Common knowledge on the selection of solenoid valves and their structural principles
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Basic knowledge for selecting solenoid valves: 1. Compatibility – The fluid in the pipeline must be the same as the medium specified for the selected solenoid valve model. The temperature of the fluid must be lower than the rated temperature of the solenoid valve used. Solenoid valves are suitable for liquids with a viscosity generally below 20 CST; if the viscosity is higher than 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 types such as the ZS, 2W, ZQDF, ZCM series should be selected ； When the minimum operating pressure difference is greater than 0.04 MPa, a pilot-operated (pressure-difference type) solenoid valve can be used ； The maximum operating pressure difference should be less than the maximum rated pressure of the solenoid valve ； Generally, solenoid valves operate in one direction only; therefore, it is necessary to pay attention to any back pressure difference, and a check valve should be installed if needed. When the fluid cleanliness is not high, a filter should be installed in front of the solenoid valve, as solenoid valves generally require the medium to have a high level of cleanliness. Pay attention to the flow orifice diameter and the pipe connection diameter ； Solenoid valves are generally controlled in only two states: on and off ； If conditions permit, install a bypass pipe to facilitate maintenance ； When water hammer occurs, it is necessary to customize the adjustment of the solenoid valve’s opening and closing time. Pay attention to the effect of ambient temperature on solenoid valves. The supply current and power consumption should be selected based on the output capacity; the supply voltage generally allows a variation of around ±10%. It is important to note that the VA value is relatively high during AC startup. II. Reliability: Solenoid valves are divided into normally closed and normally open types ； Normally, a normally closed type is chosen; it opens when powered and closes when power is cut off ； But when the on time is long and the off time is short, a normally open type should be chosen. Life testing is generally considered part of the type testing procedures in factories. To be precise, there are no specific standards for solenoid valves in our country; therefore, care should be taken when selecting a manufacturer of solenoid valves. When the actuation time is short and the frequency is high, a direct-acting type is generally chosen; for large diameters, the fast series is used. III. Safety: Ordinary 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 shortened or other problems may occur. For corrosive liquids, a fully stainless-steel model should be chosen; for highly corrosive fluids, a SLF solenoid valve is recommended. Appropriate explosion-proof products must be used in explosive environments. IV. Cost-effectiveness: Many solenoid valves can be used interchangeably, but the most cost-effective option should be chosen, provided that it meets the above three requirements. Structural principle of solenoid valves I: Direct-acting solenoid valves come in two types: normally closed and normally open. In the normally closed type, it remains closed when power is off; when current flows through the coil, an electromagnetic force is generated that enables the moving core to overcome the spring force and attract the stationary core, thereby opening the valve and allowing the medium to pass through ； When the coil is de-energized, the electromagnetic force disappears; the moving core returns to its original position under the force of the spring, thereby closing the valve directly and preventing the flow of the medium. It has a simple structure and reliable operation, functioning properly under zero pressure difference and slight vacuum conditions. The normally open type is the exact opposite. Such as solenoid valves with a flow passage smaller than φ6. (Figure 1 shows a typical structural diagram.) II. Step-type direct-acting solenoid valve: This valve combines primary opening and secondary opening into one mechanism; the main valve and the pilot valve work together to use electromagnetic force and pressure difference to directly open the main valve port. When the coil is energized, an electromagnetic force is generated that causes the moving core and the stationary core to attract each other, thereby opening the pilot valve port. The pilot valve port is located on the main valve port, and the moving core is connected to the main valve core. At this point, the pressure in the upper chamber of the main valve is released through the pilot valve port. Under the combined effect of the pressure difference and the electromagnetic force, the main valve core moves upward, allowing the medium to flow through the main valve. When the coil is de-energized, the electromagnetic force disappears; the moving core then closes the pilot valve orifice under the effect of its own weight and the spring force. As a result, the medium enters the upper chamber of the main valve core through the balance hole, causing the pressure in that upper chamber to rise. Under the combined action of the spring returning to its original position and this increased pressure, the main valve closes, and the flow of the medium is stopped. It has a reasonable structure, reliable operation, and works reliably even at zero pressure difference. Such as: ZQDF, ZS, 2W, etc. (Figure 2 is a typical structural diagram.) III. Indirect pilot-operated solenoid valve: This series of solenoid valves consists of a pilot valve and a main spool connected to form a channel assembly ； The normally closed type is in a closed state when not powered. When the coil is energized, the magnetic force generated causes the moving core and the stationary core to attract each other, opening the pilot valve port and allowing the medium to flow toward the outlet. At this point, the pressure in the upper chamber of the main valve core decreases, falling below the pressure on the inlet side; this pressure difference overcomes the resistance of the spring, causing the main valve core to move upward and thus opening the main valve port, enabling the medium to flow through. When the coil is de-energized, the magnetic force disappears, and the moving core returns to its original position under the action of the spring force, thereby closing the pilot port. At this point, fluid flows in through the balance hole, causing the pressure in the upper chamber of the main valve core to increase; under the influence of the spring force, the main valve core moves downward and closes the main valve port. The principle of normally open is exactly the opposite. Such as: SLA, DF (diameter 15 mm and above), ZCZ, etc. (Figure 3 is a typical structure diagram)
Tracing the development history of solenoid valves, to date, solenoid valves worldwide can be divided into three main categories based on their principle: direct-acting type and pilot-operated type. Further, due to differences in valve disc structure, materials, and principles, they are subdivided into six further categories: direct-acting diaphragm structure, step-type diaphragm structure, pilot diaphragm structure, direct-acting piston structure, step-direct-acting piston structure, and pilot piston structure. Principle of direct-acting solenoid valve: When electricity is applied, the electromagnetic coil generates an electromagnetic force that lifts the closing element away from the valve seat, thereby opening the valve ； When power is cut off, the electromagnetic force disappears, and the spring presses the closing element against the valve seat, causing the valve to close. Features: It can operate normally under vacuum, negative pressure, and zero pressure, but the diameter generally does not exceed 25 mm. Principle of the distributed direct-acting solenoid valve: It operates on a combination of direct-acting and pilot-operated principles. 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 closing element upward one after another, thereby opening the valve. When the pressure difference between the inlet and outlet reaches the starting value, upon power application, the electromagnetic force acts on the pilot valve; as a result, the pressure in the lower chamber of the main valve rises while the pressure in the upper chamber drops, and this pressure difference is used to push the main valve upward ； During a power outage, the pilot valve uses spring force or medium pressure to push the closing element downward, thereby closing the valve. Features: It can operate reliably under zero pressure difference or in vacuum as well as at high pressures, but it requires a large power supply and must be installed horizontally. Principle of pilot-operated solenoid valve: When electricity is applied, electromagnetic force opens the pilot hole, causing the pressure in the upper chamber to drop rapidly. This creates a pressure difference with higher pressure below and lower pressure above the closing element; the fluid pressure then pushes the closing element upward, thereby opening the valve ； When power is lost, the spring force closes the pilot hole; the inlet pressure rapidly creates a pressure difference around the valve closing element, with lower pressure at the bottom and higher pressure at the top. The fluid pressure then pushes the closing element downward, thereby closing the valve. Features: It has a high upper limit for fluid pressure range; it can be installed in any position, and customization is possible provided that the fluid pressure difference requirements are met.
Basic characteristics and related parameters of solenoid valves I. Safety: Neglecting safety can lead to disasters! 1. Corrosive media: Plastic King solenoid valves and all-stainless steel options are recommended ； For highly corrosive media, an isolated diaphragm type must be used. Example CD-F. Z3CF. In neutral media, solenoid valves with copper alloy as the valve body material are also a good choice; otherwise, rust particles often fall off from the valve body, especially in situations where the valve is not used frequently. Copper materials cannot be used for ammonia valves. 2. Explosive environments: Products with the appropriate explosion-proof rating must be selected; waterproof and dust-proof models should be used for outdoor installations or in areas with high dust levels. 3. The nominal pressure of the solenoid valve should exceed the highest operating pressure in the pipeline. II. Applicability: Not applicable means wasting money on something useless, plus it causes extra trouble! 1. Medium properties: 1.1 Different types of solenoid valves should be used for gaseous, liquid, or mixed states; for example, ZQDF is used for air, ZQDF—Y for liquids, and ZQDF—2 (or -3) for steam – otherwise, misoperation may occur. The ZDF series of multi-functional solenoid valves can be used with both gas and liquids. It is best to specify the medium condition at the time of ordering, so that the installation user does not need to adjust it again. 1.2 Products of different specifications for medium temperatures; otherwise, the coils will burn out and the seals will age, severely affecting their lifespan. 1.3 Dielectric viscosity, usually below 50 cSt. If this value is exceeded, and the passage diameter is greater than 15 mm, a ZDF series multi-functional solenoid valve must be ordered as a special order. Solenoid valves with a bore diameter of less than 15 mm for high viscosity fluids. 1.4 When the cleanliness of the medium is not high, a backflush filter valve should be installed in front of the solenoid valve; when the pressure is low, a direct-acting diaphragm solenoid valve such as CD—P can be used. 1.5 If the 1.5 medium flows in a directional manner and backflow is not allowed, ZDF—N and ZQDF—N require bidirectional flow only; special requests should be submitted in such cases. 1.6 The medium temperature should be within the range permitted by the solenoid valve. 2. Pipeline parameters 2.1 Select the valve ports and models based on the flow direction requirements of the medium and the pipeline connection method. For example, when switching from one pipe to two pipes, choose CA5 and Z3F for small diameters; for medium or large diameters, select ZDF—Z1/2. For example, to control the convergence of two pipelines, select ZDF—Z2/1, etc. 2.2 The nominal diameter is selected based on the flow rate and the Kv value of the valve; it can also be chosen to match the inner diameter of the pipeline. Please note that some manufacturers do not indicate the Kv value; often, the size of the valve orifice is smaller than the diameter of the connection pipe. Do not settle for a lower price at the risk of causing problems. 2.3 Operating pressure difference: An indirect pilot type can be used when the minimum operating pressure difference is above 0.04 Mpa ； Those with a minimum operating pressure difference close to or less than zero must use direct-acting or step-direct-acting types. 3. Environmental Conditions 3.1 The maximum and minimum temperatures of the environment should be within the allowable range; if they exceed this range, a special order must be placed. 3.2 In environments with high relative humidity or exposure to water droplets and rain, waterproof solenoid valves should be used. 3.3 In environments where there is frequent vibration, jolting, and impact, special types should be selected, such as marine solenoid valves. 3.4 In corrosive or explosive environments, corrosion resistance should be given priority in selection based on safety requirements. 3.5 If the environmental space is limited, choose a multi-functional solenoid valve, as it eliminates the need for a bypass and three manual valves and facilitates online maintenance. 4. Power supply conditions 4.1 AC and DC solenoid valves are selected accordingly based on the type of power supply. Generally, AC power is easy to obtain. 4.2 For voltage specifications, AC220V and DC24V should be given priority whenever possible. 4.3 For power supply voltage fluctuations, +%10% to -15% is typically allowed for AC, while ±10% is acceptable for DC. If these limits are exceeded, voltage stabilization measures must be taken or special ordering requirements should be submitted. 4.4 The rated current and power consumption should be selected based on the power supply capacity. It should be noted that the VA value is high during start-up under communication conditions; indirect-acting solenoid valves should be given priority when capacity is insufficient. 5. Control accuracy 5.1 Ordinary solenoid valves have only two positions: on and off. When high control accuracy and stable parameters are required, multi-position solenoid valves should be selected ； The Z3CF three-position normally open solenoid valve offers three flow states: slightly open, fully open, and closed ； The ZDF—Z1/1 combined multi-functional solenoid valve has four flow rates: full open, wide open, narrow open, and full open. 5.2 Action time: Refers to the time from when the electrical signal is applied or removed until the main valve completes its action. Only our company’s patented multi-functional solenoid valves allow for separate adjustment of the opening and closing times, which not only meets the requirements for control precision but also prevents damage caused by water hammer. 5.3 Leakage rate: The leakage rate value given for the sample corresponds to the standard economic grade; if it seems too high, please place a special order. III. Reliability: Unreliability will damage the entire system! 1. Service life: This item is not included in the factory testing procedures and falls under the category of type testing. To ensure quality, brand-name products from reputable manufacturers should be chosen. 2. Working system: There are three types, namely long-term working system, repeated short-term working system, and short-term working system. All of our company’s standard products are designed for continuous operation, meaning that the coils can remain powered on for an extended period of time. In cases where the valve remains open for a long time with only short periods of closure, an normally open solenoid valve is advisable. When used in a short working schedule with large batch sizes, it can be ordered specially to reduce power consumption. 3. Operating frequency: When a high frequency of operation is required, a direct-acting solenoid valve is the preferred choice for the structure, and alternating current is the preferred power supply. 4. Operational reliability: Strictly speaking, this test has not yet been officially included in China’s professional standards for solenoid valves; to ensure quality, it is necessary to choose branded products from reputable manufacturers. In some situations, the number of actions required is not high, but the demand for reliability is extremely high, such as in firefighting and emergency protection – these areas must not be taken lightly. It is particularly important to also use two safeguards in combination. IV. Cost-effectiveness: Being uneconomical means a waste of funds, effort, and even lives. It is one of the criteria considered, but cost-effectiveness must be achieved on the basis of safety, suitability, and reliability. Economics is not just about the price of a product; its functionality and quality, as well as the costs associated with installation, maintenance, and other accessories, should be given priority. More importantly, the cost of a solenoid valve in the entire automatic control system, and even within the production line, is minimal; choosing the wrong one in an attempt to save money can result in enormous damages. In short, cost-effectiveness refers not only to the price of a product, but also to its performance-to-price ratio as well as its overall cost-to-price ratio.