It became obvious to us many years ago that valves could save as much energy as variable speed pumps. Most jobs relied on a control valve bypass and across the line starter to insure a supply of water when the variable speed drive malfunctioned. A switch toggled the system from a variable speed drive to full speed with a valve. One particular job in 1992 used a motor that pulled 12 amps at full load. At low flow the drive would slow the motor down to 6.5 amps. Switched to full RPM with a valve controlling the flow, the amps would drop to 6.9. Only four tenths of an amp was the difference between using a valve and a variable speed on this particular pump.
The instructions and set up for the drive were very technical. Small power fluctuations kept shutting things down. Vibration in the motors was a visible problem. Spikes from the pulsing DC voltage required special insulation on motor windings. Drives added tremendous cost to our quotes with very little profit margin. Only one person out of twenty in our company was qualified to touch them. Drives were unreliable and saved no more energy than a control valve.
The affinity law states, that head produced by a pump is reduced by the square of the speed. Slowing the pumps speed by as little as 10% reduces the pressure or TDH by 19%. In most cases, the pumps output pressure can be lowered very little and still produce the "constant pressure or TDH" required by the system. If it is possible for your particular pump to be slowed by 10% and still provide the "constant pressure or TDH" needed, the horse power is only being reduced by 27%. This will only reduce the load of a 10 HP to 7.3 HP. The formula below shows how to figure minimum pump speed and minimum possible horse power when using a variable speed.
The example shows that when using a drive, a 10 HP pump will be using electricity for minimum of 7.1 HP.
When "constant pressure or TDH" must be maintained, some pumps can save more energy when restricted with a valve than when slowed with a drive. Saving energy is a natural characteristic of a centrifugal impeller. Simply restrict the flow from a pump and horsepower or power consumption decline naturally. Using a brake horsepower curve it is easy to select a good pump. Or you can use the original brake horsepower equation as follows.
This example shows that when restricted with a valve, (even at 5% efficiency) this 10 HP pump would only use 5.8 HP current, compared to the previous example using the drive with a minimum horse power of 7.1 HP.
Many times there is very little difference in power consumption of a pump between using a variable speed drive or a control valve. With this in mind, what do we really need a pump control system to do?
- Vary the flow rate and the power consumption to match the usage.
- Maintain a safe minimum flow rate to keep the pump cool.
- Turn the pump off when it's not needed and on when it is.
- Make sure rapid cycling does not happen at any flow rate, including small leaks.
- Don't cause water hammer.
- Be dependable.
For many years people have been using a mechanical control valve to vary the flow and power consumption while maintaining a "constant pressure". A hole drilled through the body or a bypass tube around the valve would insure minimum flow to cool the pump. When the system requires zero flow, the main valve closes completely and this bypass water will fill a small tank very slowly, then a mechanical pressure switch can be used to stop the pump at a higher pressure, and restart the pump at a lower pressure. The small pressure tank stores enough water to limit cycles per day when leaks are in the system. Their idea was very good but, in the field there have always been two major problems.
First, the small bypass was the only cooling for the pump. When it clogged with debris or grew closed with minerals or moss, inevitably the pump would be destroyed. Secondly, the valve operated slower than the changes in the system, bouncing the pump off and on. Speeding it up causes water hammer or surge when the valve pops open or slams shut.
Preventing the valve from closing all the way was the key. This insures the required minimum cooling flow, and eliminated the problem of clogging a bypass tube or hole. Valve speed could be increased, even super charged to react as fast as possible. Regardless of valve speed, water hammer cannot be produced by a non-closing pump control valve.
It's always the simple idea that solves the big problem. We filed for a patent in 1993 and have been replacing complicated variable speed drives with dependable "constant pressure" valves ever since. One recent example would include a system with three 200 HP turbine pumps. Variable speed equipment for this system would have cost around $200,000.00. Less than $15,000.00 was spent on "constant pressure" valves and related equipment. The simplicity of the system along with the reduced energy consumption and up front savings prompted the operator to retrofit several other systems with "constant pressure" valves.
Another example is a golf course with a 75 HP submersible in a well being controlled by a "constant pressure" valve. Pressure remains at a constant of 100 PSI regardless of the use being 800 GPM, 322 GPM, or just a single 5 GPM quick connector. If a variable speed drive had been used the submersible motor would overheat or "cycle" during low flow situations. When using a "constant pressure" valve a submersible motor can handle low or no flow conditions, and will stay cool running as low as 5 GPM, that by itself is a whole other story. (See "Cycle Stop Valve Cools Subs Better")
A smaller example would be a 5 HP booster pump for a base ball field. With a "constant pressure" valve, zones can range from 5 GPM to 80 GPM at 80 PSI.
Drives are getting smaller, less expensive, have less spikes, and are easier to set up. Low flow and vibration problems are difficult to overcome. There are some applications including sewage pumps, conveyor systems, escalators, and others that don't require "constant pressure", where energy can be saved using variable speed. Knowing when variable speed can be used without causing trouble, takes as much common sense as technical know-how.
We stopped using drives for "constant pressure" in 1992. "Constant pressure" valves are the new way, and the best way to get variable flow performance without the complications of variable speed drives. Simple technology can improve performance and dependability while complicated technology can be expensive, and because of the technical nature, usually less reliable.