A Hydraulic Accumulator Energy Engineering Essay

A Hydraulic Accumulator Energy Engineering EssayA hydraulic aggregator is a machination in which potential zilch is stored in the form of a loaded plash or spring, or by a raised metric weight unit to be aimd to exert a force against a relatively incompressible ment entirelyy ill. They are used in fluid force trunks to accumulate energy and to melted out pulsations. Accumulators store energy when hydraulic system drag is greater than the aggregator pressure and take into account 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 chassis changes (actuators moving, valves opening, etc.) are occurring within a hydraulic system. Accumulators react very fast to configuration changes, nearly forthwith for bobble accumulators. They are ordinarily used in conjunction with a pump/ ride in a hydraulic circuit. A hydraulic system utilizing an accumulator can use a small fluid pump since the accumulator stores energy from the pump during low demand periods. The pump doesnt direct to be so large to cope with extremes of demand, so that the supply circuit can respond more rapidly 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/ gaseous state.There are 4 fibers of accumulators bladder, diaphragm bladder, plumbers helper (either spring or gas controlled) and metal bellow. The choice of accumulator to use in a given application depends on required speed of accumulator rejoinder, weight, reliability and cost. Pressurized gas accumulators exit bugger off the faster dynamic response and are reliable. admixture bellows accumulators are very reliable, but will not respond as fast as a pressurized gas accumulator. Accumulators w ith impressions generally make water the lowest reliability. Accumulators are either world-wide or rounded in design. Bag, plumbers helper 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 move 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 office usually has a fitting that connects to the hydraulic reservoir (for seal leakage and to alleviate back pressure on a plumbers helper). 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 supreme sum of money of energy stored by the accumulator. As system hydraulic pressure drops, the gas will poke out pushing hydraulic fluid back into the system. The gas pre-charge level is an all-important(a) parameter for gas accumulators since the pre-charge and overall accumulator volume determine the level best 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 in that location is no hydraulic fluid in the accumulator to the desire 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 school o f a pre-charge pressure and the fluid volume ability is reduced. Furthermore, if a ravisher accumulator charge is too high than the base of operations may hit the poppet valve which could persecute the udder done repeated hits in service, or grounds 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 marginal 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 coat the minimum and pocketimum gas volumes should be computed (under worst case conditions) and analyzed to ensure piston stops are not hit or that a bag cannot richly collapse or expand completely in the accumulator. vesica AccumulatorA bladder accumulator consists of pressure vessel with an intimate elastomeric bladder with pressurized nitrogen on one side and hydraulic fluid on the other side (system side). omen 1 shows a bladder accumulator. It has 3 stages of operation The accumulator is charged with nitrogen through and 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 downriver 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 say 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 casinghttp//www.globalspec.com/NpaPics/18/146314_030520074661_ExhibitPic.JPGThe main advantages of a bladder accumulator are fast acting, no hysteresis, not hypersensitive to contamination and consistent behaviour under identical conditions. Accumulators are easy to charge with the right equip ment. 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 alliance internal to the accumulator has proven to be very reliable in service. Of course there is continuously 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 AccumulatorA diaphragm accumulator is similar to bag accumulator except an elastomeric diaphragm is used in lieu of a bag. This would typically reduce the functional 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.http//www.machinerylubrication.com/articles/200907/pg26b.gifFigure http//www.machinerylubrication.com/articles/200907/pg26b.gifThe 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 change magnitude maximum compressions ratio (maximum system pressure to pre-charge pressure) of approximately 8 to 1. speculator AccumulatorA gas piston accumulator is shown in Figure 3. A gas piston accumulator has a piston which slides against the accumulator housin g 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 TypeFigure http//www.tobul.com/index.php?option=com_contenttask=viewid=13Itemid=27A 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 countenance higher flow rates than gas accumulators for equal accumulator volumes. This is because piston accumulators can accommodate higher pressure ratios (maximum system pressu re 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 roughly 90% of minimum desired hydraulic system pressure.A schematic of a spring piston accumulator is shown in FigureAccumulator, Spring TypeIn 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 sound 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 i s set by the natural frequency, which is computed usingMetal bellows accumulators are used where a fast response time is not unfavorable yet reliability is important. nip 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 provenservice history. Metal bellows accumulators are pre-charged by supplier and then permanently sealed leading to a forethought 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 emp ty, and a long stroke relative to solid (empty) height, which gives maximum retentiveness volume for a given container size. The welded metal bellows accumulator proffers an exceptionally high level of accumulator performance, and can be call forthd with a broad(a) spectrum of alloys resulting in a broad range of fluid compatibility. Another advantage to this type is that it does not panorama issues with high pressure operation, thus allowing more energy storage capacity.Applications of Accumulators to KERSOne of the main applications of hydraulic accumulators is storing energy. Hydro-pneumatic accumulators co-ordinated 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 thoroughgoing contained volume is released, the pressure of the remaining fluid in the system will drop to zero. However, the relative incom pressibility of a hydraulic fluid makes it ideal for fluid power systems and provides quick response to power demand. The gas, however working(a) 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 bicycle 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-p neumatic regenerative braking on a bicycle. It was a redevelopment of a heavier previous go about to make a working paradigm 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.FigureIts weight is clearly impractical as it weighs almost as much as a conventional cycles/second 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 passenger a lighter wheel makes it easier to initially start a bike.Based on the team from michigans ( ) the undermentioned calculations outline the practicality of imp lementing 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 labors parkers rate the ACP series accumulators at max pressure 5000psi, if assumingA hydraulic KERS must use a hydraulic motor to provide enough torque to drive the bike as well as provide enough resistant torque to be an effective brake. A bicycle travelling at 20mph on 26 wheels spins the motor through 181 accommodate ratio of the pump gear lease which then spins the motor 4632 rpm, 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 181 gear ratio. From this brake torque is an effective brakeOn release of pressure fully charged 5000 psi accumulator generates 7.57 N-m of torques. The 141 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 141 gear ratio. This torque from fig can propel a bike atConclusionThe accumulator doesnt 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.