Leaking Plug Valve
Leaking Plug Valve
(OP)
I am stuck with this problem, and could really use some tips.
I am trying to fix a double seated 3" plug valve. the valve is passing when it should be fully closed. the positioners show the valve is closed, but as this is a double seated valve, it seems that the one of the plugs is not making contact with the seat. I am trying to figure out how much to raise or lower a seat in the valve. I know Cv value at different positions (10%, 20% etc) and i know the full flowrate through the valve at a specific pressure. I am trying to figure out the area through which the valve is passing. I do not have the pressure drop across the valve. I am open to making some assumptions, but I am not quite sure what they should be. How can I approach this problem. due to the location and other factors, the valve can not be taken off to measure the internal dimensions. Is there any way a ball park number can be obtained to calculate leak area.
Thanks





RE: Leaking Plug Valve
I believe you are describing what normally would be a globevalve with one inlet, one outlet and two closing members ('plugs' or 'clacks') mounted on a common vertical spindle, and closing against a separate seat for each clack, by operating the common spindle up or down.
(A plugvalve is normally a valve with a cone closing member where the flow is through a hole in the cone, closing by turning the cone. A 'port* is also normally the description of the inlet and outlet openings of the valve body itself (the pipeline connections), further confusing a verbal description.)
A drawing would help, showing the adjustment possibilities, and describing the purpose and function of the s valve.
Apart from that I do not believe it is possible to calculate wich port is leaking, as the variables are endless. Any repair or adjustment would require physical inspectionand tests in my opinion. Best technical solution would be to change the whole valve, as inside damage could be of different types and require quite more than an adjustment.
RE: Leaking Plug Valve
http://www.herrera.unt.edu.ar/controldeprocesos/Te...
RE: Leaking Plug Valve
There is no possible way (until now) to detect how big and where the location of leakage is. X-ray is a still photo that only captured metal parts, and definitely Ultrasonic tester (hearing aid) is subjective depends on the Inspector hearing capability, sensor sensitivity and noises outsides.
Easy way and dependable way to measure leakage rate: Block the valve (downstream side) using positive isolation (or its also may work with Tight Shut Off valve), Close your valve and bleed the pressure between the valve and the isolation, measure the built up pressure within times.
If you are willing to do all the hard work first (with possibility of gaining the same end result which is replace and repair the valve):
1. Ensure that all your Actuator and positioner variables are correct. This can be done by connecting some kind of Valve smart diagnostic (to valve actuator and positioner) and override the valve movement by field operator instead of Control room.
Check all the result is the same with what suggested by OEM valve book (value such as Hysterisis, Seating force, Dead band, etc.).
If for example seating force is less than required, it may lead to passing as well.
2. If point 1 is tackled, and you still find passing. Then valve Plug (SS 316 contact with or without Hard Facing) is for sure damage and must be repaired or replaced.
Please understand that 316 is soft material if stand alone (sensitive for indentation, etc.), and Hard Facing (Stellite, Tungsten Carbide, etc.) is brittle and sensitive to any vibration, shock, and actuator abnormality.
Hopes that provide some insights, good luck.
Regards,
MR
Greenfield and Brownfield have one thing in common; Valve(s) is deemed to "run to fail" earlier shall compared to other equipments
RE: Leaking Plug Valve
Cruel. Your problem is that this is a control valve, not an isolation valve. It is not designed to seal tight. Other than trial and error, i.e. raise one seat a mm or so first, write down which one you changed! Then try it again.if it gets worse then try the other one a but higher after returning your first one to the original position.
Even then, getting this sort of valve to try and seal tight reliably is a waste of time and effort. Use a valve designed to isolate in line with this one.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Leaking Plug Valve
The other aspect is what is the differential pressure across the valve at shutoff.
http://www.engineeringtoolbox.com/control-valves-l...
http://spiraxsarco.com/resources/steam-engineering...
This valve was never intended to be tight shut-off. You may want to ask why the double port style was chosen. This may give you a hint (from Emerson): QUOTE
Double ported designs are typically used in refineries on highly viscous fluids or where there is a concern about dirt, contaminants, or process deposits on the trim.
UNQUOTE
And as a matter of nomenclature, yes the valve has "plugs" as do most "globe type" control valves, but this is not a "plug" valve.
RE: Leaking Plug Valve
RE: Leaking Plug Valve
Also, what type of pressure drops are seen in valves at low openings. <30%?
Thanks a lot for the help guys!
RE: Leaking Plug Valve
I recommend you to contact the producer or a technical qualified representative directly, regarding allowed leakage and possible cause if out of range.
Definitly not a plug valve, but as said in data sheet and by others: a regulating valve of globe type, with two inside ports for fluid, two closing members (plugs or clacks or rounded /regulating type globes), or even also called a two- seated regulating globe valve
Note: it might be that the one port is meant to be left partially open to give a smaller stream. This type of valve is always to be mounted in series with an isolation valve, if not designed to give 100% closure and qualified by producer as isolation valve in itself.
RE: Leaking Plug Valve
Steve