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[Graph: Combined Efficiency of Boiler and Furnace Per Cent against Per Cent of Boiler's Rated Capacity Developed
Fig. 40. Approximate Variation of Efficiency with Capacity under Test Conditions]
Economical Loads—With the effect of capacity on economy in mind, the question arises as to what constitutes the economical load to be carried. In figuring on the economical load for an individual plant, the broader economy is to be considered, that in which, against the boiler efficiency, there is to be weighed the plant first cost, returns on such investment, fuel cost, labor, capacity, etc., etc. This matter has been widely discussed, but unfortunately such discussion has been largely limited to central power station practice. The power generated in such stations, while representing an enormous total, is by no means the larger proportion of the total power generated throughout the country. The factors determining the economic load for the small plant, however, are the same as in a large, and in general the statements made relative to the question are equally applicable.
The economical rating at which a boiler plant should be run is dependent solely upon the load to be carried by that individual plant and the nature of such load. The economical load for each individual plant can be determined only from the careful study of each individual set of conditions or by actual trial.
The controlling factor in the cost of the plant, regardless of the nature of the load, is the capacity to carry the maximum peak load that may be thrown on the plant under any conditions.
While load conditions, do, as stated, vary in every individual plant, in a broad sense all loads may be grouped in three classes: 1st, the approximately constant 24-hour load; 2nd, the steady 10 or 12-hour load usually with a noonday period of no load; 3rd, the 24-hour variable load, found in central station practice. The economical load at which the boiler may be run will vary with these groups:
1st. For a constant load, 24 hours in the day, it will be found in most cases that, when all features are considered, the most economical load or that at which a given amount of steam can be produced the most cheaply will be considerably over the rated horse power of the boiler. How much above the rated capacity this most economic load will be, is dependent largely upon the cost of coal at the plant, but under ordinary conditions, the point of maximum economy will probably be found to be somewhere between 25 and 50 per cent above the rated capacity of the boilers. The capital investment must be weighed against the coal saving through increased thermal efficiency and the labor account, which increases with the number of units, must be given proper consideration. When the question is considered in connection with a plant already installed, the conditions are different from where a new plant is contemplated. In an old plant, where there are enough boilers to operate at low rates of capacity, the capital investment leads to a fixed charge, and it will be found that the most economical load at which boilers may be operated will be lower than where a new plant is under consideration.
2nd. For a load of 10 or 12 hours a day, either an approximately steady load or one in which there is a peak, where the boilers have been banked over night, the capacity at which they may be run with the best economy will be found to be higher than for uniform 24-hour load conditions. This is obviously due to original investment, that is, a given amount of invested capital can be made to earn a larger return through the higher overload, and this will hold true to a point where the added return more than offsets the decrease in actual boiler efficiency. Here again the determining factors of what is the economical load are the fuel and labor cost balanced against the thermal efficiency. With a load of this character, there is another factor which may affect the economical plant operating load. This is from the viewpoint of spare boilers. That such added capacity in the way of spares is necessary is unquestionable. Since they must be installed, therefore, their presence leads to a fixed charge and it is probable that for the plant, as a whole, the economical load will be somewhat lower than if the boilers were considered only as spares. That is, it may be found best to operate these spares as a part of the regular equipment at all times except when other boilers are off for cleaning and repairs, thus reducing the load on the individual boilers and increasing the efficiency. Under such conditions, the added boiler units can be considered as spares only during such time as some of the boilers are not in operation.
Due to the operating difficulties that may be encountered at the higher overloads, it will ordinarily be found that the most economical ratings at which to run boilers for such load conditions will be between 150 and 175 per cent of rating. Here again the maximum capacity at which the boilers may be run for the best plant economy is limited by the point at which the efficiency drops below what is warranted in view of the first cost of the apparatus.
3rd. The 24-hour variable load. This is a class of load carried by the central power station, a load constant only in the sense that there are no periods of no load and which varies widely with different portions of the 24 hours. With such a load it is particularly difficult to make any assertion as to the point of maximum economy that will hold for any station, as this point is more than with any other class of load dependent upon the factors entering into the operation of each individual plant.
The methods of handling a load of this description vary probably more than with any other kind of load, dependent upon fuel, labor, type of stoker, flexibility of combined furnace and boiler etc., etc.
In general, under ordinary conditions such as appear in city central power station work where the maximum peaks occur but a few times a year, the plant should be made of such size as to enable it to carry these peaks at the maximum possible overload on the boilers, sufficient margin of course being allowed for insurance against interruption of service. With the boilers operating at this maximum overload through the peaks a large sacrifice in boiler efficiency is allowable, provided that by such sacrifice the overload expected is secured.
Some methods of handling a load of this nature are given below:
Certain plant operating conditions make it advisable, from the standpoint of plant economy, to carry whatever load is on the plant at any time on only such boilers as will furnish the power required when operating at ratings of, say, 150 to 200 per cent. That is, all boilers which are in service are operated at such ratings at all times, the variation in load being taken care of by the number of boilers on the line. Banked boilers are cut in to take care of increasing loads and peaks and placed again on bank when the peak periods have passed. It is probable that this method of handling central station load is to-day the most generally used.
Other conditions of operation make it advisable to carry the load on a definite number of boiler units, operating these at slightly below their rated capacity during periods of light or low loads and securing the overload capacity during peaks by operating the same boilers at high ratings. In this method there are no boilers kept on banked fires, the spares being spares in every sense of the word.
A third method of handling widely varying loads which is coming somewhat into vogue is that of considering the plant as divided, one part to take care of what may be considered the constant plant load, the other to take care of the floating or variable load. With such a method that portion of the plant carrying the steady load is so proportioned that the boilers may be operated at the point of maximum efficiency, this point being raised to a maximum through the use of economizers and the general installation of any apparatus leading to such results. The variable load will be carried on the remaining boilers of the plant under either of the methods just given, that is, at the high ratings of all boilers in service and banking others, or a variable capacity from all boilers in service.
The opportunity is again taken to indicate the very general character of any statements made relative to the economical load for any plant and to emphasize the fact that each individual case must be considered independently, with the conditions of operations applicable thereto.
With a thorough understanding of the meaning of boiler efficiency and capacity and their relation to each other, it is possible to consider more specifically the selection of boilers.
The foremost consideration is, without question, the adaptability of the design selected to the nature of the work to be done. An installation which is only temporary in its nature would obviously not warrant the first cost that a permanent plant would. If boilers are to carry an intermittent and suddenly fluctuating load, such as a hoisting load or a reversing mill load, a design would have to be selected that would not tend to prime with the fluctuations and sudden demand for steam. A boiler that would give the highest possible efficiency with fuel of one description, would not of necessity give such efficiency with a different fuel. A boiler of a certain design which might be good for small plant practice would not, because of the limitations in practicable size of units, be suitable for large installations. A discussion of the relative value of designs can be carried on almost indefinitely but enough has been said to indicate that a given design will not serve satisfactorily under all conditions and that the adaptability to the service required will be dependent upon the fuel available, the class of labor procurable, the feed water that must be used, the nature of the plant's load, the size of the plant and the first cost warranted by the service the boiler is to fulfill.
TABLE 60
ACTUAL EVAPORATION FOR DIFFERENT PRESSURES AND TEMPERATURES OF FEED WATER CORRESPONDING TO ONE HORSE POWER (34-1/2 POUNDS PER HOUR FROM AND AT 212 DEGREES FAHRENHEIT)
- Temperature of Pressure by Gauge Pounds per Square Inch Feed Degrees Fahrenheit 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 -+ - 32 28.41 28.36 28.29 28.24 28.20 28.16 28.13 28.09 28.07 28.04 28.02 27.99 27.97 27.95 27.94 27.92 27.90 27.89 27.87 27.86 27.83 40 28.61 28.54 28.49 28.44 28.40 28.35 28.32 28.29 28.26 28.23 28.21 28.18 28.16 28.14 28.12 28.11 28.09 28.07 28.06 28.05 28.03 50 28.85 28.79 28.73 28.68 28.64 28.60 28.56 28.53 28.50 28.47 28.45 28.43 28.40 28.38 28.36 28.35 28.33 28.31 28.30 28.28 28.27 60 29.10 29.04 28.98 28.93 28.88 28.84 28.81 28.77 28.74 28.72 28.69 28.67 28.65 28.62 28.60 28.59 28.57 28.55 28.54 28.52 28.51 70 29.36 29.29 29.23 29.18 29.14 29.09 29.06 29.02 28.99 28.96 28.94 28.92 28.89 28.87 28.85 28.83 28.82 28.80 28.78 28.77 28.76 80 29.62 29.55 29.49 29.44 29.39 29.35 29.31 29.27 29.24 29.22 29.19 29.17 29.14 29.12 29.10 29.08 29.07 29.05 29.03 29.02 29.00 90 29.88 29.81 29.75 29.70 29.65 29.61 29.57 29.53 29.50 29.47 29.45 29.42 29.40 29.38 29.36 29.34 29.32 29.30 29.29 29.27 29.25 100 30.15 30.08 30.02 29.96 29.91 29.87 29.83 29.80 29.76 29.73 29.71 29.68 29.66 29.63 29.61 29.60 29.58 29.56 29.54 29.53 29.51 110 30.42 30.35 30.29 30.23 30.18 30.14 30.10 30.06 30.03 30.00 29.97 29.95 29.92 29.90 29.88 29.86 29.84 29.82 29.81 29.79 29.77 120 30.70 30.63 30.56 30.51 30.46 30.41 30.37 30.33 30.30 30.27 30.24 30.22 30.19 30.17 30.15 30.13 30.11 30.09 30.07 30.06 30.04 130 30.99 30.91 30.84 30.79 30.73 30.69 30.65 30.61 30.57 30.54 30.52 30.49 30.47 30.44 30.42 30.40 30.38 30.36 30.35 30.33 30.31 140 31.28 31.20 31.13 31.07 31.02 30.97 30.93 30.89 30.86 30.83 30.80 30.77 30.75 30.72 30.70 30.68 30.66 30.64 30.62 30.61 30.59 150 31.58 31.49 31.42 31.36 31.31 31.26 31.22 31.18 31.14 31.11 31.08 31.06 31.03 31.01 30.98 30.96 30.94 30.92 30.91 30.89 30.87 160 31.87 31.79 31.72 31.66 31.61 31.56 31.51 31.47 31.44 31.40 31.37 31.35 31.32 31.29 31.27 31.25 31.23 31.21 31.19 31.18 31.16 170 32.18 32.10 32.02 31.96 31.91 31.86 31.81 31.77 31.73 31.70 31.67 31.64 31.62 31.59 31.57 31.54 31.52 31.50 31.49 31.47 31.46 180 32.49 32.41 32.33 32.27 32.22 32.16 32.12 32.08 32.04 32.00 31.97 31.95 31.92 31.89 31.87 31.84 31.82 31.80 31.79 31.77 31.75 190 32.81 32.72 32.65 32.59 32.53 32.47 32.43 32.38 32.35 32.32 32.29 32.26 32.23 32.20 32.17 32.15 32.13 32.11 32.09 32.07 32.05 200 33.13 33.05 32.97 32.91 32.85 32.79 32.75 32.70 32.66 32.63 32.60 32.57 32.54 32.51 32.49 32.46 32.44 32.42 32.40 32.38 32.36 210 33.47 33.38 33.30 33.24 33.18 33.13 33.08 33.03 32.99 32.95 32.92 32.89 32.86 32.83 32.81 32.79 32.76 32.74 32.72 32.70 32.68 -
The proper consideration can be given to the adaptability of any boiler for the service in view only after a thorough understanding of the requirements of a good steam boiler, with the application of what has been said on the proper operation to the special requirements of each case. Of almost equal importance to the factors mentioned are the experience, the skill and responsibility of the manufacturer.
With the design of boiler selected that is best adapted to the service required, the next step is the determination of the boiler power requirements.
The amount of steam that must be generated is determined from the steam consumption of the prime movers. It has already been indicated that such consumption can vary over wide limits with the size and type of the apparatus used, but fortunately all types have been so tested that manufacturers are enabled to state within very close limits the actual consumption under any given set of conditions. It is obvious that conditions of operation will have a bearing on the steam consumption that is as important as the type and size of the apparatus itself. This being the case, any tabular information that can be given on such steam consumption, unless it be extended to an impracticable size, is only of use for the most approximate work and more definite figures on this consumption should in all cases be obtained from the manufacturer of the apparatus to be used for the conditions under which it will operate.
To the steam consumption of the main prime movers, there is to be added that of the auxiliaries. Again it is impossible to make a definite statement of what this allowance should be, the figure depending wholly upon the type and the number of such auxiliaries. For approximate work, it is perhaps best to allow 15 or 20 per cent of the steam requirements of the main engines, for that of auxiliaries. Whatever figure is used should be taken high enough to be on the conservative side.
When any such figures are based on the actual weight of steam required, Table 60, which gives the actual evaporation for various pressures and temperatures of feed corresponding to one boiler horse power (34.5 pounds of water per hour from and at 212 degrees), may be of service.
With the steam requirements known, the next step is the determination of the number and size of boiler units to be installed. This is directly affected by the capacity at which a consideration of the economical load indicates is the best for the operating conditions which will exist. The other factors entering into such determination are the size of the plant and the character of the feed water.
The size of the plant has its bearing on the question from the fact that higher efficiencies are in general obtained from large units, that labor cost decreases with the number of units, the first cost of brickwork is lower for large than for small size units, a general decrease in the complication of piping, etc., and in general the cost per horse power of any design of boiler decreases with the size of units. To illustrate this, it is only necessary to consider a plant of, say, 10,000 boiler horse power, consisting of 40-250 horse-power units or 17-600 horse-power units.
The feed water available has its bearing on the subject from the other side, for it has already been shown that very large units are not advisable where the feed water is not of the best.
The character of an installment is also a factor. Where, say, 1000 horse power is installed in a plant where it is known what the ultimate capacity is to be, the size of units should be selected with the idea of this ultimate capacity in mind rather than the amount of the first installation.
Boiler service, from its nature, is severe. All boilers have to be cleaned from time to time and certain repairs to settings, etc., are a necessity. This makes it necessary, in determining the number of boilers to be installed, to allow a certain number of units or spares to be operated when any of the regular boilers must be taken off the line. With the steam requirements determined for a plant of moderate size and a reasonably constant load, it is highly advisable to install at least two spare boilers where a continuity of service is essential. This permits the taking off of one boiler for cleaning or repairs and still allows a spare boiler in the event of some unforeseen occurrence, such as the blowing out of a tube or the like. Investment in such spare apparatus is nothing more nor less than insurance on the necessary continuity of service. In small plants of, say, 500 or 600 horse power, two spares are not usually warranted in view of the cost of such insurance. A large plant is ordinarily laid out in a number of sections or panels and each section should have its spare boiler or boilers even though the sections are cross connected. In central station work, where the peaks are carried on the boilers brought up from the bank, such spares are, of course, in addition to these banked boilers. From the aspect of cleaning boilers alone, the number of spare boilers is determined by the nature of any scale that may be formed. If scale is formed so rapidly that the boilers cannot be kept clean enough for good operating results, by cleaning in rotation, one at a time, the number of spares to take care of such proper cleaning will naturally increase.
In view of the above, it is evident that only a suggestion can be made as to the number and size of units, as no recommendation will hold for all cases. In general, it will be found best to install units of the largest possible size compatible with the size of the plant and operating conditions, with the total power requirements divided among such a number of units as will give proper flexibility of load, with such additional units for spares as conditions of cleaning and insurance against interruption of service warrant.
In closing the subject of the selection of boilers, it may not be out of place to refer to the effect of the builder's guarantee upon the determination of design to be used. Here in one of its most important aspects appears the responsibility of the manufacturer. Emphasis has been laid on the difference between test results and those secured in ordinary operating practice. That such a difference exists is well known and it is now pretty generally realized that it is the responsible manufacturer who, where guarantees are necessary, submits the conservative figures, figures which may readily be exceeded under test conditions and which may be closely approached under the ordinary plant conditions that will be met in daily operation.
OPERATION AND CARE OF BOILERS
The general subject of boiler room practice may be considered from two aspects. The first is that of the broad plant economy, with a suggestion as to the methods to be followed in securing the best economical results with the apparatus at hand and procurable. The second deals rather with specific recommendations which should be followed in plant practice, recommendations leading not only to economy but also to safety and continuity of service. Such recommendations are dictated from an understanding of the nature of steam generating apparatus and its operation, as covered previously in this book.
It has already been pointed out that the attention given in recent years to steam generating practice has come with a realization of the wide difference existing between the results being obtained in every-day operation and those theoretically possible. The amount of such attention and regulation given to the steam generating end of a power plant, however, is comparatively small in relation to that given to the balance of the plant, but it may be safely stated that it is here that there is the greatest assurance of a return for the attention given.
In the endeavor to increase boiler room efficiency, it is of the utmost importance that a standard basis be set by which average results are to be judged. With the theoretical efficiency obtainable varying so widely, this standard cannot be placed at the highest efficiency that has been obtained regardless of operating conditions. It is better set at the best obtainable results for each individual plant under its conditions of installation and daily operation.
With an individual standard so set, present practice can only be improved by a systematic effort to approach this standard. The degree with which operating results will approximate such a standard will be found to be directly proportional to the amount of intelligent supervision given the operation. For such supervision to be given, it is necessary to have not only a full realization of what the plant can do under the best operating conditions but also a full and complete knowledge of what it is doing under all of the different conditions that may arise. What the plant is doing should be made a matter of continuous record so arranged that the results may be directly compared for any period or set of conditions, and where such results vary from the standard set, steps must be taken immediately to remedy the causes of such failings. Such a record is an important check in the losses in the plant.
As the size of the plant and the fuel consumption increase, such a check of losses and recording of results becomes a necessity. In the larger plants, the saving of but a fraction of one per cent in the fuel bill represents an amount running into thousands of dollars annually, while the expense of the proper supervision to secure such saving is small. The methods of supervision followed in the large plants are necessarily elaborate and complete. In the smaller plants the same methods may be followed on a more moderate scale with a corresponding saving in fuel and an inappreciable increase in either plant organization or expense.
There has been within the last few years a great increase in the practicability and reliability of the various types of apparatus by which the records of plant operation may be secured. Much of this apparatus is ingenious and, considering the work to be done, is remarkably accurate. From the delicate nature of some of the apparatus, the liability to error necessitates frequent calibration but even where the accuracy is known to be only within limits of, say, 5 per cent either way, the records obtained are of the greatest service in considering relative results. Some of the records desirable and the apparatus for securing them are given below.
Inasmuch as the ultimate measure of the efficiency of the boiler plant is the cost of steam generation, the important records are those of steam generated and fuel consumed Records of temperature, analyses, draft and the like, serve as a check on this consumption, indicating the distribution of the losses and affording a means of remedying conditions where improvement is possible.
Coal Records—There are many devices on the market for conveniently weighing the coal used. These are ordinarily accurate within close limits, and where the size or nature of the plant warrants the investment in such a device, its use is to be recommended. The coal consumption should be recorded by some other method than from the weights of coal purchased. The total weight gives no way of dividing the consumption into periods and it will unquestionably be found to be profitable to put into operation some scheme by which the coal is weighed as it is used. In this way, the coal consumption, during any specific period of the plant's operation, can be readily seen. The simplest of such methods which may be used in small plants is the actual weighing on scales of the fuel as it is brought into the fire room and the recording of such weights.
Aside from the actual weight of the fuel used, it is often advisable to keep other coal records, coal and ash analyses and the like, for the evaporation to be expected will be dependent upon the grade of fuel used and its calorific value, fusibility of its ash, and like factors.
The highest calorific value for unit cost is not necessarily the indication of the best commercial results. The cost of fuel is governed by this calorific value only when such value is modified by local conditions of capacity, labor and commercial efficiency. One of the important factors entering into fuel cost is the consideration of the cost of ash handling and the maintenance of ash handling apparatus if such be installed. The value of a fuel, regardless of its calorific value, is to be based only on the results obtained in every-day plant operation.
Coal and ash analyses used in connection with the amount of fuel consumed, are a direct indication of the relation between the results being secured and the standard of results which has been set for the plant. The methods of such analyses have already been described. The apparatus is simple and the degree of scientific knowledge necessary is only such as may be readily mastered by plant operatives.
The ash content of a fuel, as indicated from a coal analysis checked against ash weights as actually found in plant operation, acts as a check on grate efficiency. The effect of any saving in the ashes, that is, the permissible ash to be allowed in the fuel purchased, is determined by the point at which the cost of handling, combined with the falling off in the evaporation, exceeds the saving of fuel cost through the use of poorer coal.
Water Records—Water records with the coal consumption, form the basis for judging the economic production of steam. The methods of securing such records are of later introduction than for coal, but great advances have been made in the apparatus to be used. Here possibly, to a greater extent than in any recording device, are the records of value in determining relative evaporation, that is, an error is rather allowable provided such an error be reasonably constant.
The apparatus for recording such evaporation is of two general classes: Those measuring water before it is fed to the boiler and those measuring the steam as it leaves. Of the first, the venturi meter is perhaps the best known, though recently there has come into considerable vogue an apparatus utilizing a weir notch for the measuring of such water. Both methods are reasonably accurate and apparatus of this description has an advantage over one measuring steam in that it may be calibrated much more readily. Of the steam measuring devices, the one in most common use is the steam flow meter. Provided the instruments are selected for a proper flow, etc., they are of inestimable value in indicating the steam consumption. Where such instruments are placed on the various engine room lines, they will immediately indicate an excessive consumption for any one of the units. With a steam flow meter placed on each boiler, it is possible to fix relatively the amount produced by each boiler and, considered in connection with some of the "check" records described below, clearly indicate whether its portion of the total steam produced is up to the standard set for the over-all boiler room efficiency.
Flue Gas Analysis—The value of a flue gas analysis as a measure of furnace efficiency has already been indicated. There are on the market a number of instruments by which a continuous record of the carbon dioxide in the flue gases may be secured and in general the results so recorded are accurate. The limitations of an analysis showing only CO_{2} and the necessity of completing such an analysis with an Orsat, or like apparatus, and in this way checking the automatic device, have already been pointed out, but where such records are properly checked from time to time and are used in conjunction with a record of flue temperatures, the losses due to excess air or incomplete combustion and the like may be directly compared for any period. Such records act as a means for controlling excess air and also as a check on individual firemen.
Where the size of a plant will not warrant the purchase of an expensive continuous CO_{2} recorder, it is advisable to make analyses of samples for various conditions of firing and to install an apparatus whereby a sample of flue gas covering a period of, say, eight hours, may be obtained and such a sample afterwards analyzed.
Temperature Records—Flue gas temperatures, feed water temperatures and steam temperatures are all taken with recording thermometers, any number of which will, when properly calibrated, give accurate results.
A record of flue temperatures is serviceable in checking stack losses and, in general, the cleanliness of the boiler. A record of steam temperatures, where superheaters are used, will indicate excessive fluctuations and lead to an investigation of their cause. Feed temperatures are valuable in showing that the full benefit of the exhaust steam is being derived.
Draft Regulation—As the capacity of a boiler varies with the combustion rate and this rate with the draft, an automatic apparatus satisfactorily varying this draft with the capacity demands on the boiler will obviously be advantageous.
As has been pointed out, any fuel has some rate of combustion at which the best results will be obtained. In a properly designed plant where the load is reasonably steady, the draft necessary to secure such a rate may be regulated automatically.
Automatic apparatus for the regulation of draft has recently reached a stage of perfection which in the larger plants at any rate makes its installation advisable. The installation of a draft gauge or gauges is strongly to be recommended and a record of such drafts should be kept as being a check on the combustion rates.
An important feature to be considered in the installing of all recording apparatus is its location. Thermometers, draft gauges and flue gas sampling pipes should be so located as to give as nearly as possible an average of the conditions, the gases flowing freely over the ends of the thermometers, couples and sampling pipes. With the location permanent, there is no security that the samples may be considered an average but in any event comparative results will be secured which will be useful in plant operation. The best permanent location of apparatus will vary considerably with the design of the boiler.
It may not be out of place to refer briefly to some of the shortcomings found in boiler room practice, with a suggestion as to a means of overcoming them.
1st. It is sometimes found that the operating force is not fully acquainted with the boilers and apparatus. Probably the most general of such shortcomings is the fixed idea in the heads of the operatives that boilers run above their rated capacity are operating under a state of strain and that by operating at less than their rated capacity the most economical service is assured, whereas, by determining what a boiler will do, it may be found that the most economical rating under the conditions of the plant will be considerably in excess of the builder's rating. Such ideas can be dislodged only by demonstrating to the operatives what maximum load the boilers can carry, showing how the economy will vary with the load and the determining of the economical load for the individual plant in question.
2nd. Stokers. With stoker-fired boilers, it is essential that the operators know the limitations of their stokers as determined by their individual installation. A thorough understanding of the requirements of efficient handling must be insisted upon. The operatives must realize that smokeless stacks are not necessarily the indication of good combustion for, as has been pointed out, absolute smokelessness is oftentimes secured at an enormous loss in efficiency through excess air.
Another feature in stoker-fired plants is in the cleaning of fires. It must be impressed upon the operatives that before the fires are cleaned they should be put into condition for such cleaning. If this cleaning is done at a definite time, regardless of whether the fires are in the best condition for cleaning, there will be a great loss of good fuel with the ashes.
3rd. It is necessary that in each individual plant there be a basis on which to judge the cleanliness of a boiler. From the operative's standpoint, it is probably more necessary that there be a thorough understanding of the relation between scale and tube difficulties than between scale and efficiency. It is, of course, impossible to keep boilers absolutely free from scale at all times, but experience in each individual plant determines the limit to which scale can be allowed to form before tube difficulties will begin or a perceptible falling off in efficiency will take place. With such a limit of scale formation fixed, the operatives should be impressed with the danger of allowing it to be exceeded.
4th. The operatives should be instructed as to the losses resulting from excess air due to leaks in the setting and as to losses in efficiency and capacity due to the by-passing of gases through the setting, that is, not following the path of the baffles as originally installed. In replacing tubes and in cleaning the heating surfaces, care must be taken not to dislodge baffle brick or tile.
5th. That an increase in the temperature of the feed reduces the amount of work demanded from the boiler has been shown. The necessity of keeping the feed temperature as high as the quantity of exhaust steam will allow should be thoroughly understood. As an example of this, there was a case brought to our attention where a large amount of exhaust steam was wasted simply because the feed pump showed a tendency to leak if the temperature of feed water was increased above 140 degrees. The amount wasted was sufficient to increase the temperature to 180 degrees but was not utilized simply because of the slight expense necessary to overhaul the feed pump.
The highest return will be obtained when the speed of the feed pumps is maintained reasonably constant for should the pumps run very slowly at times, there may be a loss of the steam from other auxiliaries by blowing off from the heaters.
6th. With a view to checking steam losses through the useless blowing of safety valves, the operative should be made to realize the great amount of steam that it is possible to get through a pipe of a given size. Oftentimes the fireman feels a sense of security from objections to a drop in steam simply because of the blowing of safety valves, not considering the losses due to such a cause and makes no effort to check this flow either by manipulation of dampers or regulation of fires.
The few of the numerous shortcomings outlined above, which may be found in many plants, are almost entirely due to lack of knowledge on the part of the operating crew as to the conditions existing in their own plants and the better performances being secured in others. Such shortcomings can be overcome only by the education of the operatives, the showing of the defects of present methods, and instruction in better methods. Where such instruction is necessary, the value of records is obvious. There is fortunately a tendency toward the employment of a better class of labor in the boiler room, a tendency which is becoming more and more marked as the realization of the possible saving in this end of the plant increases.
The second aspect of boiler room management, dealing with specific recommendations as to the care and operation of the boilers, is dictated largely by the nature of the apparatus. Some of the features to be watched in considering this aspect follow.
Before placing a new boiler in service, a careful and thorough examination should be made of the pressure parts and the setting. The boiler as erected should correspond in its baffle openings, where baffles are adjustable, with the prints furnished for its erection, and such baffles should be tight. The setting should be so constructed that the boiler is free to expand without interfering with the brickwork. This ability to expand applies also to blow-off and other piping. After erection all mortar and chips of brick should be cleaned from the pressure parts. The tie rods should be set up snug and then slacked slightly until the setting has become thoroughly warm after the first firing. The boiler should be examined internally before starting to insure the absence of dirt, any foreign material such as waste, and tools. Oil and paint are sometimes found in the interior of a new boiler and where such is the case, a quantity of soda ash should be placed within it, the boiler filled with water to its normal level and a slow fire started. After twelve hours of slow simmering, the fire should be allowed to die out, the boiler cooled slowly and then opened and washed out thoroughly. Such a proceeding will remove all oil and grease from the interior and prevent the possibility of foaming and tube difficulties when the boiler is placed in service.
The water column piping should be examined and known to be free and clear. The water level, as indicated by the gauge glass, should be checked by opening the gauge cocks.
The method of drying out a brick setting before placing a boiler in operation is described later in the discussion of boiler settings.
A boiler should not be cut into the line with other boilers until the pressure within it is approximately that in the steam main. The boiler stop valve should be opened very slowly until it is fully opened. The arrangement of piping should be such that there can be no possibility of water collecting in a pocket between the boiler and the main, from which it can be carried over into the steam line when a boiler is cut in.
In regular operation the safety valve and steam gauge should be checked daily. In small plants the steam pressure should be raised sufficiently to cause the safety valves to blow, at which time the steam gauge should indicate the pressure at which the valve is known to be set. If it does not, one is in error and the gauge should be compared with one of known accuracy and any error at once rectified.
In large plants such a method of checking would result in losses too great to be allowed. Here the gauges and valves are ordinarily checked at the time a boiler is cut out, the valves being assured of not sticking by daily instantaneous opening through manipulation by hand of the valve lever. The daily blowing of the safety valve acts not only as a check on the gauge but insures the valve against sticking.
The water column should be blown down thoroughly at least once on every shift and the height of water indicated by the glass checked by the gauge cocks. The bottom blow-offs should be kept tight. These should be opened at least once daily to blow from the mud drum any sediment that may have collected and to reduce the concentration. The amount of blowing down and the frequency is, of course, determined by the nature of the feed water used.
In case of low water, resulting either from carelessness or from some unforeseen condition of operation, the essential object to be obtained is the extinguishing of the fire in the quickest possible manner. Where practicable, this is best accomplished by the playing of a heavy stream of water from a hose on the fire. Another method, perhaps not so efficient, but more generally recommended, is the covering of the fire with wet ashes or fresh fuel. A boiler so treated should be cut out of line after such an occurrence and a thorough inspection made to ascertain what damage, if any, has been done before it is again placed in service.
The efficiency and capacity depend to an extent very much greater than is ordinarily realized upon the cleanliness of the heating surfaces, both externally and internally, and too much stress cannot be put upon the necessity for systematic cleaning as a regular feature in the plant operation.
The outer surfaces of the tubes should be blown free from soot at regular intervals, the frequency of such cleaning periods being dependent upon the class of fuel used. The most efficient way of blowing soot from the tubes is by means of a steam lance with which all parts of the surfaces are reached and swept clean. There are numerous soot blowing devices on the market which are designed to be permanently fixed within the boiler setting. Where such devices are installed, there are certain features that must be watched to avoid trouble. If there is any leakage of water of condensation within the setting coming into contact with the boiler tubes, it will tend toward corrosion, or if in contact with the heated brickwork will cause rapid disintegration of the setting. If the steam jets are so placed that they impinge directly against the tubes, erosion may take place. Where such permanent soot blowers are installed, too much care cannot be taken to guard against these possibilities.
Internally, the tubes must be kept free from scale, the ingredients of which a study of the chapter on the impurities of water indicates are present in varying quantities in all feed waters. Not only has the presence of scale a direct bearing on the efficiency and capacity to be obtained from a boiler but its absence is an assurance against the burning out of tubes.
In the absence of a blow-pipe action of the flames, it is impossible to burn a metal surface where water is in intimate contact with that surface.
In stoker-fired plants where a blast is used, and the furnace is not properly designed, there is a danger of a blow-pipe action if the fires are allowed to get too thin. The rapid formation of steam at such points of localized heat may lead to the burning of the metal of the tubes.
Any formation of scale on the interior surface of a boiler keeps the water from such a surface and increases its tendency to burn. Particles of loose scale that may become detached will lodge at certain points in the tubes and localize this tendency at such points. It is because of the danger of detaching scale and causing loose flakes to be present that the use of a boiler compound is not recommended for the removal of scale that has already formed in a boiler. This question is covered in the treatment of feed waters. If oil is allowed to enter a boiler, its action is the same as that of scale in keeping the water away from the metal surfaces.
It has been proven beyond a doubt that a very large percentage of tube losses is due directly to the presence of scale which, in many instances, has been so thin as to be considered of no moment, and the importance of maintaining the boiler heating surfaces in a clean condition cannot be emphasized too strongly.
The internal cleaning can best be accomplished by means of an air or water-driven turbine, the cutter heads of which may be changed to handle various thicknesses of scale. Fig. 41 shows a turbine cleaner with various cutting heads, which has been found to give satisfactory service.
Where a water-driven turbine is used, it should be connected to a pump which will deliver at least 120 gallons per minute per cleaner at 150 pounds pressure. This pressure should never be less than 90 pounds if satisfactory results are desired. Where an air-driven turbine is used, the pressure should be at least 100 pounds, though 150 pounds is preferable, and sufficient water should be introduced into the tube to keep the cutting head cool and assist in washing down the scale as it is chipped off.
Where scale has been allowed to accumulate to an excessive thickness, the work of removal is difficult and tedious. Where such a heavy scale is of sulphate formation, its removal may be assisted by filling the boiler with water to which there has been added a quantity of soda ash, a bucketful to each drum, starting a low fire and allowing the water to boil for twenty-four hours with no pressure on the boiler. It should be cooled slowly, drained, and the turbine cleaner used immediately, as the scale will tend to harden rapidly under the action of the air.
Where oil has been allowed to get into a boiler, it should be removed before placing the boiler in service, as described previously where reference is made to its removal by boiling out with soda ash.
Where pitting or corrosion is noted, the parts affected should be carefully cleaned and the interior of the drums should be painted with white zinc if the boiler is to remain idle. The cause of such action should be immediately ascertained and steps taken to apply the proper remedy.
When making an internal inspection of a boiler or when cleaning the interior heating surfaces, great care must be taken to guard against the possibility of steam entering the boiler in question from other boilers on the same line either through the careless opening of the boiler stop valve or some auxiliary valve or from an open blow-off. Bad accidents through scalding have resulted from the neglect of this precaution.
Boiler brickwork should be kept pointed up and all cracks filled. The boiler baffles should be kept tight to prevent by-passing of any gases through the heating surfaces.
Boilers should be taken out of service at regular intervals for cleaning and repairs. When this is done, the boiler should be cooled slowly, and when possible, be allowed to stand for twenty-four hours after the fire is drawn before opening. The cooling process should not be hurried by allowing cold air to rush through the setting as this will invariably cause trouble with the brickwork. When a boiler is off for cleaning, a careful examination should be made of its condition, both external and internal, and all leaks of steam, water and air through the setting stopped. If water is allowed to come into contact with brickwork that is heated, rapid disintegration will take place. If water is allowed to come into contact with the metal of the boiler when out of service, there is a likelihood of corrosion.
If a boiler is to remain idle for some time, its deterioration may be much more rapid than when in service. If the period for which it is to be laid off is not to exceed three months, it may be filled with water while out of service. The boiler should first be cleaned thoroughly, internally and externally, all soot and ashes being removed from the exterior of the pressure parts and any accumulation of scale removed from the interior surfaces. It should then be filled with water, to which five or six pails of soda ash have been added, a slow fire started to drive the air from the boiler, the fire drawn and the boiler pumped full. In this condition it may be kept for some time without bad effects.
If the boiler is to be out of service for more than three months, it should be emptied, drained and thoroughly dried after being cleaned. A tray of quick lime should be placed in each drum, the boiler closed, the grates covered and a quantity of quick lime placed on top of the covering. Special care should be taken to prevent air, steam or water leaks into the boiler or onto the pressure parts to obviate danger of corrosion.
BRICKWORK BOILER SETTINGS
A consideration of the losses in boiler efficiency, due to the effects of excess air, clearly indicates the necessity of maintaining the brick setting of a boiler tight and free from air leaks. In view of the temperatures to which certain portions of such a setting are subjected, the material to be used in its construction must be of the best procurable.
Boiler settings to-day consist almost universally of brickwork—two kinds being used, namely, red brick and fire brick.
The red brick should only be used in such portions of the setting as are well protected from the heat. In such location, their service is not so severe as that of fire brick and ordinarily, if such red brick are sound, hard, well burned and uniform, they will serve their purpose.
The fire brick should be selected with the greatest care, as it is this portion of the setting that has to endure the high temperatures now developed in boiler practice. To a great extent, the life of a boiler setting is dependent upon the quality of the fire brick used and the care exercised in its laying.
The best fire brick are manufactured from the fire clays of Pennsylvania. South and west from this locality the quality of fire clay becomes poorer as the distance increases, some of the southern fire clays containing a considerable percentage of iron oxide.
Until very recently, the important characteristic on which to base a judgment of the suitability of fire brick for use in connection with boiler settings has been considered the melting point, or the temperature at which the brick will liquify and run. Experience has shown, however, that this point is only important within certain limits and that the real basis on which to judge material of this description is, from the boiler man's standpoint, the quality of plasticity under a given load. This tendency of a brick to become plastic occurs at a temperature much below the melting point and to a degree that may cause the brick to become deformed under the stress to which it is subjected. The allowable plastic or softening temperature will naturally be relative and dependent upon the stress to be endured.
With the plasticity the determining factor, the perfect fire brick is one whose critical point of plasticity lies well above the working temperature of the fire. It is probable that there are but few brick on the market which would not show, if tested, this critical temperature at the stress met with in arch construction at a point less than 2400 degrees. The fact that an arch will stand for a long period under furnace temperatures considerably above this point is due entirely to the fact that its temperature as a whole is far below the furnace temperature and only about 10 per cent of its cross section nearest the fire approaches the furnace temperature. This is borne out by the fact that arches which are heated on both sides to the full temperature of an ordinary furnace will first bow down in the middle and eventually fall.
A method of testing brick for this characteristic is given in the Technologic Paper No. 7 of the Bureau of Standards dealing with "The testing of clay refractories with special reference to their load carrying capacity at furnace temperatures." Referring to the test for this specific characteristic, this publication recommends the following: "When subjected to the load test in a manner substantially as described in this bulletin, at 1350 degrees centigrade (2462 degrees Fahrenheit), and under a load of 50 pounds per square inch, a standard fire brick tested on end should show no serious deformation and should not be compressed more than one inch, referred to the standard length of nine inches."
In the Bureau of Standards test for softening temperature, or critical temperature of plasticity under the specified load, the brick are tested on end. In testing fire brick for boiler purposes such a method might be criticised, because such a test is a compression test and subject to errors from unequal bearing surfaces causing shear. Furthermore, a series of samples, presumably duplicates, will not fail in the same way, due to the mechanical variation in the manufacture of the brick. Arches that fail through plasticity show that the tensile strength of the brick is important, this being evidenced by the fact that the bottom of a wedge brick in an arch that has failed is usually found to be wider than the top and the adjacent bricks are firmly cemented together.
A better method of testing is that of testing the brick as a beam subjected to its own weight and not on end. This method has been used for years in Germany and is recommended by the highest authorities in ceramics. It takes into account the failure by tension in the brick as well as by compression and thus covers the tension element which is important in arch construction.
The plastic point under a unit stress of 100 pounds per square inch, which may be taken as the average maximum arch stress, should be above 2800 degrees to give perfect results and should be above 2400 degrees to enable the brick to be used with any degree of satisfaction.
The other characteristics by which the quality of a fire brick is to be judged are:
Fusion point. In view of the fact that the critical temperature of plasticity is below the fusion point, this is only important as an indication from high fusion point of a high temperature of plasticity.
Hardness. This is a relative quality based on an arbitrary scale of 10 and is an indication of probable cracking and spalling.
Expansion. The lineal expansion per brick in inches. This characteristic in conjunction with hardness is a measure of the physical movement of the brick as affecting a mass of brickwork, such movement resulting in cracked walls, etc. The expansion will vary between wide limits in different brick and provided such expansion is not in excess of, say, .05 inch in a 9-inch brick, when measured at 2600 degrees, it is not particularly important in a properly designed furnace, though in general the smaller the expansion the better.
Compression. The strength necessary to cause crushing of the brick at the center of the 4-1/2 inch face by a steel block one inch square. The compression should ordinarily be low, a suggested standard being that a brick show signs of crushing at 7500 pounds.
Size of Nodules. The average size of flint grains when the brick is carefully crushed. The scale of these sizes may be considered: Small, size of anthracite rice; large, size of anthracite pea.
Ratio of Nodules. The percentage of a given volume occupied by the flint grains. This scale may be considered: High, 90 to 100 per cent; medium, 50 to 90 per cent; low, 10 to 50 per cent.
The statement of characteristics suggested as desirable, are for arch purposes where the hardest service is met. For side wall purposes the compression and hardness limit may be raised considerably and the plastic point lowered.
Aside from the physical properties by which a fire brick is judged, it is sometimes customary to require a chemical analysis of the brick. Such an analysis is only necessary as determining the amount of total basic fluxes (K{2}O, Na{2}O, CaO, MgO and FeO). These fluxes are ordinarily combined into one expression, indicated by the symbol RO. This total becomes important only above 0.2 molecular equivalent as expressed in ceramic empirical formulae, and this limit should not be exceeded.[75]
From the nature of fire brick, their value can only be considered from a relative standpoint. Generally speaking, what are known as first-grade fire brick may be divided into three classes, suitable for various conditions of operation, as follows:
Class A. For stoker-fired furnaces where high overloads are to be expected or where other extreme conditions of service are apt to occur.
Class B. For ordinary stoker settings where there will be no excessive overloads required from the boiler or any hand-fired furnaces where the rates of driving will be high for such practice.
Class C. For ordinary hand-fired settings where the presumption is that the boilers will not be overloaded except at rare intervals and for short periods only.
Table 61 gives the characteristics of these three classes according to the features determining the quality. This table indicates that the hardness of the brick in general increases with the poorer qualities. Provided the hardness is sufficient to enable the brick to withstand its load, additional hardness is a detriment rather than an advantage.
TABLE 61
APPROXIMATE CLASSIFICATION OF FIRE BRICK
Characteristics Class A Class B Class C Fuse Point, Degrees Safe at Degrees Safe at Degrees Safe at Degrees Fahrenheit 3200-3300 2900-3200 2900-3000 Compression Pounds 6500-7500 7500-11,000 8500-15,000 Hardness Relative 1-2 2-4 4-6 Size of Nodules Medium Medium to Medium to Large Medium Large Ratio of Nodules High Medium to High Medium Low to Medium
An approximate determination of the quality of a fire brick may be made from the appearance of a fracture. Where such a fracture is open, clean, white and flinty, the brick in all probability is of a good quality. If this fracture has the fine uniform texture of bread, the brick is probably poor.
In considering the heavy duty of brick in boiler furnaces, experience shows that arches are the only part that ordinarily give trouble. These fail from the following causes:
Bad workmanship in laying up of brick. This feature is treated below.
The tendency of a brick to become plastic at a temperature below the fusing point. The limits of allowable plastic temperature have already been pointed out.
Spalling. This action occurs on the inner ends of combustion arches where they are swept by gases at a high velocity at the full furnace temperature. The most troublesome spalling arises through cold air striking the heated brickwork. Failure from this cause is becoming rare, due to the large increase in number of stoker installations in which rapid temperature changes are to a great degree eliminated. Furthermore, there are a number of brick on the market practically free from such defects and where a new brick is considered, it can be tried out and if the defect exists, can be readily detected and the brick discarded.
Failures of arches from the expansive power of brick are also rare, due to the fact that there are a number of brick in which the expansion is well within the allowable limits and the ease with which such defects may be determined before a brick is used.
Failures through chemical disintegration. Failure through this cause is found only occasionally in brick containing a high percentage of iron oxide.
With the grade of brick selected best suited to the service of the boiler to be set, the other factor affecting the life of the setting is the laying. It is probable that more setting difficulties arise from the improper workmanship in the laying up of brick than from poor material, and to insure a setting which will remain tight it is necessary that the masonry work be done most carefully. This is particularly true where the boiler is of such a type as to require combustion arches in the furnace.
Red brick should be laid in a thoroughly mixed mortar composed of one volume of Portland cement, 3 volumes of unslacked lime and 16 volumes of clear sharp sand. Not less than 2-1/2 bushels of lime should be used in the laying up of 1000 brick. Each brick should be thoroughly embedded and all joints filled. Where red brick and fire brick are both used in the same wall, they should be carried up at the same time and thoroughly bonded to each other.
All fire brick should be dry when used and protected from moisture until used. Each brick should be dipped in a thin fire clay wash, "rubbed and shoved" into place, and tapped with a wooden mallet until it touches the brick next below it. It must be recognized that fire clay is not a cement and that it has little or no holding power. Its action is that of a filler rather than a binder and no fire-clay wash should be used which has a consistency sufficient to permit the use of a trowel.
All fire-brick linings should be laid up four courses of headers and one stretcher. Furnace center walls should be entirely of fire brick. If the center of such walls are built of red brick, they will melt down and cause the failure of the wall as a whole.
Fire-brick arches should be constructed of selected brick which are smooth, straight and uniform. The frames on which such arches are built, called arch centers, should be constructed of batten strips not over 2 inches wide. The brick should be laid on these centers in courses, not in rings, each joint being broken with a bond equal to the length of half a brick. Each course should be first tried in place dry, and checked with a straight edge to insure a uniform thickness of joint between courses. Each brick should be dipped on one side and two edges only and tapped into place with a mallet. Wedge brick courses should be used only where necessary to keep the bottom faces of the straight brick course in even contact with the centers. When such contact cannot be exactly secured by the use of wedge brick, the straight brick should lean away from the center of the arch rather than toward it. When the arch is approximately two-thirds completed, a trial ring should be laid to determine whether the key course will fit. When some cutting is necessary to secure such a fit, it should be done on the two adjacent courses on the side of the brick away from the key. It is necessary that the keying course be a true fit from top to bottom, and after it has been dipped and driven it should not extend below the surface of the arch, but preferably should have its lower ledge one-quarter inch above this surface. After fitting, the keys should be dipped, replaced loosely, and the whole course driven uniformly into place by means of a heavy hammer and a piece of wood extending the full length of the keying course. Such a driving in of this course should raise the arch as a whole from the center. The center should be so constructed that it may be dropped free of the arch when the key course is in place and removed from the furnace without being burned out.
Care of Brickwork—Before a boiler is placed in service, it is essential that the brickwork setting be thoroughly and properly dried, or otherwise the setting will invariably crack. The best method of starting such a process is to block open the boiler damper and the ashpit doors as soon as the brickwork is completed and in this way maintain a free circulation of air through the setting. If possible, such preliminary drying should be continued for several days before any fire is placed in the furnace. When ready for the drying out fire, wood should be used at the start in a light fire which may be gradually built up as the walls become warm. After the walls have become thoroughly heated, coal may be fired and the boiler placed in service.
As already stated, the life of a boiler setting is dependent to a large extent upon the material entering into its construction and the care with which such material is laid. A third and equally important factor in the determining of such life is the care given to the maintaining of the setting in good condition after the boiler is placed in operation. This feature is discussed more fully in the chapter dealing with general boiler room management.
Steel Casings—In the chapter dealing with the losses operating against high efficiencies as indicated by the heat balance, it has been shown that a considerable portion of such losses is due to radiation and to air infiltration into the boiler setting. These losses have been variously estimated from 2 to 10 per cent, depending upon the condition of the setting and the amount of radiation surface, the latter in turn being dependent upon the size of the boiler used. In the modern efforts after the highest obtainable plant efficiencies much has been done to reduce such losses by the use of an insulated steel casing covering the brickwork. In an average size boiler unit the use of such casing, when properly installed, will reduce radiation losses from one to two per cent., over what can be accomplished with the best brick setting without such casing and, in addition, prevent the loss due to the infiltration of air, which may amount to an additional five per cent., as compared with brick settings that are not maintained in good order. Steel plate, or steel plate backed by asbestos mill-board, while acting as a preventative against the infiltration of air through the boiler setting, is not as effective from the standpoint of decreasing radiation losses as a casing properly insulated from the brick portion of the setting by magnesia block and asbestos mill-board. A casing which has been found to give excellent results in eliminating air leakage and in the reduction of radiation losses is clearly illustrated on page 306.
Many attempts have been made to use some material other than brick for boiler settings but up to the present nothing has been found that may be considered successful or which will give as satisfactory service under severe conditions as properly laid brickwork.
BOILER ROOM PIPING
In the design of a steam plant, the piping system should receive the most careful consideration. Aside from the constructive details, good practice in which is fairly well established, the important factors are the size of the piping to be employed and the methods utilized in avoiding difficulties from the presence in the system of water of condensation and the means employed toward reducing radiation losses.
Engineering opinion varies considerably on the question of material of pipes and fittings for different classes of work, and the following is offered simply as a suggestion of what constitutes good representative practice.
All pipe should be of wrought iron or soft steel. Pipe at present is made in "standard", "extra strong"[76] and "double extra strong" weights. Until recently, a fourth weight approximately 10 per cent lighter than standard and known as "Merchants" was built but the use of this pipe has largely gone out of practice. Pipe sizes, unless otherwise stated, are given in terms of nominal internal diameter. Table 62 gives the dimensions and some general data on standard and extra strong wrought-iron pipe.
TABLE 62
DIMENSIONS OF STANDARD AND EXTRA STRONG[76] WROUGHT-IRON AND STEEL PIPE
___________ Diameter Circumference _____ _____ External Internal External Internal Standard ____ Standard ____ and and Nominal Extra Standard Extra Extra Standard Extra Size Strong Strong Strong Strong __ __ __ __ __ __ __ 1/8 .405 .269 .215 1.272 .848 .675 1/4 .540 .364 .302 1.696 1.144 .949 3/8 .675 .493 .423 2.121 1.552 1.329 1/2 .840 .622 .546 2.639 1.957 1.715 3/4 1.050 .824 .742 3.299 2.589 2.331 1 1.315 1.049 .957 4.131 3.292 3.007 1-1/4 1.660 1.380 1.278 5.215 4.335 4.015 1-1/2 1.900 1.610 1.500 5.969 5.061 4.712 2 2.375 2.067 1.939 7.461 6.494 6.092 2-1/2 2.875 2.469 2.323 9.032 7.753 7.298 3 3.500 3.068 2.900 10.996 9.636 9.111 3-1/2 4.000 3.548 3.364 12.566 11.146 10.568 4 4.500 4.026 3.826 14.137 12.648 12.020 4-1/2 5.000 4.506 4.290 15.708 14.162 13.477 5 5.563 5.047 4.813 17.477 15.849 15.121 6 6.625 6.065 5.761 20.813 19.054 18.099 7 7.625 7.023 6.625 23.955 22.063 20.813 8 8.625 7.981 7.625 27.096 25.076 23.955 9 9.625 8.941 8.625 30.238 28.089 27.096 10 10.750 10.020 9.750 33.772 31.477 30.631 11 11.750 11.000 10.750 36.914 34.558 33.772 12 12.750 12.000 11.750 40.055 37.700 36.914 __ __ __ __ __ __ __
Length Internal of Nominal Weight Transverse Pipe in Pounds per Area Feet per Foot Square Foot of Nominal Standard Extra External Standard Extra Size Strong Surface Strong 1/8 .0573 .0363 9.440 .244 .314 1/4 .1041 .0716 7.075 .424 .535 3/8 .1917 .1405 5.657 .567 .738 1/2 .3048 .2341 4.547 .850 1.087 3/4 .5333 .4324 3.637 1.130 1.473 1 .8626 .7193 2.904 1.678 2.171 1-1/4 1.496 1.287 2.301 2.272 2.996 1-1/2 2.038 1.767 2.010 2.717 3.631 2 3.356 2.953 1.608 3.652 5.022 2-1/2 4.784 4.238 1.328 5.793 7.661 3 7.388 6.605 1.091 7.575 10.252 3-1/2 9.887 8.888 .955 9.109 12.505 4 12.730 11.497 .849 10.790 14.983 4-1/2 15.961 14.454 .764 12.538 17.611 5 19.990 18.194 .687 14.617 20.778 6 28.888 26.067 .577 18.974 28.573 7 38.738 34.472 .501 23.544 38.048 8 50.040 45.664 .443 28.544 43.388 9 62.776 58.426 .397 33.907 48.728 10 78.839 74.662 .355 40.483 54.735 11 95.033 90.763 .325 45.557 60.075 12 113.098 108.43 .299 49.562 65.415
Dimensions are nominal and except where noted are in inches.
In connection with pipe sizes, Table 63, giving certain tube data may be found to be of service.
TABLE 63
TUBE DATA, STANDARD OPEN HEARTH OR LAP WELDED STEEL TUBES
+ -+ + + -+ + + + + -+ -+ -+ S E D B T I D Circumference Transverse Square Length Nominal i x i . h n i Area Feet in Feet Weight z t a W i t a Square Inches of per Pounds e e m . c e m + + + + + Exter Square per r e k r e Exter- Inter- Exter- Inter- -nal Foot of Foot n t G n n t nal nal nal nal Surface Exter a e a e a e per -nal l r u s l r Foot of Surface g s Length e + -+ + + -+ + + + + -+ -+ -+ 1-1/2 10 .134 1.232 4.712 3.870 1.7671 1.1921 .392 2.546 1.955 1-1/2 9 .148 1.204 4.712 3.782 1.7671 1.1385 .392 2.546 2.137 1-1/2 8 .165 1.170 4.712 3.676 1.7671 1.0751 .392 2.546 2.353 2 10 .134 1.732 6.283 5.441 3.1416 2.3560 .523 1.909 2.670 2 9 .148 1.704 6.283 5.353 3.1416 2.2778 .523 1.909 2.927 2 8 .165 1.670 6.283 5.246 3.1416 2.1904 .523 1.909 3.234 3-1/4 11 .120 3.010 10.210 9.456 8.2958 7.1157 .850 1.175 4.011 3-1/4 10 .134 2.982 10.210 9.368 8.2958 6.9840 .850 1.175 4.459 3-1/4 9 .148 2.954 10.210 9.280 8.2958 6.8535 .850 1.175 4.903 4 10 .134 3.732 12.566 11.724 12.566 10.939 1.047 .954 5.532 4 9 .148 3.704 12.566 11.636 12.566 10.775 1.047 .954 6.000 4 8 .165 3.670 12.566 11.530 12.566 10.578 1.047 .954 6.758 + -+ + + -+ + + + + -+ -+ -+
Dimensions are nominal and except where noted are in inches.
Pipe Material and Thickness—For saturated steam pressures not exceeding 160 pounds, all pipe over 14 inches should be 3/8 inch thick O. D. pipe. All other pipe should be standard full weight, except high pressure feed[77] and blow-off lines, which should be extra strong.
For pressures above 150 pounds up to 200 pounds with superheated steam, all high pressure feed and blow-off lines, high pressure steam lines having threaded flanges, and straight runs and bends of high pressure steam lines 6 inches and under having Van Stone joints should be extra strong. All piping 7 inches and over having Van Stone joints should be full weight soft flanging pipe of special quality. Pipe 14 inches and over should be 3/8 inch thick O. D. pipe. All pipes for these pressures not specified above should be full weight pipe.
Flanges—For saturated steam, 160 pounds working pressure, all flanges for wrought-iron pipe should be cast-iron threaded. All high pressure threaded flanges should have the diameter thickness and drilling in accordance with the "manufacturer's standard" for "extra heavy" flanges. All low pressure flanges should have diameter, thickness and drilling in accordance with "manufacturer's standard" for "standard flanges."
The flanges on high pressure lines should be counterbored to receive pipe and prevent the threads from shouldering. The pipe should be screwed through the flange at least 1/16 inch, placed in machine and after facing off the end one smooth cut should be taken over the face of the flange to make it square with the axis of the pipe.
For pressures above 160 pounds, where superheated steam is used, all high pressure steam lines 4 inches and over should have solid rolled steel flanges and special upset lapped joints. In the manufacture of such joints, the ends of the pipe are heated and upset against the face of a holding mandrel conforming to the shape of the flange, the lapped portion of the pipe being flattened out against the face of the mandrel, the upsetting action maintaining the desired thickness of the lap. When cool, both sides of the lap are faced to form a uniform thickness and an even bearing against flange and gasket. The joint, therefore, is a strictly metal to metal joint, the flanges merely holding the lapped ends of the pipe against the gasket.
A special grade of soft flanging pipe is selected to prevent breaking. The bending action is a severe test of the pipe and if it withstands the bending process and the pressure tests, the reliability of the joint is assured. Such a joint is called a Van Stone joint, though many modifications and improvements have been made since the joint was originally introduced.
The diameter and thickness of such flanges should be special extra heavy. Such flanges should be turned to diameter, their fronts faced and the backs machined in lieu of spot facing.
In lines other than given for pressures over 150 pounds, all flanges for wrought-iron pipe should be threaded. All threaded flanges for high pressure superheated lines 3-1/2 inches and under should be "semi-steel" extra heavy. Flanges for other than steam lines should be manufacturer's standard extra heavy.
Welded flanges are frequently used in place of those described with satisfactory results.
Fittings—For saturated steam under pressures up to 160 pounds, all fittings 3-1/2 inches and under should be screwed. Fittings 4 inches and over should have flanged ends. Fittings for this pressure should be of cast iron and should have heavy leads and full taper threads. Flanged fittings in high pressure lines should be extra heavy, and in low pressure lines standard weight. Where possible in high pressure flanges and fittings, bolt surfaces should be spot faced to provide suitable bearing for bolt heads and nuts.
Fittings for superheated steam up to 70 degrees at pressures above 160 pounds are sometimes of cast iron.[78] For superheat above 70 degrees such fittings should be "steel castings" and in general these fittings are recommended for any degree of superheat. Fittings for other than high pressure work may be of cast iron, except where superheated steam is carried, where they should be of "wrought steel" or "hard metal". Fittings 3-1/2 inches and under should be screwed, 4 inches and over flanged.
Flanges for pressures up to 160 pounds in pipes and fittings for low pressure lines, and any fittings for high pressure lines should have plain faces, smooth tool finish, scored with V-shaped grooves for rubber gaskets. High pressure line flanges should have raised faces, projecting the full available diameter inside the bolt holes. These faces should be similarly scored.
All pipe 1/2 inch and under should have ground joint unions suitable for the pressure required. Pipe 3/4 inch and over should have cast-iron flanged unions. Unions are to be preferred to wrought-iron couplings wherever possible to facilitate dismantling.
Valves—For 150 pounds working pressure, saturated steam, all valves 2 inches and under may have screwed ends; 2-1/2 inches and over should be flanged. All high pressure steam valves 6 inches and over should have suitable by-passes. All valves for use with superheated steam should be of special construction. For pressures above 160 pounds, where the superheat does not exceed 70 degrees, valve bodies, caps and yokes are sometimes made of cast iron, though ordinarily semi-steel will give better satisfaction. The spindles of such valves should be of bronze and there should be special necks with condensing chambers to prevent the superheated steam from blowing through the packing. For pressures over 160 pounds and degrees of superheat above 70, all valves 3 inches and over should have valve bodies, caps and yokes of steel castings. Spindles should be of some non-corrosive metal, such as "monel metal". Seat rings should be removable of the same non-corrosive metal as should the spindle seats and plug faces.
All salt water valves should have bronze spindles, sleeves and packing seats.
The suggestions as to flanges for different classes of service made on page 311 hold as well for valve flanges, except that such flanges are not scored.
Automatic stop and check valves are coming into general use with boilers and such use is compulsory under the boiler regulations of certain communities. Where used, they should be preferably placed directly on the boiler nozzle. Where two or more boilers are on one line, in addition to the valve at the boiler, whether this be an automatic valve or a gate valve, there should be an additional gate valve on each boiler branch at the main steam header.
Relief valves should be furnished at the discharge side of each feed pump and on the discharge side of each feed heater of the closed type.
Feed Lines—Feed lines should in all instances be made of extra strong pipe due to the corrosive action of hot feed water. While it has been suggested above that cast-iron threaded flanges should be used in such lines, due to the sudden expansion of such pipe in certain instances cast-iron threaded flanges crack before they become thoroughly heated and expand, and for this reason cast-steel threaded flanges will give more satisfactory results. In some instances, wrought-steel and Van Stone joints have been used in feed lines and this undoubtedly is better practice than the use of cast-steel threaded work, though the additional cost is not warranted in all stations.
Feed valves should always be of the globe pattern. A gate valve cannot be closely regulated and often clatters owing to the pulsations of the feed pump.
Gaskets—For steam and water lines where the pressure does not exceed 160 pounds, wire insertion rubber gaskets 1/16 inch thick will be found to give good service. For low pressure lines, canvas insertion black rubber gaskets are ordinarily used. For oil lines special gaskets are necessary.
For pressure above 160 pounds carrying superheated steam, corrugated steel gaskets extending the full available diameter inside of the bolt holes give good satisfaction. For high pressure water lines wire inserted rubber gaskets are used, and for low pressure flanged joints canvas inserted rubber gaskets.
Size of Steam Lines—The factors affecting the proper size of steam lines are the radiation from such lines and the velocity of steam within them. As the size of the steam line increases, there will be an increase in the radiation.[79] As the size decreases, the steam velocity and the pressure drop for a given quantity of steam naturally increases.
There is a marked tendency in modern practice toward higher steam velocities, particularly in the case of superheated steam. It was formerly considered good practice to limit this velocity to 6000 feet per minute but this figure is to-day considered low.
In practice the limiting factor in the velocity advisable is the allowable pressure drop. In the description of the action of the throttling calorimeter, it has been demonstrated that there is no loss accompanying a drop in pressure, the difference in energy between the higher and lower pressures appearing as heat, which, in the case of steam flowing through a pipe, may evaporate any condensation present or may be radiated from the pipe. A decrease in pipe area decreases the radiating surface of the pipe and thus the possible condensation. As the heat liberated by the pressure drop is utilized in overcoming or diminishing the tendency toward condensation and the heat loss through radiation, the steam as it enters the prime mover will be drier or more highly superheated where high steam velocities are used than where they are lower, and if enough excess pressure is carried at the boilers to maintain the desired pressure at the prime mover, the pressure drop results in an actual saving rather than a loss. The whole is analogous to standard practice in electrical distributing systems where generator voltage is adjusted to suit the loss in the feeder lines.
In modern practice, with superheated steam, velocities of 15,000 feet per minute are not unusual and this figure is very frequently exceeded.
Piping System Design—With the proper size of pipe to be used determined, the most important factor is the provision for the removal of water of condensation that will occur in any system. Such condensation cannot be wholly overcome and if the water of condensation is carried to the prime mover, difficulties will invariably result. Water is practically incompressible and its effect when traveling at high velocities differs little from that of a solid body of equal weight, hence impact against elbows, valves or other obstructions, is the equivalent of a heavy hammer blow that may result in the fracture of the pipe. If there is not sufficient water in the system to produce this result, it will certainly cause knocking and vibration in the pipe, resulting eventually in leaky joints. Where the water reaches the prime mover, its effect will vary from disagreeable knocking to disruption. Too frequently when there are disastrous results from such a cause the boilers are blamed for delivering wet steam when, as a matter of fact, the evil is purely a result of poor piping design, the most common cause of such an action being the pocketing of the water in certain parts of the piping from whence it is carried along in slugs by the steam. The action is particularly severe if steam is admitted to a cold pipe containing water, as the water may then form a partial vacuum by condensing the steam and be projected at a very high velocity through the pipes producing a characteristic sharp metallic knock which often causes bursting of the pipe or fittings. The amount of water present through condensation may be appreciated when it is considered that uncovered 6-inch pipe 150 feet long carrying 3600 pounds of high pressure steam per hour will condense approximately 6 per cent of the total steam carried through radiation. It follows that efficient means of removing condensation water are absolutely imperative and the following suggestions as to such means may be of service:
The pitch of all pipe should be in the direction of the flow of steam. Wherever a rise is necessary, a drain should be installed. All main headers and important branches should end in a drop leg and each such drop leg and any low points in the system should be connected to the drainage pump. A similar connection should be made to every fitting where there is danger of a water pocket.
Branch lines should never be taken from the bottom of a main header but where possible should be taken from the top. Each engine supply pipe should have its own separator placed as near the throttle as possible. Such separators should be drained to the drainage system.
Check valves are frequently placed in drain pipes to prevent steam from entering any portion of the system that may be shut off.
Valves should be so located that they cannot form water pockets when either open or closed. Globe valves will form a water pocket in the piping to which they are connected unless set with the stem horizontal, while gate valves may be set with the spindle vertical or at an angle. Where valves are placed directly on the boiler nozzle, a drain should be provided above them.
High pressure drains should be trapped to both feed heaters and waste headers. Traps and meters should be provided with by-passes. Cylinder drains, heater blow-offs and drains, boiler blow-offs and similar lines should be led to waste. The ends of cylinder drains should not extend below the surface of water, for on starting up or on closing the throttle valve with the drains open, water may be drawn back into the cylinders. |
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