Volume Six - Pre-Victorian to the Present Day - Select Aspects - Hydraulic Accumulators . . .

The use of separate steam-engines for these numerous machines would be costly and troublesome even for the cranes ; and a great loss of power would be involved in the transmission of steam through pipes to the machines. Water under pressure, on the contrary, can be readily conveyed for considerable distances through pipes with comparatively little loss of power, and is peculiarly adapted for intermittent working, as no loss of power is undergone when the machines are at rest, whilst the power is at hand whenever they require to be set in motion.

A steam-engine is erected on a suitable site near the dock, which is continually employed in pumping water into a vertical cylinder, from whence it flows, as required, to the various machines. Instead of effecting the pressure by a high column of water, necessitating a high tower, the water in the cylinder is pressed by a piston which can move up and down in the cylinder, and is weighted by a ballasted case suspended on it, at the top, by a cross-head. A constant pressure is thus imparted to the water pumped into the cylinder. The vertical cylinder with its weighted piston is called an accumulator, as the power is stored up in the cylinder, at a pressure of about 700 to 800 lbs. on the square inch.

When the steam-engine pumps water more rapidly into the cylinder than it is drawn off for consumption, the piston rises in the accumulator, and the steam is shut off by closing of the throttle valve when the weighted cage reaches its full height. When, on the contrary, the consumption by the machines is greater than the supply, the piston falls in the accumulator, and opening the throttle valve causes the engine to work with its full effect. The accumulator, accordingly, serves both to store up and regulate the supply.

Moreover the intermittent nature of the work of most of the machines prevents their ever being in action all together for any length of time continuously, so that any sudden unusual drain of water can be supplied by the reservoir in the cylinder of the accumulator ; and the amount of power thus expended is soon stored up again directly some of the machines cease working. On this account, it is unnecessary to provide an engine of the same maximum efficiency as if the full amount of power, liable to be expended, had to be supplied simultaneously with its use.

Frequently, when the machines are spread over a considerable area, or in groups some distance apart, the accumulators are distributed, so that the water for supplying the machines may not have to be drawn off through a long length of pipe at the moment of working, thereby diminishing the pressure and the efficiency of the machines."