A hydraulic accumulator is a pressure storage reservoir in which an incompressible hydraulic fluid is held under pressure that is applied by an external source of mechanical energy. The external source can be an engine, a spring, a raised weight, or a compressed gas. An accumulator enables a hydraulic system to cope with extremes of demand using a less powerful pump, to respond more quickly to a temporary demand, and to smooth out pulsations. It is a type of energy storage device. Compressed gas accumulators, also called hydro-pneumatic accumulators, are by far the most common type.
Types of accumulator
Towers
The first accumulators for William Armstrong's hydraulic dock machinery were simple raised water towers. Water was pumped to a tank at the top of these towers by steam pumps. When dock machinery required hydraulic power, the hydrostatic head of the water's height above ground provided the necessary pressure. These simple accumulators were extremely tall. For instance, Grimsby Dock Tower, built in 1852, is 309 feet (94 m) tall. Because of their size, they were costly, and so were constructed for less than a decade. 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. In 1892 the original Grimsby tower's function was replaced, on Fowler's advice, by a smaller weighted accumulator on an adjacent dock, although the tower remains to this day as a well-known landmark. Other surviving towers include one adjacent to East Float in Birkenhead, England, and another located at the Bramley-Moore Dock, Liverpool, England. 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.
Raised weight
A raised weight accumulator consists of a vertical cylinder containing fluid connected to the hydraulic line. The cylinder is closed by a piston on which a series of weights are placed that exert a downward force on the piston and thereby pressurizes the fluid in the cylinder. 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 pressure will decline somewhat as the cylinder is emptied due to the decline in weight of the remaining fluid.) A working example of this type of accumulator may be found at the hydraulic engine house, Bristol Harbour. The original 1887 accumulator is in place in its tower, an external accumulator was added in 1954 and this system was used until 2010 to power the Cumberland Basin (Bristol) lock gates. The water is pumped from the harbour into a header tank and then fed by gravity to the pumps. 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. The original operating mechanism of Tower Bridge, London, also used this type of accumulator. Although no longer in use, two of the six accumulators may still be seen in situ in the bridge's museum. Regent's Canal Dock, now named Limehouse Basin has the remains of a hydraulic accumulator, dating from 1869, a fragment of the oldest remaining such facility in the world, the second at the dock, which was installed later than that at Poplar Dock, originally listed incorrectly as a signalbox for the London and Blackwall Railway, when correctly identified, it was restored as a tourist attraction by the now defunct London Docklands Development Corporation. Now owned by the Canal & River Trust, it is open for large groups on application to the Dockmaster's Office at the basin and on both the afternoons of London Open House Weekend, held on the third weekend of September each year. London had an extensive public hydraulic power system from the mid-nineteenth century finally closing in the 1970s with 5 hydraulic power stations, operated by the London Hydraulic Power Company. Railway goods yards and docks often had their own separate system.
Air-filled accumulator
A simple form of accumulator is an enclosed volume, filled with air. A vertical section of pipe, often enlarged diameter, may be enough and fills itself with air, trapped as the pipework fills. Such accumulators typically do not have enough capacity to be useful for storing significant power since they cannot be pre-charged with high pressure gas, but they can act as a buffer to absorb fluctuations in pressure. They are used to smooth out the delivery from piston pumps. Another use is as a shock absorber to damp out water hammer; this application is an integral part of most ram pumps. Loss of air will result in loss of effectiveness. If air is lost over time, the design must include some way to replenish the accumulator.
Compressed gas (or gas-charged) closed accumulator
… excerpt ends here. Continue reading the full article.
