DIFFERENT TYPES OF HYDRAULIC

ACCUMULATORS & DAMPENERS

Author

Prem Baboo Sr. Manager (Prod) National Fertilizers Ltd. India F.I.E., Institution of Engineers (India) Technical Advisor & an Expert for www.ureaknowhow.com

What's an accumulator?

A hydraulic accumulator is a device in which potential energy is stored in the form of a compressed gas or spring, or by a raised weight to be used to exert a force against a relatively incompressible fluid or the pressure storage reservoir in which a non-compressible hydraulic fluid is under pressure by an external force.

How Accumulators Work

Accumulators operate by making use of the considerable difference in compressibility between a gas and fluid. Using the bladder design, the nitrogen in the bladder is highly compressible while the hydraulic oil in the fluid side of the shell is virtually non-compressible. The bladder contained in the shell is pre-charged with nitrogen gas to a pressure calculation determined by system parameters and the work to be done. After being pre-charged, the bladder occupies almost the whole volume of the shell. From there, the operation of an accumulator can be broken down into three basic stages:

(a) When the hydraulic pump in the system is turned on it causes fluid to enter the accumulator. When fluid fills the shell, accumulator charging begins as the nitrogen in the bladder is compressed by a fluid pressure greater than its pre-charge pressure. This is the source of stored energy.

(b) As the bladder compresses due to the fluid filling the shell, it "deforms" in shape, taking up less space in the shell while at the same time, pressure in the bladder increases. This bladder "deformation" ceases when the pressure of the system fluid and the now compressed nitrogen become balanced.

(c) Upon downstream system demand, fluid system pressure falls and the stored fluid is pushed out of the accumulator shell and returned to the system under pressure exerted by the compressed nitrogen, whose pressure is now greater than the fluid pressure. Upon completion of whatever hydraulic system function the accumulator was designed to do, the cycle starts all over again with step one.

One of the most important considerations in applying accumulators is calculating the correct pre-charge pressure for the type of accumulator being used, the work to be done and system operating parameters. Pre-charge pressure is generally 80 - 90% of the minimum system working pressure to allow a small amount of fluid to remain in the accumulator. This prevents the bladder, diaphragm or piston from striking the opposite end of the pressure vessel, getting fouled up in discharge valuing or blocking fluid passages. Too high or too low of a pre-charge pressure can cause accumulator damage or failure.

The accumulators use nitrogen to keep the hydraulic fluid pressurized. When the fluid is pumped into an accumulator the nitrogen (N2) inside the accumulator is compressed. When all the hydraulic fluid is in an accumulator designed for high pressure side, the pressure of the nitrogen reaches 340 bars. If empty of fluid, the pressure of the nitrogen is about 140 bars. The pressure of the nitrogen in the low pressure reservoir will vary from 4.0 bar when empty to 14.0 bar when full. They are used in fluid power systems to accumulate energy and to smooth out pulsations. A hydraulic system utilizing an accumulator can use a smaller fluid pump since the accumulator stores energy from the pump during low demand periods. This energy is available for instantaneous use, released upon demand at a rate many times greater than could be supplied by the pump alone. Accumulators can also act as surge or pulsation absorbers, much as an air dome is used on pulsating piston or rotary pumps. They will cushion

hydraulic hammer, reducing shocks caused by rapid operation or sudden starting and stopping of power cylinders in a hydraulic circuit.

Where are accumulators used?

Accumulators can be applied creatively in any number of situations,

including:

Emergency and safety:

An accumulator which is kept constantly under pressure is valuable in the

event of an electrical power failure as it can provide flow and pressure to

perform an additional function or complete a machine cycle.

Knock or pulsation dampening:

An accumulator can be used to cushion the pressure spike from sudden

valve closure, the pulsation from pumps or the load reaction from sudden

movement of parts connected to hydraulic cylinders.

Leakage compensation:

An accumulator can be used to maintain pressure and make-up for lost

fluid due to internal leakage of system components including cylinders and

valves.

Thermal expansion:

An accumulator can absorb the pressure differences caused by

temperature variations in a closed hydraulic system.

Energy conservation:

An accumulator can be used to supplement a pump during peak demand

thereby reducing the size of the pump and motor required. The

accumulator is charged during low demand portions of the pump cycle

time and then discharges during the high demand portions of the system.

Noise reduction:

An accumulator is effective at reducing hydraulic system noise caused by

relief valves, pump pulsations, system shock and other circuit generated

noises.

Improved response times: An accumulator (bladder type) has virtually

instantaneous response time that can provide fluid very quickly to fast-

acting valves such as servos and proportional to improve their

effectiveness.

FUNCTIONS.

Accumulators can provide several functions, such as

1. Store Energy

Hydro-pneumatic accumulators incorporate a gas in conjunction with a hydraulic fluid. The fluid has little dynamic power storage qualities. The fluid normally used in fluid power applications can be reduced in volume only about 1.7% under a pressure of 345 bar. Therefore when only 2% of the total contained volume is released, the pressure of the remaining oil in the system will drop to zero. However, the relative incompressibility of a hydraulic fluid makes it ideal for fluid power systems and provides quick response to power demand. The gas, on the other hand, a partner to the hydraulic fluid in the accumulator, can be compressed to high pressures and low volumes. Potential energy is stored in this compressed gas to be released upon demand. This energy can be compared to that of a raised pile driver ready to transfer its tremendous energy upon the pile. In the piston type accumulator the energy in the compressed gas exerts pressure against the piston separating the gas and hydraulic fluid. The piston in turn forces the fluid from the cylinder into the system and to the location where useful work will be accomplished

2. Absorbs Pulsation

In most fluid power applications, pumps are used to generate the required power to be used or stored in a hydraulic system. Many pumps deliver this power in a pulsating flow. The piston pump, as commonly used for higher pressures, tends to produce pulsation detrimental to a high pressure system. An accumulator properly located in the system will substantially cushion these pressure variations.

3. Cushions Operating Shock

In many fluid power applications the driven member of the hydraulic system stops suddenly, creating a pressure wave which is sent back through the system. This shock wave can develop peak pressures several times greater than normal working pressures and can be the source of system failure or objectionable noise. The gas cushion in an accumulator, properly placed in the system, will minimize this shock. An example of this application is the absorption of shock caused by suddenly stopping the loading bucket on a hydraulic front end loader. Without an accumulator, the bucket, weighing over 2 tons, can completely lift the rear wheels of a loader off the ground. The severe shock to the tractor frame and axle, as well as operator wear and tear, is overcome by the addition of an adequate accumulator to the hydraulic system.

4. Supplements Pump Delivery. An accumulator, capable of storing power, can supplement the fluid pump in delivering power to the system. The pump stores potential energy in the accumulator during idle periods of the work cycle. The accumulator transfers this reserve power back to the system when the cycle requires emergency or peak power. This enables a system to utilize a much smaller pump, resulting in savings in cost and power.

5. Maintains Pressure Pressure changes occur in a hydraulic system when the liquid is subjected to rising or falling temperatures. Also, there may be pressure drop due to leakage of hydraulic fluid. An accumulator compensates for such pressure changes by delivering or receiving a small amount of hydraulic liquid. In the event the main power source should fail or be stopped, the accumulator would act as an auxiliary power source, maintaining pressure in the system.

6. Dispenses. An accumulator may be used to dispense fluids under pressure, such as lubricating greases and oils.

DIFFERENT TYPE OF HYDRAULIC ACCUMULATOR

Accumulators basically comprise of two compartments: one of the compartments is filled with gas; the other compartment, which is connected to the hydraulic circuit, is filled with fluid. The accumulator shell is made of carbon steel, stainless steel or aluminium. Depending on separating elements, we can distinguish following types of hydraulic accumulators:

1. Bladder accumulators 2. Direct Contact type Gas filled Accumulator 3. Diaphragm accumulators 4. Piston accumulators 5. Metal Bellow Accumulator. 6. Spring type Accumulator

BLADDER ACCULUMATOR

A bladder accumulator is the most commonly used hydro-pneumatic accumulator. The bladder is filled with nitrogen and fitted in a welded or forged steel pressure vessel. The bladder is made of an elastic material (elastomer), e.g. rubber. The gas pre-charge pressure can be adapted via the gas inlet/outlet valve on top of the bladder accumulator.

If the bladder accumulator is mounted vertically or at an angle, the gas side must always be on top. When the pressure drops, the compressed gas in the bladder expands and pushes the stored fluid into the hydraulic circuit. At zero pressure, the bladder may be pushed out of the pressure vessel. To prevent this, a spring-loaded valve is provided on the fluid side.

The bladder accumulator is used when a high power output is required. Specially designed bladder accumulators are capable of operating at maximum pressures of up to 1,000 bars. The gas volume and effective hydraulic volume is medium, ranging from 0.5 l to 450 l. Fig-1

Fig-1

Advantage: Highest efficiency with tests showing 97 percent energy retainment. Disadvantage: Nitrogen will permeate the foam bladder material over time and need to be periodically recharged.

DIRECT CONTACT TYPE GAS FILLED ACCUMULATORS

Direct-contact gas-to-fluid accumulators generally are used in very large installations where it would be very expensive to require a piston-or bladder-type accumulator. This type of accumulator consists of a fully enclosed cylinder, mounted in a vertical position, containing a liquid.

Fig-2

Ammonia Feed pump suction dampener act as direct contact accumulator because the steam jacketed on top portion, so that the vaporized ammonia pushed the liquid ammonia.

Fig-3

DIAPHRAGM ACCUMULATOR

These accumulators have a rubber plate or diaphragm as the separating element the diaphragm-type accumulator is constructed in two halves which are either screwed or bolted together. A synthetic rubber diaphragm is installed between both halves, making two chambers. The compartment above the diaphragm is filled with nitrogen. Two threaded openings exist in the assembled component. Contains a screen disc which prevents the diaphragm from extruding through the threaded opening when system

pressure is depleted, thus rupturing the diaphragm. On some designs the screen is replaced by a button-type protector fastened to the center.These accumulators have a rubber plate or diaphragm as the separating element. This element is welded or screwed together between two spherical shells (or compartments). The compartment above the diaphragm is filled with nitrogen. The compartment below is directly connected to the hydraulic circuit. Diaphragm accumulators are useful if the required fluid storage capacity is low (i.e. 4 litres or less).

Fig-4

Advantages: Diaphragm accumulators have most of the advantages of bladder-type units, but can handle gas compression ratios of up to 8:1.

Disadvantages: However, they are limited to smaller volumes, and their performance can sometimes be affected by gas permeating across the diaphragm. Leakage problem also occur in this type of accumulator.

PISTON ACCUMULATOR

This accumulator consists of a cylinder assembly, a piston assembly, and two end-cap assemblies. An accumulator contains a free-floating piston with liquid on one side of the piston and pre-charged air or nitrogen on the other side. An increase of liquid volume decreases the gas volume and increases gas pressure, which provides a work potential when the liquid is allowed to dis-charge. In this type of accumulator, pressure is created when nitrogen is compressed in a thin-walled metal cylinder shell by the hydraulic fluid pushing on a metal piston. Piston accumulators use pistons as separating elements. They can handle much higher gas compression ratios (up to 10:1) and flow rates as high as 215 litres (57 gallons) per second. However, the piston inside the accumulator causes internal friction

that negatively affects reaction speed Piston accumulators also require a higher level of fluid cleanliness than bladder units. Piston accumulators are used for very large fluid storage requirements of up to 2,500 litres at very high pressures (up to 1,000 bar).

Fig-5

Advantage: Virtually no nitrogen escapes so they will not have to be recharged. Disadvantage: A bit heavier, and less efficient than the bladder mod, they are more susceptible to fluid contamination. Lower response time than the bladder and diaphragm.

METAL BELLOW ACCUMULATOR

Metal bellows are used in variety of industries to store the energy of liquid or gas The metal bellows accumulator is similar to bladder type, expect the elastic is replaced by a hermitically sealed welded metal bellows. Fluid may be internal or external to the bellows internal. It is used when a fast response time is not critical, yet reliability is important. Metal bellow types are pre-charged by supplier and then permanently sealed leading to maintenance free accumulators.

Fig-6

Advantages.

Metal bellow 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 long stroke relative solid height, which gives maximum storage volume for a given container size. It provides exceptionally high level accumulator performance. It can be produced with broad spectrum of alloys resulting broad range of fluid compatibility.

Disadvantages.

Response time is more & high cost.

SPRING TYPE ACCUMULATOR

It uses the energy stored in springs to create a constant force on the liquid contained in an adjacent ram assembly. The load characteristics of a spring are such that the energy storage depends on the force required to compress s spring. The free (uncompressed) length of a spring represents zero energy storage. As a spring is compressed to the maximum installed length, high pressure value of the liquid in a ram assembly is established. As liquid under pressure enters the ram cylinder, causing a spring to compress, the pressure on the liquid will rise because of the increased loading required to compress the spring.

Fig-7

CONCLUSION

So what are the benefits of using accumulators? Lower installed system costs, accumulator assisted hydraulics can reduce the size of the pump and electric motor which results in a smaller amount of oil used, a smaller reservoir and reduced equipment costs. Less leakage and maintenance costs, the ability to reduce system shocks will prolong component life, reduce leakage from pipe joints and minimize hydraulic system maintenance costs. Improved performance, low inertia bladder accumulators can provide instantaneous response time to meet peak flow requirements. They can also help to achieve constant pressure in systems using variable displacement pumps for improved productivity and quality.

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