Corrective maintenance for Valves


#1

Corrective maintenance, known as a breakdown or run to failure maintenance. Maintenance is only done after the equipment or machine is damaged. Corrective Maintenance is a maintenance activity carried out to overcome failures or damage found during the preventive maintenance period.

Corrective maintenance tasks for valves:

Removing the valve from the pipeline:

Remove the valve with its actuator from the pipeline. Then turn the valve to remove all process
material. If the process is toxic or corrosive, flush the valve to avoid accidents. If the valve going to return to the manufacturer for repair, do not disassemble

Disassembling the valve:

Loosen and remove each stud slowly, being cautious of any trapped fluids or leaking isolation valves. As the studs are removed and bagged, they should be checked for nicks and burrs on the threads, which could cause a faulty torque reading.

Remove the actuator installation bolts. Pull the actuator away from the stem either with a removal tool or by hand.

Inspection

Check the studs removed from the flanges for nicks and burrs on the threads, they could cause a faulty torque reading. If they are ok, it should be bagged and attached to the flanges.

Now check the coupling flanges. Surfaces must be checked for scoring and pitting. The cross sections in the flanges can leak, check it. Even if there is no cut or pit, the flange can be faced with an extremely thin coating compound to remove corrosion products from the surface.

If the valve is a diaphragm valve, then the inspection of the diaphragm should be done carefully. The diaphragm should also be checked for nicks, pitting and cracks. Replace any diaphragm that indicates these problems.

The diaphragm should be checked for minor particulate or spalling material from the piping. If such particulates are present in the diaphragm, even in small amounts, Then the I/P transducers and solenoid valves should be checked.

If there is a rusty area or rust marks on the diaphragm, it is an indication that the air in your instrument may be out of specification and the carbon steel pipes and pressure vessels may need to be checked for pitting or corrosion.

Actuator maintenance:

Along with the valve maintenance, the actuator should be refurbished. The actuator should be disassembled as per the manufacturer’s user manual and clean the parts. Sandblasting the internal diaphragm housing can eliminate rust and corrosion, but the inside must be completely free of particulates or grit prior to reassembly.

The cylinder should be well greased to permit the piston-seal to travel throughout it range.

Diaphragm Replacement Tips- Inspect the diaphragm for any manufacturing defects. The diaphragm nuts should not be over tightened, overtightening of nuts can damage the diaphragm. Do not allow the valve stem to rotate while tightening the diaphragm.

Stem Seal or O-Ring Replacement Tips- Use the proper lubricant when installing o-rings. When installing the stem through the stem seal, if there are exposed wires on the stem, make sure they do not cut the seals. Temporary use of Teflon tape around the exposed wires while the stem is being installed provides some protection.

Inspect the actuator stem where it passes through the seal rings for any wear or scratch. Replace or recondition as necessary.

Actuator Spring Replacement- All the spring associated with the actuator should be relieved. Do not allow the valve or actuator stem to turn during any operations. Lubricate all moving parts when assembling the spring.

Positioner Maintenance:

The electronic positioner should not be mounted on the valve that exihibits high vibration. Mounting the positioner to the valve is critical to its proper operation. Most positioners are aligned to the valve while the valve is at its mid-stroke.

Positioner leakage: There are two concerns with the linking of the positioner: link union and wear of the parts of the linkage. Both concerns affect the control of the valve, either in general or in one location, depending on the type of wear or joint.

Air Leaks and Relay Replacement: Check the air leaks in the positioner. Many positioners have integral “relays” that contain most of the non-metallic components of the positioner. These relays are easily replaced in the field and correct most of the problems with a positioner. When replacing the relay, some type of seal goes between the main unit and the relay. Seal the relays using proper lubricants.

I/P Transducer Maintenance:

I/P transducer cannot be mounted to the valve with high vibration. The high vibration produced by some valves affects the balance bar.

Air Leaks and Relay Replacement- Many air leaks, either internal or downstream from the transducer, are the major concern with transducers. When replacing the relay, some type of seal goes between the main unit and the relay. Seal the relays using proper lubricants.