Leaking Plug Valve

[crueltobekind]

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

 

[gerhardl]

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.

 

[crueltobekind]

hey thanks for the reply. It is a plug valve. Here are the specs for it.

http://www.herrera.unt.edu.ar/controldeprocesos/Tema_3/Ch2500.pdf

 

[Danlap]

As per your spec, agree with gerhardl that this is a Globe valve rather than Plug.

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

 

[LittleInch]

Wow, never seen one of those before!

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

 

[GHartmann]

Not sure what to call that valve. It does have double "plugs" which look a little like "cage plugs", but it operates on the principle of a globe valve. Whether on not the valve will pass depends on its shutoff classification specified for the valve. Unless there is a soft seat, more than likely the valve cannot be more than a Class III due to the double port (see attached guide).

The other aspect is what is the differential pressure across the valve at shutoff.

http://www.engineeringtoolbox.com/control-valves-leakage-d_484.html

http://spiraxsarco.com/resources/st...l-hardware-el-pn-actuation/control-valves.asp

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.

 

[Compositepro]

The title of the brochure says "globe" valve although it talks about plugs and seats. This is a pressure balanced valve where pressure on one plug cancels the effect of pressure on the other plug, so it can handle high differential pressure. As others have noted, this valve is not design for tight shut-off, it is designed to throttle.

 

[crueltobekind]

Thanks a lot for the replies guys, this has been a great learning experience, I know that this valve will always leak somewhat, I was just hoping to reduce it. I have heard that even though its a equal percentage valve, at very low flow rates, it behaves as a linear valve( not confirmed). I have the CV values and % Stem travel.I have seen online that some characteristics curve for a valves are % of total Cv vs % of stem travel, and %of total FLOW vs stem travel.I was wondering if I plot % of total Cv vs stem travel, will that curve be the same as %of total FLOW vs stem travel?

Also, what type of pressure drops are seen in valves at low openings. <30%?

Thanks a lot for the help guys!

 

[gerhardl]

Masoneilan is a well-known and normally very dependable European (Originally French) construction/series of different regulating valves and actuators, often used for more demanding regulating purposes. (Yes, there is a lot of competitors world-wide with at least equal performance).

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.

 

[SteveWag]

The valve you are talking about has about a 0.5% of full flow leakage when closed (ANSI 70.2 Class II). Any attempt to reduce this leak rate will probably damage the valve unless the valve was to be dissembled and the individual adjustable seats are optimized. I doubt that that would help unless the leakage has increased due to wear.
Steve