Where And How To Apply Hydraulic Accumulators

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 are far and away the most common.

The bladder style uses a compressible gas contained in an elastic bladder mounted inside of a tank-like shell. The shell acts as a pressure container for both the gas (in the bladder) and the hydraulic 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 the flow and pressure necessary 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 actuators.
• Leakage compensation: An accumulator can be used to maintain pressure and make-up for fluid lost 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 segments of the pump cycle time and then discharges during the high demand portions of the circuit.
• 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 calculated based on 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:

1. 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 at a pressure greater than its pre-charge pressure. This is the source of stored energy.
2. As the bladder compresses due to 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.
3. When the downstream circuit calls for flow, fluid system pressure falls and the stored fluid is pushed out of the accumulator shell. It is 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 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. This ensures a small amount of fluid will remain in the accumulator to prevent the bladder, diaphragm or piston from striking the opposite end of the pressure vessel, getting fouled up with 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 sales engineer for specific help with sizing. That said, there are some basic system requirements that must be known:

1. Total fluid volume required from all system components.
2. Minimum system working pressure.
3. Maximum system working pressure….peak demand and momentary “spikes”.
4. Fluid operating temperatures including ambient, minimum and maximum.
5. Machine cycle time / chart including “work” and “recovery” time.
6. Fluid specification.

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 system installed cost: 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 cooling capacity.
• Less leakage and maintenance cost: 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 style 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 coupled with system shock absorption lowers overall machine sound levels and results in higher operator productivity.
• Flexible design approaches: A wide range of accumulator 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.

Note: “Tech Tips” offered by Flodraulic Group or its companies are presented as a convenience to those who may wish to use them and are not presented as an alternative to formal fluid power education or professional system design assistance.