33
7.4 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 downstream pressure
during full-load conditions.
2. If the upstream pressure is correct, and stable, set the valve on dead-end conditions.
Apply full-load to the valve. If the downstream pressure drops excessively during full-load
condition, 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 in Fig. 4, page 12.
4. The point at which the external pressure control pipe is tapped into the main is in a
turbulent area. Refer to Section 3.8 for guidance.
5. Loose dirt in the pipe assembly (Item 'P' on page 31). Remove the pipe assembly. Blow
through to clear.
6. Pilot valve or pilot valve plunger is sticking. Follow Steps 7 to 11.
7. Main valve pushrod sticking. Follow Steps 36 to 43.
8. Pilot diaphragms or main diaphragms overstretched. To replace, see page 31.
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