An Introduction To Hydraulic Accumulators ,Types ,Features And Selection

An Introduction To Hydraulic Accumulators ,Types ,Features And Selection


 Introduction 
 AN ACCUMULATOR for an hydraulic system is a specific device designed for the storage of liquid under pressure. At the same time it can be effectively used to provide a number of hydraulic system improvements, such as :
1) Compensating for peak flows and pressures, by storing energy and thus economising on pump size.
2) Providing a standby power source in case of emergencies such as pump failure or fast shut down
3) Compensating for leakage losses
4) Damping out of system surges, shocks and vibration
5) Reducing pump ripple effects
6) Compensating for volume changes due to temperature and pressure variations
7) Aiding suspension damping of vehicles.
BasicalIy it is a container which includes a member which is resilient and alIows a predetermined quantity of fluid, at a finite pressure, to be stored. The hydraulic fluid has low compressibility, so some other means of resilient energy storage is required.

Use of Accumulator
1. Energy Accumulation
Accumulators are widely used as a supplementary energy source. The system in which pressurized oil discharged from accumulators is used to operate cylinders enables pumps to be smaller, shortens their cycles, and conserves energy.
Major Examples of Usage: Hydraulic presses Injection molding machines Die-cast machines Automotive brake systems Power shovels Vibration testing machines Circuit breakers for transformer substations Water supply systems Home pumps Equipment for ironwork's, power plants, and chemical plants Ship engines

FIG 1

2. Pulse Absorption
All pressurized fluid discharged from pumps has a pulse. Pulses produce noise or vibrations that can cause instability or damage devices. The use of an accumulator can attenuate pulses.
Major Examples of Usage:Machine tools Breakers for construction machinery Concrete compressors Hydraulic elevators Power sprayers Water purification plants Descaling equipment

FIG 2

3. Impact Absorption
The rapid closure of valves or sudden changes in load within a hydraulic circuit can result in impact pressure in pipes, which can then lead to noise or damage to those pipes or devices. The use of an accumulator can mitigate any such internal shock.
Major Examples of Usage: Water pipes Jet fuel injection equipment Mud-water compressors
Pipelines

FIG 3

4. Thermal Expansion Compensation
Changes in the volume of a liquid resulting from changes in the temperature within a closed circuit can increase or decrease the internal pressure. An accumulator can be used to mitigate any such fluctuations in the pressure.
Major Examples of Usage: Boilers Pressurized water heaters Central heating systems Fire extinguishing systems

FIG 4

5.Gas Spring
The use of the accumulators as a gas spring rather than a metal spring enables larger load systems to be downsized.
Major Examples of Usage: Vehicle suspensions Suspensions for construction machinery or other vehicles Agricultural machinery Coal mills Cement mills Cone crushers

FIG 5

6. Equilibrium Action
The accumulators can be used as counter balances. The accumulators smoothly balance the weight or impact of products and machinery via gas pressure.
Major Examples of Usage:Large crane systems Large-scale machinery tools Hydraulic pressure
molding machinery

FIG 6

7. Leak Compensation
The accumulators can compensate for any decreases in pressure due to internal leaks and thus retain the pressure of pressure control circuits or during any maintenance work.
Major Examples of Usage:Clamping equipment Other types of hydraulic equipment

FIG 7

8. Transfer Barrier
The use of a transfer barrier type accumulator enables transfers to take place within the fluid circuit without the different types of fluids or gases mixing.
Major Examples of Usage:Compressor lubricant supplier Boosters Sealed tanks

FIG 8

Type of Accumulator
There are three basic types of accumulator used in hydraulic system. They are:
1. Weight – Loaded, or gravity, type
2. Spring -Loaded type
3. Gas – Loaded type

1. Weight – Loaded Accumulator :
This type consists of a vertical, heavy- wall steel cylinder, which incorporates a piston with packing to pressure leakage ( Fig 9) . A dead weight is attached to the top of the piston. The force of gravity of the dead weight provides the potential energy in the accumulator. This type of accumulator creates a constant fluid pressure throughout the full volume output of the unit regardless of the rate and quantity of output. The main disadvantage of this type of accumulator is extremely large size and heavy weight which makes it unsuitable for mobile equipment.

FIG 9

2. Spring – Loaded Accumulator : 
A spring loaded accumulator is similar to the weight – loaded type except that the piston is preloaded with a spring as shown in fig 10. The spring is the source of energy that acts against the piston, forcing the fluid into the hydraulic system. The pressure generated by this type of accumulator depends on the size and pre-loading of the spring. In addition, the pressure exerted on the fluid is not a constant. The spring- loaded accumulator typically delivers a relatively small volume of oil at low pressures. Thus, they tend to be heavy and large for high- pressure, large – volume systems. This type of accumulator should not be used for applications requiring high cycle rates because the spring will fatigue and lose its elasticity. The result is an inoperative accumulator.

FIG 10

3. Gas Loaded Accumulator : Two main categories:
 A-Non separator- Type Accumulator: 
The non separator type of accumulator (fig 11)consists of a fully enclosed shell containing an oil port on the bottom and a gas charging valve on the top. The gas is confined in the top and the oil at the bottom of the shell. There is no physical separator between the gas and oil and thus the gas pushes directly on oil. The main advantage of this type is its ability to handle large volume of oil. The main disadvantage is absorption of gas in the oil due to the lack of a separator. Absorption of gas in the oil also makes the oil compressible, resulting in spongy operation of the hydraulic actuators. This type must be installed vertically to keep the gas confined at the top of the shell.

FIG 11

B. Separator – Type Accumulator : 
The commonly accepted design of gas loaded accumulators is the separator type. In this type there is a physical barrier between the gas and the oil. The three major type of separator accumulator are :

FIG 12

i) Piston type: 
The piston type of accumulator consists of a cylinder containing a freely floating piston with proper seals. The piston serves as a barrier between the gas and oil.(fig 13). The main disadvantage of the piston types of accumulator are that they are expensive to manufacture and have practical size limitation. The principal advantage of the piston accumulator is its ability to handle very high or low temperature system fluids through the utilization to compatible O- ring seals.

FIG 13

ii) Diaphragm Accumulator:
The diaphragm type accumulator consists of a diaphragm, secured in the shell, which serves as an elastic barrier between the oil and gas(fig 14). A shutoff button, which is secured at the base of the diaphragm, covers the inlet of the line connection when the diaphragm is fully stretched. The primary advantage of this type of accumulator is its small weight to – volume ratio, which makes it suitable almost exclusively for mobile applications. The restriction is on the deflection of the diaphragm

FIG 14

iii) Bladder type Accumulator: 
A bladder type- accumulator contains an elastic barrier( bladder) between the oil and gas( fig 15). The bladder is fitted in the accumulator by means of a vulcanized gas- valve element and can be installed or removed through the shell opening at the poppet valve. The poppet valve closes the inlet when the accumulator bladder is fully expanded. This prevents the bladder from being pressed into the opening. The greatest advantage of this type of accumulator is the positive sealing between the gas and oil chambers. Most widely used type of accumulator.

FIG 15

4-Metal bellows type

Figure 16 shows a metal bellows accumulator. Metal bellows accumulators are used where a fast response time is not critical yet reliability is important. Emergency brake accumulators are a good application for metal bellows accumulators.. The metal bellows accumulator consists of a pressure vessel with a metal bellows assembly separating fluid and nitrogen. The accumulator is similar to a piston accumulator, except a metal bellows replaces piston and piston seals. Metal bellows accumulators are very reliable and long life components, and have a proven service history. Metal bellows accumulators are pre-charged by supplier and then permanently sealed leading to a maintenance free accumulator. Metal bellows accumulators will be slow in responding to pressure changes due to increased mass of piston and bellows. 

FIG 16

Summary table 

Accumulator Sizing
The choice of the size of an accumulator is based on the system requirement. Initially, therefore, it is essential to determine the exact reason for including an accumulator, and what the range of its use will be. Basically the idea is to be able to supply a quantity of fluid, within a certain pressure range, over a given time. The accumulator chosen will not only need to be of a certain capacity but the precharge pressure will have to be known. Before looking at some examples, it is important to appreciate the basic equations associated with gas-type accumulators.
The nomenclature used in these equations is as follows:
Po = Gas pre-charge pressure [MPa]
PI = Minimum hydraulic fluid operating pressure [MPa]
P2 = Maximum hydraulic fluid operating pressure [MPa]
Vo = Rated volume for the accumulator [L]
VI = Gas volume at PI [L]
V2 = Gas volume at P2 [L]
n = Index for gas expansion
(Strictly speaking the ratio of the specific heats of the gas,
i.e. nitrogen - isothermal n = 1 and adiabatic n = 1.4)
For accumulators of the bladder type, the pre-charge pressure must be just below the minimum hydraulic pressure, in order to avoid the bladder coming into contact with the poppet valve. It is therefore customary to take

Po = 0.9 PI

In addition there must be a limitation on the maximum hydraulic pressure in order to avoid permanently damaging the properties of the fabric of the bladder or diaphragm. Thus

P2 :54 Po for bladder-type accumulators

P2 :58 Po for diaphragm-type accumulators

The volume of hydraulic fluid which may be accumulated in the accumulator is the difference between the maximum and minimum gas volumes. The exact value of these volumes depends on the manner in which the accumulator is used. If the process is very slow, such as with leakage compensation and the maintenance of a constant pressure in a system, then the ratios are isothermal and

PoVo = PI VI = P2 V2

If, however, the process is rapidly changing, which is the more usual operation, then the gas changes temperature and the expansion is closer to adiabatic, and

This equation enables us to determine the rated size of accumulator :

 The actual accumulator used would need to be some 1¹/² to 3 times greater than the calculated rated volume because of the restrictions mentioned earlier.

FIG 17

FIG 18

Figures 17 and 18 enable a quick assessment to be made from a knowledge of the working pressure range and the oil volume; these charts give the values for either isothermal or adiabatic operation. To use the charts, decide the range of pressure over which the accumulator is to operate (PI to P2) and determine the precharge pressure (Po), read-off the accumulator model which provides the appropriate volume. Take, for example, the need to make up fluid loss during one part of a machine operation - say 5 Lover 3 seconds within a range of 10 MPa to 15 MPa, and a precharge pressure of 7 MPa is acceptable - because of the rapid time this can be taken as an adiabatic operation and Figure 17 indicates that a nominal accumulator size of 36 L is suitable.
Where the pressures are above 20 MPa slight correction will be needed due to the nitrogen gas used deviating from an ideal gas; the correction increases with increase in pressure and may reach as low as 0.7 at 40 MPa (i.e. multiply the ideal volume by the correction factor to obtain the real volume).
Where the storage in the accumulator has been rapid there is a rise in temperature; this means that when the temperature drops to ambient, there will also be a drop in pressure.
Gas loaded accumulators with auxiliary bottle (figure 19 )

FIG 19

Fluid volume :

Precharge pressure :

Compression ratio :

FIG 20

With no gas bottle V3=0  thus

Compression ratios are generally in the range 1.5: 1 to 3: 1,depending on the application, with 2: 1 a typical average figure. This may be further modified, and the pressure difference over the working range reduced, by coupling the accumulator to an auxiliary gas bottle.
The choice of size and type of piston accumulator is largely dictated by the particular application. Thus relatively large capacities are required to cope with continuous operation and high demand. A much smaller size could be used where the accumulator has only to supply peak demand or is worked only intermittently, or is mainly employed as a shock absorber. Where the accumulator is employed solely as a source of emergency power the size can be calculated on the flow demand.
For continuous operation with piston type assemblies the pre-charge pressure (Po) should ideally be equal to the lower or cut-in value of the system working pressure (PI) as this will give the greatest swept volume over the working pressure range and thus mini mise the number of pressure cycles. For intermittent use, or where the accumulator is used as a source of emergency power, lower inflation pressures and consequently higher compression ratios can be used.

Accumulators Selection Tips
The Pressure Systems and Transportable Gas Containers Regulations 1989 have a number of highly important demands on accumulator manufacture and use. The key features may be briefly summarised as follows:
i) Safe operating limits. Checks are necessary to ensure that the maximum system pressure does not exceed the maximum working pressure of the accumulator and that all safety devices are correctly set and in operation.
ii) Relevant Records. All certificates of conformity, test certificates, material certificates and maintenance records must be retained for inspection.
iii) Correct marking. The manufacturers need to mark the shell or pressure envelope with their name, unit number, date of manufacture, design standard and, very importantly, the maximum working and test pressures.
iv) Maintenance procedure. Particular procedure is required varying on the product of system pressure (P - MPa) and accumulator volume (V -litres). The changeover point is at PV = 25, and where the PV is greater than 25 a written scheme of examination is necessary outlining the parts of the pressure system subject to examination and the type and frequency of examination ..