Types of accumulator

Gas Accumulator

The Air–Bottle or non separated, gas accumulator in Fig. 4–10 is a fully enclosed shell, mounted vertically, which has a fluid port on the bottom of the shell and a pneumatic charging valve on the top. When charged, air under pressure is confined in the upper portion of the shell by the fluid in the lower portion. There is no physical separation between the gas (air) and the fluid. These accumulators are usually fitted with some type of limit switch to control the fluid level and prevent gas from escaping out of the fluid port in the bottom. To prevent the gas from leaving the accumulator during high rates of fluid flow, about one – third of the fluid remains in the shell.

The fluid capacity of air – bottle accumulators is quite large because of their no mechanical design. The fluid capacity is only dependent on the size of the vessel used.  Although the response is fast, the major disadvantage of this accumulator is the fact that gas is absorbed or dissolved by the fluid under high pressures and can cause cavitations in the system. As a result, non separated gas accumulators should not be used with high speed pumps. Another disadvantage is that a separate compressor is usually required to charge and maintain the gas pressure in the vessel.

Bag Accumulator

Fig. 4–11 shows a Bladder or bag type accumulator. This accumulator is a seamless, high – pressure cylindrical shell that encloses a pear – shaped synthetic rubber bladder. The bladder is molded to an air stem, which contains a high – pressure air valve and is mounted with a high – pressure seal in the upper end of the shell. The bottom or fluid end of the shell is sealed by a special plug and spring – loaded poppet valve that allows fluid to flow in and out of the shell. The poppet valve is used to prevent the bladder from trying to flow out the fluid port. In addition the plug has a safety feature that prevents disassembling of the accumulator if there is any pressure in the system.

Before being put in operation, the accumulators is preloaded with air and then charged with oil from the system, thus compressing the air and the bladder. As the system needs fluid, the bladder expands; first at the top (where its diameter is largest and its wall thickness is least) and then, gradually, the bladder stretches downward and outward against the walls of the shell. This action gives the bladder its high volumetric efficiency and pressure range.

Even though the bladder has a fluid capacity of approximately 20 gallons, its response is as fast as the discharge valve will permit. The bladder also has very low inertia, making it a good pump pulsation dampener. In addition, it maintains a positive separation between the fluid and gas. This is especially important when considering the high pressure conditions for which their use is recommended.