Mechanical seal leaking only when pump is stopped

[Hamza1988]

So we have a centrifugal pump mechanical seal failure the other day. The mechanical seal leaked when the pump was stopped (Not immediately after stopping but after 1/2 an hours time. The leak was not severe and only a drop a second was seen leaking. The pump was started again to see if the seal leaks during running but it did not. The pumping fluid is hot water. I was wondering what could be the reason behind the leakage in the seal only when stopped. thanks

 

[ashtree]

Hamza1639

Depends on the style of seal.
My guess one or more of the following will be happening,

1)When the pump is running it is actually operating at negative in the seal area and is pulling air in through the seal so that little if any water leaks out.
2)The spring that pushes the faces together has lost its tension or perhaps the O Ring shaft seal is damaged or fouling on the shaft such that when there is no pressure behind it(when the pump is stopped) the seal weeps because the two faces are pushed together.
3) The housing or shaft is damaged and allows water past it when it stops but is under sufficient negative pressure when running that the leakage is minimal.
4) When stopped the pump is pressurised in some way(perhaps by another system or faulty auto priming system) that is beyond the capability of the seal to maintain when stationary.
5) The pump cools down when not pumping the hot water and the tolerances or tensions change as it cools.

It may be that the seal also leaks when the pump is running but for various reasons the drops are much harder to find or see.



Almost certainly the problem will require a pump dismantle and a seal inspection

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Hamza1988]

Thanks 'ashtree' for the reply. When we dismantled the pump seal we found that the bellow O-ring and the stationary face O-ring were very soft and almost melted. This might be the reason as you have mentioned above but then again if it is the reason why does it not leak when the pump is running with the same damaged O-ring. The other observation made was that the flange bushing or the throttle bushing was missing (might have been damaged/shattered during running). Apart from this, the seal faces were found to be in satisfactory condition.

Thanks

 

[bimr]

The seal was worn out.

The seal leaks only when the pump is not running.
•The seal is also leaking while running, but the leak is vaporizing and not visible. Hold a piece of white paper over the seal area and see if the paper becomes damp.
•A meniscus caused by centrifugal force and liquid surface tension had formed at the inside diameter of the seal faces. This prevented a leaking seal from dripping while the shaft was turning.
•You are using high temperature grade Kalrez. It's too hard at ambient temperature. It will soften and seal when the system comes back to operating temperature.
•The pump is running under vacuum and while it is running, air is being pulled into the system. The fluid leaks out when the shaft is static. This can occur if an open impeller, that was designed to be adjusted against the volute has accidentally been adjusted backwards against the back plate. The impeller "pump out vanes" can then pull a vacuum in the stuffing box. This is a common problem if you use a lot of Flowserve pumps and then bring in a few of another brand.

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