Hydraulic Accumulator

A hydraulic accumulator is a pressure vessel in hydraulic systems that stores hydraulic fluid under pressure and releases it when needed. It uses a compressible gas, a spring, or a weight as an energy storage medium to compensate for pressure fluctuations, buffer energy, or ensure emergency functions. In industrial hydraulic systems, it is one of the key functional components.

Basics and Functioning of Hydraulic Accumulators

Hydraulic accumulators serve as energy storage devices and pressure buffers in hydraulic systems. They absorb excess flow when system pressure rises and release stored fluid when pressure drops. This principle makes it possible to cover short-term power peaks without having to oversize the pump.

Design and Operating Principle

The basic design of a hydraulic accumulator consists of a pressure-resistant vessel divided into two chambers: a gas chamber and a fluid chamber. A separator—which, depending on the design, may be a bladder, a diaphragm, or a piston—ensures that the gas and hydraulic fluid do not mix. Nitrogen is typically used as the gas because it is chemically inert and poses no risk of oxidation.

During the charging phase, the rising system pressure forces hydraulic oil into the fluid chamber. The separating element retracts, the gas compresses, and the stored energy is available as potential pressure energy. During the discharge phase, the gas expands again, pushing the oil back into the line and thus maintaining system pressure even when the pump is not delivering any flow.

Thermodynamic Principles

The compression and expansion of the gas follow the laws of thermodynamics. During slow charging and discharging processes, the change of state is approximately isothermal, meaning that the temperature remains largely constant. During rapid processes, adiabatic behavior dominates, causing the gas to heat up or cool down. These differences affect the actual usable oil volume and must be taken into account during design.

Types of Hydraulic Accumulators

The choice of design depends on operating pressure, volume requirements, cycle frequency, and the specific requirements of the application. Five designs have become established in practice.

Bladder Accumulators

In a bladder accumulator, an elastic rubber bladder acts as the separator between the gas and the liquid. The bladder is pre-filled with nitrogen and is compressed by the oil pressure during the charging process. Diaphragm accumulators are characterized by fast response times and low maintenance. They cover volumes ranging from approximately 0. 2 to 50 liters and are suitable for operating pressures up to 330 bar. Typical applications include mobile hydraulics, presses, and applications with intermittent operation. The bladder is subject to aging and should be replaced after about five to ten years.

Diaphragm Accumulators

Diaphragm accumulators use a flexible diaphragm as a separating element, which is secured to the edge of the housing. They are particularly compact and cost-effective, but generally cover smaller volumes up to about 3. 5 liters. Operating pressures up to 350 bar are possible. The gas precharge pressure should not exceed 130 bar, as otherwise the diaphragm will be mechanically overloaded. Diaphragm accumulators are frequently used in stationary hydraulic systems for pulsation damping and vibration damping.

Piston Accumulators

Piston accumulators separate gas and fluid using a freely moving piston. This design can withstand the highest operating pressures, which can reach up to 700 bar, and offers precise volume control. Piston accumulators are durable and suitable for high cycle counts, such as those encountered in industrial hydraulics or offshore applications. Disadvantages include higher costs and the more complex sealing of the piston, which requires regular maintenance.

Spring Accumulators

Spring accumulators use a mechanical spring instead of gas to store energy. They do not require a gas charge, which makes them suitable for applications where gas leakage or permeation must be prevented, such as in the food industry. Their use is limited to low pressures and small volumes, as the spring characteristic curve imposes physical limits.

Weight-storage

Weight accumulators lift a mass to store energy. Their advantage lies in the nearly constant pressure throughout the entire discharge process, since the force of gravity acts independently of the stroke height. Due to their size and weight, they are used almost exclusively in large stationary systems and play a minor role in modern hydraulics.

Comparison of Design Types

Applications of Hydraulic Accumulators

Hydraulic accumulators fulfill several functions in hydraulic systems; while these functions may overlap to some extent, each places different demands on the design.

Pulsation damping and pressure surge absorption

Piston pumps generate pulsating flow rates that lead to pressure fluctuations and mechanical stress in the system. A hydraulic accumulator installed near the pump smooths out these pulsations and protects lines, valves, and connections from material fatigue. For this application, small reservoir volumes of 0. 5 to 5 liters are often sufficient. Sudden pressure surges, such as those caused by the rapid closing of valves, are also cushioned by the reservoir, thereby preventing damage to the system.

Energy Storage and Flow Rate Compensation

In machines with cyclic operating patterns, such as presses or injection molding machines, short-term power peaks occur that drive the flow rate requirement far beyond the pump’s average flow capacity. A hydraulic accumulator covers these peaks, so the pump only needs to be sized for the average demand. This reduces the installed power, lowers energy consumption, and minimizes heat generation in the system.

Emergency Power Supply and Emergency Actuation

If the pump fails—for example, due to a power outage or equipment damage—many systems require a safety function to be performed: a crane must be lowered, a clutch disengaged, or a valve moved to the safe position. A hydraulic accumulator stores the necessary pressure energy and enables emergency operation even without the pump running. The design is based on the volume requirements of the emergency function and the permissible pressure drop during discharge.

Leakage Compensation and Pressure Maintenance

Over extended periods of inactivity, hydraulic systems lose pressure due to internal leaks. A hydraulic accumulator compensates for these losses and maintains pressure in specific circuits without requiring the pump to continuously make up the pressure. This saves energy and reduces pump wear.

Design and Operating Parameters

The design of a hydraulic accumulator depends on several parameters that, in combination, determine the usable oil volume and the pressure profile.

Gas Precharge Pressure and Pressure Ratios

The gas precharge pressure—that is, the pressure of the nitrogen when the accumulator is unloaded—is a key design parameter. It must be lower than the minimum operating pressure; otherwise, the accumulator cannot accept oil. The ratio between the maximum operating pressure and the precharge pressure affects the accumulator’s efficiency. For bladder-type accumulators, this ratio should not exceed 4: 1 to protect the bladder from mechanical overload. Diaphragm-type accumulators can handle ratios up to about 8: 1, while piston-type accumulators allow for even higher pressure ratios.

Temperature and Fluid Compatibility

The permissible temperature range for hydraulic accumulators is typically between -20 and +80 degrees Celsius, but can be extended to -50 to +130 degrees Celsius with special sealing materials. Temperature affects gas pressure: as temperature rises, the precharge pressure increases; as temperature falls, it decreases. Designers must take this effect into account during the design phase, particularly for outdoor applications or those in heated process environments. Compatibility with the fluid used—such as HLP oil, HFC fluid, or HFD fluid—must be confirmed with the manufacturer.

Standards and Test Requirements

Hydraulic accumulators are pressure vessels and are therefore subject to strict legal requirements concerning both their design and operation.

DIN EN 14359 and the Pressure Equipment Directive

DIN EN 14359 governs the design, testing, and sizing of hydraulic accumulators. It defines requirements for safety equipment such as rupture discs and safety valves, as well as for the operating instructions. The European Pressure Equipment Directive 2014/68/EU classifies hydraulic accumulators into Categories I through IV based on volume and operating pressure. Accumulators in Category II and above require a conformity assessment by a notified body and bear the CE marking. Category I accumulators that are permanently installed in a machine do not fall within the scope of the directive.

Operational Safety Ordinance and DGUV Regulations

In Germany, the Industrial Safety Regulation (BetrSichV) requires operators to have hydraulic accumulators inspected regularly. DGUV Rule 113-020 specifies the requirements for hydraulic systems. Accumulators with a gas volume exceeding 10 liters are considered systems requiring monitoring and must be inspected by an authorized inspection body such as TÜV or DEKRA. For accumulators with a gas volume between 1 and 10 liters, inspection by a qualified person is sufficient.

Inspection Intervals

The inspection intervals are specified in the BetrSichV. The internal inspection must be performed at least every five years and includes checking for corrosion, cracks, and deformations using dye penetrant, magnetic particle, or ultrasonic testing methods. The pressure test must be performed at least every ten years, during which the storage tank is pressurized to 1. 3 to 1. 4 times the permissible operating pressure. Under certain conditions, this interval may be extended to 15 years. Alternative methods, such as the ID-E method, allow for inspections without disassembly and without system downtime.

Maintenance and Servicing

The reliability of a hydraulic accumulator depends largely on regular maintenance. Neglect in this area can lead to a gradual loss of performance or, in the worst case, to sudden failure.

Gas Loss and Permeation

Every gas-filled hydraulic accumulator loses part of its gas content over time due to permeation—that is, the diffusion of gas molecules through the separating element. This effect is more pronounced in bladder and diaphragm accumulators than in piston accumulators with their metallic seals. A drop in precharge pressure reduces the usable oil volume and impairs response characteristics. Regular checks of the gas pressure—about every six months—are therefore recommended. If the precharge pressure drops, the accumulator must be refilled with nitrogen. The use of oxygen or compressed air is strictly prohibited.

Signs of Wear

In bladder accumulators, wear is usually indicated by a brittle or cracked bladder. Diaphragms can tear due to overpressure or excessive pressure ratios. In piston accumulators, the seals and the piston raceway are the critical wear points. Scoring or pitting on the piston raceway leads to leakage and increased friction. Regular oil analysis provides indications of increased wear within the accumulator, such as elevated particle concentrations or gas bubbles in the fluid.

Documentation and Risk Assessment

As part of a risk assessment, the operator must specify the type, scope, and intervals of the inspections. The maximum permissible inspection intervals specified in the BetrSichV must not be exceeded. All test results must be documented in writing. If safety-related defects are identified, the hydraulic accumulator must not be operated until the defect has been rectified. The regulatory authority may impose fines if inspection intervals are exceeded.

Development Trends

Developments in hydraulic accumulators are moving toward higher efficiency, more compact designs, and smarter monitoring. Sensors that measure gas pressure, oil temperature, and fill level directly at the accumulator are increasingly being integrated into modern systems and connected to higher-level control systems. This enables condition-based maintenance instead of rigid inspection intervals. At the same time, new sealing materials and coatings are expanding the operating limits for high temperatures and aggressive fluids. In mobile hydraulics, energy recovery is gaining importance: Hydraulic accumulators store braking energy and make it available for the next acceleration, which reduces fuel consumption and CO₂ emissions.