15
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 set pressure is any lower, if whatever reason the safety valve lifts it will
not shut 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
Line or system isolation valves (A), either remotely actuated or manual, should be installed
on the upstream side of the DP17 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 DP17 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 DP17 and where
a control valve is also being used downstream of the DP17, it is recommended that the
safety valve is fitted downstream of the control valve rather than in between the DP17 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 DP17 (B) the intermediate downstream
pipework must be properly trapped (D) to ensure no condensate can buil up on the
downstream side of the DP.
Fig. 7 DP17 position in relation to other control equipment
D
C
B
A
A
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