Conventional safety valve

Tlie safety valve is similar to the relief valve except it is designed to open fiillv, or pop, with onlv a small amount of pressure over the rated limit. Conventional safety valves are sensitive to dovvmstream pressure and niav have iinsatisfactorv operating characteristics in variable back pressure applications. The balanced safety relief valve is available and minimizes the effect of dovvmstream pressure on performance. 

Figures 17. Effect of back pressure on (A) conventional safety valve and (B) piston type valve.

A bursting or rupture disk is a pressure relief device that protects a vessel or system from excess pressures. They have been commonly used in aerospace, aviation, defense, nuclear, and oilfield applications often as a backup device for a conventional safety valve. In this instance, if the pressure increases and the fitted safety valve fails to operate, the rupture disk will burst as required. The discs are usually made from thin metal foil, and gold has been used in some instances because of its ductility and resistance to corrosion. Gold discs fitted to liquid ammonia tanks, for example, have shown good durability in this application compared to other metals. The use of gold in this application was reviewed in the 1970s when this industrial application for gold was more common [10]. 

Safety Relief Valves Conventional safety relier valves (Fig. 26-14) are used in systems where built-up backpressures typically do not exceed 10 percent of the set pressure. The spring setting or the valve is reduced by the amount of superimposed backpressure expecied. Higher built-up backpressures can result in a complete loss of continuous valve 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. 

Safety Relief Valve - A safety relief valve is an automatic pressure-relieving device suitable for use as either a safety valve or relief valve, depending on application. (In the petroleum industry it is normally used in gas and vapor service or for liquid.) Safety relief valves are classified as "Conventional" or "Balanced", depending upon the effect of back pressure on their performance. 

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. 

Spring Pressure - The spring pressure is equal to the set pressure minus the superimposed back pressure for a conventional PR valve. For a balanced bellows safety relief valve, the spring pressure equals the set pressure. 

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. 

Back Pressure - In the case of a conventional PR valve, the "maximum baek pressure" column should specify the maximum superimposed baek pressure (kPa gage) under nonflowing conditions. If the diseharge is routed to the atmosphere, or to a safety valve header where the pressure is essentially atmospherie under no flow, the maximum back pressure should be specified as zero. 

The type of safety valves employed (either conventional or others) in a specific collection system dictates the level of back pressure in that system. In flare headers where multiple discharges exist, each safety valve must be checked so that it does not exceed its allowable back pressure. 

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]. 

For conventional valves, pressure drop or variations in back pressure should not exceed 10% of set pressure. Because most process safety valves are sized for critical pressure conditions, the piping must accommodate the capacity required for valve relief and not have the pressure at the end of vent or manifold exceed the critical pressure. Designing for pressure 30% to 40% of critical w ith balanced valves, yields smaller pipes yet allows proper functioning of the valve. The discharge line size must not be smaller than the valve discharge. Check the manufacturer for valve performance under particular conditions, especially with balanced valves w hich can handle up to 70% to 80% of set pressure as back pressure. 

Conventional nuclear reactors and advanced breeder reactors were America s primary energy strategy since the 1950s to resolve the fossil fuel problem but when a reactor accident occurred in 1979 at Three Mile Island in Pennsylvania, public and investor confidence in nuclear fission dropped. The accident was triggered by the failure of a feedwater pump that supplied water to the steam generators. The backup feedwater pumps were not connected to the system as required, which caused the reactor to heat up. The safety valve then failed to act which allowed a radioactive water and gas leak. This was the worst nuclear power accident in the U.S., but in this accident no one was killed and no one was directly injured. At Three Mile Island faulty instrumentation gave incorrect readings for the... 

Constant superimposed backpressure Usually backpressures that occur when a safety valve outlet is connected to a static pressure source and doesn t change appreciably under any conditions of operation. In this case, conventional valves may be used if the valve spring setting is reduced by the amount of the constant backpressure (Figure 3.8). 

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. ... 

Various proposals have been made for ways to combat these problems. One is to resettle surplus population in new lands. The problem is that most of the suitable land is already occupied. Efforts to put people into the rain forests of Brazil and Indonesia have not worked well. The land was not very suitable for conventional farming. Migration to other countries has been used as a safety valve in the past, as in the case of Ireland. There is an ethical question over whether it is right to expect a nation that has kept its population growth rate under control to absorb the surplus from ones that have not. 

---