Some of the circumstances in which a conventional pressure relief valve should not be used are described in the following sections.
1. Back Pressure
Because conventional pressure relief valves act on differential pressure, they should generally not be used whenever the back pressure at the valve’s outlet would interfere with the valve’s reliable operation. Normally not to exceed 10% of the set pressure of the lowest set PSV connected to the relief system. This interference can occur when:
a. The superimposed back pressure is sufficiently variable that a net set pressure can not be chosen to keep the valve’s set pressure both less than the protected equipment’s MAWP and greater than the equipment’s operating pressure by an amount large enough to avoid leakage through the valve (see Figure 1200-13 and the discussion below concerning high operating pressure).
b. The built-up back pressure is calculated to be sufficiently high to raise concern about causing the valve to reclose before the equipment’s pressure has fallen below its MAWP. Such reclosing can lead to the valve “chattering”
– see API Recommended Practice 520, Part I for a discussion of this behavior. The “concern level” of built-up back pressure for a conventional pressure relief valve is typically taken to be equal to the vessel’s allowable overpressure. See “Outlet Piping” on page 1200-47 for a discussion of the calculation of and acceptable values for built-up back pressure.
c. The built-up back pressure from relief valves opening first is not considered a superimposed back pressure on the other relief valves opening later in response to the same contingency.
2. High Operating Pressure
Conventional pressure relief valves should generally not be selected for applications in which the normal operating pressure can exceed 90% of the valve’s set pressure. This is because leakage of process fluid through the closed pressure relief valve, while typically zero at operating pressures below 90% of the set pressure, can increase significantly as the operating pressure increases beyond the 90% level.