Vacuum Relief/Air Inlet Valves are normally closed valves.
When the system pressure becomes negative, the valve opens, allowing air into the system to prevent a vacuum from building.
When system pressure returns to positive, the Vacuum Relief/Air Inlet Valve closes air tight.
Standard Vacuum Relief/Air Inlet Valves are designed to open with a minimal, 1/4 psi pressure differential across the orifice.
Higher or lower relief settings are available to suit the application.