What is a solenoid valve?

What is a solenoid valve?

Solenoid valve is an industrial equipment controlled by electromagnetism. It is an automatic basic component used to control fluid. It belongs to actuator and is not limited to hydraulic and pneumatic. It is used to adjust the direction, flow, speed and other parameters of the medium in the industrial control system. Solenoid valve can cooperate with different circuits to achieve the desired control, and the control accuracy and flexibility can be guaranteed. There are many kinds of solenoid valves. Different solenoid valves play a role in different positions of the control system. The most commonly used are one-way valve, safety valve, direction control valve, speed control valve, etc.

Working principle

There are closed cavities in the solenoid valve. Through holes are opened in different positions. Each hole leads to different oil pipes. The valve is in the middle of the cavity and two electromagnets are on both sides. Which side of the magnet coil is energized, the valve body will be attracted to which side, by controlling the movement of the valve body to block or leak different oil drain holes. When the oil inlet hole is normally open, the hydraulic oil will enter into different oil drainage pipes, and then push the piston of the oil cylinder through the oil pressure, which in turn drives the piston rod, and the piston rod drives the mechanical device. In this way, the mechanical movement is controlled by controlling the current of the electromagnet.

Classification of solenoid valves

Solenoid valves are divided into three categories in principle:

Direct acting solenoid valve:

Principle: when electrified, the electromagnetic coil produces electromagnetic force to lift the closing part from the valve seat and open the valve; when the power is off, the electromagnetic force disappears, and the spring presses the closing part on the valve seat, and the valve is closed.
Features: in vacuum, negative pressure, zero pressure can work normally, but the diameter is generally less than 25 mm.

Distributed direct acting solenoid valve:

Principle: it is a combination of direct acting and pilot type. When there is no pressure difference between the inlet and outlet, the electromagnetic force will directly lift the small pilot valve and the closing part of the main valve in turn, and the valve will be opened. When the inlet and outlet reach the start-up pressure difference, after power on, the electromagnetic force pilot small valve, the pressure of the lower chamber of the main valve rises, and the pressure of the upper chamber decreases, so as to push the main valve upward by using the pressure difference; when the power is off, the pilot valve uses the spring force or medium pressure to push the closing part and move downward to close the valve.
Features: it can also operate under zero pressure difference, vacuum and high pressure, but the power is large, so it must be installed horizontally.

Pilot operated solenoid valve:

Principle: when electrified, the electromagnetic force opens the pilot hole, and the pressure in the upper chamber drops rapidly, forming a high differential pressure around the closing part, and the fluid pressure pushes the closing part to move upward and the valve opens; when the power is off, the spring force closes the pilot hole, and the inlet pressure rapidly passes through the bypass hole, and the chamber forms a low pressure difference around the valve closing part, and the fluid pressure pushes the closing part downward Move and close the valve.
Features: the upper limit of fluid pressure range is high, which can be installed arbitrarily (need to be customized), but must meet the condition of fluid pressure difference.

Selection of solenoid valve

Selection basis

Firstly, the selection of solenoid valve should follow the four principles of safety, reliability, applicability and economy. Secondly, it should be selected according to six field conditions (i.e. pipeline parameters, fluid parameters, pressure parameters, electrical parameters, operation mode and special requirements).
1. According to the pipeline parameters, select the solenoid valve’s size (i.e. DN) and interface mode

• 1) The diameter (DN) is determined according to the internal diameter or flow requirements of the pipeline on site;
• 2) Generally, the flange interface should be selected when it is > DN50, and it can be freely selected according to the user’s needs if it is less than or equal to DN50.

2. According to the fluid parameters to select the solenoid valve: material, temperature group

• 1) Corrosive fluid: corrosion resistant solenoid valve and all stainless steel should be selected; edible ultra clean fluid: food grade stainless steel solenoid valve should be selected;
• 2) High temperature fluid: the solenoid valve made of high temperature resistant electrical material and sealing material should be selected, and the piston structure type should be selected;
• 3) Fluid state: gas state, liquid state or mixed state, especially when the diameter is larger than DN25, it must be distinguished;
• 4) Fluid viscosity: usually below 50cst, you can choose at will. If you exceed this value, you should choose high viscosity solenoid valve.

3. Selection of solenoid valve’s principle and structure according to pressure parameters

• 1) Nominal pressure: the meaning of this parameter is the same as that of other general valves, which is determined according to the nominal pressure of pipeline;
• 2) Working pressure: if the working pressure is low, the direct acting or step-by-step direct acting principle must be selected; when the minimum working pressure difference is above 0.04MPa, direct acting, step-by-step direct acting and pilot type can be selected.

4. Electrical selection: AC 220 V and DC24 should be preferred for voltage specification, which is more convenient.
5. Select according to the duration of the work: normally closed, normally open, or continuously energized

• 1) When the solenoid valve needs to open for a long time, and the duration is longer than the closing time, the normally open type should be selected;
• 2) If the opening time is short or the opening and closing time is not long, the normally closed type is selected;
• 3) However, for some working conditions used for safety protection, such as furnace and kiln flame monitoring, the normally open type can not be selected, and the long-term power on type should be selected.

6. Select auxiliary functions according to environmental requirements: explosion proof, check-in, manual, water mist proof, water drenching and diving.

Selection principle of solenoid valve

Security:

• 1. Corrosive medium: Plastic King solenoid valve and all stainless steel should be selected; for strong corrosive medium, isolation diaphragm type must be selected. For neutral medium, it is also appropriate to select the solenoid valve with copper alloy as the valve shell material, otherwise, rust chips often fall off in the valve shell, especially when the action is not frequent. The ammonia valve cannot be made of copper.
• 2. Explosive environment: explosion proof products must be selected. Waterproof and dust-proof products should be selected for outdoor installation or dust-proof occasions.
• 3. The nominal pressure of solenoid valve should exceed the maximum working pressure in the pipe.

Applicability:

Dielectric properties

• 1) Different kinds of solenoid valves are selected for gas, liquid or mixed state;
• 2) If the medium temperature is different, the coil will burn off and the seal will be aged, which will seriously affect the service life;
• 3) The viscosity of the medium is usually below 50cst. If it exceeds this value, the multi-functional solenoid valve is used when the diameter is greater than 15mm, and the high viscosity solenoid valve is used when the diameter is less than 15mm.
• 4) When the medium cleanliness is not high, the backwash filter valve should be installed in front of the solenoid valve. When the pressure is low, the direct acting diaphragm solenoid valve can be selected;
• 5) If the medium flows in a directional way and does not allow reverse flow, it needs two-way flow;
• 6) Medium temperature should be selected within the allowable range of solenoid valve.

Pipe parameters

• 1) According to the requirements of medium flow direction and pipeline connection mode, select the valve port and model;
• 2) The nominal diameter is selected according to the flow rate and valve Kv value, and the same pipe inner diameter can also be selected;
• 3) Working pressure difference: if the minimum working pressure difference is above 0.04MPa, indirect pilot type can be selected; if the minimum working pressure difference is close to or less than zero, direct acting type or step-by-step direct type must be selected.

Environmental conditions

• 1) The maximum and minimum temperature of the environment should be selected within the allowable range;
• 2) In the environment with high relative humidity and water dripping, waterproof solenoid valve should be selected;
• 3) In the environment, there are often vibration, bumps and shocks. Special varieties should be selected, such as marine solenoid valves;
• 4) In corrosive or explosive environment, corrosion resistant type should be preferred according to safety requirements;
• 5) If the environment space is limited, the multi-functional solenoid valve should be selected, because it eliminates the bypass and three manual valves, and is convenient for on-line maintenance.

Power supply conditions

• 1) AC and DC solenoid valves are selected according to the type of power supply. Generally speaking, AC power supply is convenient to use;
• 2) AC 220 v. DC24 V should be preferred for voltage specification;
• 3) The fluctuation of power supply voltage is usually +% 10%. – 15% for AC and ±% 10% for DC. If it is out of tolerance, voltage stabilization measures must be taken;
• 4) The rated current and power consumption should be selected according to the power supply capacity. It is necessary to pay attention to the higher VA value during AC starting, and the indirect guide solenoid valve should be preferred when the capacity is insufficient.

Control accuracy

• 1) The common solenoid valve has only two positions of on and off, so multi position solenoid valve should be selected when the control precision is high and the parameters are required to be stable;
• 2) Action time: refers to the time when the electric signal is connected or cut off to the completion time of the main valve action;
• 3) Leakage: the leakage value given on the sample is the common economic level.

Reliability:

• 1. Working life, this item is not included in the factory test item, belongs to the type test item. In order to ensure the quality, we should choose the famous brand products of regular manufacturers.
• 2. Working system: divided into long-term working system, repeated short-term working system and short-term working system. If the valve is opened for a long time and only closed for a short time, the normally open solenoid valve should be selected.
• 3. Working frequency: when the operation frequency is required to be high, the structure should be optimized for direct acting solenoid valve, and the power supply should be optimized for ac.
• 4. Action reliability: strictly speaking, this test has not been formally listed in the professional standard of China solenoid valve. In order to ensure the quality, we should choose the famous brand products of regular manufacturers. In some cases, the frequency of action is not much, but the reliability requirements are very high, such as fire protection, emergency protection, etc., we must not take it lightly. It is particularly important to adopt two combined double insurance.

Economy:

It is one of the selected scales, but it must be economical on the basis of safety, applicability and reliability.
Economy is not only the price of the product, but also its function and quality, as well as the cost of installation and maintenance and other accessories.
More importantly, the cost of a solenoid valve in the whole automatic control system, in the whole automatic control system and even in the production line is very small.

Attention to installation of solenoid valve

• 1. During installation, the arrow on the valve body should be consistent with the flow direction of the medium. Do not install in the place with direct dripping or splashing water. The solenoid valve should be installed vertically;
• 2. The solenoid valve shall ensure that the power supply voltage is within the range of 15% – 10% of the rated voltage;
• 3. After the solenoid valve is installed, there shall be no reverse pressure difference in the pipeline. It can be put into use after being electrified several times to make it suitable for temperature;
• 4. The pipeline should be cleaned thoroughly before the solenoid valve is installed. The medium should be free of impurities. A filter is installed in front of the valve;
• 5. In case of solenoid valve failure or cleaning, bypass device should be installed to ensure continuous operation of the system.

Troubleshooting of solenoid valve

The solenoid valve does not work after being energized
Check whether the power wiring is poor → rewire and connect the connector
Different kinds of solenoid valves

• Check whether the power supply voltage is within ± working range – → adjust to normal position range
• Whether the coil is desoldered → rewelding
• Coil short circuit → replace coil
• Whether the working pressure difference is not appropriate→Adjust the pressure difference→Or replace the corresponding solenoid valve
• The fluid temperature is too high → replace the corresponding solenoid valve
• The main valve core and moving iron core of the solenoid valve are jammed by impurities → clean. If the seal is damaged, replace the seal and install the filter
• The viscosity of liquid is too high, the frequency is too high and the service life is up → replace the product

The solenoid valve of the solenoid valve cannot be turned off

• The seal of the main valve core or moving iron core is damaged → replace the seal
• Whether the fluid temperature and viscosity are too high → replace the corresponding solenoid valve
• Impurities enter the valve core or moving iron core of solenoid valve → clean
• The service life of spring has reached or deformed → replace
• Orifice balance hole blocked → timely cleaning
• The working frequency is too high or the service life has reached → change products or update products

Other conditions of solenoid valve

• Internal leakage → check whether the seal is damaged and whether the spring is not assembled properly
• Leakage → loose connection or damaged seal → tighten screw or replace seal
• There is noise when power on → the fastener on the head is loose and tightened. If the voltage fluctuation is not within the allowable range, adjust the voltage. Iron core suction surface impurities or uneven, timely cleaning or replacement.

Sealing material of solenoid valve

There are three most commonly used sealing materials for solenoid valves.

NBR butadiene rubber

Obtained from emulsion polymerization of butadiene and acrylonitrile, acrylonitrile butadiene rubber is mainly produced by low temperature emulsion polymerization. It has excellent oil resistance, high wear resistance, good heat resistance and strong adhesion. Its disadvantages are poor low temperature resistance, poor ozone resistance, poor electrical performance, and slightly low elasticity.
In addition, it also has good water resistance, air tightness and excellent adhesion performance. It is widely used in the manufacture of various oil resistant rubber products, various oil resistant gaskets, gaskets, sleeves, flexible packaging, flexible hose, printing and dyeing cots, cable rubber materials, etc. it has become an essential elastic material in automobile, aviation, petroleum, copying and other industries.
NBR has excellent oil resistance, its oil resistance is second only to polysulfide rubber and fluororubber, and has wear resistance and air tightness. The disadvantage of NBR is that it is not resistant to ozone and aromatic, halogenated hydrocarbon, ketone and ester solvents, so it is not suitable for insulation material.

Main uses

NBR is mainly used to make oil resistant products, such as oil resistant pipes, tapes, rubber diaphragms and large oil bags. It is commonly used to make various oil resistant moulded products, such as O-rings, oil seals, cups, diaphragms, valves, bellows, etc., and also used to make rubber plates and wear-resistant parts.

Ethylene propylene diene monomer (EPDM)

Ethylene propylene diene monomer (EPDM) is a terpolymer of ethylene, propylene and non conjugated dienes. The world’s annual consumption is 800000 tons. The most important characteristic of EPDM is its superior oxidation resistance, ozone resistance and corrosion resistance. Because EPDM belongs to polyolefin family, it has excellent vulcanization properties. EPDM has the lowest specific gravity of all rubbers. It can absorb a large number of fillers and oil with little effect on the properties. Therefore, the rubber compound with low cost can be prepared.
Molecular structure and properties: EPDM is a terpolymer of ethylene, propylene and non conjugated dienes. Diene has a special structure. Only one of the two bonds can be copolymerized. The unsaturated double bond is mainly used as the cross chain. The other unsaturated polymer will not be the main chain, but the side chain. The main polymer chain of EPDM is completely saturated. This property makes EPDM resistant to heat, light, oxygen, especially ozone. EPDM is essentially nonpolar, resistant to polar solutions and chemicals, low water absorption and good insulation properties.

Characteristic:

• 1. Low density and high filling ability;
• 2. Aging resistance;
• 3. Corrosion resistance;
• 4. Water vapor resistance;
• 5. Resistance to superheated water;
• 6. Electrical performance;
• 7. Elasticity;
• 8. Adhesion.

Viton fluororubber (FKM)

The fluorine containing rubber has various types according to the fluorine content, i.e. monomer structure; the fluorine rubber of hexafluoride system has better high temperature resistance than silicon rubber, chemical resistance, most of oil and solvent (except ketones and esters), good weather resistance and ozone resistance, but poor cold resistance; it is widely used for sealing parts of steam turbine vehicles, class B products and chemical plants, with the service temperature range of – 20 ℃ – 2 It can be used up to – 40 ℃, but the price is high

Difference between electric valve and solenoid valve

• The solenoid valve is that the solenoid coil generates magnetic attraction after being electrified, which overcomes the pressure of the spring and drives the valve core to move. One solenoid coil is simple in structure and cheap in price and can only be switched on and off;
• The electric valve is driven by the motor to drive the valve stem to move, and the electric valve is divided into (shut-off valve) and regulating valve. The shut-off valve is a two-position work, that is, full open and full close. The regulating valve is installed with an electric valve positioner on it. The valve can be dynamically stabilized in a position through closed-loop regulation.

Application Comparison of electric valve and solenoid valve

Solenoid valve: for liquid and gas pipeline switch control, is two position do control. It is generally used for the control of small pipes.
Electric valve: it is used for analog quantity regulation of liquid, gas and air system pipeline medium flow, which is AI control. In the control of large valve and air system, electric valve can also be used as two position switch control.
Solenoid valve: can only be used as switching value, is do control, can only be used for small pipe control, common in DN50 and below pipeline.
Electric valve: can have AI feedback signal, can be controlled by do or AO, compared with large pipe and air valve, etc.

Switch type:

The solenoid valve is driven by coil and can only be opened or closed, and the action time is short.
The drive of electric valve is generally motor. It takes a certain amount of time to complete the opening or closing action, which can be adjusted.

Nature of work:

Generally, the flow coefficient of solenoid valve is very small, and the working pressure difference is very small. For example, the flow coefficient of 25 caliber solenoid valve is much smaller than that of 15 caliber electric ball valve. The drive of solenoid valve is through solenoid coil, which is easy to be damaged by voltage impact. Equivalent to the function of the switch, that is to open and close two functions.
Electric valve drive is generally motor, more resistant to voltage shock. Solenoid valve is fast open and fast close, generally used in small flow and small pressure, requiring high switching frequency of the electric valve on the contrary. The opening of the electric valve can be controlled, and the state can be open, closed, half open and half closed, which can control the flow of medium in the pipeline, but the solenoid valve can not meet this requirement.
Generally, the solenoid valve can be reset after power failure, and the electric valve needs to add a reset device for such function.

Applicable process:

Solenoid valve is suitable for some special process requirements, such as leakage, special fluid medium, etc., and its price is relatively high.
Electric valve is generally used for regulation, but also has switch value, such as: fan coil end.

Main features of solenoid valve

The external leakage is blocked, the internal leakage is easy to control, and the use is safe

Internal and external leakage is an element endangering safety. Other automatic control valves usually extend the valve stem, and the electric, pneumatic and hydraulic actuators control the rotation or movement of the valve core. It is necessary to solve the leakage problem of the long-term dynamic seal of the valve stem; only the electromagnetic force acts on the iron core sealed in the magnetic isolation sleeve of the electric control valve, and there is no dynamic seal, so the leakage is easy to be blocked. The torque control of the electric valve is not easy, and it is easy to produce internal leakage and even break the valve stem head; the structure of the solenoid valve is easy to control the internal leakage until it drops to zero. Therefore, the use of solenoid valve is particularly safe, especially suitable for corrosive, toxic or high and low temperature media.

The system is simple, easy to connect to the computer, and the price is low

The structure of solenoid valve is simple and its price is low. Compared with other kinds of actuators such as control valve, it is easy to install and maintain. What’s more, the automatic control system is much simpler and the price is much lower. Because the solenoid valve is controlled by switch signal, it is very convenient to connect with industrial computer. In today’s computer popularization, the price drops sharply, the advantage of solenoid valve is more obvious.

Action express, small power, light shape

The response time of solenoid valve can be as short as several milliseconds, and even the pilot operated solenoid valve can be controlled in tens of milliseconds. Because of its own loop, it is more sensitive than other automatic control valves. Properly designed solenoid valve coil power consumption is very low, belongs to energy-saving products; can also achieve only trigger action, automatically maintain the valve position, usually no power consumption. The size of solenoid valve is small, which not only saves space, but also is light and beautiful.

The adjustment accuracy is limited and the applicable medium is limited

Solenoid valve usually only has two states of switch, the spool can only be in two limit positions, can not be continuously adjusted, so the adjustment accuracy is also limited.
The electromagnetic valve has high requirements on the cleanliness of the medium, and the medium containing particles cannot be used. If it is impurities, it must be filtered first. In addition, viscous media can not be applied, and the viscosity range of specific products is relatively narrow.

Various models, wide range of uses

Although the solenoid valve has inherent deficiencies, the advantages are still very prominent, so it is designed into a variety of products to meet various needs, and has a wide range of uses. The progress of solenoid valve technology is also around how to overcome the inherent deficiencies and how to better play the inherent advantages.

Selection of solenoid valve

• 1) The main control parameters of solenoid valve are diameter, design nominal pressure, allowable temperature range of medium, interface size, etc.
• 2) Solenoid valve is to use electromagnet to push the valve to open and close. It is usually used in two position control with diameter less than 40mm, especially for connecting, cutting off or changing gas and liquid circuits.
• 3) The sealing performance of valve is one of the main indexes to evaluate the quality of valve. The sealing performance of the valve mainly includes two aspects: internal leakage and external leakage. Internal leakage refers to the sealing degree of the medium between the valve seat and the closing part. The leakage refers to the leakage of the valve stem packing, the middle mouth gasket and the casting defects of the valve body. Leakage is not allowed.
• 4) The main advantages of solenoid valve are small volume, reliable action, convenient maintenance and low price. It is necessary to select the normally open or normally closed type according to the process requirements.

Common faults of solenoid valve

Solenoid valve is composed of solenoid coil and magnetic core, which is a valve body containing one or several holes. When the coil is energized or de energized, the operation of the magnetic core will cause the fluid to pass through the valve body or be cut off, so as to change the direction of the fluid. The electromagnetic components of the solenoid valve are composed of fixed iron core, moving iron core and coil; the valve body is composed of slide valve core, slide valve sleeve, spring base, etc. The electromagnetic coil is directly installed on the valve body, and the valve body is enclosed in the sealing pipe, forming a simple and compact combination. Our commonly used solenoid valves in production are two three-way, two four-way, two-way five-way and so on. Here first talk about the meaning of two: for the solenoid valve is charged and lost power, for the valve controlled is open and close.
The failure of solenoid valve will directly affect the action of switching valve and regulating valve. The common fault is that the solenoid valve does not act. The following aspects should be investigated:

• 1. If the connector of solenoid valve is loose or the wire end falls off, the solenoid valve shall not be electrified, and the wire end can be tightened.
• 2. If the solenoid valve coil is burnt out, the wiring of the solenoid valve can be removed and measured with a multimeter. If the solenoid valve coil is open circuit, the solenoid valve coil is burnt out. The reason is that the coil is damp, which leads to poor insulation and magnetic leakage, resulting in excessive current in the coil and burning. Therefore, it is necessary to prevent rainwater from entering the solenoid valve. In addition, if the spring is too hard, the reaction force is too large, the number of turns of the coil is too small, and the suction force is not enough, the coil can also be burned. In case of emergency treatment, the manual button on the coil can be turned from “0” position to “1” position during normal operation to open the valve.
• 3. Stuck solenoid valve: the fit clearance between the spool sleeve and the spool of the solenoid valve is very small (less than 0.008mm). It is usually assembled in one piece. When mechanical impurities are brought in or there is too little lubricating oil, it is easy to get stuck. The treatment method can be used to poke steel wire through the small hole in the head to make it bounce back. The fundamental solution is to remove the solenoid valve, take out the valve core and valve core sleeve, and clean it with CCI4, so that the valve core can move flexibly in the valve sleeve. During disassembly, attention should be paid to the assembly sequence and external wiring position of each component, so as to facilitate the re assembly and correct wiring. Also, check whether the fuel injection hole of the oil mist atomizer is blocked and whether the lubricating oil is sufficient.
• 4. Air leakage: air leakage will cause insufficient air pressure, which makes it difficult to open and close the forced valve. The reason is that the sealing gasket is damaged or the slide valve is worn, resulting in air leakage in several cavities. When dealing with the solenoid valve fault of the switching system, we should choose the appropriate time to deal with it when the solenoid valve is in power loss. If it can not be handled completely within a switching gap, the switching system can be suspended and handled calmly.

Source: China Valve Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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