The passage through the pump should be so timed as to produce a gradually increasing velocity in the water until it reaches the circumference of the fan, then a gradually decreasing velocity until it is discharged from the pipe. These conditions are met by having a conical end to the suction pipe, and a spiral casing surrounding the fan. The form of the casing should be such that the water flowing round the casing will move with the same velocity as that issuing from the fan; the casing enlarges from that locality into the discharge pipe.
A small increase in the number of revolutions of the fan after the pump commences to discharge produces a large increase in the volume of water delivered.
[Illustration: FIG. 496.—For description see page 216.]
Fig. 495, upon the previous page, is intended to show a _Boggs & Clarke hydraulic dredging or sand centrifugal pump_. This is a heavy strong pump fitted with flap valve, _without close fitting joints_, but with ample room for the water to wash away the sand from the working parts. The cut shows the pump with ejector for priming and large elbow to discharge through. The pump is lined with sheet steel fitted so that it can be easily and cheaply replaced. The diameters in which these pumps are made run from 4 to 12 inches inclusive.
=========+======================+=========+================== Size of | Pipe Size of Flanges | Size of | Capacity per Hour Pump | Discharge | Suction | Pulley | Cubic Yards Sand ---------+------------+---------+---------+------------------ 4 | 4 | 5 | 12 × 10 | 30 to 40 5 | 5 | 6 | 12 × 10 | 40 to 60 6 | 6 | 8 | 18 × 12 | 60 to 80 8 | 8 | 10 | 24 × 12 | 80 to 125 10 | 10 | 12 | 30 × 12 | 150 to 250 12 | 12 | 14 | 36 × 14 | 250 to 400 ---------+------------+---------+---------+------------------
Smaller sizes for pumping sand and gravel are made with cast chilled linings with chilled piston, and brass covered shaft especially adapted for stone and marble mills to carry the sand to the saws, or for mining where there is a large quantity of sand to be pumped with water.
The table will convey an idea of the capacity, sizes, etc.
For pumping sand or heavy material the speed of pump should be increased 25% more than for water.