What you didn't need to know about Hydraulic Accumulators Hydraulic Accumulators first started being used in the simplest form during the Industrial Revolution. Hydraulic ring mains were used at most docks around the country, using water as the hydraulic fluid. The water was pumped into piston type holding tanks which were put under pressure by having large weights on top of the piston, and thus allowing the effect of gravity to create the pressure. Description and Operation A hydro-pneumatic accumulator is a device that can store large amounts of energy within small spaces in a hydraulic system. Since liquids are practically incompressible and therefore unsuitable as energy accumulators, compressible gases are used for this purpose. In a metal casing (the accumulator's body), an elastic diaphragm or bladder separates the liquid from the gas chamber. Inert gas (nitrogen) is let into the accumulator at pressure P1, suitable for that particular application; the gas fills the entire volume of the accumulator V1. When the systems pressure P2 exceeds the accumulator filling pressure, the poppet valve lifts allowing fluid into the accumulator and the volume of gas in the bladder is reduced to V2. When the pressure is further increased to P3, the gas volume is reduced to V3 and its pressure increases to balance the liquid pressure. The usable stored volume V = V2 - V3 The theory behind accumulator sizing is shown as a sample calculation sheet. The theory is based on thermodynamics and the behaviour of gases.