Page 219 - Valve Selection Handbook

206 Valve Selection Handbook

Figure 5-38. Pilot-Operated Pressure Relief Valve, Diaphragm Type for Overpressure
and Vacuum Relief Modes, Balanced Non-Flowing Pilot, Modulating Action.
Application: Low Pressure Gas. (Courtesy of Anderson, Greenwood & Co.)

As a carryover of the time when above standard did not apply to
pilot-operated valves, most U.S. manufacturers also offer non-standard
pilot-operated pressure relief valves, referred to as full-bore valves. The
term full-bore applies to the nozzle size, which approaches the size of
the bore of the inlet pipe.
Figure 5-36 shows an example of a full-bore valve. It is noteworthy in
this case that some bodies can accommodate a number of orifice sizes by
adjusting the length of the piston stroke by means of an adjustable length
bolt. The orifices involved cover sizes D through T according to API 526.
The piston is provided in this presentation with a pressure-actuated
drag ring designed to suppress rapid oscillating movements of the piston
due to resonant reinforcement of the assembly and inlet piping.