Direct control valves are a type of control valve that operates directly in response to changes in the control signal, without the need for additional auxiliary devices such as positioners or pilots. They are widely used in various industrial processes to regulate the flow, pressure, temperature, or level of fluids within a system. Direct control valves can be classified based on several factors:

1. Actuation Method:

   • Manual Control Valve: Operated manually by hand, typically using a handwheel, lever, or wrench to adjust the valve position.
   • Automatic Control Valve: Actuated automatically in response to changes in the control signal. These valves may be operated by pneumatic, hydraulic, electric, or electronic actuators.

2. Valve Type:

   • Globe Valve: Features a linear motion valve plug or disc that moves perpendicular to the flow path to regulate flow.
   • Butterfly Valve: Utilizes a rotating disc or vane mounted on a shaft to control flow.
   • Ball Valve: Employs a rotating ball with a hole (bore) through the center to regulate flow.
   • Plug Valve: Uses a cylindrical or tapered plug with a passage to control flow through rotation.
   • Needle Valve: Features a long, tapered needle-like stem to precisely control flow rates.
   • Diaphragm Valve: Uses a flexible diaphragm to regulate flow by deforming the diaphragm.

3. Function:

   • Flow Control Valve: Regulates the flow rate of fluid passing through the valve.
   • Pressure Control Valve: Controls the pressure within a system by adjusting the flow rate.
   • Temperature Control Valve: Regulates the temperature of a process fluid by adjusting the flow of heating or cooling media.
   • Level Control Valve: Maintains a constant level of liquid within a tank or vessel by controlling inlet or outlet flow.

4. Design and Construction:

   • Straight-Through Valve: Allows fluid to flow directly through the valve body with minimal resistance.
   • Angle Valve: Features an angled flow path, often used in applications where space is limited or where flow direction changes are required.
   • Tee Valve: Has a T-shaped configuration with three openings, used for diverting or mixing flows.

5. Control Signal Compatibility:

   • Analog Control Valve: Responds to continuous analog control signals (e.g., 4-20 mA) for precise control over a wide range of flow rates or pressures.
   • Digital Control Valve: Operates based on digital control signals (e.g., on/off signals or digital protocols) for discrete control actions.

These are some common classifications of direct control valves based on various factors such as actuation method, valve type, function, design, and control signal compatibility. Proper selection of the appropriate type of direct control valve is essential to meet specific application requirements and achieve optimal performance and reliability.