Hydropneumatic Tanks are ASME vessels that contain both water and air under pressure.
The compressed air acts as a cushion that can exert or absorb pressure as required.
A small air compressor may be used to replenish the air volume periodically as
it is absorbed into the water. Large hydropneumatic tanks are usually horizontal with
volumes of 2,000 gallons or more. Normal operating pressure for a water system
is 60 to 75 psi. Therefore, common design pressure for a hydropneumatic tank is 100 psi.
They provide an adequate "quick demand" water supply and help regulate
proper system pressure range, both necessary for the efficient operation of
a water supply system.