Leaking Mechanical Seal 2

[Dhondt]

We are having aproblem with amechanical-seal that keeps leaking on aJohnson FREF pump (Free Flow Centrifugal pump with an extended shaft). The pump is used to clean chemical tanks with water on a ship. The old seal was broken, not completely worn, and the retainer bold of the impeller was broken. Although there was no visible damage to the impeller (possibly it just stayed in its place) we installed a new impeller, seal and mechanical-seal. When the pump was reinstalled on the ship it kept leaking. The pump runs at a constant speed and ofthen has to operate with its discharge trottled (when cleaning only one tank at a time). The ships crew filled the compartiment that holds the mechanical-seal with oil, is this correct? It was this oil that came out through the vent hole, and then after this water came out. When the pump was reopend no significant problem a new damage was found. We did observe that the sealspring was only being compressed for about 1.5 mm. Is this enough for a spring that is +/- 20 mm. long. We also found that the extended shaft was bend 0.1mm. But no vibration occured when we ran the pump. Then a college of mine installed a (self made) shim behind the pumps backplate to increase springpressure. This cured our leaking problem but this olso affected impellerclearance with a pressuredrop of about 2.5 bar (cavitation?). Anyway the leak came back after a day. Seals were checked for size and lenght and spring stifness and as far as we can see they are the same. So wehere do we look now?

 

[pants]

Firstly what type of mechanical seal are you using please give manufacture/size/materials.?
Is the seal a new one or has it been refurbished ?
What materials/elastomers etc
Send more information/

 

[Dhondt]

First of all, thank you for showing an intrest pants. As you might have guessed i'm kind'a new to this mechanical seal business. So it's nice to have some help. As for more information i'm affraid i don't have much to offer. The seal was a new one, and it was an original Johnson pump seal. As always, and especially with tankers, the problem was time. So, we didn't order those parts, the customer himself drove to Holland to pick up the seal and other parts at a Johnson dealer. Therefore i have no more information to offer. Since this ship frequents both the port of Antwerp and Rotterdam they have probably contacted this dealer on arrival in Rotterdam. So i assume the problem has been solved.But i still have questions. The oil in the sealchamber seems to be the right procedure. As for spring load, i still don't know if there is a general rule to go by? Then there is the question why the retainerbold of the impeller broke? The pump is only two years old, so unless something got stuck in that pump, cuasing the damage, i asume there is something wrong with the whole instalation. For instance the rule to keep at least ten diameters between the pump suction and the first elbow was not kept at all, max. two diameters. What if for some reason both seal parts would not be parallel to each other, would this cause the seal to leak or would the spring compensate for that? To be honest, because four people were involved in repairing this pump, i can not even rule out backwards instalation of the seal ring. Well i'm going to leave it at this, but any information that can help me to understand that kind of questions or problems better are welkom. Thanks anyway.

 

[longeron]

I hope this helps. For more information look at some of the major seal OEM websites, Chesterton, Burgman, John Crane, or Flowserve.

A mechanical seal works best when the forces that act upon it are balanced in a way. There is a force from the pressure of the fluid in the stuffing box, the housing around the seal, there is a force from the pressure of the atmosphere, and lastly, a spring force. The spring/s in your seal need to be compressed a certain amount for the seal to operate correctly. In a component seal, I assume, like the one you are talking about, the working height (the height of the seal when the spring is properly compressed) of the seal is critical.

Write the following things down:

1. The pump manufacturer, model, shaft size, and serial number.

2. The information about the usual operating conditions, suction and discharge pressure, the shaft speed, the fluids that are pumped.

3. The mechanical seal manufacturer, model, size, assembly number, materials of construction, and drawing number.

Now, call the seal or pump representative with all of the information you have. Ask for a pamphlet on the installation of the seal and a drawing for this installation. Then ask for the working height of the seal.

Here is are some basic component seal installation instructions once you know the working height:

- With the shaft in it’s working position use a ruler to mark the face of the stuffing box or backlpate on the sleeve. (Y)

- Find the distance from the lapped face of the stationary seat, or mating ring, to the face of the seal gland plate that will mate against the backplate. (X)

- Measure and mark the distance between Y and X on the shaft towards the bearing housing.

- From X measure towards the impeller the distance of the working height. Mark this position on the shaft.

- The back of the retainer or the spring holder, should be set screwed to this position.

These measurements and installation should not be performed until after the impeller clearances are set. Be aware that some pumps use a shaft step to hold the seal in place.


Good luck