Steam, Its Generation and Use Part 7

Further, the circulation results directly from the design of the boiler and requires no a.s.sistance from "r.e.t.a.r.ders", 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 a.s.sures that all portions of the pressure parts of the Babc.o.c.k & Wilc.o.x 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 sh.e.l.l boiler. The headers in a Babc.o.c.k & Wilc.o.x 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 cla.s.s in comparison with fire-tube boilers.

[Ill.u.s.tration: McAlpin Hotel, New York City, Operating 2360 Horse Power of Babc.o.c.k & Wilc.o.x 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-cla.s.s 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 ma.s.s, 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 sh.e.l.l 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 Babc.o.c.k & Wilc.o.x boiler is obviously a distinguis.h.i.+ng advantage where safety is a factor. It is of interest to note, in the article on the evolution of the Babc.o.c.k & Wilc.o.x boiler, that staybolt construction was used in several designs, found unsatisfactory and unsafe, and discarded.

Another feature in the design of the Babc.o.c.k & Wilc.o.x 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 cla.s.s lends itself more readily than does the fire-tube boiler to a variation in the relation of grate surface, heating surface and combustion s.p.a.ce has been already pointed out. In economy again, the construction made possible by the use of headers in Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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, baga.s.se, tan bark, natural gas, blast furnace gas, by-product c.o.ke oven gas and for the utilization of waste heat from commercial processes. The great number of Babc.o.c.k & Wilc.o.x boilers now in satisfactory operation under such a wide range of fuel conditions const.i.tutes 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 cla.s.s. In the Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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-pa.s.sed directly from the furnace to the superheater chamber without pa.s.sing 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--Babc.o.c.k & Wilc.o.x 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, Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x boiler, resulting as it does in the securing of a positive circulation.

In the design of the Babc.o.c.k & Wilc.o.x boiler sufficient s.p.a.ce is provided between the steam outlet and the disengaging point to insure the steam pa.s.sing from the boiler in a dry state without entraining or again picking up any particles of water in its pa.s.sage 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 Babc.o.c.k & Wilc.o.x boiler to deliver dry steam, result from the proper proportioning of the steam and water s.p.a.ce 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 s.p.a.ce 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x boiler as to its ability to deliver dry steam.

Experience has shown the absolute lack of basis for any such objection.

The Babc.o.c.k & Wilc.o.x 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 pa.s.sage to the drum. Gla.s.s 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 pa.s.sage through the horizontal circulating tubes, which in the boiler in question were but 50 inches long.

[Ill.u.s.tration: Northwest Station of the Commonwealth Edison Co., Chicago, Ill. This Installation Consists of 11,360 Horse Power of Babc.o.c.k & Wilc.o.x Boilers and Superheaters, Equipped with Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 cla.s.s 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 Babc.o.c.k & Wilc.o.x boiler, steam may be raised more rapidly than in practically any other water-tube design.

In starting up a cold Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 12 minutes after lighting the fire. The boiler also responds quickly in starting from banked fires, especially where forced draft is available.

In Babc.o.c.k & Wilc.o.x boilers the water is divided into many small streams which circulate without undue frictional resistance in thin envelopes pa.s.sing 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 s.p.a.ce. 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 s.p.a.ce, 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 s.p.a.ce, therefore, may be over rated, but if this be too small, the steam pa.s.sing off will carry water with it in the form of spray. Too great a water s.p.a.ce results in slow steaming and waste of fuel in starting up; while too much steam s.p.a.ce adds to the radiating surface and increases the losses from that cause.

That the steam and water s.p.a.ce of the Babc.o.c.k & Wilc.o.x boiler are the result of numerous experiments has previously been pointed out.

Accessibility--Cleaning. That water-tube boilers are more accessible as a cla.s.s 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.

[Ill.u.s.tration: Rear View--Longitudinal Drum Vertical Header Boiler, Showing Access Doors to Rear Headers]

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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 a.s.sure 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 makes.h.i.+ft and unworkmanlike character.

By the removal of handhole plates, all tubes in a Babc.o.c.k & Wilc.o.x boiler may be inspected for their full length either for the presence of scale or for suspected corrosion.

Repairs--In Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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 Babc.o.c.k & Wilc.o.x 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.