One of the least understood components within hydraulic circuits is also one of the most important, as it can bring a raft of benefits to virtually all hydraulic applications, whether they be stationary or in motion. The authors take a 'back-to-basics' look at accumulators and discuss how virtually all those that employ the features of fluid-based systems can use them. Using the simple interaction of an inert gas (nitrogen) on one side of a moveable barrier and a hydraulic fluid on the other, accumulators are very simple in operation but tremendously effective in what they achieve.
Used to moderate and maintain hydraulic circuits, their method of operation is not 'rocket science' although tight specifications must be determined prior to use to get the very best out of them.
Put simply, accumulators are used to both store and deliver hydraulic potential whenever a system experiences a peak or trough in pressure.
'When correctly specified, accumulators can be used for a variety of functions to enhance system efficiency,' explains Lindgren.
'These include: reducing the shock effects generated by inertia or external mechanical forces; system pressure maintenance; compensation for leakage; and as a back up power supply when system demands are higher than the pump can deliver.' Three common types exist, all of which exhibit specific features that make them more suitable to different applications.
The first, bladder-type accumulators, employ a gas filled bladder contained within a vessel that is attached to the hydraulic circuit.
As the hydraulic pressure increases, hydraulic fluid enters the vessel and compresses the bladder, which, in its 'resting state', is at a known pressure or precharge.
Fluid will continue to compress the bladder until equilibrium is reached, i e, until the gas and hydraulic fluid are at the same pressure - this is why the precharge pressure is important.
Once the system pressure starts to fall, the gas under pressure, acting like a spring, forces hydraulic fluid back into the system until the pressures balance.
The second type of accumulator, widely used in mobile and off-road applications, uses a piston instead of a bladder to contain the gas in the closed end of a cylinder.
As with the bladder accumulator, the device is tailored to the application by selection of size and volume of the accumulator, and by means of precharge pressure adjustment.
The third and final variant is the diaphragm accumulator.
Instead of a bladder, it used a rubber diaphragm to separate the gas and liquid in the two halves of a vessel.
Its method of operation is essentially identical to the other two types.
As mentioned earlier, each accumulator variant exhibits operational features and constraints that make them more or less suitable to particular applications.
Bladder accumulators offer quick response times, hysteresis-free operation and good tolerance to contamination of the system fluid.
However, they only offer a limited compression ratio (up to 4:1) and limited flow rates when compared to piston accumulators.
The failure mode of bladder accumulators is immediate - there are no warning signs - a valuable feature in industrial applications where a consistent product is required, as a reduction in quality is immediately apparent and waste product can be avoided.
Piston accumulators offer extremely high flow rates, a wide operating temperature range, high compression ratios (up to 10:1) and resistance to external forces.
They also offer virtually unlimited size availability, large gas ports for use with gas bottles, position monitoring of the piston and more versatile mounting possibilities.
On the 'down side' they require a higher level of fluid cleanliness than bladder types, have lower response times at low pressures (p<35 bar) and exhibit hysteresis.
Their 'non-catastrophic' or progressive failure mode makes them particularly suitable for mobile steering and braking applications as early warning of deteriorating performance is easily identified.
Diaphragm variants will withstand compression ratios of up to 8:1 and exhibit the same advantages as bladder types.
They can suffer from gas permeation across the diaphragm and, like bladder accumulators, their failure mode is immediate.
'When selecting an accumulator for a particular application, both system and performance criteria should be considered,' says Killing.
'To ensure long and satisfactory service life, many factors should be taken into account.' The factors Killing is alluding to include failure mode, output volume, flow rate, fluid type, response time, shock suppression, high frequency cycling, mounting position, external forces, sizing information, certification and safety.
All of these can have an effect on the optimum operation of the accumulator and, in most cases, external expertise is the best option.
With this fact in mind, Lindgren is keen to point out that potential users do not need to worry about performing these calculations as Parker's engineers are used to bespoke specification and will happily design the optimum system for a user's application.
'We find ourselves acting as consultants for the majority of our customers,' he explains.
'With so much knowledge of the hydraulic market we are more than happy for customers to take advantage of our expertise in these types of applications.' One of the most critical factors in the operation of an accumulator is precharging.
Correct precharging involves accurately filling the gas side of an accumulator with pure nitrogen before admitting fluid to the hydraulic side.
It is important to precharge an accumulator to the correct specified pressure as the precharge pressure determines the volume of fluid held in the accumulator at maximum and minimum system pressures.
In an energy storage application, a bladder accumulator is typically precharged to 80% of minimum system pressure, and a piston accumulator to 7 bar below, or 90% of, minimum system pressure.
The ability to correctly carry out and maintain precharging is an important factor when choosing the type of accumulator for an application.
Accumulator failure is often defined as the inability to accept and exhaust a specified amount of fluid when operating over a specific system pressure range.
Failure often results from unwanted loss or gain of precharge pressure.
Correct precharge pressure is the single most important fact in prolonging the life of the accumulator.
It is for this reason that Parker supplies complementary equipment to ensure that precharge pressure can be maintained at its optimum level.
This equipment includes a remote charging and gauging block, (Note well - this is more for the industrial than the mobile sector) which enables operators to pre-charge a newly installed accumulator and to check and adjust the pre-charge of an existing unit.
Initial filling of the gas pre-charge and adjustment where necessary can be carried out using Parker's UCA charging and gauging assembly.
A huge amount of accumulator options are also available to engineers.
Piston, bladder and diaphragm variants come in a variety of sizes and volumes and all can be supplied pre-charged to the customer's specification.
Piston accumulators, due to the nature of their construction and operation, are the most flexible in design.
Indeed, massive early examples were used to store energy within London's Tower Bridge for raising and lowering of the two halves of the roadway.
Accumulators can also be ganged into multiple groups to act as an emergency power reserve in case of system delivery failure.