Accumulators usually are in-stalled in hydraulic systems tostore energy and to smooth out

pulsations. Typically, a hydraulic sys-tem with an accumulator can use asmaller pump because the accumulatorstores energy from the pump during pe-riods of low demand. This energy isavailable for instantaneous use, re-leased upon demand at a rate manytimes greater than what could be sup-plied by the pump alone.

Accumulators also can act as surgeor pulsation absorbers, much as an airdome is used on pulsating piston or ro-tary pumps. Accumulators will cushionhydraulic hammer, reducing shockscaused by rapid operation or suddenstarting and stopping of power cylin-ders in a hydraulic circuit.

There are four principal types of ac-cumulators: the weight-loaded pistontype, diaphragm (or bladder) type,spring type, and the hydro-pneumaticpiston type. The weight-loaded typewas the first used, but is much largerand heavier for its capacity than themodern piston and bladder types. Bothweighted and spring types are infre-quently found today. Hydro-pneumaticaccumulators, Figure 1, are the typemost commonly used in industry.

Functions Energy storage — Hydro-pneu-

matic accumulators incorporate a gas inconjunction with a hydraulic fluid. Thefluid has little dynamic power-storagequalities; typical hydraulic fluids canbe reduced in volume by only about1.7% under a pressure of 5000 psi.(However, this relative incompressibil-

fluid. The piston in turn, forces thefluid from the cylinder into the systemand to the location where useful workwill be accomplished.

Pulsation absorption — Pumps, ofcourse, generate the required power tobe used or stored in a hydraulic system.Many pumps deliver this power in apulsating flow. The piston pump, com-monly used for its high pressure capa-

1998/1999 Fluid Power Handbook & Directory A/5

ity makes them ideal for power trans-mission, providing quick response topower demand.) Therefore, when only2% of the total contained volume is re-leased, the pressure of the remaining oilin the system drops to zero.

On the other hand, gas, the partner tothe hydraulic fluid in the accumulator,can be compressed into small volumesat high pressures. Potential energy isstored in the compressed gas to be re-leased upon demand. Such energy canbe compared to that of a raised piledriver ready to transfer its tremendousenergy upon the pile. In the piston typeaccumulator, the energy in the com-pressed gas exerts pressure against thepiston separating the gas and hydraulic

Shell

Charging valve

Bladder

Poppet

Spring

Hydraulic cap

Charging valve

Gas cap

Body

Piston

Hydraluic cap

Bladder Gas Piston

Fig. 1. Cross-sectional views of typical of bladder and piston-type accumulators.

For more information about usingaccumulators in hydraulic sys-tems, please refer to the Basic Cir-cuits section which appears else-where in this handbook.

Accumulators

bility, can produce pulsations detri-mental to a high-pressure system. Anaccumulator properly located in thesystem will substantially cushion thesepressure variations.

Shock cushioning — In many fluidpower applications, the driven memberof the hydraulic system stops suddenly,creating a pressure wave that travelsback through the system. This shockwave can develop peak pressures sev-eral times greater than normal workingpressures and can be the source of sys-tem failure or objectionable noise. Thegas cushion in an accumulator, prop-erly placed in the system, will mini-mize this shock.

An example of this application is theabsorption of shock caused by sud-denly stopping the loading bucket on ahydraulic front end loader. Without anaccumulator, the bucket, weighing over2 tons, can completely lift the rear

wheels of a loader off the ground. Thesevere shock to the tractor frame andaxle, as well as operator wear and tear,is overcome by adding an adequate ac-cumulator to the hydraulic system.

Supplementing pump flow — Anaccumulator, capable of storing powercan supplement the hydraulic pump indelivering power to the system. Thepump stores potential energy in the ac-cumulator during idle periods of thework cycle. The accumulator transfersthis reserve power back to the systemwhen the cycle requires emergency orpeak power. This enables a system toutilize a much smaller pump, resultingin savings in cost and power.

Maintaining pressure — Pressurechanges occur in a hydraulic systemwhen the liquid is subjected to rising orfalling temperatures. Also, there maybe pressure drop due to leakage of hy-draulic fluid. An accumulator compen-

sates for such pressure changes by de-livering or receiving a small amount ofhydraulic fluid. If the main powersource should fail or be stopped, the ac-cumulator would act as an auxiliarypower source, maintaining pressure inthe system.

Fluid dispensing — An accumula-tor may be used to dispense small vol-umes of fluids, such as lubricatinggreases and oils, on command.

Operation When sized and precharged prop-

erly, accumulators normally cycle be-tween stages (d) and (f), Figure 2. Thepiston will not contact either cap in apiston accumulator, and the bladderwill not contact the poppet or be com-pressed so that it becomes destructivelyfolded into the top of its body.

Manufacturers specify recom-mended precharge pressure for their ac-

A/6

Gas��������(a)�����������������(b)� ���� � ������ ��� �(d) ���������������(f) ����� � ���� �� ��(e)����������������������(c)

Fig. 2. Six stages of operation for bladder- and piston-type accumulators: stage (a), the accumulator is empty -- no gas charge; stage(b), the accumulator has been precharged with dry nitrogen; stage (c), system pressure exceeds precharge pressure, and hydraulic fluidflows into accumulator; stage (d), system pressure peaks, maximum fluid has entered the accumulator, and system relief opens; stage(e), system pressure drops, precharge pressure forces fluid from the accumulator and into the system; and stage (f), system pressurereaches minimum needed to do work.

A C C U M U L A T O R S

cumulators. In energy-storage applica-tions, a bladder accumulator typicallyis precharged to 80% of minimum hy-draulic system pressure and a piston ac-cumulator to 100 psi below minimumsystem pressure. Precharge pressuredetermines how much fluid will remainin the accumulator at minimum systempressure.

Correct precharge involves accu-rately filling an accumulator’s gas sidewith a dry inert gas, such as nitrogen,while no hydraulic fluid is in the fluidside. Accumulator charging then be-gins when hydraulic fluid is admittedinto the fluid side, and occurs only at apressure greater than the prechargepressure. During charging, the gas iscompressed to store energy.

A correct precharge pressure is themost important factor in prolonging ac-cumulator life. The care with whichprecharging must be accomplished andmaintained is an important considera-tion when choosing the type of accumu-lator for an application, all else beingequal. If the user tends to be carelessabout gas pressure and relief valve set-tings, or adjusts system pressures with-out making corresponding adjustmentsto precharge pressure, service life maybe shortened, even if the correct type ofaccumulator was selected. If the wrongaccumulator was selected, prematurefailure is almost certain.

Mounting positionThe optimum mounting position for

any accumulator is vertical with the hy-draulic port down. Piston models canbe horizontal if the fluid is kept clean.When solid contaminants are present orexpected in significant amounts, hori-zontal mounting can result in uneven oraccelerated seal wear. Maximum ser-vice life can be achieved in the horizon-

tal position with multiple piston sealsto balance the piston’s parallel surface.

A bladder accumulator also can bemounted horizontally, Figure 3, but un-even wear on the bladder as it rubsagainst the shell while floating on thefluid can shorten life. The amount ofdamage depends on fluid cleanliness,cycle rate, and compression ratio (de-fined as maximum-system-pressure/minimum-system-pressure). In ex-treme cases, fluid can be trapped awayfrom the hydraulic end, which reducesoutput or may elongate the bladder toforce the poppet closed prematurely.

Sizes and outputsAvailable sizes and capacities also

influence which accumulator type tochoose. Piston accumulators of a par-ticular capacity often are supplied in achoice of diameters and lengths, Table1. Furthermore, piston designs can bebuilt to custom lengths for little or noprice premium. Bladder accumulatorsare offered only in one size per capac-ity, with fewer capacities available.

The inherently higher output of thepiston accumulator may make it thebest alternative when space is tight.

Table 1 lists outputs for 10-gal pistonand bladder accumulators operatingisothermally as auxiliary powersources over a range of minimum sys-tem pressures. The differences inprecharge pressure, columns 3 and 4,(determined by 80% of minimum sys-tem pressure for bladder models, 100psi below minimum for piston) lead toa substantial difference in outputs,columns 5 and 6.

To prevent excessive bladder defor-mation and high bladder temperatures,also note in Table 1 that bladder accu-mulators should be specified with com-pression ratios greater than 3:1.

Multiple componentsAlthough bladder designs are not

available in sizes over 40 gal, pistondesigns are currently supplied up to200 gal in a single vessel. Economicsand available installation space haveled engineers to consider multiple com-ponent installations. Two of these cancover most high-output applications.

The installation in Figure 4 consistsof several gas bottles serving a singlepiston accumulator through a gas mani-fold. The accumulator portion must be

1998/1999 Fluid Power Handbook & Directory A/7

�� ���� � Fig. 3. Horizontally mounted accumulatorcan cause uneven bladder wear and trapfluid away from the hydraulic valve.

������������Gas manifold��������Fluid����Fig. 4. Piston accumulators used in conjunction with gas bottles.

Table 1–Relative outputs, 10 gal accumulator

System pressure,psi

maximum 1 minimum 2 bladder 3 piston 4 bladder 5 piston 6

1.5 3000 2000 1600 1900 2.53 3.002.0 3000 1500 1200 1400 3.80 4.41

3.0 3000 1000 800 900 5.06 5.706.0 3000 500 – 400 – 6.33

Compressionratio 1/2

Recommendedprecharge, psi

Output,gal

A C C U M U L A T O R S

sized so the piston does not repeatedlystrike the caps while cycling. One draw-back of this arrangement is that a singleseal failure could drain the gas system.Because gas bottles often are less ex-

pensive than accumulators, one advan-tage of this setup might be lower cost.

Several accumulators, either pistonor bladder design, can be mounted on ahydraulic manifold, Figure 5. If usingpiston accumulators, the piston withthe least friction will move first and oc-casionally could bottom on the hy-draulic cap. In slow or infrequentlyused systems, this is insignificant.

Gas bottle installationsRemote gas storage offers flexibility

in large and small systems, Figure 6.The gas bottle concept is generally de-scribed with this simple formula: accu-mulator size minus required fluid out-put equals gas bottle size. For example,an application that calls for a 30-gal ac-cumulator may only require 8 to 10 galof fluid output. This application, there-fore, could be satisfied with a 10-galaccumulator and a 20-gal gas bottle.

An accumulator used with remote gasstorage generally has the same size portat the gas end as at the hydraulic end toallow unimpeded flow of gas to andfrom the gas bottle. The gas bottle has anequivalent port in one end and a gascharging valve at the other. These two-piece accumulators can be configured orbent at any angle to fit available space.

The gas bottle concept is suitable foreither bladder or piston accumulators.Note that bladder accumulators requirea special device called a transfer bar-rier at the gas end to prevent extrusionof the bladder into the gas bottle piping.

Again, a piston accumulator shouldbe sized to prevent piston bottoming ateither end of the cycle. Bladder designsshould be sized to prevent filling tomore than 85% or discharging to morethan 85% empty. The flow rate be-tween the bladder transfer barrier andits gas bottle will be restricted by the

neck of the transfer barrier tube. Be-cause of these drawbacks, bottle/blad-der accumulators should be reservedfor special applications.

Flow rates and response timesTable 2 suggests maximum flow

rates for representative accumulatorsizes and types. The larger standardbladder designs are limited to 220 gpm,although the rate can be boosted to 600gpm using an extra-cost, high-flowport. The poppet controls flow rate; ex-cessive flow causes the poppet to closeprematurely. Multiple accumulatorsmounted on a common manifold areneeded to achieve flows that are greaterthan 600 gpm.

Allowable flow rates for piston accu-mulators generally exceed those forbladder designs. Flow is limited by pis-ton velocity, which should not exceed10 ft/sec to avoid piston seal damage.In high-speed applications, high sealcontact temperatures and rapid decom-pression of nitrogen that has permeatedinto the seal material can cause blisters,cracks, and pits in the rubber.

Bladder accumulators respond morequickly to system pressure variationsthan do piston types for two reasons:

1. Rubber bladders do not have toovercome the static friction which apiston seal must, and

2. The piston mass does not need tobe accelerated and decelerated.

In practice, though, the difference inresponse may not be as great as com-monly believed, and is probably insig-nificant in most applications.

Shock suppressionTests at the University of Wisconsin,

Madison, indicate that shock controldoes not necessarily demand a bladderaccumulator. With system flow at a

A/8

Table 2–Maximum recommended accumulator flow rates

Bladder capacity

2 1 qt 100 60 –4 1 gal 400 150 –6 2.5 gal 800 220 600

7 1200 220 6009 larger than 2.5 gal 2000 220 600

12 3400 220 600

Piston bore, in. Piston Standard High-flow

Tubing lengthmakes pump-to-servovalvedistance 118 ft

LVDT�����Fig. 6. A small accumulator may do the jobif it is remotely connected to an auxiliarygas bottle.

Fig. 7. Test circuit to generate and mea-sure shock waves in system.

A C C U M U L A T O R S

Bladder

Gpm at 3000 psi

Fig. 5. Several accumulators may be man-ifolded to provide large system flows.�����������������Fluid manifold

nominal 30 gpm in the test circuit, Fig-ure 7, an internally piloted directionalcontrol valve, 118 ft away from thepump, closes to generate a shock. Asthe shock wave travels from the valveback through the hydraulic lines andaround corners and various restrictions,some portion of its energy is consumedwhile accelerating the mass of fluid inthe lines.

With 11/4-in. tubing, a 2750-psi reliefvalve setting, and no accumulator in thecircuit, oscilloscope trace A, Figure 8,shows a pressure spike of 385 psi overthe relief valve setting. Adding a 1-galpiston accumulator at the valve reducesthe transient to 100 psi over relief valvesetting, trace B. Substituting a 1-galbladder accumulator cuts the transientto 78 psi over relief valve setting, trace

C, only 22 psi better than the piston-type protection.

A second, similar test with 5/8-in.tubing and a relief valve setting of 2650psi results in a pressure spike of 2011psi over relief valve setting without anaccumulator, trace A, Figure 9. A pis-ton accumulator damps the transient to107 psi over relief valve setting, traceB, while a bladder accumulator dampsthe transient to 87 psi over relief valvesetting, trace C. The difference be-tween accumulator types in shock sup-pression again was negligible.

Servo equipmentAnother common misconception

says that all servo applications requirea bladder accumulator. Experienceshows that only a small percentage ofservos require response times of 25 msor less, the region where the differencein response between piston and bladderaccumulators becomes material. Blad-der accumulators should be used forapplications requiring less than a 25-msresponse, and either type when re-sponse of 25 ms or greater is adequate.

Setup and maintenance: precharging On newly repaired bladder accumu-

lators, the shell ID should be lubricatedwith system fluid before precharging.This fluid acts as a cushion, and lubri-cates and protects the bladder as it un-winds and unfurls. When prechargingbegins, the initial 50 psi of nitrogenshould be introduced slowly.

Neglecting these precautions couldresult in immediate bladder failure.

High-pressure nitrogen, expandingrapidly and thus cold, could channel thelength of the folded bladder and con-centrate at the bottom. The chilled brit-tle rubber expanding rapidly could rup-ture in a starburst pattern, Figure 10(a).The bladder also could be forced underthe poppet, resulting in a C-shaped cutin the bladder bottom, Figure 10(b).

The fluid side of piston accumulatorsshould be empty during precharging sothat gas-side volume is at a maximum.Little damage, if any, can take placeduring precharging.

Too high a precharge pressure or re-ducing the minimum system pressurewithout a corresponding reduction inprecharge pressure may cause operat-ing problems or damage to accumula-tors. With excessive precharge pres-sure, a piston accumulator will cyclebetween stages (e) and (b), Figure 2,and the piston will range too close tothe hydraulic end cap. The piston couldbottom at minimum system pressure toreduce output and eventually causedamage to the piston and its seal. Thebottoming of the piston often can beheard; the sound serves as a warning ofimpending problems.

Too high a precharge in a bladder ac-cumulator can drive the bladder into thepoppet assembly when cycling betweenstages (e) and (b), Figure 2. This couldcause fatigue failure of the spring andpoppet assembly, or a pinched and cutbladder if the bag gets trapped beneaththe poppet as it is forced closed. Toohigh a precharge pressure is the mostcommon cause of bladder failure.

Fig. 10. Starburst rupture in end of bladder,(a), could indicate loss of elasticity of blad-der material due to embrittlement fromcold nitrogen gas during precharge. If blad-der is forced under poppet, (b), bladdercould sustain C-shaped cut from poppet.

Fig. 8. Graph indicates results of shock wave tests. Fig. 9. Results of second test using smaller-diameter tubing.

Trace C

Trace BTrace A

Time – ms

0 1000 2000

2000

4000

Pre

ssur

e –

psi

4000

2000

1000 20000Time—ms

Pre

ssur

e—ps

i

Trace A

Trace B

Trace C

(a)

1998/1999 Fluid Power Handbook & Directory A/9

A C C U M U L A T O R S

(b)

Too low a precharge pressure or anincrease in system pressure without acompensating increase in prechargepressure also can cause operatingproblems, with possible accumulatordamage. With no precharge in a pis-ton accumulator, the piston likely willbe driven into the gas end cap andprobably will remain there. A singlecontact is unlikely to cause damage.

For bladder accumulators, too lowor no precharge can have severe con-sequences. The bladder may becrushed into the top of the shell, thenmay extrude into the gas valve and bepunctured, Figure 11. One such cycleis sufficient to destroy a bladder. Pis-

ton accumulators, therefore, are moretolerant of improper precharging.

External forcesAny application subjecting an accu-

mulator to acceleration, deceleration,or centrifugal force may have a detri-mental effect on accumulator opera-tion. Forces along the axis of an accu-mulator’s tube or shell normally havelittle effect on a bladder model but mayincrease or decrease gas pressure in apiston type because of the mass of thepiston affects the force.

Forces perpendicular to an accumu-lator’s axis should not affect a pistonmodel, but fluid in a bladder accumula-

tor may be thrown to one side of theshell, displacing the bladder and flat-tening and lengthening it, Figure 12.With this distortion, fluid dischargecould cause the poppet valve to pinchand cut the bladder.

Failure predictionSeveral methods can be used to mon-

itor the precharge pressure of piston ac-cumulators:

With the hydraulic system shutdown — A pressure transducer or gagelocated in the gas end cap, Figure 13(a),indicates the true precharge pressureafter a working hydraulic system hascooled, and the accumulator does notcontain fluid.

With the hydraulic system operat-ing — On request, accumulator manu-facturers will install a piston-positionsensor in an accumulator’s hydraulicend cap, Figure 13(b). This sensor canbe connected to a number of electronicspackages. With an accurate prechargeand after enough system operation forthermal stability, the electronics can becalibrated to provide continuous read-out of precharge pressure that corre-sponds accurately to the true precharge.

With the accumulator coupled to agas bottle — A ferrous or nonferroussensor can be installed in the accumula-tor gas end cap, Figure 13(c), to detectwhen the piston comes within 0.125 in.of the cap. This warning indicates thatprecharge pressure has dropped, andthe system should be shut down andchecked.

A/10������������Pressuretransducer

Fluid(a)��������Sonar absolute

position sensor

Pressuretransducer

Fluid(b)��� ����������������

Pressuretransducer

Fluid (c)�� ������ ��Hall proximitysensor

Fig. 11. If pressure fluid is allowed intouncharged accumulator, bladder could becrushed or extruded into gas valve andpunctured.

Fig. 12. Forces applied perpendicular tobladder accumulator vertical axis can dis-tort bladder shape and risk bladder punc-ture.

Fig. 13. With pressure transducermounted in cap of piston-type ac-cumulator, (a), actual prechargewill be indicated after workingsystem has become dormant andcooled. Piston-position sensor,(b), can provide continuous read-out of precharge when connectedto proper electronics package.With Hall-effect sensor installed,(c), close proximity of piston toend cap can be indicated. Note:pistons in (b) and (c) must be flatfor use with sonar or Hall-effectsensors.

���������� � ��� Acceleration� ��� ��A C C U M U L A T O R S