Page 222 - Valve Selection Handbook

Pressure Relief Valves 209

• Field test connection for the in-service verification of the set pressure.
• Field test indicator for the in-service verification of set pressure of
modulating type pilots only.
• Backflow preventer to prevent accidental reverse flow through the
valve if outlet pressure ever exceeds inlet pressure.
• Pilot supply filter to protect the pilot from entry of significant impuri-
ties carried by the fluid traveling in the pressure sense line. In the case
of liquid service, the filter is particularly appropriate in connection with
flowing type modulating pilots.
• Pressure spike snubber to attenuate pressure spikes in gas systems
downstream of reciprocating compressors.
• Manual or remote unloader permits the main valve to be opened
remotely to depressurize the system.
• Pilot lift lever to permit manual testing of the pressure relief valve oper-
ation. (Note: Lift lever does not permit checking the set pressure as
sometimes assumed.)
• NACE material option for sour gas service in accordance with the
requirements of NACE MR0175 for both pilot and main valve.

Stable Operation of Valves with On/Off Pilots

These are the rules:
The pressure loss in the valve inlet pipeline section between the inte-
gral sense pressure pick-up point and the source of pressure, as with
spring-loaded pressure relief valves, must be less than the pilot blow-
down. If this is not observed, the valve will reclose after opening and
then reopen again in rapid succession until the valve has destroyed itself
or the system pressure has risen sufficiently to stop valve cycling. It is
prudent to ensure that the blowdown is at least 2% longer than the pres-
sure loss in the pipe section upstream of the sense pressure pick-up point.
Note that modulating pilots overcome the potential for cycling in this sit-
uation.
The length of remote pressure sense lines is determined by the dis-
tance between the location of the pressure relief valve and the pressure
sensing point. Such lines must be run in a manner that prevents conden-
sate from blocking the flow passage. In the case of pressure relief valves
with non-flowing pilots, pressure sense lines of 200 feet (60 m) length
have been reported, using a size 1"NB (DN 25) pressure sense line in
compliance with the applicable piping code for the system. In the case of
steam service and other services with the potential of high condensate