PISTON VALVES
The inside of a piston valve contains a piston that blocks the output when the valve is pressurized, and a volume of air behind the piston. When the pressure behind the piston is released the piston is pushed back by the force of the pressure from the input.
Piston actuated valves use an external control medium to pilot the actuator, where a piston is directly connected to the main seal that closes onto the main orifice, thereby controlling the flow of liquids and gases.