How to match a pump a a varying system load? 1

[Yobbo]

Matching a pump to a system load is quite straight forward as you try to get a pump with the BEP as close as possible to the actual working point. Now comes the additional question. What to do when the load is varying in a rather random way, due to controlling with a valve in the delivery line or to controlling of the pump rotational speed? If I were to have a frequency histogram of the different flows required, would it be possible to determine a sort of equivalent ( average ) working point, that I should try to match with a pump having a BEP as nearby a possible?

 

[BigInch]

I think that your real goal is to minimize operating cost.

If you have a varying load, whether random or otherwise, you need to make a Flowrate vs Percent_of_Operation_Time chart.

As an initian approximation, Look at a BEP around
the Sum of all values of (Flowrate x Percent_Op_Time)
That's your expected average flowrate.

For good general pump operating characteristics, some
Additional constraints are
minimum flowrate >= 50% BEP and
Maximum flowrate <= 115% of BEP.

If you can't meet the addtional constraints, you may want to think of multiple pumps in parallel, or
perhaps a VFD, but be very carefull when looking at VFDs by calculating exact power consumption operating costs, including the efficiencies of the VFD and motor at partial loads and that the head at low flows and low VFD rpms is sufficient for your system curve.

That's only an approximation of average flowrate. The actual cost of operation could still vary considerably, if the flowrate range is wide, due to the variable head of the pump curve over a wide range of flows and the varying efficiency at each flowrate. Using the average alone, while still having substantial operating times away from the average florate, will not give you the lowest operating cost, so you'll still have to calculate the operating cost for each flowrate multiplied by the percent time operating at that flowrate. Then minimize that value by choosing a pump curve from several possible pumps that all give the highest operating efficiencies in the most utilized flowrate ranges.



"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[Yobbo]

Hello Biginch,

I thank you for your extensive reply to my question. I am going to work it out further for myself. Thank you again.

 

[kiran21]

hi,

how about load sensing pump

 

[DieselTwitch]

VFD with a pressure sensor?

 

[BigInch]

I think the OP's objective is first, to find the best pump, rather than the best VFD.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[Yobbo]

That's absolutely right.

Karel Postulart, The Netherlands
Nuon Power Generation