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"Constant Pressure" Eliminates Water Hammer, Saving Water

Constant Pressure Systems differ from old fashioned hydro pneumatic and water tower systems in that the pump or pumps are made to produce exactly the same flow rate that is being used. The flow from a constant speed pump can be throttled with a "constant pressure valve" to vary the flow produced to match the usage. A variable speed drive can also be used to vary the RPM of a pump and motor to produce a flow rate exactly equal to the flow being used. If the pump produces even a small amount more flow than is being used the pressure will increase. If the pump produces any less flow than is being used the pressure in the system will decrease. In order to maintain a "constant pressure" the flow produced by the pump must exactly and instantly match the amount of water being used.

With an old fashioned hydro pneumatic or water tower system, water will always be produced at the maximum flow rate that the pump or pumps can produce. When the hydro tank or water tower is filled to a higher pressure the pumps are shut off. When the hydro tank or water tower is empty, the lower pressure will start the pump or pumps again and this process is repeated over and over. The repeated starting and stopping of the pumps is what produces transient pressure waves which causes water hammer and subsequent damage to the entire system.

Large water systems such as a city with more than 100 homes, cattle feed yard operations, and golf courses never get to a zero flow condition. Even during times when no water is actually being used, a dripping faucet here, a small leak there, and several seeps that are not large enough to ever show at the surface, add up to a continuous small flow rate. These larger systems can benefit from a constant pressure system using multiple size pumps. A jockey pump or pressure maintenance pump is a small pump that basically never shuts off and is used to efficiently keep up with leaks and small water usage. When more water is needed than the jockey pump can supply, a larger pump comes on line to make up the difference. When even more flow is needed, additional pumps come on line to supply exactly the flow rate that is being used. As the system requires less and less flow, pumps shut down when they are not needed, until only the jockey pump remains running to supply the small flows and leaks at off peak times such as in the middle of the night. This insures that the right number and size of pumps are running to efficiently supply any flow rate required. Constant Pressure systems also insures that the body of water in the pipe system remains in motion from the supply to the demand which eliminates water hammer, eliminating leaks and saving millions of gallons of water.

With "Constant Pressure" in large water systems the pump or pumps are continuously supplying the exact same amount of water that is being used. No extra water is produced, so water towers and big hydro tanks are no longer needed and an entire city can operate from a single 80 gallon bladder tank. Emergency water for fire protection can be delivered with a diesel powered pump or a backup generator for a fraction of the cost of a water tower. Not to mention that a backup pump lets you use the full volume of the reservoir for emergencies, instead of being limited to the small amount of water stored in a water tower.

With constant pressure controls for small water systems such as a single home, the body of water will stay in motion as long as any water is being used. Without constant pressure controls the pump will cycle on and off 3 or 4 times while someone takes a shower and will cycle multiple times when a sprinkler is left running. Constant Pressure controls will vary the output of the pump to exactly match the usage. The pressure remains constant and the pump runs steady any time someone is taking a shower or even if a sprinkler is left running indefinitely. These smaller water systems will have times when absolutely no water is being used. These systems need to be able to transition from having the body of water in the pipeline at rest, to being in motion, then back to rest again as smoothly as possible. This transition is handled differently with variable speed pump systems than with constant pressure valves.

With variable speed pumps the transition between no flow, flow, and back to no flow is met by ramping up and then back down the RPM of the motor. A slight drop in pressure will cause the system to ramp the motor up to the speed needed to deliver the water required. A slight increase in pressure will cause the system to ramp down the motor. Usually after the motor stays in a ramped down condition for a few seconds it is completely turned off. This keeps the pressure fairly constant which means the house is always at say 50 PSI. This also means the pump must start for every glass of water, for the ice maker, or even just to wash a tooth brush. A dripping faucet or running toilet can cause the pump to ramp up and down continuously. Any delay in response from the electronic controller to the motor RPM will cause a dip in pressure on startup and a spike in pressure before the motor shuts down. Variable speed pumps have computerized electronic controls and are only as dependable as other electronic devices such as computers, cell phones, and televisions.

Constant Pressure Valves will throttle the flow from a full speed pump to keep the house at 50 PSI as long as any water is being used. When all the taps are closed, about 1 GPM will still be flowing through the constant pressure valve. This 1 GPM has no place left to go except in to a bladder tank. The pressure will increase to 60 PSI and a standard pressure switch will shut off the pump. Water can then be used from the bladder tank for ice makers or a few flushes before the pressure drops to 40 PSI and the pressure switch starts the pump. When a tap is opened, the initial flow is started in motion by the pressure in the tank. When the tank is empty and the pump starts, the constant pressure valve is in the 1 GPM position which is a mechanical soft start. The 40/60 pressure bandwidth is what allows the draw down from a tank to be used so the pump does not have to start for every glass of water. The constant pressure valve is also in the 1 GPM position during the time when the pump is stopped. With a mechanical soft start and stop the transition between flow and no flow is made as smooth as possible. As long as a shower, a sprinkler, or anything else is running the constant pressure valve keeps the system at a steady 50 PSI. Constant Pressure Valves are simple, dependable, have no electronics and work with standard pumps, motors, starters, and pressure switches.

Smaller water systems have very few connections and short runs of underground pipe. Leaks are usually not a problem until water hammer makes the faucets and toilet float valves start leaking. Constant Pressure control will eliminate water hammer from the pump starting and stopping which can save thousands of gallons of water from being wasted to leaking faucets and toilets.

Constant Pressure systems can only eliminate water hammer during normal operations. Abnormal operations include things like initial startup and power out conditions. For these abnormal circumstances it is important to have safety equipment installed. Equipment that will help during a power outage include surge tanks, gas vessels, surge anticipator valves, and pressure relief valves. This kind of equipment cannot stop water hammer but will absorb some of the extremes in high and low pressures when needed. Software is available to help design these safety systems and can show you where in the piping system is the best place to install the safety devices.

Constant pressure valves make pumps, motors, starters, pressure switches, bladder tanks, and piping systems last longer. Constant pressure is a fundamental system change from filling and draining hydro tanks and water towers to varying the flow to exactly match the usage. Constant pressure may be the best way to completely eliminate water hammer. Water hammer not only causes equipment failure but is a major waste of fresh water and can cause contamination even in a pressurized line.

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Cycle Stop Valves, Inc.
10221 CR 6900
Lubbock, Texas 79407
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