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Accumulator, Hydraulic - Description

Accumulators store hydraulic energy and then provide this energy back to the system when required. Accumulators store energy when hydraulic system pressure is greater the accumulator and provide hydraulic energy when the accumulator pressure is greater than the system pressure. By storing and providing hydraulic energy, accumulators can perform 5 basic functions for hydraulic systems:

Almost all aerospace hydraulic systems use accumulators for one of the above reasons. In fact, most hydraulic systems use an accumulator to dampen pressure transients in the power generation system. The pressure transients result from pump ripple, opening/closing of valves, actuators bottoming out and so on. Some practitioners believe accumulators are over utilized and systems can be designed without an accumulator in the power generation system. However, this has not been standard practice and if an accumulator is not used, other design considerations should be considered. The selection and design characteristics of accumulators will vary between the applications.

Hydraulic accumulators store hydraulic fluid under pressure. Pressure is supplied through a bag, diaphragm or piston by either a spring, or pressured gas (most common). 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 instanteously for gas accumulators. The capability and affect of the accumulator is determined by the overall volume of the accumulator and preload/precharge of the spring/gas

Gas accumulators take advantage of the fact that the gas (nitrogen) is compressible. A gas accumulator has a gas precharge that is less than 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 precharge level is an important parameter for gas accumulators since the precharge and overall accumulator volume determine the maximum amount of hydraulic energy that will be available to the system.

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 aluminum or a composite (filament wound) material.

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 with the 3 stages of operation, plus an overexpanded bag schematic. The accumulator is charged with nitrogen through a valve installed in the top. The accumulator will be precharged to nominal pressure when the pumps are not operating, shown in Figure 1a. When nominal hydraulic system pressure is applied the bag will be compressed to its fully compressed state as shown in Figure 1b. When the bag is fully compressed, the nitrogen pressure and the hydraulic pressure are equal. As system pressure drops the bag expands, forcing fluid from the accumulator into the system as shown in Figure 1c. 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 as shown in Figure 1d (an undesirable situation). A poppet valve keeps the bag in accumulator from being pulled into the downstream tubing should the bag overexpand. 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.

The main advantages of a bladder accumulator are fast acting, no hysteresis, not susceptible to contamination and consistent behavior 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 precharge pressure) which is limited to approximately 4 to 1. Hence gas accumulators will be larger than other accumulators for the same flow requirements. The precharge pressure is typically set to approximately 80% of the minimum desired hydraulic system pressure.



Figure 1 Bladder Accumulator Schematic


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. The behavior characteristics of a diaphragm accumulator are similar to a bag accumulator.



Figure 2 Diaphragm Accumulator Schematic


Diaphragm accumulator’s behavior will be similar to a bladder 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 precharge 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.


Figure 3 Piston Accumulator Schematic


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 precharge 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 precharge 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 4.


Figure 4 Spring Controlled Accumulator Schematic

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

(rad/sec)


Metal Bellows

Figure 5 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. A metal bellows accumulator is shown in Figure 5. 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.


Figure 5 Metal Bellows Accumulator Schematic


Gas Accummulator Precharging

The precharge is the pressure of the gas in the accumulator without hydraulic fluid in the fluid side. A gas accumulator is precharged with nitrogen gas when there is no hydraulic fluid in the accumulator to the desired pressure. A rule of thumb for bladder accumulators is to set the precharge pressure to approximately 80% of the desired minimum hydraulic system pressure. A rule of thumb for gas piston accumulators is to set the precharge pressure to approximately 90% of the of the desired minimum hydraulic system 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 precharge 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 precharge pressure and the accumulator may not maintain desired minimum hydraulic system pressure. Also a low precharge 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 precharge 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.


Accumulator Design Considerations

The most important characteristics for hydraulic accumulators are listed below.

Accumulator Type – as described above there are 4 basic types of hydraulic accumulators (bladder, diaphragm, piston, and metal bellows). Each type has advantages and disadvantages and the specifications will vary between the 4 types.

Accumulator Volume – total volume of the accumulator (both gas and fluid volume)

Nominal Hydraulic System Pressure – this is the nominal hydraulic system pressure in the system, which will usually be the no flow rating of the hydraulic pump

Minimum Hydraulic System Pressure – this is the minimum pressure that the accumulator must maintain in the hydraulic system. This is a design requirement used to size the accumulator.

Precharge Pressure – precharge is the pressure of the nitrogen in an accumulator without any hydraulic fluid in the accumulator. The precharge pressure determines the amount of fluid that an accumulator can hold at the system pressure and the desired minimum hydraulic system pressure.

Required Flow Rate – to maintain minimum hydraulic system pressure, the accumulator must be able to supply sufficient flow over a determined period of time. The required flow rate is a key requirement that drives the size of the accumulator. The accumulator volume for hydraulic flow is equal to Q * t (required flow rate times the time required for this flow). The accumulator must provide this flow when the gas (or spring) is between the nominal hydraulic system pressure and the minimum desired hydraulic system pressure. Note that the flow rate provided by the accumulator will be nonlinear because as the gas expands the pressure drops off nonlinearly. This is a design requirement used in sizing accumulators.

Output Volume Capability – the output volume capacity of the fluid volume the accumulator is capable of providing between the nominal hydraulic system pressure and the required minimum hydraulic pressure. This volume must be provided at the required flow rate (see Required Flow Rate). The value is also called the working volume.

Response Time – this is the time for the accumulator to provide the desired fluid volume. The response time times the output volume capability equals the flow rate of the accumulator. This will be a function of precharge value and the flow opening (orifice) in the accumulator.

Recharge Time – this is the time fully charge an accumulator from a fully drained state (i.e., at minimum volume, which is the volume at the minimum desired hydraulic system pressure). This should be evaluated when there is a fast duty cycle requirement. The recharge time will be the amount of time for fluid to fill the accumulator based on the available flow rate from the pump (minus other system demands).

High Frequency Cycling Capability – only a concern when accumulators are used for damping of pressure pulsations or very fast pressure transients. In this type of application, the frequency response capability of the accumulator should be computed to ensure it is compatible with the transient phenomena.

Fluid Type – accumulator seals and elastromeric bladder/diaphragm material must be compatible with the hydraulic fluid used in the system

Failure Modes – the main failure modes for an accumulator will be failure of a bladder or piston seal, or a pressure vessel burst. The affects of a loss of accumulator performance should be evaluated in the hydraulic system to ensure no unacceptable affects may occur within the system. For a potential pressure vessel burst, the installation should be reviewed with respect to surrounding components and also for drainage of fluid and compartment ventilation.

Operating Temperature Range – the behavior of the gas (nitrogen) varies with temperature. Accumulator performance should be evaluated over the expected temperature range of the nitrogen.

Mounting Position – vertical is always preferred with fluid outlet at the bottom. Horizontal installations will tend to wear a bladder or diaphragm on the down side leading to earlier failures and lower reliability. For piston accumulators, the seals will also tend to wear unevenly leading to earlier leakage. If a non-vertical installation is required some evaluation of accumulator life should be accomplished and the appropriate maintenance inspections (or life limits) put in place.

Mounting Flange – Determine method of mounting accumulator is acceptable in your application and that the mount is capable of withstanding all mounting forces, including crash g loads. Analysis should use the mass of the accumulator when fully charged with fluid.

Connection Fitting – the hydraulic interface fitting must be known so that a mating fitting can be included in the design of the hydraulic system.


Applications of Accumulators

One of the main applications of hydraulic accumulators is to supply flow for brief periods of time when a pump cannot keep up. A benefit of using an accumulator in this regard is that it allows the pump size to be smaller. Usually the accumulator only assists during a worst case duty cycle or after a particular failure has occurred in the system. This requires having an accumulator of sufficient volume to supply the flow needs while still maintaining adequate system pressure. The approach to sizing an accumulator for this application is shown in the sizing section (see Accumulators, Hydraulic – Sizing).

Another application of accumulators is to damp pressure spikes from pumps or downstream configuration changes (such as actuators hitting stops and valves closing). This is most often done in the power generation portion of a hydraulic system, but accumulators can be put anywhere in the system for pressure pulsation damping. In this application, as a pressure wave moves up and down the piping, the energy is partially absorbed by the accumulator each time the wave flows by the accumulator. Hence the wave damps out much faster than in a system without an accumulator. Standard practice has shown this to be a proven technique, but no well proven design procedure exists for both sizing and placement of accumulators for pressure pulsation damping. Hence some experimentation may be required if an initial design does not achieve the desired results. One of the reasons for experimentation is that laboratory research has shown pressure waves in pipe to be both a function of time and location along a pipe. Thus at some locations along a tube there will only be small changes in pressure magnitude (high peak to low peak) while at other locations the pressure fluctuations (high peak to low peak) will be much larger.

A secondary function of accumulators is to absorb volume changes in fluid due to temperature fluctuations. If an accumulator is not used and a rise in temperature increases pressure above system pressure, then the fluid must flow through a thermal relief valve to the reservoir. This is wasted flow and hence results in wasted energy thereby decreasing system efficiency. Furthermore, a pressure relief valve exhibits hysteresis and must flow a sufficient fluid so that pressure drops below the level where the valve will close (which could be a significant flow amount). With an accumulator, reasonable volume changes can be accommodated without having flow to the reservoir. Computing accumulator size to accommodate temperature variations is relatively straightforward.

Of course, an accumulator can be sized and installed to do multiple functions. Therefore, a single accumulator can perform any or all of the above functions. The size and type of accumulator chosen will depend on the functions that accumulator is addressing.

Lastly, the loss of pressurized gas in a sealed accumulator (or spring failure in spring accumulators) is generally a latent failure. This latency may be an issue when conducting a safety analysis on a system where the accumulator plays an important role (such as emergency gear extend or emergency braking). In this case, an acceptable functional test procedure will usually need to be implemented at an appropriate interval.