stauff accumulators · accumulators add efficiency, smooth operation and even allow unpowered...
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STAUFF Accumulators
Knowledge and Safety
www.stauff.com
Accumulators add efficiency, smooth operation and even allow unpowered functions to operate in hydraulic circuits every day, in hundreds of different applications.
The gas-charged accumulator provides a safe and efficient manner of removing unwanted and damaging pulsating pressures, storing energy for supplying flow to a system when power is unavailable, supplementing hydraulic pump flow to meet surge demands, thus reducing running power and providing a hydraulic spring to act as a suspension medium in a variety of applications.
When properly installed and maintained, the accumulator does its work without human intervention, and the machine operator need not even be aware of its presence. But what is it and are there regulations that govern the use of Hydraulic Accumulators..?
The Hydraulic Accumulator is available in three distinct types, yet the operating principle remains the same in every occasion. A chamber, usually cylindrical, is exposed at one end to the hydraulic system, allowing hydraulic fluid to enter the chamber. The opposite end of the chamber is filled with Nitrogen gas, and a barrier is provided to prevent the Hydraulic fluid and Nitrogen gas from intermixing. The barrier is flexible such as the bladder in a bladder accumulator or the piston plunger in the case of a piston accumulator. The principle relies on the inherent incompressibility of hydraulic fluid, also the fact that any gas can be compressed and thus vary its volume with pressure.
In a bladder and diaphragm gas-charged accumulator, the gas is pre-charged to a value which is lower than the system operating pressure, which is generally 80% - 90% for Storage and Shock applications and ~60% for Pulsation Dampener applications. As an example, a normal hydraulic system pressure varies from 100 Bar to 150 bar; thus accumulator gas should be pre charged to 80 to 90 bar. Where a Piston gas-charged accumulator is used in storage applications, the pre-charge pressure is normally set to -5 to -10 bar of minimum operating pressure.
In all cases it is preferred that the bladder, diaphragm or piston does not reach the bottom of the accumulator during normal operation; as this would result in the bladder, in the case of a bladder accumulator, nearly filling the volume of the chamber. When a surge in pressure occurs in a hydraulic system, perhaps due to rapid closure of a valve or a suddenly applied load, damage can occur due to the pressure spike exceeding equipment ratings. Additional damage may be caused by fatigue effects of spikes occurring repeatedly over long periods of time.Spike pressures can be relieved by a hydraulic accumulator.
As the hydraulic fluid pressure rapidly increases in the accumulator chamber, it compresses the gas in the chamber which contracts in volume according to the Boyle–Mariotte law of P1.V1 = P2.V2 this reduction in volume consumed by the gas, produces room for additional hydraulic fluid to enter the chamber, thus increasing the available volume for fluid and absorbing the pressure spike. As the hydraulic fluid pressure returns to normal levels the gas expands to again consume most of the volume of the chamber thus returning hydraulic fluid to the system and the accumulator is ready to absorb the next pressure spike.
Hydraulic accumulators are pressure vessels, and in many cases, may be subject to regulatory control. So, what is a pressure vessel? A simple definition offered by Australia’s WorkCover Plant Division is that “if it holds pressure, then it is a pressure vessel”. Any pressure vessel represents a potential hazard; however the size of the hazard may be determined by the volume of the vessel, the pressure it holds and the medium that is used in the vessel.
The Australian standard AS4343 defines levels of hazard into 5 categories. According to a table and simple formula these hazard levels escalate from E through to A, with hazard level E being the least dangerous level and not subject to regulatory control. Any vessel of hazard level D through to A must be of a design registered with Work Cover in the relevant state and subject to regular in-service inspections by an approved inspector. A simple formula for a gas-charged accumulator which is used with a non-hazardous fluid and a non-hazardous gas is to take the volume of the chamber in litres and multiply by the design pressure in Megapascals. If the answer is a value of thirty or greater, the vessel represents a hazard greater than level E and must be of a design registered with Work Cover in the relevant State or Territory. This is an important feature to remember with gas-charged accumulators because these devices are frequently fitted on imported machinery and not properly reviewed for compliance with Australian law upon the machinery’s purchase from overseas.
Note that a pressure vessel of Hazard level D or greater should be stamped with the design code, the Australian design registration number, its design pressure and temperature and its latest test date. Also the plant owner should be in possession of a Manufacturer’s Data Report (MDR) and a copy of the registration certificate for every vessel of hazard level D or greater.
Accumulators - Overview
Safety Standards
Bladder type Hydraulic AccumulatorsAs its name would suggest this type of accumulator uses a rubber bladder to contain the gas charge. These vessels are available in sizes ranging from 1 Litre through to 55 litres. They are a repairable device, as the bladder is easily removed and replaced in the event off failure. The bladder accumulator uses large orifice openings in the fluid port for rapid discharge and, as a result, remains virtually insensitive to debris. Due to the free movement of the bladder they do not suffer from hysteresis and are very fast to react.
It is preferred that bladder accumulators are operated in the vertical plane, however in low flow and low cycle applications they may be mounted in the horizontal position – however a slight reduction in stored volume may be experienced as a result.
The bladder type accumulator is generally limited to a 4:1 ratio of maximum pressure to gas-charged pressure, which is necessary to protect the bladder from excessive distortion stresses.
Piston type Hydraulic AccumulatorsThis device is essentially a hydraulic cylinder without a rod.
On one side of the Piston is charged gas, whilst hydraulic fluid is admitted to the opposite end. The piston accumulator is repairable by simply replacing the Piston seals. Because they do not use a bladder, distortion problems do not occur. They can be operated at very high compression ratios, which are often limited only by the design pressure of the vessel. These accumulators are sensitive to debris because of the sliding contact nature of the dynamic piston seals, and they also suffer from hysteresis, due to the friction action of these seals. As such, piston accumulators are not generally used in applications where a fast response is required such as a valve closure or as pulsation dampers.
High flow rates and piston speed velocity should also be considered. In general, the piston speed velocity should be limited and can vary between 1 m/s to <4 m/s - depending on the piston accumulator brand used.
Diaphragm type Hydraulic AccumulatorsDiaphragm accumulators are normally restricted to the smaller sizes of 0.07 through to 3.5 litres. These vessels are not repairable. Their design employs a diaphragm to separate the gas and fluid chambers. They are virtually insensitive to debris and are very responsive to quick changes in pressures as they are hysteresis free.
The diaphragm is not distorted to the same level as the bladder in bladder type accumulators in operation, and as such, diaphragm accumulators may be exposed to compression ratios of up to 8:1. They may be mounted in both the horizontal and vertical position.
Construction The construction method of gas accumulator falls into three basic types:
ThermodynamicsHydraulic accumulators rely on the compressibility of gas in order to store potential energy for their operation. The potential energy either when being stored in the accumulator or when discharging may be subject to the phenomena of adiabatic compression / expansion. Anyone who has pumped up their bicycle tyres very quickly and then touched the tyre valve will know that it gets very hot; this is due to adiabatic compression. Similarly anyone who has opened the drain valve on their compressor receiver and observed that it becomes frosty cold, even frozen, has experienced adiabatic expansion. Adiabatic simply means that the gas has been compressed or allowed to expand so quickly that the normal transfer of heat to the surrounding atmosphere cannot occur. When a hydraulic accumulator is caused to charge adiabatically the compressed gas gets hot and, as a result, tries to expand. Similarly when the accumulator is discharged quickly the gas gets cold and, as a result, tends to contract. This phenomena means that under adiabatic conditions, an accumulator delivers less hydraulic fluid because the contraction and expansion of the gas is limited.
A slower rate of expansion or contraction, where the heat transfer can occur, is known as an Isothermal change. Therefore most sizing charts and formulae resolve to a larger accumulator for adiabatic conditions than for isothermal conditions – where very little potential energy is lost due to the effects of change in temperature. In reality, most applications never achieve an isothermal or adiabatic condition but are said to be in a Polytropic state. Depending on the application,
this can be somewhere between an isothermal and adiabatic condition. To understand the effect of this, more information is required regarding the system - in particular the charging and discharging times of the accumulator, and the ambient temperatures.
Accumulators may be used as energy storage devices, pulsation dampeners, and hydraulic springs or as a means of supplementing pump flow during peak demands.
SummaryIn summary, the gas-charged hydraulic accumulator offers a safe and convenient method of enhancing the performance of fluid power systems. The purchaser should consider the type of accumulator that best suits the application and allows for an appropriate size for the anticipated charging discharging cycle rates. And the purchaser should also ensure that they receive the necessary Manufacturer’s Data Report and proof of design registration where appropriate.
Author: Ted Thomas - STAUFF National Engineering Manager [email protected]
Adelaide
SA AustraliaTel: (08) 8415 1000Fax: (08) 8341 1604
Auckland
New ZealandTel: (09) 271 4812Fax: (09) 271 4832
Brisbane
QLD AustraliaTel: (07) 3716 1000Fax: (07) 3715 8960
Melbourne
VIC AustraliaTel: (03) 8542 5400Fax: (03) 9543 5422
Sydney
NSW AustraliaTel: (02) 8787 8000Fax: (02) 9725 2744
Wollongong
NSW AustraliaTel: (02) 4271 9000Fax: (02) 4271 8432
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WP/STAUS/ACCUM/03.2015.2