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Saturday, July 5, 2008

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A question raised by a young engineer in this blog...

Earlier discussion in "Pump Efficiency Estimation Without Vendor Information", shaft power of pump can be estimated using the following equation :
For high flow, the efficiency can be estimated according to equation to "Estimate Pump Efficiency base on Specific Speed (Ns)".

How to estimate power consumption for pump during normal operation without motor vendor information ?

To estimate power consumption of a motor, first thing is to estimate the pump shaft power (Ps) as shown above. The power consumption of motor (Pm) can be related with pump shaft power (Ps) with motor efficiency (Eff,m). One special characteristic is motor efficiency is higher the motor rating, higher the efficiency.

Pm = Ps / Eff,m

Pm = Power consumption of motor,
Ps = Pump shaft power
Eff,m = Motor efficiency (see following table)

The following table tabulate the motor rating and associates motor efficiency, assuming at best efficiency point (BEP).

Source : Pocket Guide to Chemical Engineer

For example,
A pump with capacity of 440 gpm, pump head of 264.1 ft and pump speed of 2950 rpm, calculate the power consumption of motor.

Step 1 : Estimate pump shaft efficiency
Calculate Specific speed (Ns). Refer to Estimate Pump Efficiency base on Specific Speed (Ns).
Ns = 996. Pump shaft efficiency, Eff,s = 74%.

Step 2 : From above equation, Pump shaft power, Ps = 37.7 HP (~ 28.1 kW).

Step 3 : From above table, Pre-select motor and its motor efficiency. Motor rating = 40 HP with motor efficiency, Eff,m = 89%, delivered power = 40 x 89% = 35.6 HP, less than Pump Shaft Power (Ps) of 37.7 HP. The motor is not acceptable.

Next size motor, Motor rating = 50 HP with motor efficiency (Eff,m) = 89%, delivered power = 50 x 89% = 44.5 HP, more than Pump Shaft Power (Ps) of 37.7 HP. The motor is acceptable.

Step 4 : From above equation, power consumption, Pm = 37.7 / 89% = 42.4 HP

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posted by Webworm, 10:36 PM


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