Estimated reading time: 21 minute
The accumulator is a kind of energy storage device, its function is to store a certain volume of pressure oil, and release it when the system needs it for use by the system. Since the compressibility of the liquid itself is very small, a heavy hammer or other elastic elements are needed to store and release energy (such as springs, compressible gas, etc.). The accumulator is the energy storage element of the hydraulic system. It is often used for auxiliary oil supply or emergency energy, maintaining system pressure, and absorbing shock and pressure fluctuations.
Question 1: Calculation of The Effective Volume of The Accumulator
The function of the accumulator in the system is different, and its effective volume calculation is also different.
When used as auxiliary fuel supply or emergency energy, there must be sufficient capacity to ensure that the system can meet the speed requirements or move the actuators to the corresponding position (generally return to the original position) in an emergency.
Question 2: Design of The Position of The Accumulator
According to the different roles of the accumulator in the hydraulic system, place the accumulator in a reasonable position, as shown in Figure 1-1.
Question 3: Globe Valve Problem
1. A shut-off valve should be installed between the accumulator and the system. This valve is used for charging, checking, and repairing the accumulator or when it is shut down for a long time.
2. The diameter of the special stop valve for the accumulator should be appropriately selected according to the system usage.
3. When choosing a special stop valve for accumulators, they often pay more attention to the nominal pressure and interface size of the valve, and pay little attention to its diameter. Reasonably choose the diameter of the special stop valve, which should be based on the actual use of the accumulator in the system. To decide. If the accumulator needs to supply oil to the system instantaneously or absorb shock and pulsation, a large diameter should be selected to improve the response time, and a small diameter should be selected for a branch to improve the pressure holding effect.
Question 4: The Problem of Liquid Level Control
When the accumulator is used in the hydraulic system, in order to prevent the compressed gas in the liquid tank from entering the hydraulic system, a liquid level controller must be installed. The function of the liquid level controller is mainly to display the liquid level in the high-pressure closed container-the liquid tank so that the operator can control the relevant equipment in time to ensure production safety. At present, in gas-liquid direct contact accumulators, electrical control is mostly used for liquid level control. When the liquid in the liquid tank is in various positions, the liquid level controller is used to operate the pump and the valve to turn on or off, and send out various light signals according to different situations.
Commonly used liquid level controllers in my country include mercury contact magnet buoy type and mercury tank. The photoelectric method is also used to measure the liquid level.
Question 5: The Connection Problem Between The Accumulator And The Hydraulic Pump
Since the accumulator stores a certain pressure of oil during the working process, when the system stops working, the oil pressure in the accumulator will directly act on the outlet of the hydraulic pump, causing the hydraulic pump to reverse and affect the service life of the hydraulic pump. After the one-way valve is set between the accumulator and the hydraulic pump, it can effectively prevent the oil from flowing back, as shown in Figure 1-2.
Question 6: Problems That Easily Occur in The Installation of Accumulators
- Processing and welding on the accumulator are not allowed. The accumulator is a high-pressure vessel, and the national standard has strict requirements for it.
Surface processing and welding will cause damage to it, and the weakening of surface stress and strength is very dangerous
According to relevant standards
Seeking truth is absolutely not allowed.
2. When installing an accumulator with compressed gas, pay attention to the following points.
- The accumulator used to buffer the impact is generally as close as possible to the place where the impact occurs and installed vertically with the oil port down and the gas part up. If it is true that it is not possible to install it vertically due to its status, it should be installed horizontally.
- When installing the accumulator on the pipeline, the accumulator must be fastened with support or support frame to avoid flying accidents.
- Should be kept away from heat sources.
3. The installation direction of the accumulator should be reasonable. For non-isolated accumulators and gas-generating accumulators, they must be placed upright, and the oil should be in the lower part of the accumulator, as shown in Figure 1-3.
4. When installing the accumulator, it is taboo to use pipelines as support. As the accumulator of a high-pressure vessel, its own weight and volume are very large. If it is directly supported by pipelines, its stability is poor, which will cause local pipelines to be stressed and deformed. If a hydraulic shock occurs, it will increase the size of the pipeline. The amplitude of the circuit causes the joint to loosen and leak oil. Therefore, it is stipulated in the standard of general technical conditions for hydraulic systems: pipelines are not allowed to be used to support equipment or oil circuit blocks. Therefore, the accumulator as a high-pressure vessel is not allowed to be supported by pipelines, as shown in Figure 1-4.
- The installation location of the accumulator should be convenient for maintenance and far away from heat sources. When the installation position of the fuel tank is greatly restricted by the on-site environment, the position of the accumulator should be selected as far as possible to avoid blind spots and set it in a position that is easy for maintenance personnel to operate. If necessary, a separate accumulator mounting frame can be set up.
- The accumulator must be firmly fixed on the bracket or foundation, so as to prevent the accident of flying and injuring people when the accumulator is detached from the fixed part.
- The nameplate of the accumulator should be placed in a conspicuous position.
- The accumulator used to reduce noise, absorb pulsation and hydraulic shock should be as close as possible to the vibration source.
- The accumulator in the oil-filled state cannot be disassembled.
- Non-isolated accumulators and bladder accumulators should be installed vertically in principle with the oil port facing downwards and the inflation valve facing upwards.
Because of the simplified power source circuit in Figure 1-5, under the prerequisites of appropriate settings, the method of linearization with small increments near the given equilibrium point is used to study the nonlinear problem.
From calculation and analysis, it can be seen that the smaller the hydraulic resistance of the pipeline before the accumulator, the smaller the amplitude of the pressure pulsation, and the better the effect of the accumulator circuit in absorbing the pulsation. Therefore, when the pulsation frequency of the pump is known, the effective volume of the accumulator should be reasonably selected, the equivalent mass of the oil in the accumulator pipeline and the oil in the accumulator, and the cross-sectional area of the accumulator oil, Make the hydraulic resistance of the pipeline before the accumulator as small as possible to make the loop absorb the pulsation best.
Analysis result: In the circuit of this example, inspection and analysis found that, except for the unreasonable selection of the structural parameters of the accumulator, the pipeline before the accumulator is 4m long and far away from the pump, so the accumulator has no obvious effect of absorbing pressure fluctuations.
Question 7: Inflation of The Accumulator
- Check the charging pressure of the accumulator: install the pressure gauge near the oil port of the accumulator, fill the accumulator with oil with a pump, then stop the pump, and let the pressure oil pass through the valve connected to the accumulator Slowly flow from the accumulator. Observe the pressure gauge during the process of oil discharge. The pointer of the pressure gauge slowly drops. When the charging pressure value of the accumulator is reached, the poppet valve in the accumulator is closed, so the pointer of the pressure gauge quickly drops to zero. The reading on the pressure gauge before the pressure drops rapidly is the charging pressure of the accumulator. In addition, you can use the inflation tool to directly check the inflation pressure. Because a little gas is released every time you check, this method is not suitable for small-capacity accumulators.
- It is forbidden to fill the accumulator with oxygen to avoid explosion.
- Problems that should be paid attention to when charging the accumulator. As a high-pressure vessel, the accumulator must be filled with inert gas with good stability, such as nitrogen. The charge value must be less than the rated pressure of the accumulator, and the charge value should not be determined arbitrarily. It should be determined after calculating with the corresponding formula according to the system usage. The pressure gauge on the inflation tool is required to be sensitive, difficult to determine, and the range meets the requirements of use, and the inflation valve is required to be flexible and air-tight.
- Problems to pay attention to when choosing nitrogen-filling tools. A Nitrogen charging tool is an essential accessory of the accumulator. When selecting, pay attention to that the nominal pressure should be consistent with that of the accumulator. The pressure gauge should be sensitive, the range and accuracy should meet the requirements of use, and the length of the atmospheric hose should be appropriate. , It is convenient to use.
Question 8: Selection of Circuit Capacity of Accumulator For Special Machine Tools
Figure 1-9 shows the hydraulic circuit of a special machine tool. To eliminate the impact of the reversing impact of the electromagnetic reversing valve on the processing quality of the workpiece, the circuit adopts a bladder-type accumulator 3 to absorb the hydraulic impact. Hydraulic pump 1 is quantitative, the working pressure of the circuit is controlled by the overflow valve? Set, the total volume of the accumulator 3 is 0.25L, the inner diameter of the pipe is 10mm, and the length of the pipe before the electromagnetic reversing valve is 20m.
Existing problem: It was found that the effect of the accumulator in eliminating hydraulic shock was not good during the system debugging.
When the liquid flows in the pipeline because the control valve is suddenly closed, the liquid flow suddenly stops flowing, the kinetic energy of the liquid becomes pressure energy, and high pressure is generated in front of the valve. The high-pressure area propagates in the pipeline in the form of pressure waves, forming a hydraulic shock. The pressure rise may be several times higher than the normal pressure and may endanger the instruments, components, and sealing devices in the hydraulic system, thereby affecting the system’s Works normally. In addition, it can also make the system produce noise and vibration. It is one of the effective measures to reduce the hydraulic shock to install an accumulator in the part where the hydraulic shock is generated in the system. Since the magnitude of hydraulic shock pressure is determined by the rate of change of the momentum of the liquid in the pipeline to time, when the pressure increases, the accumulator can absorb the liquid, which slows down the rate of change of the liquid momentum in the pipeline, thereby reducing Impact pressure.
The total capacity of the accumulator used to absorb hydraulic shock is calculated in section 6.1. From the calculation, the total capacity of the accumulator for the system to obtain the best effect of absorbing hydraulic shock should be 0.63L, while the actual accumulator used in the system is only It can be seen that improper selection of accumulator specifications is the main reason for the problems in the system.
Select the appropriate accumulator according to the above calculation results, and pay attention to the following issues in the installation.
- Install as close as possible to the shock source, that is, near the electromagnetic reversing valve, to obtain the best effect of absorbing hydraulic shock.
- A shut-off valve should be installed between the accumulator and the pipeline system for inflation and maintenance.
- A check valve should be installed between the accumulator and the hydraulic pump to prevent the pressure oil stored in the accumulator from flowing backward when the hydraulic pump is stopped.
The improved circuit is shown in Figure 1-9. After the improvement, the effect of the improved accumulator in absorbing hydraulic shock is significantly improved, and the processing quality of the workpiece is guaranteed.
Some auxiliary parts in the hydraulic system can only achieve the best use effect if they are matched with the system structure reasonably.
When choosing these auxiliary parts, it is best to calculate and determine the actual structure size according to the system requirements. It is difficult to be accurate based on experience and estimation, so it is not easy to achieve the expected effect.
Question 9: The Role And Performance of The Oil Filter
The role of the oil filter
In the hydraulic system, to ensure that the oil is clean and clean, and oil filter must be installed to filter out the impurities in the oil. Impurities in the oil are caused by external dust, dirt, residues (oxidized scales, cuttings, etc.) in the components and tubing of the device and the oil mixed with the oil due to oxidative deterioration. These impurities can cause scratches, abrasion, and even seizure on the surface of the relatively moving parts, block the small holes of the pipe and the throttle valve port, and affect the working performance of the machine tool, resulting in the system not working properly. At the same time, with the continuous improvement of the accuracy of hydraulic components, the requirements for the cleanliness of the oil are getting higher and higher. Therefore, the impurities and pollutant particles in the hydraulic oil must be cleaned up. At present, the most effective way to control the cleanliness of hydraulic oil is to use an oil filter. The main function of the oil filter is to filter the hydraulic oil and control the cleanliness of the oil.
Performance index of oil filter
The main performance indicators of the oil filter include filtration accuracy, flow capacity, pressure loss, etc., of which filtration accuracy is the main indicator.
- Filter accuracy. The working principle of the oil filter is to filter the dirt with a filter element with a certain size of filter hole. The filtration accuracy refers to the maximum size of the impurity particles (expressed by the average diameter d of the dirt particles) that the oil filter can filter out from the hydraulic oil.
The oil filters currently used can be divided into four levels according to the filtering accuracy: coarse oil filters (d≥0.1mm), ordinary oil filters (d≥0.01mm), refined oil filters (d≥0.001mm), and Special oil filters (d≥0.0001mm).
- Flow capacity. The flow capacity of the oil filter is generally expressed by the rated flow, which is proportional to the filter area of the oil filter element.
- Pressure loss refers to the pressure difference between the inlet and outlet ports of the oil filter under the rated flow. Generally, the better the flow capacity of the oil filter, the smaller the pressure loss.
- Other performance The other performance of the oil filter mainly refers to qualitative indicators such as the strength of the filter element, the life of the filter element, and the corrosion resistance of the filter element. These performances of different oil filters will be quite different, you can determine their pros and cons through comparison.
The working capacity of the oil filter depends on the content of solid particles in the oil, the pressure difference between the oil before and after filtration, the viscosity and temperature of the oil, and the performance of the filter medium itself. The greater the pressure applied to the surface of the oil filter and the lower the resistance, the greater the oil output capacity of the oil filter. The lower the speed of oil flowing through the filter element and the lower the surface pressure, the higher the filtration accuracy. The filter element with low hydraulic resistance should be selected as much as possible because in the working process, the pressure loss increases, and the flow rate decreases due to continuous clogging. When the pressure loss reaches a certain allowable value, the oil filter must stop working for cleaning. The life of the oil filter (that is, the filter cycle of the filter element) is determined by the pressure drop of the oil filter. The design of the oil filter is mainly to select the filter element material according to the requirements of working pressure and filtration accuracy, and calculate the filter area according to the selected filter element material and flow requirements.
Question 10: Determine The Flow Capacity of The Oil Filter
- The flow capacity of the oil filter should not be too low. The oil filter in the hydraulic system is generally located at the pump inlet, the system oil return pipe, the key element inlet, and the separate filter passage. When the flow capacity is low, it is easy to produce a large pressure drop on the filter. At the pump inlet, the inlet pressure of the pump is too low, which is easy to cause damage to the pump. In the oil pressure or return pipe, the system efficiency is reduced. In addition, When the pressure difference that the oil filter bears is too large, the filter element will be damaged. Generally, the oil-passing capacity of the oil filter should be more than twice the flow rate during normal operation.
- The precision of the suction oil filter should not be too high. The suction oil filter is located in the inlet pipeline of the hydraulic pump. Its function is to prevent larger pollutant particles from entering the hydraulic pump. Its flow capacity is very important for the normal operation of the hydraulic pump.
- The accuracy of the oil filter is too high, and its flow capacity is weakened. At the same time, it is easy to accumulate pollutants on the filter, thereby further reducing the flow capacity. Generally, use a coarse oil filter or ordinary precision oil filter on the oil suction pipe.
- The flow capacity of the return oil filter cannot be simply selected according to the flow rate of the hydraulic pump. Generally, the flow capacity of the filter in the hydraulic system should be determined according to the flow of the hydraulic medium flowing through it. Regarding the oil-passing capacity of the return oil filter, the maximum return oil flow rate during the working process of the system should be considered when selecting. Generally, hydraulic cylinders in hydraulic systems often use single-rod cylinders. At this time, special attention should be paid to the difference in flow caused by the effective area on both sides of the piston, which cannot be simply selected according to the flow rate of the pump.
Question 11: Filtering Precision Selection
- The principle of selection of filtration accuracy is to make the size of the filtered dirt particles smaller than half of the size of the sealing gap of the hydraulic components. The higher the system pressure, the smaller the fit clearance of the relative moving parts in the hydraulic parts, therefore, the higher the filtering accuracy of the oil filter is required. The filtration accuracy of the hydraulic system mainly depends on the pressure of the system.
- The filtering accuracy of the oil filter should meet the requirements of the element for the accuracy of the medium. Because the coordination accuracy of various hydraulic components and the importance of the system application is different, the cleanliness indicators required to ensure the reliable operation of the system are also different, So the filter accuracy of the oil filter is required to reach the cleanliness index required by the system and components after circulating filtration.
- The general hydraulic valve system
- It is recommended to use 10~20um; the proportional valve system is recommended to use 1~10um; the servo system is recommended to use an oil filter with a filtration accuracy of 1~3um.
- According to actual investigation and research, different systems, and different working conditions, oil filters with different filtration accuracy can be selected according to Table 6-1. Generally speaking, the selection of high-precision oil filters can greatly improve the working reliability of the hydraulic system and the life of the components. However, the higher the filtering accuracy of the oil filter, the faster the filter element of the oil filter is often blocked, the shorter the cleaning or replacement period of the filter element of the oil filter, and the higher the cost. Therefore, when selecting an oil filter, the filter accuracy of the oil filter should be selected reasonably according to the specific situation to achieve the required oil cleanliness.
Question 12: Filter Element Selection Problem
When selecting an oil filter for a specific purpose, the main factors that must be considered are the nature of the filtrate and the compatibility with the filter material; the flow rate through the oil filter and the degree of change and fluctuation of the flow rate; the working pressure of the system and Whether the pressure is steady-state or time-varying; the operating temperature of the system, and the filtering accuracy required by the system, etc.
Influencing factors of filter element selection
- Fluid properties. The materials of the filter element, accessories, and housing that constitute the oil filter must be compatible with the filtered fluid. For example, some liquids may corrode the filter element or housing, and this corrosion, in turn, contaminates the filtrate. Therefore, the first thing to determine is whether the liquid is acidic, alkaline, or neutral.
- Flow rate. After the filter flow rate (the volume of fluid flowing in a unit time) is determined, the specifications of the oil filter should be selected according to the sample regulations. When conditions permit, you can choose a larger flow oil filter, but it is not allowed to choose a too small oil filter.
- Temperature. The temperature of the filtrate affects the viscosity of the fluid, the corrosion rate of the shell, and the compatibility of the filtrate with the filter material. As the temperature increases, the viscosity of the liquid generally decreases. If the fluid is too thick, it can be preheated appropriately or a belt preheater can be installed in the oil filter. It is important to determine the viscosity of the liquid according to the working temperature to select the filter element reasonably.
- High temperatures also tend to accelerate corrosion and weaken the seal of the oil filter housing. It is for this reason that people often choose porous metal materials as filter materials and high-temperature resistant sealing materials.
- Filter accuracy. For an oil filter for a specific purpose, it must be able to effectively filter out polluting particles in the liquid to achieve the required cleanliness level. According to the size of the pollutant particles to be filtered, an oil filter with a certain absolute filtration accuracy (or a required filtration ratio) can be selected.