Estimation variable speed. Savings
Estimation variable speed. Savings
(OP)
I would estimate savings by installing a variable speed in a centrifugal pump of 355 kW, 1480 rpm. Could anyone give me an idea about how much a variable speed regulator would cost?
Thanks in advance





RE: Estimation variable speed. Savings
Do you have a reasonably constant flowrate (within 60% to 110% of BEP flowrate) that isn't practical to smooth to a constant flow by filling and withdrawing from a storage tank?
Do you have a high static head at low flows (Head >= 50% of the head needed at the BEP flowrate)?
If you answered yes to either one, I doubt you need a VSD.
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RE: Estimation variable speed. Savings
The total head at 2200 m3/h is 50 m (Hst= 40m)
The total head at 168 m3/h is 46 m (Hst=40 m)
Is not worth it to use the vsd? In which cases it would be a good way of flow regulation?
thanks again
RE: Estimation variable speed. Savings
To generate the 40 m head, you'd have to run at about 90% speed, giving a minimum flowrate of around 2000 m3/h from the pump. If that matches your system curve and if you could plan on doing that most all of the time, maybe you could reach a 10% on your power bill.
Looks like good old fashioned flow control valves, but 10% is 10%, so if that saving potential looks attractive, do a more in depth study that includes estimating the running time at various flowrates to get a better idea of the actual expected power cost and decide based on that result.
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RE: Estimation variable speed. Savings
Taking the basic cost of a VSD is only part of the calcualtion to determine a return on your investment (assuming you have answered the first part of my post). You need to take into consideration the following:
1) Purchase cost of VSD
2) Installation costs including labour and additional materials e.g. cabling (shielded from VSD to motor), maybe specific drive enclosure for the environment, EMC filters, maybe harmonic filters or reactors, control strategy (you will need some sort of intelligence to 'tell' the VSD what sort of speed the pump should be running at), plant downtime if the pump is existing etc.
3) If it is an existing pump, then you need to check the motor is suitable for inverter use. If not, then the cost of a suitable motor may be required or some sort of sinewave filter on the output to ensure the PWM waveform is not detrimental to the motor life.
4) I've already mentioned the issue of harmonics but 355kW is quite a large motor/drive. If your power system is not large enough then the harmonic distortion could be an issue. It needs investigating and maybe analysing. .I know some water companies in the UK would demand a 12-pulse configured drive or other similar type to ensure harmonic distortion is kept to a minimum. This is expensive.
All these cost money and need to be included in any calculation you do to determine any potential savings.
But as I say at the start, you will only see 'savings' when you compare it to another method of flow control.
RE: Estimation variable speed. Savings
For estimates, I have the habit of using about 10% of pump cost for providing a VSD option. Do you think that's a reasonable number?
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RE: Estimation variable speed. Savings
Check the power at partial closed condition of the valve corresponding to 1680m3/hr flowrate and see if you have any significant difference in the savings.
I, generally, consider 2 years as payback period for viability.
RE: Estimation variable speed. Savings
To be honest, I couldn't be sure on that figure. It wouldn't be linear is my guess due to the 'additional' items needed as drives get bigger.
There's an interesting article published here concerning Pump life cycle costs and it includes some estimates on different control strategies:
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RE: Estimation variable speed. Savings
pdf looks usefull.
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RE: Estimation variable speed. Savings
RE: Estimation variable speed. Savings
You will save energy compared to throttling a valve but the key is whether it will pay back your investment in a reasonable time.
2 years ROI seems to be the norm now. I remember when 5 years was acceptable....
RE: Estimation variable speed. Savings
RE: Estimation variable speed. Savings
RE: Estimation variable speed. Savings
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