Pressure Tank Size for a Variable Speed Submersible Pump
Maintaining 40-50 psi is the key. Yes you can do it with a 7 gallon bladder tank and it would be better to go to a 35 gallon bladder tank but depending on your flow you could set your pump to run at 80% all day and maintain 50 psi so you really don’t need much of a pressure tank in that case.
If your pump is producing 40 gallons at 70% then you want a big tank to reduce cycling of the pump.
Water cooled 3 phase pumps can start more often than single phase pumps because 3 phase start better than single phase with a capacitor.
You don’t want to cycle your pump because this is where the cost $$$ to run comes in. Remember a motor draws 3-5x normal full load amperage at startup and even higher amounts called locked rotor amperage until they start turning. Starting often can double your cost to run so that 35 gallon tank provides just under a minute were 7 gallon only a few seconds.
Will it hurt your system to go big on the tank if it would run at 70% 50 psi nope won’t hurt anything but it provides a buffer if a head gets plugged so you are not pumping enough for constant run.
So go big with the pressure tank it will help in most cases and won’t hurt. Note I did not talk about dual set points 50 or 60 psi pump drops to 70 then at 40 it jumps to 120% never stopping lowest power consumption and the pressure tank being big helps maintain control. Of pressure because there is a slight lag from pressure reading, pump commanded up to pressure increased.