Drainage - Basic Sizing
Drainage Pipe Sizing
Foul Water Pipe Sizing
The following method is one way of sizing
pipework.
1. Choose a minimum gradient
for all pipes, say 1:80
2. Use the table below to calculate the total number of discharge units in pipe.
|
No. |
Appliance |
No. of units |
Total units |
|
|
WC |
14 |
|
|
|
basin |
3 |
|
|
|
bath |
7 |
|
|
|
shower |
4 |
|
|
|
sink |
6 |
|
|
|
washing machine |
4 |
|
|
|
dish washer |
4 |
|
3. Size section from pipe manufacturers’ graphs.
An example of a
pipe-sizing graph is shown below.
Example
Size the foul
water pipework for 12 houses from the
DATA below in the table.
1. Use a minimum gradient
of 1:80 for all pipes.
2. Discharge units from each house:
|
No. |
Appliance |
No. of units |
Total units |
|
2 |
WC |
14 |
28 |
|
2 |
basin |
3 |
6 |
|
1 |
bath |
7 |
7 |
|
1 |
shower |
4 |
4 |
|
1 |
sink |
6 |
6 |
|
0 |
washing machine |
4 |
0 |
|
0 |
dish washer |
4 |
0 |
|
|
|
Total |
51 |
12 houses x 51 =
612 discharge units.
Flow graph gives 150mm-dia. foul drain since the
convergence of the two lines on the graph is between the pipe size 100mm
diameter and 150mm diameter.
Surface
Water Pipe Sizing
The following method is one way of sizing
pipework.
1. Choose a minimum gradient for all pipes, say 1:80
2.
Use the table below to
calculate the flow rate in each
section.
|
SURFACE TYPE |
AREA (A) m2 |
IMPERMEABILITY FACTOR (f) |
TOTAL (A x f) |
|
Road or pavement |
|
0.90 |
|
|
Roof |
|
0.95 |
|
|
Path |
|
0.75 |
|
|
Garden |
|
0.25 |
|
|
Access road, parking |
|
0.90 |
|
|
|
|
Total |
|
3.
The area of each surface
is calculated from drawings.
4.
The impermeability factor allows for water, which
runs off each surface.
5.
The flow rate (Q) for each house can be calculated
from:
Q = area drained x rainfall intensity x impermeability factor
6.
If Rainfall intensity = 50mm/hr,
then Q becomes:
Q = A x 50 x f
Q = (A x f) 50 ((litres/hour)
7.
Divide Q by 3600 to get
value in litres/second.
8.
Multiply Q by number of
houses to get Total Q.
9.
Estimate pipe size from
Pipe Sizing graph.
For example, size
the pipework for 12 houses from the drawing
Example
Size the surface
water pipework for 12 houses using the
DATA below in the table.
1. Choose a minimum gradient for all pipes,
say 1:80
2. Surface water flow from each house.
|
SURFACE TYPE |
AREA (A) m2 |
IMPERMEABILITY FACTOR (f) |
TOTAL (A x f) |
|
Road or pavement |
20 |
0.90 |
18.00 |
|
Roof |
40 |
0.95 |
38.00 |
|
Path |
15 |
0.75 |
11.25 |
|
Garden |
68 |
0.25 |
17.00 |
|
Access road, parking |
25 |
0.90 |
22.50 |
|
Total |
192 |
Total |
103.50 |
3. Rainfall intensity 50mm/hr.
Q = area
drained x rainfall intensity x impermeability factor
Q = A x 50 x f
Q = (A
x f) 50
Q = 103.50
x 50 = 5175
litres/hour
Q = 1.438
litres/second per house X 12 houses.
Q = 17.25 litres/second.
Flow graph gives 150mm dia. surface water drain since the point on
the graph lies between 100mm and 150mm
Drainage - Basic Sizing