In the last 15 to 20 years
there has been an increase in the misapplication of parallel relief
valves, (pressure safety valves). By misapplication I mean two or
more relief valves of the same fully sized capacity are
left open to
the process. This practice apparently began when operators
decided to open a standby valve in the well intentioned, but mistaken
belief that if one is safe then two are safer.
Why is this practice dangerous? Because the relief
system is grossly oversized. Under no process conditions, with
any relief valve type or manufacturer, at any ASME setpoint offset, is
this practice acceptable.
If this is not understood then please contact me for more
information. I am published on this issue and have a PowerPoint
summary of that paper1.
It has been a long process but ASME is working on a clarification to
the standards relating to pressure safety valve sizing and parallel
installations based on the issues raised in my paper. The
ASME Subcommittee on Safety Valves reference is Item
are my comments to ASME on 14-909.
James R. Lawrence Sr.
Control Systems Engineer (retired)
1 Relief Valves in
Parallel, James R. Lawrence Sr, Mary Kay O'Connor Process Safety
Center, International Symposium, October 2010.