Do I need a pressure sustaining valve on a 2 inch deep well jet pump
I suspect you may be referring to maintaining pressure in the pipe between the pump and cistern while the pump is not operating.
I am pumping into an underground tank, so I have no pressure on the discharge side.
It seems, you maybe utilizing a float switch within the cistern, to control the pump based on the water level within the cistern. If so, with the end of the supply line remaining open, no pressure will remain in the supply line once the pump is no longer operating.
Assuming there is a foot valve within the 2" casing, and also assuming the water inlet is at the top of the cistern above the water level, if the supply inlet is lower in elevation than any part of the supply line, the supply line could drain into the cistern, resulting in at least some of the supply line becoming filled with air whenever the pump is not running.
To keep any pressure within the supply line, will require a valve to shut off incoming water flow into the cistern. This valve could be a float controlled valve, or could be an electric solenoid valve which the power is controlled by a float operated electric switch.
To maintain pressure in the piping between the foot valve and cistern valve whenever the pump is not running, will require a pressure tank and pressure switch, located somewhere between the pump and cistern valve. Because a liquid cannot be compressed, compressed air is utilized within the pressure tank, which will continue to push down on the water within the tank, which will will maintain pressure within the system.
Because your well pump is only supplying the cistern directly, the pressure switch setting may be calibrated to a low pressure range such as to activate the pump once the system pressure has been reduced to 10 psi, and shut off the pump once the system pressure has been increased to 30 psi. A lower pressure range will permit the pump to fill the cistern at a higher flow rate compared to when using a higher pressure range such as 40/60.
