Valve adjustment mechanism

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

From the structural point of view, the adjustment mechanism consists 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 a controlled fluid flows. It is used to connect a pipe and realize a fluid passage, and provides support of a valve seat and the like. The valve trim is a component that directly contacts the controlled medium inside the valve and includes a valve core, a valve seat, a valve stem, a guide sleeve, a sleeve, a seal ring, and the like. Typically, the upper bonnet assembly includes an upper bonnet, a packing cavity, a packing, an upper cover plate, a connecting bolt, and the like. In some adjustment mechanisms, the lower valve cover acts as part of the valve body and does not separate. The lower valve cover is used for a bottom-guided adjustment mechanism, which includes a lower valve cover, a guide sleeve, and a discharge screw. For ease of installation and maintenance, the upper valve cover of some adjustment mechanisms is united with the valve body, and 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 valve body structure, it can be divided into single-seat valve body with one valve seat and one valve core, two-seat valve body with two valve seats and one valve core, two with one connection population and one connection outlet. A three-way valve body that has three connection ports (a split of one population and two outlets or a confluence of two populations and one outlet).

From the displacement of the spool, the adjustment mechanism is divided into a linear displacement valve and an angular displacement valve. They are used with linear displacement actuators and angular displacement actuators, respectively. Straight valve, angle valve, sleeve valve, etc. are linear displacement valves, also known as sliding valve stem valve (SlidingStemValve) o butterfly valve, eccentric rotary valve, ball valve, etc. are angular displacement valves, also known as rotary valve (Ro-taryValve) o In recent years, some manufacturers have also introduced control valves for moving seats. They cooperate with the quarter-turn actuators, but they are still linear displacements from the relative displacement of the valve cores, such as the Nufflo control valve.

Seen from the spool guide, it can be divided into top guide, top guide, sleeve guide, stem guide and seat guide. For fluid control and closure, etc., the orientation of the spool is important. The spool guide is used to align the spool and seat. The top guide adopts a guide sleeve or packing structure inside the valve cover or the valve body to achieve the guide; the top and bottom guide adopts the guide sleeve of the valve cover and the lower valve cover to realize the guide; the double seat valve and the adjustment mechanism that needs precise guide need to adopt the top and bottom guide. The sleeve guide adopts the outer surface of the valve core to guide the inner surface of the sleeve. This guide method has self-centering performance and can accurately realize the centering of the valve core and the valve seat; the stem guide adopts the upper valve cover. The guide sleeve and the seat ring are centered, and the shaft sleeve and the stem are used to guide; the seat guide is used in the small flow control valve and it is directly centered with the valve seat.

From the unbalanced force experienced by the spool, the spool of the adjusting mechanism is unbalanced and balanced. The balanced valve core is a valve core with a balancing hole in the valve core. When the valve core moves, the upper and lower parts of the valve core are connected by a balance hole. Therefore, most of the pressure difference between the two sides is offset and greatly reduced. The effect of the unbalanced force on the spool is that the balanced spool needs to balance the chamber. Therefore, the seal must be sealed. Depending on the direction of flow, the pressure on the balanced spool can be pre-valve pressure (outward center flow) or post-valve pressure (outward flow to center). The balanced valve plug can be used for the valve plug of the sleeve structure, and can also be used for the valve plug of the plunger structure. The two sides of the unbalanced spool are the pressures of the front and rear of the control valve respectively. Therefore, the unbalanced force of the spool is large, and the actuator with the same caliber control valve needs more thrust to operate.

Seen from the pressure drop of the valve core, the valve core structure has a single-step buck and multi-step buck. Because of the large pressure difference at both ends, the single-stage pressure-reducing structure is suitable for applications where noise is low and cavitation is not severe. In high noise reduction requirements, severe cavitation occasions.

In a multi-stage pressure-reducing structure, the pressure difference across the control valve is decomposed into several pressure differences, so that the pressure difference in each stage is small, no cavitation and flashing phenomenon will occur, thus preventing cavitation and flash evaporation. Health, but also greatly reduce the noise.

From the flow characteristics, according to the different changes in the flow area, can be divided into linear special, equal percentage characteristics, quick opening characteristics, parabolic characteristics, hyperbolic characteristics and some correction characteristics. The flow characteristic, Jé™›, indicates the relationship between the displacement of the stem and the fluid flow. In general, the flow characteristics are used to compensate for the non-linear characteristics of the controlled object. The shape of the valve plug or the shape of the sleeve opening determines the flow characteristics of the control valve. Straight line, Cheng valve core can be divided into flat type (for quick opening), plunger type, window type and sleeve type. Due to the different opening area changes, the flow area is also different when the valve core moves, so that the required flow characteristics are achieved. The ram valve and window type valves can also have different shapes depending on the desired flow characteristics. The rotary valve spool also has different shapes, for example, the traditional valve plate and dynamic contour valve plate for the butterfly valve; O-shaped opening, V-shaped opening and modified opening for the ball valve.