Hydraulic Accumulator

What sets a hydraulic accumulator apart

A hydraulic pump delivers a steady flow, but many machines need a short burst of much higher flow, or must keep working after the pump has stopped. An accumulator solves both problems. Hydraulic oil is pushed into a vessel that holds a pre-charge of nitrogen behind a bladder, piston or diaphragm. The gas compresses and stores the energy, and when the system pressure falls the gas pushes the oil back out. On a ship this is the reserve that closes a watertight door when power is lost, supplies a steering gear or a deck machine for a short period, damps the pressure shocks that follow a sudden valve closure and smooths pulsation in a pump line. On offshore and subsea equipment, accumulators supply the energy for safety functions that must work with no supply at all.

Main designs

• Bladder accumulators, where a flexible elastomer bag holds the gas inside a steel shell; fast response and widely used for general duty
• Piston accumulators, where a free piston separates gas and oil; suited to large volumes, high pressures and high flow rates
• Diaphragm accumulators, with a flexible membrane in a compact, often spherical shell for smaller volumes
• Subsea and pressure-balanced designs, adapted to work under external water pressure
• Gas bottles and bottle banks, which add gas volume to piston accumulators so the pressure drops less as the oil is delivered

Main components

• Pressure shell, forged or welded steel, designed and tested as a pressure vessel
• Gas valve for the nitrogen pre-charge, with a protective cap
• Bladder, piston or diaphragm with its seals, the part that wears
• Oil port with a poppet or anti-extrusion valve that protects the bladder when the accumulator empties
• Safety block, usually with an isolating valve, a pressure relief valve, a pressure gauge and a manual discharge valve
• Mounting clamps or brackets, which hold the accumulator against vibration and ship motion

Selection and sizing

• Required oil volume, the minimum and maximum working pressure, and how fast the energy must be delivered
• Pre-charge pressure, set in relation to the minimum working pressure as the manufacturer prescribes
• Type: bladder, piston or diaphragm, chosen for pressure, volume, response speed and the fluid
• Elastomer and seal materials that suit the hydraulic fluid and the temperature range
• Mounting position, since bladder types are normally installed with the gas valve at the top
• Connection size, and the safety block that matches it, so isolation and discharge are always possible

Regulations and class

An accumulator is a pressure vessel and must be designed, built and tested to a recognised pressure-vessel code, with a certificate that states its allowable pressure, volume and test pressure. Classification societies approve accumulators for essential systems such as steering gear and watertight doors, and offshore and subsea units follow additional rules. Periodic surveys include the condition of the shell, the safety devices and the records of pre-charge checks. A replacement must carry a certificate for the same pressure rating, and the relief valve must be set and sealed for the system.

Typical faults

• Loss of pre-charge through the gas valve or a worn seal, which cuts the stored energy long before anything looks wrong
• Bladder rupture, which lets gas enter the oil circuit and shows as foaming, noise and a collapse of the reserve
• Piston seal wear, with nitrogen leaking into the oil side or oil into the gas side over time
• External corrosion at clamps and welds, or damage to the coating on subsea units
• Contaminated fluid, which wears seals and sticks the anti-extrusion valve
• Safety devices that were removed, blocked or never tested, which turns a normal fault into a hazard

What to look for in a supplier

• A certificate that names the exact model, the allowable pressure and the volume, with the approval of the classification society where the system requires it
• Pre-charge equipment, seal kits, bladders and gas valves as spare parts for the same series
• Safety block, relief valve and gauges that are rated for the accumulator's pressure
• Written instructions for charging and for safe dismantling, in the language of the crew
• Availability of test and re-certification services in the ports on the ship's route

Charge an accumulator with nitrogen only, never with air or oxygen, and always release the hydraulic pressure and then the gas before opening it. Compressed air mixed with oil vapour is an explosion risk, and a pressurised accumulator that is dismantled can kill.