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Low pressure a problem for Shurflo 2088?

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  • Low pressure a problem for Shurflo 2088?

    I have a Shurflo 2088 that I want to use to pressurize a water tank. It will have unrestricted water from a cistern but with virtually no psi. Will that be a problem? The Shurflo manual states "If the plumbing is restrictive or the flow rate is very low, the pump may re-pressurize the outlet faster than the fluid is being released causing rapid cycling." I could also potentially put the pump and pressurized tank at a lower elevation where it will have more head and slightly more psi.

  • #2
    No pressure between the cistern and pump should not be a problem as long as it unrestricted and the lift is within the pumps specs.

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    • #3
      From what you write, seems to me they may be telling you to avoid upstream plumbing designs that would promote pump cavitation.

      Add: More thought = While inadequate pump inlet line sizes can cause cavitation and pressure fluctuations that can, depending on the design, lead to rapid pump cycling, they're probably referring to rapid cycling caused by higher pressure transients from downstream piping design inadequacies such as a combination of small line sizes and long runs/lots of bends, etc. Design problems can happen on either side of a pump.
      Last edited by J.P.M.; 01-18-2018, 12:11 PM.

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      • #4
        These pumps are activated by a drop in pressure. They will continue pumping until they reach a pre set pressure. That being said they need to output into a closed system. I would think that using them to fill a tank you would need to be able to manually shut them off. Possibly a float switch could be set up to start/stop the pump.
        Last edited by littleharbor; 01-19-2018, 09:39 AM.
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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        • #5
          Originally posted by BryanB View Post
          I have a Shurflo 2088 that I want to use to pressurize a water tank. It will have unrestricted water from a cistern but with virtually no psi. Will that be a problem? The Shurflo manual states "If the plumbing is restrictive or the flow rate is very low, the pump may re-pressurize the outlet faster than the fluid is being released causing rapid cycling." I could also potentially put the pump and pressurized tank at a lower elevation where it will have more head and slightly more psi.
          They do not need pressurised feed but like any pump can only lift so much, the lift spec should be in the pump info.

          The rapid cycling is when it reaches pressure and cuts out via its internal switch. normally about 40psi standard setting.

          Set up your water system, if you have trouble with it rapid cycling when it reaches pressure to cut out then install an accumulator.

          Shurflo make small ones, you can also make your own.

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          • #6
            Thanks everybody for your help!!!

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            • #7
              Originally posted by BryanB View Post
              I have a Shurflo 2088 that I want to use to pressurize a water tank. It will have unrestricted water from a cistern but with virtually no psi. Will that be a problem? The Shurflo manual states "If the plumbing is restrictive or the flow rate is very low, the pump may re-pressurize the outlet faster than the fluid is being released causing rapid cycling." I could also potentially put the pump and pressurized tank at a lower elevation where it will have more head and slightly more psi.
              Just a bit more info:

              Pumps need some head pressure ( outlet pressure) to operate correctly, so they can either have an outlet that is smaller than the intake to create pressure in the pump or use the head pressure of the water.

              If a pump does not have a restriction, or head pressure, it can throw the water out faster than it can suck it in causing cavitation.

              The shurflo if a diaphragm pump so they dont matter so much but you still want to keep the outlet to keep the pump full.

              As long as the tank you are pressurising is at the same level or only slightly higher than the pump this will be achieved.

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              • #8
                Originally posted by Bala View Post

                Just a bit more info:

                Pumps need some head pressure ( outlet pressure) to operate correctly, so they can either have an outlet that is smaller than the intake to create pressure in the pump or use the head pressure of the water.

                If a pump does not have a restriction, or head pressure, it can throw the water out faster than it can suck it in causing cavitation.

                The shurflo if a diaphragm pump so they dont matter so much but you still want to keep the outlet to keep the pump full.

                As long as the tank you are pressurising is at the same level or only slightly higher than the pump this will be achieved.
                And a bit more:

                I think you mean inlet pressure, or inlet head, If a pump is operating correctly, it will produce outlet head. Both inlet and outlet head are composed of system pressure, static head (elevation differences ) and velocity head. The head added by a pump in an atmospheric system with no outlet restrictions such as nozzles, valves or piping for example, that will be discharging directly to the atmosphere will be expressed mostly or entirely as velocity head, not pressure. If the discharge is to pressurize a tank, for example, the outlet head will be tank pressure + velocity head plus line pressure drop.

                Cavitation happens when the pressure on a fluid in the pump is less than the vapor pressure of the fluid at it's transit temp. through the pump as a result of the pump characteristics. If the NPSH available is < the NPSH required, some of the fluid in the pump will, in effect, boil (vaporize) and then the vapor "bubbles" will collapse as the pump adds pressure. Cavitation usually sounds like stones or marbles in the pump, or not unlike the sound an Otto cycle ICE makes when it's knocking.

                Relative pipe sizes can be larger or smaller, inlet or outlet, depending on the system requirements. In a well designed system, there is rarely, if ever, any direct relation between those line sizes with respect to cavitation possibilities. As long as the system is designed such that the NPSH, available is > the NPSH, required, the inlet line size relative to the outlet line size doesn't matter, at lest not for cavitation considerations.

                The available inlet pressure, or "net positive suction head, available" (NPSH, available), must be greater than the "net positive suction head, required" (NPSH, required) at the suction port (the inlet) of the pump to prevent cavitation.

                The idea is to have sufficient NPSH, available to avoid cavitation, not necessarily lowering the NPSH, required by throttling the outlet to avoid cavitation. Throttling the outlet will effectively reduce the (absolute) required inlet suction to maybe/probably lower that NPSH, available, enough to stop or prevent cavitation, but that's a poor way to get around a bad design. Correct way is to size and select the correct type of pump for the application and for the flow required, and then design the inlet piping to give an available NPSH that's high enough to prevent cavitation at max. operating NPSH of the pump. Design the outlet piping for max. flow, at allowable pressure drop and fluid mechanical considerations like vibration, flow maldistribution, pipe themmal expansion/contraction and support. Throttle the outlet to control the flow rate, not to prevent cavitation.

                Diaphragm pumps, like all pumps, have NPSH requirements, but because diaphragm pumps are a type of positive displacement pump, the NPSH requirements are usually calculated differently because of what are usually cyclic fluid acceleration characteristics common with positive displacement pumps.

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