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Paper Abstract: As an actuator for automated instruments, solenoid valves have seen a sharp increase in usage in recent years. This article analyzes the main characteristics of solenoid valves, and provides a brief overview of the development trends of mature domestic technologies from an application perspective. It examines advancements in the areas of simplification, intelligence, versatility, and specialization, and also discusses briefly the social factors that influence the development of solenoid valve technology. I. Main features of solenoid valves: (1) External leakage is prevented, internal leakage is easy to control, ensuring safe use. Internal and external leaks are factors that pose a threat to safety. Other automatic control valves typically extend a valve stem, with electric, pneumatic, or hydraulic actuators controlling the rotation or movement of the valve core. All of this is necessary to address the problem of external leakage in the dynamic sealing of long-term operating valve stems ； Only the solenoid valve operates by using electromagnetic force on an iron core sealed inside a magnetic insulation sleeve; there is no moving seal, so external leaks can be easily prevented. Torque control of electric valves is difficult; internal leakage can occur, and in some cases the head of the valve stem may even break off ； The design of solenoid valves makes it easy to control internal leakage until it is reduced to zero. Therefore, solenoid valves are particularly safe to use, especially for corrosive, toxic, or high/low temperature media. (2) The system is simple, easy to connect to a computer, and inexpensive. Solenoid valves have a simple structure and low cost; they are also easier to install and maintain compared to other types of actuators such as control valves. More significantly, the self-control system formed is much simpler, and its cost is much lower. Since the solenoid valve is controlled by switch signals, it is very convenient to connect it to an industrial computer. In today’s era of widespread computer use and significantly reduced prices, the advantages of solenoid valves become even more apparent. (3) Fast action, low power consumption, and compact design. The response time of solenoid valves can be as short as a few milliseconds, and even pilot-operated solenoid valves can have their response time kept within dozens of milliseconds. Due to its self-contained circuit, it responds more sensitively compared to other automatic control valves. Well-designed solenoid valves have very low power consumption, making them energy-efficient products ； It is also possible to have the valve position automatically maintained simply by triggering the action, with no power consumption at all in normal operation. Solenoid valves have a small size, which saves space and makes them lightweight and attractive in appearance. (4) The adjustment accuracy is limited, and the applicable media are limited. Solenoid valves usually have only two states: on and off; the valve core can only be in two extreme positions and cannot be adjusted continuously. (There are many new ideas aimed at overcoming this limitation, but they are still in the experimental stage), so the accuracy of adjustment remains somewhat limited. Solenoid valves have high requirements regarding the cleanliness of the medium; media containing particles are not suitable, and any impurities must be filtered out first. Additionally, viscous media are not suitable, and the viscosity range of media suitable for specific products is relatively narrow. (5) Various models available, with wide applications. Although solenoid valves have certain inherent limitations, their advantages remain very significant; as a result, they are designed in a variety of forms to meet different needs, making them extremely versatile in use. Advances in solenoid valve technology also focus on how to overcome inherent shortcomings and how to make better use of their natural advantages. II. An Introduction to the Development of Solenoid Valve Technology 2.1. Development in the Direction of Simplification: Perfection lies in simplicity; only the simple can endure over time. This is also the perpetual pursuit of scientists and engineers. (1) Simplified control circuit: In the past, actuators largely used pneumatic and electric control circuits, which increased the complexity of the system; whereas pilot-operated solenoid valves create a control circuit within the valve using the working medium itself, resulting in a very simple structure. In the past, various technical parameters of solenoid valves both domestically and internationally were limited; nowadays, the diameter of solenoid valves in China has been increased to 300 mm ； The medium temperature ranges from as low as –200 °C to as high as 450 °C ℃ ； Operating pressure ranges from vacuum to 25 MPa. The action time ranges from over ten seconds to a few milliseconds. The recent advancements in these technologies have made it possible to completely replace the traditional, bulky and expensive two-position control quick shut-off valves and pneumatic on/off valves. Electric on/off valves can also partially replace the pneumatic and electric control valves used for continuous regulation. (How to better meet the requirements for adjustment accuracy will be discussed further below.) In foreign countries, industries such as textiles, light industry, and urban construction have widely adopted solenoid valves, while industries such as metallurgy and chemicals are taking the lead in using an increasing number of solenoid valves in their auxiliary systems. More and more domestic professionals in automatic control are paying attention to solenoid valves. (2) Simplify the piping system: When automatic control valves are in use, some auxiliary valves and pipe fittings are also required in the piping system. For example, the isolation bypass shown in Figure 1 represents a typical installation method, which requires three manual valves; among them, manual valve 1 is the bypass valve, used as a backup. Manual valves 2 and 3 are isolation valves, ensuring online maintenance of automatic control valve 5. Of course, two tees 4 and union joints 6 are also required. This type of piping system takes up a lot of space, is time-consuming to install, and is prone to leaks. The ZDF series of multi-functional solenoid valves cleverly eliminates the need for these additional accessories while still retaining the function of isolation bypass, and for this reason it was awarded the Golden Prize for New Technologies at the Geneva International Exhibition. A filter must be installed in front of the automatic control valve. When multiple automatic control valves are used together, check valves are often required to prevent interference between the pipes. Now, one-way solenoid valves, combined solenoid valves, and solenoid valves with filtration have all played a role in simplifying piping systems. (The company has also introduced a backflow filter valve that combines switching and filtering functions, and allows the filter element to be cleaned without removing the valve; it has obtained invention patents in both China and the United States. ) (3) Simplify the valve structure and manufacturing process. Solenoid valves are automatic control valves with simple principles and structures, but further simplification is still possible. The solenoid valves used in washing machines and refrigerators in the home appliance industry have component structures and manufacturing processes that are suitable for mass production. The coil components of ordinary solenoids are now enclosed in plastic, which reduces the risk of wire breaks and also makes it easier to meet requirements for protection against water and explosions. Components such as the valve body and valve cover are manufactured using precision casting, forging, and other processes, and engineering plastic solenoid valves are now available in bulk. Domestic explosion-proof solenoid valves are no longer limited to the flameproof type; there are also pot-sealed and intrinsically safe types. The structure and manufacturing process of solenoids for high pressure and high temperature conditions have also been simplified. 2.2 Development in the direction of intelligence: This primarily refers to how solenoid valves can work more effectively in conjunction with intelligent instruments, thereby improving the control accuracy and reliability of the system. It is generally believed that on-off control is difficult to achieve high precision; however, in many cases, by using appropriate methods, a fairly high level of precision can be attained, thereby **saving costs. It is a combination of solenoid valves of different sizes. The fueling units that are used on a large scale have a fuel measurement accuracy of up to 0.02%, and all of them utilize dual solenoid valves. The large valve opens, allowing the refueling to be nearly completed in a short period of time ； Then the large valve closes, with the small valve providing compensation to ensure flow accuracy. This technology has been in use abroad for many years, and the products are manufactured by world-renowned specialist companies in solenoid valves such as ASCO and Honeywell. Our company’s dual-electromagnetic valves now meet the standards of imported products, and they also possess unique performance features. Incidentally, for high-flow metering in domestic oil depots, complex and less stable multi-stage shut-off valves of foreign origin are still being introduced or replicated; such products are not manufactured by world-renowned specialty companies, as the market demand is not high enough to arouse their interest. In fact, our oil depots use high-flow dual combined valves produced domestically, which yield much better results. Dual combination valves are also commonly used to control parameters such as temperature, pressure, and liquid level. The large valve ensures the base amount, while the small valve provides the compensatory amount. In systems with relatively stable parameters, this on-off control can achieve quite high precision. In many systems where various parameters change significantly, the rate of these changes is still subject to certain limitations; for example, temperature control systems are still affected by thermal inertia. Therefore, a properly utilized dual combination valve can significantly reduce costs compared to control valves, while still meeting the control requirements. (2) Three-position solenoid valves and digital valves: The spool of a three-position solenoid valve has three operating positions; when no power is applied, it remains in a slightly open state, allowing the valve to provide an initial flow rate ； When an electrical signal is applied, the solenoid valve opens fully, allowing a high flow rate ； When another electrical signal is applied, the valve closes. The valve is also equipped with a manual mechanism, allowing it to remain closed for extended periods without consuming power. The three-position solenoid valve can be regarded as a more compact dual solenoid valve that enables easy three-position control, and it has found wide application. For the ZDF multi-functional solenoid valve, an additional approach is to use n solenoid valves of sizes in a certain proportion to generate 2n different flow rates; similar to digital circuits, such combined valves are referred to as digital valves. In fact, two double or three-way valves of different sizes can generate 24=16 or 32=9 different flow rates, achieving a very high level of precision. In principle, it has advantages when used in conjunction with industrial control computers, but since there has not been a large demand in various application areas, this technology is still in the experimental stage. (3) Self-holding solenoid valve: A self-holding solenoid valve requires only an instantaneous power supply to complete the opening and closing action of the valve, and no electricity is needed to maintain the position of the valve core. Its advantage lies in energy savings, especially when powered by batteries; moreover, since temperature rise need not be considered, the coil has a long lifespan, offering high safety in applications involving extreme temperatures or explosion-proof environments. The common types are mechanical retention and permanent magnet retention, which are further divided into dual-coil and single-coil versions; the single-coil magnetic retention structure is the simplest. It changes the two states of the corresponding valve by altering the polarity of the DC power supply. When installed separately, this solenoid valve requires a changeover switch for external wiring, but it is very convenient when used in conjunction with smart meters. The solenoid valves used in IC card pre-paid gas meters and water meters are one of the key factors determining the success of these meters, and the self-holding technology is an important element in ensuring the proper functioning of these solenoid valves. Self-holding solenoid valves in China have become increasingly mature, and our company has supplied them to numerous manufacturers of smart meters. 2.3 Development in the direction of generalization: Solenoid valves are widely used, but their application scenarios vary greatly. Efforts have been made in recent years to enhance their versatility and reduce costs related to manufacturing, procurement, storage, installation, and maintenance. (1) Adjustable response time: The time from the application of an electrical signal to the completion of the valve core’s movement is referred to as the response time. Different applications require different switching speeds, with four combinations: fast open and fast close – used in systems that need rapid response ； Fast open, slow close – used to prevent water hammer ； Slow start, fast stop – used for gradual startup and immediate shutdown ； Open and close slowly – to ensure a smooth transition. The ZDF multi-functional solenoid valve can be conveniently adjusted to any state, offering great flexibility in application. (2) Scope of application for extended media. Electromagnetic valves differ from other automatic control valves in that they have a relatively narrow range of acceptable medium viscosities; therefore, the structural parameters for pilot-operated electromagnetic valves vary depending on the medium such as air, water, steam, or oil ； Generally speaking, it cannot be used universally. The ZDF multi-functional solenoid valve uses just one small screw, yet it enables use with air, water, and oil; moreover, it widens the applicable range of oil viscosities, which is why it is very popular. (3) Adjustable opening and manual function – When using solenoid valves in kilns, it is often desired that they also have a manual function as a backup in case of power outages or failures. Generally, manual devices can only be used to open or close the valve manually; those that have both functions are referred to as fully manual. Manual devices can also preset the valve opening degree; generally, manual control allows only presetting of the valve’s maximum or minimum opening degree, while full manual control enables separate presetting of both the maximum and minimum opening degrees. The ZDF multi-functional solenoid valve is of the completely manual type. Other models of solenoid valves can be equipped with simple conventional manual controls as needed. 2.4 Development in the direction of specialization: In recent years, the sales volume of solenoid valves has surpassed that of control valves, mainly due to the sharp increase in demand for various specialized solenoid valves. The following is an introduction by medium type: (1) Gas solenoid valves. Due to the adjustment of China’s fuel structure, the proportion of gaseous fuels is increasing continuously, which in turn leads to a growing demand for gas solenoid valves. There are various types of gas, including natural gas, industrial gas, city gas, and liquefied petroleum gas; they contain different levels of impurities and exhibit varying degrees of corrosiveness, which in turn results in different requirements for valves. Moreover, the pressure levels vary greatly: natural gas is compressed to as high as 21–25 Mpa, while the pressure in gas appliances is as low as 2 Kpa. Our country already has mature products in each of these areas. Our company’s CE series gas solenoid valves have been exported to the EU. It is worth mentioning that dual-fuel vehicles are being increasingly promoted in China as environmental regulations become more stringent. Twelve cities have modified tens of thousands of vehicles, and currently imported fuel supply systems are still predominantly used, with the solenoid valves also being imported as part of these systems. In fact, these solenoid valves are not produced by specialized foreign companies in this field either, and the results of their performance tests do not all meet the relevant technical standards of China’s gas industry. Our company now has more reliable and cost-effective dedicated solenoid valves, which are gradually replacing imported solenoid valves. (2) Steam solenoid valve: The primary challenge for a steam solenoid valve is to ensure high-temperature resistance; therefore, the sealing materials, as well as the materials used for conductivity and magnetism, must be carefully selected. The corrosion of the steam pipes is quite severe, and it needs to be taken seriously. More importantly, there is a lot of condensed water in the steam pipes after work at the factory; the solenoid valves need to function reliably whether gas or liquid fluid is passing through them, and their factory testing involves multiple strict procedures. Our company’s ZQDF-3 type solenoid valve was designated as a **-grade new product in 1992; it has withstood the test of time among users, and in recent years an improved CC series of steam solenoid valves has been introduced. (3) Water solenoid valves: Water solenoid valves are generally considered to be the simplest type. In reality, water comes in various forms such as tap water, industrial pure water, industrial wastewater, irrigation water, and seawater; each of these has different levels of corrosiveness and purity, so different solenoid valves should be used for each case. Although tap valves are the most common, different applications require varying performance standards from these valves. For example, in fire-fighting solenoid valves, operational reliability is of utmost importance, as the consequences of a failure would be severe. Even if a regular manual valve is not opened or closed for a long time, it is still difficult to open quickly. In recent years, high-rise buildings have sprung up like mushrooms after rain, and fire protection solenoid valve technology in our country has also matured as a result. For example, water treatment plants need to use solenoid valves in siphon pipelines; such valves must function reliably both under pressure and under vacuum conditions, with no internal leakage, and they should still operate smoothly even when the water quality is not high. Our solenoid valves designed specifically for water treatment plants have been used in hundreds of water treatment plants across the country. It is the only solenoid valve recommended by the China Water Association. In addition, sprinkler solenoid valves have also been gradually adopted with the development of China’s agriculture and horticulture industries. (4) Oil solenoid valves: There are many types of oils, and seal technology is one of the key factors. Oil-resistant rubber is merely a category name; different oils, or even the same oil with varying impurities, require the use of different types of oil-resistant rubber. For example, a vehicle that can handle leaded gasoline may not be able to handle unleaded gasoline. In recent years, as the sales of dedicated solenoid valves have increased, the lifespan of specialized oil-resistant rubber has also improved. The viscosity of oils varies greatly, requiring solenoid valves with different structures to be used. Oils with high viscosity often flow at high temperatures as well, so solenoid valves also need to address the issue of heat resistance. Fuel also needs to meet explosion-proof requirements. In recent years, our company has successively introduced various specialized solenoid valves for gasoline, diesel, heavy oil, etc., which have been widely used. (5) Solenoid valve for air conditioning: Air conditioning operates in heating and cooling modes. There have been significant changes in refrigerants in recent years: ammonia and Freons are gradually being reduced, while new types of refrigerants such as lithium bromide are increasing in use. Issues related to sealing, corrosion, and lifespan require careful selection of materials, proper design, and advanced manufacturing processes. It is worth noting that air conditioners primarily control temperature and humidity by adjusting the air volume, and continuous adjustment of valve flow is not entirely necessary. Valveless air conditioning systems are also in use now, but they have been criticized for issues such as dew formation and dripping. Therefore, solenoid valves with a simple structure and low cost are the best choice. Under the influence of foreign traditions, electric valves with complex structures and difficult-to-control torque are still widely used in China. During the maintenance work on the Memorial Hall in 1997, our company’s air-conditioning solenoid valves were used for the air-conditioning system, which ensured such high precision in preserving the remains, achieving satisfactory results. **Not long ago, the director of the Management Office of the Memorial Hall in the Central Office personally delivered the certificate of honor to our company; in fact, it is the courage and wisdom of the engineering designers who deserve to be recognized. III. Social progress related to the development of solenoid valve technology. In the past, scientific and technical papers could only focus on technology; discussing social management in such papers was often seen as ridiculous. It is now a common understanding that technological development relies on and drives social progress. The author is engaged in both technological inventions and business management, and it seems necessary to discuss some related topics. Solenoid valves are important industrial products that are directly related to safe production in enterprises. In China, serious accidents such as boiler explosions and failed fire suppression systems have occurred due to the poor quality of solenoid valves. The former Ministry of Machinery had already listed solenoid valves as products whose production and sale could only be permitted after obtaining a license. There are only about ten licensed companies in the country. Compared to large professional companies abroad, there is a significant gap in terms of technical and economic capabilities. After the reform and opening up, private technology enterprises emerged; they hold patents approved in the United States, Japan, and Europe, and their products have gradually entered developed markets. Their economic strength has also been growing steadily, surpassing that of similar small and medium-sized enterprises abroad. However, there are dozens of unlicensed manufacturing enterprises, and fake and substandard products have led many designers to mistakenly believe that the quality of electromagnetic valves in China is not up to standard. Advances in the industry in recent years have brought about the following positive changes. (1) The mentality of worshipping foreign powers and showing subservience to them remains severe, while national confidence is continuously increasing. The introduction of complete sets of equipment in the 1980s brought in many solenoid valves. In the 1990s, well-known foreign companies established offices in China, and the use of imported solenoid valves once became a trend in automation projects. The designers knew that imported products were clearly at a disadvantage in terms of price, delivery time, after-sales service, etc., but they still chose them in the end. Some are certainly necessary, while for others ordinary products from smaller companies in places such as Taiwan, South Korea, and Italy are chosen. Solenoid valves are nothing like space shuttles; after all, they are just small products, and the Chinese certainly have the capability to gain an advantage in this market. In recent years, our company’s solenoid valves have been successfully awarded contracts for use at satellite launch sites, the Daya Bay Nuclear Power Plant, and the maintenance projects at Shanghai Baosteel, reflecting the growing confidence of our nation. As our company’s products are exported to Europe, the French Committee for the Supervision and Evaluation of Technology and Quality has also listed our company and its products as those recommended for the EU. (2) Unfair competition is extremely severe, and the awareness of the rule of law is also on the rise. Today, unfair competition in the solenoid valve industry is extremely severe; unlicensed production and sales, patent counterfeiting, trademark counterfeiting, fake factory names, false advertising, and counterfeit products are everywhere, causing great harm to consumers. There were cases where immobile solenoid valves requested replacement from our company due to the (YJ) trademark. Upon seeing that its connection flange is 2\", while the actual diameter is less than 30 mm, it is clear that it is a fake and inferior product. Now, designers indicate the certified manufacturing units on the drawings, and purchasers also know to contact the certified units and their authorized dealers directly. In recent years, as the IS09000 quality system certification has been gradually implemented in China, people have increasingly tended to choose certified enterprises. All of this shows that the concept of the rule of law is continuously strengthening. (3) The selection of solenoid valves is complex, requiring more dedicated technical personnel. Solenoid valves have never been included in textbooks on automatic control; many people think that selecting them is a simple task, but only experienced designers understand that this small device requires careful consideration. There are dozens of types of solenoid valves classified by their structure and principle, and when various technical parameters are taken into account, the total number of possible combinations reaches tens of thousands. Design and procurement personnel have gradually learned to make the right choices amid the abundance of options. In addition to collecting and studying sample data, they also consult directly with manufacturing companies; for large-scale or important orders, they personally assess the economic and technical capabilities as well as the technical assurance capabilities of the production facilities, and visit their existing customers to gain firsthand insight into how the products are used in practice. Now everyone has to do cost calculations, but this is by no means a simple comparison of quotes. In addition to properties such as safety, suitability, durability, and aesthetics that affect the price, it is also related to the performance and cost of the entire automatic control system as well as the piping system; this has become a comprehensive field of study. Our company has a technical services department that attaches great importance to services such as pre-sales consultation, design of shapes, and pipeline system design. The designers, purchasers, and managers take great responsibility in making careful selections, and their dedication drives the continuous advancement of solenoid valve technology. China’s solenoid valves have developed by learning from advanced foreign technologies and through continuous innovation of its own. Its development prompts people to continuously update their concepts. At the same time, only when social laws and regulations are more robust and the overall quality of the population improves can solenoid valve technology develop more rapidly and effectively.