Building Services Engineering 5th Edition Handbook

75
Below-ground drainage 233
40
Water flow rate (I/s)
30
20
10
0
0 1 2 3 4 5 6 7 8 9× 10 3
Discharge units
9.4 Simultaneous flow data for foul- and surface-water drains (reproduced from data in CIBSE Guide
(CIBSE, 1986) by permission of the Chartered Institution of Building Services Engineers).
1000
500
0.4
375
0.6
300
Gradient of 1 in
200
100
0.8
Diameter (mm) 150
100
1.0
225
Velocity (m/s)
50
1.5
20
2.0
2 3 4 5 7 10 20 30
Flow (I/s)
3.0
50 100
9.5 Sizes for two-thirds full spun precast concrete drains (reproduced from IHVE Guide [IHVE, 1970], now
superseded, by permission of the Chartered Institution of Building Services Engineers [CIBSE, 1986]).
EXAMPLE 9.1
The flow from a 100 mm stack is equivalent to 750 discharge units and is to run underground
for 30 m before entering the foul sewer at a depth 375 mm lower than at the
building end. Find a suitable diameter for a spun precast concrete drain so that it will not
be more than two-thirds full at maximum flow rate.