FRICTION LOSS & FLOW RATES - Supadiverta - syphonic rainwater diverter

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Understanding pipe sizes and flow rates

TO ACHIEVE FULL FLOW
To remain primed, the vertical drop must not be angled or increased in diameter.

FRICTION LOSS
The vertical drop's potential flow rates are reduced by friction losses in the connected pipe work but losses are minimised by connecting the vertical drops to larger horizontal pipes.
This is easy to calculate and is explained with examples in the PIPES & FITTINGS page.
The simple adage is: Larger pipes = less friction loss = higher flow rates.
Polypipe 19mm vertical drops are often connected to 25mm (78% greater volume) horizontal polypipe, depending on intended usage. Polypipe has the advantage of low cost, ease of fitting and is easily altered.
For the recommended class 9 & 12 PVC pressure pipe, the 20mm vertical drops can be connected to 25mm, 32mm, 40mm and even 50mm PVC pressure pipe and fittings. PVC pipes maintain higher flow rates and are more suited to permanent connections than polypipe. Class 9 PVC pipe is cheaper than class 12. Poly and PVC pipe is easily mixed and matched.

PVC pressure pipe is sold as a 'nominal size'… the actual internal diameters (ID) are larger!
For example, 32mm class 12 PVC pipe has an ID of 37.5mm and the 32mm class 9 PVC pipe has an ID of 38.5mm.


SELF CLEANING PIPEWORK
Air flushing requires a flow velocity of .8 metres per second.
Sediment flushing requires a flow velocity of .6 metres per second.
Nominal
ID
Wall
litres/metre
litres/min @ 0.8m/sec
CLASS 9 PVC
25
30.5
1.5
0.8
35
32
38.5
1.9
1.2
56
40
44.1
2.1
1.5
73
50
55.2
2.6
2.4
115
CLASS 12 PVC
20
23.65
1.5
0.4
21
25
29.75
1.9
0.7
33
32
37.45
2.4
1.1
53
40
42.75
2.7
1.4
69
50
53.65
3.3
2.3
109
POLYPIPE
19
19
0.3
14
25
25.4
0.5
24


LESS FRICTION LOSS = HIGHER FLOW RATES

FLOW RATES FOR CLASS 12 PVC PRESSURE PIPE
Litres per minute flow rates with 20 metres of friction loss
HEAD
20mm
25mm
32mm
40mm
50mm
0.25m
12
23
42
51
108
0.50m
18
33
60
87
156
1.0m
26
48
88
126
227
1.5m
33
60
110
157
283
2.0m
38
70
128
184
330
Litres per minute flow rates with 30 metres of friction loss
HEAD
20mm
25mm
32mm
40mm
50mm
0.25m
10
18
33
47
86
0.50m
16
27
49
69
125
1.0m
21
39
71
100
182
1.5m
26
48
88
125
227
2.0m
31
56
103
146
265
OF INTEREST: Doubling a pipe's internal diameter increases the volume x4 and if the same head pressure is maintained, the flow rate increases roughly x6.
25mm class 12 PVC pipe has 38% more capacity than 25mm poly pipe and 20mm class 12 PVC pipe has 55% more capacity than 19mm poly pipe! 20mm horizontal pipe is rarely used and 40mm pipe is commonly used. Further details can be found on the PIPES & FITTINGS page.
POLY PIPE AND FLOW RATES
A single 19 mm poly pipe vertical drop connected to a 25 mm horizontal poly pipe can also generate impressive flow rates.
A Supadiverta fitted with a single 19 mm poly pipe vertical drop joined to a 13 metre horizontal length of 25 mm poly pipe diverted to a tank’s bottom inlet was found to flow at 25 litres per minute (1,500 litres per hour) with a 1 metre head during flow testing. This is equivalent to a steady rainfall of nearly 43 mm per hour on a 35 sq m roof area harvested by one downpipe.
Substituting the 25 mm horizontal poly pipe with 19 mm poly pipe in the same situation and head pressure still produced a flow rate of 15 litres per minute.
Shorter or larger internal diameter horizontal pipes have higher flow rates.
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