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An extraction pump is an important part of a feed water system.Generally, fitted in between the condenser and de-aerator, the extraction pump works as an excellent tool for generating the requisite pressure, to deliver the feed water from a condenser under vacuum to de-aerator or a feed pump inlet.
Extraction pump is usually a vertical shaft, two stage, centrifugal pump, which is used in applications involving high pressure and high volume. Centrifugal pumps are of the type, which due to their high capacity, require a specific minimum suction head to operate.
Almost all the feed systems having an extraction pump, use a condensate level controller system, which is attached to the condenser and perform two main functions:
- To maintain the amount of condensate in the condenser
- To provide the sufficient suction head to the extraction pump
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Construction & Working
A centrifugal pump, consists of three main parts - impeller, diffuser and volute casing.
A two stage centrifugal pump consists of two impellers arranged as shown in the figure. The high temperature water, which is almost boiling due to the low vacuum condition, enters the first stage impeller from the suction line. The high temperature water receives slight positive pressure from the first stage impeller and it then passes through the second stage impeller. In the second stage impeller, water gets more pressurized, almost equal to the required system pressure. The pressurized water then passes through the diffuser, where the strategically arranged vanes pushes the water towards the volute casing at an even higher pressure. The volute casing removes the water through the water outlet at the appropriate system pressure.
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The feed systems in which the condenser sump level is of a fluctuating type or where there are chances of the pump running dry, a self-regulating type of extraction pump is used. A self-regulating type of extraction pump aligns its required suction head according to the condensate level in the condenser. But this type of extraction pump often faces the problem of cavitation, which occurs when the suction head falls to a very low level. Cavitation is a phenomena in which the vapour bubbles inside the pump collapse, resulting in the reduction of pump discharge rate to almost zero. The cavitation will only go when the pump suction head again increases, which will help in getting rid of the vapour bubbles trapped inside the pump. Once the suction head increases, the pump begins to discharge again.
Cavitation can also lead to the damage of the internal parts of the pump, but this occurs at high pressure. At low pressure conditions, the pump would not be affected. The biggest damage due to cavitation occurs to the impeller of the pump.Nowadays the impellers are designed in such a way that the air bubbles, even if gets produced, collapse away from the impeller.
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