Straight through single seat control valve has a spool and a seat. The valve stem is connected with the valve core. When the actuator makes a linear displacement, the valve core is driven to move through the valve stem. The upper cover plate is used to compress the packing, and the upper cover plate is connected with the valve body by bolts, which is used for the center positioning of the valve stem and valve core. The valve seat and the upper valve cover are used to ensure the center positioning of the valve core and the valve seat, and change the flow area of the fluid when the valve core moves, so as to change the control variables and realize the function of adjusting the fluid flow. The valve core in the figure adopts top guide mode. Some straight through single seat control valves adopt top and bottom guiding mode to improve guiding accuracy. Some small flow through control valves often use seat guide. Straight through single seat control valve has only one spool and one seat, which is a very common control valve.

The features of single seat control valve are as follows:

1. Small leakage, easy to achieve strict sealing and cut-off, for example, metal to metal hard seal, or metal to polytetrafluoroethylene or other composite material soft seal, the standard leakage is 0.0l% C (C is the rated flow coefficient).

2. The allowable pressure difference is small, for example, the allowable pressure difference of dnl00 valve is only 120kPa.

3. The flow capacity is small, for example, the flow capacity of dnl00 through valve is only 100.

4. Due to the large thrust of the fluid medium on the valve core, that is, the imbalance force is large, therefore, in the application of high pressure difference and large diameter, it is not suitable to use this kind of control valve. In recent years, through control valve with cage structure has been widely used because of its advantages of simple maintenance and changing valve flow characteristics by changing valve seat.

In order to reduce the influence of the unbalanced force on the valve core, the high thrust actuator can be used, the straight through double seat valve structure can be adopted, and the cage structure of the balanced valve core can also be adopted. The sliding piston ring lubricated with graphite is used between the upper part of the valve core and the cylinder body of the valve cage to seal between the valve core and the valve cage. The bonnet gasket and spiral gasket are also used to seal between the cage and the upper bonnet to ensure that the upstream high-pressure fluid will not enter the downstream low-pressure chamber. In addition, the fluid acts on the bottom and top of the valve core through the balance connection hole at the top of the valve core, which eliminates most of the static unbalanced force and has a certain damping effect, reducing the influence of vibration and other disturbances caused by fluid flow.