Turbine-Driven Centrifugal Pumps for Waterworks Service.

Turbine-Driven Centrifugal Pumps for Waterworks Service.

It is only recently that centrifugal pumps have received the serious consideration of waterworks engineers, although their economy and efficiency have long since been demonstrated for supplying water in large quantities to industrial plants. It is our purpose here to show that the steam turbine driven centrifugal pump is able to compete successfully wherever the price of coal is not excessive, with reciprocating pumping engines of the high duty vertical triple-expansion or other type. The high duty shown by reciprocating engines in waterworks service involving the pumping of large quantities of water against high heads under practically uniform load conditions, has long and justly been regarded as a supreme achievement. Due to the direct connection of the pump and engine cylinders and to the high etticiency realized in the pump when the proper attention is paid to its packings, the triple expansion reciprocating pumping engine has been able to develop duties that are unapproachable by rotary units consisting of turbines and centrifugal pumps. However, when a careful comparison is made of the total costs of pumping, it will be found that the balance is entirely reversed; that is, the high first cost of the reciprocating unit together with the cost of the attendance, foundations and buildings it requires, introduce annual charges for interest, upkeep and depreciation which more than offset the greater steam consumption of the turbine unit. Of the several types of pumping engines the following are chiefly used for waterworks service: Low duty compound condensing engines, costing with foundations, piping and appurtenances about $2,300 per million gallons capacity per 24 hours; low duty triple expansion condensing horizontal engines, costing about $2,800 per million gallons capacity; cross-compound condensing horizontal fly-wheel engines, costing about $3,300 per million gallons capacity; and high-duty triple expansion vertical condensing engines, costing about $4,800 per million gallons capacity. The figures just given are from a paper by the late Charles A. Hague, presented before the American Society of Civil Engineers, May 17, 1911. Mr. Hague selected the high-duty triple expansion vertical engine as the only one to be considered in most cases, and it is with this type of engine that we shall make our comparison. It would appear that the relative merits of the two types can be most clearly brought out by a couple of examples drawn from recent practise, and a tabulation of the costs involved. In preparing these comparisons we have

YEARLY CHARGES, TURBINE AND REC IPROCATING PUMPING UNITS.

made use of the following values:

These charges include all costs incident to supplying steam at the engine throttle, such as fuel, labor, interest, depreciation and upkeep of boilers, boiler house, real estate, etc. The first cost per b.h.p. we have taken to he $30 complete, with piping, chimney capacity, grates, etc. The use of a lower figure would tend to favor the turbo pump as compared with the high duty engine. Fuel costs are based on a boiler efficiency of 65 per cent, heat content of 14,500 B. T. U. per pound of coal and 24 hours per day operation. The prices given for pumping units are taken from recent waterworks practise and include condensers, piping and foundations complete. The duties given are on a basis of 150 pounds steam

-Read at the Convention of American W’ater Works Association, in Louisville, Ky., June 5, 1212. pressure and 28-inch vacuum. Two total results are given, one based on coal at $2 per ton and the other on coal at $3 per ton. Where coal can be obtained cheaper than $2 per ton the advantages of the turbo pump are still more clearly marked.

it may be objected that whereas, in the case of high-duty vertical triple expansion engines, a large part of the cost is made up of fixed charges, such as interest which will not increase or may diminish, the larger part of the cost of pumping with steam turbine driven centrifugal pumps, on the other hand, consists of fuel cost, which will probably increase as time goes on. Such increase m the cost of fuel, however, will not be rapid nor excessive, as long as competition still obtains among the producers of coal. In England, for instance, where exhaustion of the coal supply is already in sight, the price of coal has not risen appreciably and large quantities are exported to other countries, in competition with coal from America and other producers whose supply may still be regarded as almost unlimited. In fact, it may be said that in the absence of artificial disturbing causes the price of coal will be determined largely by the cost of mining, and that this will diminish as time goes on, due to improvements in methods and machinery, so that no radical increase in the cost of fuel is to be expected during the life of pumping engines installed at the present time. The centrifugal pump, if properly constructed, maintains its original efficiency indefinitely; the reciprocating pump unless subject to constant and expert attendance and frequent leakage tests may easily develop a slip of 10 per cent. In making comparisons ol cost between turbine and engine equipments, the matter of foundations should not be overlooked, since, apart from its own cost the turbine may also reduce tha expense of installation from forty to eighty per cent. The turbine must also be credited with any saving in floor space, buildings, etc., which it may effect as compared with the reciprocating engine. Inasmuch as power plant buildings rarely have a useful life greater than that of the machinery which they house, the same rate of deprication should be charged against the buildings, foundations, etc., as against the machinery itself. The foundation costs of a 15-million gallon capacity vertical triple-expansion fly-wheel engine may easily be $10,000 more than that required for a turbo pump of the same capacity. A recent example of floor space occupied by a 15-million gallon capacity unit is 30 x 20 x 35 for the reciprocating engine as against 20 x 10 x 7 for the turbo pump.

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