Vacuum in the Box? 3

[Milkboy]

I recently came across an application where a seal leaked when the pump was stationary but the leakage stopped when the pump was running.

The seal was removed and as expected the SiC v SiC faces were heavily worn due to running dry.

Several theories were banded about (trapped air, inproper venting etc).
A gauge was attached to the box which recorded a vacuum!

I had never seen this and would have said its not possible.
Its, as far as I know, a standard centrif pump with a single mech seal.

How is the box pressure going so low and onto vacuum?

Hope there is enough info - its about all I have.

TIA

-
Milkboy

 

[rbcoulter]

It looks as though the pump is creating a vacuume when running. This is normal since that is how pumps work. What is not normal is that the mechanical seal should not be allowed to run dry. Some type of fluid must be provided to the seal faces for lubrication. Is there a purge line directing pressurized fluid to the seal region? Is it plugged? This could be faulty design. The mechanical seal should not be situated so that it is exposed to the suction generating area of the pump.

 

[JJPellin]

It sounds to me as if the design of the pump would be to maintain some positive pressure on the pummp suction. You can design a pump to run with vacuum on the suction, but then you wounld not normally design the seal chamber to be at suction pressure. I would check for a suction restriction. The most obvious would be a partially plugged suction strainer. If it is normal to have vacuum on the suction, which I think is unlikely, then I would suggest adding a Plan 11 flush from discharge with a close clearance throat bushing to build positive pressure in the seal chamber. A single seal with hard faces cannot run under vacuum with good reliability.

 

[jet1749]

I have seen this where the impeller is fitted with pump-out vanes on the back shroud, especially if the impeller/shaft position is adjustable, and it has been set too close to the back cover. What make/type of pump is it?

 

[Milkboy]

Thanks All

Just to clarify though JJPellin. Am I to understand from what you say, you would not usually expect to see vac in a box?
Usually the box pressure is somewhere between suction and discharge? Getting a vac could well be a total error?

We should be looking for the cause of the error/vac and possible adding a Plan 11 and bush.


Jet, good point, I will fond out if the impeller is adjustable. This could be the cause.


Once again, thanks for your time





-
Milkboy

 

[JJPellin]

Designing for vacuum in the seal chamber would be unusual. We have a number of pumps that come off the bottom of vacuum towers. The suction to the pump always runs with a hard vacuum. But the pump impellers are designed to hold higher pressure at the seal when the pump is running so you always have pressure above atmospheric. Ours accomlish this using back wear rings and balance holes through the impeller. Back pump-out vanes could have the same issues. But my main point would be to first check the system to see if the suction pressure to the pump is lower than it should be before jumping right in with modifications to the pump. A plan 11 could solve your problem for the short term. But if the problem is a restriction in the suction screen or the Johnson screen in the bottom of a fractionating tower, this will only mask the real problem which will eventually cause cavitation, loss of performance and pump failure.

 

[Artisi]

You need to look carefully at the pump / impeller design to make any sense of your problem. If the impelller has pump-out vanes it could be the cause, if the impeller has balance holes through the impeller shroud and there is a negative pressure on the inlet side - this will be transfered to the "back-side" of the impeller and could account for the negative pressure in the stuffing box. However, irrespective of the cause you need to supply product from the discharge side to the stuffing box or form an outside source for seal lubrication and cooling.

Naresuan University
Phitsanulok
Thailand