Author: Cast Steel Valves

(1) Classification of control valves
The adjustment mechanism of the control valve is a device that converts the change in the output displacement of the actuator into a change in the flow area between the valve core and the valve seat of the control valve. Usually, the regulating mechanism is called a valve. According to the nominal diameter, it can be divided into small diameter (DN<40mm), medium diameter and large diameter (DN>350mm); according to the nominal pressure, it can be divided into vacuum (50mm< DN <300mm) (PN<atmospheric pressure), low pressure (PN<1.6MPa), medium pressure (2.5MPa<PN<6.4MPa), high pressure (10.0MP a<PN<80.0MPa) and ultra high pressure (PN>80.0MPa), etc. According to the fluid working temperature, it can be divided into ultra-low temperature (t<-100℃), low temperature (-100℃ <t<-29℃), normal temperature (-29℃<t<120℃), medium temperature (120℃ <t< 450 ℃) and high temperature (t>450℃); according to the connection with the pipeline, it can be divided into flange connection, threaded connection, welded connection, clamp connection and ferrule connection, etc.; according to the mesh material, it can be divided into non-metallic Material valve, metal material valve and metal valve body lining valve, etc.; according to the application object, it can be divided into flow, pressure, temperature, liquid level, etc.

(2) The function and characteristics of the regulating valve
1. From the structure. The adjustment mechanism is composed of a national body, a valve trim, an upper valve cover, and a lower valve cover. The valve body is the device through which the controlled fluid flows. It is used to connect the bamboo path and realize the fluid passage. The well provides the support of the valve seat and other wide internal parts. Because the internal parts are components that directly contact the controlled medium in the room, including the valve core, the seat, the mesh rod, the guide sleeve, the sleeve, the sealing ring, etc., the upper valve passage component is usually composed of the upper reading cover, the packing cavity, It is composed of packing, upper cover plate and connecting bolts. In some adjustment mechanisms, the lower valve cover is a part of the reading body and is not separated. The lower valve is used for the adjustment mechanism with bottom guide, which includes the lower valve cover, guide sleeve and discharge screw. In order to facilitate installation and maintenance, the upper valve cover of some adjustment mechanisms is integrated with the valve body, while the lower valve cover is separated from the valve body, which is called a valve body separation valve. For example, some high-pressure valves and valve body separation valves.
2. From the perspective of the valve body structure, it can be divided into a single seat valve body with a valve seat and a valve core, a double mesh seat valve body with two valve seats and a valve core, a connecting port and a connecting outlet Two-way valve body, a three-way valve body with three connection ports (division of one inlet and two outlets or confluence of two inlets and one outlet).
3. From the perspective of spool displacement, the adjustment mechanism is divided into linear displacement valve and angular displacement valve. They are used in conjunction with linear displacement actuators and angular displacement actuators respectively. Straight-way valves, angle valves, sleeve valves, etc. are linear displacement valves, also known as sliding stem valves. Butterfly valves, eccentric rotary valves, ball valves, etc. are angular displacement valves, also known as rotary valves (Rotary Valve) . In recent years, some manufacturers have also introduced a control valve with a movable valve seat, which cooperates with a quarter-turn actuator, but from the relative displacement of the valve core, it is still a linear displacement. For example, Nufflo control valve.
4. From the perspective of the valve core guide, it can be divided into top guide, top bottom guide, sleeve guide, valve stem guide and valve seat guide. For fluid control and closure, the valve core guide is very important, and the valve core guide is used to align the valve core and the valve seat. The top guide adopts the bonnet or a guide sleeve or packing structure in the valve body to realize the guide; the top and bottom guide adopts the guide sleeve of the bonnet and the lower bonnet to realize the guide, and the top and bottom guide is required for the double seat valve and the adjustment mechanism that requires precise guidance. ; Sleeve guide adopts the outer surface of the valve core and the inner surface of the sleeve to guide. This guide method has self-centering performance and can accurately realize the centering of the valve core and valve seat; the valve stem guide adopts the upper valve cover The guide sleeve and the valve seat ring are centered, and the shaft sleeve and the valve stem are used for guidance; the valve seat guide is used in the small flow control valve, and it uses the valve seat to directly center.
5. From the perspective of the unbalanced force on the spool, the spool of the regulating mechanism has two types: unbalanced and balanced. A balanced spool is a spool with a balance hole on the spool. When the spool moves, the upper and lower parts of the spool are connected by a balance hole. Therefore, most of the pressure difference on both sides is offset, which greatly reduces The effect of unbalanced force on the valve core. The balanced spool needs a balanced chamber, therefore, it needs to be sealed by a sealing device. Depending on the flow direction, the pressure on the balance spool can be the pressure in front of the valve (flowing from the center to the outside) or the pressure after the valve (flowing from the outside to the center). The balanced valve core can be used for the valve core of the sleeve structure and the valve core of the plunger structure. The two sides of the unbalanced spool are the pressures before and after the control valve. Therefore, the unbalanced force on the spool is large, and the control valve of the same caliber requires a larger thrust actuator to operate.
6. From the perspective of spool pressure reduction, the spool structure has single-stage pressure reduction and multi-stage pressure reduction. Due to the large pressure difference between the two ends of the single-stage pressure reduction structure, it is suitable for occasions with low noise and low cavitation. In situations with high noise reduction requirements and severe cavitation, a multi-stage pressure reduction structure should be adopted. In the multi-stage pressure reduction structure, the pressure difference between the two ends of the control valve is broken down into several smaller pressure differences, so that the pressure difference in each stage is small, and cavitation and flashing will not occur, thereby preventing cavitation And the flash evaporates, also greatly reduces the noise.
7. From the perspective of flow characteristics, it can be divided into linear characteristics, equal percentage characteristics, quick opening characteristics, parabolic characteristics, hyperbolic characteristics, and some correction characteristics according to different changes in the flow area. Flow characteristics indicate the relationship between valve stem displacement and fluid flow. Usually, the flow characteristics are used to compensate the nonlinear characteristics of the controlled object. The shape of the spool or the shape of the sleeve opening determines the flow characteristics of the control valve. The straight stroke valve core can be divided into flat type (for quick opening), plunger type, window type and sleeve type. Due to the different changes in the opening area, the flow area is also different when the spool moves, so as to achieve the required flow characteristics. Plunger valve and window valve can also have different shapes according to the required flow characteristics. The spools of quarter-turn valves also have different shapes, such as traditional valve plates and dynamic contour valve plates for butterfly valves; O-shaped openings, V-shaped openings, and modified openings for ball valves.
8. From the perspective of the interchangeability of valve trims, the trims of some adjustment mechanisms can be easily replaced and maintained. For example, the sleeve valve can be easily replaced to achieve different flow characteristics; the top and bottom oriented valve trim can be easily replaced and maintained. Conveniently flip the valve core and valve seat to realize the replacement of the positive valve and the reverse valve, thereby realizing the replacement of air opening and air closing; the valve body separation valve can be easily disassembled for valve seat replacement and cleaning.
9. From the perspective of the upper bonnet structure, according to different application requirements, ordinary bonnets, long-neck bonnets, long-neck bonnets with heat dissipation or heat absorption fins, and bellows-sealed bonnets can be used. The long-necked bonnet is used for high temperature and low temperature applications. It protects the stem packing to avoid sticking, seizing, leakage or reducing lubrication effect caused by the temperature of the medium. In addition to the long-necked bonnet that extends the bonnet to keep the packing temperature away from the working temperature of the medium, heat dissipation or heat absorption fins can also be added to make a long-neck bonnet with heat dissipation or heat absorption fins to keep the medium temperature Be reduced or improved. Generally, the cast long neck bonnet has better heat dissipation and higher high temperature adaptability, and is used in high temperature applications; the long neck bonnet assembled by stainless steel has lower thermal conductivity and better low temperature adaptability It is used in low temperature applications. When the controlled medium is not allowed to leak, the upper valve cover with common packing structure cannot be used, and the upper valve cover with bellows seal must be used. This structure adopts a bellows seal, which can make the controlled medium be sealed in the valve body without contacting the packing and prevent fluid leakage. The pressure resistance and temperature influence of the bellows should be considered when selecting.
10. From the perspective of the connection between the adjusting mechanism and the pipeline, there are several types such as screw thread connection, flange connection, flangeless clamp connection and welding connection. Small-scale control valves often use screw-type pipe thread connection, the valve body connection end is a tapered pipe female thread, and the pipe connection end is a tapered pipe male thread. This connection method is suitable for the connection between the control valve body and the pipeline with a caliber less than 2in0, and is not suitable for high temperature conditions. Due to the difficulty of maintenance and disassembly, unions need to be installed upstream and downstream of the control valve. The flange connection adopts the flange matched with the control valve, and connects with bolts and gaskets. The matching flange is welded to the pipeline. According to the different connecting flanges of the control valve, there are different matching flanges, for example, there are flat flanges, raised face flanges, ring type joint flanges, etc. The flange used should be compatible with the rated working pressure and temperature of the control valve. When connecting a flat flange, a gasket can be installed between the two flange surfaces, which is suitable for the installation and connection of low pressure, cast iron and copper control valves; the raised flange is processed with a tightening line, which is concentric with the flange Small groove, when the gasket installed between the two flanges is compressed under the action of bolts, the gasket will enter the groove of the tightening line to make the sealing of the connection tighter. The convex flange connection is suitable for most applications Cast steel and alloy steel control valves used in occasions; The ring-shaped joint flange is used for the connection of high-pressure control valves. The lens gasket is used. When the gasket is compressed, the gasket is pressed into the U-shape on the convex surface of the flange In the groove, a tight seal is formed; the clamp connection is suitable for the connection of low-pressure and large-diameter control valves such as gate valves and butterfly valves. The control valve is clamped by an external flange, and gaskets are placed on the connection surface, and the flange is tightened with bolts. The connection of the valve and the pipeline; The welding connection directly welds the control valve to the pipeline, and can adopt socket welding or butt welding. The advantage of welding connection is that it can achieve strict sealing. The disadvantage is that the welding connection requires that the valve body material can be welded and is not easy to disassemble from the pipeline. Therefore, welding connection is generally not used.

The characteristics of the control valve adjustment mechanism are as follows.
1. There are various types of control valves, and their application occasions are different. Therefore, the type of control valve should be selected reasonably according to the requirements of the process and production process.
2. The control valve is divided into two types: air open and air close. When the air-open control valve is in a fault state, the control valve is closed, and when the air-close control valve is in a fault state, the control valve is opened. Some auxiliary equipment can be used to form a retaining valve or make the control valve self-locking, that is, when a fault occurs, the control valve maintains the valve opening before the fault.
3. The way of air opening and air closing can be realized by the type of positive and negative actuators and the combination of positive and negative valves. When using a valve positioner, it can also be realized by a valve positioner. 0 The material of the valve trim changes with temperature. Therefore, the influence of thermal expansion at different temperatures should be considered, and the change of pressure resistance at high temperature should also be considered. The corrosion resistance and fatigue resistance of the material should be considered.

Three-way control valve
The three-way control valve is a type of multi-port control valve. According to the action mode of the fluid, the three-way control valve can be divided into two types: converging valve and diverging valve. The confluence valve has two populations and flows out from one outlet after confluence. The diverter valve has a fluid population, which is divided into two streams and flows out from two outlets. Shown in the figure is the structure diagram of the split three-way regulating valve. The structure of the confluence three-way control valve is similar to that of the split three-way control valve. The characteristics of the three-way control valve are as follows.
1. The three-way regulating valve has two valve cores and valve seats, and its structure is similar to that of a double seat valve. However, in a three-way control valve, when the flow area between one valve core and the valve seat increases, the flow area between the other valve core and the valve seat decreases. In a double seat valve, the flow area between the two valve cores and the valve seat increases or decreases at the same time.
2. The air opening and closing of the three-way regulating valve can only be achieved by selecting the positive and negative effects of the actuator. The change of the air opening and closing of the double seat valve can be realized by directly installing the valve body or valve core and the valve seat in reverse.
3. When the three-way regulating valve is used in a control system that requires fluid to be proportioned, since it replaces an air-open control valve and an air-close control valve, it can reduce costs and reduce installation space.
4. The three-way regulating valve is also used in places where the bypass is controlled. For example, one fluid passes through the heat exchanger to exchange heat, and the other fluid does not exchange heat. When the three-way regulating valve is installed in front of the heat exchanger, the split three-way regulating valve is adopted; when the three-way regulating valve is installed behind the heat exchanger, the converging three-way regulating valve is adopted. Since the fluid flowing in the three-way regulating valve installed in front of the heat exchanger has the same temperature, the leakage is small; the fluid flowing in the three-way regulating valve installed behind the heat exchanger has different temperatures, The expansion degree of the valve core and the valve seat is different, therefore, the leakage is larger. Generally, the temperature difference between the two fluids should not exceed 150C.
5. The leakage of the three-way control valve has a great relationship with the structure, and the leakage level can be from II to IV.
The three-way control valve shown in the figure adopts a cage structure with a balance hole and is guided by the cage. Therefore, the unbalanced force can be greatly reduced. The early three-way control valve used a cylindrical thin-walled window and was guided by the side of the spool. Although it could reduce the unbalanced force, there was still a large unbalanced force when a stream of fluid was close to closing (flow direction). Moreover, as the valve opening changes, the unbalanced force changes. The cage structure with balance holes as shown in the figure can eliminate the unbalanced force and has a damping effect, which is beneficial to the stable operation of the control valve. The three-way control valve guided by the valve stem has a relatively large balance force, and the required driving thrust is related to the fluid outlet, the population pressure, the friction force and the pressing force.
Due to the large leakage of the three-way control valve, in applications where a small leakage is required, two control valves (and a three-way connector) can be used to split or merge fluids, or to control the proportion of fluids.
Three-way valve and sleeve valve

Sleeve control valve
Sleeve control valve as shown in the figure, also known as cage valve. The structure of the valve body is similar to that of a straight-through control valve, and it is also composed of a valve body, a valve core, a guide sleeve, an upper valve cover, an upper cover plate, a valve stem, and packing. Its structural feature is the use of valve core and valve cage (sleeve) for internal parts. The sleeve regulating valve is guided by the inner surface of the cage, and the cage is used to throttle the opening to meet the required flow characteristics.
The features of the sleeve control valve are as follows.
1. Convenient installation and maintenance: The valve seat is tightly pressed on the valve body through the valve cover, and no threaded connection is used, which is convenient for installation and maintenance.
2. Convenient to change the flow characteristics: The flow out of the sleeve in the sleeve regulating valve is called the outward flow from the center, and vice versa, the flow from the outside to the center. Shown in the figure is the structure diagram of the straight-through sleeve regulating valve that flows from the outside to the center. However, it is usually used to flow from the lower part of the sleeve and flow out through the opening of the valve cage, which is opposite to the flow direction in the figure. 3, 4 or .6 throttle openings are symmetrically distributed on the sleeve. The shape of the throttle opening is related to the required flow characteristics. Therefore, the sleeve (the shape of the throttle opening) can be easily replaced to change the control The flow characteristics of the valve.
3. Noise reduction and cavitation reduction: In order to reduce the noise of the control valve, there is a type of sleeve control valve with noise reduction valve trim. Its sleeve and valve core have multiple small holes, which use small holes to increase resistance, convert the speed head into kinetic energy, and reduce noise. Generally, this kind of sleeve regulating valve can reduce noise by more than 10dB, so it is widely used in noise reduction occasions. In order to reduce the control valve noise, a multi-stage pressure reduction method can also be used. The total pressure drop at both ends of this type of noise reduction control valve is distributed to all levels, so that each level will not cause flashing and cavitation of the fluid, thereby reducing the noise of the control valve, and weakening and preventing flashing and cavitation Scour and wear caused by. The bottom of the sleeve control valve spool is flat. If cavitation occurs, the impact generated by the bubble burst will not act on the spool, but will be absorbed by the medium itself. Therefore, the sleeve control valve has little cavitation impact and has a long service life.
4. Leakage is larger than single-seat valve: Because there is a graphite piston ring seal between the sleeve and the valve core, after long-term operation, the wear of the seal ring makes the leakage of the sleeve control valve larger than that of the single-seat valve.
5. High interchangeability and versatility: Replace different sleeves to obtain different flow coefficients and different flow characteristics.
6. Reduce the impact of unbalanced force: Generally, there are two types of sleeve control valves, one is a balanced valve, and the other is an unbalanced valve. The valve core of the balance valve is provided with a balance hole, which greatly reduces the unbalanced force on the valve core. At the same time, it has a damping effect, which is beneficial to the stable operation of the control valve. Therefore, this type of control valve is often used in applications where the pressure difference is large and low noise is required.
7. Since the valve core can be disassembled from the lower part of the anti-body sleeve regulating valve, it is especially suitable for occasions where the internal parts of the valve need frequent inspection and maintenance.

Pinch valves are divided into two types: support sleeve and unsupported sleeve. The picture shows a pinch valve without a support sleeve, which is a commonly used pinch valve. An elastic sleeve is installed in a split metal valve body so that the fluid only contacts the sleeve. The metal valve body is a pressure vessel made of cast iron or aluminum, and the operating air pressure should be greater than the fluid pressure. The diameter of the sleeve is the same as that of the pipe. When the pinch valve is fully opened, there is no pressure drop at both ends of the valve. The sleeve and the valve body are pressurized with compressed air, liquid or process fluid. When pressure squeezes the sleeve, the sleeve pipe restricts the fluid, forming a pressure drop, and adjusting the fluid flow. When the pressure disappears, the original shape is restored under the elastic force of the sleeve. This kind of pinch valve is suitable for DN25~DN1200mm, which can contain solid particles.
The pinch valve supporting the sleeve is supported by a pipe with slots of equal height, which is sealed in the pressure tank. The support pipe is divided into an inlet and an outlet by a formed plug. The plug is used to prevent fluid from directly entering the pipeline. The fluid entering the valve is forced to exit the pipe radially through the slot. In this way, the elastic sleeve is lifted away from the valve seat it surrounds, and the fluid must flow radially back to the pipe and exit the valve. The fluid pressure lifts the sleeve away from the support tube valve seat. The force of the fluid is offset by the pressure of the control fluid and the elastic force generated by the elastic sleeve. This kind of pinch valve is suitable for clean fluid with rated working pressure <10MPa and DN<300mm.
The sleeve should be made of thermoplastic, elastic materials with good wear resistance and chemical compatibility, such as chlorinated butyl rubber, ethylene propylene rubber, neoprene rubber, polyurethane, and fluoro rubber.
The characteristics of the pinch valve are as follows.
1. The adjustable is relatively small, generally 8~26 according to the structure of the sleeve.
2. There is no valve stem and packing, etc., with good sealing performance, which can meet the requirements of bubble sealing.
3. It can handle fluids with entrained solids.
4. For split pipe clamp valves, serious accidents such as leakage of the controlled fluid due to the failure of the sleeve will occur.
5. Due to the limitation of elastic materials, the temperature and pressure resistance are poor. Generally, the temperature for continuous application should not exceed 200″C.

(1) Precautions before disassembling the floating ball valve
For safety, the following items must be noted before the valve is removed from the pipeline or disassembled:
① When disassembling the valve, keep your hands away from the inner cavity of the valve. The remotely operated valve will be closed at any time to injure the operator.
②You should understand the medium in the pipeline, and if you have any questions, you can check with the relevant personnel.
③ Wear protective clothing or appropriate equipment when contacting media.
④When the valve and pipeline are depressurized, open the valve before removing the valve from the pipeline to drain the pressure that may be left in the valve and keep the valve in the open position. After removing the valve and before disassembling, place the valve vertically and open and close the valve repeatedly to drain the residual medium.
⑤Rated value of sealing seat and valve body one-the safe and correct use of this type of valve mainly depends on the rated value of sealing seat and valve body. The rating on the nameplate should be understood. There are many sealing seat materials for this type of valve, and some sealing seat materials are rated less than the valve body rating. The overall rating of the valve body and the sealing seat also depends on the type, specification, sealing seat material, bolt material and stability of the valve. The operating conditions of the valve cannot exceed these ratings.

(2) Installation method of floating ball valve
① The valve is in the open position.
②The valve allows fluid to flow in from either side. To ensure a good installation, it must be installed in accordance with the actual conditions of standard pipeline transmission.
③Use flange bolts of corresponding specifications, and tighten the flange bolts according to the gasket manufacturer’s requirements.
④If there is leakage at the valve stem seal during installation, this is because the valve has undergone a large temperature change during transportation, and the seal can be restored by simply adjusting the packing as described in the “Maintenance” section.

(3) Maintenance method of floating ball valve
① Regular maintenance requires regular inspection to ensure that the valve is operating well. Daily maintenance includes tightening the gland bolts regularly to compensate for the wear of the valve stem seal.
②Overhaul includes replacement of sealing seat and packing. You can purchase a repair kit, and refer to the following “disassembly” and “assembly” methods for replacement.
③Maintenance spare parts package Standard maintenance spare parts package, including sealing seat, valve stem seal and PTFE gasket.

Floating ball valve disassembly principle diagram

(4) Schematic diagram of disassembly of floating ball valve
①Fix the right valve body flange of the ball valve downward on the workbench, and turn the valve stem to close the ball valve.
②Unscrew the connecting flange nut and take off the left valve body.
③Remove the ball from the inner cavity of the right valve (be careful not to bump the ball).
④ Take out the sealing seat from the left and right valve bodies.
⑤Remove the stem retaining ring and wrench.
⑥Unscrew the gland and two bolts, and remove the positioning block and gland.
⑦ Take out the valve stem from the inner cavity of the right valve.
③Take out 4 packings from the middle port (1 upper packing, 2 middle packing and 1 lower packing).
④Remove the gasket from the right valve body.
①Remove the thrust washer from the valve stem, and remove the upper bushing from the gland.
Check all parts for damage, bumps and corrosion, replace the gaskets and damaged, worn or corroded parts, and repair the damaged parts.

(5) Assembly schematic diagram of floating ball valve (as shown in the figure)
① All metal parts are cleaned with oil-free cleaning agent.
②Assemble the sealing seat on the right valve body and the left valve body.
③Put the thrust washer on the valve stem.
④Fix the right valve body flange down on the workbench, put the valve stem into the packing hole from the right valve body cavity, and then install a set of packing (4 in total).
⑤ Put the upper lining into the gland, put the gland into the middle port, and pre-tighten the two bolts.
⑥ Rotate the valve stem to make the flat body at the end of the valve stem consistent with the channel, and then align the groove on the ball with the flat body of the valve stem, and put the ball into the right valve body.
⑦ Put the gasket into the right valve body, install the left valve body, turn the valve stem to make the ball completely in the open position, and then pre-tighten the opposite nut on the connecting flange. Turn the valve stem to open and close the ball valve several times to make the ball in the open position, tighten all the nuts on the connecting flange, and then open and close the ball valve several times. It should be flexible and free from jamming during the opening and closing process. Install the valve stem on the positioning block, snap on the shaft retaining ring, and place the ball valve in a fully open or fully closed state.

(6) Maintenance and use of ball valve with actuator
① For the valve part, refer to the valve product instruction manual.
② For the actuator (electric, pneumatic and worm gear drive, etc.), refer to the instruction manual of the actuator.

(7) Common faults and troubleshooting methods of floating ball valves
① Leakage at the right and left valve body. Trim the sealing surface, remove dirt, replace the gasket, and tighten the nuts evenly and crosswise.
② Leakage of filler. Tighten the gland bolts or replace the packing (1 upper packing, 2 middle packing, 1 lower packing). Be careful not to trim the edges during installation, and tighten the gland bolts evenly.
③The valve stem is not flexible. Loosen the gland bolts, evenly loosen the left and right body connecting nuts or thicken the gasket to reduce the pre-tightening force, and be careful not to leak.
④ Seal leakage. Tighten the left and right connecting nuts evenly and crosswise to increase the pre-tightening force, or replace the sealing seat and gasket and clean the sealing groove.
⑤ When the ball valve cannot be screwed, do not put auxiliary levers or other tools on the wrench to harden it, which will damage the valve stem. If the operation is too strenuous, the reason must be analyzed. The closing parts of some valves are difficult to open due to thermal expansion. At this time, the gland bolts can be loosened, and the valve stem stress can be removed before opening. When the packing is too tight, the packing can be properly loosened.
⑥The ball valve can only be used for fully open or fully closed. Generally, it is not allowed for adjustment and throttling. The ball valve with a wrench is fully open when the wrench is parallel to the channel, and fully closed when it is turned 90°.
⑦Seal leakage. Tighten the left and right connecting nuts evenly and crosswise to increase the pre-tightening force, or replace the sealing seat and gasket and clean the sealing groove.