THE ELECTRICAL OPERATION OF MUNICIPAL PLANTS
Since steam pumping machinery in large works may generally be operated at the highest economy of any class of steam apparatus of similar size, the greatest opportunities for the introduction of electrical pumping are to be found in the small cities and towns. Here the independent operation of the pumping plant will generally be found less modern and far _____ess economical than in the large cities. Also, the systems in use will generally lend themselves more advantageously to electrical operation. If the pumping is performed with direct-acting deep well pumps or direct-acting pressure pumps which make uneconomical use of the steam expansion, large savings may be made by using electric motor drive. As a rule, the small city and village pumping stations feed into an elevated tank from which water is supplied to the mains by gravity. A system of this kind is almost ideal ior the central station to serve since the pumping may be done at off peak hours and at a time of lightest load conditions. This is highly desirable as many of the fixed losses in the steam system are so large that at light load periods a small increase in load does not make a proportionate increase in the fuel expense. In the medium sized and large cities where direct pressure systems arc the rule the value of the load to the central station is not quite as great as in the former case. The load in this instance will more nearly follow the conditions of the regular power and lighting loads on the plant. The pumping being a long hour load, however, causes it to remain in the highly desirable class of prospects. In the large cities, there is a held for motordriven centrifugal pumps to op-rate as boosters in outlying districts where the piessures arc low. Numerous installations of tb s character are giving excellent satisfaction. The city of Minneapolis uses central station service to operate two 1,200-horsepower centrifugal pump units in such an auxiliary station. The city of Chicago is planning to use electric power from the sanitary district on a large scale in its pumping stations. New York City is using motor-driven centrifugal pumps on its high pressure fire system. In considering the best kind of equipment for electrical pumping we have two systems to consider, hirst: The plant pumping direct from a stream, shallow well or reservoir into the main. Second : The plant pumping from deep wells to a reservoir and from the reservoir to the mains, in the first instance, either the plunger type or the centrifugal pump will give satisfactory service when driven by motors. When considering both repairs and efficiency, there is practically no choice between the centrifugal and triplex pumps. The efficiency of the triplex pump ranges from 55 to 75 per cent., and that of the centrifugal pump front 50 to 70 per cent. In the second place, when pumping from deep wells of moderate depth, the very small plant with limited finances and service may use a plunger type of pump satisfactorily. When considerable quantities of water are pumped or the lift is high, the reciprocating deep well pump is usually troublesome and repair bills excessive. In general, however, where the small plant can make use ot it the deep well centrifugal pump will give the best service. This pump is of the vertical shaft type and is sunk deep in the well below the water level. Where the quantity of water to be pumped is from 250 to 1,000 gallons per minute, this type of installation is probably the most satisfactory known. The efficiency of this type of pump is from 50 to 60 per cent.
While the air lift is not very economical, it proves to he very satisfactory in many systems. In operation the air lift pump compresses air which is discharged from a small pipe into a larger pipe sunk below water level. The air thus forces the water to the surface reservoir. The efficiency of the air litt may he anywhere from 20 to 50 per cent., depending on installation conditions. The depth to which the air pipe is submerged and the air pressure necessary have an important bearing on the efficiency. The first cost of the air lift is less than the deep well centrifugal pump, and it eliminates some of the uncertainty of service and cost of repairs of the latter, hence may he more desirable for small plants with limited reserve capacity.
The cost of pumping naturally varies largely since the conditions under which the water is pumped vary so greatly. One company, operating a triplex pump driven by a 35-horsepower motor and pumping against 100 feet head, reports an average current consumption of 700 watts per 1,000 gallons pumped. Another company operating a motor-driven deep well plunger pump reports a consumption of 1,333 watts per 1,000 gallons pumped against a head of 120 feet. Another plant reports a current consumption of 1,220 watts per 1,000 gallons pumped by a motordriven deep well plunger pump lifting 110 feet. One waterworks plant in a city of 12,000 inhabitants installed a generator that supplied current to a 7-horsepower motor placed at each oi nine 65-foot wells which fed a reservoir. A 100-horsepower motor driving a triplex pump delivered 1,000,000 gallons of water per day to an elevated tank and the distributing system. As a result the fuel cost was reduced one-half and the number of employes from 13 to three.
A small municipal pumping plant by expending $6,000 in order to change from steam to electric drive, claims to have reduced its operating expense $5,000 the first year. Power was purchased on an annual basis of $26.40 per horsepower.