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.
|Table 1 - Relative outputs, 10-gal accumulator|
|System pressure, psi||Recommended precharge, psi||Output, gal|
|maximum 1||minimum 2||bladder 3||piston 4||bladder 5||piston 6|
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.
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.