The adjustment mechanism 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. The regulating mechanism is usually called a valve, such as a straight-through single-seat valve, an angle valve, and so on. Its structural characteristics can be analyzed from the following aspects. From the structural point of view, the adjustment mechanism is composed of the valve body, the valve trim, the upper valve cover assembly, and the lower valve cover. The valve body is a device through which the controlled fluid flows. It is used to connect pipes and realize fluid passages, and to provide support for valve internals such as valve seats. Valve trim is a component that directly contacts the controlled medium inside the valve, including valve core, valve seat, valve stem, guide sleeve, sleeve, sealing ring, etc. Generally, the upper valve cover assembly includes an upper valve cover, a packing cavity, a packing, an upper cover plate, and connecting bolts. In some adjustment mechanisms, the lower valve cover is a part of the valve body and does not separate. The lower valve cover is used for the adjustment mechanism with bottom guide, which includes the lower valve cover, guide sleeve and drain 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, such as some high-pressure valves and valve body separation valves. 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-seat valve body with two valve seats and a valve core, and a two-seat valve body with a connecting port and a connecting outlet. A three-way valve body, a three-way valve body with three connecting ports (division of one inlet and two outlets or confluence of two inlets and one outlet). From the perspective of spool displacement, the regulating 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 (SlidingStemValve) o Butterfly valves, eccentric rotary valves, ball valves, etc. are angular displacement valves, also known as rotary valves (Ro-taryValve). In recent years, some manufacturers and 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 spool, it 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 and bottom guide, sleeve guide, valve stem guide and valve seat guide. For fluid control and closing, 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 the 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 guiding; the valve seat guide is adopted in the small flow control valve, and it uses the valve seat to directly center.
From the perspective of the unbalanced force on the spool, the spool of the adjustment mechanism is unbalanced and flat. Two types of scales. 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 between the two sides is offset and greatly reduced. The effect of unbalanced force on the valve core, the balanced valve core needs to balance the chamber, therefore, the sealing device is required to seal. 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 pressure 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. From the perspective of spool depressurization, the spool structure has single-stage depressurization and multi-stage depressurization. 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 occasions with high noise reduction requirements and severe cavitation. In the multi-stage pressure reduction structure, the pressure difference between the two ends of the control valve 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 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. The flow rate and J Bi indicate the relationship between the valve stem displacement and the fluid flow rate. Usually, the flow characteristics are used to compensate the non-linear 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. Straight travel, the valve core 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, the flow area is also different when the spool moves, so as to achieve the required flow characteristics. Plunger-type valves and window-type valves 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. From the perspective of the interchangeability of valve trims, the trims of some adjustment mechanisms can be easily replaced and maintained. For example, sleeve valves can be easily replaced with sleeves to achieve different flow characteristics; top-bottom-oriented trims can be easily replaced Flip the valve core and valve seat to realize the replacement of the positive valve and the reverse valve, thereby realizing the replacement of the air-opening and air-closing modes; the valve body separation valve can be easily disassembled for replacement and cleaning of the valve seat. From the perspective of the upper bonnet structure, according to different application requirements, ordinary bonnets can be used, long-neck bonnets or long-neck bonnets with heat dissipation or heat absorption fins, and bellows-sealed bonnets can also be used. The long-necked bonnet is used for high and low temperature applications to protect the stem packing from the temperature of the medium and prevent sticking, seizing, leakage or reducing the lubrication effect. In addition to the long-necked bonnet that extends the bonnet to keep the temperature of the packing 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 and temperature effects of the bellows should be considered when selecting. From the perspective of the connection between the adjustment mechanism and the pipeline, there are several types of screw 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 diameter of 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 a union upstream and downstream of the control valve. Flange connection Use flanges that match the control valve, connect them with bolts and gaskets, and weld the matching flanges to the pipeline. According to the different connecting flanges of the control valve, there are different matching flanges, such as flat flanges and raised face methods. Flange, ring-shaped joint surface flange, etc.. The flange used should be compatible with the rated working pressure and temperature of the control valve. When the flat flange is connected, a gasket can be installed between the two flange surfaces. It is suitable for low pressure, cast iron and copper The installation and connection of the quality control valve. The raised face flange is processed with a tightening line, 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 tightening line makes the sealing of the connection tighter. The convex flange connection is suitable for the control valve of cast steel and alloy steel used in most applications. The ring joint flange is used for the connection of the high pressure control valve. 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. Clamp the control valve by the flange, and place a gasket on the connecting surface, and use bolts to press the flange to complete the connection between the valve and the pipeline. The welding connection directly welds the control valve to the pipeline, which can be socket welding or butt welding. Welding connection The advantage is that strict sealing can be achieved, but 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.