What is an accumulator?
An accumulator is an energy storage device. It stores potential energy through the compression of a dry inert gas (typically nitrogen) in a container open to a relatively incompressible fluid (typically hydraulic oil). There are two types of accumulators commonly used today. The first is the bladder type (including diaphragm designs) and the second is the piston type. While other types of accumulator designs exist, compressed gas accumulators ar far and away the most common.
The bladder style uses a compressible gas contained in an elastic bladder mounted inside a shell. The shell acts as a pressure container for both the gas (in the bladder) and the hydrauic fluid. The bladder provides the barrier between the inert gas and the fluid to prevent intermixing. The piston style uses a cylinder with a floating piston. The cylinder serves as the pressure container for both the gas and fluid while the piston provides the barrier between the gas and the oil to prevent intermixing. Note that oxygen is never used as it can be explosive when mixed with oil under high pressure.
Where are accumulators used?
Accumulators can be applied creatively in any number of situations, including:
- Emergency and safety: An accumulator which is kept constantly under pressure is valuable in the event of an electrical power failure as it can provide flow and pressure to perform an additional function or complete a machine cycle.
- Shock or pulsation dampening: An accumulator can be used to cushion the pressure spike from sudden valve closure, the pulsation from pumps or the load reaction from sudden movement of parts connected to hydraulic cylinders.
- Leakage compensation: An accumulator can be used to maintain pressure and make-up for lost fluid due to internal leakage of system components including cylinders and valves.
- Thermal expansion: An accumulator can absorb the pressure differences caused by temperature variations in a closed hydraulic system.
- Energy conservation: An accumulator can be used to supplement a pump during peak demand thereby reducing the size of the pump and motor required. The accumulator is charged during low demand portions of the pump cycle time and then discharges during the high demand portions of the system.
- Noise reduction: An accumulator is effective at reducing hydraulic system noise caused by relief valves, pump pulsations, system shock and other circuit generated noises.
- Improved response times: An accumulator (bladder type) has virtually instantaneous response time that can provide fluid very quickly to fast-acting valves such as servos and proportionals to improve their effectiveness.
How do accumulators work?
Accumulators operate by making use of the considerable difference in compressibility between a gas and fluid. Using the bladder design, the nitrogen in the bladder is highly compressible while the hydraulic oil in the fluid side of the shell is virtually non-compressible. The bladder contained in the shell is pre-charged with nitrogen gas to a pressure calculation determined by system parameters and the work to be done. After being pre-charged, the bladder occupies almost the whole volume of the shell. From there, the operation of an accumulator can be broken down into three basic stages:
(a.) When the hydraulic pump in the system is turned on it causes fluid to enter the accumulator. When fluid fills the shell, accumulator charging begins as the nitrogen in the bladder is compressed by a fluid pressure greater than its pre-charge pressure. This is the source of stored energy.
(b.) As the bladder compresses due to the fluid filling the shell, it “deforms” in shape, taking up less space in the shell while at the same time, pressure in the bladder increases. This bladder “deformation” ceases when the pressure of the system fluid and the now compressed nitrogen become balanced.
(c.) Upon downstream system demand, fluid system pressure falls and the stored fluid is pushed out of the accumulator shell and returned to the system under pressure exerted by the compressed nitrogen, whose pressure is now greater than the fluid pressure. Upon completion of whatever hydraulic system function the accumulator was designed to do, the cycle starts all over again with step one.
One of the most important considerations in applying accumulators is calculating the correct pre-charge pressure for the type of accumulator being used, the work to be done and system operating parameters. Pre-charge pressure is generally 80 – 90% of the minimum system working pressure to allow a small amount of fluid to remain in the accumulator. This prevents the bladder, diaphragm or piston from striking the opposite end of the pressure vessel, getting fouled up in discharge valving or blocking fluid passages. Too high or too low of a pre-charge pressure can cause accumulator damage or failure. Conversely, a properly designed and maintained accumulator should operate trouble-free for years.
What must I know to size and select an accumulator?
Based on what the accumulator is being tasked to do, there are a variety of questions, formulas and charts that factor into the actual sizing, application and placement of accumulators. This article is intended to give an overview of accumulator operation and application, not a lesson in isothermal or adiabatic sizing. Please contact your RHM Fluid Power sales or application engineer for specific help with sizing. That said, there are some basic system requirements that must be known:
Total fluid volume required from all system components
Minimum system working pressure
Maximum system working pressure
Fluid operating temperatures including ambient, minimum and maximum
Machine cycle time chart including “work” and “recovery” times
With these basic system parameters, we can calculate proper pre-charge pressures, accumulator size, bladder materials, accumulator type and placement in the system.
Conclusion: So what are the benefits of using accumulators?
A properly designed accumulator circuit can offer many advantages to hydraulic system operation. Key among them:
Lower installed system costs…..accumulator assisted hydraulics can reduce the size of the pump and electric motor which results in a smaller amount of oil used, a smaller reservoir and reduced equipment costs.
Less leakage and maintenance costs…..the ability to reduce system shocks will prolong component life, reduce leakage from pipe joints and minimize hydraulic system maintenance costs.
Improved performance…..low inertia bladder accumulators can provide instantaneous response time to meet peak flow requirements. They can also help to achieve constant pressure in systems using variable displacement pumps for improved productivity and quality.
Reduced noise levels…..reduced pump and motor size couple with system shock absorption lowers overall machine sound levels and results in higher operator productivity.
Flexible design approaches…..a wide range of accumlator types and sizes, including accessory items, provides a versatile and easy to apply design approach.
Reduced energy costs…..cost savings of up to 33% are achievable in high performance industrial machinery using accumulators.