Wastewater Plug Valve

[JWShira]

Is it correct to expect a plug valve to hold full test pressure in both flow directions?
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?

 

[Joesteam]

I guess it depends upon exactly the manufacturer and model used, but in general most plug valves I have run into should hold the rated leakage in both directions.

Joe Lambert

http://www.control-associates.com/

 

[gerhardl]

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).


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.

This should not be the case for your type of valve.

Something is wrong here! Are you talking about the same type of valve, for the same nominal pressure rating?

 

[JWShira]

My understanding has always been as you describe, that a typical resilient seat plug valve, designed for a typical municipal wastewater environment, should be able to hold test pressure regardless of which side of the valve pressure is applied. In this case, the contractor needs to test a section of new force main, and pressure will be held by two plug valves, one at each end of the new force main section. The contractor is claiming something I'd never heard before, and in all my searching of manufacturers data, I've not found anything to indicate that this type of plug valve is "pressure tight" in only one flow direction. I think what I have is a contractor who is grasping at straws in case his test fails, he wants to blame a valve.

 

[gerhardl]

A PS! Sure that the main is not tested at about 1,5 times nominal pressure?

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.

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.