Text № 16. Pump Motors

Pumping is one of the major duties performed by powerhouse auxiliary equipment, and usually the largest motors in the station are those that drive the boiler-feed-pumps. In a typical station the total horsepower rating of the boiler-feed-pump motors is between five and six percent of the kilowatt rating of the associates turbine. At least two and usually three boiler-feed pumps of equal rating are used. These pumps operate against a very high head of water and require 3,600-rpm driving motors.

The output of the boiler-feed pumps is controlled by throttling or by varying the speed of the pump. The latter method is attractive because of reduced operating cost. Variable-speed control, when used, is achieved with a variable-speed coupling or by using a wound-rotor motor and a liquid rheostat.

The torque requirements of boiler-feed pumps and most of other pumps are satisfied by motors with low starting current and normal starting torque. Most boiler-feed pump motors are rated for a temperature rise of 40 degrees C above ambient and have class A insulation. Where the ambient temperature is above 40 degrees, class B insulation is used.

Although drip-proof construction is usual, special enclosures are sometimes use to reduce the noise level of the motor or to protect the motor from flyash and other unfavorable atmospheric conditions. Noise can be reduces by using pipe-or base-ventilated motors in which the inlet and exhaust for cooled by air-to-air at a remote location. In particularly dirty locations enclosed motors are used. Air for such motors is cooled by either an air-to-air or an air-to-water heat exchanger. Since outside air is never drawn into the motor, the windings are protected from contamination.

In addition to boiler-feed pump, numerous other pumps are associated directly with the water cycle of the plant or perform auxiliary functions. These include pumps for handling circulating water, condensate, drain water, raw water, water-purification chemicals, ash, flood water, water for fire protection, sump water, lubricating oil, and station water supply. Usually the largest of these are the circulating water pumps. In a typical station, there are two circulating water pumps per turbine with the total horsepower for slightly less than one percent of the turbine rating. The size od driving motor in a particular application is determined by the head and capacity requirements, which are influenced by the nature of the water source.

Дата добавления: 2015-08-20