The advantages of the Babcock & Wilcox boiler may perhaps be most clearly set forth by a consideration, 1st, of water-tube boilers as a class as compared with shell and fire-tube boilers; and 2nd, of the Babcock & Wilcox boiler specifically as compared with other designs of water-tube boilers.
WATER-TUBE VERSUS FIRE-TUBE BOILERS
Safety—The most important requirement of a steam boiler is that it shall be safe in so far as danger from explosion is concerned. If the energy in a large shell boiler under pressure is considered, the thought of the destruction possible in the case of an explosion is appalling. The late Dr. Robert H. Thurston, Dean of Sibley College, Cornell University, and past president of the American Society of Mechanical Engineers, estimated that there is sufficient energy stored in a plain cylinder boiler under 100 pounds steam pressure to project it in case of an explosion to a height of over 3½ miles; a locomotive boiler at 125 pounds pressure from one-half to one-third of a mile; and a 60 horse-power return tubular boiler under 75 pounds pressure somewhat over a mile. To quote: “A cubic foot of heated water under a pressure of from 60 to 70 pounds per square inch has about the same energy as one pound of gunpowder.” From such a consideration, it may be readily appreciated how the advent of high pressure steam was one of the strongest factors in forcing the adoption of water-tube boilers. A consideration of the thickness of material necessary for cylinders of various diameters under a steam pressure of 200 pounds and assuming an allowable stress of 12,000 pounds per square inch, will perhaps best illustrate this point. Table 1 gives such thicknesses for various diameters of cylinders not taking into consideration the weakening effect of any joints which may be necessary. The rapidity with which the plate thickness increases with the diameter is apparent and in practice, due to the fact that riveted joints must be used, the thicknesses as given in the table, with the exception of the first, must be increased from 30 to 40 per cent.
Steam, Its Generation and Use by Babcock & Wilcox Company, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. ADVANTAGES OF THE BABCOCK & WILCOX BOILER
ADVANTAGES OF THE BABCOCK & WILCOX BOILER
The advantages of the Babcock & Wilcox boiler may perhaps be most clearly set forth by a consideration, 1st, of water-tube boilers as a class as compared with shell and fire-tube boilers; and 2nd, of the Babcock & Wilcox boiler specifically as compared with other designs of water-tube boilers.
WATER-TUBE VERSUS FIRE-TUBE BOILERS
Safety—The most important requirement of a steam boiler is that it shall be safe in so far as danger from explosion is concerned. If the energy in a large shell boiler under pressure is considered, the thought of the destruction possible in the case of an explosion is appalling. The late Dr. Robert H. Thurston, Dean of Sibley College, Cornell University, and past president of the American Society of Mechanical Engineers, estimated that there is sufficient energy stored in a plain cylinder boiler under 100 pounds steam pressure to project it in case of an explosion to a height of over 3½ miles; a locomotive boiler at 125 pounds pressure from one-half to one-third of a mile; and a 60 horse-power return tubular boiler under 75 pounds pressure somewhat over a mile. To quote: “A cubic foot of heated water under a pressure of from 60 to 70 pounds per square inch has about the same energy as one pound of gunpowder.” From such a consideration, it may be readily appreciated how the advent of high pressure steam was one of the strongest factors in forcing the adoption of water-tube boilers. A consideration of the thickness of material necessary for cylinders of various diameters under a steam pressure of 200 pounds and assuming an allowable stress of 12,000 pounds per square inch, will perhaps best illustrate this point. Table 1 gives such thicknesses for various diameters of cylinders not taking into consideration the weakening effect of any joints which may be necessary. The rapidity with which the plate thickness increases with the diameter is apparent and in practice, due to the fact that riveted joints must be used, the thicknesses as given in the table, with the exception of the first, must be increased from 30 to 40 per cent.
In a water-tube boiler the drums seldom exceed 48 inches in diameter and the thickness of plate required, therefore, is never excessive. The thinner metal can be rolled to a more uniform quality, the seams admit of better proportioning, and the joints can be more easily and perfectly fitted than is the case where thicker plates are necessary. All of these points contribute toward making the drums of water-tube boilers better able to withstand the stress which they will be called upon to endure.
The essential constructive difference between water-tube and fire-tube boilers lies in the fact that the former is composed of parts of relatively small diameter as against the large diameters necessary in the latter.
The factor of safety of the boiler parts which come in contact with the most intense heat in water-tube boilers can be made much higher than would be practicable in a shell boiler. Under the assumptions considered above in connection with the thickness of plates required, a number 10 gauge tube (0.134 inch), which is standard in Babcock & Wilcox boilers for pressures up to 210 pounds under the same allowable stress as was used in computing Table 1, the safe working pressure for the tubes is 870 pounds per square inch, indicating the very large margin of safety of such tubes as compared with that possible with the shell of a boiler.
A further advantage in the water-tube boiler as a class is the elimination of all compressive stresses. Cylinders subjected to external pressures, such as fire tubes or the internally fired furnaces of certain types of boilers, will collapse under a pressure much lower than that which they could withstand if it were applied internally. This is due to the fact that if there exists any initial distortion from its true shape, the external pressure will tend to increase such distortion and collapse the cylinder, while an internal pressure tends to restore the cylinder to its original shape.
Stresses due to unequal expansion have been a fruitful source of trouble in fire-tube boilers.
In boilers of the shell type, the riveted joints of the shell, with their consequent double thickness of metal exposed to the fire, gives rise to serious difficulties. Upon these points are concentrated all strains of unequal expansion, giving rise to frequent leaks and oftentimes to actual ruptures. Moreover, in the case of such rupture, the whole body of contained water is liberated instantaneously and a disastrous and usually fatal explosion results.
Further, unequal strains result in shell or fire-tube boilers due to the difference in temperature of the various parts. This difference in temperature results from the lack of positive well defined circulation. While such a circulation does not necessarily accompany all water-tube designs, in general, the circulation in water-tube boilers is much more defined than in fire-tube or shell boilers.
A positive and efficient circulation assures that all portions of the pressure parts will be at approximately the same temperature and in this way strains resulting from unequal temperatures are obviated.
If a shell or fire-tubular boiler explodes, the apparatus as a whole is destroyed. In the case of water-tube boilers, the drums are ordinarily so located that they are protected from intense heat and any rupture is usually in the case of a tube. Tube failures, resulting from blisters or burning, are not serious in their nature. Where a tube ruptures because of a flaw in the metal, the result may be more severe, but there cannot be the disastrous explosion such as would occur in the case of the explosion of a shell boiler.
To quote Dr. Thurston, relative to the greater safety of the water-tube boiler: “The stored available energy is usually less than that of any of the other stationary boilers and not very far from the amount stored, pound for pound, in the plain tubular boiler. It is evident that their admitted safety from destructive explosion does not come from this relation, however, but from the division of the contents into small portions and especially from those details of construction which make it tolerably certain that any rupture shall be local. A violent explosion can only come from the general disruption of a boiler and the liberation at once of large masses of steam and water.”
Economy—The requirement probably next in importance to safety in a steam boiler is economy in the use of fuel. To fulfill such a requirement, the three items, of proper grate for the class of fuel to be burned, a combustion chamber permitting complete combustion of gases before their escape to the stack, and the heating surface of such a character and arrangement that the maximum amount of available heat may be extracted, must be co-ordinated.
Fire-tube boilers from the nature of their design do not permit the variety of combinations of grate surface, heating surface, and combustion space possible in practically any water-tube boiler.
In securing the best results in fuel economy, the draft area in a boiler is an important consideration. In fire-tube boilers this area is limited to the cross sectional area of the fire tubes, a condition further aggravated in a horizontal boiler by the tendency of the hot gases to pass through the upper rows of tubes instead of through all of the tubes alike. In water-tube boilers the draft area is that of the space outside of the tubes and is hence much greater than the cross sectional area of the tubes.
Capacity—Due to the generally more efficient circulation found in water-tube than in fire-tube boilers, rates of evaporation are possible with water-tube boilers that cannot be approached where fire-tube boilers are employed.
Quick Steaming—Another important result of the better circulation ordinarily found in water-tube boilers is in their ability to raise steam rapidly in starting and to meet the sudden demands that may be thrown on them.
In a properly designed water-tube boiler steam may be raised from a cold boiler to 200 pounds pressure in less than one-half hour.
For the sake of comparison with the figure above, it may be stated that in the U. S. Government Service the shortest time allowed for getting up steam in Scotch marine boilers is 6 hours and the time ordinarily allowed is 12 hours. In large double-ended Scotch boilers, such as are generally used in Trans-Atlantic service, the fires are usually started 24 hours before the time set for getting under way. This length of time is necessary for such boilers in order to eliminate as far as possible excessive strains resulting from the sudden application of heat to the surfaces.
Accessibility—In the “Requirements of a Perfect Steam Boiler”, as stated by Mr. Babcock, he demonstrates the necessity for complete accessibility to all portions of the boiler for cleaning, inspection and repair.
Cleaning—When the great difference is realized in performance, both as to economy and capacity of a clean boiler and one in which the heating surfaces have been allowed to become fouled, it may be appreciated that the ability to keep heating surfaces clean internally and externally is a factor of the highest importance.
Such results can be accomplished only by the use of a design in boiler construction which gives complete accessibility to all portions. In fire-tube boilers the tubes are frequently nested together with a space between them often less than 1¼ inches and, as a consequence, nearly the entire tube surface is inaccessible. When scale forms upon such tubes it is impossible to remove it completely from the inside of the boiler and if it is removed by a turbine hammer, there is no way of knowing how thorough a job has been done. With the formation of such scale there is danger through overheating and frequent tube renewals are necessary.
In Scotch marine boilers, even with the engines operating condensing, complete tube renewals at intervals of six or seven years are required, while large replacements are often necessary in less than one year. In return tubular boilers operated with bad feed water, complete tube renewals annually are not uncommon. In this type of boiler much sediment falls on the bottom sheets where the intense heat to which they are subjected bakes it to such an excessive hardness that the only method of removing it is to chisel it out. This can be done only by omitting tubes enough to leave a space into which a man can crawl and the discomforts under which he must work are apparent. Unless such a deposit is removed, a burned and buckled plate will invariably result, and if neglected too long an explosion will follow.
In vertical fire-tube boilers using a water leg construction, a deposit of mud in such legs is an active agent in causing corrosion and the difficulty of removing such deposit through handholes is well known. A complete removal is practically impossible and as a last resort to obviate corrosion in certain designs, the bottom of the water legs in some cases have been made of copper. A thick layer of mud and scale is also liable to accumulate on the crown sheet of such boilers and may cause the sheet to crack and lead to an explosion.
The soot and fine coal swept along with the gases by the draft will settle in fire tubes and unless removed promptly, must be cut out with a special form of scraper. It is not unusual where soft coal is used to find tubes half filled with soot, which renders useless a large portion of the heating surface and so restricts the draft as to make it difficult to burn sufficient coal to develop the required power from such heating surface as is not covered by soot.
Water-tube boilers in general are from the nature of their design more readily accessible for cleaning than are fire-tube boilers.
Inspection—The objections given above in the consideration of the inability to properly clean fire-tube boilers hold as well for the inspection of such boilers.
Repairs—The lack of accessibility in fire-tube boilers further leads to difficulties where repairs are required.
In fire-tube boilers tube renewals are a serious undertaking. The accumulation of hard deposit on the exterior of the surfaces so enlarges the tubes that it is oftentimes difficult, if not impossible, to draw them through the tube sheets and it is usually necessary to cut out such tubes as will allow access to the one which has failed and remove them through the manhole.
When a tube sheet blisters, the defective part must be cut out by hand-tapped holes drilled by ratchets and as it is frequently impossible to get space in which to drive rivets, a “soft patch” is necessary. This is but a makeshift at best and usually results in either a reduction of the safe working pressure or in the necessity for a new plate. If the latter course is followed, the old plate must be cut out, a new one scribed to place to locate rivet holes and in order to obtain room for driving rivets, the boiler will have to be re-tubed.
The setting must, of course, be at least partially torn out and replaced.
In case of repairs, of such nature in fire-tube boilers, the working pressure of such repaired boilers will frequently be lowered by the insurance companies when the boiler is again placed in service.
In the case of a rupture in a water-tube boiler, the loss will ordinarily be limited to one or two tubes which can be readily replaced. The fire-tube boiler will be so completely demolished that the question of repairs will be shifted from the boiler to the surrounding property, the damage to which will usually exceed many times the cost of a boiler of a type which would have eliminated the possibility of a disastrous explosion. In considering the proper repair cost of the two types of boilers, the fact should not be overlooked that it is poor economy to invest large sums in equipment that, through a possible accident to the boiler may be wholly destroyed or so damaged that the cost of repairs, together with the loss of time while such repairs are being made, would purchase boilers of absolute safety and leave a large margin beside. The possibility of loss of human life should also be considered, though this may seem a far cry from the question of repair costs.
Space Occupied—The space required for the boilers in a plant often exceeds the requirements for the remainder of the plant equipment. Any saving of space in a boiler room will be a large factor in reducing the cost of real estate and of the building. Even when the boiler plant is comparatively small, the saving in space frequently will amount to a considerable percentage of the cost of the boilers. shows the difference in floor space occupied by fire-tube boilers and Babcock & Wilcox boilers of the same capacity, the latter being taken as representing the water-tube class. This saving in space will increase with the size of the plant for the reason that large size boiler units while common in water-tube practice are impracticable in fire-tube practice.
BABCOCK & WILCOX BOILERS AS COMPARED WITH OTHER WATER-TUBE DESIGNS
It must be borne in mind that the simple fact that a boiler is of the water-tube design does not as a necessity indicate that it is a good or safe boiler.
Safety—Many of the water-tube boilers on the market are as lacking as are fire-tube boilers in the positive circulation which, as has been demonstrated by Mr. Babcock’s lecture, is so necessary in the requirements of the perfect steam boiler. In boilers using water-leg construction, there is danger of defective circulation, leaks are common, and unsuspected corrosion may be going on in portions of the boiler that cannot be inspected. Stresses due to unequal expansion of the metal cannot be well avoided but they may be minimized by maintaining at the same temperature all pressure parts of the boiler. The result is to be secured only by means of a well defined circulation.
The main feature to which the Babcock & Wilcox boiler owes its safety is the construction made possible by the use of headers, by which the water in each vertical row of tubes is separated from that in the adjacent rows. This construction results in the very efficient circulation produced through the breaking up of the steam and water in the front headers, the effect of these headers in producing such a positive circulation having been clearly demonstrated in Mr. Babcock’s lecture. The use of a number of sections, thus composed of headers and tubes, has a distinct advantage over the use of a common chamber at the outlet ends of the tubes. In the former case the circulation of water in one vertical row of tubes cannot interfere with that in the other rows, while in the latter construction there will be downward as well as upward currents and such downward currents tend to neutralize any good effect there might be through the diminution of the density of the water column by the steam.
Further, the circulation results directly from the design of the boiler and requires no assistance from “retarders”, check valves and the like, within the boiler. All such mechanical devices in the interior of a boiler serve only to complicate the design and should not be used.
This positive and efficient circulation assures that all portions of the pressure parts of the Babcock & Wilcox boiler will be at approximately the same temperature and in this way strains resulting from unequal temperatures are obviated.
Where the water throughout the boiler is at the temperature of the steam contained, a condition to be secured only by proper circulation, danger from internal pitting is minimized, or at least limited only to effects of the water fed the boiler. Where the water in any portion of the boiler is lower than the temperature of the steam corresponding to the pressure carried, whether the fact that such lower temperatures exist as a result of lack of circulation, or because of intentional design, internal pitting or corrosion will almost invariably result.
Dr. Thurston has already been quoted to the effect that the admitted safety of a water-tube boiler is the result of the division of its contents into small portions. In boilers using a water-leg construction, while the danger from explosion will be largely limited to the tubes, there is the danger, however, that such legs may explode due to the deterioration of their stays, and such an explosion might be almost as disastrous as that of a shell boiler. The headers in a Babcock & Wilcox boiler are practically free from any danger of explosion. Were such an explosion to occur, it would still be localized to a much larger extent than in the case of a water-leg boiler and the header construction thus almost absolutely localizes any danger from such a cause.
Staybolts are admittedly an undesirable element of construction in any boiler. They are wholly objectionable and the only reason for the presence of staybolts in a boiler is to enable a cheaper form of construction to be used than if they were eliminated.
In boilers utilizing in their design flat-stayed surfaces, or staybolt construction under pressure, corrosion and wear and tear in service tends to weaken some single part subject to continual strain, the result being an increased strain on other parts greatly in excess of that for which an allowance can be made by any reasonable factor of safety. Where the construction is such that the weakening of a single part will produce a marked decrease in the safety and reliability of the whole, it follows of necessity, that there will be a corresponding decrease in the working pressure which may be safely carried.
In water-leg boilers, the use of such flat-stayed surfaces under pressure presents difficulties that are practically unsurmountable. Such surfaces exposed to the heat of the fire are subject to unequal expansion, distortion, leakage and corrosion, or in general, to many of the objections that have already been advanced against the fire-tube boilers in the consideration of water-tube boilers as a class in comparison with fire-tube boilers.
Aside from the difficulties that may arise in actual service due to the failure of staybolts, or in general, due to the use of flat-stayed surfaces, constructional features are encountered in the actual manufacture of such boilers that make it difficult if not impossible to produce a first-class mechanical job. It is practically impossible in the building of such a boiler to so design and place the staybolts that all will be under equal strain. Such unequal strains, resulting from constructional difficulties, will be greatly multiplied when such a boiler is placed in service. Much of the riveting in boilers of this design must of necessity be hand work, which is never the equal of machine riveting. The use of water-leg construction ordinarily requires the flanging of large plates, which is difficult, and because of the number of heats necessary and the continual working of the material, may lead to the weakening of such plates.
In vertical or semi-vertical water-tube boilers utilizing flat-stayed surfaces under pressure, these surfaces are ordinarily so located as to offer a convenient lodging place for flue dust, which fuses into a hard mass, is difficult of removal and under which corrosion may be going on with no possibility of detection.
Where stayed surfaces or water legs are features in the design of a water-tube boiler, the factor of safety of such parts must be most carefully considered. In such parts too, is the determination of the factor most difficult, and because of the “rule-of-thumb” determination frequently necessary, the factor of safety becomes in reality a factor of ignorance. As opposed to such indeterminate factors of safety, in the Babcock & Wilcox boiler, when the factor of safety for the drum or drums has been determined, and such a factor may be determined accurately, the factors for all other portions of the pressure parts are greatly in excess of that of the drum. All Babcock & Wilcox boilers are built with a factor of safety of at least five, and inasmuch as the factor of the safety of the tubes and headers is greatly in excess of this figure, it applies specifically to the drum or drums. This factor represents a greater degree of safety than a considerably higher factor applied to a boiler in which the shell or any riveted portion is acted upon directly by the fire, or the same factor applied to a boiler utilizing flat-stayed surface construction, where the accurate determination of the limiting factor of safety is difficult, if not impossible.
That the factor of safety of stayed surfaces is questionable may perhaps be best realized from a consideration of the severe requirements as to such factor called for by the rules and regulations of the Board of Supervising Inspectors, U. S. Government.
In view of the above, the absence of any stayed surfaces in the Babcock & Wilcox boiler is obviously a distinguishing advantage where safety is a factor. It is of interest to note, in the article on the evolution of the Babcock & Wilcox boiler, that staybolt construction was used in several designs, found unsatisfactory and unsafe, and discarded.
Another feature in the design of the Babcock & Wilcox boiler tending toward added safety is its manner of suspension. This has been indicated in the previous chapter and is of such nature that all of the pressure parts are free to expand or contract under variations of temperature without in any way interfering with any part of the boiler setting. The sectional nature of the boiler allows a flexibility under varying temperature changes that practically obviates internal strain.
In boilers utilizing water-leg construction, on the other hand, the construction is rigid, giving rise to serious internal strains and the method of support ordinarily made necessary by the boiler design is not only unmechanical but frequently dangerous, due to the fact that proper provision is not made for expansion and contraction under temperature variations.
Boilers utilizing water-leg construction are not ordinarily provided with mud drums. This is a serious defect in that it allows impurities and sediment to collect in a portion of the boiler not easily inspected, and corrosion may result.
Economy—That the water-tube boiler as a class lends itself more readily than does the fire-tube boiler to a variation in the relation of grate surface, heating surface and combustion space has been already pointed out. In economy again, the construction made possible by the use of headers in Babcock & Wilcox boilers appears as a distinct advantage. Because of this construction, there is a flexibility possible, in an unlimited variety of heights and widths that will satisfactorily meet the special requirements of the fuel to be burned in individual cases.
An extended experience in the design of furnaces best suited for a wide variety of fuels has made The Babcock & Wilcox Co. leaders in the field of economy. Furnaces have been built and are in successful operation for burning anthracite and bituminous coals, lignite, crude oil, gas-house tar, wood, sawdust and shavings, bagasse, tan bark, natural gas, blast furnace gas, by-product coke oven gas and for the utilization of waste heat from commercial processes. The great number of Babcock & Wilcox boilers now in satisfactory operation under such a wide range of fuel conditions constitutes an unimpeachable testimonial to the ability to meet all of the many conditions of service.
The limitations in the draft area of fire-tube boilers as affecting economy have been pointed out. That a greater draft area is possible in water-tube boilers does not of necessity indicate that proper advantage of this fact is taken in all boilers of the water-tube class. In the Babcock & Wilcox boiler, the large draft area taken in connection with the effective baffling allows the gases to be brought into intimate contact with all portions of the heating surfaces and renders such surfaces highly efficient.
In certain designs of water-tube boilers the baffling is such as to render ineffective certain portions of the heating surface, due to the tendency of soot and dirt to collect on or behind baffles, in this way causing the interposition of a layer of non-conducting material between the hot gases and the heating surfaces.
In Babcock & Wilcox boilers the standard baffle arrangement is such as to allow the installation of a superheater without in any way altering the path of the gases from furnace to stack, or requiring a change in the boiler design. In certain water-tube boilers the baffle arrangement is such that if a superheater is to be installed a complete change in the ordinary baffle design is necessary. Frequently to insure sufficiently hot gas striking the heating surfaces, a portion is by-passed directly from the furnace to the superheater chamber without passing over any of the boiler heating surfaces. Any such arrangement will lead to a decrease in economy and the use of boilers requiring it should be avoided.
Capacity—Babcock & Wilcox boilers are run successfully in every-day practice at higher ratings than any other boilers in practical service. The capacities thus obtainable are due directly to the efficient circulation already pointed out. Inasmuch as the construction utilizing headers has a direct bearing in producing such circulation, it is also connected with the high capacities obtainable with this apparatus.
Where intelligently handled and kept properly cleaned, Babcock & Wilcox boilers are operated in many plants at from 200 to 225 per cent of their rated evaporative capacity and it is not unusual for them to be operated at 300 per cent of such rated capacity during periods of peak load.
Dry Steam—In the list of the requirements of the perfect steam boiler, the necessity that dry steam be generated has been pointed out. The Babcock & Wilcox boiler will deliver dry steam under higher capacities and poorer conditions of feed water than any other boiler now manufactured. Certain boilers will, when operated at ordinary ratings, handle poor feed water and deliver steam in which the moisture content is not objectionable. When these same boilers are driven at high overloads, there will be a direct tendency to prime and the percentage of moisture in the steam delivered will be high. This tendency is the result of the lack of proper circulation and once more there is seen the advantage of the headers of the Babcock & Wilcox boiler, resulting as it does in the securing of a positive circulation.
In the design of the Babcock & Wilcox boiler sufficient space is provided between the steam outlet and the disengaging point to insure the steam passing from the boiler in a dry state without entraining or again picking up any particles of water in its passage even at high rates of evaporation. Ample time is given for a complete separation of steam from the water at the disengaging surface before the steam is carried from the boiler. These two features, which are additional causes for the ability of the Babcock & Wilcox boiler to deliver dry steam, result from the proper proportioning of the steam and water space of the boiler. From the history of the development of the boiler, it is evident that the cubical capacity per horse power of the steam and water space has been adopted after numerous experiments.
That the “dry pipe” serves in no way the generally understood function of such device has been pointed out. As stated, the function of the “dry pipe” in a Babcock & Wilcox boiler is simply that of a collecting pipe and this statement holds true regardless of the rate of operation of the boiler.
In certain boilers, “superheating surface” is provided to “dry the steam,” or to remove the moisture due to priming or foaming. Such surface is invariably a source of trouble unless the steam is initially dry and a boiler which will deliver dry steam is obviously to be preferred to one in which surface must be supplied especially for such purpose. Where superheaters are installed with Babcock & Wilcox boilers, they are in every sense of the word superheaters and not driers, the steam being delivered to them in a dry state.
The question has been raised in connection with the cross drum design of the Babcock & Wilcox boiler as to its ability to deliver dry steam. Experience has shown the absolute lack of basis for any such objection. The Babcock & Wilcox Company at its Bayonne Works some time ago made a series of experiments to see in what manner the steam generated was separated from the water either in the drum or in its passage to the drum. Glass peepholes were installed in each end of a drum in a boiler of the marine design, at the point midway between that at which the horizontal circulating tubes entered the drum and the drum baffle plate. By holding a light at one of these peepholes the action in the drum was clearly seen through the other. It was found that with the boiler operated under three-quarter inch ashpit pressure, which, with the fuel used would be equivalent to approximately 185 per cent of rating for stationary boiler practice, that each tube was delivering with great velocity a stream of solid water, which filled the tube for half its cross sectional area. There was no spray or mist accompanying such delivery, clearly indicating that the steam had entirely separated from the water in its passage through the horizontal circulating tubes, which in the boiler in question were but 50 inches long.
Northwest Station of the Commonwealth Edison Co., Chicago, Ill. This Installation Consists of 11,360 Horse Power of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers
These experiments proved conclusively that the size of the steam drums in the cross drum design has no appreciable effect in determining the amount of liberating surface, and that sufficient liberating surface is provided in the circulating tubes alone. If further proof of the ability of this design of boiler to deliver dry steam is required, such proof is perhaps best seen in the continued use of the Babcock & Wilcox marine boiler, in which the cross drum is used exclusively, and with which rates of evaporation are obtained far in excess of those secured in ordinary practice.
Quick Steaming—The advantages of water-tube boilers as a class over fire-tube boilers in ability to raise steam quickly have been indicated.
Due to the constant and thorough circulation resulting from the sectional nature of the Babcock & Wilcox boiler, steam may be raised more rapidly than in practically any other water-tube design.
In starting up a cold Babcock & Wilcox boiler with either coal or oil fuel, where a proper furnace arrangement is supplied, steam may be raised to a pressure of 200 pounds in less than half an hour. With a Babcock & Wilcox boiler in a test where forced draft was available, steam was raised from an initial temperature of the boiler and its contained water of 72 degrees to a pressure of 200 pounds, in 12½ minutes after lighting the fire. The boiler also responds quickly in starting from banked fires, especially where forced draft is available.
In Babcock & Wilcox boilers the water is divided into many small streams which circulate without undue frictional resistance in thin envelopes passing through the hottest part of the furnace, the steam being carried rapidly to the disengaging surface. There is no part of the boiler exposed to the heat of the fire that is not in contact with water internally, and as a result there is no danger of overheating on starting up quickly nor can leaks occur from unequal expansion such as might be the case where an attempt is made to raise steam rapidly in boilers using water leg construction.
Storage Capacity for Steam and Water—Where sufficient steam and water capacity are not provided in a boiler, its action will be irregular, the steam pressure varying over wide limits and the water level being subject to frequent and rapid fluctuation.
Owing to the small relative weight of steam, water capacity is of greater importance in this respect than steam space. With a gauge pressure of 180 pounds per square inch, 8 cubic feet of steam, which is equivalent to one-half cubic foot of water space, are required to supply one boiler horse power for one minute and if no heat be supplied to the boiler during such an interval, the pressure will drop to 150 pounds per square inch. The volume of steam space, therefore, may be over rated, but if this be too small, the steam passing off will carry water with it in the form of spray. Too great a water space results in slow steaming and waste of fuel in starting up; while too much steam space adds to the radiating surface and increases the losses from that cause.
That the steam and water space of the Babcock & Wilcox boiler are the result of numerous experiments has previously been pointed out.
Accessibility—Cleaning. That water-tube boilers are more accessible as a class than are fire-tube boilers has been indicated. All water-tube boilers, however, are not equally accessible. In certain designs, due to the arrangement of baffling used it is practically impossible to remove all deposits of soot and dirt. Frequently, in order to cheapen the product, sufficient cleaning and access doors are not supplied as part of the boiler equipment. The tendency of soot to collect on the crown sheets of certain vertical water-tube boilers has been noted. Such deposits are difficult to remove and if corrosion goes on beneath such a covering the sheet may crack and an explosion result.
It is almost impossible to thoroughly clean water legs internally, and in such places also is there a tendency to unsuspected corrosion under deposits that cannot be removed.
In Babcock & Wilcox boilers every portion of the interior of the heating surfaces can be reached and kept clean, while any soot deposited on the exterior surfaces can be blown off while the boiler is under pressure.
Inspection—The accessibility which makes possible the thorough cleaning of all portions of the Babcock & Wilcox boiler also provides a means for a thorough inspection.
Drums are accessible for internal inspection by the removal of the manhole plates. Front headers may be inspected through large doors furnished for the purpose. Rear headers in the inclined header designs may be inspected from the chamber formed by such headers and the rear wall of the boiler. In the vertical header designs rear tube doors are furnished, as has been stated. In certain designs of water-tube boilers in order to assure accessibility for inspection of the rear ends of the tubes, the rear portion of the boiler is exposed to the atmosphere with resulting excessive radiation losses. In other designs the means of access to the rear ends of the tubes are of a makeshift and unworkmanlike character.
By the removal of handhole plates, all tubes in a Babcock & Wilcox boiler may be inspected for their full length either for the presence of scale or for suspected corrosion.
Repairs—In Babcock & Wilcox boilers the possession of great strength, the elimination of stresses due to uneven temperatures and of the resulting danger of leaks and corrosion, the protection of the drums from the intense heat of the fire, and the decreased liability of the scale forming matter to lodge on the hottest tube surfaces, all tend to minimize the necessity for repairs. The tubes of the Babcock & Wilcox boiler are practically the only part which may need renewal and these only at infrequent intervals When necessary, such renewals may be made cheaply and quickly. A small stock of tubes, 4 inches in diameter, of sufficient length for the boiler used, is all that need be carried to make renewals.
Repairs in water-leg boilers are difficult at best and frequently unsatisfactory when completed. When staybolt replacements are necessary, in order to get at the inner sheet of the water leg, several tubes must in some cases be cut out. Not infrequently a replacement of an entire water leg is necessary and this is difficult and requires a lengthy shutdown. With the Babcock & Wilcox boiler, on the other hand, even if it is necessary to replace a section, this may be done in a few hours after the boiler is cool.
In the case of certain staybolt failures the working pressure of a repaired boiler utilizing such construction will frequently be lowered by the insurance companies when the boiler is again placed in service. The sectional nature of the Babcock & Wilcox boiler enables it to maintain its original working pressure over long periods of time, almost regardless of the nature of any repair that may be required.
Durability—Babcock & Wilcox boilers are being operated in every-day service with entirely satisfactory results and under the same steam pressure as that for which they were originally sold that have been operated from thirty to thirty-five years. It is interesting to note in considering the life of a boiler that the length of life of a Babcock & Wilcox boiler must be taken as the criterion of what length of life is possible. This is due to the fact that there are Babcock & Wilcox boilers in operation to-day that have been in service from a time that antedates by a considerable margin that at which the manufacturer of any other water-tube boiler now on the market was started.
Probably the very best evidence of the value of the Babcock & Wilcox boiler as a steam generator and of the reliability of the apparatus, is seen in the sales of the company. Since the company was formed, there have been sold throughout the world over 9,900,000 horse power.
A feature that cannot be overlooked in the consideration of the advantages of the Babcock & Wilcox boiler is the fact that as a part of the organization back of the boiler, there is a body of engineers of recognized ability, ready at all times to assist its customers in every possible way.
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This book is part of the public domain. Babcock & Wilcox Company (2007). Steam, Its Generation and Use. Urbana, Illinois: Project Gutenberg. Retrieved
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