A recent article by the Hydraulic Institute in the October issue of Pumps and Systems magazine list several "Opportunities to Save Power" with pump systems. While the items in the list are correct, giving VFD the credit for saving energy is wrong. When the Hydraulic Institute, who says they have been "creating pump standards since 1917" cannot get their facts correct, how can we expect the average person to understand? I sometimes feel like I am the only person willing to stand up and say "the emperor is not wearing any cloths" and "VFD does not save energy". I have written many articles and letters to the editors dispelling the myth that variable speed pumps save energy. No one has yet been able to argue with my facts, but the articles and letters do not get published because they go against the popular misconception that VFD saves energy. Everyone wants so badly for the VFD to be the magic elixir that cures all pump problems that they cannot see the facts staring them in the face.
Under HI's list of (Opportunities to Save Power) are:
- Maintain pumps close to as-new condition
- Do not allow for excess margins
- Use a more efficient pump design
- Use two or more smaller pumps instead of one larger pump
- Use pumps as turbines to recover energy when possible. To this list should be added the most important power saving possibility
- Increase pipe size to keep system head as flat as possible
System head curves that increase the head required as the flow increases are caused by the piping system being too small. Increasing the pipe size will allow for smaller pumps to be installed to start with. When considerable more head is required at high flow than at low flow, velocity at high flow rates can cause destruction of the pipe line itself. Also when the head pressure is increased to supply customers at the end of the system with adequate pressure during times of high flow, customers close to the pump system will experience high pressure.
The article from HI starts out with, "the most cost effective change that can be made is converting to variable speed drives for your pumps". This statement makes the article from HI incorrect.
From the example curve that follows we have a pump running at full speed (3550 RPM) and simply choked back with a valve from 1,200 GPM to 100 GPM. The power requirement drops from 100 HP to about 42 HP.
Using the same pump and reducing the RPM with a drive until the pump is producing 100 GPM at 100 PSI we are still using 38 HP current which is 2.38 gallons per horse power compared to 2.63 gallons per horse power with valve control. When using a drive there are 3% to 5% losses in the drive itself, so we will actually be using at least 103 HP to pump 1,200 GPM which is only 11.65 gallons per horse power compared to 12 gallons per horse power with valve control. The longer the pump is being used at 1,200 GPM the more energy is burned by the drive using 3% more compared to across the line using a control valve. When the flow required is only 100 GPM, the drive is only saving about 10% using 38 HP instead of 42 HP. When the required flow rate is 1,200 GPM for 12 hours per day and 100 GPM for 12 hours per day the kilowatt hours used are as follows.
Example from curve:
- 231' TDH, 1200 GPM 12 hours per day, 100 GPM 12 hours per day
at $0.10 per KW hour, 100 HP drive cost $10,000.00
KW to run pump with valve:
- 1 KW per HP X 100 HP X 12 hours = 1200 KWhrs. 1 KW per HP X 42 HP X 12 hours = 504 KWhrs. Total 1704 KWhrs.
KW to run pump with drive:
- 1 KW per HP X 103 HP X 12 hours = 1236 KWhrs. 1 KW per HP X 38 HP X 12 hours = 456 KWhrs. Total 1692 KWhrs.
Cost of running pump with valve control:
- 1704 X .10 = $170.40 per day
Cost of running pump with drive control:
- 1692 X .10 = $169.20 per day
Cost difference valve verses drive = $1.20 per day or less than 1%
If the drive cost $10,000.00 more than a control valve, it would take 22.8 years to save enough energy to payback the cost of the drive. Not one or two years payback that you normally hear from drive manufacturers. If a drive lasted for 23 years, then it would start saving 1% in energy. Since drives are computers, like any other computer the technology changes so frequently that they are obsolete in about 2 years. Some drive manufacturers have started to guarantee availability of parts for 10 years. If that means that having problems after 10 years means having to purchase a new drive, then a drive will never last long enough to pay for itself.
If a valve burns energy then so does a drive. A VFD burns almost the same amount of energy as a valve at low flow rates and burns even more energy at high flow.
It is the "counter intuitive" property of a centrifugal impeller that makes the horsepower requirement go down as the back pressure on the pump goes up. This along with the Affinity Law, which causes pumps to lose head by the square of the RPM, makes this concept difficult to understand.
Give credit where credit is due. There are lots of good uses for drives and lots of good ways to save energy. But, drives themselves can save no more energy than control valves. Anytime a pump control system can be done with anything other that a VFD, a multitude of negative side effects are eliminated. A few of the negative side effects caused by VFD are; harmonics fed back into the grid, mechanical frequency vibration, electrical fluting of ball bearings, voltage spikes, RF interference, energy loss compared to across the line, skin effect, high purchase cost, and specialized maintenance and installation.
I have read about lots of situations where drives are getting the credit for saving energy. In reality a better designed pump control system was used which allowed for a smaller pump to be installed or a dump valve to be discontinued. Other than better controlling the pump, which could also be done with a control valve, the drive has nothing to do with energy savings and can actually increase the energy used per gallon. Again, VFD does not save energy and the emperor is still naked.