The basic knowledge and common errors and solutions of solenoid valve (1)

Solenoid valve is an industrial equipment controlled by electromagnetic, which
adjusts the direction, flow, speed and other parameters of the medium in the
industrial control system. Focusing on structural principle, classification,
characteristics, selection, maintenance, errors and solutions of solenoid valves to
discuss.

1. Structural principle of solenoid valve

(1. Valve body 2. Air inlet 3. Air outlet 4. Conductor 5.Plunger)
When the solenoid valve is not set on, the valve needle blocks the passage of valve body under the action of spring, and solenoid valve is in the cut-off state. When the coil is powered on, the coil generates a magnetic force. The valve core is lifted up against the spring force, the channel in valve is opened, and the solenoid valve is in a conducting state.

2. Solenoid valve classification
Local and international solenoid valves are divided into three categories in principle (like: direct-acting, step-by-step direct-acting, pilot-operated). They are divided into six branches from the difference in structure and material of valve disc and the difference in principle (direct-acting diaphragm structure, step-by-step diaphragm structure, pilot-operated diaphragm structure, direct-acting piston structure, step-by-step piston structure, pilot piston structure). We would introduce three aspects: introduction, principle and characteristics.

1. Direct acting solenoid valve

Introduction:
There are two types: normally closed and normally open. The normally closed type is in a closed state when the power is off. When the coil is energized, an electromagnetic force is generated, so that the moving iron core overcomes the spring force and the static iron core is attracted to directly open the valve, and the medium is in a path. When the coil is powered off, the electromagnetic force disappears, the moving iron core is reset under the action of spring force, the valve port is directly closed, and the medium is blocked. The structure is simple, the action is reliable, and it works normally under zero pressure difference and micro-vacuum. Normally open is just the opposite, such as a solenoid valve with a flow diameter smaller than φ6.

Principle:
When the normally closed type is energized, the electromagnetic coil generates electromagnetic force to lift the opening piece from the valve seat, and the valve opens. When the power is off, the electromagnetic force disappears, the spring presses the opening piece on valve seat, and valve opens. (Normally open is the opposite of this)
Features:
It can work normally in vacuum, negative pressure and zero pressure, but the diameter generally does not exceed 25mm.

2. Processing direct-acting solenoid valve

Introduction:
This kind of valve adopts the one-time opening valve and the second-time opening valve to connect together. The main valve and the pilot valve step by step make the electromagnetic force and pressure difference directly open the main valve port. When the coil is energized, electromagnetic force is generated to make the moving iron core and static iron core pull together, the pilot valve port is opened and pilot valve port is set on the main valve port, and the moving iron core and the main valve core are connected together. The pressure of cavity is unloaded through the pilot valve port, and the main valve core moves upward under the simultaneous action of pressure difference and the electromagnetic force, opening the main valve medium to flow. When coil is powered off, the electromagnetic force disappears. At this time, the moving iron core closes pilot valve hole under the action of its own weight and spring force. At this time, the medium enters the upper cavity of main spool in the balance hole, which increases pressure in the upper cavity. The main valve is closed under the action of spring return and pressure, and the medium is cut off. Reasonable structure, reliable action, and reliable operation under zero pressure difference, such as: ZQDF, ZS, 2W, etc.

Principle:
It is a combination of direct action and pilot type. When there is no pressure difference between the inlet and the outlet, after the power is turned on, the electromagnetic force directly lifts the pilot valve and the main valve closing member upwards in turn, and the valve opens. When the inlet and outlet reach the starting pressure difference, after the power is turned on, the electromagnetic force pilots the small valve, the pressure in the lower chamber of the main valve rises, and the pressure in the upper chamber drops, so that the main valve is pushed up by the pressure difference. When the power is off, the pilot valve uses a spring. The force or medium pressure pushes the closing member, moving downward, causing the valve to close.

Features:

It can also operate under zero pressure difference or vacuum and high pressure, but the power is large and it must be installed horizontally.

3. Indirect pilot solenoid valve
Introduction:
This solenoid valve is composed of a pilot valve and a main spool to form a channel combination; the normally closed type is in a closed state when it is not energized. When the coil is energized, the generated magnetic force makes the moving iron core and the static iron core pull together, the pilot valve port is opened, and the medium flows to the outlet. At this time, the pressure in the upper chamber of the main valve core is reduced, which is lower than the pressure on the inlet side, forming a pressure difference to overcome the spring. The resistance will move upwards accordingly to achieve the purpose of opening the main valve port, and the medium will flow. When the coil is powered off, the magnetic force disappears, and the moving iron core resets and closes the pilot port under the action of the spring force. At this time, the medium flows in from the balance hole, the pressure in the upper cavity of the main spool increases, and moves downward under the action of the spring force. Close the main valve port. The normally open principle is just the opposite.

3. Features of solenoid valve
1. External leakage is blocked, internal leakage is easy to control, and it is safe to use
Internal and external leaks are elements that endanger safety. Other automatic control valves usually extend the valve stem, and the rotation or movement of the valve core is controlled by electric, pneumatic and hydraulic actuators. All this needs to solve the problem of external leakage of the dynamic seal of the long-term action valve stem. Only the solenoid valve is completed by electromagnetic force acting on the iron core sealed in magnetic isolation sleeve, and there is no dynamic seal, so the external leakage is easy to block. The torque control of electric valve is not easy, it is easy to cause internal leakage, and even the head of valve stem is broken. The structure of solenoid valve is easy to control the internal leakage until it drops to zero. Therefore, the solenoid valve is particularly safe to use, especially for corrosive, toxic or high and low temperature media.

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

The solenoid valve itself has a simple structure and low price, and is easier to install and maintain than other types of actuators such as control valves. What is more remarkable is that the composed 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 the industrial computer. In today's era of computer popularization and price drops, the advantages of solenoid valves are even more obvious.

3. Action, small power, light appearance
Solenoid valve response time can be as short as several milliseconds, and even pilot-operated solenoid valves can be controlled within tens of milliseconds. Because of its own loop, it is more sensitive than other automatic control valves. A properly designed solenoid valve coil has low power consumption and is an energy-saving product. It can also be achieved only by triggering the action to automatically maintain the valve position without power consumption at all. The solenoid valve has a small size, which saves space and is light and beautiful.

4. The adjustment accuracy and applicable medium are limited
Solenoid valves usually only have two states of on and off, the valve core can only be in two extreme positions, and cannot be adjusted continuously. The solenoid valve has high requirements on the cleanliness of the medium, and the medium containing particles cannot be applied. If it is an impurity, it must be filtered out first. In addition, viscous media cannot be used, and the range of media viscosity suitable for specific products is relatively narrow.

5. Diverse models and wide range of uses
Although the solenoid valve has inherent deficiencies, its 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 advancement of solenoid valve technology also revolves around how to overcome congenital deficiencies and how to make better use of inherent advantages.

4. Solenoid valve selection
1. Economical
There are many solenoid valves that can be used in general. In the process of selection, the most economical product should be selected, but it must be economical on the basis of safety, application and reliability. Economy is not only the price of product, but also its function and quality, as well as the cost of installation, maintenance and other accessories. The most important is the cost of a solenoid valve in entire automatic control system and even in the production line is very small. If it is greedy for small and cheap and wrongly selected, it will cause huge damage. In a word, economy not only refers to product price, but also product performance-price ratio to comprehensive cost-price ratio.

2. Security

(1) Corrosive medium: plastic king solenoid valve and all stainless steel should be used. For strong corrosive medium, isolation diaphragm type must be used. For neutral medium, it is also advisable to use a solenoid valve with copper alloy as the valve casing material. Otherwise, rust chips often fall off in the valve casing, especially in the occasions where the action is not frequent. Ammonia valves cannot be made of copper.
(2) Explosive environment: Products with corresponding explosion-proof grades must be selected, and waterproof and dust-proof varieties should be selected for outdoor installation or in dusty occasions.
(3) The nominal pressure of solenoid valve should exceed the maximum working pressure in the pipe.

3. Reliability
(1) Working life, this item is not included in the factory test item, but belongs to the type test item. In order to ensure the quality, brand-name products from regular manufacturers should be selected.
(2) Work system: There are three types of long-term work system, repeated short-time work system and short-time work system. For the case where the valve is opened for a long time and only closed for a short time, a normally open solenoid valve should be used. When used in short-time working system and large batches, special orders can be made to reduce power consumption.
(3) Operating frequency: When the operating frequency is required to be high, the structure should preferably be a direct-acting solenoid valve, and the power supply should preferably be AC.
(4) Action reliability: Strictly speaking, this test has not been officially included in the professional standards of Taiwan's solenoid valves. In order to ensure the quality, brand-name products from regular manufacturers should be selected. In some occasions, the number of actions is not many, but the reliability requirements are very high, such as fire protection, emergency protection, must not be taken lightly. It is especially important to take two consecutive double insurances.

4. Applicability
(1) Medium characteristics
1.1 Choose different types of solenoid valves for gas, liquid or mixed state. For example, ZQDF is used for air, ZQDF-Y is used for liquid, and ZQDF-2 (or -3) is used for steam, otherwise it is easy to cause malfunction. ZDF series multi-function solenoid valve can be connected to gas and liquid. It is best to inform the state of the media when ordering, so that the installation user does not have to adjust the mode.
1.2 The medium temperature is different for products of different specifications, otherwise the coil will burn out and the seals will be aged, which will seriously affect the service life.
1.3 Medium viscosity, usually below 50cSt. If it exceeds this value, the ZDF series multi-function solenoid valve should be used for special order if the diameter is larger than 15mm. The diameter is less than 15mm to order high viscosity solenoid valve.
1.4 When the cleanliness of the medium is not high, a recoil filter valve should be installed in front of the solenoid valve. When the pressure is low, a direct-acting diaphragm solenoid valve can be used.
1.5 If the medium is in directional circulation, and reverse flow is not allowed, ZDF-N and ZQDF-N only need to use bidirectional circulation, please make special requirements.
1.6 The medium temperature should be selected within the allowable range of the solenoid valve.

(2) Pipeline parameters
2.1 Select the valve diameter and model according to the medium flow direction requirements and the pipeline connection method.
2.2 Select the nominal diameter according to the flow rate and Kv value of valve, or the same as the inner diameter of pipeline.
2.3 Working pressure difference, indirect pilot type can be used when the minimum working pressure difference is above 0.04Mpa; direct-acting type or step-by-step direct type must be used when the minimum working pressure difference is close to or less than zero.

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