Valves have a wide variety of uses, including controlling industrial processes, water for irrigation and waste management, residential water systems and appliances. They are used in almost all industrial processes, such as manufacturing, mining, processing of gases and oils, power generation, transport and military, medical and many other applications.
A valve is one of the most common natural and mechanical devices. Designed to regulate, control and direct the flow of any fluid substance, including gases as well as liquids and liquidised solids (slurries), it works by opening, partially obstructing or closing off various passageways. The oldest and simplest type of valve is a freely hinged flap, or check valve, which drops down to check or obstruct the flow of fluid in one direction, while flow in the other direction pushes it closed.
Valves are very diverse and can be classified into several basic types, including the check or non-return valve, safety or relief valve, sampling valve, flow control valve, poppet regulator valve, pressure reducing and thermal expansion valve. They may also be classed by actuation type, as manual, motor, hydraulic, solenoid and pneumatic. Manual valves are operated by a pedal, wheel, lever or handle, while mechanical valves act by means of an actuator driving a piston or diaphragm from a signal of compressed air.