1. Back Pressure
While designed to minimize the effects of back pressure on their performance, balanced pressure relief valves are not immune to these effects. In general, the flow capacity of balanced valves is reduced at total back pressures greater than roughly 17% of set pressure for liquid flow and 30% of set pressure for vapor flow. An individual balanced valve’s manufacturer should be consulted regarding the onset and rate of capacity reduction as a function of back pressure, and for possible onset of unstable performance (i.e., possible chattering) at high levels of built-up back pressure. Bellows-type PSVs can be used with header back pressures as high as 40% of the set pressure.
Because the bellows itself is fabricated from thin, corrugated metal available in a limited number of materials, it may have a pressure rating that is below the rating of the valve body and outlet flange. Thus, the selection of a bellows type valve may impose a limit on the absolute value of the back pressure (rather than its fraction of set pressure) that would not be present if a conventional valve were selected.
2. Potential Atmospheric Discharge
The bonnet of a balanced pressure relief valve must be vented to a location of constant low pressure; the atmosphere is the typical choice of venting location. If the bellows should fail, fluid from the discharge side of the valve may be discharged from the bonnet vent. Therefore, careful consideration should be given to the nature (particularly the toxicity) of the discharge fluids and the safety of the bonnet vent’s ultimate discharge location.
3. High Operating Pressure
Balanced pressure relief valves exhibit the same behavior as conventional valves with respect to seat leakage as the operating pressure exceeds 90% of set pressure. Therefore, they should generally not be selected for applications with high ratios of maximum operating pressure to set pressure.
4. Periodic Inspection
When bellows-type PSVs are installed, it is necessary to periodically check that the bellows is intact. A leaking bellows does not provide back pressure compensation and it allows the relief header to leak to atmosphere.