What is a diaphragm valve?
Diaphragm valve is a stop valve that uses diaphragm as the opening and closing part to close the flow channel, block the fluid, and separate the inner cavity of valve body and bonnet. Diaphragms are usually made of elastic, corrosion-resistant and impermeable materials such as rubber and plastic. The valve body is mostly made of plastic, FRP, ceramic or metal rubber lining materials. The utility model has the advantages of simple structure, good sealing and anti-corrosion performance and small fluid resistance. It is used for medium with low pressure, low temperature, strong corrosivity and suspended substances. According to the structural form, it can be divided into ridge type, cut-off type, ram type, etc. According to the driving mode, there are manual, pneumatic and electric.
Working principle and composition of diaphragm valve
Composition of diaphragm valve
The structural form of diaphragm valve is very different from that of general valves. It is a new type of valve and a special form of block valve. Its opening and closing part is a diaphragm made of soft material, which separates the inner cavity of valve body from the inner cavity of valve cover and driving parts. It is now widely used in various fields. Common diaphragm valves include rubber lined diaphragm valve, fluorine lined diaphragm valve, unlined diaphragm valve and plastic diaphragm valve.
The diaphragm valve is equipped with a flexible diaphragm or combined diaphragm in the valve body and valve cover, and its closing part is a compression device connected with the diaphragm. The valve seat can be a weir or a pipe wall through the flow channel. The advantage of diaphragm valve is that its control mechanism is separated from the medium path, which not only ensures the purity of the working medium, but also prevents the possibility of medium in the pipeline impacting the working parts of the control mechanism. In addition, there is no need to use any form of separate seal at the valve stem, unless it is used as a safety facility in controlling harmful media. In the diaphragm valve, since the working medium contacts only the diaphragm and valve body, both of which can adopt a variety of different materials, the valve can ideally control a variety of working media, especially those with chemical corrosivity or suspended particles. The working temperature of diaphragm valve is usually limited by the materials used for diaphragm and valve body lining, and its working temperature range is about – 50 ~ 175 ℃. The diaphragm valve has a simple structure and is only composed of three main components: valve body, diaphragm and valve head assembly. The valve is easy to disassemble and repair quickly, and the replacement of diaphragm can be completed on site and in a short time.
Classification of diaphragm valves
There are many kinds of diaphragm valves, and there are many classification methods. This paper mainly introduces the diaphragm valves according to their structural forms.
1. Weir type (ridge type) diaphragm valve
The diaphragm valve with ridge structure and diaphragm as the sealing pair in the valve body channel is called weir type (ridge type) diaphragm valve.
As a part of the valve body, the diversion weir of the weir diaphragm valve (Fig. 1) is equivalent to the valve seat, and the diaphragm is pressed on it to cut off the medium flow. This type of diaphragm valve is generally larger in size. The raised diversion weir reduces the stroke of the diaphragm valve from fully open to fully closed, thus reducing the stress and strain of the diaphragm. When closed, the valve disc with diaphragm moves down to seal it with the weir (back) of the valve body to meet the requirements of cutting off fluid.
Short stroke, large flow resistance coefficient (relative to straight through type), low requirements for diaphragm flexibility and no dead angle of silting medium. It is the most widely used diaphragm valve. It is divided into unlined, plastic lined, rubber lined and enamel lined. It has certain throttling characteristics, good corrosion resistance and particle resistant medium, and reliable sealing.
The weir type (ridge type) diaphragm valve is designed to have a short stroke between the closed and full open positions. In this way, the flexible stress of the diaphragm is small, so the service life of the diaphragm is long. Due to the short stroke, the diaphragm made of PTFE can be used. For a long stroke, the flexibility of Pt-Fe is not enough. However, when PTFE is used, the back of the diaphragm must be lined with rubber to ensure the necessary elasticity during tight closing.
Standard weir diaphragm valves can also be used in vacuum. However, when used in high vacuum, the diaphragm must be specially reinforced.
Because the diaphragm area is larger than the cross-sectional area of the fluid channel, the fluid will produce a relatively large pressure on the arched diaphragm, which makes the closing torque of the diaphragm valve very large, thus limiting the development of the diaphragm valve size series. For example, the maximum size of a typical weir diaphragm valve is DN350 (14 in). The larger weir diaphragm valve shown in Figure 2 is equipped with double diaphragm, and the maximum size is DN500 (20in).
Conventional weir diaphragm valves can also be used in horizontal pipelines with automatic discharge. The valve rod can be lifted 15 ° ~ 20 ° from the horizontal position to make the horizontal pipeline have some drop, so as to realize the automatic discharge of the pipeline.
If the diaphragm breaks, it is necessary to install an O-ring on the valve stem close to the bonnet to prevent fluid overflow.
2. Direct flow diaphragm valve
The direct flow diaphragm valve has the characteristics of stop valve and ridge diaphragm valve. The valve seat seal adopts the metal valve disc coated with non-metallic materials, which has high bearing capacity and tightness. In addition, as the middle flange seal and valve stem packing seal, as well as the barrier separating the valve body and bonnet, the diaphragm has reliable sealing. The flow channel of the valve body is linear, with good flow capacity and small flow resistance coefficient. The inner cavity of the valve body can be lined with plastic and rubber.
3. Straight through diaphragm valve
The fluid channel of the straight through diaphragm valve is approximately straight, with small flow resistance and long stroke (compared with the weir type). It has high requirements for diaphragm flexibility, good cutting performance and circulation performance, corrosion resistance, can contain granular media, and the inner cavity is divided into unlined and lined with plastic, rubber and enamel.
When the straight through valve is opened, its diaphragm rises to make it flow smoothly in any direction; When the valve is closed, the diaphragm is tightly sealed, which can ensure that it can be absolutely closed when there is sand or fiber in the pipeline.
Compared with weir diaphragm valve, straight through diaphragm valve requires diaphragm to have more elasticity. Therefore, this structure limits the range of elastic materials for the diaphragm.
Because the diaphragm of straight through diaphragm valve is a high elastic material and has a large area, the diaphragm is easy to expand and enter the flow channel under high vacuum conditions. The expansion degree of the diaphragm is related to the manufacture. The expansion will cause the reduction of the valve flow path area or the collapse of the diaphragm. To avoid the latter, the bonnet must be moved upward to balance the pressure on the diaphragm. When using this valve in high vacuum air, consult the manufacturer.
Straight through diaphragm valves also have full bore and reduced bore channels. For valves with reduced flow path, a smaller bonnet assembly needs to be installed. For example, a reduced flow path valve with a nominal size of DN50 (NPS2) should be installed with a nominal size of Dn40 (nps1 ½) Remove the valve cover. Its bonnet structure is similar to that of weir diaphragm valve. The size series of typical straight through diaphragm valves has reached the nominal size DN350 (nps14).
Full channel valves are mostly used in beverage industry. It can be cleaned with a ball brush with steam or caustic soda (Fig. 6) without opening or removing the valve from the pipeline.
4. Stop diaphragm valve
The structure of the valve body channel and sealing surface of the stop diaphragm valve (Fig. 7) is similar to that of the stop valve. The valve disc adopts a diaphragm coated with metal and non-metallic materials as a barrier to isolate the valve body and bonnet. Rotate the hand wheel to raise and lower the valve stem, press the diaphragm on the sealing surface of the valve body through the valve disc to achieve sealing fit, and open and close the valve.
The stop diaphragm valve has good sealing performance, but the flow resistance is larger than the ridge diaphragm valve. It has high bearing capacity, sealing performance and throttling characteristics. It can be used for low vacuum (less than or equal to 101325pa).
5. Angle diaphragm valve
The inlet and outlet of the angle diaphragm valve (Fig. 8) form a 90 ° angle. There is a diaphragm under the valve disc as a barrier to isolate the valve body and bonnet. It has good cutting performance and flow capacity, reliable sealing, good flow capacity, small flow resistance and no dead angle of silting medium.
Working principle of diaphragm valve
For diaphragm valve, the valve body with corrosion-resistant lining and corrosion-resistant diaphragm are used to replace the valve core assembly, and the movement of diaphragm is used to adjust. The valve body of diaphragm valve is made of cast iron, cast steel or cast stainless steel, lined with various corrosion-resistant or wear-resistant materials, diaphragm material rubber and polytetrafluoroethylene. The lining diaphragm has strong corrosion resistance and is suitable for the regulation of strong corrosive media such as strong acid and alkali.
The diaphragm valve has the advantages of simple structure, small fluid resistance and large flow capacity compared with other types of valves of the same specification; No leakage, can be used for the regulation of high viscosity and suspended particle media. The diaphragm isolates the medium from the upper chamber of the valve stem, so the medium will not leak out without packing. However, due to the limitations of diaphragm and lining materials, the pressure resistance and temperature resistance are poor. Generally, it is only applicable to 1.6Mpa nominal pressure and below 150 ℃.
The flow characteristic of diaphragm valve is close to the fast opening characteristic, which is approximately linear before 60% stroke, and the flow changes little after 60%. The pneumatic diaphragm valve can also be attached with feedback signal, limiter, positioner and other devices to meet the needs of automatic control, program control or flow regulation. The feedback signal of pneumatic diaphragm valve adopts contactless sensing technology. The product adopts a film type propulsion cylinder to replace the piston cylinder, which eliminates the disadvantage that the piston ring is easy to be damaged, resulting in leakage and unable to push the valve to open and close. When the air source fails, the hand wheel can be operated to open and close the valve.
Sealing principle of diaphragm valve
The sealing principle of diaphragm valve is to press down the diaphragm or diaphragm assembly by the downward movement of the operating mechanism and close the channel of weir lined valve body or straight lined valve body to achieve sealing. The sealing specific pressure is realized by the downward pressure of the closing part. Because the valve body can be lined with various soft materials, such as rubber or polytetrafluoroethylene; The diaphragm is also made of soft materials, such as PTFE lined with rubber or synthetic rubber, so it can be completely sealed with less sealing force[ 2]
Diaphragm valve has only three main parts: valve body, diaphragm and bonnet assembly. The diaphragm separates the inner cavity of the lower valve body from the inner cavity of the upper valve cover, so that the valve stem, valve stem nut, valve disc, pneumatic control mechanism, electric control mechanism and other parts above the diaphragm do not contact with the medium, and there will be no medium leakage, eliminating the sealing structure of the stuffing box.
Advantages of diaphragm valve
1. With simple structure and few parts, the diaphragm valve is only composed of three main components: valve body, diaphragm and bonnet assembly.
2. The disassembly and maintenance are convenient. Generally, the damaged part of the diaphragm valve is the diaphragm. When replacing the diaphragm, there is no need to remove the valve from the pipeline, just remove the valve cover, and the replacement of the diaphragm can be completed in a short time on site.
3. Good sealing performance. Because the diaphragm is made of soft materials such as rubber or plastic, it has good sealing performance, but the mechanical life of the diaphragm is short. The medium in the valve will not leak from the connection of valve body and bonnet. The upstream and downstream of the valve can be completely cut off without internal leakage.
4. There is no dead angle in the valve. The flow channel of the valve body is streamlined, smooth and free of dead angle. The medium flowing through the valve cannot deposit in the valve and become moldy and deteriorated, and it is easy to wash. The medium flowing through, such as fiber or particles, will not get stuck.
5. The diaphragm valve without packing structure does not need packing and is not corrosive to the valve stem and bonnet (because the diaphragm separates the lower valve body from the inner cavity of the upper bonnet).
6. It can be used for cutting off and throttling.
7. Corrosion resistance due to the diversity of lining, it has good chemical resistance.
8. The diaphragm valve is suitable for various working conditions, especially for dangerous chemicals and radioactive fluids. Because the valve will not produce pollution, it is widely used in product processing, pharmaceutical, food, nuclear industry and other industries that do not allow pollution.
Application of diaphragm valve
Diaphragm valve is a special form of block valve. Its opening and closing part is a diaphragm made of soft material, which separates the inner cavity of the valve body from the inner cavity of the valve cover.
Due to the limitation of valve body lining process and diaphragm manufacturing process, it is difficult to manufacture larger valve body lining and diaphragm, so diaphragm valve is not suitable for larger pipe diameter and is generally applied to pipelines ≤ DN200.
Due to the limitation of diaphragm material, diaphragm valve is suitable for occasions with low pressure and low temperature. Generally, it shall not exceed 180 ℃. As the diaphragm valve has good anti-corrosion performance, it is generally used in devices and pipelines with corrosive media. Due to the service temperature of diaphragm valve, the applicable medium is limited by the lining material of diaphragm valve body and diaphragm material.
Selection Guide for diaphragm valves
(1) The shape and structure of the valve core are mainly considered according to the selected flow characteristics and unbalanced force.
(2) Wear resistance when the fluid medium is a suspension containing high concentration wear particles, the valve core and valve seat joint surface will be seriously rubbed every time they are closed. Therefore, the flow path of the valve should be smooth and the internal material of the valve should be hard.
(3) Corrosion resistance because the medium is corrosive, the valve with simple structure shall be selected as far as possible under the condition of meeting the regulation function.
(4) Medium temperature and pressure when the medium temperature and pressure are high and change greatly, the valve with small temperature and pressure change of valve core and valve seat materials shall be selected.
(5) Prevent flash and cavitation. Flash and cavitation are only produced in liquid medium. In the actual production process, flash and cavitation not only affect the calculation of flow coefficient, but also form vibration and noise, which shortens the service life of the valve. Therefore, when selecting the valve, flash and cavitation should be prevented.
(6) Manufacturers must choose regular, large and professional ones.
Maintenance of diaphragm valve
1. The diaphragm valve shall be stored in a dry and ventilated room and shall not be stacked.
2. Both ends of the passage of the inventory valve must be sealed to prevent foreign matters from entering the inner cavity and damaging the sealing parts. And avoid contact with oil or other inflammables.
3. The metalworking surface of the valve shall be cleaned of dirt and coated with rust inhibitor.
4. Do not brush grease on the surface of rubber lining or rubber diaphragm to avoid rubber swelling and affecting the service life of the valve.
5. During storage or shutdown, rotate the hand wheel counterclockwise to make the valve slightly open, so as to avoid losing the elasticity of the diaphragm due to long-term pressure.
6. Slings shall not be tied to the handwheel or valve stem during transportation or installation. It is strictly prohibited to collide with other metals or hard objects to prevent damage to parts and lining.
7. Before installation, carefully check whether the operating conditions and medium of the pipeline are consistent with the application scope of the valve, so as to avoid unnecessary losses caused by improper selection and accidents.
8. In addition to not suitable for vacuum pipeline, the valve can be installed at any position of the pipeline for two-way flow, but it shall ensure the convenience of operation and maintenance.
9. During installation, clean the inner cavity of the valve body to prevent dirt from blocking or damaging the sealing parts, and check whether the connecting bolts at all parts are tightened evenly.
10. During operation, the parts in contact with the medium must be checked frequently, and the vulnerable parts must be replaced regularly according to the actual use.
Precautions for diaphragm valve
1. When the diaphragm needs to be replaced, be careful not to tighten or loosen the diaphragm too tightly.
2. If the valve is applied to the intermittent operation pipeline, the parts in contact with the medium shall be cleaned during shutdown to prolong the service life.
3. When manually operating the longitudinal valve, do not use the auxiliary lever to open and close the valve, so as not to damage the driving parts or sealing parts due to excessive torque.
4. Before application, the repaired valve shall be subject to sealing test according to relevant test regulations, and can be installed only after passing the test. And make maintenance records for future reference.
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