VALVE TESTING zero leak capability ( typically soft-seated valves ) or use devices like double block and bleed ( in critical services ). In all other locations , what most users really need is for the flow to be “ almost ” stopped , not necessarily “ completely ” stopped . This implies that the actual seat leakage of the valve is rarely important in most applications - if the valve stops the bulk of the flow , that is enough ! Another aspect is the disparity in conditions under which a new valve is tested for seat leakage and what the valve experiences in service . One of the most common differences is the way the valve is “ fully ” closed . While a factory test under controlled , ideal conditions would ensure proper closure , in actual use , the operator may hardly ever close a valve fully . Except in the case of quarter-turn valves , it may be difficult for the operator to gauge whether a valve is fully closed based solely on the tactile feedback of the operating torque they apply . To compound matters , in many services handling “ not so clean ” fluids , debris in the media settled at the seating area might obstruct full closure . Therefore , in actual usage , a “ fully closed ” valve may not be closed fully , which makes the factory seat test irrelevant in actual use .
Prototype tests
1 . Tests that establish the range of seat leakage for a valve design ( for on-off valves ; for control valves , seat leakage should , ideally , be irrelevant ).
2 . Tests that establish flow capacity and characteristic ( for control valves ; for on-off valves , flow capacity is rarely important as “ type and size ” effectively define the ballpark range of capacity , which is almost always sufficient . A Cv test for on-off valves should be optional ).
3 . Life tests to establish reasonable service life ( number of operation cycles ) during which the valve can be expected to retain all design parameters . Typically , seat leakage tests are conducted using specified fluid , conditions , and test pressure . However , in practice , users
Seat leakage Differential pressure (% of rated pressure ) with increasing differential pressure , but in some valve designs , it could decrease - as , for example , in pressure-assisted sealing valves such as unidirectional , metal-seated knife gate valves . Therefore , seat leakage data at different test pressures , rather than at one test pressure , would be more logical . If these leakage flows are expressed as a percentage of the maximum flow that the valve will pass at the same differential pressure , the data would be more comprehensible to most users as well . Tests may be conducted on multiple samples of the same size and design , and the maximum leakage observed in any of the samples could be used to determine the conforming seat leakage class . For example , a seat leakage test would involve obtaining the following data for the valve sample :
10 % 25 % 70 % 75 % 90 % 100 % Max Class Max 0.01 % 0.01 % 0.02 % 0.03 % 0.07 % 0.09 % 0.09 % A 0.1 % 0.1 % 0.2 % 0.35 % 0.4 % 0.6 % 0.9 % 0.9 % B 1.0 % 1 % 2 % 2.1 % 3.2 % 3.6 % 4.1 % 4.1 % C 5.0 %
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A logical testing protocol
Considering the above , a more logical valve testing protocol may be followed . It may be divided into prototype tests ( done on a set of samples ) and production tests ( done on each valve shipped to users ).
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operate on-off valves at any pressure from insignificant levels ( say , as in gravity flow ) to the rated maximum pressure of the valve . For most valves , the seat leakage will vary according to the differential pressure applied across the seat . Usually , leakage will increase |
In the above example , the figures in italics are actual test data from three different valves , and the valve with data in the second row would have a seat leakage designation of Class B ( seat leakage max 1 %). |
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