Conventional pressure relief valve

If the superimposed back pressure is less than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is unaffected and back pressure is not a factor. However, builtup back pressure on a conventional pressure relief valve will affect its flow capacity and operating characteristics, and should not exceed 100% of its set pressure. If total back pressure (superimposed plus built-up) is greater than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is affected, and total back pressure is incorporated into the sizing procedure. Any back pressure reduces the capacity of a conventional PR valve in liquid service, and... 

Figure 4.1 Conventional pressure relief valve with a single adjusting ring for blowdown control. (Courtesy Anterican Petroleum Institute, 1220 L Street, NW, Washington, D.C.)...

Conventional pressure relief valve (vapor service)... 

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. 

Superimposed Back Pressure - Is the pressure at the outlet of the pressure relief valve while the valve is in a closed position. This type of back pressure comes from other sources in the discharge system it may be constant or variable and it may govern whether a conventional or balanced bellows valve should be used in specific applications. 

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 PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

The back pressure developed at the downstream section of any pressure-relief valve connected to the same headers should not exceed the allowable limit, i.e., 10 percent of the set pressure in psig for the conventional type and 40 to 50 percent of the set pressure in psia for the balanced type valve. 

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]. 

Before initiating any calculations, it is necessary to establish the general category of the pressure relief valve being considered. This section covers conventional and balanced spring-loaded types. 

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. 

API Standard 527, Seat Tightness of Pressure Relief Valves This standard describes tests with air, steam and water to determine the seat tightness of metal- and soft-seated PRVs. Valves of conventional, bellows- and pilot-operated designs are covered. Acceptable leakage rates are defined for gas, steam and liquid. 

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... 

The safety valve is a pressure relief valve that is designed to open fully, or pop, with only a small amount of pressure over the rated limit. Where conventional safety valves are sensitive to downstream pressure and may have unsatisfactory operating characteristics in variable backpressure applications, pressure-balanced safety relief valve designs are available to minimize the effect of downstream pressure on performance. 

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

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. ... 

Mechanical Relief Devices. The water seals discussed above in Section 9.1.10.1 are effective only at very low differential pressures. After the compression of chlorine, and particularly in liquefaction and storage systems, more conventional relief devices, rupture discs and pressure relief valves, are used. With some fluids, there is a simple choice to be made between discs and valves. While the former are less likely to permit bypassing of small quantities of fluid, they are destroyed when they open. A release will continue even after the pressure on the system drops below the set point of the disc. Relief valves have the opposite characteristics. 

The SPWR adopts a water-filled CV. The RPV is covered with a water-tight shell. A mirror insulator of laminated thin stainless steel plates is mounted inside the shell. Heat loss is estimated to be below 1 MW. The space between the water-tight shell and the RPV corresponds to a drywell of the suppression type CV of conventional BWRs. To allow for a pipe rupture in this space, the shell is equipped with pressure relief valves. The advantages of the water-filled CV are the compactness of the reactor plant and ease of application of a passive decay heat removal system. 

---