Hydraulic Accumulators: Selection and Adjustment - Cylinder, Piston and Diaphragm Systems

1. Introduction: Engineering Challenge and Production Reliability

In today's industrial hydraulic systems operating under significant loads and dynamic conditions, ensuring stability and efficiency is critical. Hydraulic accumulators play a key role in achieving these goals by performing the functions of energy storage, pressure pulsation damping, volume compensation and hydraulic shock absorption. Incorrect battery selection or configuration can lead to reduced system performance, reduced component life, and potential emergency situations. For maintenance and reliability engineers, understanding the nuances of hydraulic accumulator technology is fundamental to keeping production facilities running smoothly and optimizing operating costs.

This article is an in-depth technical guide covering the main types of hydraulic accumulators - cylinder, piston and diaphragm - their construction, principles of operation, selection criteria and pre-charging procedures. We will review the applicable standards, engineering calculation methods and practical recommendations to ensure maximum efficiency and safety.

2. Fundamental Principles: Physics and Mechanics

The basis of hydraulic accumulators is the use of gas compressibility to store hydraulic energy. Most batteries work on the principle of gas charging, where compressed gas (usually nitrogen) is separated from the working fluid by an elastic element. When the pressure in the hydraulic system increases, the working fluid enters the accumulator, compressing the gas. When the pressure drops, the compressed gas expands, displacing the liquid back into the system. This process is governed by the Boyle-Marriott law (isothermal process) or Poisson's law (adiabatic process) depending on the rate of change of gas volume:

• Isothermal process (slow changes): P₁V₁ = P₂V₂
• Adiabatic process (rapid changes): P₁V₁^k = P₂V₂^k

Where P₁V₁ is the initial pressure and gas volume, P₂V₂ is the final gas pressure and volume, and k is the adiabatic index (for nitrogen k ≈ 1.4).

2.1. Balloon Accumulators

In cylinder batteries, the gas is contained inside an elastic cylinder (rubber bulb), which is located in a metal case. The working fluid surrounds the cylinder. This ensures complete separation of gas and liquid, preventing them from mixing. Balloon accumulators are characterized by high reaction speed and efficiency, since the balloon can change shape to maximize the use of volume. The typical operating pressure range for cylinder accumulators reaches 350 bar, with a volume from 0.075 l to 100 l.

2.2. Piston Accumulators

In piston accumulators, gas and liquid are separated by a floating piston equipped with seals. The gas is on one side of the piston, and the liquid is on the other. This type of batteries is more resistant to liquid contamination and high temperatures. They are suitable for very large volumes (up to 1000 l and more) and extremely high pressures (up to 1000 bar). Piston accumulators have longer seal life when properly maintained, but may have a slightly slower response than cylinder accumulators due to piston friction.

2.3. Diaphragm Accumulators

Diaphragm accumulators use an elastic diaphragm to separate gas and liquid. They are compact and have low inertia, making them ideal for systems that require fast ripple damping and volume compensation. Typical volumes vary from 0.075 l to 3.5 l, and the maximum working pressure can reach 350 bar. Diaphragm accumulators are an economical solution for small systems.

3. Technical Specifications and Standards

The selection and operation of hydraulic accumulators must comply with strict engineering standards and safety directives. In Ukraine, there are harmonized national standards (DSTU), which are based on European (EN) and international (ISO) standards.

• DSTU EN 14359:2017: The main standard regulating the requirements for gas-filled accumulators for hydraulic drives. It lays down rules for the materials, construction, manufacture, testing, inspection and configuration of safety equipment for cylinder, diaphragm, piston and transfer accumulators.
• DSTU EN ISO 4413:2018: Defines general rules and safety requirements for hydraulic systems and their components, ensuring the integration of accumulators into a safe and reliable circuit.
• DSTU ISO 5596: Defines standardized ranges of pressure and volume, as well as characteristic values ​​for gas-filled accumulators with a separator. This is important for the unification of characteristics and compatibility of components.
• DSTU series EN 13445: Since the battery housing is a pressure vessel, it must meet the requirements for stationary pressure vessels, in particular:
• DSTU EN 13445-1: General provisions.
• DSTU EN 13445-2: Requirements for materials.
• DSTU EN 13445-3: Design and calculation.
• DSTU EN 13445-5: Verification and testing procedures.

In addition, all hydraulic accumulators operating under a pressure of more than 0.5 bar must comply with the Technical Regulations for equipment operating under pressure, approved by Resolution of the Cabinet of Ministers of Ukraine No. 27 of January 16, 2019. This regulation is harmonized with EU Directive 2014/68/EU (PED) and requires the UA TR conformity mark. All UNITEC-D components meet these standards and have the necessary CE and UkrSEPRO certificates.

4. Selection and Calculation Guide

The correct choice of hydraulic accumulator depends on the specific function it must perform in the system.

4.1. Main Applications:

• Energy storage: To power hydraulic cylinders or hydraulic motors during a short period when the pump cannot provide peak consumption.
• Rulse Damping: Smoothing out pressure ripples created by reciprocating pumps, extending component life and reducing noise.
• Амортизація гідравлічних ударів: Поглинання різких стрибків тиску, що виникають при швидкому закритті клапанів або зміні напрямку потоку.
• Volume compensation: Maintaining a constant pressure in the system when the volume of the liquid changes due to thermal expansion/compression.
• Emergency energy reserve: Provision of power to perform emergency functions (eg closing valves) in case of main pump failure.

4.2. Calculation of the Battery Volume

For energy storage, the required effective gas volume (V₀) of the battery can be calculated by the formula (taking into account the adiabatic process for fast cycles):

$$V_0 = \frac{Q_p \\cdot \\Delta t \\cdot P_{max}}{P_{min} \\cdot \\left( \\left(\frac{P_{max}}{P_{min}}\right)^{\frac{1}{k}} - 1 \right)}$$

• $Q_p$ – required flow from the battery (l/min)
• $\\Delta t$ - battery discharge time (min)
• $P_{max}$ – максимальний робочий тиск системи (бар)
• $P_{min}$ is the minimum working pressure of the system (bar)
• $k$ is the adiabatic index for nitrogen (about 1.4)

Important: The resulting V₀ is the effective gas volume, but a standard battery size with a larger nominal volume should be selected to ensure efficient operation.

4.3. Determination of Precharge Pressure (P₀)

The gas precharge pressure P₀ is a critical parameter. It should always be installed when the hydraulic circuit is not under pressure (0 bar) and the working fluid temperature corresponds to the operating conditions.

• For energy storage and volume compensation: P₀ is usually 80% - 90% of the minimum operating pressure of the system (P_min). For example, if P_min = 100 bar, P₀ = 80-90 bar. This ensures a sufficient volume of liquid in the battery and its effective displacement.
• For pulsation damping and shock absorption: P₀ is usually 60% - 70% of the average operating pressure or 60% - 70% of the pump's peak pressure. Наприклад, для демпфування пульсацій від насоса з піком 200 бар, P₀ = 120-140 бар. This allows the battery to effectively absorb and smooth out short-term fluctuations.

UNITEC-D recommends always consulting a technical specialist for accurate calculation and setting of P₀.

Table 1: Battery Type Selection Criteria

5. Best Practices for Installation and Commissioning

Proper installation and commissioning are critical to the longevity and safety of a hydraulic accumulator. Failure to follow these instructions can result in serious malfunctions and personal injury.

• Safety: Always install the battery in a safe place protected from mechanical damage and excessive heat. Use safety valves with the appropriate operating pressure. Make sure the system can be completely de-energized and depressurized before any work.
• Orientation: Cylinder accumulators are usually installed vertically with the gas valve facing up to minimize cylinder wear. Piston and diaphragm accumulators can be installed in any position, but a vertical position is often preferred to ensure optimal drainage and ventilation.
• Preload:
1. Переконайтеся, що гідравлічна система повністю знеструмлена і знята з тиску.
2. Use only nitrogen (N₂) of high purity. Oxygen or air can create an explosive mixture with hydraulic fluid when compressed, and also accelerate the aging of elastic elements.
3. Use a specialized battery charging kit that includes a pressure reducer, pressure gauge and connection adapter.
4. Charge the accumulator to the calculated pressure P₀. The pressure should be measured at a temperature close to the expected operating temperature, as temperature has a significant effect on gas pressure.
5. After charging, check that there are no gas leaks from the valve.
• Protection against pollution: Before connecting to the system, make sure that the connections are clean. Contamination is one of the main causes of failure of hydraulic components, including accumulators.
• Documentation: Keep records of pre-charge pressure, date of installation and maintenance.

6. Failure Modes and Root Cause Analysis

Even the best hydraulic accumulators can fail due to improper operation, wear or external factors. Understanding typical failure modes allows timely intervention and prevention of more serious system damage.

6.1. Typical Failure Modes:

• Loss of precharge pressure: Most common failure. Occurs due to gas leakage from the charging valve, damage to the cylinder/diaphragm (puncture, tear) or worn piston seals.
• Hydraulic Fluid Contamination: Dirt particles or abrasives can damage the internal surfaces of piston accumulators or cause cylinder/diaphragm wear.
• Overheating/subcooling: Elastic elements (cylinders, diaphragms, seals) have a limited temperature range. Operation outside this range (eg above 80°C or below -20°C for standard NBR cylinders) results in accelerated aging and degradation.
• Corrosion of casing: External or internal corrosion can lead to weakening of the walls and potential rupture under pressure, especially in aggressive environments or when using unsuitable fluids.
• Втома матеріалу: Повторювані цикли заряджання/розряджання з великими перепадами тиску можуть спричинити втому металевих частин корпусу або кріплень.
• Incorrect P₀ pressure: Too low P₀ leads to frequent contact of the cylinder/diaphragm with the metal walls, their damage and reduction of the effective volume. A P₀ that is too high reduces the useful volume of the battery and can lead to insufficient damping.

6.2. Visual Indicators and Diagnostics:

• Unstable pressure in the system: Indicator of loss of damping capacity.
• Noise and Vibration: Undamped pressure pulsations cause increased pump and component noise.
• Slow or uneven operation of executive mechanisms: Indicates insufficient energy reserve or loss of compensatory ability.
• Liquid/gas leak: Visual signs of damage to seals or housing.
• Change in color or odor of fluid: May indicate overheating or contamination.

UNITEC-D recommends regular visual inspection and inspection of P₀ as part of the standard maintenance procedure.

7. Projected Maintenance and Condition Monitoring

Implementing Predictive Maintenance (PMT) strategies allows you to optimize service intervals, reduce the risk of unplanned downtime and extend the life of hydraulic accumulators.

7.1. Monitoring methods:

• Regular Precharge Pressure Check: Use a pressure gauge to check P₀ at least every 6-12 months or more often for critical systems. A drop in P₀ of 20% or more is a signal for intervention.
• Temperature Monitoring: Thermal imaging cameras or contact thermometers can detect abnormal heating of the battery case or hydraulic fluid, which may indicate cylinder problems or contamination.
• Hydraulic fluid analysis: Regular laboratory tests for metals, water and mechanical impurities help detect wear of seals, pistons or system contamination affecting the battery. Fluid compliance with the ISO 4406 standard (fluid purity) is mandatory.
• Vibration Analysis: Although less applicable to accumulators than to pumps, abnormal vibrations in the accumulator area may indicate uneven flow or mounting problems.
• Ultrasonic diagnostics: Allows you to detect gas leaks from the charging valve or microcracks in the battery case.

The implementation of SCADA systems or specialized sensors for continuous monitoring of key parameters can significantly increase the effectiveness of PTO. The data collected from such systems allows you to analyze trends and predict potential failures long before they occur.

8. Матриця Порівняння Типів Гідравлічних Акумуляторів

Choosing the optimal accumulator type for a specific application is a key decision that affects the efficiency, reliability and cost of a hydraulic system. Below is a comparison table to help engineers in this process.

Table 2: Comparison of Hydraulic Accumulator Types

9. Conclusion

Hydraulic accumulators are an integral component of reliable and efficient industrial hydraulic systems. The correct choice of battery type, accurate calculation of pre-charge volume and pressure, as well as compliance with strict installation and maintenance rules are fundamental to ensure their long-term and trouble-free operation. Understanding engineering principles, standards and potential failure modes enables UNITEC-D maintenance engineers to optimize system performance and minimize operational risks.

UNITEC-D GmbH is a reliable partner that supplies high-quality hydraulic accumulators and provides a full range of technical advice and services. We offer a wide range of components that meet the strictest international and Ukrainian standards (EN, ISO, DSTU) and have CE and UkrSEPRO certificates.

For a detailed overview of the range of hydraulic accumulators and other components that increase the reliability of your equipment, visit our e-catalog: https://www.unitecd.com/e-catalog/

10. Links

• DSTU EN 14359:2017. Gas-filled accumulators for hydraulic drives.
• DSTU EN ISO 4413:2018. Volumetric hydraulic drives. General safety rules for systems and their components.
• DSTU ISO 5596. Volumetric hydraulic drives. Accumulators are gas-filled with a separator. Pressure and volume ranges, as well as characteristic values.
• DSTU EN 13445 (series). Stationary pressure vessels.
• Bosch Rexroth. Hydraulic Accumulators. Technical Information.
• Parker Hannifin. Accumulator Engineering Handbook.