Any vessel that contains both water and air under pressure can be called a hydropneumatic tank. The two most common applications are in well water systems, or as buffer tanks to absorb water hammer shocks in large capacity pumping systems. In both cases, the captive compressed air acts as a cushion which can exert or absorb pressure as required.
Large hydropneumatic tanks are usually horizontal and 2000 gallons or more. Hanson Tank has manufactured hydropneumatic tanks up to 30,000 gallon capacity. The size is determined by the needs of the system, and is approximately the drawdown gallons times four. A small air compressor can be included to replenish the air volume periodically as it is absorbed into the water. Sensors monitor the water level and pressure and automatically control compressor operation as required. These vessels are usually lined to prevent corrosion and have a design working pressure of 125# ASME. Hanson Tank can manufacture the basic tank or supply all the components for a complete system ready to install.
Tanks that include an internal bladder are usually smaller, and sometimes called captive air or bag tanks. There are styles that are designed to replace the expansion tank in a domestic hot water system. All bladder tanks are usually a standard vertical design. On larger versions, the bladder is often replaceable through a flange, which can be either on the top or bottom of the tank depending on the application.
Vertical hydropneumatic tanks can also have a diaphragm which can be fixed or floating. This virtually eliminates the absorption of the air into the water over time and makes the compressor unnecessary. These are built to order only.