Pump Motor Selection

Many pump suppliers have web sites that will do all the work for you.

• Pump Sizing Worksheets / Calculators - We offer Unitra pumps ...
• Pump and Motor Calculators : Hydradyne Hydraulics
• Pump Efficiency with Electric Motor Driver - PumpCalcs
• Pump Sizing Calculation - Scribd

Then, hopfully you can make your own informed decision.

thank u sir

These links also help:

http://www.gemi.org/waterplanner/excel/GEMI-Equations-Tool.xls

http://cr4.globalspec.com/thread/50082

http://www.mcnallyinstitute.com/06-html/6-04.html

- MS

The capacity of the motor is larger than the volumetric flowrate multiplied by the rise in pressure divided by the pump's efficiency. At 75%, the minimum motor size for that duty is therefore 3kW.

You don't have all the information you need to accurately determine the required motor capacity. Firstly, do we know if the flow and head you state are maximum figures or at duty point? (If you are reading off the nameplate, it is usually the case that off-the-shelf pumps show maximum figures, whilst bespoke pumps show duty point figures). More importantly, perhaps, you also need to know the pump efficiency (at duty point) which you can only get from the pump curve or pump manufacturer.

You can get a rough approximation if we assume, for example, that the pump efficiency is between 60%-75% (it could be higher or lower) using the formula:

Absorbed power (kw) = Flow (m3/hr) x head (M) x SG / 3.687 x pump efficiency (%)

If we assume the figures you give are duty point, then absorbed power would be 3.9hp at 75% efficiency and 4.9hp at 60% efficiency.

So the chances are a 5hp motor will be OK, although you might not be leaving much margin if the pump efficiency is not so good.

Motors don't like too much heat, and overheating is apparently the biggest cause of motor failure. Your motors might not be designed for an ambient temp of 50-55 deg C (the motor manufacturer/supplier should tell you). You might need to consider beefing up the bearings and/or bearing lubrication. Alternatively, since the pumps are for chilled water circulation, perhaps you could tap off a loop to provide cooling for the motor.

5 HP motor is more than enough.

for your information

pump power = volume flow in meter cub /scond -multyplied by pressure in bar multyplied by100 = power in Kw which in ths case will be 2.25 Kw

taking pump effeciency and electric motor effeciency , into cinsideration so 5 HP is good enough.

Heating of bearing needs checking the alignement.

Sarwat

You can only say that a 5hp motor is more than enough if you totally ignore pump efficiency, but efficiency is in fact an important part of the equation (see post #5 for the complete formula). Your figure of 2.25kw is meaningless (unless you know of a centrif that will reach 100% efficiency, if so, please let me know about it).

If the pump efficiency at duty point is 60%, then the 5hp motor is only just big enough, with virtually no safety margin. If you allow the usual 25% safety margin (for motors up to 7.5kw), then you would require a pump efficiency at duty point close to 75%.

If you consider that this is an old pump recovered from a scrapyard, and that even for new pumps in this flow/head range it is not unusual to have efficiencies of less than 60% at BEP (Best Efficiency Point), and that a larger pump with an efficiency of, say, 80% (approaching the top end for centrifs of this size) is very likely to have an efficiency of less than 60% at half the flowrate at BEP, then it doesn't look so promising.

If it were up to me, and considering also the high ambient temperature, I would be inclined to fit a 7.5hp motor (subject to no complications with frame-size, shaft-size, interface, etc.) if I wanted to be certain of achieving the stated flowrate, or fit a 5hp motor and be prepared to throttle back for a lower flowrate.