Selfpriming Pumps

The basic requirement for a self-priming centrifugal pump is that the pumped liquid must be able to entrain air in the form of bubbles so the air will be removed from the suction side of the pump. This air must be allowed to separate from the liquid after the mixture of the two has been discharged by the impeller, and the separated air must be allowed to escape or to be swept out through the pump discharge. Such a self-priming pump therefore requires, on its discharge side, an air-separator, which is a relatively large stilling chamber, or reservoir, either attached to or built into the pump casing. Alternatively, a small air bleed line can be installed from the discharge pipe between the pump and the discharge check valve back to the suction source.

There are two basic variations of the manner in which the liquid from the discharge reservoir makes the pump self-priming: (1) recirculation from the reservoir back to the suction and (2) recirculation within the discharge and the impeller itself.

Recirculation to Suction In such a pump, a recirculating port is provided in the discharge reservoir, communicating with the suction side of the impeller. Before the first time the pump is started, the reservoir is filled. As the pump is started, the impeller handles whatever liquid comes to it through the recirculating port plus a certain amount of air from the suction line. This mixture of air and liquid is discharged to the reservoir, where the two elements are separated, the air passing out of the pump discharge and the liquid returning to the suction of the impeller through the recirculation port. This operation continues until all the air has been exhausted from the suction line.

The vacuum thus produced draws the liquid from the suction supply up the suction piping and into the impeller. After all the air has been exhausted and liquid is drawn into the pump, the pressure difference between the pump body and the inlet causes the priming valve, which permits communication between discharge and suction passages, to close. It is essential that the reservoir in the suction side remain filled with liquid when the pump is stopped, so the pump is ready to restart. This is accomplished by incorporating either a valve or some sort of trap between the suction line and the impeller.

Pumps with recirculation to suction are seldom used today, and by far the most common arrangement is that with recirculation at the discharge.

Recirculation at Discharge This form of priming is distinguished from the preceding method by the fact that the priming liquid is not returned to the suction of the pump but mixes with the air either in the impeller or at its periphery. The principal advantage of this method, therefore, is that it eliminates the complexity of internal valve mechanisms.

One such self-priming pump is illustrated in Figure 10. An open impeller (A) rotates in a volute casing (B), discharging the pumped liquid through passage C into the reservoir (D). When the pump starts, the trapped liquid carries entrained air bubbles from the suction to the discharge chamber. There, the air separates from the liquid and escapes,

FIGURE 10 Self-priming pump with recirculation at discharge (Flowserve Corporation)
FIGURE 11 Self-priming pump with recirculation at discharge (Peabody Barnes)

whereas the liquid in the reservoir returns to the impeller through the recirculation port (F), reenters the impeller and, after mixing once more with air bubbles, is discharged through passage C. This operation is repeated continuously until all the air in the suction line has been expelled. After the pump is primed, a uniform pressure distribution is established around the impeller, preventing further recirculation. From this moment on, the liquid is discharged into the reservoir bath at C and at F.

Figure 11 illustrates another form of self-priming pump with recirculation at the discharge. In this arrangement, the return of the liquid to the impeller periphery takes place through a communicating passage (A) located at the bottom of the pump casing. After the pump is primed, liquid is delivered into the reservoir at the discharge both through its normal volute discharge and through port A. In the pump illustrated in Figure 11, a check valve at the pump suction (B) acts to prevent the draining of the pump after it has been stopped.

Regenerative Turbine Pumps Because these pumps can handle relatively large amounts of gas, they are inherently self-priming as long as sufficient liquid remains in the pump to seal the clearance between the suction and discharge passages. This condition is usually met by building a trap in the pump suction.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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