Keeping up the pressure

The shock absorbing characteristics of accumulators are often relied upon where hydraulic shock suppression is required, but how to choose between piston and bladder type? Andrew Delaney from Parker Hannifin Cylinder Division explains.

The benefits of incorporating accumulators into hydraulic systems are widely recognised and include improved efficiency by the reduction of shock effects, maintenance of system pressure and the utilisation of stored energy to compensate for system demand peaks.

The shock absorbing characteristics of accumulators are frequently relied upon where hydraulic shock suppression is desirable during valve closure in hydraulic power systems, or when loads start/stop. Shock absorption is also desirable when loads are carried on lift trucks, and earth moving equipment.

Accumulators are used on mobile equipment to eliminate pitching oscillations – set up in articulated wheel loaders when running with no load – which could cause the operator to lose control of the vehicle.

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. 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.

Two types of hydraulic accumulator are in common use, namely bladder type and piston type, each with specific merits depending on the type of hydraulic system application.

Both types of accumulator operate on the hydropneumatic principle and use a dry, inert gas such as nitrogen. Accumulators are precharged 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 precharge pressure determines how much fluid remains in the accumulator at a minimum system pressure; precharging involves accurately filling the accumulator’s gas side before hydraulic fluid is admitted to the fluid side.

In broad terms bladder accumulators may be specified for applications where very 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 compensation 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 further consideration when making a choice between the two types of accumulator is their performance in the event of a failure. Bladder types, for example, in which failure is normally sudden as a result of rupture or physical damage to the bladder material, 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, and 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 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.

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 either 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 which may lead to seal wear and ultimate failure

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