TechArchive

Original article date: February 1999

Two of the types of hydraulic accumulator in common use are the bladder type and the piston type. But which to use and when? This advice from ANDREW DELANEY of Parker Hannifin Cylinder Division.

The main task of an accumulator is to take a specific amount of fluid under pressure from the hydraulic system and store it until it is required within the system thereby compensating for system demand peaks. Other benefits of incorporating accumulators into hydraulic systems include improved efficiency by the reduction of shock effects and maintenance of system pressure.

The shock absorbing characteristics of accumulators are frequently relied upon for example where hydraulic shock suppression is desirable during valve closure in hydraulic power systems or fluid transmission lines. They are also valuable when loads start stop or are reversed in machinery such as hones and planes. Shock absorption is also desirable when loads are carried on lift trucks tractor shovel loaders and earthmoving equipment and to compensate for load reactions on the pressure rolls of papermaking machines and printing presses.

Accumulators are used on mobile equipment to eliminate pitching oscillations which could cause the operator to lose control of the vehicle. These oscillations can be set up in articulated wheel loaders and end loaders for example when running with no load.

Accumulators are also employed to maintain system pressure compensating for pressure losses due to leakage and increases due to thermal expansion or for holding pressure while a pump does other work. In the case of emergencies such as power or pump failure accumulators also serve to maintain pressure to support oil pressure until a turbine can be stopped or where fluid power is needed to withdraw tools to avoid work spoilage. An accumulator can provide power for emergency valve closure to ensure fail-safe operation of systems and processes.

In other applications accumulators are a valuable means of storing energy and are often used on electric circuit breakers and switchgear and on hydraulic starters for diesel engines. They may be used to dispense fluid in a pressurised lubrication system and to supplement pump delivery in machines capable of performing multiple operations. One type of accumulator the bladder is also useful as a fluid barrier where two different fluids are used in the same system.

Both bladder type and piston type accumulators operate on the hydropneumatic principle and use a dry inert gas such as nitrogen. Pre-charging involves accurately filling the accumulator’s gas side before hydraulic fluid is admitted to the fluid side. Accumulators are pre-charged with the gas to a specified pressure typically 80% of minimum hydraulic system pressure in the case of bladder accumulators and 100psi below minimum system pressure in the case of piston accumulators. The pre-charge pressure determines how much fluid remains in the accumulator at minimum system pressure.

In broad terms bladder accumulators may be specified for applications where rapid cycling is likely to be encountered when very short response times are required and when there is a possibility of contamination of hydraulic fluid. They can be used to maintain pressure supplement pump delivery dispense fluid and act as a fluid barrier.

Piston accumulators offer higher efficiency and flexibility in most applications allowing higher compression ratios to be used and greater flow rates to be achieved. The piston type also has an inherently higher output which makes it the best choice when space is a consideration. Piston accumulators can be used to absorb shock compensate for leakage and provide emergency power supply.

A third type of accumulator – the membrane accumulator – is used for small gas volumes. Available as a welded or screwed construction the advantages include a good seal and long service life. They may be installed in any position and they operate without inertia.

A further consideration between piston and bladder accumulators is their performance in the event of a failure. In bladder types failure is normally sudden and occurs as a result of rupture or physical damage to the bladder material. These should be chosen where product quality is a function of hydraulic system pressure – a failure which can be detected immediately results in a minimum of scrap product.

Conversely piston accumulator failure tends to be gradual rather than catastrophic. This behaviour makes them more suitable for applications where safety could be compromised by failure. A sudden failure in a brake or steering circuit for example could have disastrous results – clearly a piston accumulator is the best choice for applications of this type.

The type of system fluid also influences choice of accumulator type. Water systems for example with their tendency to carry higher levels of solid contaminants are better served by bladder types which resist corrosion better than piston accumulators. Exotic fluids and fluids at unusual temperatures however are better suited to piston types of accumulator as seals are cheaper and easier to mould from special rubber compounds than bladders.

Response times to system pressure variations may also influence choice of accumulator type. Bladder accumulators respond more quickly because unlike piston accumulators they do not need to overcome static friction. Bladder types are therefore recommended where response times of less than 25ms are required with either accumulator type being suitable for responses of 25ms or more.

A common misconception exists over servo applications however where bladder accumulators are frequently specified. In practice such applications rarely require a response of less than 25ms and could be served by wither type.

Where high frequency system pressure cycling occurs bladder accumulators are the correct choice. Such conditions can cause a piston to “dither” cycling rapidly back and forth in a distance less than its seal width which causes heat build-up and subsequent lack of lubrication. This in turn may lead to seal wear and ultimate failure.

Other factors such as the effects of external forces flow rate limitations and the possible need for multiple accumulator installations will also influence the selection of the correct type of accumulator for any given application.

• Parker Hannifin Cylinder Division
• 01923 492000
• Andrew Delaney

February 1999