Hydraulic accumulator
The first accumulators for William Armstrong's hydraulic dock machinery were simple raised water towers.
When dock machinery required hydraulic power, the hydrostatic head of the water's height above ground provided the necessary pressure.
Around the same time, John Fowler was working on the construction of the ferry quay at nearby New Holland but could not use similar hydraulic power as the poor ground conditions did not permit a tall accumulator tower to be built.
By the time Grimsby was opened, it was already obsolete as Armstrong had developed the more complex, but much smaller, weighted accumulator for use at New Holland.
The latter tower is to be renovated as part of plans for the proposed development of the area associated with the construction of a new football stadium for Everton F.C.
A raised weight accumulator consists of a vertical cylinder containing fluid connected to the hydraulic line.
In contrast to compressed gas and spring accumulators, this type delivers a nearly constant pressure, regardless of the volume of fluid in the cylinder, until it is empty.
The working pressure is 750 psi (5.2 MPa, or 52 bar) which was used to power the cranes, bridges and locks of Bristol Harbour.
A compressed gas accumulator consists of a cylinder with two chambers that are separated by an elastic diaphragm, a totally enclosed bladder, or a floating piston.
The other chamber contains an inert gas (typically nitrogen), usually under pressure, that provides the compressive force on the hydraulic fluid.
Inert gas is used because oxygen and oil can form an explosive mixture when combined under high pressure.
If the pressure does not vary over a very wide range this can be a cost effective way to reduce the size of the accumulator needed.
The advantages to the metal bellows type include exceptionally low spring rate, allowing the gas charge to do all the work with little change in pressure from full to empty, a long stroke that allows efficient usage of the casing volume, and the bellows can be built to be resistant to overpressure that would crush a bladder-type separator.
An accumulator can maintain the pressure in a system for periods when there are slight leaks without the pump being cycled on and off constantly.
Its size helps absorb fluid that might otherwise be locked in a small fixed system with no room for expansion due to valve arrangement.
The gas precharge in an accumulator is set so that the separating bladder, diaphragm or piston does not reach or strike either end of the operating cylinder.
