Backflow is the reversal of the flow of water from a fire suppression system. A backflow preventer is a mechanical device with check valves, air inlets, and relief valves installed on water piping that serves to prevent pressurized water from flowing from the fire suppression system into the public water supply. Specialized backflow preventer valves are used where there is not sufficient vertical clearance or physical space to install an air gap, or when pressurized operation or other factors rule out use of an air gap.