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Solenoids can be classified into three main categories based on their principle of operation: 1) Direct-acting solenoids: Principle: When electricity is applied, an electromagnetic force is generated by the coil, which lifts the sealing element away from the valve seat, thereby opening the valve; when the power is cut off, this electromagnetic force disappears, and the spring pushes the sealing element back against the valve seat, closing the valve. Feature: It can operate normally under vacuum, negative pressure, or zero pressure, but the diameter usually does not exceed 25mm. 2) Direct-acting solenoid valve: Principle: It operates based on a combination of direct-acting and pilot-operated mechanisms. When there is no pressure difference between the inlet and outlet, when electricity is applied, the electromagnetic force lifts the pilot valve and the main valve’s sealing element upward in sequence, thereby opening the valve. When the inlet and outlet reach the starting pressure difference, upon power being applied, the electromagnetic force acts on the pilot valve, causing the pressure in the lower chamber of the main valve to rise while the pressure in the upper chamber drops. This pressure difference then pushes the main valve upward. When power is cut off, the pilot valve uses the force of a spring or the pressure of the medium to push the sealing element downward, thereby closing the valve. Features: It can operate at zero pressure difference or in vacuum as well as under high pressure, but it requires a higher power level and must be installed horizontally. 3) Pilot-operated solenoid valve: Principle: When powered, electromagnetic force opens the pilot hole, causing the pressure in the upper chamber to drop rapidly. This creates a pressure difference with lower pressure below and higher pressure above the closing element, and the fluid pressure pushes the closing element upward, thus opening the valve. When power is cut off, the spring force closes the pilot hole, and the inlet pressure acts through the bypass hole, creating a pressure difference with lower pressure below and higher pressure above the closing element. The fluid pressure then pushes the closing element downward, closing the valve. Features: High upper limit for fluid pressure scale; can be installed in any configuration (customization required), but it is necessary to meet the fluid pressure difference requirements. 2. Based on differences in valve structure, materials, and operating principles, solenoid valves can be divided into six subcategories: direct-acting diaphragm structure, step-type diaphragm structure, pilot diaphragm structure, direct-acting piston structure, step-type direct-acting piston structure, and pilot piston structure. Points to consider when selecting solenoid valves I: Compatibility The fluid in the pipeline must be the same as the medium specified for the series type of solenoid valve being chosen. The temperature of the fluid must be lower than the rated temperature of the solenoid valve used. The solenoid valve is suitable for liquids with a viscosity generally below 20 CST; if the viscosity is above 20 CST, this should be specified. For operating pressure differences, when the maximum pressure difference in the pipeline is less than 0.04 MPa, direct-acting and step-direct-acting valves such as those of the ZS, 2W, ZQDF, ZCM series should be used; when the minimum operating pressure difference is greater than 0.04 MPa, pilot-operated (pressure-difference type) solenoid valves can be chosen. The maximum operating pressure difference should be below the maximum rated pressure of the solenoid valve. Solenoid valves generally operate in one direction, so it is necessary to pay attention to whether there is any reverse pressure difference, and if so, a check valve should be installed. When the fluid cleanliness is not high, a filter should be installed in front of the solenoid valve, as solenoid valves generally require a high level of cleanliness in the fluid passing through them. Pay attention to the flow aperture and receiving diameter; solenoid valves usually only need to be controlled in two positions – on and off. If conditions permit, install a bypass pipe to facilitate maintenance; when water hammer occurs, it is necessary to adjust the opening and closing timing of the solenoid valve. Pay attention to the impact of ambient temperature on solenoid valves. The supply current and power consumption should be selected based on the output capacity; the supply voltage is generally allowed to vary within ±10%. It is important to note that the VA value is high during AC startup. II. Reliability Solenoid valves are divided into two types: normally closed and normally open. The normally closed type is usually chosen, as it opens when power is applied and closes when power is cut off; however, the normally open type should be used when the valve needs to remain open for a long time and close for a short period of time. Life expectancy tests are usually classified as part of type testing. To be precise, China does not yet have any specific standards for solenoid valves; therefore, care must be taken when selecting manufacturers of such valves. When the actuation time is very short and the frequency is high, a direct-acting type is usually chosen; for large diameters, the quick series is used. III. Safety Normally, solenoid valves are not waterproof; in situations where water resistance is required, please choose a waterproof version, which can be customized by the factory. The maximum rated pressure of the solenoid valve must exceed the highest pressure in the pipeline; otherwise, its service life will be reduced or other problems may occur. For corrosive liquids, a fully stainless-steel model should be chosen; for highly corrosive fluids, a Plastic King solenoid valve is recommended. In explosive environments, it is necessary to use appropriate explosion-proof products. IV. Economy First, the four principles of safety, reliability, suitability, and economy should be followed in sequence; thereafter, the selection is made based on six aspects of operational conditions on-site (namely, pipeline parameters, fluid parameters, pressure parameters, electrical parameters, operation methods, and special requirements). Selection criteria: 1. The solenoid valve is selected based on pipeline parameters: diameter standard (i.e., DN) and connection type. 1. The diameter (DN) is determined according to the inner diameter of the pipelines in place or the required flow rate. 2. For interface types, flange interfaces are usually required for sizes >DN50, while those ≤DN50 can be chosen freely according to the user’s requirements. II. Selection of solenoid valves based on fluid parameters: material and temperature range 1. Corrosive fluids: Corrosion-resistant solenoid valves made of all-stainless steel are recommended; for food-grade ultra-pure fluids, solenoid valves made of food-grade stainless steel materials should be used. 2. High-temperature fluids: Electromagnetic valves made from high-temperature resistant electrical materials and sealing materials should be selected, and those with a piston-type structure are preferred. 3. Fluid state: It can be gaseous, liquid, or in a mixed state; in particular, this distinction must be made when placing orders for pipes with a diameter larger than DN25. 4. Fluid viscosity: Generally, any value below 50 cSt can be chosen; if it exceeds this value, a high-viscosity solenoid valve should be used. III. Selection of solenoid valves based on pressure parameters: Principles and structural types 1. Nominal pressure: This parameter has the same meaning as that of other common valves; it is determined based on the nominal pressure of the pipeline. 2. Operating pressure: If the operating pressure is low, a direct-acting or step-by-step direct-acting principle must be used; when the minimum operating pressure difference is above 0.04 Mpa, direct-acting, step-by-step direct-acting, and pilot-operated types can all be employed. IV. Electrical selection: For voltage standards, AC220V and DC24 are preferable as they are more convenient to use. V. Selection based on the duration of continuous operation: normally closed, normally open, or capable of remaining powered on. If the time it is in the open state is short, or if there are not many cycles of opening and closing, then a normally closed type should be chosen. However, for some applications related to safety maintenance, such as flame monitoring in furnaces and kilns, normally open types cannot be used; instead, types that can remain powered on for an extended period should be chosen. VI. Select auxiliary functions based on environmental requirements: explosion-proof, check valve, manual, fog-resistant, water spray-resistant, submersible. 1. In explosive environments, solenoid valves with the appropriate explosion-proof rating must be used. 2. When there is backflow of fluid in the pipeline, solenoid valves with check valve functionality can be chosen. 3. When on-site manual operation of the solenoid valve is required, a solenoid valve with a manual function can be selected. 4. For outdoor installations or environments with high dust levels, waterproof and dust-proof models (with a protection rating of IP54 or higher) should be selected. 5. For fountains, it is necessary to use submersible solenoid valves (with a protection rating of IP68 or higher). 6. When the solenoid valve needs to remain open for an extended period, and the time it is closed is shorter than the time it is open, a normally open type should be used.