11 replies to this topic

[drdave]

Sir,

One centrifugal pump delivers chilled water (at 4 deg_C ) 43 cu.m./hr at TDH of 37MLC(Meter liquid column).
Running of standby pump in parallel just increases flow to ~50 cu.m./hr at TDH of 39 MLC.
WHY DOES PARALLEL PUMP RUN NOT INCREASE FLOW RATE TO ALMOST DOUBLE (ie ~82 cu.m./hr)?
Does it only because of system resistance?
May you please guide me on this?

[les]

should be check the standby pump maybe a bad performance impeller or restriction on pipe/checkvalve .
Check this first to take futher steps. Or try 't with trottled discharge to increase pressure/flow.


les

[drdave]

mvancleave

Thank you very much for your educative feedback in nutshell.

Will only Increase of discharge header sizing help to get benefit of almost double capacity by parallel run of pumps?
What preventive measures to be considered later on for occasional single run?

Thanks once again.

[drdave]

Cooling water pumps in parallel performs well during test in workshop but in actual operation ,There is 20% less flow available obviously with higher discharge pressure indicating system resistance.My queries are :
[1] What are the ways & means to increase capacity in the available pumping system as shown in the sketch(keeping two pumps in parallel)?
Say, each pump capacity is 10000 m3/hr @ 3.5 bar_a.
[2] How system resistance curve will be developed in this case? Here, 15 meters back pressure of CW return header to top of cooling tower will be there.
[3] What are effects of suction velocity,submergence and capacity control during parallel pumping of two with the third one idle in common suction pit?

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[fallah]

[1] What are the ways & means to increase capacity in the available pumping system as shown in the sketch(keeping two pumps in parallel)?
Say, each pump capacity is 10000 m3/hr @ 3.5 bar_a.

In practice, for two parallel working pumps ,we can not obtain two times of capacity of a single pump , and for increasing total capacity of two existing parallel pumps (going toward double capacity) we must decrease the dynamic resistance of the system , for example , increasing the piping size.

regards

[AA Mishra]

You can increase motor frequency

Regards

[Padmakar Katre]

Dear A A Mishra,
I would like to bring into your notice that double flow we don't get because of the system resistance and not because of the Pumps or its drives are limiting in this case. Only option to get the total flow for what these two pumps are designed is to design your discharge line/system that can sustain or take this much of the quantum and same is achieved through increasing the discharge line size for the required flow and pressure.But in case the frequency of running both pumps at a time is rare then need not to go for increase the line size as we can achieve some increased flow with an existing line when both pumps will run i.e. with an existing flow design margin of the discharge line.
Now to check or change in the pumps' drives or its parameters will come into picture only when there is problem to get the rated flow through pump for what it is designed then you have to have an inspection of pumps or the cross-check of its drives provided rest of the things are ok ( like density,viscosity,temperature of the liquid.suction and discharge conditions like line sizes and all)
I hope you will agree with my opinion,Please if you have any opinion different from above I will like to see in this forum.
Cheers......Njoy........

Regards,
Padmakar

[kanzure]

Say, what does parallel pumping look like? Is it something like the following ASCII art?

| -------------------- |
| chamber |
= <-- pump1 |
= <-- pump2 |
| -------------------- |


- Bryan

[djack77494]

The performance of two pumps operating in parallel depends strongly on the shape of the pump curves. As an extreme, the pump curve of a positive displacement pump is essentially a straight vertical line - the flowrate is the same regardless of the discharge pressure. In this case, employing two pumps in parallel WILL DOUBLE the flowrate. For a centrifugal pump, and especially one with a "flat" curve, increases in the system resistance will decrease the pump's capacity, perhaps quite quickly. If a control valve contributes significantly to the system resistance, it could open to allow for a larger flowrate. Alternately, specify pumps with steeper curves and you can get a more significant capacity increase by operating your pumps in parallel.
Doug