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Текст 1. Pump types






The conditions under which liquids are to be transported vary widely and re- quire a careful analysis before the proper selection of a pump can be made.

The conditions that will influence the selection of the type of pump are: 1) the type of liquid to be handled, that is, its viscosity, cleanliness, temperature and so on, 2) the amount of liquid to be handled, 3) the total pressure against which the liquid is to be moved, 4) the type of power to be used to drive pumps.

Pumps may be divided into four major classifications:

1) Piston pumps or reciprocating pumps driven by engine or electric motors.

2) Centrifugal pumps driven by steam turbines or electric motors.

3) Rotary pumps driven by steam turbines or electric motors.

4) Fluid-impellent (жидкостный) pumps which are not mechanically operated but are fluid-pressure operated.


Centrifugal pumps

The centrifugal pump consists of an impeller or rotating section to produce the flow and a casing to enclose the liquid and to direct it properly as it leaves the impeller at its center and parallel to the shaft. The velocity of the liquid with re- spect to the impeller is in a direction opposite to the impeller motion. The impeller blades are curved backward to permit the liquid to flow to the rim (край) of the impeller with minimum friction. As the liquid leaves the impeller, it is thrown in a spiral motion forward with a certain velocity.

The water is graded away from the impeller by two basic types of casing: the volute and the turbine or diffuser. Liquid enters the impeller in the center, is thrown to the outside, and leaves the pump through the expanding spiral or volute casing. The casing has the volute shape to permit flow with a minimum of friction and to convert a part of the velocity head into static head. The static head is the head that overcomes resistance to flow.

The turbine or diffuser pump has the same type of impeller as the volute pump. The casing has a circular shape, and within the casing is a diffuser ring on which are placed vanes (лопасть). The vanes direct the flow of liquid and a de- crease in the velocity of the liquid occurs because of an increase in the area through which the liquid flows. Thus, part of the velocity head is converted into static head as in the volute pump. For multistage pump the diffuser pump has a more compact casing than the volute pump. Generally, the volute pump will be used for low-head high capacity flow requirement and diffuser pump for high-head requirement.

Both volute and diffuser pumps are classified by the type of impeller, the number of stages and the type of suction used.

When two or more impellers are mounted on the same shaft and act in series, the pump is called a multistage pump, the number of stages corresponding to the number of impellers. Usually each stage produces the same head, and the total head developed is the number of heads produced per stage.


The types of impellers installed in centrifugal pumps are as numerous as the uses to which the pumps are put. Each of the impeller types has a specific purpose. The axial-flow type is used to pump large quantities of fluid against a rela- tively small static head. It is not a true centrifugal pump but is designed on the principles of airfoil shapes. The radial pump is used for handling smaller quantities of fluid against a high head, because the centrifugal force is high but the flow path is small and restrictive. The open impeller is designed to handle dirty liquids such as sewage, where the flow path must be less restrictive. The partially radial impel-

ler covers (отвечает) intermediate pumping conditions.

 

 






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