Choosing the right accumulator is an important part of hydraulic-system design. An accumulator is not simply a container for hydraulic oil. It is a pressure-storage device that can help a hydraulic circuit manage changing loads, peak flow requirements, pressure fluctuations, and emergency operating conditions.
There are several types of hydraulic accumulators, and the right choice depends on the pressure, volume, temperature, flow rate, duty cycle, and purpose of the application.
What Does an Accumulator Actually Do?
A hydraulic accumulator stores energy in the form of pressurized hydraulic fluid. When the system has available hydraulic power, fluid enters the accumulator and compresses a gas, normally nitrogen.
When additional hydraulic energy is needed, the compressed gas expands and pushes the stored fluid back into the circuit.
This makes an accumulator useful for applications where hydraulic demand changes during operation.
For example, a machine may normally require a moderate flow rate but need a much higher flow for a short period. Instead of sizing the entire hydraulic power unit around that short peak, an accumulator can provide part of the additional flow.
Common Reasons for Using Hydraulic Accumulators
Hydraulic accumulators can perform several functions within the same system.
Energy Storage
The accumulator stores hydraulic energy so it can be used later. This can be particularly useful in systems with intermittent hydraulic demand.
Peak Flow Assistance
When an actuator suddenly requires additional flow, stored fluid from the accumulator can supplement the pump.
Pressure Maintenance
An accumulator can help compensate for small pressure losses and changes in fluid volume in appropriately designed circuits.
Shock Absorption
Sudden stopping or starting of hydraulic components can create pressure spikes. An accumulator can help absorb some of this energy and reduce hydraulic shock.
Pulsation Damping
Pressure pulsations from pumps can affect pipes, valves, instruments, and actuators. Properly selected hydraulic accumulators can help reduce these fluctuations.
Emergency Operation
Stored hydraulic energy can be used to support selected functions during temporary loss of the primary hydraulic power source.
Understanding the Bladder Accumulator
One of the commonly used designs is the bladder accumulator.
A bladder accumulator contains a flexible bladder that separates the hydraulic fluid from compressed nitrogen. When hydraulic fluid enters the accumulator, the nitrogen is compressed. When system pressure decreases, the nitrogen expands and pushes the hydraulic fluid back into the circuit.
The design allows relatively rapid charging and discharging, making bladder accumulators useful for applications where quick hydraulic response is required.
They can be used for energy storage, pulsation damping, shock absorption, and peak-flow support, depending on the system requirements.
How Is the Right Accumulator Selected?
Before selecting an accumulator, it is important to understand the actual hydraulic application.
1. Determine the Pressure Range
Identify the minimum and maximum operating pressure of the system. The accumulator must be suitable for the intended pressure range and operating conditions.
2. Calculate the Required Fluid Volume
Determine how much hydraulic fluid needs to be supplied during the accumulator's discharge cycle.
The required volume can vary considerably depending on the application. An accumulator used for shock absorption may have different requirements from one used for energy storage.
3. Consider Flow Rate
The accumulator must be capable of supplying and receiving fluid at the required flow rate.
For applications involving rapid actuator movement or sudden pressure changes, response time can be particularly important.
4. Establish the Pre-Charge
Nitrogen pre-charge has a major influence on accumulator behavior. The correct value depends on the application and pressure conditions.
Incorrect pre-charge can reduce usable volume and affect system performance.
5. Check Operating Temperature
Temperature affects gas pressure and hydraulic-fluid characteristics. The accumulator and its materials must be suitable for the expected operating temperature range.
6. Consider the Duty Cycle
An accumulator that charges and discharges frequently may have different requirements from one that operates only occasionally.
Cycle frequency should therefore be considered when selecting the accumulator design and maintenance schedule.
Bladder vs. Piston Accumulator
Both bladder and piston accumulators can store hydraulic energy, but they have different construction and operating characteristics.
A bladder accumulator uses an elastomeric bladder, while a piston accumulator uses a movable piston to separate the gas from the hydraulic fluid.
Bladder designs can be well suited to applications requiring fast response. Piston designs can be useful in applications requiring larger fluid volumes or specific operating characteristics.
The decision should be based on engineering requirements rather than simply choosing one type for every application.
What Can Cause Accumulator Performance Problems?
Several factors can affect accumulator performance.
Incorrect pre-charge can change the available working volume and system response.
Incorrect sizing may prevent the accumulator from supplying the required flow or energy.
Temperature changes can influence gas pressure and therefore accumulator behavior.
Wear or component damage can result from unsuitable operating conditions or inadequate maintenance.
Poor installation can also affect performance and service life.
For these reasons, the accumulator should be considered as part of the complete hydraulic circuit.
Maintenance of Hydraulic Accumulators
Regular inspection helps maintain reliable performance.
Depending on the accumulator design, maintenance may include checking nitrogen pre-charge, connections, valves, seals, external condition, and possible leakage.
For a bladder accumulator, the condition of the bladder and related components should also be considered during appropriate service intervals.
All maintenance should follow the manufacturer's instructions and applicable safety requirements. Accumulators contain stored pressure, so servicing should only be performed using the correct safety procedures.
Frequently Asked Questions
What is an accumulator?
An accumulator is a device that stores hydraulic energy and releases pressurized hydraulic fluid when the system requires it.
What is a bladder accumulator?
A bladder accumulator uses a flexible bladder to separate compressed nitrogen from hydraulic fluid.
What are hydraulic accumulators used for?
They can be used for energy storage, peak-flow support, pressure maintenance, pulsation damping, shock absorption, thermal compensation, and emergency hydraulic functions.
How do I know what size accumulator I need?
Accumulator size depends on the system's pressure range, required fluid volume, flow rate, pre-charge, temperature, and operating cycle. Proper sizing should be based on the actual application.
Is a bladder accumulator suitable for industrial machinery?
It can be suitable for many industrial applications, particularly where fast response and hydraulic energy storage are required. The specific operating conditions should be evaluated before selection.
Why is nitrogen pre-charge important?
Pre-charge determines how the accumulator responds during charging and discharging and affects the amount of usable hydraulic fluid available.
Conclusion
Selecting an accumulator should begin with the application rather than the accumulator itself. Understanding pressure, flow, volume, temperature, pre-charge, and duty cycle makes it easier to select an appropriate solution.
A bladder accumulator can provide rapid energy storage and discharge for many hydraulic applications, while other types of hydraulic accumulators, such as piston and diaphragm designs, may be better suited to different requirements.
When correctly sized, installed, and maintained, an accumulator can become an important part of a reliable and efficient hydraulic system.
For additional information about hydraulic accumulators and their applications, you can refer to FEC International's resource:
