Pressure relief valve rated relieving capacity

Back-pressure can affect either the set pressure or the capacity of a relief valve. The set pressure is the pressure at which the relief valve begins to open. Capacity is the maximum flow rate that the relief valve will relieve. The set pressure for a conventional relief valve increases directly with back-pressure. Conventional valves can be compensated for constant back-pressure by lowering the set pressure. For self-imposed back-pressure—back-pressure due to the valve itself relieving—-there is no way to compensate. In production facility design, the back-pressure is usually not constant. It is due to the relief valve or other relief valves relieving into the header. Conventional relief valves should be limited to 10% back-pressure due to the effect of back-pressure on the set point. 

When the rupture disk is installed on the inlet side of the pressure relief valve (see Figures 7-10, 7-11 and 7-12), the ASME code requires that for untested disk-valve combinations that the relieving capacity of the combination be reduced to 80% of the rated relieving capacity of the pressure relief valve [ 1 ]. 

Safety-Relief Valve this is an automatic pressure-relieving device actuated by the static pressure upstream of the valve and characterized by an adjustment to allow reclosure, either a pop or a non-pop action, and a nozzle type entrance and it reseats as pressure drops. It is used on steam, gas, vapor and liquid (with adjustments), and is probably the most general tyqDe of valve in petrochemical and chemical plants (Figures 7-3, 7-3A, and 7-4). Rated capacity is reached at 3% or 10% overpressure, depending upon code and/or process conditions. It is suitable for use either as a safety or a relief valve [1,10]. It opens in proportion to increase in internal pressure. 

Relief Valve a relief valve is an automatic spring loaded pressure-relieving device actuated by the static pressure upstream of the valve, and which opens further with increase in pressure over the opening pressure. It is used primarily for liquid service [1,10] (Figure 7-1A and 7-1B). Rated capacity is usually attained at 25 percent overpressure. 

The SCS relief valves are sized, based on an inadvertent safety injection actuation signal (SIAS) with full pressurizer heaters operating from a water-solid condition. The analysis assumes simultaneous operation of four SIS pumps and one charging pump with letdown isolated. The resulting flow capacity requirement for water is 3934 gpm. The analysis in Section 5.2.2.10.2.1 assumed that either SCS relief valve relieved water at this rate. 

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