VALVE TESTING
Rethinking valve testing protocols
A thought-provoking article that challenges conventional valve testing methods and proposes a more efficient , cost-effective and informative protocol for manufacturers and end-users alike .
By Dr . S Vijayakrishnan for improved efficiency
Every day , thousands of valves leave manufacturers ’ facilities to end users after final inspection and testing . Typically , this involves conducting the following tests , which are often witnessed by customers and / or third-party inspectors : 1 . A test on the valve ’ s capability to hold fluid under maximum rated pressure without structural damage or deformation .
2 . A test on the valve ’ s capability to hold fluid under maximum rated pressure without leakage to the environment .
3 . A test on the valve ’ s capability to hold the fluid at rated maximum pressure with specified maximum leakage past the closed valve to the downstream side . In the case of control valves , the following additional test may also be done : 4 . A test to confirm the rated capacity ( Cv ) of the valve and / or its flow characteristic . Tests ( 1 ) and ( 2 ), often conducted as a single test , concern operational safety and are therefore necessary . These tests can detect serious defects that may pass undetected through the manufacturing process despite checks . However , test ( 4 ) is not conducted routinely but only at the request of the user , often for an additional fee . The discussion in this article focuses on test ( 3 ), the seat leakage test .
Seat leakage
What constitutes seat leakage ? It is the passing of a fluid across a closed valve , which is not desirable .
What determines the seat leakage of a valve ? Typically , the following factors influence the leakage capability of the valve : valve type , size / flow capacity of the valve , seating materials , manufacturing processes ( particularly of the seating components ), pressure across the closed valve , and age of the valve . Tests can be conducted at the prototype stage to determine the seat leakage for a specific valve design . Further units of the valve manufactured under the same design and process are bound to conform to the same range of seat leakage , within a specified range . Since the affecting parameters are defined and guaranteed by the manufacturer with their internal records , there is really no need to individually test each valve for seat leakage . However , there is another small chance that the valve may leak beyond the specified range . This occurs when the valve does not close fully mechanically . Despite the controlled materials and processes used , a minor defect that escaped detection may cause this to happen .
How are on-off valves used in service ?
Valves used for on / off applications are typically kept fully open or fully closed . Well , when fully closed , do users need or expect the flow to completely stop ? Yes , in some locations . For example , drain valves , custody transfer valves , line isolation valves , and the like . Users would then specify valves with
22 Valve World September 2024 www . valve-world . net