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.

Parallel Relief Valves

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 presentation 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 14-909.  Here are my comments to ASME on 14-909.

Contact Information:

James R. Lawrence Sr.
Control Systems Engineer (retired)
(832) 387-1153

1 Relief Valves in Parallel, James R. Lawrence Sr, Mary Kay O'Connor Process Safety Center, International Symposium, October 2010.

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