PUMPS, ROTARY. Centrifugal Pumps,----Apaper by John Richards, of San Francisco, published in the Proceedings of the Institution of Mechanical Engineers, February, 1888, con tains much valuable information on the subject of centrifugal pumps. We abstract from it as below: Elsewhere it has not been common to recommend centrifugal pumps for high lifts, and they have been considered less economical than piston pumps; but the opinions hitherto entertained regarding them have been much modified by their work in California. A head of 100 ft., however, for a centrifugal pump to work against is a very different thing from a head of only 10 ft. : the impact or mechanical push of the vanes, which is a very important factor, diminishes as the head increases, and as the speed of the tips of the vanes exceeds that of the water in the volute casing. When the head exceeds 40 ft., efficiency declines rapidly, but not to such an extent as to outweigh the great economic advantages of centrif ugal pumps for heads up to 100 ft. or even more. For lifting water from the gravel strata in California, four kinds of centrifugal pumps have been employed, namely: firstly, the com mon make with open vanes revolving in a plain volute casing; secondly, wheels with shielded or encased vanes, the water being drawn in at the center and discharged from the circum ference ; thirdly, compound pumps with two or more wheels acting in succession upon the water during its passage through the pump ; and, fourthly, balanced pumps receiving the water at one side, whence it is deflected in an easy curve to the circumference by a conical disk on which are formed the vanes. These various forms of the centrifugal pump may be regarded as phases of development, adapted in some cases for particular objects, but generally reverting from encased vanes. compound or double wheels, and other features, back to the original simple form of the first pumps in use prior to 1820. The wheels with encased vanes, for example, have been a feature of the earlier practice with most prominent makers. These
wheels were made in America as early as 1831, mainly with the object of partly avoiding side thrust when a single inlet was employed Centrifugal Pumps with Open were at first employed for lifts up to 30 ft,, and were usually arranged, as shown in Fig. 1, at the bottom of redangnlar pits sunk to the depth required for bringing the pumps within suc tion distance of the water, The pits have often to be sunk 50 ft. or more below the surface, and are usually 10 to 12 ft. long, and 4 to 6 ft. wide. The pump, P, is driven by a vertical shaft, which is mounted in pivoted bearings, each having a sup ported collar for carrying the weight of the shaft and pump wheel.
Centrifugal Pumps with Shrouded or Encased all makers of centrifugal pumps in California and elsewhere have at first followed Sir Henry Bessemer's plan of more than 30 years ago (Proceedings, 1852), employing a shrouded wheel, in which the sides of the vanes, V, are attached to two enclosing disks that revolve with them, as shown in Fig. 2, and in the plan of the wheel, Fig. 3. The difference is very great between a wheel or runner constructed in this manner with closed sides, and an open wheel without enclosing disks attached to the wanes. With the shrouded wheel a water-tight joint must be maintained all round the inlet orifice, otherwise the water would only circulate through the pump, passing from the circumference back to the inlet. Such leakage is increased by the pressure, which at all points on the sides of the wheels is the same as in the discharge pipe or at the discharge orifices of the wheels. The skin friction of the water is no less with a shrouded wheel ; the water instead of being driven round in contact with the sides of the stationary casing, flows through the wheels as it does through the pipes, without any greater skin friction in passing through the wheel than for an equal distance in the pipes; lint on the other hand there is an equal skin friction of the outside of the wheel itself.