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