Type of Actuator Problems

The problems associated with actuators are normally easy to repair because the actuators do not have to be removed from the fluid system. Many times a visual inspection of the actuator in the field gives an immediate indication of poor performance.

Erratic or jerky throttling:

Erratic or jerky throttling can be caused by a fluctuating air supply, an unsteady electronic/pneumatic signal, or a binding packing problem. A few minutes of watching the valve cycle can indicate which of these problems is the cause. Adjustment of the positioner or other control devices can produce the desired result.

Failure to fully retract or fully extend:

Failure to fully retract or fully extend is treated as the same problem for linear actuators. The usual causes are:

  • Low supply air pressure. This could also include air systems on infrequently used valves. Also, the number of instruments being supplied by an airline might have a greater volume that can be made up through the airline. In this case, the valve might not close or open, or might perform these actions extremely slowly.

  • Air leaks. This can cause insufficient air pressure to the actuator.

  • Travel stops not properly set. On rotary actuators, the mechanical stops can be set so that the actuator travel stops prior to the valve closure.

  • Incorrect coupling between the actuator and the valve stem. This and wrong travel stops have similar symptoms.

  • Limit switches out of proper adjustment. This can cause the actuator to cease movement prior to opening or closing.

  • “Gummed-up” solenoid valves. Lubrication of piston actuators can gum up solenoid valves. This can keep the vent valve open and cause the piston to stay in the mid position or spring detent.

  • Higher actual process pressure than used for actuator selection.

  • Improper stem alignment, cage-to-plug alignment, or bonnet-to-body alignment.

  • Damaged cylinder or piston rings are problems peculiar to piston actuators. Any damage that permits air flow from the high- to the low-pressure side of the piston can end up causing a pneumatic lock or insufficient force to move the actuator.

  • A temporary fix might be to add an O-ring to the piston exterior and add a lubricator. If the air leaks are small and the pressure is low, this repair can hold up for an extended time, possibly until the next outage.

  • Elastomers in either diaphragms or seals that start to crack or break are a major cause of actuator failure and degradation.