The Choice of Pumping Engines for Municipal Water Supply
Since water to the inhabitants of the various towns and cities is frequently furnished from a publicly owned water works system, the problem of deciding upon the best pumping equipment to be installed for delivering this water is often met. I regret to say that after some years of experience it is generally found by the young engineer that the controlling Board of many city works insist on their own superior judgment to decide such matters and keep the engineer as a ready “goat” on whom to heap the blame when very bad results are finally obtained. Again, there are some who may be so fortunate as to serve some of the few public officials who have the wisdom to leave the technical matters to the decision of the engineer chosen for that purpose. For the service in villages and the smaller towns where the regular domestic supply is from an elevated tank, the demand is for a pump to fill the tank at intervals and to furnish direct pressure for fire fighting purposes. If there is available a reliable source of cheap electric power, motor driven turbine pumps, or triplex plunger pumps are desirable, the latter driven through gears by electric motors. If such electric power is not available, triplex plunger pumps driven by gasoline or kerosene engines are due careful consideration. These machines are highly desirable for intermittent service as they deliver their full output at once when started, instead of requiring such time as is necessary to raise steam in a boiler for a steam driven pump. In addition to the great loss of heat from the boiler while the pump is not in actual operation, the notorious inefficiency of. all small direct acting steam pumps almost eliminates this type, except when operated in connection with an electric lighting system which requires that steam be kept up most of the time anyway. In considering the service for towns which use. from one to five million gallons a day there is more field for careful consideration of the actual conditions to be met. When the supply is taken from a river and first delivered to filter beds, there is nothing better than the turbine pump for this service as a low and practically constant head is to be operated against. The turbine pump may be driven either by an electric motor or steam engine, as the local conditions demand. Where the source of supply is wells in which the water does not rise to the surface, the engineer may consider several methods. One is the direct acting deep well pump with the steam cylinder at the surface and the water piston near the elevation of the sources of water, actuated through a long vertical piston rod. One of the best examples of this system is at Memphis, Tcnn. The Memphis supply is interesting because such a large city is supplied from a great many wells scattered for a considerable distance and flowing into a masonry tunnel which is nearly 100 feet below the surface of the ground and which discharges by gravity into a sump at the pumping station. The steam cylinders of the vertical pumping engines are in the engine room at the surface of the ground while the water cylinders are far below near the surface of the water. These engines have given excellent service. The pumps at Memphis are in a sump large enough to give ample access even at the bottom for attention and repairs to the water cylinders. In the rice fields of Louisiana and Texas a submerged vertical turbine pump in the bottom of the well, dnven through a vertical shaft either with a belt or electric motor has found considerable favor. For a reliable method of raising water from a considerable depth probably there is nothing better than compressed air. The air lift leaves much to be desired in the matter of total efficiency, but its reliability of operation and durability offset this objection. The entire engine is in the pumping station. One compressor can raise water from one well or many and they may be scattered widely. When the available funds will provide a compressor with Corliss valve, steam cylinders, and condensors, and two stage compression, if the required pressure warrants this, very satisfactory operating efficiency is obtained. In the choice of pumping engines for the smaller cities, which require between 1,000,000 and 4,000,000 gallons daily, there is a slightly wider range of equipment from which to make a choice to meet the conditions of the individual installation. In this class of pumping station twenty-four hour attention is usual. Most such cities operate an electric lighting system in connection with the pumping station, which requires that boilers be kept in operation constantly. Under this condition the choice lies between several arrangements with steam as the motive power. When the engines operating the generators are of high efficiency, ample capacity and in duplicate, electric motor driven pumps give excellent total efficiency. The higher steam efficiency obtainable from the steam engine often more than balances the loss in electrical equipment. Either turbine pumps, direct connected to electric motors, or triplex plunger pumps, motor driven through gears, are available. It is usually demanded of municipal service pumping engines that they deliver the normal domestic consumption, against about 50 pounds to the square inch, for more than 95 per cent, of the time in operation, and yet be capable of furnishing this same output and considerably more for fire streams, at 125 pounds to the square inch, or more. This condition means the engine must operate 95 per cent, of the time, at about 40 per cent, load, or at times of fire at more than 100 per cent, over-load, depending on whether the engine is designed for its rated service at domestic or at fire pressure. To meet these requirements 2 turbine pumps can be operated in series for fire pressure and either operated singly for domestic service. To have duplicate equipment, either set of which is capable of meeting any reasonable daily. demand for both domestic and fire service, is absolutely essential for a first class water works pumping station. To meet this condition will require much idle equipment, or operation at small load factor, with turbine pumps. Triplex plunger pumps, motor driven, give good efficiency under domestic service pressure, and will deliver the same volume at double the pressure when required. They are reliable and worthy of careful consideration in this connection. For service of this character, duplex directing acting, tandem compound, or triple expansion, condensing pumping engines are probably the most used. Under favorable operating conditions a duty of 70,000,000 foot pounds per thousand pounds of steam may be approached. In the smaller stations of this class, one more expensive compound engine may be installed, which will do most of the work with reasonable efficiency, and one Standard Underwriter’s . Steam Fire Pump, which will serve to aid in giving fire service and can furnish the domestic, supply also when the more efficient engine must, be stopped for repairs. In many of the stations delivering 5,000,000 gallons a day or more, vast sums of money are wasted, cither through the operation of equipment unsuited to the conditions, or that would be inefficient under any conditions. Here it becomes possible to make interesting comparisons between present and final economy. Large pumping engines may be obtained that will give very high operating efficiency. Some in daily operation are giving a duty of about 200,000,000 foot pounds per 1,000 pounds of steam. These engines, ot course, represent considerable capital, and the problem before the engineer is to determine how large an investment the proposition will warrant. It is possible to purchase costly, high-duty engines and operate them under such conditions that the interest on the first cost of the engine is more than the saving effected by their higher efficiency in operation. For the smaller stations in this class the horizontal cross compound crank and fly-wheel type of condensing engine offers a satisfactory total efficiency; its first cost is tnuch less than the vertical triple expansion, crank and fly-wheel type, and a duty of 125 to 150 million foot pounds can be obtained. In the larger stations using only engines of a daily capacity of 12,000,000 gallons or more, every factor before any but the vertical triple expansion type of engine is chosen should be considered, if the necessary money can be secured to install the more costly type. Data should be obtained as to the actual daily probable output during the term of service of the engine and the cost and efficiency of the two general types of machines. In the larger pumping stations, when the city also owns its electric lighting system, there is a possibility of securing considerable saving in the joint operation of the two stations by having a turbine pump designed to deliver at best efficiency the average domestic consumption so that it will be running practically constantly; this pump to be driven by a synchronous motor so designed and operated as to improve the power factor on the electric circuit. In the field of pumping machinery the engineer has considerable equipment from which to choose, that which will give the best total efficiency under the conditions at hand. He may take the first which is offered and make some salesman happy. He may misjudge the future conditions, and even after faithful effort fail in securing the best efficiency—it is human to err. He may consider carefully, judge correctly and secure excellent results and he may get the credit for it; this he should not expect.
Engineer, Board of Commissioners Jacksonville, Fla.