Pumps in parallel with different discharge pipework 1

[bouk]

Hi All,

In many references I have come across regarding use of pumps in parallel, all examples and theory covers a situation where the discharge of the second pump's pipework is identical to the first pump's pipework, up until their point of rejoining.

However, in reality, this sometimes isn't the case/can't be the case - there might be a need for more bends on the second pump's discharge and some more pipework, for example, before the common point of rejoining.

In my theory courses, I have learned to develop a system curve with just one pump in operation. So my question is, how do I show such a case of the additional pipework and fittings that are required in the second pump's case on the "system curve"? Or am I going to need two "system curves" for each pump - but then how is the overall system represented?

 

[vpl]

My two cents: You need curves that show Pump 1 in operation, Pump 2 in operation and both Pumps 1 and 2 together in operation. If you don't it is quite possible for one of the pumps to be deadheaded when they're running in parallel.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

One will take more load and shift the other off set point, or they might oscillate trying to share load.

If you only have fixed speed units, do as vpl says. If you have variable speeds, you'll have to do as many curve combinations as you can make from some set of discrete operating points chosen from your speed range. Its much better to use a pipeline simulation program in that case.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[chicopee]

In addition to what vpl says, plot the discharges of the two pumps separately on a graph paper, then add the GPM's at constant pressuresof the pumps. The resulting third curve will a double hump curve. Then plot a line friction loss curve and where it intersect the resulting third graph will be the water available to the piping. You can also tell from the third graph which pump is deadheaded.

 

[bouk]

Thanks all for the prompt and descriptive responses. From what has been discussed above, I shall develop 3 system curves - pump 1, pump 2 and combined pumps. I'll then plot these against the pump manufacturer curves and check that I don't have any problems with one pump taking more of a load than the other pump.

If anyone additionally has any recommended websites with an example problem, it would be greatly appreciated - as they say, a picture is worth a thousand words!

 

[chicopee]

I should be able to send you some examples but it will not be for another eight to ten days.

 

[bingopin]

Be aware of where the system curve intersects the 2 pump operation curve. If this intersection is in a "flat" section
of the composite curve, the parallel pump operation could be very sensitive and/or unpredictable.

 

[stanier]

Download the tools from this website and model your system accurately.

http://www.pumpsystemsmatter.org/content_detail.aspx?id=110

 

[itsmoked]

Nice. Thanks stanier.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[chicopee]

It's always nice to have a computer program that can do the calculations for you and you should couple those results with hand calculations and graphs that were mentioned in the previous responses to ensure the results are within ballpark of each other.

 

[bouk]

Thanks for the link stanier. Chicopee, if you additionally have any examples as well to send that too would be appreciated.

Kind regards

 

[stanier]

The developers of the software that I have mentioned by giving a link had this to say in their Users Guide to the full blown software. It applies equally to Pump Masters version:-

CAUTION!
AFT Fathom is a sophisticated pipe flow analysis program designed for qualified engineers with experience in pipe flow analysis and should not be used by untrained individuals. AFT Fathom is intended solely as an
aide for pipe flow analysis engineers and not as a replacement for other design and analysis methods, including hand calculations and sound engineering judgment. All data generated by AFT Fathom should be independently verified with other engineering methods.

AFT Fathom is designed to be used only by persons who possess a level of knowledge consistent with that obtained in an undergraduate engineering course in the analysis of pipe system fluid mechanics and are familiar with standard industry practice in pipe flow analysis.

By all means learn your fluid dynamics by using this software as a tool. Recognise the limits of boundary conditions. Use other resources such as books and websites.

http://www.mcnallyinstitue.com

&

http://www.pipingdesign.com

etc.

Download tools from the pump suppliers such as Sulzer, Goulds, ITT Fluids, Flowserve etc etc