What causes water leak with no pressure? 1

[floattuber]

We have a cartridge water filter that's been giving us a bit of a problem lately. The body (looks like a tall pitcher complete with handle) screws onto a top cover and seals with an O-Ring sandwiched between the two. It looks like it's leaking out of the O-ring and the strange thing is that it only leaks when we turn off the pump. The rate can be as small as one drop every ten seconds or a steady stream of water. The O-ring looks good with no cracks but we'll probably replace it anyway.

What could cause this?

 

[Tmoose]

If The o-ring groove or sealing face is scratched, or the groove is too shallow, the added pressure will force the o-ring into the defect and seal it. The o-ring should be compressed about 10%, and the surface finish should be 32 or better.

http://www.americanseal.com/technical/images/oring_function_2_fig2.jpg

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[timeline1968]

While I can't explain exactly what is happening, I can say that just because you turn off the pump, doesn't mean that there's no pressure in the system. There is still pressure from the water that is above the filter (in the upper floors of the house, if this is a house water filter, as I suspect it is).

I get the feeling that there's some strange back pressure or something going on when you shut off the pump. I don't claim to know the answer, but I can still see that it might leak in the opposite fashion from what you'd expect.

 

[TheTick]

There should still be pressure with the pump off, due to pressure tank/reservoir, which has an air bladder to maintain and moderate system pressure.

 

[tomwalz]

Check the groove. Check the "O" ring (especially the joint). Try a little Vaseline or similar on the "O" ring. Tighten the cylinder until you can feel the "O" ring compress.

If it truly is a case or pressure / no pressure then something is being shoved into the leak when the system is under pressure. When the system is not pressurized (or the pressure is reduced) then whatever is plugging the hole is allowed to relax and move out of (or away from) the hole.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[TheTick]

Don't use Vaseline, as petroleum may harm O-ring. Go to hardware store and find appropriate lubricant. Buy a new O-ring while you're at it.

Don't be too frugal with plumbing. Replace whatever is necessary to keep system integrity, for sake of health and property.

 

[hydromech]

Tmoose is correct in that the O-ring will be forced into a sealing position when the pump runs.

O-rings are hydraulic seals and rely on pressure to energise them.

The variable leak rate would indicate that the o-ring is moving when the pump is switched off.

Do not assume that the O-ring is the correct size. Measure the groove and get a suitable replacement.

Hydromech...

 

[floattuber]

Thanks everybody. Actually this is part of a heat exchanger for a machine. The DI water that is leaking is part of the system that gets circulated to the machine and cools it. The chilled water is part of a seperate circuit that comes from a house supply.

We put a pressure guage and a strip chart on the inlet and didn't see any pressure spikes. Also, when we turn off the pump the pressure is low enough that it doesn't register on the guage. Of course the guage range is pretty high, over 100 psi, so the low end isn't going to register well.

The filter had been running fine for a while and suddenly started leaking. We hardly ever open it up.

Last night we replaced the top cover and the O-ring. We'll see if that helps.

 

[RobWard]

Are you on the pressure or suction side of the pump?
(Just a thought....)

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

 

[GregLocock]

O rings seal by exploiting the pressure difference on the two sides of the seal. If there is no pressure difference then the ring has a more difficult job. I was told by an experienced submarine designer that it is much easier to get a seal at 1000 ft than at the surface.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[automatic2]

Your O ring can become brittle over time, particularly if you have some conditioning agents in the fluid. Low pressure leaks are typical early signs. Use an O ring material that is compatible with your fluids, making sure the gland is clean and intact. I would have no problem with using vaseline or an assembly grease.

 

[TheTick]

On household filter canisters I have seen, O-rings face seal and under compression. This should seal even with no differential pressure.