charge of air or an inert gas such as nitrogen.Sometimes the amount of air charge is limited tothe volume within the accumulator; otherinstallations may use separate air flasks which arepiped to the air side of the accumulator. Pistonaccumulators may be mounted in any position.The gas portion of the accumulator may belocated on either side of the piston. For example,in submarine hydraulic systems with tailrodpistons, the gas is usually on the bottom and thefluid on top; in surface ships with floating pistons,the gas is usually on the top. The orientation ofthe accumulator and the type of accumulator arebased upon such criteria as available space,maintenance accessibility, size, need for externalmonitoring of the piston’s location (tailrodindication), contamination tolerance, seal life, andsafety. The purpose of the piston seals is to keepthe fluid and the gas separate.Usually, tailrod accumulators use two pistonseals, one for the air side and one for the oil side,with the space between them vented to theatmosphere through a hole drilled the length ofthe tailrod. When the piston seals fail in this typeof accumulator, air or oil leakage is apparent.However, seal failure in floating piston ornonvented tailrod accumulators will not be asobvious. Therefore, more frequent attention toventing or draining the air side is necessary. Anindication of worn and leaking seals can bedetected by the presence of significant amountsof oil in the air side.BLADDER-TYPE ACCUMULATORSBladder- or bag-type accumulators consist ofa shell or case with a flexible bladder inside theshell. See figure 9-7. The bladder is larger indiameter at the top (near the air valve) andgradually tapers to a smaller diameter at thebottom. The synthetic rubber is thinner at the topof the bladder than at the bottom. The operationof the accumulator is based on Barlow’s formulafor hoop stress, which states: “The stress in acircle is directly proportional to its diameter andwall thickness.” This means that for a certainthickness, a large diameter circle will stretch fasterthan a small diameter circle; or for a certaindiameter, a thin wall hoop will stretch faster thana thick wall hoop. Thus, the bladder will stretcharound the top at its largest diameter and thinnestwall thickness, and then will gradually stretchdownward and push itself outward against thewalls of the shell. As a result, the bladder iscapable of squeezing out all the liquid from.Figure 9-7.—Bladder-type accumulator.the accumulator. Consequently, the bladderaccumulator has a very high volumetric efficiency.In other words, this type of accumulator iscapable of supplying a large percentage of thestored fluid to do work.The bladder is precharged with air or inert gasto a specified pressure. Fluid is then forced intothe area around the bladder, further compressingthe gas in the bladder. This type of accumulatorhas the advantage that as long as the bladder isintact there is no exposure of fluid to the gascharge and therefore less danger of an explosion.DIRECT-CONTACT GAS-TO-FLUIDACCUMULATORSDirect-contact gas-to-fluid accumulatorsgenerally are used in very large installations whereit would be very expensive to require a piston-or bladder-type accumulator. This type ofaccumulator consists of a fully enclosed cylinder,mounted in a vertical position, containing a liquid9-6
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