Actuator: A device that operates a valve by utilizing electricity, pneumatics, hydraulics, or a combination of one or more of these energies. Sometimes actuators are referred to as operators. In this chapter, the word operator will be used for a person who operates any equipment, machine, plant, or system.
Ambient conditions: The pressure and temperature of the environment surrounding a valve.
Back-flow: The flow that occurs in the opposite direction of the normal or expected fluid flow.
Back pressure: The static pressure existing at the outlet of a pressure-relief device due to pressure in the discharge system.