Accumulators

Accumulators start with a pre-charged gas and no fluid. By increasing the fluid pressure, the accumulator fills with oil and the gas is compressed. When the pressure in the fluid side is reduced, the gas expands to equalize and pushes the fluid out of the accumulator and into the fluid system.

There are three main types of accumulators, gas (compressibility of gas), dead weight (gravity) and spring (mechanical). The three main construction types of gas-charged accumulators are diaphragm, bladder, and piston.

Diagraphm Accumulators

  • Lightweight and compact in design
  • Higher pressure ratios than bladder-type accumulators
  • Pressure ratios are typically 8:1 and range from 4:1 to 10:1
  • Used more with small volume and flow
  • May be mounted in any position
  • Low maintenance

Bladder Accumulators

  • Feature fast response
  • They have good dirt tolerance and are mostly unaffected by particle contamination in the hydraulic fluid
  • Their preferred mounting position is vertical to prevent the possibility of fluid getting trapped between the bladder and the shell.

Piston Accumulators

  • Can handle much higher gas compression ratios (up to 10:1)
  • Flow rates as high as 214 liters (57 gallons) per second
  • Can be mounted in any position
  • Require a higher level of fluid cleanliness than bladder units
  • Slower response times (greater than 25 milliseconds)
  • Exhibit hysteresis

Applications

There are many advantages for using an accumulator including the following:

  • Energy storage - An essential function of accumulators is their ability to store energy. Particularly in cyclic or varying operations, accumulators discharge in times of high demand and recharge during periods of low demand. They are often used to supplement pump flow during peak demand. Without an accumulator, the pump and motor must be sized to handle peak power requirements even if maximum power is only required momentarily.
  • Emergency backup - Accumulators can maintain a high-pressure charge almost indefinitely and serve as an emergency power source should a machine lose electric power, or a pump fails.
  • Vibration and shock reduction - Mounting a small accumulator near the outlet of the pump can absorb pulsations, minimize vibration and provide smoother operation. Also, adding an accumulator into the return line of machines can mitigate shock.
  • Leakage & temperature compensation - An accumulator can maintain constant pressure even if fluid slowly leaks internally past piston seals or valve clearances and compensate for temperature-related pressure differences in a closed hydraulic system. Only when circuit pressure drops below preset limits does the pump fire up and recharge the accumulator.
  • Faster response - Bladder and diaphragm accumulators have virtually instantaneous response and can quickly supply fluid to valves and improve their performance. Accumulators can also immediately meet peak flow requirements.

Standards

Accumulators are pressure vessels and as such are manufactured, tested and certified according to statutory standards. In the United States, for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.

All pressure vessels manufactured to these standards are considered to have a finite service life depending on the number of pressure cycles experienced during normal operation. The typical design life for a hydraulic accumulator is 12 years.

In many jurisdictions, periodic inspection and recertification is required. This particularly applies to hydraulic accumulators which have relatively large volumes and operate at high working pressures. Inspection may be required at predetermined intervals (i.e. every two, five or 10 years) or when a certain percentage of usable design life is deemed to have been reached.