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A accumulator is an energy storage device that can store hydraulic pressure in a pressure-resistant container and release it when needed. Accumulators are important auxiliary components in hydraulic systems; they play a crucial role in ensuring the normal operation of the system, improving its dynamic performance, maintaining operational stability, extending its service life, and reducing noise. The benefits that accumulators bring to the system in terms of cost savings, energy efficiency, safety, reliability, and environmental protection are very significant. Its use is particularly worth promoting in modern large-scale hydraulic systems, especially those with intermittent operating conditions. 1.1 Working principle of accumulators Hydraulic oil is an incompressible fluid; therefore, it cannot be used to store pressure energy. Other media must be employed to convert and store pressure energy. For example, a bladder-type pneumatic accumulator developed by utilizing the compressible property of gas (nitrogen) is a device for storing hydraulic oil. The bladder-type accumulator consists of an oil section and a gas section with gas seals; the oil surrounding the bladder is connected to the oil circuit. When the pressure increases, oil enters the accumulator and the gas is compressed, so the pressure in the system pipelines no longer rises ； When the pipeline pressure drops, the compressed air expands and forces oil into the circuit, thereby slowing down the decline in pipeline pressure. There are various types of accumulators, each with complex functions. Different hydraulic systems have different requirements regarding the functions of accumulators. Only by thoroughly understanding and knowing the types and functions of accumulators can one select the appropriate ones for different operating conditions, thereby enabling them to fulfill their roles effectively and improving the performance of the system. 1.2 Types of accumulators Accumulators can be classified into spring-type, weight-type, and gas-type according to the method of loading. ①A spring-type accumulator relies on compressed springs to convert the excess pressure energy in the hydraulic system into spring potential energy, which is stored and released when needed. It has a simple structure and low cost. However, due to the limited expansion and contraction range of the spring, as well as its insensitivity to pressure changes, which results in poor vibration damping performance, it is only suitable for low-capacity, low-pressure systems (P≦1.0～1.2MPa), or for use as a buffering device. ②A weight-type accumulator converts the pressure energy in the hydraulic system into gravitational potential energy by lifting a mass loaded on a sealed piston. It has a simple structure and stable pressure. The downside is that its installation is limited, as it can only be installed vertically ； Difficult to seal ； The mass block has high inertia and is not sensitive. Such accumulators are intended only for temporarily storing energy. These two types of accumulators are rarely used due to their limitations. It is worth noting, however, that some research institutions have made structural improvements to these two types of accumulators from an economic perspective, thereby overcoming their shortcomings to a certain extent. For example, a domestic factory has adopted an improved structure for spring-type accumulators. Increasing the outer diameter of the spring (to be larger than the diameter of the hydraulic chamber) and limiting the spring’s stroke (by keeping the maximum load on the spring within the allowable limit) improve the operating pressure and capacity of the accumulator. External spring-type accumulator ③ The working principle of a gas accumulator is based on Boyle’s law (PVn=K=constant); energy conversion is achieved by compressing gas. When in use, gas at a predetermined pressure is first filled into the accumulator. When the system pressure exceeds the pressure inside the accumulator, the oil compresses the gas, converting the pressure in the oil into internal energy of the gas ； When the system pressure is lower than the pressure inside the accumulator, the oil in the accumulator flows toward the external system under the action of high-pressure gas, thereby releasing energy. Choosing the appropriate inflation pressure is key to this type of accumulator. Based on their structure, such accumulators can be classified into pipeline vibration dampers, those with direct gas-liquid contact, piston-type ones, diaphragm-type ones, airbag-type ones, etc. ④A bladder-type accumulator consists of a pressure tank made by casting or forging, a bladder, a gas inlet valve, and an oil inlet valve. The material of the casing follows standards, and typically includes materials such as nitrile rubber (R), butyl rubber (IR), fluororubber (FKM), and ethylene oxide-chloropropylene oxide rubber (CO). Figure 3: Bladder-type accumulator. ⑤ The bladder-type accumulator consists of a pressure-resistant housing, an elastic bladder, a filling valve, a lift valve, oil ports, etc. These accumulators can be manufactured in various sizes and are suitable for hydraulic systems of all sizes ； Capsules have low inertia and are highly responsive, making them suitable for eliminating pulsations ； It doesn’t leak easily, and there’s no risk of oil and gas mixing ； It is easy to maintain, has few accessories, is simple to install, and convenient to inflate, which is why it is the most widely used at present. ⑥A pipeline vibration damper is a short tubular accumulator that is installed directly on the pipelines of high-pressure systems. This type of accumulator has good response performance and is capable of effectively eliminating high-frequency oscillations in high-pressure, high-frequency systems; it is widely used in high-pressure vibration damping systems. 1.3 Functions of the accumulator The functions of an accumulator mainly fall into four categories: storing energy, absorbing hydraulic shocks, eliminating pulsations, and recovering energy. Category 1: Storing energy. These benefits can be further subdivided in practical use into: ① serving as an auxiliary power source to reduce the required installed capacity ； ②Compensate for leakage ； ③For thermal expansion compensation ； ④As an emergency power source ； ⑤It forms a constant-pressure oil source. The principles behind these five functions are essentially the same; they all make use of the ability of accumulators to store a large amount of energy. The main difference lies in the selection of parameters; by using different parameter selection formulas, the desired functions can be achieved to meet the required specifications. Category 2: Absorbing hydraulic shock. Sudden reversal of the directional valve or an abrupt stop in the movement of the actuator can both generate pressure surges in the hydraulic system, causing the system pressure to rise rapidly in a short period of time. This can lead to damage to instruments, components, and sealing devices, as well as the occurrence of vibrations and noise. To ensure effective absorption, the accumulator should be placed near the point of impact; therefore, it is generally installed in front of the source of impact such as control valves or hydraulic cylinders, allowing it to effectively absorb and cushion hydraulic shocks. Third category: Eliminate pulsation and reduce noise. In hydraulic systems that use piston pumps with a small number of pistons, periodic changes in pump flow cause vibration in the system. By installing an accumulator, it is possible to absorb a large amount of energy from pressure and flow fluctuations over one cycle of these fluctuations. The portion of oil with an instantaneous flow rate higher than the average flow rate is absorbed by the accumulator, while the portion with a flow rate lower than the average flow rate is replenished by the accumulator. This absorbs the energy in the fluctuations, reduces these fluctuations, and thereby minimizes the damage to sensitive instruments and equipment. Category 4: Energy recovery. Recovering energy using accumulators is an area that has received significant research attention. Energy recovery can improve energy utilization efficiency and is an important way to save energy. Since accumulators can store energy temporarily, they can be used to recover potential energy from various functions and locations. The main research in this area includes: ① recovering braking energy from vehicles ； ②Recovering the potential energy of the boom mechanism in construction machinery ； ③Recovering braking energy from hydraulic excavator turntables ； ④Recovering the gravitational potential energy of pipes from oil workover rigs and drilling rigs ； ⑤Recycle the gravitational potential energy of the elevator during descent. 1.4 Use and Maintenance of Accumulators The use and maintenance of accumulators mainly includes their installation and upkeep, fault diagnosis and troubleshooting, as well as repair. The installation of a accumulator includes pre-installation inspections, installation, nitrogen charging, etc. Proper installation, fixation, and inflation are essential conditions for the accumulator to operate properly and fulfill its intended function. The measurement of parameters and the proper use of various tools and instruments cannot be ignored. During use, accumulators need to be protected from vibration, high temperatures, contamination, and leaks; it is also necessary to regularly check the airtightness of the air bags as well as other aspects. Therefore, regular inspection and maintenance are essential. Routine inspections involve using simple methods such as visual inspection, auditory testing, tactile examination, and instruments to check the appearance and condition of the equipment; during these inspections, both individual parts and the overall equipment need to be considered. Abnormalities detected during inspection that prevent the accumulator from continuing to function must be addressed urgently ； The others should be carefully observed and recorded, with issues resolved during regular maintenance. Some damaged parts also need to be replaced promptly. Proactive maintenance is a new equipment management theory that has been proposed internationally in recent years, following fault repair, preventive maintenance, and condition-based maintenance. Its definition is: fixing the root cause parameters that lead to equipment damage, thereby effectively preventing failures and extending the equipment’s service life. Proactive maintenance involves taking measures to address the root causes of wear before it occurs, effectively controlling wear and failure and thereby significantly extending the repair cycle. Proactive maintenance not only ensures the reliable operation of hydraulic equipment and components, but also significantly reduces maintenance costs. Accumulators are hazardous components in hydraulic systems, so special attention must be paid to safety during operation. The diagnosis and troubleshooting of accumulators involves both the diagnosis and troubleshooting of faults in the accumulator itself, as well as those in the hydraulic system in which the accumulator is located; these two aspects are interrelated. The main tasks of fault diagnosis include: ① Determining the nature and severity of the fault. Based on the conditions at the site, determine whether there is a fault, what type of issue it is (pressure, speed, operation, or something else), and the severity of the problem (normal, minor fault, moderate fault, or severe fault). ②Identify the faulty components and their locations. Based on the symptoms and relevant information, identify the fault point in order to further troubleshoot the issue; here the focus is on figuring out \"where the problem lies\". ③Further investigate the initial cause of the failure. Such as hydraulic oil contamination, low component reliability, and unsuitable environmental conditions. Here, we mainly identify the external causes of the fault. ④Mechanism analysis. Conduct an in-depth analysis and exploration of the causal chain of the failure to understand the full background of the issue. ⑤Predict the development trend of faults. Based on the current status and rate of system wear and degradation, as well as theoretical and empirical data on component lifespan, the future condition of the accumulator or hydraulic system is predicted. Analyze, compare, statistically process, generalize, and synthesize to identify patterns. 1.5 Historical Review and Current Research Status The 17th and 18th centuries were the peak periods for the development of hydraulic theory. Theories such as hydrostatic transmission theory, modern fluid dynamic lubrication theory, and fluid dynamics, which were developed and matured during this period, essentially laid the foundation for modern hydraulic theory. And due to the requirements of practical applications, some simple accumulators have also been developed, such as weight-type accumulators that use containers filled with water as mass blocks. In the later stages of World War II, hydraulic machinery became popular. The use of hydraulic servo drives in the manufacturing of military weapons contributed to the development of hydraulic drive and control technologies. Advances in hydraulic control technology, material sealing and lubrication techniques, as well as automatic control technologies, also laid a theoretical foundation for the development of hydraulic control theory. The technologies developed for military needs after the war gradually shifted to industrial and civilian applications, where they began to thrive. It was from this period onward that theoretical research on accumulators, focusing on mature hydraulic control theories and practical technologies, began to receive increasing attention. Some universal types of accumulators have emerged, such as spring-based accumulators, the more mature weight-type accumulators, and some simple gas accumulators. Starting from the 1970s, researchers began to focus on the study of the basic theories of accumulators (such as parameter selection formulas and frequency calculation formulas), and continued to develop and refine them. In the late 1970s, the development of automotive energy-saving technologies spurred research on accumulators and energy-saving techniques using them; attention began to be paid to the role of accumulators in saving energy in hydraulic systems. In the 1980s, the structure, types, forms, and functions of accumulators began to diversify, and the development of various types of accumulators became a major area of research. In the 1990s, the development of new computer software, hardware, and control technologies provided advanced research tools and methods for the study of hydraulic systems and intelligent hydraulic components, which posed new requirements for the research of accumulators. The development of hydraulic theory and technology is inseparable from the research, development, and creation of new hydraulic components. Currently, research on accumulators at home and abroad mainly focuses on the following aspects. ①In line with the development of research on new hydraulic systems, more studies have been conducted in the area of technical applications. As hydraulic systems evolve toward higher pressure, higher speed, and greater precision, many specialized systems have emerged. These systems often have very high requirements in certain aspects, and improving other components alone is not sufficient to meet those demands; therefore, it is necessary to develop special accumulators as a solution. For example, to address absorption of pulsations, Shini-chi YOKOTA in Japan developed a new type of active accumulator driven by multi-stage PED (Piezo-Electric Device) units, which can effectively eliminate high-frequency pulsations (500–1000 Hz) caused by hydraulic components. Another example is a series-type bladder accumulator developed by Xing Keli and others from Xi’an Jiaotong University, which exhibits good absorption performance for pulsations with frequencies ranging from 112 to 288 Hz; moreover, compared to conventional accumulators, it has a wider attenuation bandwidth. ②By combining existing accumulator theories with new analytical methods and control theories, innovation is achieved theoretically; that is, based on existing theories, more advanced research methods and approaches are employed to yield more valuable theoretical results. For example, Chen Zhaodi and others from Harbin Institute of Technology used keygraph theory to analyze the impact of accumulators on pressure spikes in pipeline systems. They established a dynamic mathematical model of the accumulator using keygraph theory, demonstrated the accumulator’s ability to suppress pressure shocks, and proposed valuable theories regarding the accumulator’s function in absorbing pressure fluctuations. This method can also be extended to the dynamic analysis of other hydraulic systems containing accumulators. ③Based on existing theories of accumulators and hydraulic systems, and supported by newly emerging design methods and computing software, software has been developed for the auxiliary design, calculation, or testing of accumulator circuits. For example, the Sharp EL512 calculator offered by Par.ker Hannifin Corp can assist users in selecting accumulator parameters. Furthermore, Wu Xiaoming and others from Yanshan University, based on thorough research into accumulators and their related theories, utilized the theory of \"embedded\" expert systems to develop intelligent software for accumulators and their associated circuits. The resulting software for the auxiliary design of accumulators and their circuits helps system designers select appropriate accumulators more easily. Currently, there are no highly effective methods for testing the characteristics of accumulators, which directly results in imperfect parameters for these devices and unclear dynamic properties. Understanding aspects such as their optimal operating range is also limited, posing significant difficulties in selecting the appropriate accumulators and indirectly leading to errors in such selections ； Furthermore, the selection system cannot determine these parameters accurately based on the dynamic characteristics of the hydraulic circuit, such as the nitrogen filling pressure; in other words, the issue of matching accumulator parameters to the application environment has not been resolved at all. Developing testing techniques for the dynamic performance of accumulators holds great practical value. Applying virtual instrument technology to the testing of accumulators takes full advantage of its advantages in terms of simplicity, speed, efficiency, and accuracy in detection, enabling accurate measurement of the dynamic performance parameters of accumulators and ensuring that the system meets the performance requirements and usage conditions of such accumulators. By conducting online and simulated tests on accumulators, the performance curves of accumulators of different specifications can be obtained. With the development of hydraulic systems, the requirements for these systems are increasing steadily, and the basic theories underlying existing accumulators as well as their structural designs can no longer meet the needs of research in hydraulic systems and hydraulic components. The main reason is that most of the existing basic theories on accumulators were developed in the 1970s and 1980s, based on empirical observations; as a result, these theories are often empirical in nature, lacking standardization and consistency. They can only provide preliminary guidance for system design, while actual application requires continuous adjustment and selection by operators. Moreover, the structure of existing accumulators means that once they are installed in a system, their parameters cannot be changed dynamically to meet the various requirements of the system. This poses obstacles to the research of hydraulic systems and their application in engineering practice. 1.6 Future Research Directions By summarizing and analyzing the research achievements of peers at home and abroad in the field of accumulators, and combining these with one’s own theoretical research and practical experience, the main future directions for accumulator research are predicted as follows. ①For the purpose of meeting technical application needs, research on structural improvements will continue to advance. It mainly includes the following two aspects: a. Taking the skin-type gas accumulator as the object of study, corresponding improvements are made according to the requirements of the actual system in order to maximize the improvement of the system’s performance ； b. Focusing on spring-type or weight-type accumulators, improve them from the perspective of economic efficiency while meeting the system requirements. Research on structural improvements can meet the needs of certain special systems without requiring much effort, so it will continue to receive attention from engineering and technical researchers. ②As engineering practices place higher demands on the performance of hydraulic systems, research focused on accumulators for absorbing hydraulic shocks, eliminating high-frequency pulsations, and recovering energy has become a priority and continues to advance. The main tasks include: a. With the emergence of large hydraulic systems, developing corresponding large-sized accumulators or those equipped with auxiliary devices to absorb hydraulic shocks ； b. With the emergence of high-frequency oscillation systems, it is necessary to develop accumulators capable of absorbing these high-frequency oscillatory waves in order to reduce the damage caused by such oscillations to the system ； c. As the concept of green energy conservation gains increasing attention in practical applications, further research is being conducted on energy-saving technologies using accumulators for different systems. ③Research on the basic Li theory of accumulators will receive attention. The existing basic theories of accumulators are already too outdated; therefore, it is imperative to improve these basic theories, especially by formulating accurate and comprehensive formulas for accumulators. This is a groundbreaking work. ④Research on intelligent accumulators will be carried out step by step. The development and use of new hydraulic components endow hydraulic systems with advantages such as high pressure, high speed, and high precision; simple improvements to existing accumulators based on their traditional structure alone will not be sufficient to meet these requirements. Furthermore, due to the uncertainty in system operating conditions and its inherent complexity and non-linearity, it is not possible to strictly separate the functions of the accumulator. This requires the ability to adjust various parameters of the accumulator in real time during the operation of the system, so as to utilize its different functions to meet the system’s needs. Therefore, it is necessary to develop an accumulator that can monitor changes in system parameters in real time, process them promptly, and issue commands to adjust various parameters of the accumulator, in order to meet the requirements of such complex systems. ⑤In terms of maintenance, new theories and methods will be gradually applied to the diagnosis, monitoring, and repair of accumulators. The accumulator and hydraulic system are complex mechanical, electrical, and hydraulic integrated systems, characterized by mechanical-hydraulic coupling, time variability, and non-linearity. The occurrence of failures is somewhat random. The diversity of faults, the complexity of causal relationships, and the reliance of fault diagnosis on the experience of domain experts are important characteristics of hydraulic faults; intelligent diagnosis systems are increasingly demonstrating their importance and irreplaceability. With the continuous development of modern mass production and ongoing advancements in science and technology, mechanical equipment is evolving toward being larger, faster, more precise, capable of continuous operation, and featuring more complex structures. Online monitoring of accumulators and hydraulic equipment is an important measure to ensure their safe, stable, long-term, full-load, high-performance, high-precision, and low-cost operation. The Intelligent Maintenance System (IMS) provides accumulator manufacturers with intelligent tools to monitor the operation of their accumulators via the Internet or wireless communication systems. The most important feature in IMS is its predictive diagnosis capability; by assessing the degradation of the accumulator’s performance, it enables timely and appropriate preventive maintenance, thereby reducing damage to the accumulator and downtime, as well as eliminating unnecessary periodic maintenance tasks.