A balanced pressure relief valve is designed to be much less dependent than a conventional valve on the back pressure present at its outlet. To achieve this goal, balanced valves most often contain a bellows that serves to isolate a portion of the valve disk from the back pressure. The inside of the bellows is in pressure communication with the valve bonnet, which must be vented to a location of constant low pressure, typically the atmosphere. Other balanced valve designs exist (see API Recommended Practice 520 Part I), but the bellows valve is by far the most prevalent and has been shown to be the most reliable in most installations. With the exception of the incorporation of the vented bellows for pressure balancing, the construction and characteristics of a bellows valve are identical to those of a conventional relief valve. In fact most conventional valves can be converted to balanced performance merely by installation of the bellows and its gaskets and removal of a plug in the bonnet vent. Figure 1200-14 shows a schematic diagram of a typical balanced bellows pressure relief valve. Balanced PSVs are most often used to tie a new low-pressure relief load into an existing heavily loaded relief header or to protect the PSV topworks from corrosive gases in the relief header.
A balanced pressure relief valve is generally the preferred relief device whenever back pressure conditions preclude the use of a conventional pressure relief valve. Because the bellows isolates many of the internal parts from the fluid on the discharge side of the valve, bellows-type balanced valves also find application when the discharge system contains corrosive or fouling fluids. These advantageous properties of a balanced valve come at the expense of a higher cost and increased maintenance requirements relative to a comparable conventional valve.