Balanced-bellows reliefs

Balanced Bellows Safety Relief Valve - A balanced safety relief valve incorporates means for minimizing the effect of back pressure on the performance characteristics opening pressure, closing pressure, lift and relieving capacity. This is usually achieved by the installation of a bellows. 

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. 

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 4. Typical balanced bellows safety relief valve.

Figure 6. Forces acting on discs of conventional and balanced bellows safety relief valves.

In general, the total back pressure on a balanced bellows pressure relief valve (superimposed plus built-up) should be limited to 50% of set pressure, because of the marked effect of higher back pressures on valve capacity, even when appropriate correction factors are used in sizing. In exceptional cases, such as a balanced bellows PR valve discharging into another vessel, total pressure up to 70% of set pressure may be used. 

VARIABLE OR CONSTANT BACKPRESSURE SIZING FACTOR. K,. FOR BALANCED BELLOWS SAFETY RELIEF VALVES (VAPORS AND GASES)... 

Figure 17. Variable or constant back pressure sizing factor, Kw for 25% overpressure on balanced bellows safety relief valves (liquids only). The curve represents conqiromise of the valves reconunended by a number of relief valve manufacturers. This curve may be used wiien the make of dw valve is not known. When the make is known, the manufacturer should be consulted for the correction factor.

For balanced bellows pressure relief valves, the maximum superimposed baek pressure should be specified. (For balanced bellows valves discharging to a safety valve header which is at atmospheric pressure under non-flowing conditions, the maximum back pressure is zero.)... 

In the case of a balanced bellows pressure relief valve, to the maximum pressure permitted by considerations of bellows and bellows bonnet flange mechanical strength. This maximum pressure may be obtained by applying the following correction factor to the maximum back pressure listed for 38 °C. 

Other types of pressure-relief valves do not depend upon the back pressure for their performances. However, to ensure that the safety valves work at their maximum capacity, back pressure is limited to 50 percent of the relief valve set pressure. In the balanced bellows type valve, the spring does not act directly on the disk. Instead, it serves on a bellows first, which in turn acts on the disk. In case of the piston type, it works on the same principle as the bellows type, except that the bellows is replaced by a piston (see Figure 17B). The cross-sectional area of both the piston and the bellows is the same as the inlet nozzle of the valve and the effect of the back pressure on the top and the bottom of the disk creates equal balancing forces. That is, P,A is always equal to F, as shown in Figure 17B. 

I here are three types of relief valves conventional, balanced-bellow s, and spring loaded. 

Figure 13-4. Balanced-bellows relief valve. (Couriesy of APi. ...

Figure 13-5. Operation of balanced-bellows sofety-relief valve. (Reprinted with permission from APi 520.1...

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

The set points for pilot-operated and balanced-bellows relief valves are unaffected by back-pressure, so they are able to tolerate higher backpressure than conventional valves. For pilot-operated and balanced-bellows relief valves, the capacity is reduced as the back-pressure goes above a certain limit. 

Figure 13-8 Bock-pressure sizing factor K, for balanced-bellows pressure relief valves—vapors ono gases.

When using a balanced/bellows relief valve in the sub-critical, use Equations 7-18 through 7-22 how ever, the backpressure correction factor for this condition should be supplied by the valve manufacturer [33A]. For sub-critical, conventional valve ... 

There are two general categories of relief devices (spring-operated and rupture discs) and two major types of spring-operated valves (conventional and balanced-bellows), as illustrated in Figure 8-7. 

For the balanced-bellows design the bellows on the backside of the valve seat ensures that the pressure on that side of the seat is always atmospheric. Thus the balanced-bellows valve will always open at the desired set pressure. However, the flow through the balanced-bellows relief... 

Figure 9-4 Backpressure correction Kb for 25% overpressure on balanced-bellows reliefs in liquid service. Source API RP 520, Recommended Practice for the Sizing, Selection, and Installation of Pressure Relieving Systems in Refineries, 6th ed. (1993), p. 35. Used by permission of the American Petroleum Institute, Washington, DC.

The backpressure correction Kb is used only for balanced-bellows-type spring reliefs and is given in Figure 9-4. This correction compensates for the absence of backpressure on the back... 

The backpressure correction is not required because this is not a balanced-bellows spring relief. Thus Kb = 1.0. 

Kb is the backpressure correction and depends on the type of relief used. Values are given in Figure 9-5 for conventional spring reliefs and in Figure 9-6 for balanced-bellows reliefs. 

Thermal Relief Semi Nozzle Full Nozzle Snap Acting Proportional High Performance Metal Seated Sort Seated Steam Valve Liquid Valve Gas Valve conventional Valve Balanced Bellow... 

Conventional balanced bellows and pilot operated safety relief valves... 

Figure 5-6. Back pressure sizing factor, K, for balanced-bellows pressure 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 permitted distinct values for the maximum allowable pressure drop depend, among other things, on the exact design of the valve. It is common practice to limit the maximum permitted pressure drop for the inlet pipe to 10% of the set pressure. For the outlet vent line from a balanced bellows safety relief valve a 30% pressure drop with respect to the set pressure is regarded as permissible. The manufacturers of relief valves provide the exact limit values. Therefore the data given here should be looked upon as recommendations without any mandatory character. 

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