A straight-through single-seat control valve consists of a valve body, valve seat, valve core, guide sleeve, valve cover, valve stem, and packing. The valve core and valve stem are connected together. The connection method can be interference fit pin fixation, threaded connection pin fixation, or the valve stem and valve core can be machined as a single piece. A guide sleeve is provided between the valve cover and the valve body to guide the up and down movement of the valve core. A small hole on the guide sleeve connects the inner cavity of the valve body to the valve outlet end. The medium in the upper cavity of the guide sleeve can easily flow into the valve outlet end through the small hole without affecting the movement of the valve core. A straight-through single-seat control valve has only one valve core and one valve seat in its valve body. Its characteristics include low leakage and easy sealing. Structurally, there are regulating and shut-off types. Their difference lies in the shape of the valve core. The regulating type valve core is plunger-shaped; the shut-off type valve core is flat or conical. Another characteristic is that the medium exerts a large force on the valve core, i.e., a large unbalanced force, especially under high pressure differential and large diameter conditions, so it is only suitable for low pressure differential applications. Otherwise, an actuator with a large thrust should be selected, or a valve positioner should be used.
Straight-through single-seat control valves have two types of valve cores: upright and reverse-mounted. When the valve core moves downward, the flow area between the valve core and the valve seat decreases, which is called upright mounting; conversely, it is called reverse mounting. The nominal size DN and valve seat diameter dN of the control valve indicate the size of the valve. For single-guided valve cores with a nominal size DN < 2.5mm, upright mounting is possible, but reverse mounting is not allowed. Therefore, air-to-open valves must use a reverse-acting actuator. Air-to-open control valves have a flow area that increases with increasing signal pressure; while air-to-close valves have the opposite effect, with the flow area decreasing as the signal pressure increases.