[1] Hydropneumatic refers to the pneumatic (gas) and hydraulic (water) components needed for operation of the devices.
Hydropneumatic water hammer preventers are chambers of sufficient volume to allow an extension of time in which a given flow may be accelerated or decelerated without sudden large change in pressure.
The controllers are used in a circuit after a pump that is followed by a valved-side branch, and beyond a check valve, so that this device can only discharge liquid volume by a pressure fall of the system.
Variations on the design include Hydropneumatic pulsation filters provide means of reducing the amplitude of pressure changes the velocity of which is in the order of 1.4 km/s.
Lack of any sudden change in cross section area of flow path that would reflect a pressure wave, i.e. no orifice plate(s).
Variations include Combination "dual purpose" devices addressing "acceleration head reduction" by means of a gas containment.
This acceleration head needs to be reduced to prevent damage to pump components and excessive noise.
[3] These devices are typically mountable in any orientation such that the device is connectable directly to the suction check valve beneath the pump or directly to any vertical or horizontal discharge check valve; minimizing the length of any liquid column mass that will experience velocity change.