitude and the rangeability (Tip #65).
A sliding stem valve has a direct connection between the actuator shaft and the internal flow element trim (e.g., plug). The motion is linear and direct. There is no backlash from shaft connections, translation from linear to rotary motion, linkages, or rotary element seals (e.g., ball and butterfly seals). The only stiction is from valve packing and the seating of the plug in the trim. A diaphragm actuator and digital positioner have a threshold sensitivity of better than 0.1%.
Sliding stem valves offer a better installed characteristic than that of rotary valves. The installed characteristic for rotary valves tends to become too flat at upper valve positions (i.e., more open); valve gain becomes too low. Sliding stem valves also offer more choices for inherent trim characteristics.
The most precise sliding stem valve is a roller diaphragm valve, designed for low flow, viscous flow, and sanitary service. The flow is forced to be laminar.
Concept: Sliding stem valves inherently have the best precision. Nearly all processes have limit cycles from deadband, resolution, and threshold sensitivity limits. The limit cycles from sliding stem valves can be less than the process and measurement noise and the historian data compression. However, the flow path is tortuous, with pockets and crevices that can accumulate process fluid or solids. Plugging and erosion can occur. The roller diaphragm design solves these problems but is only suitable for low flows.
Details: Precision determines the limit cycle amplitude and rangeability of control valves. Use sliding stem valves when size permits and where erosion and accumulation of solids do not cause a maintenance or safety problem. Use low friction packing. Use balanced trim to reduce seating friction if leakage is not a problem. The precision of the valve with a diaphragm actuator and digital positioner should approach 0.1% for the entire throttling range. Use a roller diaphragm valve for low flows in sanitary service (e.g., food and drug applications) and for low flow rates of viscous reagents (e.g., 98% sulfuric acid). Add an on-off valve to provide tight shutoff (isolation).
Watch-outs: Piping valve manufacturers may offer a piping globe or pinch valve as a sliding stem valve. Excessively tightened stem packing, lined valves, and soft seats can cause excessive stiction. Marginally sized actuators can cause stiction and erratic action near the seat (Tip #85). Sliding stem valve sizing should include the loss of capacity if flashing occurs. Special trim may be needed to prevent cavitation. Valves inadvertently installed in the “flow to close” direction will behave erratically near the seat due to the “bath tub stopper” effect.
Exceptions: Some highly reactive monomers, such as hydrogen cyanide, will exothermically polymerize in crevices and stagnant areas, creating a dangerous safety hazard besides a maintenance problem. Sliding stem valves cannot generally be used on slurries. Applications with a large, erosive, or corrosive flow require a rotary valve.
Insight: Sliding stem valves from a control valve manufacturer offer the tightest control.
Rule of Thumb: Use a sliding stem valve with diaphragm actuator and digital positioner whenever size and process conditions permit.
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.
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.