Q. Pressure Relief Valve Application?

A general comparison of operational characteristics is given for the different types of pressure relief valves in common industrial use. The influence on operating margin, from set pressure, is considered.

Operational characteristics, of direct spring operated and pilot operated pressure relief valves, should be known by the user as well as the designer. Equipment with an MAWP of less than 15 PSIG does not require relieving devices that meet ASME Code requirements. Direct spring and pilot operated relief valves are available for use on applications that must meet ASME Code requirements. The recommended range of opening pressures available for direct spring valves is 10 PSIG (69 kPa) and higher, and 0.5 PSIG and higher for pilot operated valves. The approximate reseating pressure for direct spring operated valves is 93% of the set pressure in gas or vapour service, and 85% of set pressure for National Board tested safety relief valves in liquid service. Many older liquid service safety valves, requiring 25% overpressure to be full open, have a reseating pressure as low as 70% of the set pressure. The reseating pressure for pilot operating valves is typically specified in the same range as the direct spring valves. However, the reseating pressure of pilot operated valves can be lowered to a value slightly above atmospheric, by adding a manual blowdown connection which can be operated either locally or remotely. Pilot operated valves are used in this fashion as remote, manual, emergency, blowdown valves. The versatile pilot operated valve has some significant application limitations.

The point where leakage begins to be a concern, when using direct spring operated valves, depends on the disk seat design. Metal-to-metal contact seats will begin to leak at about 90% of set pressure. O-ring soft seat disk type direct spring operated valves will not leak below 95% of set pressure. Pilot operated valves will not leak below 98% of set pressure. The recommended maximum equipment operating pressure is slightly below, but many times considered to be equal to, the start-to-leak limit for the valve.