Drainage Pipes ~ sizes for normal domestic foul water applications:-
<20 dwellings = 100 mm diameter
20†150 dwellings = 150 mm diameter
Exceptions: 75 mm diameter for waste or rainwater only (no WCs)
150 mm diameter minimum for a public sewer
Other situations can be assessed by summating the Discharge Units
from appliances and converting these to an appropriate diameter
stack and drain, see BS EN 12056-2 (stack) and BS EN 752 (drain).
Gradient will also affect pipe capacity and when combined with
discharge calculations, provides the basis for complex hydraulic
theories.
The simplest correlation of pipe size and fall, is represented in
Maguire's rule:-
400 (100 mm) pipe, minimum gradient 1 in 40
600 (150 mm) pipe, minimum gradient 1 in 60
900 (225 mm) pipe, minimum gradient 1 in 90
The Building Regulations, approved Document H1 provides more
scope and relates to foul water drains running at 0„75
proportional depth. See Diagram 9 and Table 6 in Section 2 of the
Approved Document.
Other situations outside of design tables and empirical practice can
be calculated.
eg. A 150 mm diameter pipe flowing 0„5 proportional depth.Applying the Chezy formula for gradient calculations:-
v=cpffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffim¾i
where: v = velocity of flow, (min for self cleansing = 0„8 m/s)
c = Chezy coefficient (58)
m = hydraulic mean depth or;
area of water flowing
wetted perimeter for 0„5 p:d:= diam=^4
i = inclination or gradient as a fraction 1/x
Selecting a velocity of 1 m/s as a margin of safety over the minimum:-1=58ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi0 : 15 = (^4) ¾i
q
i = 0„0079 where i = 1/x
So, x = 1/0„0079 = 126, i.e. a minimum gradient of 1 in 126
Drainage---Pipe Sizes and Gradients