Parallel Pumps and efficiency

[Boguslav99]

Ok, here is my dilema.

Recently I was placed on a six sigma project, which is tackling an efficiency issue at a pumping station that is running 6 electric motors. All are 2300V running 1800RPM and have 500HP. The current efficiency for the station is 40% and i would like to know what should i expect. Ideally i would like to understand how bad this number is.

I know that putting pumps in parallel decreses the efficiency by a lot. But if there is anyone out there who knows how to get an approximate efficiency coefficient. The would help a lot in identifying success of the project.

blester@dow.com or boguslav99@yahoo.com

 

[TD2K]

How are you defining the efficiency? For the size of pumps and motors I would expect a lot higher efficiency unless you are way off the BEP or are recirculating a lot of water.

 

[25362]

TD2K is absolutely right. Assuming excessive flow rates for the individual pumps and a consequent lack of sufficient NPSHa aren't a factor, have you considered stopping one pump (out of the six operating in parallel) while observing what happens with the overall station efficiency ?

 

[Artisi]

something is wrong , very wrong with a station efficiency of only 40%.
25362 could be pointing you in the right direction, adding pumps in parallel does not necessarily add much to the overall output of a pumping system if the total head on the system is excessive due to inadequate pipeline sizing for the required thru'put. To take this argument to the extreme, you can reach a point at which it doesn't matter how many additional pumps you add to the system, the flow rate will not increase.
It seems that a full hydraulic study is required to:
1. Establish individual pump performance in terms of flow / head/ and overall pump/motor efficiency.
2. Measure or calculate the actual total head at various flow rates for the station which you can readily undertake as you have a multi-pump installation.
3. Once you have this information, some meaningful study can be made and recommendations made to assist you.

Naresuan University
Phitsanulok
Thailand

 

[scalleke]

All the comments are very much valid.

Can I suggest to have the pump manufacturer involved in the exercise? At this kind off efficiency we are talking about mega$ that can be saved by increaseing the efficiency and both the pumps and the system should be looked into.

What type of pumps are we talking about?
What are the flows and heads.
Can any improvements be made on the Electrical supply that would gain a percentage point?

Best regards.

scalleke

 

[BobPE]

scalleke:

I don't see where the pump is broken from what I read in the posts, I wouldn't run to the manufacturer right away. I would think it to be a system type problem and no pump vendor should be helping you there anyway. We really dont know anything about the pumping system, so 40% may be a good number?? Artisi hit the nail on the head though, performing an in depth engineering study would be cheap and may have great$$$ benefit.

let us know how you make out....

 

[vanstoja]

If you are pumping cold water(SG=1.0) and are using all of the motors' 500HP at an overall efficiency of 40% for each unit then the combined product of QH (flow times head) is 792,000. For a single pump flow range from 1000 to 5000 GPM, the heads obtained were put in the specific speed equation for 1800 RPM running speeds and pump hydraulic efficiency at best efficiency flowrate was found from typical Worthington plots to range from 60% to 88% from lowest to highest flow. Assuming 90% motor efficiency gives overall pumping efficiencies from 54% to 79.2%, all of which are higher than your cited 40% overall plant efficiency. Flowrates below 2000 GPM yield implausibly high heads above 396 feet for centrifugal pumps suggesting that the overall efficiency range would really be about 71.1 to 79.2%. This crude estimate confirms what others have said that you are either operating far off best efficiency conditions for the pumps or are including factors other than pumps and drivemotors in the plant efficiency determination.