Safety Valve; Position In Relation To Other Control Valves; Dp27 Position In Relation To Other Control Equipment - Spirax Sarco DP27 Installation And Maintenance Instructions Manual

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
DP27
A
B
A
D
Fig. 7 DP27 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 DP27 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 DP27 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 DP27 and where
a control valve is also being used downstream of the DP27, it is recommended that the safety
valve is fitted downstream of the control valve rather than in between the DP27 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 DP27 (B) the intermediate downstream pipework
must be properly trapped (D) to ensure no condensate can build up on the downstream side
of the DP27.
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