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Pump Power Savings

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Wattman

Electrical
May 2, 2001
24
An Application utilizes two pumps to fill a water tower. When the water level reaches the lower setpoint the first pump starts and runs until the high setpoint is reached at which the pump is stopped. (Just a typical pumping scenario.)

The cycle of the pumps appear to be a runtime of about 30 min then shut down for 20 min. But I'm sure this would vary somewhat over a 24/7 period. The water in the tower is used to supply water to a large industrial plant which operates 24/7, the demands of water usage depends on the number of boilers online and other process variables. The application probably does not have the demand changes that residential water customers would impose.

Question: The pumps use softstarts for starting and stopping, and I have learned that the power bill is approx. 2800 per month and feel that changing out the softstarts and installing A/C Drives would probably save power but I'm unsure of how to calculate the savings?

I would think allowing the pump to speed up/down to maintain a set level in the tower would be best. And I understand some of the complications involved with the pumps operating at a slower speed.

I plan to install a data logger to record the starting and stopping of the pumps with time and date stamps in order to evaluate the application, and I feel there would be more advantages than the power savings alone.

Is there a "Rule of thumb"????

Please Advise
 
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Hello;
I doubt that there will be any energy savings at all. For one thing the motor and probably the pump effiency will decrease with the lower spped. Then you have the power losses within the VFD. You might actaully use more power!
 
I agree there will be new losses in the drive and increased losses in the motor due to frequency-dependent losses.

There is some savings in fluid friction losses (i.e. in transfer piping) which are approx proportional to flow-squared. If you decrease your flow-rate to 1/2 and double your run time, then you have 1/4 of the power loss (watts) from friction times twice the duration gives 1/2 the energy (joules) for the same volume of fluid.

I think that some pump losses will behave similar to the friction losses discussed above.

Whether or not those savings will exceed the increased drive/motor losses and also justify the captial outlay would require some study.

In general you would get less wear and tear on the motor from fewer starts, but if you've already got soft start that is not much of an issue.
 
It would seam that if I could keep the pump running to maintain the level. There would not be as many start cycles and the second pump would not need to come on to assist in meeting the High demands. Them the utility peak would be lower for the month.

The system works off of a 3 min delayed telemetry system now. I could utilize the new wireless technology which would allow the level to update quicker.
I'm not second guessing anyone; I just need to generate some numbers to confirm it.

Please advise
 
You could reduce demand if you could anticipate when other plant loads peak and make sure that the tank is filled before the peak. This would avoid having the pumps running at the peak demand time.
 
Suggestion: If you happened to have the pump motor characteristics that would include efficiency versus motor speed, it would be a good start to prove issues. It appears that the motor efficiency would decrease with the motor slowdown. How much, that will be taken off from the efficiency versus speed plot.
Data loggers will also be useful as you already know.
There may also be hidden aspects, such as motor-pump frequent starting and stopping that causes the equipment wear and the shorter life-cycle.
 
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