Centrifugal pumps running in parallel, one pump with high vibration

[Mike9208]

Hi to all colleagues out there,

In the ethylene plant that company where I work is biulding there are three ITT Goulds Non-API pumps OH1, namely P-1234A/B/C, running with equalized water, rated capacity each one 300m3/h@40.3m.

There are several operating cases, but the normal one requires two pumps running in parallel. When we run pump A and B, the pump B is having a higher vibration than A. When runs B and C, again B has higher vibration than C. Last case is when A and C runs in parallel both has similar vibration and under normal ISO limits for newly commisioning machines. in the physical arrangement the pump B is in the middle, and suction piping layout is not symetrical. I have attached a snapshot of the 3D. Vibration measurements found 4X component. Also, found sort of cyclic sound coming from the piping. Current absorption of pump motor B is lower than other two motors.

my first thought is that 4X is coming from vane pass frequency due to operation too much to the left of BEP which is somehow confirmed by current absoption being less than other pumps. However I dont have any explanation of why this happens only with one pump. someone suggested the problem comes from the suction piping layout.

Any ideas of why only one pump is having this behaviour and any suggestions of how to solve? Client wants to rotate the two pumps in operation to have "uniform" wear in all the three pumps so now I need to find a way to reduce vibration of pump B.

Appreciate any support.

 

[EdStainless]

slap mechanical pressure gauges on the inlet of A and B.
If A is steady and B is fluctuating then you are starving/cavitating at the inlet to B.

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P.E. Metallurgy, consulting work welcomed

 

[moideen]

Simply swap the pumps so that Pump B moves to Pump A's position and vice versa, then observe the performance.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[LittleInch]

"Simply swap"?? There's a lot of things which are simple but that is a decent piece of work, cost and downtime for maybe no better information.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mike9208]

Thank you all for valuable comments.

the plant is in operation so, every test or modification needs to go under carefully planning. Operation team agreed to operated pump A and C, and keep pump B in stand-by. Once we have time window, we will inspect pump A and C to see condition of shaft and impeller.

At this time, we have many hyposthesis. Our piping stress engineer made some modification on the suction piping supports and no change at all. Our process engineer reviewed calculations and according to what whe have seen, based also on vibration spectra, the issue is not likely to be cavitation, enough NPSH, level in the suction pit has been always high enough.

We have somehow discarded a mechanical issue on the pump B itself since when it runs alone vibration is very low, and current absorption is as expected and same than other 2 pumps.

I am taking all the comments here as a starting point to make a sort of workshop with all the team and see what can be done. I will provide update.

 

[electricpete]

We have somehow discarded a mechanical issue on the pump B itself since when it runs alone vibration is very low, and current absorption is as expected and same than other 2 pumps

Pump B could be the weakest pump. So when run alone it is at a happy point on the curve but when run in parallel it is pushed too far to the left where the vibration may be higher and the current may be lower.

 

[EdStainless]

I do like Pete's thought.
I have had experience with parallel pumps and interactions between the intakes is not predictable unless you are delivering >4x the required NPSH.
Our pumps required high NPSH and we nearly always used a charge pump in parallel operations for safety.

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P.E. Metallurgy, consulting work welcomed