Globe valve leak

[Shiras]

Hi All,

We have installed a 6'' #600 hand wheel operating globe valve in a reverse osmosis plant.
Valve shall be normally at closed condition. Inlet side shall remain closed (10 bar maximum) outlet side holding pressure 55-60 bar while running the system. We noticed back flow from out let to inlet & pressure is exceeding at inlet side. As per valve drawings inlet flow is through above the disc & outlet is holding the pressure below the disc. We have already replaced one valve due to this problem. Please clarify whether it is happening because of any design fault or valve defect.

In my view point, globe valve only can use to hold the pressure at inlet side. Is it true. Please help me to resolve this issue.

 

[Latexman]

A globe valve is not a wise choice if no leakage is critical. A globe valve is designed for control. One of the best valves for total shut off is a ball valve.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[LittleInch]

Can you post the valve drawing as this looks quite strange, but might be correct. In a classic globe valve I would tend to agree with you, but there are many different forms and versions. However latexman is correct, globe valves are not normally very reliable at sealing closed for long periods of time or at very low leakage rates. You might need to use or add a better isolation valve on the outlet side of your globe valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

Always back up a globe with a ball, gate valve.

 

[LittleInch]

I wish you wouldn't double post - see

http://www.eng-tips.com/viewthread.cfm?qid=383885

as this has a diagram of the valve.

It looks a little odd - I've never seen what appears to be a two stage opening, but in this case the operation looks correct. When working as designed, the inlet big disc is held against the seat whilst the initial upward movement of the spindle lifts up the small disc and allows fine control over the initial flow. Then more turns leads to much larger flow as the big disc is lifted off its seat.

In your case it doesn't work well as the force required by the spindle down is high, the contact point with the big disc is small (the small disc) and any contaminants or particles will result in a passing valve.

Do you need this sort of valve? If not I would replace it with a more normal type or add an isolating valve.

I would love to see the valve characteristics...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

No room to insert a blind plate with thickness suitable for 60barg +?

 

[bimr]

When ordering a valve, it is essential to be clear about the functionality required – valves which seal against the
seats in the closed position only or valves which seal against the seats in the open and closed positions; unidirectional or bi-directional sealing.

The configuration of the flow path is normally only suitable for uni-directional flow and globe valves should be mounted in the line such that flow is in a direction from beneath the disk.

 

[bimr]

http://www.seridium.com/downloads/4_Guidance_on_Valve_Type_Selection.pdf

 

[Shiras]

DEar All,

Sorry i was not in the situation to use internet. Thanks for your suggestions & support.

Little Inch & Latex man, hope that the ball valve is not suitable in this line. Because outlet holding a pressure (60 Bar) more than inlet (10bar max.). Am not sure about it. We are only traders and repeatedly the replaced valve also reported the same issue, so that i came to this forum for a solution. I just want to know if the valve installed in a reverse direction, is there wil be any flow? Your continuous support is requested.

 

[LittleInch]

Shiras,

As said, this particular valve looks quite strange to me as it is a two part flat disc designed only to work with pressure on the inlet side higher than the outlet. A more normal globe valve with pressure normally coming from under the plate subject to reverse pressure differential would probably seal better, but you may find opening it under reverse differential pressure difficult or impossible.

If you swing this valve around it would seal better under reverse pressure differential, but if used in the "normal" direction of flow would be difficult to control. It looks to me to fundamentally the wrong type of valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

How is this valve used?

Does it control the flow at some partially open position? Or, is it either wide open or all the way closed?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Shiras]

Dear All,

Me too confused, however we have decided to do seperate hydrotest for a better coclusion.

 

[BigInch]

Best not to hydrotest the valve.

 

[bimr]

Globe valves are not bi-directional so the flow is in a direction from beneath the disk.

Why do you have the arrows on your drawing showing flow opposite of pressure?

 

[Shiras]

in some cases inlet port shall be applied through above the disc

 

[LittleInch]

Bimr,

Normally i would agree, but look closely at the valve. There are two discs. The smaller one relies on the larger disc being forced intro the seat by pressure. Only when you open up the valve by more than a couple of turns does it pull the big disc off the seat. All quite strange and nothing like your admirable section drawing.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

It is critical to this discussion to know, what is the primary function of the valve. Is it a control valve or an ON/OFF valve?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[bimr]

Double Disc Globe Valve : It features two discs bearing on separate seats spaced apart on a single shaft, which frees the operator from stresses set up by the conveyed fluid pressing into the valve. It is principally used on control valves and pressure regulators for steam and other gases. Tight shut-off is not possible.

It seems to be a misapplication of the valve since a double disc valve is not capable of the tight shut-off that the OP is requesting.

 

[georgeverghese]

May be this globe valve is being used for 2 purposes at the moment

a) Some kind of throttling operating where 60barg water is being letdown into the 10barg stream

b) Isolation between 60barg and 10barg when not in use.

Function (b) appears to have not been engineered properly - it is not possible use a single block valve of ANY type as positive isolation. Leakage is to be expected, but the bigger concern is misoperation.

If both these functions are still required, then the solution lies not in the type of valve, but in the excess pressure control and overpressure protection measures on the 10barg side.

 

[Shiras]

george varghese you are correct the operations is same as you described.