EXCAVATING MACHINERY USED IN LAND DRAINAGE. 53 
The required horsepower of the engine may be obtained by multi- 
plying the weight of the mixture transported by the total head in 
feet, and a coefficient dependent upon the combined efficiencies of the 
pump and engine. Efficiencies over 50 per cent are rarely obtained 
in a hydraulic dredge pump. Table 22, compiled from data pub- 
lished by manufacturers of hydraulic dredges, gives the capacities 
and required power for pumps of various sizes. 
TABLE 22.—Capacities and required power of hydraulic dredges. 
Solids pumped per hour. Approxi- 
Diameter Se el nace 
of horse- 
suction Normal power 
an capacity. 10 15 20 required 
3 ne ea per cent. | per cent. | per cent. on enc 
head. 
Gallons 
per Cubic Cubie Cubic 
Inches. | minute. yards. yards. yards. 
4 450 12 18 24 0.4 
5 700 20 30 40 6 
6 1, 000 30 45 60 8 
8 1, 800 50 75 100 ie 
10 2, 800 90 135 180 2.5 
12 4, 000 130 195 260 3.0 
15 6, 300 200 300 400 5.0 
18 9, 000 300 450 600 7.0 
20 13, 000 375 560 750 8.0 
24 17,000 500 750 1,000 10.0 
Suppose it is desired to select pump and power equipment for a 
hydraulic dredge to be used in building a levee with a top elevation 
20 feet above the water surface, the maximum length of discharge 
pipe to be 2,000 feet, the material to be pumped being sand which it 
is desired to pump at the rate of 130 cubic yards of solids per hour. 
In pumping sand it is assumed that 10 per cent of the discharge is 
solids. Table 22 shows that a 12-inch pump will deliver the required 
amount of material. 
To determine the size of the power unit to work the pump efli- 
ciently it is necessary to determine the head against which the pump 
is to operate. This includes static head, entrance loss, and friction in 
suction and discharge pipes. Table 23 shows the velocity of flow 
and friction head per 100 feet for water in clean iron pipe, as given 
by pump manufacturers. To obtain the friction head for the mix- 
ture pumped by a hydraulic dredge, the figures given in Table 23 
should be increased from 40 to 75 per cent, depending upon the 
character of the material pumped. 
