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How to Determine the Reliability of an Actuator

 

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 #9, and was written by Hunter.

 

Over the course of many years of plant experience, I have come to a simple conclusion in regards to selecting on/off actuators. Despite innumerable glossy, color sales brochures and sales presentations to the contrary, the failure of an on/off actuator can usually be attributed to three things. Two of the items will make ANY actuator fail – undersizing and poor air quality. The third item is rather subtle, yet can be a surprisingly accurate predictor of how well an actuator will hold up in service.

 

Concept: Several different on/off actuator designs are available today. Some employ a scotch yoke mechanism, others use a rack and pinion, and undoubtedly many others exist. While the vendors will argue the pros and cons of one design versus the other, plant experience suggests that the diminutive piston o-ring design can be a very good indicator of how well a particular actuator will hold up in service.

Details: An on/off actuator converts air pressure to a 90-degree turn movement that actuates the valve. Most of the actuator failures can be attributed to three things:

  1. The actuator was undersized from the start (see Tip #85).
  2. Poor instrument air quality – If water and/or particulates are in the instrument air system every valve, solenoid, and actuator in the plant will be failing prematurely.
  3. The piston o-ring fails.

If the actuator is properly sized and the air quality is good, the typical point of failure will almost always be the piston o-ring that seals against the cylinder. Once this o-ring begins to wear, it will allow the air pressure to “blow by” the piston robbing it of torque. Eventually the actuator will not stroke at all. The design of this o-ring is what usually determines how long an actuator will last in service.

A cheap design will employ a single round o-ring on the piston. Such a design works wonderfully when new, but quickly wears and begins leaking air. By contrast, a better design will employ multiple o-rings or a wide, flat ring around the circumference of the piston. Either of the designs will last much longer.

 

Watch-Outs: A poor o-ring design will make an actuator fail quickly. But also watch out for actuator limit switch covers that employ individual screws that are not captive in the cover. (In other words, the bolts fall out when they are unscrewed rather than being held in the cover by a slip ring.) Captive screws will not seem like such a big deal until one is on the 5th floor of an open structure pulling a cover to set a limit switch and the screw falls out and bounces off several vessels and pipes on the way to the ground 100' below. At that point, the utility of captive screws in the cover becomes a very obvious thing!

Exceptions: Some designs use a shorter stroke and a diaphragm instead of a piston with an o-ring. Obviously, this particular design is not susceptible to the o-ring issue.

Insight: Find at least two acceptable actuator designs and get both vendors on your bid list. Having two sources keeps the pricing low and limiting the actuator types to only two cuts down on spare parts. Once a good actuator design is chosen, always oversize the actuator.  If the instrument air quality is good, the actuators will provide years of maintenance free service.

Rule of Thumb: Pick a good design, and size the actuator for at least one and half times the maximum torque required. (Note that actuators have different torque values at either end of the stroke so be sure to check both ends of the table when doing the sizing.)

 

About the Author

 

About the Author
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.

 

 

Image Source: Wikipedia-1; Wikipedia-2; Wikipedia-3

 

Greg McMillan
Greg McMillan
Greg McMillan has more than 50 years of experience in industrial process automation, with an emphasis on the synergy of dynamic modeling and process control. He retired as a Senior Fellow from Solutia and a senior principal software engineer from Emerson Process Systems and Solutions. He was also an adjunct professor in the Washington University Saint Louis Chemical Engineering department from 2001 to 2004. Greg is the author of numerous ISA books and columns on process control, and he has been the monthly Control Talk columnist for Control magazine since 2002. He is the leader of the monthly ISA “Ask the Automation Pros” Q&A posts that began as a series of Mentor Program Q&A posts in 2014. He started and guided the ISA Standards and Practices committee on ISA-TR5.9-2023, PID Algorithms and Performance Technical Report, and he wrote “Annex A - Valve Response and Control Loop Performance, Sources, Consequences, Fixes, and Specifications” in ISA-TR75.25.02-2000 (R2023), Control Valve Response Measurement from Step Inputs. Greg’s achievements include the ISA Kermit Fischer Environmental Award for pH control in 1991, appointment to ISA Fellow in 1991, the Control magazine Engineer of the Year Award for the Process Industry in 1994, induction into the Control magazine Process Automation Hall of Fame in 2001, selection as one of InTech magazine’s 50 Most Influential Innovators in 2003, several ISA Raymond D. Molloy awards for bestselling books of the year, the ISA Life Achievement Award in 2010, the ISA Mentoring Excellence award in 2020, and the ISA Standards Achievement Award in 2023. He has a BS in engineering physics from Kansas University and an MS in control theory from Missouri University of Science and Technology, both with emphasis on industrial processes.

Books:

Advances in Reactor Measurement and Control
Good Tuning: A Pocket Guide, Fourth Edition
New Directions in Bioprocess Modeling and Control: Maximizing Process Analytical Technology Benefits, Second Edition
Essentials of Modern Measurements and Final Elements in the Process Industry: A Guide to Design, Configuration, Installation, and Maintenance
101 Tips for a Successful Automation Career
Advanced pH Measurement and Control: Digital Twin Synergy and Advances in Technology, Fourth Edition
The Funnier Side of Retirement for Engineers and People of the Technical Persuasion
The Life and Times of an Automation Professional - An Illustrated Guide
Advanced Temperature Measurement and Control, Second Edition
Models Unleashed: Virtual Plant and Model Predictive Control Applications

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