Hydraulic accumulator
A fluid is stored under pressure in a hydraulic accumulator ( hydraulic accumulator , hydropneumatic accumulator or accumulator ) . Hydraulic energy can be released when unloading .
The liquid pressure compresses a gas or a spring or lifts a weight. When the volume is withdrawn, the storage gas expands or the storage spring relaxes, whereby the pressure is reduced. In the case of weight-loaded accumulators, the pressure remains almost constant.
Tasks of a hydraulic accumulator are u. a .:
- Leakage compensation
- Weight compensation
- Energy storage
- Emergency function
- Pulsation damping
- Shock absorption
- Covering short-term volume flow requirements
- Media separator (without gas side)
origin
The hydropneumatic storage principle was already known in antiquity and was e.g. B. Applied in music when building organs.
The invention of the organ dates back to 246 BC. BC back. Its inventor was probably the engineer Ktesibios . He lived first in Alexandria, later in Aspendus. Both cities claim to be the place where the organ was invented. The organ that Ktesibios invented was called hydraulos , a word made up of hydor (water) and aulos (oboe-like instrument). It contained a piston pump filled with water, a wind tunnel, a single row of pipes and a keyboard. The ratio of keys, of which there were about 13 on the organ at that time, was equal to the width of the pipe spacing, each key was about a hand's width. Depressing the keys required so much force that fast play was not yet possible.
The prototype of the Ktesibio was soon followed by further developed instruments with several rows of pipes that could be played individually as well as together. Vitruvius and Heron described the technical aspects of hydraulics. It is said to have consisted of reed pipes as well as open and covered labial pipes. The instrument (portative), which was small and portable in antiquity, quickly spread throughout the Hellenistic and Roman Empire. It was used for domestic music purposes, in the theater and as a propaganda tool. In the 1st century AD, organ playing was so widespread that public organ competitions were held in Greece. It is also known that several emperors of the Roman Empire played the organ themselves and played organ music in their palaces. Because of the obesity of the hydraulos, small oil lamps in the shape of an organ were made in Carthage and sold in large numbers.
The first indications of an air-powered bellows are from the 2nd century AD.
The instrument was called hydraulis .
The Frenchman Jean Mercier (1901–1971) is considered to be the inventor of the modern hydro bladder accumulator. He was known as the father of the bladder accumulator. Mercier fled France to the United States during the Nazi invasion and eventually settled in New York. After meeting Edward M. Greer in 1940, the two founded Greer Hydraulics in 1942 to manufacture hydraulic aircraft components.
function
A hydraulic fluid is pressed under pressure into a pressure vessel filled with gas (usually nitrogen ) . The hydraulic fluid compresses the gas and is available as "stored energy" at a later point in time.
A special case is the use as a vibration damper or pulsation damper and pressure shock absorber (surge damper) in hydraulic systems.
The gas and the hydraulic fluid are usually separated from one another by a separator ( membrane , elastomer bladder , piston or metal bellows ). The separation of the gas from the liquid and its escape ( permeation ), especially when the hydraulic system is depressurized, is greatly reduced, but not entirely avoided. The gas loss depends essentially on the elastomer used in the separating membrane, the liquid, the storage gas (molecular size), the thickness of the membrane, the pressure difference between the gas and the liquid side, the number of load cycles during operation and the temperature of the storage. Metal bellows accumulators, on the other hand, are practically gas-tight and have no permeation, as the metal membrane is "gas-tight" even at high temperatures.
Another use of hydraulic accumulators is as a media separator. Liquids are used on both sides of the hydraulic accumulator. The separator (piston, membrane, bladder) separates the two mostly different liquids. Application is, for example, an underwater compensator that transfers the seawater pressure to an underwater system and thus compensates for the pressure in the system. Compensators on the blowout preventer .
Individual storage tanks can be interconnected to form storage batteries. A very large amount of energy can be stored in this way (storage volumes of a few thousand liters on the hydraulic media side and a few thousand liters of nitrogen on the gas side below 40–1000 bar are built). Such systems are used in power plants and steelworks , but also in theaters and on offshore platforms. The advantage of such systems is that this high level of stored energy is available even when the drive energy is no longer available ( emergency operation ). Another application is the provision of a briefly high volume flow within hydraulic controls, e.g. B. with electro-hydraulic controls . The pump volume flows can be kept low; only the volume flow required on average is conveyed. Accordingly, the power of the drive unit can be kept low, so that a better overall efficiency is achieved. The heat balance is also improved, so that external cooling of the hydraulic unit can often be dispensed with.
The formulas for changes in the state of gases can be used to design the memory. Nitrogen is usually used as the storage gas . Other gases or gas mixtures such as helium, CF4 or air are also possible. There are also hydraulic accumulators with additional foam filling on the gas side. The open-pored foam is introduced in addition to the gas on the gas side of bubbles, (pistons) and membrane accumulators. Downstream nitrogen bottles are also filled with foam. The aim is to increase the heat capacity on the gas side and to change the storage behavior towards isothermal. In some cases, energy savings of up to 30% are possible.
The calculation is often based on ideal changes in gas state. At high pressures (200–1500 bar) real gas behavior must be expected. The formulas for this are relatively unwieldy and only lead to precise values for pressure, temperature and volume change with iterative calculations. Some memory manufacturers provide free software.
Designs
There are several types of hydraulic accumulators:
- Diaphragm accumulator
- Bladder accumulators
- Piston accumulator
- Maintenance -free metal bellows accumulator
- Spring accumulator (special form of piston accumulator, instead of a gas a spring is preloaded)
- Air chamber special design without separator
Laws
As pressure equipment, hydraulic accumulators are subject to the RL 2014/68 / EU Pressure Equipment Directive in Europe, for example , and the ASME code in the USA . The interpretation according to European regulations can e.g. B. to DIN EN 14359 or DIN EN 13445-1 . The company is subject to the industrial safety regulations in Germany
