IM-P316-03 CTLS Issue 14
16
It can be seen that the safety valve does not re- seat when the pressure falls to the safety valve set pressure.
There must be adequate differential between the maximum system operating pressure and the safety valve
set pressure if the valve is to re-seat.
The maximum system operating pressure is most likely to occur during no -load conditions. Figure 10 shows
the range within which the valve must be set.
10.1 Choice of set pressure
A decision must be made regarding the pressure at which the safety valve should be set.
The maximum set pressure, in accordance with BS 5500, is the safe working pressure of the plant it protects
and the valve must achieve its rated capacity with an overpressure of no more than 10%.
The SV615 achieves its rated capacity at 5% overpressure on steam, 10% on air. If the valve is set too close to
the operating pressure of the system the valve may operate prematurely. It will also fail to close satisfactorily
when normal operating pressure is restored.
Figure 9 outlines the way a safety valve operates.
Fig. 9
(Valve operates)
Time
Re-seat differential
Pressure
Set
pressure
Full lift position
5% of set pressure
Re-seat pressure
(10% of set pressure)
10. Guidelines for setting
(By authorised and competent persons only)
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