For single stage centrifugal pump, if my rated flow is 135% higher than the normal flow; then what about my BEP and pump actual performance in the field? I am not having pump curves, we are just deciding over the flow design margin as our desing flows are 20% higher than normal, add on that we have taken more 10% margin of desing flow. is worth designing pump this way?
I am quite worried about pump performance and efficiencies!

If the rated flowrate is (too) much higher than the BEP, then:
* reliability falls (due to increased wear and recirculation)
* pump efficiency decreases
* power consumption goes up (increasing the flowrate by 10% increases the friction losses by 21% in turbulent flow).
* NPSH Required increases.

So I suggest you not to over-specify the pump's flowrate and specify only the design flowrate (normal flowrate + design margin, usually between 10 and 25% to allow for calculation errors - to be checked with your Process Design Basis/Criteria document).

The manufactuer will then select the closest model pump that fits these conditions.

After, once you get the selected pump curve from the pump manufacturer, check that the rated flowrate (which is the actual capacity realized by the actual, supplied centrifugal pump when running at the design speed and TDH) falls betwen 80% and 110% of the BEP flowrate (caution: this range varies from pump to pump). In any case the BEP should be between the rated point and the normal operating point on the pump curve.

Thanks Lionel, agreed. Means if my design margin is way off the process requirement will cost me substandard pump performance in the field. Please let me know if the things are specified in design basis but actually on field 10-15% margine is more than sufficient then will it affect the capital and fixed costs to my client?

@ Bharat Kadu - I notice that you are Process Engineer with Jacobs Engineering, Mumbai.

Taking that as a clue, I would guess that your terms of "rated" flow and "Normal flow" seem to be from system designer's point of view. Meaning of these terms would be different from pump-performance point of view.

In a system-designer's point of view, and to speak by numbers, if NORMAL requirement of the system is 100 m3/h, the purchase-specification may seek pump for a RATED flow of 135 m3/h providing 35 % margin.

Coming to specifying RATED Head, assuming Static Head in the system is 20m and piping is selected to have Hf as 5m at NORMAL flow of 100 m3/h, at RATED flow 135 m3/h Hf will be higher in square proportion. It will be 5 * (1.35)^2 = 5 * 1.8225 = 9.1125 m So your RATED head at RATED flow will have to be 20 + 9.1125 = 29.1125, say 29.5m.

NORMAL system requirement will be 100 m3/h; 25 m RATED values will be 135 m3/h; 29.5 m ! Motor rating will be higher by the ratio (135*29.5) / (100*25) = 1.593 !

Conversely if you select the piping to have Hf = 5 m at RATED flow 135 m3/h, Hf at NORMAL flow of 100 m3/h will be 5 / 1.8225 = 2.7435 m

Then also NORMAL system requirement will be 100 m3/h; 22.7435 m RATED values will be 135 m3/h; 25 m ! Motor rating will be higher by the ratio (135*25) / (100*22.7435) = 1.484 !

Capital cost of piping selected for Hf = 5m at RATED flow of 135 m3/h will be higher than for piping for Hf = 5m at 100 m3/h. But Operating cost to the user will be higher with economical capital cost !!

How pump-performance will be affected by how the purchase specification is drawn whether for RATED flow and head, 135 m3/h 29.5 m or for NORMAL flow and head i.e. 100 m3/h 25 m.

Pump-vendor will try to offer the pump with good efficiency at the purchase specification. Then again the duty specified in purchase specification may not match with BEP of pump. And pump-performance will be affected depending upon how much to the left or right of BEP will the duties of RATED and NORMAL conditions will happen in actual operation of the system.

Please note following correction in the above -->
....
Conversely if you select the piping to have Hf = 5 m at RATED flow 135 m3/h, Hf at NORMAL flow of 100 m3/h will be 5 / 1.8225 = 2.7435 m

Then also NORMAL system requirement will be 100 m3/h; 22.7435 m RATED values will be 135 m3/h; 25 m ! Motor rating will be higher by the ratio (135*25) / (100*22.7435) = 1.484 !

@ Bharat Kadu - Since you are in Process Engineering, I would note that the phrase In your post, "...Is it worth designing pump this way?.." is not correct. I take it that you are not designing the pump. You are designing the system and you would specify purchase specification for the pump to work in your system, right ?

Thanks SL. Its very good information for me. If you have more literature on this topics will be very much helpful me.
bharat4power@gmail.com

Why would you need such a huge safety margin between normal and rated flows?
Are you trying to cover a future operating condition or are you being ultra conservative and oversizing your system "just in case"?
Oversizing by 10% is considered "normal" in the API business.
But sometimes everybody adds their 10% at every stage of the design and you end up with a pump operating nearer to min con flow than BEP!

Best is to tell the pump suppliers what you are trying to achieve and why, both nowand in the future.
For example a pump manufacturer may be able to design for the future condition by changing out an impeller from a low flow design to a high flow design (in the same pump case) and not compromise on your current design condition.
Or variable speed may be a solution

@ Simon - Agreed, 10 % is suffcient, and VFD will work if at all required. For the this type of pumps we also have minimum flow bypass with control valve and we generally consider 30% of the design flow as minimum recirculation.
What is the creteria to select minimum recirculation?
i think, all the time the pump output has to greater than 50% of the design flow so as to avoid cavitation and overheating which may kill pump.
please suggest which is the creteria to select minimum recirculation flow.
Thanks All.

Yes 30% for a continuous bypass is about right.
Of course by doing this you are wasting energy and oversizing the pump.
You might like to consider a 3 way ARC (auto recirculation) valve like those made by Yarway & Schroedahl.
These bypass only when you need it and so save you 30% on your energy cost, & size of pump.
Of course these are not cheap.
"You pays your money and you makes your choice"

Pump manufacturers establish min con flow in the test lab. It being the minimum flow at which vibration levels are within limits.
Thus it is the flow at which a pump can run 24/7/365 without appreciable reduction in bearing life and mechanical seal life.
30% of BEP is a good rule of thumb for API process pumps.
Another rule of thumb is the flow rate at which the manufacturer terminates his NPSH curve.
This is because below this flow the NPSH curve probably starts to rise and vibrations to increase.
I like to consider 2 min con flows....the CONTINUOUS flow and the INTERMITTENT flow
The latter is approx half the min continuous flow and is good for running the pump for 2-3 hours at a time followed by return to normal flow
If you run at this regularly you will shorten seal and bearing life but do little harm
It is good for commissioning, startup, emptying tanks...
Disregard Minimum Thermal Flow. If you ever run at this flow you will be doing serious mechanical damage to the pump.