Borehole pump motor efficiency

[valdc]

I have a pump rated to deliver 75 gpm at a head of 740 feet
coupled to motor rated 20 Hp

The pump currently discharges 60 gpm and draws an average of 47.6 amps on the phases with 230 volts.

Question is how do i compute for the effeciency?

would it be-

[Pout = 14920 (from 20 Hp)] /
[Pin = A (from meter) * V (from meter) * 1.73 *.8 pf]

id get = 98%

or:

from the pump power curve, from the manufacturer, i've plotted the kw, against the current discharge of 60 gpm

ive got a value of 12.4kw = P2

P2 / P1 =
12.4kw / 15kw = 83% = p1 by the way is the pump motor requirement given from the manufacturers curve...

im confused. thanks!

 

[Artisi]

Are you trying to establish the overall pump and motor efficiency or just the motor efficiency as it will make a difference to the answers you are likely to get back?

 

[Artisi]

Forgot to add, if you are only after the motor efficiency - why?

 

[BigInch]

I doubt that your pump is running normally and at full RATED power. I would imagine that normal operation is at 5 to 10% less than that, or even more 15% less.

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[valdc]

i'm trying to determine the overall pump and motor effeciency of the system on about the consumption of energy against the pump output...

the first equation indeed pertains to motor efficiency.. and ive found out that the correct means would be to get the

eff = (Mechanical Power output) / (Electrical Power input)

if im not mistaken..

my concern now is it possible to calculate a specific efficiency for a pump motor system, like a base data so that if id compute this efficiency, i'd know when im straying too far from that base data...

thanks

 

[Artisi]

One more basic question so it is clear what we are talking about.
What depth is the pump setting below your measuring point - which I assume is ground level?

 

[BigInch]

Most newer motors run around 0.84 a(smaller) to 0.94 (larger) and pumps running at BEP anywhere from 0.55 to 0.85, so if you can determine what those are, just multiply those two efficiencies together to get a close overall efficiency. If you can't determine what each are, then you'll have to calculate power delivered by the pump.

Pump Power Consumed (Hydraulic Power) = flowrate * density * Differential Head

Divide that by your electrical power input to get overall eff.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

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[valdc]

Atisi, that would be 460 feet

 

[Artisi]

valdc,
Before doing anything you need to establish accurate flow, discharge head at a convenient point at ground level, power input and the static lift from the water level in the bore to the measuring point with the pump operating.

As an example -
(you fit your own figures to suit the site conditions)

60gpm
200 ft discharge head (gauge reading)
Static lift 400 ft
12.4kW input


This gives a WHP of 60 x 600 / 3960 = 9.1 hp (6.74 kW)

overall efficiency is 6.74 / 12.4kW = 54.3%

Now bear in mind that this is the o/a efficiency of the pump, motor and all losses from the pump inlet to the head measurement point at ground level which includes any discharge pipe, bends, valves plus the static lift to this point plus all the losses on the discharge side of the measuring point.

If you wish to calculate (estimate) the pump / motor efficiency at the pump discharge, you can try and calculate the losses for all the discharge pipe below ground level any valves etc before the head measuring point plus the standing water level below your measuring point with the pump operating, this can be added to the discharge head that you measured and the efficiency re-calculated as follows,

60gpm
200 ft head + (calculated head losses including static lift) lets say 500 ft = 700 ft

re-calculating
60 x 700 / 3960 = 10.6hp = 7.85 kW

Overall pump /motor efficiency is 7.85 / 12.4 = 63%


Of course is all this worth the time and effort which at the end of the day is still only an estimate.

If you need a reliable baseline - just use the flow / head / power input of the installaton - any drop-off over time will be measureable against these figures.