Tuesday, December 6, 2011

Centrfugal Pumping Principle

Many hydraulic systems employ centrifugal pumps to move fluid through a piping system. These pumps all rely on centrifugal force as the fundamental principle by which they operate. Centrifugal force affects an object or material moving in a circular pattern by causing it to pull away from the central axis or center point of the path along which it travels. This force can be used to regulate the pressure and motion within a pumping unit, and when applied in combination with a number of other centrifugal pumping principles, forms an integral part of hydraulic mechanisms.

Generally, a centrifugal pump is based around a casing filled with fluid, usually water. A special unit within the casing exerts fast rotary motion that causes the water to spin, generating centrifugal force that channels it through a discharge outlet. Discharged water creates a vacuum for atmospheric pressure to force more water out of the casing. It is a continuous process, dependant mostly on continued rotary motion and a constant supply of water. Most centrifugal pumps rely on rotating impellers or vanes to provide rotary motion, though the design and implementation of these systems can vary according to capacity and project requirements.

Basic Centrifugal Pumping Concepts

To better illustrate the essential principles of centrifugal pumping, it may be useful to consider a simplified version of an industrial pumping mechanism. A cylindrical can with a pair of rotary vanes along its interior can be attached to a shaft. This shaft has a pulley unit responsible for agitating the can with rotational movements. Once the can is filled with water, the pulley begins to rotate the shaft at high speed, causing the can to spin. As the water in the can rotates, centrifugal force pushes it out toward the walls of the can where it is pressed against the edges of its container.

Because the water cannot continue moving outward through the walls of the can, it begins to climb upward, eventually overflowing while the water at the center point is drawn downward. The overflowing water moves at the same velocity as it does at the rim, meaning that the kinetic energy it produces can be maintained if the water is caught and more water is supplied to the pump. Therefore, a receiving container is usually employed to catch the spilling water and a surplus tank is attached to the shaft to maintain a continuous supply of liquid. The same centrifugal force effect can be achieved without the pulley mechanism by rotating only the vanes or impellers within the can.

Vane and Impeller Functions

The radial vanes within a water casing cause the water to spin when the casing rotates or when the vanes themselves are rotated, making them crucial components in most centrifugal pumping systems. Likewise, the impeller is an integral pumping unit because it provides the rotational force that moves the vanes. Common types of vane and impeller designs include:

Straight Vane: In this basic configuration, water enters the casing through an inlet on the impeller. The impeller rotates its blades, causing the water to spin and generating centrifugal force that creates pressure along the impeller’s outer diameter. When enough force has been applied, the water pushes outward from the impeller and moves through a discharge channel at one end of the casing.

Curved Vane: This design features both curved vanes and a curved casing. An inlet pipe channels water toward the center, or “eye,” of the impeller unit, where curved vanes begin to push it toward the edge of the casing in a spiral pattern. As the rotary force continues to apply pressure, the water is directed into a discharge channel.

Volute: The volute is a single-plane spiraled curve that recedes from a central point. It is designed to match the shape of the casing surround the impeller in a centrifugal pump¸ and forms a passageway for discharged water. The volute expands at certain intervals, growing wider the further along the water flows.

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