Is it correct to expect a plug valve to hold full test pressure in both flow directions?Balance Valves
In this case, I'm looking at a resilient seat eccentric plug valve (6") in a new wastewater force main. Contractor claims full test pressure (150 psi) can only be held in one direction, and not against the "back" of the valve. Is this correct? Instrumentation Ball Valves

Some double eccentric plug valves will have poor sealing, but in my knowledge only if the sealing is metallic or not proper soft sealing.

The construction itself (double eccentric plug) are in general constructed to give a long-standing excellent seat-sealing both ways.

In general a proper constructed valve, knife-gate, gate or double eccentric plug, should be good for the nominal operation pressure both ways, and able to be tested for this
(in your case 150LB if this is the valve nomination, or for 1,1 times this).Needle & Gauge Valves


Other type of valves, for instance some knife-gate valves, are constructed in such a way that the knife gate bottom (port) sealing is below lowest port  opening section, and the pressure from the liquid is necessary to give a proper sealing by pressing the gate towards the port sealing, eg. one sided sealing, and tendency to poor sealing either way at low pressures.Valve seats are normally fabricated or by norm required and guaranted to keep droptight up to an operational pressure (when closed) at max 1,1 times operational (normal) pressure. Whith open valve the house of a valve is tested at 1,5 times OP.

Occasionally (depending on construction) some valves may be allowed (by manufacturor) to be tested with closed seat up to the pressure/test normally used for pipelines and valve housing.Instrument Manifolds

In this case the supplier may well be right: pressure in one direction (normal flow direction) could be allowed upped to 1,5 , and construction require that the opposite direction max is kept at 1,1 to avoid seat damage.

Unless the valves are especially required in contract to allow for the valves to be tested with closed seats at normal pipe test pressure and in both direction (about 1,5 times nominal), you will have to use blinds for the pipeline test, and accept the valves as normal qualified valves.Cast Steel Valves

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