Page 185 - Valve Selection Handbook

172 Valve Selection Handbook

Figure 5-7. Conventional Safety Relief Valve. (Courtesy
of Anderson, Greenwood & Co.)

Figure 5-8. Conventional Safety Relief Valve.
(Courtesy of Anderson, Greenwood & Co.)

Prior to valve opening, the forces from back pressure acting on both
sides of that portion of the disc that overhangs the nozzle balance each
other while the area of the back covering the valve nozzle is unbalanced.
Superimposed back pressure acting on the unbalanced area thus raises
the set pressure by the amount of the superimposed back pressure. The
bench pressure setting must therefore be lowered by the amount of super-
imposed back pressure to allow the valve to open at the operational set
pressure. For this reason, conventional safety relief valves are not suit-