Hotline FAQ: Pressure Tank for Constant Pressure Systems

Do I still need a pressure tank with a Franklin constant pressure system?

Variable Frequency Drive systems including Franklin’s MonoDrive and SubDrive still use a bladder tank but are typically much smaller than one sized for a standard system.

In a SubDrive or MonoDrive installation the tank’s role is to capture water turbulence, not store water. The MonoDrive and SubDrive systems are designed to hold water pressure to plus or minus one psi of the set point. With a switch this sensitive, any water turbulence around the pressure sensor would cause wider pressure swings. The tank’s job is to capture any wave action in the plumbing coming from the pump side, so the water moving around the pressure sensor is smooth and steady.

The gallons per minute, or gpm rating of the pump, determines the size of the tank needed. The tank’s air pre-charge is also different than a conventional system. Please see your installation manual for complete instructions.

See previous frequently asked questions on our FAQ page.

8 thoughts on “Hotline FAQ: Pressure Tank for Constant Pressure Systems

  1. There doesn’t seem to be any reference to tank sizing for true VFD’s that I can find. Bob McClain told me that, ‘tank sizing is typically 10% of your pumping capacity”. Although that statement maybe true, I need something in writting for a submittal.

  2. From Rick on the Hotline:
    For commercial, or what you call true VFD applications, Franklin Electric recommends the tank size be at least 10% of the anticipated max flow rate. For example, a system capable of 100 gpm would use a 10-gallon tank minimum. Franklin Electric offers a commercial size VFD called the SubDrive HPX. Your tank sizing information is found on page 13 of the HPX installation manual that can be found at the link Tammy posted above.

  3. From Rick on the Hotline:
    The only thing Franklin currently offers for SubDrive HPX tank sizing is in the Installation Guide. The Installation
    Guide also provides company information, which should satisfy as a submittal. Thanks for your interest.

    • SubDrives and HPX units are only a small part of the water well industry. Just hard for me understand that since Franklin Electric has been in operation some 60 plus years. VFD’s have been around for 40 some. Franklin Electric does not have any ‘recommendations’ for installation for commercial VFD’s on their motors?

      • Kurt,
        We completely understand your need, but as different combinations of VFDs and tanks may affect performance, your question is best directed to the VFD manufacturer. Accordingly, we can only publish recommendations as they apply to our own drives. We’d be happy to answer any further questions if you would like to call our Technical Service Hotline at 800-348-2420.

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