Fault Finding of Pilot Operated Pressure Reducing Valves

Preliminary procedure of Pilot Operated Pressure reducing valves:
 Before undertaking the following fault finding procedure, ensure that the alve has been isolated and that upstream and downstream pressures are zero. Possible fault checks are given in a logical order below.

Downstream pressure zero or too low

If downstream pressure drops below set pressure or is zero, check the following:

1. No high pressure steam reaching the pressure reducing valve. Check that steam is truend on and that the strainer is clear. (To assist with commisioning and fault finding it is recommended to fit an upstream pressure gauge.)

2. Pressure adjustment spring broken.

3. Pipe assembly blocked. Remove by uncoupling the union nuts and blow through to clear obstruction.

4. Control orifice blocked. Unscrew from the outlet port and clear out obstruction. For identification this coupling has a groove around the hex.

5. Main diaphragms fractured. Replace the diaphragms.

6.Pilot valve plunger too short.

7. Valve capacity insufficient for downstream conditions.

   a). Check that the upstream pressure correct. If it is low, the capacity of the valve will be reduced.

   b). Ensure that the pressure control pipe is fitted as recommended and if necessary fit an external pressure control pipe as described. If the downstream pressure is still too low, then a larger valve with more capacity is required.

Downstream pressure too high

If the pressure on the downstream side of the pressure reducing valve has risen above the required set pressure, check the following:

1. Downstream pressure control pipe blcoked. Dismantle and blow through.

2. Control Orifice blocked. Unscrew the pipework from the side of the body and clean out. For identification this coupling has a groove around the hex.

3. Pilot valve diaphragms fractured. Check replace it.

4. Pilot valve or pilot valve plunger is sticking.

5. Main valve not seating. Check it.

6. Main valve pushrod sticking.

7. Pilot valve plunger too long.

8. Pilot valve not seating.

Hunting

Hunting may coincide with variations in steam load. If this is the case, the following checks should be made before dismantling the valve:

1. Check that the upstream pressure is stable. If the pressure drops during full-load conditions, it is possible that there is a partial blockage upstream, or that the upstream pipework is undersized. If the upstream pressure is low, the effect will be to reduce the capacity of the valve with the possibility of not maintaining the downsteam pressure during full-load conditions.

2. If the upstream pressure is correct, and stable, set the valve on dea-end conditions. Apply full-load to the valve. If the downstream pressure drops excessively during full-load conditions, it is likely that the valve is undersized, in which case it should be replaced.

Once it has been determined that the upstream pressure is correct and stable, and that the valve is correctly sized, the following check should be carried out on the valve:

3. Steam is very wet. Ensure the valve installation is as recommended.

4. The point at which the external pressure control pipe is tapped into the main is in a tubulent area.

5. Loose dirt in the pipe assembly. Remove the pipe assembly. Blow throught to clear.

6. Pilot valve or pilot valve plunger is sticking.

7. Main valve pushrod sticking.

8. Pilot diaphragms or main diaphragms overstretched.