PUMPING MACHINERY TEST DUTY VERSUS OPERATING RESULTS’
This paper is the result of an effort prompted by a request from our Secretary starting with the statement that “we now install pumping engines of 200,000,000-foot pounds guaranteed duty, but get much less every day efficiency,” and ending with the admonition that we “dwell on what the coal piles should give us in water pumped.” From physics we learn that heat and energy, or work, are equivalent and mutually convertible one to the other. Our pump station results represent work. The source of the heat to perform this work is coal, and the means for converting the heat of the coal into work is our steam plant equipment. The per cent, of the heat contained in the coal that may be realized in useful work or energy, represented by the water delivered into our mains, and why the losses is the problem before us. The coal we buy contains, we will say, 100 per cent, of heat, not that coals do not differ in heat producing or thermal values, but let us assume that the coal we have contains heat represented by 100 per cent., then to those familiar with the subject it is well known that with the contrivances in use to this date, we have been able to make apparent in work done as represented by the water column delivered with the most efficient machines, less than 20 per cent, of the 100 per cent, of heat in the coal fired under our boilers and, in many instances, the net result does not exceed 5 per cent. Why this loss? In the boilers, due to the heat that must necessarily pass out through the stack so that the temperatures of the hot gases in the boilers may be in excess of the temperature of the steam generated and thus effective throughout their entire travel through the boiler, coupled with the radiation of the exterior parts of the boiler and the incomplete combustion of fuel represented by the combustible matter lemaining in the ash, test boiler efficiencies are not obtained greatly in excess of 70 per cent., or otherwise stated, we realize only in net heat delivered as dry steam from the boiler, 70 per cent, of the total heat contained in the coal, which we termed 100 per cent. Next comes the steam lines and be they long or short, properly or poorly proportioned, bare or well covered, some radiation must take place, and some loss due to friction must be further incurred, for friction represents energy and energy is heat and so, under the best of conditions, we can expect to deliver less than 70 per cent, of heat of the coal converted into steam and less than 70 per cent, at the engine throttle Then due to the ineffective utilization of the heat delivered at the throttle by the highest grade steam motors or engines in use to-day, which seldom exceeds 20 per cent., we find the net results left of our original 100 per cent, in the coal approximately 15 per cent. As a concrete illustration of the above, recent final and preliminary duty and efficiency tests at Erie, Pa., showed that of the available heat units determined by analysis of the coal fired, the boilers delivered at their outlet nozzles 71 per cent., and of the available heat units received by the steam pipe at the discharge nozzle of the boil ers it delivered to the throttle at the engine 99.6 per cent.; and that the engine converted into useful work but 21.5 per cent, of the available heat units delivered at its throttle by the steam pipe, or that the entire combination, not considering auxiliaries of any sort, showed an overall efficiency of 15.25 per cent., and still the engine on preliminary test (final test not yet run) performed a duty of 205,348,000 foot pounds for each 1,000 pounds of steam consumed. Rut probably, to be more readily understood by the water works operator, we would better restate this problem in the terms of the relations between test duties based on 1,000 pounds of dry steam and obtained on tests for acceptance as compared with every day operating results per 100 pounds of coal fired under the boilers. Before getting too deeply in either discussion, let it be known that we have accorded the field of plants consuming one million and less to the internal combustion engine and electric motor driven pump. For pumping units on which the demand is 1,000,000 ga_____ons or more daily, practical test duties vary from 50,000,000 to 200,000,000 foot pounds of useful work performed for each 1,000 pounds ot dry steam delivered to such unit, bor the same machines, the station duty wouid be expected to vary irom 20,000,000 to 125,000,000 foot pounds of work done for each 100 pounds of coal tired under the boilers. Why the difference between the maximum steam duty ot 200,000,000 and the coal duty of 125,000,000? Also the minimum steam duty of 50,000,000 and the minimum coal duty of 20,000,00? Pump station resubs, as must oi us know, are affected by the following: 1. Quality of coal. 2. Efficiency of boilers. 3. Efficiency of steam lines. 4. station capacity. 5. Head against which water is delivered. 6. Load factor. 7. Adaptability of machinery. 8. Compactness of station. 9. Low vacuum in condensing units. 10. Care in operation and many other minor elements. In the average wacer works station with the best coal, Pennsylvania or West Virginia product, we are able to obtain but an average of about 8 pounds of water evaporated per pound of coal burned. Consequently, if nothing more than this existed, our steam duty of 100,000,000 would be reduced to 80,000,000 in station duty. Besides, we cannot all locate our stations within a practical shipping distance from the Pennsylvania or West Virginia coal fields, and those who obtain their coal from the Ohio districts must expect about 7 pounds of water per pound of coal, and from the Illinois and Indiana districts, in the neighborhood of 6 pounds of water per pound of coal, so that the latter has reduced the 100,000,000 duty to 60,000,000 duty without further inroads. So much for the quality of coal. The above evaporative results are based on the ordinary boiler efficiency of about 65 per cent., which calls for intelligent firing, the boiler operating, within at least 80 per cent, of its rated capacity, grates that will prevent there remaining in the ash more than 5 per cent, combustible, a stack draft that will supply the coal being consumed with the necessary amount of air, boiler walls sufficiently tight to prevent the admission of air above and beyond the grates in quantities that will interfere with the best combustion, and the whole exterior of such a construction as to provide an insulation that will keep down the loss of heat by radiation to the minimum. Unfortunately, the above are seldom found in the ordinary small water works plant in a combination that provides a boiler efficiency greatly in excess of 50 per cent., and efficiencies running as low as 40 per cent, are frequently found, so in the latter case, even when firing a coal from which a high evaporation might be expected, a much lower net result is obtained. Much more may depend upon whether the steam lines are well proportioned, for if too large and the velocity sluggish, the condensation will be increased. If too small, a portion of the heat must be lost in friction. If bare, a large amount of useful energy escapes through radiation, and in 90 per cent, of our steam plants the covering has been done to no specifications other than the will of the company manufacturing and installing such commodities, wherein the covering generally stops short of flanges or fittings about 2 inches on each side, seldom fits the pipe tight, and in consequence, is but 50 per cent, as efficient as though it were continuous over fittings and provided against air circulating between the covering and the steam pipe. A 10 per cent, loss of energy between boilers and engines is not an uncommon one, when by proper care in design and construction it can be kept within 1 per cent. The next great loss comes from a source which has been frequently referred to in the meetings of this association, and while many of the pump and boiler builders think it is a hobby, yet you may be assured it exists in 90 per cent, of our plants in stem reality. It is the varying demand and head and consequent load factor of the plant, and it is not a matter that either the designing engineer or the operating officials can correct. Metering intensifies it, so even when brought to
the minimum, it still remains a large factor, causing inroads on economy; for though the water may be pumped once and then to a reservoir, a well selected engine should, at the time it is purchased, exceed in capacity the demands to be made upon it, and although when operated the reservoir may permit it to deliver at its rated capacity while in operation and thus the maximum of economy from this standpoint obtained, it must, a portion of its time, be either shut down or operated below rated capacity, and so subject its economical possibilities to the inroads of the fixed thermal charges that continue whether the pump is in operation at full or half . speed. With 75 per cent, of our water works plants there exists no high level reservoir and, consequently, the delivery of the pump is governed by the demand on the mains, which varies throughout the twenty-four hours in the ratio of its extremes of about one to three. Then again, in many plants having a capacity of less than 10,000,000 gallons daily, fire service is rendered by direct pressure, which calls for an increase when the alarm comes in of from twenty to sometimes 10C per cent., for all of which the boiler power must be kept in constant readiness, thus increasing grossly the fixed thermal losses necessarily to be charged against the station’s efficiency. When an engine is tested for acceptance and to prove the builder’s guarantee of efficiency, we unfortunately too often set up in the purchaser’s mind a standard which he hopes to but never in the future realizes, not only for the reasons above discussed, but others to follow, one of which is that during this duty test, we simply measure the steam used by the engine, neglecting, of course, that used by many of the auxiliaries such as the feed pump, the electric light engine now to be found in a majority of stations; sometimes the blowing engine that produces the draft, and frequently the air compressor that charges the chambers, the heating of the station and frequently adjacent buddings, all of which may represent a draft on the boilers from ten to 100 per cent, of that of the main engine. In recent years, due to our constantly increasing population and consequent congested districts with modern sanitary methods, the majority of our water supplies, to be safe, must be filtered, and the installation of purification plants generally necessitates the water being pumped from the source to the filters and again repumped to the mains whence the consumption is drawn. Notwithstanding this, that individual, to whom the man immediately responsible for the economy of this station must report, forgets this fact and is prone to demand, as a station duty, the test duty of but what may be termed the high service unit, ignoring all of the elements enumerated above and which must necessarily make inroads on the station’s efficiency. There is too great a tendency on the part of the station or operating engineer to neglect or disregard his means for removing the back pressure from the exhaust side of the low pressure piston in his engines, otherwise known as his condenser and air pump or vacuum may be roughly reckoned in reciprocating engines at 1 per cent, for each inch of mercury. Steam turbines require, for economical operation, a much greater vacuum than 26, and while the condensing facilities may be nursed at the time of the test to produce the desired results, the greater is the fall when the lower vacuum of every day results becomes effective. Taking up the last producing facilities, for with even reciprocating engines, nothing less than 26 inches, as indicated by the mercury column, should be tolerated, while unexpected visits generally find in the neighborhood of 20 inches. The loss due to low vacuum of the major elements that tend to reduce plant efficiency, we come to what is too frequently found, to wit: The inadaptability of many of the steam producing, pumping and auxiliary units due many times, to be sure, to changes that have taken place that are beyond the control of those responsible for their presence, but often, to the exercise of bad judgment, or none at all, in their original selection and installation from an unwillingness to incur expense of expert advice.
Abstract of paper read at American Water Works Convention.