There has been considerable discussion at various times during the last three or four years, in the engineering societies and waterworks associations, as to the possibilities of the steam turbine centrifugal pump compared with the reciprocating pump or pumping engine for waterworks use. The same sort of discussion preceded this a few years in reference to the steam turbine generator; and the general use of which in the modern plant is evidence that these units can compete with the reciprocating type of prime mover for powerhouse use. In considering the economical advantages of the various types of engines or pumps, it is necessary to take into account not only the first cost or interest upon invested capital, but the depreciation and maintenance as well as the operating expense; and, unless all of these items are considered it is not possible to come to a proper conclusion as to which would be the most economical type. In the discussion at the meeting of the American Water Works Association three years ago, the statement that the centrifugal pump could show an economy, when everything is considered, against the crank and fly-wheel type, was challenged by representatives of some of the large engine builders. It is a, noticeable fact, however, that some of these same builders are now offering on the market turbine-driven centrifugal pumps and guaranteeing duties between 95,000,000 and 100,000,000 foot pounds per 1,000 pounds of steam.

One of the features which is often overlooked in the purchase or designing of a pumping plant, is the fact that the machinery may or may not be in continuous operation; also, that it may not always be operated at its full-load condition. The specifications for large pumping engines almost invariably specify that the duty trial shall be made under full load, whereas, in practical operations, particularly in towns having a directpressure system, it is almost impossible to reproduce these test conditions for any considerable period, and it often occurs that the machinery is operated during the greater portion of the time at from one-half to three-fourths of its capacity rating. It is well known to engineers, superintendents and those familiar with waterworks service, that the total capacity of a pumping equipment must be largely in excess of the average requirement; provision must not only be made for peak loads in times of extraordinary demands, but there must also be spare units to be brought into service at times of repairs, and it is for such service that the turbine centrifugal, on account of its first low cost, makes a very favorable showing in net economy during the annual period. In comparing these two types of pumps, it is necessary not only to consider the cost of the units themselves, but also the cost of foundations. buildings and boiler plant, as it will be readily seen that the total cost of delivering water will include both the fixed charges and operating expense of the entire plant, as it is obvious that a high fixed charge may mean a low cost of operation, or a smaller investment in the pumping equipment, and correspondingly lower duty, a higher boiler room expense and additional boiler capacity.

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