Type DB – Pipe interrupter with atmospheric vent and moving element

A mechanical backflow prevention device with an air inlet closed by a moving element when the device is in normal use but which opens and admits air if the water pressure upstream of the device falls to atmospheric pressure, the device being installed so that the flow of water is in a vertical, downward direction. 

Maximum permissible fluid category against which a Type DB device is accepted as adequate backflow protection:

Backflow by back pressure:  not acceptable for protection against back pressure from any fluid category 

Backflow by back siphonage: fluid category 4

Note: Arrangements incorporating a Type DB device shall have no control valves on the outlet of the device. The device shall be fitted not less than 300mm above the spillover level of an appliance and discharge vertically downwards. 

Interpretation given in Regulators’ Backflow Specification - backflow prevention arrangements and devices: backflow prevention arrangements