Sewerage and Sewage Treatment

by Harold E. Babbitt

Fig. 56.—Marine Type Thrust Bearing.
Courtesy, DeLaval Steam Turbine Co.

Thrust bearings or thrust balancing devices are used to take up the end thrust which occurs in even the best designed pumps. To overcome this pumps are designed with double suction, opposed impellers, or two pumps with their impellers opposed may be placed on the same shaft. Due to inequalities in wear, workmanship or other conditions, end thrust will occur and must be cared for. Various types of thrust bearings are in successful use, such as: the piston, ball, roller or marine types. The marine type thrust bearing is shown in Fig. 56. The piston type of hydraulic balancing device is shown in Fig. 57. In the figure A represents the impeller, and B a piston fixed to the shaft and revolving with it. There is a passage for water through the openings (1), (2), and (3) leading from the impeller chamber to the atmosphere or to the suction of the pump. If the impeller tends to move to the right opening (1) is closed resulting in pressure on the right of the impeller forcing it to the left. If the impeller moves to the left (1) is opened thus transmitting pressure to the piston B forcing the impeller to the right. The flange C is not essential, but is advantageous in pumps handling gritty matter. As the channel (2) wears larger the pressure against the piston decreases allowing it to move to the left. This partially closes (3) building up the pressure again.

Fig. 57.—Piston Type of Thrust Balancing Device.

Flexible shaft couplings should be used if the shaft of the driving motor and the pump are in the same line, as direct alignment is difficult to obtain or to maintain. Where connected to steam turbines, reduction gearing and rigid couplings are usually used on sewage pumps to obtain slow speed and permit large passages. Flexible couplings are of various types, one of which is shown in Fig. 50. A rigid coupling would be formed by bolting the flanges firmly together. Shaft couplings are usually not necessary where the pump is driven by belt connection to the engine or motor, or where the pump and pulley rest on only two bearings.

The stuffing box shown in Fig. 50 is packed loosely with two layers of hemp between which is a lantern gland, in order to permit a small amount of leakage. A drip box is placed below this gland to catch the leakage and return it to the pump. The leakage is permitted as it aids in lubrication and the tightening of the gland will cause binding of the shaft. The gland on the suction side of the pump should be connected by a small pipe to the discharge chamber in order to keep a constant supply of water for lubrication and to prevent the entrance of air to the suction end of the pump.

78. Centrifugal Pump Characteristics.—The capacity of a centrifugal pump is fixed by the size and type of its impeller and by the speed of revolution. Roughly, the capacity of a pump, for maximum efficiency, varies directly as the speed of revolution, the discharge pressure varies as the square of the speed, and the power varies as the cube of the speed. These relations are found not to hold exactly in tests because of internal hydraulic friction in the pump.

The characteristic curves for a centrifugal pump, or the so-called pump characteristics, are represented graphically by the relation between quantity and efficiency, quantity and power necessary to drive, and quantity and head, all at the same speed. The quantities are plotted as abscissas in every case. The curve whose ordinates are head and whose abscissas are quantities is known as “the characteristic.” The curve showing the relation between quantities and speeds is sometimes included among the characteristics. Characteristics of pumps with different styles of impellers are shown in Fig. 58. Fig. 59 shows the characteristics of a pump run at different speeds, the efficiencies at these speeds when pumping at different rates, and the maximum efficiency at different speeds. It is to be noted that the information given in this figure is more extensive than that in Fig. 58. The operating conditions under any head, rate of discharge, and speed are given. The curves of constant speed are parallel, and their distances apart vary as the square of the speed. The line of maximum efficiency is approximately a parabola.

Fig. 58.—Characteristics of Centrifugal Pumps with Different Styles of Impellers at Constant Speed.