Hydraulic accumulator
Encyclopedia
A 'hydraulic accumulator' is an energy storage
device. It is a pressure
storage reservoir in which a non-compressible hydraulic fluid
is held under pressure by an external source. That external source can be a spring
, a raised weight
, or a compressed gas
. The main reasons that an accumulator is used in a hydraulic system are so that the pump doesn't need to be so large to cope with extremes of demand, so that the supply circuit can respond more quickly to any temporary demand and to smooth pulsations.
Compressed gas accumulators are by far the most common type. These are also called hydro-pneumatic accumulators.
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The first accumulators for Armstrong's hydraulic dock machinery were simple raised water tower
s. 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 towers were extremely tall. One of the best known, Grimsby Dock Tower
opened in 1852, is 300 feet (91.4 m) tall. The size of these towers made them expensive to construct. By the time Grimsby was opened, it was already obsolete as Armstrong had developed the more complex, but much smaller, weighted accumulator. These simple tower accumulators were constructed for less than a decade. In 1892 the original Grimsby tower's function was replaced by a smaller weighted accumulator on an adjacent dock, although the tower remains to this day as a well-known landmark.
See also
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 energizes 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 external accumulator was added around 1920. 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 is 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.
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, a notable example of an early accumulator, dating from 1869 being at the Regent's Canal Dock of the Regent's Canal Company at Limehouse
, London
. The artifact has been converted into a visitor attraction which is open yearly during London Open House weekend, usually the third weekend in September.
) 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. As the volume of the compressed gas changes the pressure of the gas, and the pressure on the fluid, changes inversely. Existing hydraulic accumulator designs are large and heavy due to the need for two storage tanks and do not have the high energy density needed for many applications.
It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the gas side of the accumulator. This is mainly done since a gas bottle normally is cheaper to produce than an accumulator.
The compressed gas accumulator was invented by Jean Mercier
, for use in variable pitch propellers.
The need for two storage tanks and two accumulators can be eliminated and the entire hydraulic energy storage system is an open loop. The storage requirement is smaller because depressurized air is not stored. The hydraulic open loop accumulator works by drawing air in from the atmosphere and expelling air into the atmosphere. A separate hydraulic pump maintains the pressure balance of the air by increasing the amount of hydraulic fluid in the system. This results in a steady pressure of air and up to 24 times the energy density of a standard hydraulic accumulator.
This hydraulic energy storage system has applications in energy storage for wind turbines, regenerative braking systems for hybrid cars that could partially power the car, and energy storage for power construction equipment.
the magnitude of the force exerted by a spring is linearly proportional to its extension. Therefore as the spring compresses, the force it exerts on the fluid is increased linearly.
. Fluid may be internal or external to the bellows. 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, and a long stroke relative to solid (empty) height, which gives maximum storage volume for a given container size. The welded metal bellows accumulator provides an exceptionally high level of accumulator performance, and can be produced with a broad spectrum of alloys resulting in a broad range of fluid compatibility. Another advantage to this type is that it does not face issues with high pressure operation, thus allowing more energy storage capacity.
An accumulator is placed close to the pump with a non-return valve preventing flow back to it. In the case of piston-type pumps this accumulator is placed in the best place to absorb pulsations of energy from the multi-piston pump
. It also helps protect the system from fluid hammer
. This protects system components, particularly pipework, from both potentially destructive forces.
An additional benefit is the additional energy that can be stored while the pump is subject to low demand. The designer can use a smaller-capacity pump. The large excursions of system components, such as landing gear on a large aircraft, that require a considerable volume of fluid can also benefit from one or more accumulators. These are often placed close to the demand to help overcome restrictions and drag from long pipework runs. The outflow of energy from a discharging accumulator is much greater, for a short time, than even large pumps could generate.
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. When temperature changes cause pressure excursions the accumulator helps absorb them. 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. The design precharge normally ensures that the moving parts do not foul the ends or block fluid passages. Poor maintenance of precharge can destroy an operating accumulator. A properly designed and maintained accumulator should operate trouble-free for years.
Energy storage
Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator....
device. It is a pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
storage reservoir in which a non-compressible hydraulic fluid
Hydraulic fluid
Hydraulic fluids, also called hydraulic liquids, are the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water...
is held under pressure by an external source. That external source can be a spring
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
, a raised weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
, or a compressed gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
. The main reasons that an accumulator is used in a hydraulic system are so that the pump doesn't need to be so large to cope with extremes of demand, so that the supply circuit can respond more quickly to any temporary demand and to smooth pulsations.
Compressed gas accumulators are by far the most common type. These are also called hydro-pneumatic accumulators.
Towers
Water tower
A water tower or elevated water tower is a large elevated drinking water storage container constructed to hold a water supply at a height sufficient to pressurize a water distribution system....
s. Water was pumped to a tank at the top of these towers by steam pumps. When dock machinery required hydraulic power, the hydrostatic head
Hydrostatic head
When generating hydropower, the head is a general term used to describe the distance that a given water source has to fall before the point where power is generated. Ultimately the force responsible for hydropower is gravity, so a hydroelectricity plant with a tall/high head can produce more...
of the water's height above ground provided the necessary pressure.
These simple towers were extremely tall. One of the best known, Grimsby Dock Tower
Grimsby Dock Tower
Grimsby Dock Tower is a hydraulic accumulator tower and a famous maritime landmark in Grimsby, North East Lincolnshire, England. It was completed on 27 March 1852 with the purpose of containing a 30,000 UK gallon hydraulic wrought iron reservoir at a height of , to provide hydraulic power to...
opened in 1852, is 300 feet (91.4 m) tall. The size of these towers made them expensive to construct. By the time Grimsby was opened, it was already obsolete as Armstrong had developed the more complex, but much smaller, weighted accumulator. These simple tower accumulators were constructed for less than a decade. In 1892 the original Grimsby tower's function was replaced by a smaller weighted accumulator on an adjacent dock, although the tower remains to this day as a well-known landmark.
See also
- Wallasey Great Float
Raised weight
A working example of this type of accumulator may be found at the hydraulic engine house, Bristol Harbour
Hydraulic engine house, Bristol Harbour
The Hydraulic engine house is part of the "Underfall Yard" in Bristol Harbour in Bristol, England.The octagonal brick and terracotta chimney of the engine house dates from 1888, and is grade II* listed, as is the hydraulic engine house itself. It replaced the original pumping house which is now...
. The external accumulator was added around 1920. 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
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
(5.2 MPa, or 52 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
) which is used to power the cranes, bridges and locks of Bristol Harbour
Bristol Harbour
Bristol Harbour is the harbour in the city of Bristol, England. The harbour covers an area of . It has existed since the 13th century but was developed into its current form in the early 19th century by installing lock gates on a tidal stretch of the River Avon in the centre of the city and...
.
The original operating mechanism of Tower Bridge
Tower Bridge
Tower Bridge is a combined bascule and suspension bridge in London, England, over the River Thames. It is close to the Tower of London, from which it takes its name...
, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
, 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.
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
London Hydraulic Power Company
The London Hydraulic Power Company was set up by an Act of Parliament in 1883 to install a hydraulic power network of high-pressure cast iron water mains under London. It was the successor to the Steam Wharf and Warehouse Company, founded in 1871 by Edward B Ellington...
. Railway goods yards and docks often had their own separate system, a notable example of an early accumulator, dating from 1869 being at the Regent's Canal Dock of the Regent's Canal Company at Limehouse
Limehouse
Limehouse is a place in the London Borough of Tower Hamlets. It is on the northern bank of the River Thames opposite Rotherhithe and between Ratcliff to the west and Millwall to the east....
, London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...
. The artifact has been converted into a visitor attraction which is open yearly during London Open House weekend, usually the third weekend in September.
Compressed gas (or gas-charged) closed accumulator
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. One chamber contains hydraulic fluid and is connected to the hydraulic line. The other chamber contains an inert gas under pressure (typically nitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
) 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. As the volume of the compressed gas changes the pressure of the gas, and the pressure on the fluid, changes inversely. Existing hydraulic accumulator designs are large and heavy due to the need for two storage tanks and do not have the high energy density needed for many applications.
It is possible to increase the gas volume of the accumulator by coupling a gas bottle to the gas side of the accumulator. This is mainly done since a gas bottle normally is cheaper to produce than an accumulator.
The compressed gas accumulator was invented by Jean Mercier
Jean Mercier
Jean Mercier , a Frenchman, was known as the father of the bladder-type hydraulic accumulator. Mercier fled France for the United States during the Nazi invasion, eventually settling in New York City. After meeting Edward M...
, for use in variable pitch propellers.
Compressed gas open accumulator
Hydraulic energy storage systems store energy by compressing air similar to a battery storing energy in an electric circuit.The need for two storage tanks and two accumulators can be eliminated and the entire hydraulic energy storage system is an open loop. The storage requirement is smaller because depressurized air is not stored. The hydraulic open loop accumulator works by drawing air in from the atmosphere and expelling air into the atmosphere. A separate hydraulic pump maintains the pressure balance of the air by increasing the amount of hydraulic fluid in the system. This results in a steady pressure of air and up to 24 times the energy density of a standard hydraulic accumulator.
This hydraulic energy storage system has applications in energy storage for wind turbines, regenerative braking systems for hybrid cars that could partially power the car, and energy storage for power construction equipment.
Spring type
A spring type accumulator is similar in operation to the gas-charged accumulator above, except that a heavy spring (or springs) is used to provide the compressive force. According to Hooke's lawHooke's law
In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law...
the magnitude of the force exerted by a spring is linearly proportional to its extension. Therefore as the spring compresses, the force it exerts on the fluid is increased linearly.
Metal bellows type
The metal bellows accumulators function similarly to the compressed gas type, except the elastic diaphragm or floating piston is replaced by a hermetically sealed welded metal bellowsMetal bellows
Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape .Bellows technology of the 20th and 21st century is centered on metal...
. Fluid may be internal or external to the bellows. 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, and a long stroke relative to solid (empty) height, which gives maximum storage volume for a given container size. The welded metal bellows accumulator provides an exceptionally high level of accumulator performance, and can be produced with a broad spectrum of alloys resulting in a broad range of fluid compatibility. Another advantage to this type is that it does not face issues with high pressure operation, thus allowing more energy storage capacity.
Functioning of an accumulator
In modern, often mobile, hydraulic systems the preferred item is a gas charged accumulator, but simple systems may be spring-loaded. There may be more than one accumulator in a system. The exact type and placement of each may be a compromise due to its effects and the costs of manufacture.An accumulator is placed close to the pump with a non-return valve preventing flow back to it. In the case of piston-type pumps this accumulator is placed in the best place to absorb pulsations of energy from the multi-piston pump
Axial piston pump
An axial piston pump is a positive displacement pump that has a number of pistons in a circular array within a cylinder block. It can be used as a stand-alone pump, a hydraulic motor or an automotive air conditioning compressor.-Description:...
. It also helps protect the system from fluid hammer
Water hammer
Water hammer is a pressure surge or wave resulting when a fluid in motion is forced to stop or change direction suddenly . Water hammer commonly occurs when a valve is closed suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe...
. This protects system components, particularly pipework, from both potentially destructive forces.
An additional benefit is the additional energy that can be stored while the pump is subject to low demand. The designer can use a smaller-capacity pump. The large excursions of system components, such as landing gear on a large aircraft, that require a considerable volume of fluid can also benefit from one or more accumulators. These are often placed close to the demand to help overcome restrictions and drag from long pipework runs. The outflow of energy from a discharging accumulator is much greater, for a short time, than even large pumps could generate.
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. When temperature changes cause pressure excursions the accumulator helps absorb them. 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. The design precharge normally ensures that the moving parts do not foul the ends or block fluid passages. Poor maintenance of precharge can destroy an operating accumulator. A properly designed and maintained accumulator should operate trouble-free for years.