What is the optimum Design margin for Drv HP for pump motor

[MIANCH]

Hi,
Can any one let me know, what is the optimum design margin must be take in account during design phase for a pump beyond the suction and design head requirements.If some one have practical experience and hand on best margin please share with me or if have reading material please post it here.
Thanks

MIANCH

 

[micalbrch]

10-15 % margin is often used. Min. 10 % is what API 674 requires.

 

[Artisi]

Depends where in the world you are located and the motor design, Australian, UK and most European motor don't have a safety factor margin, there is a full load current and that's it.

 

[MIANCH]

Artisi,
Thank you for your input, we have bad experience with 10% margin, Pump is not able to pump design flow and taking hi amps.
Pump was designed for 70m^3/hr but now pumping less than 50m^3/hr.
I have done all calculation and finally come to this point may be Driver have less capcity.
Thanks
MIANCH

 

[BigInch]

So you think you need 150% ???

10% is reasonable, sometimes more is used but usually only when standard size motors are not convenient for closer matching to the actual required power.

I would imagine that you have some other problem.

 

[Artisi]

You need to supply more data if you want to receive useful information, what you have told us so far is useless. If you are pumping 50m3/hr and the pump/motor was sized for 70m3/hr then something is way wrong - if the flow is reduced the power should also be reduced.

 

[ash9144]

Your problem is not in the overdesign margin percentage. It is in the design itself. When selecting a pump you need to get a pump curve for the potential pump and thoroughly understand what it is telling you. Sounds like you have a motor that is too small on a pump that may or may not provide your needed flow.

 

[MIANCH]

Here is Design data from pupm data sheet, please have look on this and let me know if driver Kw is ok or not.
Capacity Normal/ Rated m3/hr= 72.8/80
Suction pr. Max/ Rated barg= 4.3/ 2.08
Discharge Pressure barg= 8.21
Differential Pressure barg= 6.13
Diff Head "m"= 108
suction Head "m" @ top of FND= 4.3
Pumping Temperature "0C"= 228
Vap Pressure barg= 2.8
Sp Gr= 0.58
Viscosity= 0.15
Imp Dia, Rat/MAX/MINI "mm"= 297/350/280
Rated Power Kw= 23
Efficiency %= 59
Minimuim Continuous Flow m3/hr= 20/ 30
Allowable operating range m3/hr= 30-120
Max Head @ rated imp m3/hr= 122
Max power @ rated imp Kw= 26.7
NPSHR @ rat Capacity @ TOF "m"= 3
NPSHR @ rat Capacity @ Impeller center "m"= 2.65
Suction Specific Speed "GPM.ft"= 10690
Suction Nozzal Ø inches= 4
Discharge Nozzal Ø= 3
I have found one difference in viscosity, in design they used 0.15cp and Distillate have 0.38cp (Distillate IBP /FBP is 145/ 316C) and SP Gr is 0.785.
Hope you will come up with good results.
Thanks
MIANCH

 

[TenPenny]

If I read that correctly, your max power at installed impeller diameter is 26.7 kW, but that's based on 0.58 SG. Where you have .785 SG distillate, that would make your theoretical max power 36 kW.

If it were my pump, I would make sure the motor was at least 37 kW for this installation.

 

[Artisi]

And without the pump curve, who would know.

 

[MIANCH]

Drive HP and Dishcharge head is from pump curve.2nd- Sp Gr is 0.58 they used and 0.785 is from our daily analysis. may be on high temp Sp Gr is 0.58 (at 228^0C).
Can you please post some calculation method.
Thanks
MIANCH

 

[Artisi]

Do you think it might be time you stopped running in circles and got someone qualified to investigate the problem?

Back to your original question - "what is the optimum design margin must be take in account during design phase" -motor size for at the design stage should be, end of curve requirement + 20% or the closest standard motor available.