Full port gates at relief valves

[crazypiper]

We have several relief valve stations that require full port gates (a common requirement at relief valves). This is not a problem for sizes 3" and above, because gates in these sizes are full port by design in most cases. But gate valves 1 1/2" and below (forged) are supplied as reduced port by default and 2" could be either reduced or full (forged or cast).

OK - the problem is that these small bore relief stations are already built and we didn't purchase full port valves (1 1/2" and less). oooooops.

We are trying to justify the use of reduced port valves by comparing the port area of the reduced port gate valve and the port area of the relief valve and saying, "the port area of the gate is much larger than the port area of the relief valve, so reduced port gates should be OK".

Questions -

1. What are the reasons for using full port gates at relief stations, other than the obvious port area/flow issues?

2. Do thermal reliefs need full port gates?

 

[insult2injury]

Check the effect on the SRV inlet piping pressure drop.

I2I

 

[JimCasey]

You can do it.
Here is an excerpt from ASME SecVIII:
"UG-135 INSTALLATION
(a) Pressure relief devices for vapor application shall
be connected to the vessel in the vapor space above any
contained liquid or to piping connected to the vapor
space in the vessel which is to be protected.
(b) The opening through all pipe and fittings between a
pressure vessel and its pressure relieving device shall
have at least the area of the pressure relieving device inlet,
and the flow characteristics of this upstream system shall
be such that the pressure drop will not reduce the relieving
capacity below that required or adversely affect the proper
operation of the pressure relieving device. The opening in
the vessel wall shall be designed to provide direct and
unobstructed flow between the vessel and its pressure
relieving device. (See Appendix M.)"