Actuator maintenance


#1

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At the same time as the restoration of the valve, it is necessary to maintain the actuator of the valve. Each of the main components will be repaired according to the manufacturer’s guidelines or replaced. The proof of repairs will be on the success of the retest and the test bench.

Steps for actuator maintenance:

  • The actuator should be disassembled in accordance with the vendor’s manual.

  • The parts should be cleaned

  • Sand blasting the internal diaphragm housing can eliminate rust and corrosion, but the inside must be completely free of particulates or grit prior to reassembly.

  • All elastomers must be replaced, and sealing surfaces should present a mirror-like finish that is completely free of nicks and burrs.

  • If the actuator is piston type, the cylinders should be well greased to permit the piston-seal to travel throughout its range.

  • The grease should maintain its lubricating qualities throughout the range of environmental temperatures that the AOV will be subjected to.

  • If lubricators are used, it is necessary that the solenoid vent path not be subjected to hydrocarbon vapors.

Diaphragm Replacement:

  • Keep the diaphragm clean. Wear clean gloves to keep deteriorating materials from coming into contact with diaphragm.

  • Inspect the diaphragm for any manufacturing defects.

  • When tightening the diaphragm to the diaphragm plate, do not allow the valve stem to rotate.

  • Don’t overtighten the parts such as bolts, diaphragm casing caps, it can damage the diaphragm.

  • Once torqued, do not go back and re-torque after the diaphragm has set.

Stem Seal or O-Ring Replacement:

  • Use the proper lubricant when installing the O-rings.

  • When installing the stem through the stem seal, if there are exposed threads on the stem make sure that these do not cut the seals. The temporary use of Teflon tape around the exposed threads while the stem is being installed affords some protection.

  • Inspect the actuator stem where it goes through the seal rings for any wear or scratches. Replace or refurbish as required.

Actuator Spring Replacement:

  • Relieve all spring compression prior to working on the actuator.

  • Do not allow the valve or actuator stem to turn during any operations.

  • Lubricate all moving parts when assembling the spring.

  • Adjust the spring so that it just starts to move at the lower end of the bench set pressure as recorded on the actuator nameplate.

Bench Set:

“Bench assembly” is the specification of the identification plate that is used to verify the correct operation of the actuator. The bench set is expressed as the pressure range from the start of the actuator stroke to the nominal travel of the valve. Because the spring rates of the actuator are not very consistent, it is reasonable to assume that only one of the bank’s set points can be met and that the critical value should be the one that fits.

  • On air-to-open valves, the start pressure is critical for a valve that requires positive shutoff by the spring.

  • Adjust the spring to satisfy the critical end of the stroke. The full stroke differential pressure can be changed only by replacing the actuator spring.

When conducting the bench set test, the configuration of the actuator and valve is defined by the valve manufacturer. The most common configuration is to have the actuator disconnected from the valve stem.

If there is also conventional packing, the packing load must be relieved. The force of the bellows seal compression is significant and must be factored into the bench set of the actuator.