Hydraulic Accumulator


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1. Introduction 1.1 History of hydraulic accumulator 2. Types of hydraulic accumulator 2.1 Tower type accumulator 2.2 Raised weight accumulator 2.3 Spring-type accumulator 2.4 Compressed-gas accumulator a) Bladder type accumulator b) Diaphragm type accumulator c) Piston type accumulator d) Metal bellow type accumulator 3. Hydraulic symbols 4. Functions. 5. Design consideration 5.1 calculations 5.2 calculation of useful volume of the 6. Applications 6.1 Injection molding machine hydraulic circuit 6.2 Leakage Oil Compensation hydraulic circuit 6.3 Shortening of Stroke Time hydraulic circuit 7. Conclusion


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HYDRAULIC ACCUMULATOR Chandan Nagaraja 1RV08ME021 R.V.College of Engineering, Bengaluru


OVERVIEW Introduction History Types of H ydraulic Accumulator Functions of Accumulator Accumulator design consideration Application Conclusion


INTRODUCTION A Hydraulic Accumulator is energy storage device. It is pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. The external source used can be a spring, a raised weight, or a compressed gas. The main reasons that an accumulator is used in a hydraulic system, is that the pump doesn’t need to be so large to cope with extremes of demand and supply circuit can respond quickly to any temporary demand and to smooth pulsation.


HISTORY First Hydraulic accumulator was made by William Armstrong in 1846 built a crane powered by water of town mains at Newcastle, United Kingdom. Later 1852 ,Armstrong used Grimsby Dock Tower for the constant pressure for cranes, lock gates and sluices. Grimsby Dock Tower is hydraulic accumulator is used for the purpose of containing a 30,000UK gallon (136 m 3 ) hydraulic wrought iron reservoir at a height of 300feet (91 m) to provide power for lock gates and cranes of Grimsby Dock.

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Tower type accumulator Raised weight accumulator Compressed-gas accumulator Bladder type accumulator Diaphragm type accumulator Piston type accumulator Metal bellow type accumulator Spring accumulator TYPES OF HYDRAULIC ACCULUMATOR


1. TOWER TYPE ACCUMULATOR The first accumulators for Armstrong’s hydraulic dock machinery were simple raised water tower. It uses hydraulic head It delivers constant pressure to the output LONDON TOWER BRIDGE


2. RAISED WEIGHT ACCUMULATOR A raised weight accumulator consists of a vertical cylinder containing fluid 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. Gravity acts on the weight to pressurize the hydraulic system fluid, thus storing energy.


ADVANTGES : Used to meet heavy industrial requirements. Extremely high capacity at low cost. Rugged and durable construction. LIMITATIONS : Extremely bulky and heavy. Problem in sealing. In contrast to compressed gas accumulator, this type delivers a nearly constant pressure, regardless of the volume of fluid in the cylinder, until it is empty Hydraulic engine house, Bristol Harbor.


3. COMPRESSED GAS ACCUMULATOR It is widely used accumulator in present scenario. It is popularly known as “hydro-pneumatic accumulator”. It apply force to the liquid by using a compressed gas that acts as the spring. It uses inert gas (nitrogen) under pressure that provides the compressive force on fluid. Oxygen is not used because oxygen and oil can form an explosive mixture when combined under pressure As the volume of the compressed gas changes the pressure of the gas, and pressure of the fluid, changes inversely.


a) BLADDER TYPE ACCUMULATOR A bladder accumulator consists of seamless high-pressure cylinder with an internal elastomeric bladder with pressurized nitrogen on it and hydraulic fluid on the other(external) side. The accumulator is charged with nitrogen through a valve installed on the top. The accumulator will be pre-charged to nominal pressure when the pumps are not operating. The maximum flow rate of the accumulator is controlled by the opening orifice and the pressure difference across the opening. Bladder material widely used are epichlorohydric rubber(ECO) and Acrylonitrile butadiene rubber (NBR).

Bladder Accumulator (Various stages of operation):

Bladder Accumulator ( Various stages of operation)

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ADVANTAGES : Fast acting Not susceptible to contamination Consists behavior under similar condition LIMITATIONS : Compressed ratio is limited, approximately 4:1 Bladder failure.


b) DIAPHRAGM TYPE ACCUMULATOR A similar to bladder type, expect an elastomeric diaphragm is used in place of a bag. This would typically reduce the usable volume of the accumulator, so the diaphragm accumulator may not have volume capacity of a bladder accumulator. Diaphragm accumulator may be spherical or cylindrical. The main difference with bladder accumulator is an increased maximum compression ratio of approximately 8:1 It is low weight, compact design and good for shock applications (good response characteristics)






c) PISTON TYPE 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-charged.


PISTON TYPE ACCUMULATOR ADVANTAGES : High compression ratio up to 10:1 Higher flow rate than bladder type . LIMITATIONS : They are more susceptible to fluid contamination Lower response time than the bladder and diaphragm


d) METAL BELLOW ACCUMULATOR 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. 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 a maintenance free accumulator. Internal


METAL BELLOW ACCUMULATOR ADVANTGES : 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. LIMITATIONS : Response time is more High cost E xternal External

4. Spring type accumulator:

4. 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.

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It acts according to Hook’s law, magnitude of the force exerted by the spring is linearly proportional to its extension. Spring loaded accumulator





FUNCTIONS : Where are accumulators used?:

FUNCTIONS : 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. Shock 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. Noise reduction: An accumulator is effective at reducing hydraulic system noise caused by relief valves, pump pulsations, system shock and other circuit generated noises.

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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. 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 Discharge valve, electric ( optional ) Accumulator P-port Gauge port Shut-off valve Manifold Block Pressure-relief valve Discharge, manual Hydraulic Symbol Adapter fitting, Flange

ACCUMULATOR DESIGN CONSIDERATION : (What must I know to size and select an accumulator?):

ACCUMULATOR DESIGN CONSIDERATION : ( What must I know to size and select an accumulator ?) Accumulator type Accumulator volume Nominal hydraulic system pressure Minimum and Maximum hydraulic system pressure Pre-charge pressure Required flow rate Output volume capacity Recharge time Response time Fluid type

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Pressures : For the calculation of an accumulator, the following pressures are of decisive significance: p 0 = gas charge pressure at room temperature and drained fluid chamber p 0 (t ) = gas charge pressure at operating temperature p 0 ( tmax ) = gas charge pressure at max. operating temperature p 1 = minimum operating pressure p 2 = maximum operating pressure pm = medium operating pressure V0 = nominal capacity of the accumulator To achieve the best possible utilization of the accumulator capacity and a long service life, it is recommended that the following values be adhered to: p0 ( tmax ) ≈0.9* p1 ( 1 ) The highest hydraulic pressure should not exceed the quadruple of the pre-charge pressure; otherwise, the elasticity of the bladder is overstressed and excessive variations in the compression result in strong heating up of the gas: p2 ≤ 4 * p0 (2) CALACULATIONS

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a) In the case of isothermal changes of state of gases , that is , when the gas buffer changes so slowly that enough time is available for a complete heat exchange between the nitrogen and its surroundings and the temperature therefore remains constant, the following is valid : (3.1) b ) In the case of an adiabatic change of state , that is, with a rapid change of the gas buffer, in which the temperature of the nitrogen changes as well, the following is valid: (3.2) γ = ratio of the specific heat of gases (adiabatic exponent) for nitrogen = 1.4 In practice, changes in state rather follow adiabatic laws. Charging is often isothermal, discharging adiabatic. Oil volume : Pressures p0 … p2 determine gas volumes V0 … V2. Here, V0 is also the nominal capacity of the accumulator. The available oil volume V corresponds to the difference between gas volumes V1 and V2: dV ≤ V1 – V2 (4) p0 • V0 = p1 • V1 = p2 • V2 p0 • V0^ γ = p1 • V1^ γ = p2 • V2^ γ Cont ………

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Calculation of the useful volume of the accumulator. On the basis of the hydraulic cylinder the useful volume of the hydraulic accumulator (V)can be calculated according to following formula… + d = bore diameter of the hydraulic cylinder D = piston diameter of the hydraulic cylinder s = stroke length of the hydraulic cylinder a = number of complete extending stroke e = number of complete retracting stroke x = number of residual extending stroke z= number of residual retracting stroke

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measuring jar 4/3 directional valve with solenoid actuation Cylinder assembly Accumulator Hydraulic Pump power unit Check valve Hydraulic –accumulator circuit PRV

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Electrical circuit


APPLICATIONS Accumulators are devices used to store fluid power to do the following: Dampen pulsations and shocks of a periodic nature I ncrease the speed of the operational circuit. Clamping devices to hold the jaw vices and fixtures Standby power supply circuits. Surge reduction circuits Agricultural Machinery & Equipment Forestry Equipment Oil Field & Offshore Machine Tools and Off- Road Equipment Mining Machinery & Equipment Mobile & Construction Equipment Suspension in vehicles

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Hydro electric P ressure Switch Accumulator safety block for diaphragm-type Energy Storage in an Injection Molding Machine

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Leakage Oil Compensation circuit & Shortening Time circuit of Stroke Time

CONCLUSION So what are the benefits of using accumulators?:

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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Reduced noise levels, reduced pump and motor size couple with system shock absorption overall machine sound levels and results in higher operator productivity . Flexible design approaches. A wide range of accumulator types and sizes, including accessory items, provides a versatile and easy to apply design approach . Reduced energy costs, cost savings of up to 33% are achievable in high performance industrial machinery using accumulators.

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