Seal and Bearing Failure approx. every 6 months 5

[mikeyd2]

Hello All,

I am working in a Pulp Mill and have been given the task of troubleshooting a pump in which mechanical seal failure is causing water contamination and failure of the front bearing. We are having to replace the bearing approximately every 6 months. The front feet of the pump is mounted to channel which is welded to the base. The welds have continously broken over time and have been rewelded. The channel has also cracked and been re-welded.

The pump is running on a fairly high efficiency part of its curve based on flow data taken from the DCS, so run out or low flow condition is not the problem. The fluid is pulp stock and I am not certain of the temperature (probably around 50C). The ambient temperature is ~8C (winter) and ~23C (Summer).

Some think that the problem is from entrained air due to the agitator that is installed on the same tank 5 feet away from the pump suction connection. This is a concrete, tile lined tank. I have been informed by the pump manufacturer that placing the agitator close to the pump suction is a common practice and diverts the flow around the circumference of the tank and into the suction.

There is excessive vibration when this pump is operating. I do not think cavitation is the culprit as there is no pitting or corrosion on the impeller or casing when it is re-built.

The pump suction is horizontal and is a straight run of 24" diameter SS pipe (~4 ft long) from the tank followed by a 24x14 exccentric reducer into the pump suction. Could thermal expansion in the suction be causing this problem? It seems like a short run but the tank it is connected to will not flex like a thinner walled metal tank.

Another reason why I think piping loads may be to blame is the arrangement of the discharge piping. The pump discharges vertically up about 9 ft before turning horizontally. Directly after it turns horizontal there is a guided support which only allows axial growth. This support looks like it is restraining the piping from growing up from the pump and putting the thermal expansion loads back into the pump.

Does it sound reasonable that piping loads may be causing this problem? Does anyone have any other ideas at what may be causing this?

Thanks.

 

[Artisi]

Get the suction pipe supported ASAP - you could nearly lay bets on this being the problem.
If you are running 3.5% upwards I would give serious thought to the flat plate reducer ["Dutchman reducer" - don't ask why they are called this cause I don't have any idea] this will eliminate 1 likely problem.

Guess you have checked flow carefully, the pump is running where you think it is?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[PumpSmart]

Mikeyd2,
Vibrations:
You have very high values
I understand that with agitator on you have 0.921 inch/sec RMS and with agitator off you have 0.635 inch/sec so when is agitator off vibration level decrease but 0.635 inch/sec is also high value.
This test indicates that pump has turbulence or air on suction.
That corresponds one factor of vibration, hydraulic factor.
I see very short suction length pipe between tank and pump, so agitator on tank make turbulence and vortices that air entrained and this turbulence go thru the pump. So you could improve piping support but turbulence or air entrained does not dessapear and continue creating vibration.

For poor suction piping support you could make a cold stress on piping to be sure if this problem exist or not.
With cold stress suction piping test, you will detect if you have this problem or not and if you put suction piping support you see if this new suction piping support will make well or not. So it is important to do this test, because it detects if you make those things well.

Vibration readings with 1X dominant of FFT Analysis could indicate mechanical problem like casing distorsion make for stress on suction piping or mechanical shaft misaligment.
Vibration readings with Vane pass freq dominant of FFT Analysis could indicate hydraulic problem like turbulence and air entrained. So will be an important test also.

I hope this help.