Expanding Gate Valves - Bad Experiences 1

[monaco8774]

I am presently working on a facility where they have a large number of valves from an italian manufacturer (who shall remain nameless). Apart from problems with real variable stiffness from one valve to another we have a major issue with the double seated expanding gate valves. These are used in a pigging service. They pass a pressure test and then whenever we have pigged for the first time they just leak. We have even pulled these valves out sent them off cleaned them up at the manufacturer and reinstalled them, they pass a pressure test clean, and then after first pig run, same thing happens they leak. The valves as found condition is always in good condition but they are extremely difficult to close , its a long shot but I was wondering if anyone could shed some light since I have never come across an expanding double seated gate valve before. One thing is there is a bleed valve in the cavity that we cannot use as it is not piped up to anything, is it necessary to bleed the cavity to make these valves seal?

 

[NGiLuzzu]

monaco8774,
what is the fluid passing through those valves? At what temperatures and pressures?

How did the Manufacturer analyze the occurrence?

As you talk about "cleaning", the problem could be related to the dirt content within the fluid, of course... but in any case some more information about the service may help answering your questions.

Regards, 'NGL

 

[monaco8774]

Sorry about delay in reply have been on a location with no access to internet.

Pressures circa 270 bar(g)
Temperatures around 60Celsius
Fluid is hydrocarbon gas with no condensate downstream of glycol contactor so very low water dew point
Some solids in line due to internal corrosion in yard when pipes were left unpreserved

 

[11echo]

It sounds like these are "double block and bleed" type valves. Once closed the bleed valve is opened to ensure there is no leakage between the two seats at the gate. To get a "good" seal it is not necessary to bleed this cavity, it's just for insurance in the double block and bleed philosophy.
My question to you is do you see any damage to the gate after this pig passes through? It sounds like the gate is not being fully pulled out of the stream and the pig is clipping the gate as it passes through...would also explain that after this event the valve is difficult to completely close. With a slightly damaged gate the valve would be difficult to completely seal. My $0.02 good luck!

 

[HotStab]

monaco8774,
I would recommend taking a look at the seat x body sealing system. Some sealing systems are not robust enough to a piggable valve. Delicate seals can be damage by the dirty pushed into the seat x body gap due to pig action.
Regards,
HCKS.

 

[DrJim]

I trust these are the old WKM syle gate valve.
A couple questions.

Are the valves installed with the gate portion of the split gate assy downstream? The part that attaches to the stem is typically called the gate and the other half is called the segment. If installed backward, high torque is guaranteed.

What size are the valves? Those over 4" bore normally have levers on the side of the gate assy to hold the segment close to the gate during operation. This lever can be out of trim of just screwed at the manufacturer - if so, sorry.
Those 4" and less have a couple of springs holding the gate assy together. Sometime they work well and other times they do not.

Are the two sides of the gate parallel when in the split apart position? If not, a seal can be challenging.

They call these dual block and bleed valves - pressure from both sides and vent the center cavity. Do not bet on it. Most never reach that standard, at best they are downstream sealing. Sometimes you can pressure the center cavity and have both seats seal - whatever that proves.

In you application is sounds like the pig is focing debris between the seat and gate and messing up the seal and possible not allowing the gate to colapse when being operated - a possible source of the high torque. Sometimes cycling the valve will wipe the debris out and other times it will scratch the gate and a regrind is needed to solve the problem.

Another point, the gate can be machined wrong causing it to bind on the stem and stop the gate from sealing properly.

And the potential problems are endless for this type of valve.

Let us know what you come up with.