Safety Valve - Spirax Sarco DP27T Installation And Maintenance Instructions Manual

3.10 Bypass
If it is essential to maintain a constant supply of steam and the reducing valve station does
not include duplication or stand-by equipment, it may be necessary to install a bypass to
ensure continuation of supply when the reducing valve is being serviced (see Figure 2).
The bypass valve will normally be the same size as the reducing valve. The handwheel
should be padlocked to prevent use by unauthorised personnel, and when in use should
be under constant manual supervision.
The bypass may be arranged above or to the side of the main assembly but never below it.

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 DP 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 DP 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 DP and where
a control valve is also being used downstream of the DP, it is recommended that the safety
valve is fitted downstream of the control valve rather than in between the DP 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 DP (B) the intermediate downstream pipework
must be properly trapped (D) to ensure no condensate can build up on the downstream
side of the DP.
Fig. 3 DP position
in relation to other
control equipment
C
B
A A
D
14 IM-P240-02-EN-ISS1 CTLS