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Centrifugal Pumps: A New Concept in Pumps industries

The operating manual of any centrifugal pump often starts with a general statement, centrifugal pump will gives one completely trouble free and satisfactory service only on the condition that it is installed and operated with due care and is properly maintained.

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.
Despite all the care in operation and maintenance, engineers often face the statement “the pump has failed i.e. it can no longer be kept in service”. Inability to deliver the desired flow and head is just one of the most common conditions for taking a pump out of service. There are other many conditions in which a pump, despite suffering no loss in flow or head, is considered to have failed and has to be pulled out of service as soon as possible. These include seal related problems (leakages, loss of flushing, cooling, quenching systems, etc), pump and motor bearings related problems (loss of lubrication, cooling, contamination of oil, abnormal noise, etc), leakages from pump casing, very high noise and vibration levels, or driver (motor or turbine) related problems. The list of pump failure conditions mentioned above is neither exhaustive nor are the conditions mutually exclusive. Often the root causes of failure are the same but the symptoms are different. A little care when first symptoms of a problem appear can save the pumps from permanent failures. Thus the most important task in such situations is to find out whether the pump has failed mechanically or if there is some process deficiency, or both. Many times when the pumps are sent to the workshop, the maintenance people do not find anything wrong on disassembling it. Thus the decision to pull a pump out of service for maintenance / repair should be made after a detailed analysis of the symptoms and root causes of the pump failure. Also, in case of any mechanical failure or physical damage of pump internals, the operating engineer should be able to relate the failure to the process unit’s operating problems. Any operating engineer, who typically has a chemical engineering background and who desires to protect his pumps from frequent failures must develop not only a good understanding of the process but also thorough knowledge of the mechanics of the pump. Effective troubleshooting requires an ability to observe changes in performance over time, and in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem from re-occurring.

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.
Centrifugal pumps are used to induce flow or raise pressure of a liquid. Its working is simple. At the heart of the system lies impeller. It has a series of curved vanes fitted inside the shroud plates. The impeller is always immersed in the water. When the impeller is made to rotate, it makes the fluid surrounding it also rotate. This imparts centrifugal force to the water particles, and water moves radially out. Since the rotational mechanical energy is transferred to the fluid, at the discharge side of the impeller, both the pressure and kinetic energy of the water will rise. At the suction side, water is getting displaced, so a negative pressure will be induced at the eye. Such a low pressure helps to suck fresh water stream into the system again, and this process continues.
From foregoing discussions it is clear that, the negative pressure at the eye of the impeller helps to maintain the flow in the system. If no water is present initially, the negative pressure developed by the rotating air, at the eye will be negligibly small to suck fresh stream of water. As a result the impeller will rotate without sucking and discharging any water content. So the pump should be initially filled with water before starting it. This process is known as priming.

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