Installation manual

© 2014 EcoInnovation Ltd (NZ) Page 32
Table 1. Pipes common in NZ (Rural Direct)– indicative prices 2014
Pipe
OD
Pipe
ID
Material
Pressure
rating
Pressure
rating
Pressure
rating
Pressure
rating
Approx
cost/m
Approx
cost/ft
mm
mm
PSI
M
kPa
Bar
NZ$
US$
57
50
MDPE
102
70
700
7
4.00
1.05
63
53
HDPE
131
90
900
9
5.00
1.30
63
50
HDPE
174
120
1200
12
7.00
1.80
75
65
HDPE
116
80
800
8
6.80
1.75
90
79
HDPE
116
80
800
8
8.60
2.25
110
94
HDPE
116
80
800
8
11.50
3.00
Bold indicates the change from ID to OD sizing
4.7.4. PVC pipes
PVC pipes are widely used in applications ranging from low cost road culverts to mains
pressure water distribution networks in cities. PVC pipe sizes vary around the world (see
Annex II: Common PVC pipe sizes) and frequently the available pipe sizes differ between
countries. Most countries seem to either use the American or British pipe size dimensions, or
develop their own standards for pipe sizes.
PVC pipes are often more cost effective than PE pipes in sizes above 110 mm. As PVC
pipes glue together the cost to join them is low, so short lengths can be used (normally 4-6
m). They can be bent in-situ by applying heat to the tension side of the bend. We therefore
see them mainly used at lower head sites where more water flow is available and often on
sites running multiple turbines from a common pipe line.
PVC is not as durable as PE and can be shattered by falling rocks and trees. Where these
risks can be managed and the price is right for the application they are commonly used. PVC
left in direct sunlight will weaken and become brittle with age.
We see larger PVC pipes (150 mm and larger) used for lower head applications below 30 m
and often with less than 200 m of pipe needed. Our PowerSpout TRG turbine has been
specifically designed for this application.
PVC culvert grade farm pipes glued together are the lowest cost PVC pipe you can obtain.
There are also larger sized HDPE culvert pipes up to 450 mm but these often require
expensive joiners as they cannot be glued together, though plastic welding is possible.
4.7.5. Pipe myths
We often get told that the pipe has to reduce in size in order to keep up the pressure. This is
a huge misconception and arises from confusion with irrigation schemes. If you decrease the
pipe size you increase the friction, which will actually decrease the final pressure.
The pipe for an irrigation scheme supplying many farms will reduce in size as the last farm
has to convey a smaller amount of water. The start of the pipe has to be larger because it
has to convey the water needed for all the farms on the line. The pipe myth arises because
pictures of irrigation schemes have often been incorrectly used to depict hydro schemes.
People also confuse pressure with velocity; if you increase the pipe velocity by reducing pipe
size the pressure at the turbine will decrease. Reducing pipe size increases water velocity,
which increases pipe friction and reduces even further the pressure in the pipe, resulting in
less power generation.