Large water systems, which operate at high pressures during peak water usage can save millions of gallons of water by using a lower pressure during times of off peak usage. An example would be a system that needs 120 PSI to operate sprinklers. When the sprinklers are not running, the only usage is a few leaks and some hand watering with garden hoses, which do not need to operate at 120 PSI. Changing the system pressure from 120 PSI during peak usage to 50 PSI during off peak usage will reduce the pressure on the leaks in the system. Leaks at 50 PSI compared to 120 PSI will reduce the amount of water wasted tremendously.
Changing the off peak pressure from 120 PSI to 50 PSI on an example system reduces the leaks from 15 GPM to 6 GPM. This will reduce the amount of water wasted by 8,640 gallons in the 16 hours of off peak each day. Each year this will be a savings of our precious water resources of three million one hundred fifty three thousand six hundred gallons. (3,153,600 gallons) Reducing the number of gallons used also saves electricity. Gallons pumped is energy used and producing these gallons at a lower pressure also reduces energy consumption. The following are a few ways to easily use FBPM or dual pressure systems.
Systems with one small pump and one large or two alternating large pumps can be adapted to a dual pressure system easily. For this example the small pump will produce 50 GPM and the large pump will make 1,000 GPM. The sprinklers on this system require 120 PSI and put out 65 GPM each. The small pump will be set up to come on at 50 PSI, go off at 60 PSI, and the attached Cycle Stop Valve will hold 50 PSI constant. The large pump will come on at 40 PSI, go off at 130 PSI, and it's Cycle Stop Valve will be set to hold a constant 120 PSI. A small bypass line should be plumbed around the Cycle Stop Valve on the large pump. This bypass line should have a small ball valve included to adjust the rate of bypass. It has been determined that there is about 15 GPM leaking in the system at 120 PSI. The small bypass line should be adjusted to allow about 30 GPM past the large Cycle Stop Valve.
The Cycle Stop Valve on the large pump will try to hold 120 PSI constant. When demand on the system is less than 30 GPM (no sprinklers running) the bypass line will allow the pressure tank to fill to 130 PSI and a pressure switch will shut off the large pump. The leaks in the system will continue to drain the pressure tank and the pressure will lower from 130 PSI to 50 PSI. At 50 PSI a pressure switch will start the small pump. The Cycle Stop Valve on the small pump will try to maintain 50 PSI constant. The small pump can produce up to 50 GPM, so as long as the system uses less than 50 GPM the system will stay at 50 PSI. The Cycle Stop Valve will vary the flow from the small pump to match the leaks and small usage from 5 GPM to 50 GPM. If the system was able to stop all leaks and go to zero flow, the 5 GPM minimum through the small Cycle Stop Valve will fill the pressure tank to 60 PSI and the pressure switch will shut off the small pump. However, we know that there is 15 GPM leaking when the system is at 120 PSI. The leaks will reduce considerably when the system is maintained at 50 PSI but, there is still 6 GPM leaking at this lower pressure so the small pump will continue to run producing 6 GPM exactly. The small pump on this system is designed for continuous duty and to be efficient at flows between 5 GPM and 50 GPM. It is good for a pump to run continuously, as it will, as long as at least 5 GPM is leaking or being used.
If even one of the 65 GPM sprinklers is turned on, the 50 GPM pump will no longer be able to maintain it's design point of 50 PSI. When the pressure is lowered to 40 PSI a pressure switch starts the large pump. The setting of this Cycle Stop Valve and the design point of this large pump are both 120 PSI. As this 1,000 GPM pump comes on the Cycle Stop Valve was closed to 5 GPM at start up but instantly opened up to allow pressure to build to 120 PSI. As the system pressure increases going through 60 PSI a pressure switch shuts off the small pump. Pressure continues to increase to 120 PSI. At 120 PSI the Cycle Stop Valve on the large pump begins to throttle back the discharge from the pump. Any flow required from one 65 GPM sprinklers to 15 sprinklers which require 975 GPM will be met by the Cycle Stop Valve. If pressure rises to 121 PSI the Cycle Stop Valve throttles back flow from the pump. If pressure falls to 119 PSI the Cycle Stop Valve opens up to allow more flow into the system from the pump. This keeps the system at a constant 120 PSI regardless of the number of sprinklers on at any one time. When all the sprinklers have been turned off and the system flow is less than 30 GPM, the flow remaining from the 30 GPM bypassing around the Cycle Stop Valve will increase the system pressure to 130 PSI. At 130 PSI the pressure switch shuts off the large pump. When the leaks in the system have lowered the pressure to 50 PSI the small pump is restarted and again takes care of the leaks and small water demands at 50 PSI.
The small pump and large pump or pumps can be installed side by side or in different locations of the system. These pumps must only be pumping into a common system with pressure allowances made for elevation differences.
Another way to easily use this dual pressure system is when boosting from a low pressure to a higher pressure. An example would be boosting city water pressure from 50 PSI to 120 PSI for the same reasons as above with a single or alternating duplex pumps. The city maintains 50 PSI on our supply line, which is enough pressure for small usage or leaks in the system. A 1,000 GPM pump or a pair of alternating 1,000 GPM pumps are controlled with a Cycle Stop Valve set at 120 PSI. A pressure switch starts the pump at 40 PSI and turns it off at 130 PSI. A small line and ball valve is used to allow 30 GPM to bypass the Cycle Stop Valve. A backpressure pilot must also be used to keep water from flowing through the pump unless that pump is started.
Another line with a check valve and a ball valve is used to bypass water from the inlet of the pump to the discharge past the Cycle Stop Valve. A small line adjusted to allow 50 GPM past the pump station would mean that any flow below 50 GPM would be supplied directly from the city line at 50 PSI without the pump running. Any flow above 50 GPM would cause the system pressure to drop to 40 PSI where a pressure switch would start the pump. The Cycle Stop Valve would allow the pump to increase system pressure to 120 PSI. The system would remain at 120 PSI as long as flow between 30 GPM and 1,000 GPM was being used. When the sprinklers are turned off, the flow remaining from the 30 GPM bypass around the Cycle Stop Valve would increase the pressure to 130 PSI, and the pump is shut off. The leaks in the system will lower the pressure to 50 PSI and the city line can again take care of the leaks and small usage.
Another example system might only need to increase pressure when flow increases to a point where friction loss becomes a problem. The city water pressure or small pump running at 50 PSI may be all that is needed until flow is increased above 500 GPM. These larger flows up to 1,000 GPM add 30 PSI of friction loss. Bypass the entire pump station with a check valve, gate valve, and bypass line of sufficient size to allow 500 GPM through. When more than 500 GPM is being used the gate valve on the bypass restricts the flow. The pressure in the system drops to 40 PSI starting the pump. The Cycle Stop Valve allows the pump to bring the pressure up to 80 PSI. When flow required is above 500 GPM and less than 1,000 GPM the Cycle Stop Valve maintains the system at 80 PSI. When the sprinklers are turned off, a bypass line around the Cycle Stop Valve allows the system to increase in pressure until the pump is shut off at 90 PSI. Then the leaks in the system lower the pressure until the pump station bypass begins to supply flow at 50 PSI.
Systems with more than one main pump or two alternating pumps can also be controlled at dual or more than one pressure. These systems may require a flow meter and a PLC to control the start and stop of the additional pumps.