Leakage in Ball valve

[thought2007]

Hello!

I am finding leakage in ball valve,when it is in closed condition.
The Teflon seat is loaded by spring force from either end of ball(Inlet & outlet flow).
However experts opinion is at cold flow(-20 degF to +275degF)),Teflon doesnt give good sealing.

My question

1.what are the characteristics of seat(PTFE) & Ball (steel 440C) which drivers for leakage
2.How to determine leakage in between Ball & seat.

Would appreciate your inputs for my thought process

Thanks

John

 

[israelkk]

The problem to my opinion is not the Teflon material. It is usually as a result of the seal pass over the hole in the ball. If the hole edges are not carefully designed and machined/grinded/polished it can scratch the Teflon seal with every pass of the seal over the hole. Dirt, dust and small particles embeded on the ball can scratch the seal too. It is a good practice to use a fine filter on the valve inlet and outlet and use clean fluid.

Depends on the size of the leakage is it a liquid or gas? if it gas and it is not too excessive you can immerse the valve inside a vessel filled with aliquid such as water of alchol (EPA) (if you can not use water) and countes the bubbles or accumulate them into a glass vessel. Or use leak detector.

 

[zdas04]

What you described sounds like a trunnion ball valve. These come with body bleeds. With the valve shut, you can pipe the body bleed to a lab flow meter. Also you can put a pressure gauge on the body to see which seal is leaking. It is pretty unusual for both seats to leak, but it happens.

David

 

[gerhardl]

There is a number of reasons why a ball valve PTF sealing may leak.

The main point is, however, that at below minus 15 -20 deg C, here down to almost minus 29 deg C, you will tend to go outside the 'normal' application for both metallic materials (House and ball etc.) and soft sealing materials.

Not that this is not relatively wide available for low temperatures, but standard products are not normally tested and approved for this temperature.

Checkpoints:
a) Are you sure the valve is supplied and guaranteed from supplier for existing, media, pressure and temperature?
b) If not: get correct valve, if yes, size and pressure class and operating conditions? Ask supplier for leakrate at this condition.
d) Leakrate acceptable?

...etc.

PTF is not a 'constant quality' supplied material, but may vary in structure. This may give different shrinkage and swelling (fluid absorbing), and aging (hardening) over time.

Is ice (water ice) or solids from fluid (abrasion on seals) an issue?

The seal itself could, as you state, of course be the problem alone, but the other points must at least be eliminated as problem sources.

 

[Shimples]

I would think the first step would be to strip the valve down and inspect the ball to seat sealing area. You can check this with engineers blur to ensure there is no deformation in the seat.

 

[thought2007]

Hello,

Thanks to all for sharing knowledge with me.

I completely agree with Israelkk, however undulations after machining between ball & seal also drives for the leakage cause.

I am unable to correlate leakage with machining errors
between contact surfaces(Teflon seal& Spherical ball).

Is there any ways to build analytics for leakage,cuz presently working on concept design,need to ensure customer,leakage value when its is in closed condition

John

 

[NGiLuzzu]

thought2007,
may I ask some more detail about what You generically call "leakage"?
How do You detect and measure that?
Are you testing with gas or liquid?
Which are the test conditions (pressure, temperature, etc.) and how do their change affects leakage? If You increase pressure, for example, does the leak rate increase too or not?

For general reference about leak testing, may be you'll find interesting to take a look at thread408-150132 within this Forum.


Thanks and Regards, 'NGL

 

[thought2007]

Anegiri,

Its internal leakage between Spherical ball & Teflon seal

When valve in closed condition(isolation of flow)
leakage is admissible 0.5cc/min at 70psid,fluid temperature -20degF to +275degF.

I am working on analytics(concept design) for liquid leakage not testing.

Presently working on concept design,indeed specification states above mentioned leakage admissible requirements.

thanks&regards
John