Accumulators

 

Supplementing pump flow - An accumulator, capable of storing power can supplement the hydraulic pump in delivering power to the system. The pump stores potential energy in the accumulator during idle periods of the work cycle. The accumulator transfers this reserve power back to the system when the cycle requires emergency or peak power. This enables a system to utilize a much smaller pump, resulting in savings in cost and power.

Maintaining pressure - Pressure changes occur in a hydraulic system when the liquid is subjected to rising or falling temperatures. Also, there may be pressure drop due to leakage of hydraulic fluid. An accumulator compensates for such pressure changes by delivering or receiving a small amount of hydraulic fluid. If the main power source should fail or be stopped, the accumulator would act as an auxiliary power source, maintaining pressure in the system.

Fluid dispensing - An accumulator may be used to dispense small volumes of fluids, such as lubricating greases and oils, on command.

Operation

When sized and precharged properly, accumulators normally cycle between stages (d) and (f), Figure 2. The piston will not contact either cap in a piston accumulator, and the bladder will not contact the poppet or be compressed so that it becomes destructively folded into the top of its body.

Manufacturers specify recommended precharge pressure for their accumulators. In energy-storage applications, a bladder accumulator typically is precharged to 80% of minimum hydraulic system pressure and a piston accumulator to 100 psi below minimum system pressure. Precharge pressure determines how much fluid will remain in the accumulator at minimum system pressure.

Correct precharge involves accurately filling an accumulator's gas side with a dry inert gas, such as nitrogen, while no hydraulic fluid is in the fluid side. Accumulator charging then begins when hydraulic fluid is admitted into the fluid side, and occurs only at a pressure greater than the precharge pressure. During charging, the gas is compressed to store energy.

A correct precharge pressure is the most important factor in prolonging accumulator life. The care with which precharging must be accomplished and maintained is an important consideration when choosing the type of accumulator for an application, all else being equal. If the user tends to be careless about gas pressure and relief valve settings, or adjusts system pressures without making corresponding adjustments to precharge pressure, service life may be shortened, even if the correct type of accumulator was selected. If the wrong accumulator was selected, premature failure is almost certain.

Mounting position

The optimum mounting position for any accumulator is vertical with the hydraulic port down. Piston models can be horizontal if the fluid is kept clean. When solid contaminants are present or expected in significant amounts, horizontal mounting can result in uneven or accelerated seal wear. Maximum service life can be achieved in the horizontal position with multiple piston seals to balance the piston's parallel surface.

A bladder accumulator also can be mounted horizontally, Figure 3, but uneven wear on the bladder as it rubs against the shell while floating on the fluid can shorten life. The amount of damage depends on fluid cleanliness, cycle rate, and compression ratio (defined as maximum-system-pressure/ minimum-system-pressure). In extreme cases, fluid can be trapped away from the hydraulic end, which reduces output or may elongate the bladder to force the poppet closed prematurely.

Sizes and outputs

Available sizes and capacities also influence which accumulator type to choose. Piston accumulators of a particular capacity often are supplied in a choice of diameters and lengths, Table 1. Furthermore, piston designs can be built to custom lengths for little or no price premium. Bladder accumulators are offered only in one size per capacity, with fewer capacities available.

The inherently higher output of the piston accumulator may make it the best alternative when space is tight. Table 1 lists outputs for 10-gal piston and bladder accumulators operating isothermally as auxiliary power sources over a range of minimum system pressures. The differences in precharge pressure, columns 3 and 4, (determined by 80% of minimum system pressure for bladder models, 100 psi below minimum for piston) lead to a substantial difference in outputs, columns 5 and 6.

To prevent excessive bladder deformation and high bladder temperatures, also note in Table 1 that bladder accumulators should be specified with compression ratios greater than 3:1.

Table 1 - Relative outputs, 10-gal accumulator
Compression ratio 1/2	System pressure, psi		Recommended precharge, psi		Output, gal
	maximum 1	minimum 2	bladder 3	piston 4	bladder 5	piston 6
1.5 2.0	3,000 3,000	2,000 1,500	1,600 1,200	1,900 1,400	2.53 3.80	3.00 4.41
3.0 6.0	3,000 3,000	1,000 500	800 -	900 400	5.06 -	5.70 6.33

 

Fig. 4. Piston accumulators used in conjunction with gas bottles.

Fig. 5. Several accumulators may be manifolded to provide large system flows.

Fig. 6. A small accumulator may do the job if it is remotely connected to an auxiliary gas bottle.

Fig. 7. Test circuit to generate and measure shock waves in system.

Fig. 8. Graph indicates results of shock wave tests.

Fig. 9. Results of second test using smaller-diameter tubing.

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

 

 

Multiple components

Although bladder designs are not available in sizes over 40 gal, piston designs are currently supplied up to 200 gal in a single vessel. Economics and available installation space have led engineers to consider multiple component installations. Two of these can cover most high-output applications.

The installation in Figure 4 consists of several gas bottles serving a single piston accumulator through a gas manifold. The accumulator portion must be sized so the piston does not repeatedly strike the caps while cycling. One drawback of this arrangement is that a single seal failure could drain the gas system. Because gas bottles often are less expensive than accumulators, one advantage of this setup might be lower cost.

Several accumulators, either piston or bladder design, can be mounted on a hydraulic manifold, Figure 5. If using piston accumulators, the piston with the least friction will move first and occasionally could bottom on the hydraulic cap. In slow or infrequently used systems, this is insignificant.