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Sizing a Tank for Residential Systems with Leaks When using a Cycle Stop Valve, small pressure tank and a pressure switch for controlling a small pump system, consider the system leaks. Always the worst case scenario is a demand or leak on the system that releases less than the amount of the minimum flow of the Cycle Stop Valve. With a one inch Cycle Stop Valve the minimum flow is set at about 1 GPM. If we have ½ of a GPM leaking in the system, the Cycle Stop Valve will always allow the pump to produce a minimum of 1 GPM. ½ of this GPM will be going directly to the leak and the other ½ of a GPM will fill the pressure tank until the pressure switch shuts off the pump. Then the ½ GPM leak will slowly use all the water in the tank and the pressure will drop to the point where the pump will be restarted by the pressure switch. Use a pressure tank that will keep the number of starts per day lower than the maximum allowed by the motor manufacturer. Using a motor with a maximum of 100 starts per day allowed we would figure the tank size as follows. 1440 minutes per day divided by 100 maximum starts allowed equals one start every 14.4 minutes. Adjusting the Cycle Stop Valve and pressure switch to get a three minute run time during a zero flow condition means that the pump must remain off for the other 11.4 minutes to get a minimum of 14.4 minutes per start. Using the worst case of ½ GPM leaking we would multiply 11.4 minutes times the ½ GPM getting a drawdown required of 5.7 gallons. Using a pressure tank with 5.7 gallons of drawdown and having a leak of ½ GPM it will take 11.4 minutes to drain the tank from 60 PSI to 40 PSI causing the pressure switch to start the pump. It will take a 50 PSI Cycle Stop Valve a minimum of 3 minutes (if the leak stops for that period of time) and a maximum of 6 minutes (if the leak continues) to refill the tank from 40 PSI to 60 PSI so the pressure switch can shut off the pump. On for three minutes and off for 11.4 minutes gives us a minium cycle time of once every 14.4 minutes or 100 starts per day worst case. Double the size of the tank and it cuts the number of cycles per day in half. When using a Cycle Stop Valve the worst case number of cycles per day can be easily controlled by the amount of drawdown in a tank. (See also "VFD Maximum Number of Starts per Day") |
| Cycle Stop Valves® is a registered trademark. All right reserved unless prior authorization is obtained. Cycle Stop Valves are patented: Patent number 5,988,984 and other patents pending. |
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