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[Repost] Current Status and Future Prospects of Pressure Control Valves Abstract: Pressure control valves are key control components in hydraulic transmission systems, and they represent an important aspect of research in hydraulic transmission technology. This article introduces some of the main types and performance characteristics of hydraulic control valves both domestically and internationally, analyzes several typical structural forms and features of such valves, summarizes the primary engineering materials used in current hydraulic control valves, and provides an outlook on their development and application prospects. Keywords: hydraulic control valve ； Typical products ； Structural type ； Engineering Materials ； Current Development Status 1. Introduction Hydraulic transmission technology is a new type of transmission technology that uses natural fresh water or seawater directly as a substitute for mineral oil as the working medium in hydraulic systems. Water has advantages such as wide availability, cleanliness, and safety; using it as a working medium in hydraulic systems meets the requirements of environmental protection and sustainable development. At present, hydraulic drive technology has received widespread attention from various developed countries around the world, and has become one of the key research areas in the field of international fluid power and control technology in the 21st century. As a key control element in hydraulic transmission systems, the performance of pressure control valves directly affects the reliability and service life of such systems. Therefore, the development of hydraulic control components with excellent performance has become one of the key technologies in current research on hydraulic drive systems. 2. Introduction to the main products of pressure control valves. Some developed countries abroad, such as Denmark, Germany, the United States, Japan, the United Kingdom, Finland, etc., began researching pressure-driven technology in the early 1980s. To date, various pressure control valve products have been developed, and their performance has reached or is close to that of similar hydraulic control valve products; these valves are being used widely in industrial production. (1) Pressure control valve: Figure 1 shows a direct-acting relief valve produced by the German company Hauhinco. The maximum operating pressure of this valve is 35 MPa, with a pressure adjustment range of 2 to 32 MPa. In addition, the direct-acting pressure relief valves produced by this company can achieve a maximum operating pressure of 50 MPa, with a pressure adjustment range of 3 to 50 MPa. The VRH series of pressure control valves produced by the Danish company Danfoss comes in three models: VRH30, VRH60, and VRH120. Their maximum flow rates are 30 L/min, 60 L/min, and 120 L/min respectively. The pressure adjustment range for the VRH30 valve is 2.5 to 14 MPa ； Both the VRH60 valve and the VRH120 valve come in two pressure adjustment ranges: 2.5–8 MPa and 8–14 MPa, respectively. (2) Flow control valve: Figure 2 shows the VOH 30PM pressure-compensated manual throttle valve produced by the Danish company Danfoss. The valve is equipped with a pressure compensator that maintains a constant steady-state flow rate, regardless of changes in system pressure; this enables stable control of the water flow rate in the system. Its maximum inlet pressure is 14 MPa, the flow rate adjustment range is 2–30 L/min, and the maximum and minimum pressure drops across the valve are 14 MPa and 1.5 MPa, respectively. In addition, the flow control valves produced by the American company ELWOOD can operate at a pressure of 41.5 MPa. (3) Direction control valve: Figure 3 shows the VDH 60EC three-position four-way direction control valve produced by the Danish company Danfoss. The valve is composed of four seat valves, two inlet valves and two outlet valves, with each seat valve being controlled by its own pilot valve. The maximum pressure of the valve is 14 MPa, the minimum inlet pressure is 0.5 MPa, the maximum flow rate is 60 L/min, the minimum flow rate is 1 L/min, and the maximum leakage rate is 5 mL/min. The opening and closing times of the valve port during direction change are 110 ms and 130 ms respectively. The two-position three-way pilot-operated solenoid directional valves developed by the German company Hauhinco feature ceramic spools, with a maximum operating pressure of 32 MPa. The three-position four-way manual slide valve directional control valve produced by the American company Elwood has a maximum pressure of 42 MPa and a maximum flow rate of 42 L/min. (4) Proportional control valve: Figure 4 shows the hydraulic proportional control valve produced by the Japanese company Ebara. The valve spool is supported statically at both ends, and corresponding damping holes are provided to compensate for the poor lubricity and viscosity of the water medium, thereby improving the movement accuracy of the spool and reducing leakage from the valve. The valve’s rated pressure is 7 MPa, its rated flow rate is 35 L/min, the internal leakage rate is less than 0.7 L/min, and the switching frequency is 25 ms. The VOH 30PE proportional flow control valve produced by the Danish company Danfoss can be controlled via a PC or PLC. Its rated flow rate is 30 L/min, with maximum and minimum operating pressures of 14 MPa and 1.5 MPa respectively. This valve features good static and dynamic characteristics; the transition lag between the minimum and maximum flow rates is less than 8%, and the response time for changes in the control signal from 0% to 100% is less than 150 ms. (5) Servo control valve: Figure 5 shows the hydraulic servo control valve produced by the Japanese company Ebara; the valve body and its main components are made of SUS 304 stainless steel. This servo control valve consists of a nozzle-plate pilot valve and a three-position four-way power stage main valve. The spool is supported by hydrostatic forces at both ends; the hydrostatic working chamber is connected to the orifice of the nozzle-plate valve, which effectively reduces flow loss and helps to mitigate and suppress the valve’s nonlinear operating behavior. The valve spool is driven by nozzle pressure; an inductive displacement sensor detects the position of the spool and feeds an electrical signal back to the servo amplifier, thus creating a closed-loop system. The valve’s rated flow rate is 80 L/min, and its rated pressure is 14 MPa. The seawater servo control valve developed by the American company Moog has an operating pressure of 7 MPa, a rated flow rate of 2.3 L/min, a dead zone of less than 1%, and a hysteresis of less than 5%. China began research on hydraulic drive technology in the 1990s, and it is still at the stage of basic technical research. In the early 1990s, with funding from the National Natural Science Foundation, Huazhong University of Science and Technology took the lead in conducting research on seawater hydraulic transmission technology. To date, it has developed seawater relief valves and flow control valves with a rated flow rate of 40 L/min and a rated pressure of 14 MPa. The **Key Laboratory of Fluid Power Transmission and Control** at Zhejiang University has also carried out research on hydraulic power transmission technology with the support of the Ministry of Education’s “211” Project; to date, a series of hydraulic relief valves, directional control valves, and throttle valves with a rated pressure of 14 MPa and a rated flow rate of 100 L/min have been developed. In addition, some universities such as Beijing University of Technology, Southwest Jiaotong University, and Kunming University of Science and Technology have also begun research on hydraulic drive technology. 3. Typical structural types and characteristics of pressure control valves for water. Water has physical and chemical properties that are quite different from those of mineral oil; therefore, the structural designs of traditional hydraulic control valves cannot be directly applied to the design of pressure control valves for water. Therefore, it is necessary to research and design new structures that can adapt to the physical and chemical properties of water, in order to meet the demands of the development of hydraulic drive technology and to improve the performance and lifespan of hydraulic control valves. (1) Improving existing pneumatic valves into low-pressure hydraulic control valves. The high cost of hydraulic control valves is an important factor affecting their widespread use and adoption. Pneumatic valve components are inexpensive, operate at pressure levels comparable to those of low-pressure hydraulic systems, and the materials used in pneumatic valves are usually corrosion-resistant. Therefore, reducing costs is possible by improving existing pneumatic valves to produce low-pressure hydraulic control valves. The University of Technology in Tampere, Finland, modified a pilot-operated 2/5-way on-off pneumatic valve into a hydraulic proportional control valve; experiments have shown that this valve can be used in most low-pressure water systems, as shown in Figure 6. This valve replaces the original pilot valve with a proportional solenoid; an adjusting plate is installed between the proportional solenoid and the valve body, and a spring with greater stiffness replaces the original return spring. Tampere University of Technology conducted performance tests on this valve and compared it with a pressure control valve that is already in use in high-pressure hydraulic systems. Experiments show that the improved hydraulic control valve can handle higher flow rates; aside from a greater delay, most of its performance characteristics are similar to those of another reference valve, making it suitable for use in low-pressure hydraulic transmission systems. (2) A new type of secondary structure. The secondary structure is a design commonly used in solenoid valves. Due to the limited output force and power of solenoids, using them to directly drive the valve core is only feasible in low-pressure systems with low flow rates. When the system requires higher flow rates and pressures, an additional power stage is needed to create a valve with a secondary or multi-stage structure. Proportional solenoid, adjusting piece, valve spool, valve body, valve sleeve, spring, valve spool travel stop, valve spool travel stop. Figure 7a shows the schematic diagram of the structure of a conventional two-stage hydraulic control valve. As can be seen from the diagram, the hydraulic control valve has damping holes in the auxiliary piston, which create a pressure difference on the upper and lower sides of the main spool, thereby controlling the operation of the main valve. Due to the low viscosity of water, applying such a structure directly to hydraulic control valves results in high leakage rates from the valves, as well as unstable pressure differences on both the upper and lower sides of the main spool. The Department of Mechanical Control Engineering at Tokyo Institute of Technology in Japan has developed a high-speed solenoid valve operated by hydraulic pressure, which utilizes a new two-stage structure as shown in Figure 7b. This valve eliminates the auxiliary piston and uses the leakage volume between the main valve spool and the valve sleeve as the control flow for the pilot valve. Experiments show that the internal leakage of this valve is zero, and its steady-state flow characteristics are excellent. The switching time for opening and closing the valve is approximately 2 ms; the maximum flow rate is 9 L/min, and the maximum pressure is 14 MPa. By controlling this valve using PWM, it is possible to regulate the speed of the hydraulic motor. (3) Application of hydrostatic pressure technology: Water has poor lubricity, and the friction and wear between the valve core and the valve sleeve in water pressure control valves is particularly severe, which reduces the control accuracy of the valve. Currently, hydrostatic technology is used in some pressure proportional control valves and pressure servo control valves to support the valve spool, thereby improving the control accuracy of the valves. Figure 8 shows a schematic diagram of the hydrostatic support structure used in the hydraulic control valve, with water at a certain pressure supplied by a hydraulic pump. If an eccentricity downward as shown in the figure occurs between the valve core and the valve sleeve, the gap between the upper part of the valve core and the valve sleeve increases, resulting in a decrease in pressure ； The gap between the lower part of the valve core and the valve sleeve decreases, resulting in increased pressure; the pressure difference on the upper and lower sides generates an upward force that pushes the valve core toward the center. The use of hydrostatic pressure ensures that the space between the valve core and the valve sleeve is always filled with water, thereby separating the two friction surfaces, reducing the friction force during the movement of the valve core, and improving its movement accuracy. Furthermore, the hydraulic pressure control valves and static pressure support chambers mentioned in the literature are all connected to their respective valve chambers, which not only effectively reduces the flow loss of the valves but also enables the flow through the valve orifices to exert a stabilizing effect on the system. (a) Two-stage structure used in traditional hydraulic valves; (b) Two-stage structure used in new hydraulic valves. Figure 7 shows a comparison of the two two-stage structures. The pilot ball valve, the main valve spool, the main flow rate, spring leakage, pilot flow rate, auxiliary piston spring, pilot flow rate, main flow rate, main valve spool – the valve sleeve’s damping holes provide a certain degree of damping, thereby making the system more stable. (4) Application of multi-stage throttling structures: Multi-stage throttling structures are a type of design that can effectively reduce cavitation and erosion. Figure 9 shows two commonly used two-stage throttling structures. This structure consists of two throttle orifices connected in series, which share the pressure difference at the throttling section; as a result, the pressure difference at each orifice is reduced, thereby minimizing the occurrence of cavitation and erosion. A pilot-operated hydraulic relief valve developed by the **Key Laboratory of Fluid Power Transmission and Control at Zhejiang University employs this structure, and experiments have shown that it exhibits good cavitation resistance. Figure 9: Secondary throttling structure. In addition to the several typical structural designs mentioned above, there are other designs as well; for example, combined sealing rings can be used between the valve core and the valve sleeve for sealing, or the valve core can be given a special shape to reduce the occurrence of phenomena such as cavitation and erosion. Therefore, adopting a new structure can effectively improve the performance and service life of hydraulic control valves, and it is a necessary and effective approach for developing such valves. 4 Material selection for hydraulic control valves: Due to the high corrosivity of water, the materials used in traditional hydraulic control valves cannot be directly applied in the manufacture of hydraulic control valves. Common engineering materials used in such valves today include stainless steel, corrosion-resistant alloys, engineering ceramics, and engineering plastics. Among them, stainless steel has a relatively low price and can be used to manufacture valve bodies ； Corrosion-resistant alloys possess extremely high corrosion resistance, but they are expensive; they are generally used to manufacture key components of pressure control valves. Engineering ceramics possess high strength and hardness, as well as excellent corrosion and wear resistance, and can be used to manufacture components such as valve cores and valve sleeves. Engineering plastics have a very low coefficient of friction and excellent self-lubricating properties, and can also be used to manufacture hydraulic components. Furthermore, the use of some effective surface protection and modification techniques can also improve the wear resistance and corrosion resistance of water pressure control components. Currently, the ceramic coating process is widely used in hydraulic control valves. 5 Outlook: Hydraulic transmission technology uses water as the working medium, causes zero environmental pollution, and holds great potential for development. However, the development of hydraulic drive technology is relatively recent; its theory is not yet mature, and several key technologies need to be addressed urgently. At present, the performance of most hydraulic control valve products has not yet reached the level of similar hydraulic control valve products; they have low operating pressure and flow rates, poor control precision, a short service life, and unstable performance. Therefore, developing hydraulic control valves with excellent performance is key to the advancement of hydraulic drive technology. It is necessary to select materials with good corrosion resistance, design new structures, and employ advanced manufacturing processes to ensure precision in fabrication. At the same time, it is also necessary to reduce costs in the design, manufacturing, and distribution processes, as well as to lower the prices of pressure control valves, in order to facilitate the promotion and application of pressure-driven technology.