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This article serves as a technical guide for specifying bellows balanced pressure relief valves , detailing when and why to use them instead of conventional valves , particularly for applications involving variable backpressure or challenging fluids .
By Artur Cardozo Mathias
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In an industrial process , there are several types of fluids that have characteristics that can result in the jamming of the moving components of a conventional design pressure relief valve , such as the disc support in the guide , including the stem in the upper spring support and in the spring adjustment screw . They can be with or without a manual lever . Applications subject to backpressure – bellows with balancing function
When a conventional design pressure relief valve discharges into a manifold that is kept constantly pressurized , the pressure existing at the discharge connection ( outlet flange ) of the valve before it opens is called “ constant superimposed backpressure ”. In such cases , if the fluid is clean , non-viscous , non-corrosive and has no tendency to crystallize or polymerize , the user can specify a conventional valve . The valve manufacturer should be informed during purchase of the backpressure value . In this way , the manufacturer makes a differential adjustment ( CDTP ) in the set pressure , that is , the value of the constant superimposed backpressure is subtracted from the value of the set pressure for the valve to open in the process , and under real operating conditions . In turn , the manufacturer will select a spring whose set pressure desired by the user is within the spring adjustment range . When the backpressure is of the variable superimposed type , that is , its value can range from zero ( if the collector is empty at the time of valve actuation ) to a maximum value pre-established during the collector design phase , then the user must specify a balanced pressure relief valve with bellows . The bellows have the function of canceling the effects of the variable superimposed backpressure on the value of the valve set pressure . This cancellation occurs in both the radial and axial directions of the bellows , since the effective area of the bellows is equal to or greater than the sealing area of the disc and nozzle . Consequently , the valve will open at the same value obtained on the test bench ( within the tolerance allowed by the ASME Code – Section XIII – in Table 3.6.3.1- 2 – 2023 Edition ), under ambient temperature
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Balancing bellows . Manufacturer : Crosby . Model JBS .
and atmospheric backpressure , considering the compensation for the cold differential adjustment to correct the effects of the fluid temperature on the mechanical characteristics of the spring . Balanced pressure relief valves with bellows are also recommended for applications in which the build up backpressure is greater than the overpressure value achieved under relief conditions . In a conventional valve , the limit of this type of backpressure is 10 % of the valve set pressure for an overpressure also of 10 %; consequently , one value cancels the other and the disc lifting stroke is not reduced , just as the valve flow capacity is not reduced .
Bellows with isolation function
However , there are applications in which the fluid may be corrosive , viscous , with a tendency to crystallize or polymerize . If such fluids are lodged in the gap between the guide bore and the disc support shaft in a conventional valve , the fluid may fill that gap , rendering the valve inoperative . To avoid these situations , it is recommended to specify balanced valves with bellows , with the bellows having an isolation function . Even when the backpressure is of the constant superimposed type , but the fluid is dirty , corrosive , containing solid particles in suspension , with a tendency to crystallize or polymerize or very viscous , it is also recommended to specify a balanced pressure relief valve with bellows . In these applications , even if the backpressure is of the constant superimposed type , there is no need for
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