The capacity of the receiving well should be so designed that the pump when operating will be working at its maximum capacity, and the period of rest during conditions of average rate of flow should be in the neighborhood of 15 to 20 minutes. For example, assume an average rate of flow of 2 cubic feet per second, with a maximum rate of double this amount. The pump should have a capacity of 4 cubic feet per second, and if the receiving well is to be filled in 15 minutes by the average rate of sewage flow its capacity should be 15 × 5 × 60 × 7.5 or 14,000 gallons. Under these circumstances the pump will operate 15 minutes and rest 15 minutes, during average conditions of flow. =74. Types of Pumping Machinery.=—The two principal types of pumping machines for lifting sewage are centrifugal pumps and reciprocating pumps. A centrifugal pump is, in general, any pump which raises a liquid by the centrifugal force created by a wheel, called the impeller, revolving in a tight casing, as shown in Fig. 50. A reciprocating pump is one in which there is a periodic reversal of motion of the parts of the pump. Centrifugal pumps are sometimes classified as volute pumps and turbine pumps. A volute pump is a centrifugal pump with a spiral casing into which the water is discharged from the impeller with the same velocity at all points around the circumference, as shown in Fig. 51. A turbine pump is a centrifugal pump in which the water is discharged from the impeller through guide passages into a collecting chamber, in such a manner as to prevent loss of energy in changing from kinetic head to pressure head. A turbine pump is shown in section in Fig. 51. Centrifugal pumps are sometimes classified as single stage and multi-stage. A centrifugal pump from which the water is discharged at the pressure created by a single impeller is called a single-stage pump. If the water in the pump is discharged from one impeller into the suction of another impeller the pump is known as a multi-stage pump. The number of impellers operating at different pressures determines the number of stages of the pump. A three-stage pump is shown in Fig. 52. [Illustration: FIG. 50.—Section through de Laval Single-Stage, Double Suction Centrifugal Pump. ] 375 Lubricating ring. 380 Oil hole cap. 383 Oil drain tube. 404 Shaft sleeve lock nut. 440 Driving coupling. 441 Driven coupling. 443 Coupling check nut. 450 Coupling bolt. 451 Coupling bolt nut. 452 Coupling rubber. 453 Coupling rubber steel tube. 500 Pump case. 550 Bearing bracket cap. 551 Bearing. 552 Shaft. 553 Shaft sleeve, right hand thread. PW Impeller. 554 Shaft sleeve, left hand thread. 555 Shaft stop collar, inner. 555–1 Shaft stop collar, outer. 556 Guide ring. 560 Packing gland. 563 Bearing. 567R Impeller protecting ring, right hand thread. 567L Impeller protecting ring, left hand thread.