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Friday, April 24, 2009

Propeller Pump

Originally, the term of vertical propeller pump was applied to vertical wet pit diffuser or turbine pumps with a propeller or axial flow impellers, usually for installation in an open sump with a relatively short setting. Operating head exceeding the capacity of a single stage axial flow impeller might call for a pump of two or more stages or single stage pump with a lower specific speed and a mixed flow impeller. High enough operating heads might demand a pump with mixed flow impellers and two or more stages. For lack of a more suitable name, such high head design have usually been classified as propeller pump also.

Although vertical turbine pump and vertical modified propeller pump are basically the same mechanically and could even be of the same specific speed hydraulically, a basic turbine pump design is suitable for a large number of stage. A modified propeller pump, design, however, is basically intended for a maximum of two or three stages.

Most wet pit drainage, low head irrigation, and storm water installation employ conventional propeller or modified propeller pumps. These pumps have also been used for condenser circulating services, but a specialized design dominates this field. As large power plants are usually located in heavily populated areas, they frequently have to use badly contaminated water (both fresh and salt) as a cooling medium. Such water quickly shortens the life of fabricated steel. Cast iron, bronze, or an even more corrosion resistant cast metal must therefore be used for the column pipe assembly. This requirement means a very heavy pump if large capacities are involved. To avoid the necessity of lifting this large mass for maintenance of the rotating parts, some design are built so that the impeller, diffuser and shaft assembly can be removed from the top without disturbing the column pipe assembly. These design are commonly designated as pullout design.

Like vertical turbine pumps, propeller and modified propeller pumps have been made with both open, and enclosed line shafting except for condenser circulating services, enclosed shafting, using oil as a lubricant but with a grease lubricated tail bearing below the impeller, seems to be favored. Some pumps handling condenser circulating water use enclosed shafting but with water (often from another source) as the lubricant, thus eliminating any possibility of oil getting into circulating water and coating the condenser tubes.

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