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:
- The actuator was undersized from the start (see Tip #85).
- 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.
- 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.)