A Hydraulic Accumulator Energy Engineering Essay

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. They are used in fluid power systems to accumulate energy and to smooth out pulsations. Accumulators store energy when hydraulic system pressure is greater than the accumulator pressure and provide hydraulic energy when the accumulator pressure is greater than the system pressure. By storing and providing hydraulic energy, accumulators can be used as a primary power source. Accumulators are inherently dynamic devices, they function when configuration changes (actuators moving, valves opening, etc.) are occurring within a hydraulic system. Accumulators respond very fast to configuration changes, nearly instantaneously for gas accumulators. They are usually used in conjunction with a pump/motor in a hydraulic circuit. A hydraulic system utilizing an accumulator can use a smaller fluid pump since the accumulator stores energy from the pump during low demand periods. The pump doesn’t need to be so large to cope with extremes of demand, so that the supply circuit can respond more quickly to any temporary demand and to smooth pulsations. The capability and affect of the accumulator is determined by the overall volume of the accumulator and preload/pre-charge of the spring/gas.

There are 4 types of accumulators: bladder, diaphragm bladder, piston (either spring or gas controlled) and metal bellows. The choice of accumulator to use in a given application depends on required speed of accumulator response, weight, reliability and cost. Pressurized gas accumulators will have the faster dynamic response and are reliable. Metal bellows accumulators are very reliable, but will not respond as fast as a pressurized gas accumulator. Accumulators with seals generally have the lowest reliability. Accumulators are either spherical or cylindrical in design. Bag, piston and metal bellows accumulators are cylindrical. Diaphragm accumulators may be spherical or cylindrical. Accumulators are usually manufactured into 2 halves which are either welded or threaded together. A fill port is installed at one end of a gas accumulator and the hydraulic connection fitting (with poppet valve, if required) is installed at the opposite end. For a spring accumulator, the non pressure side usually has a fitting that connects to the hydraulic reservoir (for seal leakage and to alleviate back pressure on a piston). Materials are usually steel, but accumulators may also be made from aluminium or a composite (filament wound) material.

Compressed gas accumulators are by far the most common type; these gas accumulators take advantage of the fact that the gas is compressible. A gas accumulator has a gas pre-charge that is less than the nominal hydraulic system pressure. As hydraulic fluid enters the accumulator the gas is compressed to the nominal system pressure, which is an equilibrium position and represents the maximum amount of energy stored by the accumulator. As system hydraulic pressure drops, the gas will expand pushing hydraulic fluid back into the system. The gas pre-charge level is an important parameter for gas accumulators since the pre-charge and overall accumulator volume; determine the maximum amount of hydraulic energy that will be available to the system.

The pre-charge is the pressure of the gas in the accumulator without hydraulic fluid in the fluid side. A gas accumulator is pre-charged with nitrogen gas when there is no hydraulic fluid in the accumulator to the desired pressure. The gas accumulator pre-charge is a very important variable for ensuring optimal accumulator performance and maintaining long life of the accumulator. Too high of a pre-charge pressure and the fluid volume capacity is reduced. Furthermore, if a bag accumulator charge is too high than the bag may hit the poppet valve which could damage the bag through repeated hits in service, or cause a fatigue failure in the poppet valve assembly. For a piston accumulator, the piston may be driven into the stops repeatedly affecting seals or cause a fatigue failure in the piston stop. Too low of a pre-charge pressure and the accumulator may not maintain desired minimum hydraulic system pressure. Also a low pre-charge pressure will allow a piston accumulator to repeatedly hit the “up” stops leading to premature failure of the accumulator. For a bag accumulator, the bag may be forced into an unnatural shape (e.g.,with folds) leading to bag damage and premature bag failure. When sizing an accumulator the pre-charge pressure is an input to the sizing process. However, once the accumulator is sized the minimum and maximum gas volumes should be computed (under worst case conditions) and analyzed to ensure piston stops are not hit or that a bag cannot fully collapse or expand completely in the accumulator.

Bladder Accumulator

A bladder accumulator consists of pressure vessel with an internal elastomeric bladder with pressurized nitrogen on one side and hydraulic fluid on the other side (system side). Figure 1 shows a bladder accumulator. It has 3 stages of operation: The accumulator is charged with nitrogen through a valve installed in the top. The accumulator will be pre-charged to nominal pressure when the pumps are not operating. Secondly when nominal hydraulic system pressure is applied the bag will be compressed to its fully compressed state. When the bag is fully compressed, the nitrogen pressure and the hydraulic pressure are equal. Finally as system pressure drops the bag expands, forcing fluid from the accumulator into the system. As the bag expands pressure in the bag decreases. The bag will continue to expand until the bag pressure equals the hydraulic pressure (which will be lower than nominal system pressure) or the bag fills the entire accumulator volume which is an undesirable situation. A poppet valve keeps the bag in accumulator from being pulled into the downstream tubing should the bag over-expand. If the bag was pulled into the downstream tubing, the accumulator would never recharge and normal flow from the pump would be constricted. The maximum flow rate of the accumulator is controlled by the opening area (orifice) and the pressure difference across the opening.

Figure http://www.globalspec.com/NpaPics/18/146314_030520074661_ExhibitPic.JPGAccumulator, Bladder Typehttp://www.globalspec.com/NpaPics/18/146314_030520074661_ExhibitPic.JPG

The main advantages of a bladder accumulator are fast acting, no hysteresis, not susceptible to contamination and consistent behaviour under similar conditions. Accumulators are easy to charge with the right equipment. Because there is no piston mass, the speed of the bladder accumulator is governed by the gas, which reacts very fast to changes in hydraulic system pressure. Hence bladder accumulators are the best choice for pressure pulsation damping. Also, the bladder attachment internal to the accumulator has proven to be very reliable in service. Of course there is always the potential for bladder failure, which is a failure that would not usually be detectable in service. Also, temperature differences on the gas will have some affect on performance.

The main limitation of bladder accumulators is the compression ratio (maximum system pressure to pre-charge pressure) which is limited to approximately 4 to 1. Hence gas accumulators will be larger than other accumulators for the same flow requirements. The pre-charge pressure is typically set to approximately 80% of the minimum desired hydraulic system pressure.

Diaphragm Accumulator

A diaphragm accumulator is similar to bag accumulator except an elastomeric diaphragm is used in lieu 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. A schematic of a diaphragm accumulator is shown in Figure 2.

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Figure http://www.machinerylubrication.com/articles/200907/pg26b.gif

The behaviour characteristics of a diaphragm accumulator are similar to a bag accumulator and have the same advantages and disadvantages. However a diaphragm accumulator may be spherical or cylindrical (or possibly other shapes) which may be an advantage in some installations. The main difference with bladder accumulators is an increased maximum compressions ratio (maximum system pressure to pre-charge pressure) of approximately 8 to 1.

Piston Accumulator

A gas piston accumulator is shown in Figure 3. A gas piston accumulator has a piston which slides against the accumulator housing on seals. On one side of the piston is nitrogen and on the other side is the hydraulic fluid and connection to the system. A fill port allows pressurization of the nitrogen.

Accumulator, Piston Type

Figure http://www.tobul.com/index.php?option=com_content&task=view&id=13&Itemid=27

A gas piston accumulator will not respond to transient pressures as fast as a bladder accumulator due to the mass of the piston (frequency characteristics depend on piston mass and spring characteristics of the nitrogen). However, a piston accumulator will have better damping due to hydraulic leakage (viscous damping) and friction between the piston and housing (coulomb friction & seal friction). Piston accumulators may also be more prone to leakage than other types of accumulators due to the seals. Piston accumulators will generally provide higher flow rates than gas accumulators for equal accumulator volumes. This is because piston accumulators can accommodate higher pressure ratios (maximum system pressure to pre-charge pressure) than gas accumulators, up to 10 to 1, compared with bladder accumulator ratios of 4 to 1.

The disadvantages of piston accumulators are that they are more susceptible to fluid contamination, have a lower response time than bladder (unless the piston accumulator is at a very high pressure) and will have hysteresis from the seal friction. The pre-charge for a gas piston accumulator is typically set to around 90% of minimum desired hydraulic system pressure.

A schematic of a spring piston accumulator is shown in Figure

Accumulator, Spring Type

In a spring accumulator, the spring applies a force to a piston which compresses (or pressurizes) the fluid in the accumulator. As normal system pressure, the spring will be fully compressed. As system flow demands exceed the pump capacity, the spring will extend pushing the piston which in turn pushes fluid into the adjoining pipe. Hence the accumulator supplements pump flow. The maximum response time of the accumulator is set by the natural frequency, which is computed using

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.

The advantages to the metal bellows 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 a long stroke relative to solid (empty) height, which gives maximum storage volume for a given container size. The welded metal bellows accumulator provides an exceptionally high level of accumulator performance, and can be produced with a broad spectrum of alloys resulting in a broad range of fluid compatibility. Another advantage to this type is that it does not face issues with high pressure operation, thus allowing more energy storage capacity.

Applications of Accumulators to KERS

One of the main applications of hydraulic accumulators is storing energy. Hydro-pneumatic accumulators incorporate a gas in conjunction with a hydraulic fluid. The fluid has little dynamic power storage qualities. The fluid can only be reduced a small amount in volume even under high pressure. Therefore when only a small amount of the total contained volume is released, the pressure of the remaining fluid 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, however working with 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. 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.

On this basis, with respect to all the types of accumulator a hydro-pneumatic accumulator would be ideal for storing the energy taken out of a bike whilst braking.. Of course the loss of pressurized gas in a sealed accumulator is a failure critical to safety when it plays such an important role as braking. A team of engineering students from university of Michigan undertook a project to use a hydro-pneumatic regenerative braking on a bicycle. It was a redevelopment of a heavier previous attempt to make a working prototype to fit within a 29″ front wheel. They use a 0.5L accumulator and believed this to be sufficient in storing the required energy at a maximum working system pressure of 5000psi.

They failed to test and thus supply conclusive results for the performance characteristics but through theoretical analysis they prescribe the key parameters fig.

Figure

Its weight is clearly impractical as it weighs almost as much as a conventional bike at 13kg. In terms of weight of a bicycle with respect to saving weight, it is more important to have lighter wheels than a lighter bike frame. This is because the rolling resistance is applied at the wheels although it carries half the overall weight of bike and rider a lighter wheel makes it easier to initially start a bike.

Based on the team from michigans ( ) the following calculations outline the practicality of implementing a hydraulic KERS. Firstly for a hydraulic system to be implemented the storage must be addressed the capacity must be determined and pressures needed to store the kinetic energy. A bike braking from 20mph requires 5000J of energy to power. From Parkers website a manufacture of accumulator and motors parker’s rate the ACP series accumulators at max pressure 5000psi, if assuming

A hydraulic KERS must use a hydraulic motor to provide enough torque to drive the bike as well as provide enough resistive torque to be an effective brake. A bicycle travelling at 20mph on 26″ wheels spins the motor through 18:1 gear ratio of the pump gear train which then spins the motor 4632rpm, corresponding to 4.52 N-m torques at 3000 psi. This translates to a braking torque of about 81.36 N-m applied to the main gear due to the 18:1 gear ratio. From this brake torque is an effective brake

On release of pressure fully charged 5000 psi accumulator generates 7.57 N-m of torques. The 14:1 gear ratio of the motor gear train applies a 105 N-m torque to the main bicycle cluster gear.

7.57 N-m corresponds to around 800 rpm from its torque rpm curve, which turns the main gear at around 57 rpm due to the 14:1 gear ratio. This torque from fig can propel a bike at

Conclusion

The accumulator doesn’t need to be an excessively large capacity to release enough energy to propel a bike 20mph, upon releasing the energy at a pre-charge of 3200psi. But a larger accumulator is needed for the accumulator to give more than one bursts using its full capacity. A hydraulic motor can produce 81.36N-m braking torque which is an effective brake. Furthermore an accumulator can power a hydraulic motor provide an accelerating torque to propel a bicycle. However based on the weight of the design from univerty of Michigan their prototype was 13kg, they used two accumulators plus they attached it to a bracket that probably contributed to the majority of the weight.