Pressure Relief Valves
ORIFICES ( pol 2 ) LIMITS in psi ( bar ) ORIFICES ( pol 2 ) LIMITS in psi ( bar )
D to J ( 0.110 to 1,238 ) 230 ( 16 ) Q ( 11,05 ) 70 ( 4.8 )
K ( 1,838 ) 150 ( 10.5 ) R ( 16 ) 60 ( 4,1 )
L ( 2,853 ) 100 ( 7.0 ) T ( 26 ) 30 ( 2.0 )
M to P ( 3,6 to 6,38 ) 80 ( 5.5 ) – –
For corrosive environments , this relief hole must be routed to a safe location to prevent the disc holder from jamming with the valve guide and also the stem from jamming with the spring adjustment screw . For toxic or flammable fluids , this hole must also be routed to a safe location .
Reduction in the flow capacity of the bellows balanced valve
The presence of the bellows , due to the space occupied inside the valve , reduces its flow capacity whenever it is operating under backpressure conditions , in addition to increasing the fluid flow speed . To correct this reduction in flow
capacity , correction factors ( Kw and Kb ) are used whenever the fluid is a liquid or gas , respectively . The valve manufacturer should be consulted for the value of the correction factor to be applied in the calculations for sizing the nozzle area . Due to the reduction in flow capacity and the use of the correction factor , the calculated minimum required nozzle area results in a larger area than if there were no backpressure or bellows .
Backpressure test on bellowsbalanced valves
Any valve that discharges into a manifold , whether conventional or
balanced , must be subjected to a backpressure test in which the test pressure must be at least 30 psig ( 2.1 barg ). Specifically for balanced valves , care must be taken to prevent bellows collapse due to pressure during the test . This test is a requirement of the ASME Code Section XIII , in paragraph 3.6.2 – 2023 Edition . It is required for conventional or balanced design valves with inlet size greater than 1 ” and which and discharge into a manifold . When the valve is of the conventional type and discharges to the atmosphere , and the bonnet is of the closed type , the backpressure test can be dispensed with .
Conclusion
Balanced valves with bellows should be specified and applied whenever there is a variable backpressure in the manifold or a build up backpressure in the discharge piping at a value greater than the maximum overpressure achieved under relief conditions for a conventional design valve or when the fluid is dirty , viscous , corrosive , and may crystallize or contain solid particles in suspension .