Development status of ultra-high pressure technology
In some specific ultra-high pressure occasions, such as hydraulic support, hydraulic tools, powder metallurgy, ultra-high pressure jet, metal extrusion and ultra-high pressure pressure, blasting test, etc., the working pressure of the hydraulic system is usually around 700bar, or even higher pressure. At present, compared with general hydraulic technology, ultra-high pressure hydraulic technology has the following main characteristics:
1) Ultra-high pressure and small flow rate. When the hydraulic system operates at a certain power, due to the high working pressure required by the ultra-high pressure hydraulic system, the system flow under high pressure conditions is very small, generally in 1L/min.
2) There are special requirements for hydraulic media. Because the viscosity and elastic modulus of the hydraulic system are greatly affected by the system pressure and temperature, the ultra-high pressure hydraulic system often requires a specific hydraulic medium. Generally speaking, the viscosity of hydraulic medium in hydraulic system increases with the increase of pressure, while the fluidity decreases. The viscosity decreases with the increase of temperature, and it is easy to cause the system oil leakage. Therefore, in the ultra-high pressure hydraulic system for different applications, there are often specific special hydraulic media. For example, emulsion is commonly used in hydraulic supports, and water medium is used in some giant die forging hydraulic presses.
3) The sealing requirements are very strict. In the ultra-high pressure hydraulic application environment, sealing is a very important link. The pressure of the ultra-high pressure hydraulic system is very high, and even a small leakage will produce local high temperature because of its high energy, and it is easy to break down the seal. At the same time, the same sealing gap in the ultra-high pressure environment leakage is much larger than the ordinary pressure, and the ultra-high pressure hydraulic system flow is generally very small, so it will have a great impact on the normal operation of the hydraulic system. At present, the main methods to improve the sealing performance are the selection of suitable materials, the design of suitable sealing structure and manufacturing technology.
4) More stringent requirements for ultra-high voltage components. The strength and sealing problems brought about by the ultra-high pressure hydraulic system make the ultra-high pressure hydraulic components have more stringent requirements, and the choice of structural forms is much less than ordinary pressure. Plunger pumps and superchargers are commonly used in the booster elements of ultra-high pressure hydraulic systems. Because the plunger pump has high volumetric efficiency, it can be well adapted to the sealing under ultra-high pressure. The supercharger can generate ultra-high pressure through the relationship of area ratio under lower system pressure, and different ultra-high pressure hydraulic media can also be used. All kinds of valves in the ultra-high pressure hydraulic system adopt cone valve and ball valve structure to reduce the difficulty of processing and reduce the leakage.
However, with the development of hydraulic technology, ultra-high pressure has shown great advantages in many heavy industrial occasions. Mainly because, with the improvement of system pressure, the host structure is greatly simplified, the weight is greatly reduced, the unit weight output ratio is increased, the manufacturing cost is reduced, the energy consumption in manufacturing is reduced, the industrial emission pollution is reduced, and it is easy to achieve green production. With the growth of demand for important defense industry equipment such as giant die forging hydraulic press, rubber bag hydraulic forming machine, synthetic diamond synthetic press, deep-sea oil and gas development equipment, UPLFHE demand is increasing. Therefore, at present, the ultra-high pressure technology is changing from the ultra-high pressure small flow to the ultra-high pressure large flow direction, but also from the switch type components to high-performance proportional components.
In China, more mature ultra-high pressure hydraulic systems are mainly concentrated in hydraulic tools, hydraulic supports and hydraulic testing industries. Ultra-high pressure hydraulic tools themselves are small in size, light in weight and easy to carry, but the input force or torque can be large, and the hydraulic system is simple, the research and development cost is low, and the sales volume of ultra-high pressure hydraulic tools is large, so most domestic manufacturers are willing to produce. The ultra-high pressure and large flow detection system designed for hydraulic support hydraulic parts test testing in 2016, its hydraulic system uses a double-acting supercharger as a booster element,which can produce 1000bar,0.62L/ time and 750bar, 1.57L/ time ultra-high pressure hydraulic source. At the same time, emulsion and No. 63 anti-wear hydraulic oil are used as hydraulic medium.Then through the external large flow emulsion source, it can meet the test work of most hydraulic parts used in hydraulic support. Hu Yang of Yanshan University reformed a hydraulic cylinder test bench in 2014 and successfully raised the test pressure to 520bar for hydraulic cylinder testing of hydraulic support. It also uses a hydraulic supercharger to obtain an ultra-high pressure hydraulic source. Zhao Dechun et al.used a supercharger with a large booster ratio to design an ultra-high pressure hydraulic system with a pressure of 2000bar for the blasting test of aircraft hydraulic accessories and ducts. Zhang Xinwu also demonstrated through simulation that the hydraulic supercharger was used to generate ultra-high pressure for the bursting test of the pressure vessel, and verified the feasibility of applying automatic control technology to the ultra-high pressure hydraulic system. Luo Meiwei of Shandong University designed a test bench for testing ultra-high pressure hydraulic components in 2005,the test pressure can reach 700bar, and the pump performance test, cylinder performance test, relief valve and reversing valve partial performance test can be completed. However, the design and introduction of the ultra-high pressure hydraulic system is less in the article, focusing on the design of the test bench control system, but it can be seen that the ultra-high pressure proportional relief valve used in the system is the PMVPS series of HAWE company, and the system flow will not be too large.
In terms of ultra-high pressure oil and seal, Shang Jianguo made a detailed introduction to ultra-high pressure static and dynamic seals. Ultrahigh pressure static seals are mostly metal seals, and O-shaped seals with high hardness of Shore can also be used. The dynamic seal can be made of clearance seal, rubber seal ring and special self-tightening seal. Tian Xiaojing et al from Dalian Jiaotong University carried out numerical simulation on the piston clearance seal of the ultra-high pressure hydraulic cylinder of the ultra-high pressure bolt tensioner. The results show that it is a very good strategy to adopt the clearance seal in the ultra-high pressure hydraulic cylinder that needs stable operation at lowspeed. However, the width of the sealing clearance is not the smaller the better, and there should bea reasonable clearance value. Zhu Renzong of Xi 'an Jiaotong University has made a certain analysis of the influence of hydraulic oil viscosity on ultra-high pressure hydraulic system, which has some practical significance for the selection of hydraulic oil type in ultra-high pressure hydraulic system.
Foreign hydraulic industry has developed earlier, especially in some developed countries such as Europe and the United States, hydraulic technology has been in a leading position. Pressure in 315bar hydraulic component technology has been widely used, and local use of ultra-high pressure technology, pressure can reach 420bar, this technology has also been quite mature. With the development of market demand, foreign hydraulic technology is also developing, in addition to the previous accumulation of hydraulic technology development, research and development strength, there are all kinds of hydraulic simulation and calculation software, which provides a strong guarantee for the research and development of ultra-high pressure components. With the growth of global demand for giant die forging hydraulic press, some hydraulic manufacturers in Europe and the United States specially developed supporting hydraulic ultra-high pressure technology. All kinds of ultra-high pressure hydraulic components, pressure levels from small to large, flow specifications from small to large, have a fairly comprehensive ultra-high pressure series. It can be said that foreign ultra-high pressure hydraulic technology has become increasingly mature, but it is mainly monopolized by a few manufacturers. Because of this, foreign scholars have less research on ultra-high pressure hydraulic components, which should be mostly independent research and development by enterprises, and the technology is confidential. In the academic papers that have been published, there are only a few basic studies on the characteristics of ultra-high pressure hydraulic fluids. Karjalainen et al. began to measure the properties of various types of hydraulic oils in 2006. They designed a test bench with a static pressure of up to 1500bar, and performed sound velocity, density, and Young's modulus equivalents at different pressures and temperatures. The results show that when the pressure reaches 500bar, the viscosity of the oil increases more than twice, and the Young's modulus also increases greatly. In 2009, he also summarized the relationship between oil characteristics and pressure through experiments. Density, Young's modulus, sound velocity, etc., can be fitted and estimated by quadratic polynomial, and modified the Young's modulus formula of adiabatic process and isothermal process, and also gave the formula of density and kinematic viscosity about pressure and temperature.