Introduction to valve regulating mechanism

The regulating mechanism is a device that converts changes in the output displacement of the actuator into changes in the flow area between the control valve spool and the valve seat. The regulating mechanism is usually called a valve, such as a straight-through single-seat valve, an angle valve, etc. Its structural characteristics can be analyzed from the following aspects. ​

From a structural point of view, the regulating mechanism consists of the valve body, valve trim, upper valve cover assembly, lower valve cover, etc. The valve body is a device through which the controlled fluid flows. It is used to connect pipes and realize fluid passages, and provides support for valve internal parts such as valve seats. Valve trims are components inside the valve that are in direct contact with the controlled medium, including valve core, valve seat, valve stem, guide sleeve, sleeve, sealing ring, etc. Usually, the upper valve cover assembly includes upper valve cover, packing chamber, packing, upper cover plate and connecting bolts. In some regulating mechanisms, the lower valve cover is part of the valve body and is not separated. The lower valve cover is used for the adjustment mechanism with bottom guide, which includes the lower valve cover, guide sleeve and discharge screw. For the convenience of installation and maintenance, the upper valve cover of some regulating 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 type valve, such as some high-pressure valves and valve body separation valves. From the perspective of valve body structure, it can be divided into single-seat valve body with one valve seat and one valve core, double-seat valve body with two valve seats and one valve core, two-seat valve body with one connecting port and one connecting outlet. One-way valve body, three-way valve body with three connection ports (divergence of one inlet and two outlets or confluence of two inlets and one outlet). ​

From the perspective of valve core 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. belong to linear displacement valves, also known as sliding stem valves (SlidingStemValve). Butterfly valves, eccentric rotary valves, ball valves, etc. belong to angular displacement valves, also known as rotary valves (Ro-taryValve). In recent years, some manufacturers have introduced control valves with movable valve seats, which are matched with quarter-turn actuators, but the relative displacement of the valve core is still a linear displacement, such as the Nufflo control valve. ​

From the perspective of valve core guide, it can be divided into top guide, top bottom guide, sleeve guide, ceramic valve stem guide and valve seat guide. For the control and closure of fluids, the guidance of the valve core is very important. The valve core guide is used for the centering and matching of the valve core and the valve seat. The top guide uses a guide sleeve or packing structure in the valve cover or valve body to achieve guidance; the top and bottom guide uses the guide sleeves of the valve cover and the lower valve cover to achieve guidance. Top and bottom guides are required for double-seat valves and adjustment mechanisms that require precise guidance. ; The sleeve guide uses the outer surface of the valve core and the inner surface of the sleeve for guidance. This guide method has self-centering performance and can accurately achieve centering of the valve core and valve seat; the valve stem guide uses the upper valve cover. The guide sleeve is aligned with the valve seat ring, and the shaft sleeve and the valve stem are used to achieve guidance; the valve seat guide is used in small flow control valves, and it is directly aligned with the valve seat. ​

Judging from the unbalanced force on the valve core, the valve core of the regulating mechanism has two types: unbalanced and balanced. The balanced valve core is a valve core with a balance hole on the valve core. When the valve core moves, the upper and lower parts of the valve core are connected by the balance hole. Therefore, most of the pressure difference on both sides is offset and greatly reduced. The effect of unbalanced force on the valve core. The balanced valve core needs to balance the chamber, so it needs to be sealed by a sealing device. Depending on the flow direction, the pressure on the balance valve core can be the pressure in front of the valve (flow from the center outward) or the pressure behind the valve (flow from the outside to the center). The balanced valve core can be used for the valve core of the sleeve structure or the valve core of the plunger structure. The two sides of the unbalanced valve core are the pressure in front of the control valve and the pressure behind the valve. Therefore, the valve core is subject to a large unbalanced force, and the same caliber control valve requires an actuator with greater thrust to operate. ​

From the perspective of valve core pressure reduction, the valve core structure can be divided into single-stage pressure reduction and multi-stage pressure reduction. The single-stage buck structure has a large pressure difference at both ends, so it is suitable for situations where the noise is small and cavitation is not serious. In situations where noise reduction requirements are high and cavitation is severe. ​

In the multi-stage pressure reduction structure, the pressure difference at both ends of the control pressure relief valve spring is decomposed into several pressure differences, so that the pressure difference in each stage is small and cavitation and flash evaporation will not occur, thereby preventing cavitation and flash evaporation. It also greatly reduces the noise. From the perspective of flow characteristics, according to different changes in the flow area, it can be divided into linear characteristics, equal percentage characteristics, quick opening characteristics, parabolic characteristics, hyperbolic characteristics and some correction characteristics. Flow rate and JB represent the relationship between valve stem displacement and fluid flow rate. Usually, flow characteristics are used to compensate for the nonlinear characteristics of the controlled object. The shape of the valve plug or sleeve opening determines the flow characteristics of the control valve. Straight travel, stroke valve cores can be divided into flat type (for quick opening), plunger type, window type and sleeve type, etc. Due to the different changes in the opening area, when the valve core moves, the flow area is also different, thereby achieving the required flow characteristics. Plunger type valves and window type valves are also available in different shapes depending on the required flow characteristics. The valve cores 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. Quick opening linear equal percentage

From the perspective of the interchangeability of valve trims, the valve trims of some regulating mechanisms can be easily replaced and maintained. For example, sleeve valves can easily replace the sleeve to achieve different flow characteristics; top-bottom guided valve trims can be easily replaced. Flip the valve core and valve seat to replace the positive body valve and the reverse body valve, thereby realizing the replacement of air-opening and air-closing modes; the valve body separation valve can be easily disassembled for valve seat replacement and cleaning. ​

From the upper valve cover structure, according to different application requirements, ordinary valve covers can be used, long-neck valve covers or long-neck valve covers with heat dissipation or heat-absorbing fins, and bellows-sealed valve covers are also available. The long-neck valve cover is used in high and low temperature applications to protect the valve stem packing from being affected by the medium temperature and prevent sticking, seizure, leakage or reduction of lubrication effect. In addition to the long-neck valve cover that extends the valve cover to keep the temperature of the packing away from the working temperature of the medium, heat dissipation or heat-absorbing fins can also be added to make a long-neck valve cover with heat dissipation or heat-absorbing fins to keep the medium temperature be lowered or raised. Generally, cast long-neck valve covers have better heat dissipation and higher high-temperature adaptability and are used in high-temperature applications; stainless steel-assembled long-neck valve covers have lower thermal conductivity and better low-temperature adaptability. properties and are used in low temperature applications. When the leakage of the controlled medium is not allowed, 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 bellows sealing, which can seal the controlled medium in the valve body without contact with the packing and prevent fluid leakage. The pressure and temperature effects of the bellows need to be considered when selecting. ​

From the perspective of the connection between the adjusting mechanism and the pipe, there are several types: screw-type pipe thread connection, flange connection, flangeless clamping connection and welding connection. Small control valves often use screw-type pipe thread connections. 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 diameter less than 2″. No, it is suitable for high temperature working conditions. Due to the difficulty of maintenance and disassembly, it is necessary to install union joints upstream and downstream of the control valve. Flange connection Use the flange matching the control valve, connect it with bolts and gaskets, and the matching flange is welded to the pipe.

According to the different connecting flanges of the control valve, there are different matching flanges, such as flat flanges, raised surface flanges, annular joint surface flanges, etc. The flange used should be compatible with the rated operating pressure and temperature of the control valve. When connecting flat flanges, 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. There is a tensioning line processed on the convex flange, which is a small groove concentric with the flange. When the gasket installed between the two flanges is compressed under the action of the bolt, the gasket will enter the groove of the tensioning line. To make the connection more tightly sealed, the raised face flange connection is suitable for cast steel and alloy steel control valves used in most applications. The annular joint surface flange is used to connect the high-pressure control valve and uses a lens gasket. When the gasket is pressed, the gasket is pressed into the U-shaped groove on the convex surface of the flange to form a tight seal. The clamp connection is suitable for the connection of low-pressure and large-diameter control valves such as gate valves and butterfly valves. The external flange is used to clamp the control valve, a gasket is placed on the connection surface, and the flange is tightened with bolts to complete the connection between the valve and the pipeline. The welding connection welds the control valve directly to the pipeline, and can use socket welding or butt welding. The advantage of welded connection is that it can achieve strict sealing. The disadvantage is that welded connection requires the valve body material to be weldable and is not easy to disassemble from the pipeline. Therefore, welded connection is generally not used.