Devices to prevent pollution by backflow of potable water - Air gap with non circular overflow (unrestricted) family A, type B.
Calculations based on a maximum allowable flow rate of 3m/s unless a lower flow rate is known.
Input data required: Number of cistern inlets and their respective diameters "D", width "l" and % open area of slot, weir thickness "Cw" and unobstructed vertical deptth of wier sill upstream face "Uw"
Select "Calculate" and total maximum flow rate "Q", air gap "A", head over weir "h" and weir slot height "Ow" are automatically calculated.
Results valid for single or multiple inlet arrangements if the following conditions are met.
"h" = head over Weir at Total Maximum Flow.
Width (l) is greater than or equal to 10h at the spillover level. (If not, increase l)
Weir crest thickness (Cw) is less than or equal to 5h
Upstream face of weir (Uw) is vertical to a depth greater than or equal to 5h
The depth of the weir (Ow) is greater than or equal to 2D+h and never less than 20mm. For multiple inlets (D) is the largest feed pipe bore diameter.
All dimensions in mm
Acceptable Weir Sizes
Validation Criteria see above
l Weir Width
Ow (4) Weir Height
Unrestricted Weir. (No insect screen)
Weir Width l increased to account for insect screen obstruction
Slot Height Ow increased to account for insect screen obstruction