281
OF ENGINEERING FORMULÆ.
W ater- Works—continued.
Filters for Water-works.
1 square yard of filter for each 700 gallons in 24 hours;
formed of 2 ft. 6 in. of fine sand.
» 6 in. of common sand.
„ 6 in. of shells.
,, 2 ft. 6 in. of gravel.
Perforated pipes laid in the lowest stratum.
Hawksley’s Rule fok the Stobage of Water.
D = N umber of days’ supply to be stored = -°^ •
. . V K .
t1 =s Mean annual rainfall in inches of three consecutive
dry years, say J of the average annual rainfall.
Storage capacity in England varies from 25,000 to 50 000
cubic feet per acre of catchment area.
Coating for Pipes.
The pipes are lowered into a bath containing a composition
of gas-tar, Burgundy pitch, oil, and resin, heated to 400° Fahr
and remain until they attain the heat of the bath, they are
then placed in an upright position to allow the superfluous
coating to drain off.
Pumping Engines.
Compound reciprocating engines, with double-acting
pumps, are chiefly used for water-works.
They give a duty of from 95 to 100 (duty reduced to 112 lbs
of coal), compared with a duty of from 50 to 70 for Cornish
engines.
Large air-vessels are now very generally used in preference
to stand-pipes, but arrangements should be made for supply-
ing the air-vessel with air.
Mains or Pipes.
The velocity of water in the pipes should not exceed 3 feet
per second.
Reflux flaps should be placed at intervals in long pumping
mains, to prevent back pressure on the engine. Relief valves
should also be used to prevent excessive pressure. Pipes
should not rise more than 22 feet above the mean hydraulic
gradient, and air-valves should be placed at high places in
the main where air is likely to accumulate.