1919 .] 
Parry.—Power from Town Water-mains. 
377 
POWER FROM TOWN WATER-MAINS. 
By E. Parry, Public Works Department, Wellington. 
Attempts are made from time to time to derive a supply of power from 
town mains, sometimes in small units for industrial purposes, and some¬ 
times in larger quantities on a more ambitious scale for pumping sewage, or 
for generating electric power for general distribution. In every case which 
has come to my notice the result has not come up to expectations. The 
reason why will be evident on perusal of the present article, the object of 
which is to discuss the subject and to elucidate the underlying principles. 
It is not generally realized how small a flow of water is required to 
satisfy the domestic and ordinary requirements of a community, nor, on the 
other hand, how large, comparatively speaking, is the flow required to generate 
a very moderate amount of power at ordinary town pressures; and when it 
is attempted to draw the larger quantity through long pipes of small diameter 
designed for quite another purpose the result is naturally disappointing. 
These principles may be better elucidated by reference to a typical 
case of town water-supply service—for example, that of Wellington. The 
principal supply is obtained from Wainuiomata, approximately fourteen 
miles distant. More precisely, the main supply is brought through 25,400 ft. 
of 30 in. main, and 46,700 ft. of 24 in. main in parallel with a 20 in. main. 
The difference in level between the service reservoir at Wainui and the 
end of the main in Thorndon is 400 ft. The mains and branches are fully 
adequate for the ordinary needs of the community, but most engineers 
even would be somewhat astonished to learn that the utmost possible 
amount of power which could be obtained at the end of the mains at 
Thorndon, with full use of the mains and without any regard to the 
exigencies of control or governing, would be about 270 b.h.p., more or less 
according to the state of the pipe-surface and the efficiency of the water¬ 
wheel. The maximum amount of power which could be obtained from 
the full use of the same mains if the fall of pressure be limited to, say, 
5 per cent., so as to secure good governing, is about 155 b.h.p. Now, it 
is obvious that the cost of providing power in this manner would be 
absolutely prohibitive ; and it follows that, inasmuch as the charge for 
domestic water-supply is a most reasonable one, it must be based upon 
a limited and intermittent use of the water, and that, on the other hand, 
the cost of supplying water for continuous use over long periods, such as 
power for industrial purposes, or the continuous use of water for any 
purpose such as watering a garden, would also be prohibitive if the price 
were fairly and justly assessed. 
The amount of power which could be obtained in the aggregate at the 
end of the branches and services would diminish rapidly with the extent 
and number of subdivisions, because the friction losses in small pipes 
increase rapidly as the diameters of the pipes diminish, and the difference 
in this respect between a large and a small pipe is not adequately realized. - 
The following example taken from a standard table exhibits the contrast 
in a marked manner. 
A pipe 3 in. in diameter will discharge 16-253 gallons of water per 
minute under a certain head, whilst a pipe 12 in. in diameter will discharge 
520-61 gallons per minute under the same head : i.e., a pipe of 16 times 
the cross-sectional area discharges 32 times as much. 
