Rated flow of and minimum re-circulation line of centrifugal pump

[Vinu1991]

Hi All,

1. I have been preparing some centrifugal pump datasheets and in this context I want to clarify the difference between the 'Maximum operating flow of the pump' and the 'Rated Flow of the pump'.

2. Can someone please explain the real purpose behind having a minimum re-circulation line in a centrifugal pump. I have read that there are different schemes involved in designing the re-circulation line for a pump(e.g. having an FO, Control Valve, ARC etc). Also any help/advice as to how the selection of the re-circulating scheme is made would be useful.

3. Can someone explain how the Minimum Continuous Stable Flow (MCSF) would compare with the minimum re-circulation flow for a centrifugal pump?

Kind Regards,
Vinayak

 

[Artisi]

If you need to ask these basic question, how are you able to prepare a meaningful pump data sheet?

The "rated flow" is the normal design flow to achieve the requested application duty, whereas the "Maximum operating flow" is the flow that may need to be met at various times during operation without causingel any pump hydraulic upsets.
A centrifugal pump is not designed to operate at zero flow a maximum head or near minimum flow approaching maximum head, hence the need to by-pass some of the flow to waste or back to the source, how to achieve this is really beyond normal in discussion here.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[shvet2008]

For more information about spillback schemes see here.

 

[lilliput1]

You have to look at the pump curve and plot the system curve. Select the pump impeller and motor hp required. The intersection of the selected pump curve and the system curve is the rated operating point. The maximum flow would be on the pump curve and at the maximum pump motor hp. The minimum flow would be such that if all the pump energy goes into heating the water, its temperature would be below boiling point.

 

[moltenmetal]

Pumps have a minimum flow well below the point at which the recirculated energy is "boiling the water", lilliput1. It depends on the pump and seal type, and on the size of the pump, and sometimes can be a fairly significant fraction of the rated flow.

 

[JJPellin]

The pump manufacturer should provide the minimum stable continuous flow which could be based on thermal considerations or increased vibration. In most API pumps, the increase in vibration at low flow would drive this limit and not the point of thermal instability.

Johnny Pellin

 

[pumpking]

Generally speaking (for small overhung Impeller pumps) we generally use the formula of:

10% of rated flow at BEP for intermittent duties
20% of rated flow at BEP for continuous duties

A lot of our kit is magnetic drive which adds to heat generation in the pump/magnet assembly (through eddy currents). It is also important to mention that vibration, hydraulic thrusts etc are also a big concern when operating at this point of the curve too, especially when you get to larger API style pumps etc so really needs some consideration adn advice when selecting or sizing up plant.

http://www.cdrpumps.co.uk

Ash Fenn

http://www.cdrpumps.co.uk

 

[electricpete]

Pumps have a minimum flow well below the point at which the recirculated energy is "boiling the water", lilliput1. It depends on the pump and seal type, and on the size of the pump, and sometimes can be a fairly significant fraction of the rated flow.

I think you meant well above? To my thinking low flow is the condition where you are worried about boiling the water, especially in a high head low flow pump. Or maybe I'm misunderstanding your point.


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(2B)+(2B)' ?

 

[lilliput1]

I just saw on youtube ad explaination of what makes the noise on the submarine screw propeller. The speed and shape of the propeller create a vacuum where bubbles are formed. The bubbles then burst creating noise and this is cavitation. So I would think pumpking's formula presented is a good guideline. I have heard also that 20% speed is the minimum recommended speed for variable frequency drives.

 

[Artisi]

There is no formula to derive the minimum flow /head, this might be supplied by the manufacturer based on design or testing- and then it might not be. pumpking's guideline is based on their experience for the pumps they are involved with and cannot be applied across the board for all pumps and all operating conditions.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[dbill74]

There is a reason you don't see "data sheets" for pumps from manufacturers, there is too much information about any given pump to be presentable in a "data sheet" format. Pump curves present the information much more cleanly and concisely. I recommend abandoning the idea of creating meaningful data sheets.

The reason you can't find a single scheme for designing re-circulation lines is because there isn't one. You need to design the recirculation pipe and flows based on the main use of the fluid. Pump and circulation requirements for a closed-loop heating/cooling system are different than that of an open-loop potable hot water system.

Ideally you want your pump operating at peak efficiency, which is something that is included on the afore mentioned pump curves.
Generally speaking, you will want the minimum flow rate through the pump to be no less than ~20% of peak flow. This isn't because it can't go slower, but you have to take into consideration the accuracy of your flow measuring device, which (for the most part) start to loose accuracy below ~20% peak flow. With variable speed drives, the efficiency of the pump drops off as well.

 

[lilliput1]

Also note that as you follow the pump curve to the left it rises then drops back down so at a certain pressure (2) flow points can be possible. This would be the pump surge region. Set minimum flow so pump curve does not reach this region.

 

[Artisi]

Not all pumps have a curve that rises and then reduces as it nears the minimum flow, pumps of this design can be troublesome if not applied correctly - - yes, can have 2 operating points.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)