Conventional safety relief

Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes of the back pressure on the valve. 

Figure 2. Typical conventional safety relief valve.

The operation and characteristics of a conventional safety relief valve are illustrated diagrammatically in Figure 3. The action of the valve as pressure rises from the initial normal operating pressure (assuming no back pressure) is described below. The effect of back pressure on PR valve operation is described later. 

Conventional safety relief valves, as usually installed, produce unsatisfactory performance w hen variable back pressure exists [10, 33]. See Figure 7-6. The same variable back pressure forces affect the set pressure release also. At low back pressures, the valve flow falls rapidly as compared with the flow for a theoretical nozzle. See Figures 19 and 20 in Ref. [33a]. 

Safety Relief Valves (SRVs) Conventional safety relief valves are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting of the valve is reduced by the amount of superimposed backpressure expected. Higher built-up backpressures can result in a complete loss of continuous valve relief capacity. The designer must examine the effects of other relieving devices connected to a common header on the performance of each valve. Some mechanical considerations of conventional relief valves are presented in the ASME code however, the manufacturer should be consulted for specific details. 

Where conventional Safety Relief Valves are used, the relief manifold system should be sized to limit the built-up back pressure to approximately 10% of the set pressure... 

CONVENTIONAL SAFETY-RELIEF VALVE CAP, SCREWED------fgfe... 

Effect of Back Pressure on Set Pressure of Conventional Safety-Relief Valves... 

Figure 5-3. Conventional safety relief valve. (Reprinted courtesy of the American Petroleum Institute.)...

Figure 5-5. Constant back pressure sizing factor, for conventional safety relief valves in vapor or gas service. (API Recommended Practice 520, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1, 5th ed., 1990. Reprinted courtesy of the American Petroleum Institute.)...

The maximum allowable back pressure (gauge) for a conventional safety relief valve is 10% of the set pressure (gauge). If the maximum allowable back pressure is exceeded during any relief situation, the size of the relief lines or headers should be increased until the back pressures are sufficiently reduced. ... 

Comment Conventional safety relief valves are for applications where excessive variable or built-up backpressure is not present in the system into which the valve discharges (Fig. 5.191). 

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