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3.11 Safety valve
A safety valve should be fitted to protect the downstream equipment from excessive
pressure. It should be set to lift below the safe working pressure of the downstream equipment,
and will normally be sized to pass the full capacity of the PRV should the PRV fail in the fully
open position. The safety valve set pressure should take account of its reseat characteristic
and the 'No-load' pressure setting of the PRV. For example, the typical blowdown value
(reseat differential) for a DIN type safety valve is 10% of set pressure. The minimum possible
safety valve set pressure must therefore equal the no-load set pressure of the reducing valve
plus the blowdown value of the safety valve plus a small margin of at least 0.1 bar. If the safety
valve lifts and the working pressure is too close then it will not be able to close properly and
will simmer, creating a leak which is often wrongly diagnosed as a result of a leaking reducing
valve.
Discharge pipework should be taken to a safe place.
3.12 Position in relation to other control valves
C
DP27S
A
B
A
D
Fig. 5 DP27S position in relation to other control equipment
Line or system isolation valves (A), either remotely actuated or manual, should be installed on
the upstream side of the DP27S reducing valve.
Where there is downstream control equipment (B), particularly when it is fast acting (for
instance pulsed piston actuated valves) ensure the control equipment is at least 50 pipe
diameters away from the DP27S to prevent pressure pulses being transmitted back causing
unstable operation and premature wear or if this is impractical an intermediate vessel can
provide a similar benefit.
Where a safety valve (C) is required to protect the system downstream of a DP27S and where
a control valve is also being used downstream of the DP27S, it is recommended that the safety
valve is fitted downstream of the control valve rather than in between the DP27S and the control
valve. If any slight leakage occurs this will avoid any pressure build-up causing nuisance
operation of the safety valve but provide complete protection for the downstream system.
Where valves are installed downstream of the DP27S (B) the intermediate downstream
pipework must be properly trapped (D) to ensure no condensate can build up on the downstream
side of the DP27S.
IM-IBR16-27IN Issue 6
11
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