On-off valves, also known as shut-off valves or isolation valves, are a type of industrial valve used to completely stop or allow the flow of fluids (liquids, gases, or vapors) in a process system. Unlike control valves that provide continuous and precise flow control, on-off valves have a simple binary function of either being fully open or fully closed.

When the on-off valve is open, it allows the fluid to flow freely through the valve, facilitating the movement of the fluid within the system. When the valve is closed, it blocks the flow of fluid, effectively isolating or shutting off the process.

On-off valves are typically operated manually or automatically. Manually operated on-off valves are often operated by hand using a lever or a handwheel, while automatically operated on-off valves are actuated by various means, such as electric, pneumatic, hydraulic, or self-actuated mechanisms.

Applications of on-off valves include:

1. Emergency Shut-off: On-off valves are used as emergency shut-off devices to rapidly stop the flow of fluid in critical situations.

2. Start/Stop Operations: On-off valves are used to start or stop processes at specific intervals or based on specific conditions.

3. Safety Applications: On-off valves play a crucial role in safety systems to prevent potential hazards by isolating the fluid flow.

4. Flow Diversion: On-off valves are used to divert the flow of fluid from one line to another.

5. Simple On/Off Control: On-off valves are employed in systems where continuous and precise control is not required, and simple on/off control is sufficient.

Common types of on-off valves include ball valves, gate valves, butterfly valves, and plug valves. The choice of the on-off valve type depends on the specific application, fluid type, pressure and temperature conditions, and other relevant factors.

There are several types of on-off valves, each designed for specific applications and with different mechanisms for controlling the flow of fluids. Some common types of on-off valves include:

1. Ball Valves: Ball valves have a spherical disc with a hole in the center that rotates to open or close the flow passage. They provide quick and reliable on-off control and are widely used in various industries.

2. Gate Valves: Gate valves have a sliding gate or wedge that moves perpendicular to the flow path to open or close the valve. They are suitable for applications where a tight shut-off is required.

3. Butterfly Valves: Butterfly valves have a disc that rotates around an axis to control the flow. They are lightweight and have a compact design, making them suitable for large diameter applications.

4. Plug Valves: Plug valves have a cylindrical or conical plug that rotates to control the flow. They are commonly used in on-off and throttling applications.

5. Diaphragm Valves: Diaphragm valves use a flexible diaphragm to regulate flow, creating a tight seal when closed. They are often used in applications with slurry or corrosive fluids.

6. Pinch Valves: Pinch valves use a pinching mechanism to control flow by pinching or releasing a flexible tube or sleeve. They are ideal for applications with abrasive or corrosive media.

7. Globe Valves: Globe valves have a linear motion valve plug and seat arrangement, providing excellent throttling and on-off control capabilities.

8. Knife Gate Valves: Knife gate valves have a sharp-edged gate that cuts through and isolates the flow. They are commonly used in applications with viscous or high-solid content fluids.

9. Check Valves: Check valves allow flow in one direction only and prevent backflow. They act as on-off valves when preventing reverse flow.

10. Stop Valves: Stop valves are basic on-off valves used to stop or start fluid flow in a pipeline or system.

The choice of the on-off valve type depends on factors such as the type of fluid, pressure and temperature conditions, flow requirements, space limitations, and the specific needs of the application. Each type of on-off valve has its advantages and limitations, making it suitable for different scenarios where simple start-stop or isolation control is required.