US2412886A - Boiler construction and method of producing same - Google Patents

Description

Patented Dec. 17, 1946 BOILER CONSTRUCTION AND METHOD Oh PRODUCING SAME Frederick P; Huston, Fanwood, and Lawton A.

Burrows, Woodbury, N. 3., and Walter E. Lawson, Wilmington, DeL; said Huston assignor to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware; said Burrows and Lawson assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application February 2, 1943, Serial No. 474,480 In Canada December 16, 1 942 pecially of the type having fire sheets and wrapper sheets and to staybolt connections between the fire sheets and wrapper sheets of the fire boxes of such boilers.

The conventional practice for fitting staybolts between the fire sheets and wrapper sheets of boiler fire boxes may be described as the screwedthrough ends-riveted type of construction. In fitting rigid staybolts in this type of construction the fire sheets and wrapper sheets were provided with aligned threaded openings through which the staybolt was screwed so that the ends extended beyond the outer faces of the fire sheet and wrapper sheet a distance at least equal to two threads on the bolt. Flexible staybolts extendedthrough the fire sheet in a similar manner. These extending ends were then upset so as to form a spread rounded head. Considerable difficulty has beenexperienced with this type of construction due to leakage of steam and/or water between the male and female threads where the bolt passes through the sheet. This dimculty has been particularly acute on the fire side of the fire box.

Dimculty was experienced not only in obtaining a tight fit between the staybolts and the fire sheets but also, particularly with staybolts of the rigid tym, it had been found that numerous bolts broke in service. It has been almost universal practice to construct fire boxes with wrapper sheets of heavier gauge than the fire sheets and it was found that the stay bolts which broke in service almost always fractured substantially in the plane of the inner surface of the wrapper sheet.

We have invented a method of expanding the ends of staybolts in the sheets or plates of heat exchange apparatus which provides fluid tight to provide an improved boiler construction in which the fire sheets and wrapper sheets are of substantially the same gauge.

A further object of the present invention is the 2 Claims. (Cl. 122-493) provision of staybolts in which concentration 01' stresses in the plane oi the fire sheet and/or wrapper sheet due to the notch efiect of the threads is largely prevented. 5 The present invention also contemplates the provision of an improved construction for heat exchange apparatus generally and especially for use inthe chemical field for catalysis and the like.

Other objects and advantages of the present invention will become apparent from the following detailed description of the invention taken in conjunction with the drawings in which:

7 Fig. 1 shows a fragmentary sectional. view of a locomotive boiler fire box; 7

Fig. 2 is an enlarged fragmentary sectional view of one end of a staybolt provided with a charged explosive capsule, said staybolt being screwed into a tapped hole of a fire sheet;

Fig. 3 depicts a similar fragmentary sectional view of a reduced, body staybolt screwed into a tapped hole in the fire sheet, the bolt having a charge of explosive within a bore provided in this end;

Fig. i is a fragmentary sectional view showing a bolt similar to that of Fig. 3 in expanded condition following detonation of the explosive charge; 1

Fig. 5 is a sectional view of a heat exchange 39 apparatus especially adapted to catalysis; and

' Fig. 6 is a plan view of said heat exchange apparatus with parts broken away to reveal details of structure.

Referring now more particularly to Fig. 1, reference character 2 represents the foundation ring of a fire box having an outside fire box sheet or wrapper sheet t and an inside fire box sheet or fire sheet 6. The fire sheet and, wrapper sheet are provided with aligned threaded holes 8 in which staybolts represented generally by the reference character S are fitted. Two such staybolts are illustrated in Fig. 1. One of these,- designated by the reference numeral i0}, is a straight body, rigid type of stay, while the other, designated by reference character 52, is a rigid stay of the reduced body type.

Straight body rigid stays ordinarily are provided with threads throughout their length. The reducedbody rigid stays, on the contrary, are threaded only at the ends and the center region of the stay has a body of reduced diameter with respect to the threaded ends. Both types of rigid stays may be provided with a tell-tale hole, such as that illustrated at M, or they may be hollow 55 throughout their entire length as illustrated at I6.

sheet. In the improved boiler embodying our invention we prefer to make the fire sheet and wrapper sheet of the same gauge so that stresses 1 in the bolt at the plane of the inner face of the fire sheet and wrapper sheet are substantially of the same order. Moreover, in order to reduce the notch effect of the threads, the threads may be machined off, as indicated at 22, to provide a bolt having the lowest practical value of localized stress in the plane of the inner face of the wrapper sheet and fire sheet. The machining should extend only slightly under the root diameter of the threads so as not to reduce the net diameter or net 1 area of the bolt appreciably.

The fire sheet and wrapper sheet ordinarily will be made of rolled steel plates or sheets. The staybolts may likewise be made of steel but we prefer age by extending the stays beyond the plate not less than two threads when installed and thereafter upsetting the end to provide a rounded head such as that illustrated in Fig. 1 by reference character 24. This expedient did not solve the difficulty since leakage still occurred in boilers having the staybolts .headed in this manner.

hole. Since controlled compression of the charge is advantageous, it will ordinarily be preferred to precharge the explosive into a cylindrical container adapted to fit into the hole.- This will ob-. viate the hazards present in handling and pressing exposed explosive charges under shop conditions. Various methods of bringing about the explosion are suitable, such as the application of heat, percussion, the flame from a fuse orother source, electrical methods, etc.

. In Figure'2, an assembly is shown similar to that of Figure 3, with the preferred method of loading and firing illustrated more in detail.

After the bolt S is sc'rewed into the fire wall 6 so that the former extends slightly beyond the outer .edge of said wall, a metal shell is inserted into the bore hole 26 in the bolt and contains a compressed base charge 28 of a high strength, high velocity explosive extending slightly beyond both edges of the fire wall. A compressed primer charge 32 of the type of lead azide and mercury fulminate is adjacent to the base charge and is adapted to detonate said base charge at its maximum velocity. The ignition charge 33 'is fired by means of the fuse 34.

Referring'now to Figures 5 and 6, the steel case 35 contains within it a catalyst chamber 36, which is spaced from the walls of the case by means'of the staybolts 31, providing an intervening space for thepassage of a heat exchange agent such as Dowtherm, molten salts,

Moreover, in certain areas of the fire box, the

, rounded heads of the staybolts were rapidly cut away by'the flow of cinders until the bolt was substantially flush with the exposed face of the fire sheet, thus destroying the sealing effect that the rounded heads were designed to produce.

We have discovered that completely fluid-tight joints may be easily and inexpensively produced by expanding the ends of the staybolts through the use of explosivecharges. For this purpose the end of the staybolt which is to be expanded is provided with a bore 26 adapted to receive an explosive charge. This bore should extend at least to the plane of the inner face of the fire sheet or wrapper sheet and preferably extends at least wrapper sheet in the usual manner and it may extend only flush with the exposed face as shown at 27 in Fig. 1, or it may extend any desired distance beyond, as illustrated in Figs. 2, 3 and 4; In

Figs. 2 and 3 the play between the bolt S and the sheet 6 'prior to expanding the end of the bolt has been exaggerated for purposes of illustration.

Either before or after the bolt isin position, an explosive charge 28 is placed in tlie bore 26. The

I The staybolt is screwed into the threaded holes in the fire and/or explosive charge 28 may be loaded directly into the borehole, if desired, as illustrated in Fig, 3. In such case it maybe desirable to insert a disc or closing device at the base of the hole to keep the water, steam, oil, and/or other suitable materials. Inlet 30 and outlet 39 are provided in the walls of the case for the passage of the heat ex.-

change medium. Tubular entries 40 and 4! extend into the catalyst chamber for the passage of material to be treated therein. An enlarged entry .42 is also provided extending into "said chamber. The chamber contains the catalyst 43. The staybolts holding the case and chamber in spaced relationship are expanded within the walls of each by In carrying out our invention, various explosives may be used. It is preferred, however, to use the explosives described in greater detail in the copending application, Serial No. 426,150.

It is essential that the explosive be one of the class designated as high velocity explosives, in other words, one that possesses a normal velocity of detonation above 1000 meters per second when shot under the usual conditions of determining velocity. The primary detonating compounds may be employed for example, mercury fulminate, lead azide, diazodinitrophenol. Preferably,

however, we use explosives suchas tetryl, nitroshould be sufll'ciently large to accommodate a charge of the proper amount. At the'same time,

the thickness of metal between the center hole and the outer bolt wallmust be enough to give the necessary strength when the metal has been expanded. An advantage of expanding the staybolt in accordance with our invention over mechanical expanding methods lies in the uniformity of expansion in all directions.

The following examples illustrate specific 'embodiments, of staybolts joined to metal walls explosive means, as indicated where the ends of said bolts have been expanded by means of explosive charges.

Example No. 1

A threaded bolt of "A" diameter made of the nickel-copper alloy sold under the trade-mark Monel was screwed into the tapped hole of a fire sheet of A?" thickness until it extended htly beyond the outside face of said sheet. A borehole of 0.235" diameter and of /a" depth had been provided at the end of the bolt adjacent to the fire sheet and into this bore was inserted a cylindrical metal shell, adapted to fit said bore and containing a compressed charge of grains of pentaerythritol tetranitrate at its base, a primer charge of 2 grains of compressed lead azide adjacent to the base charge, and an ignition charge of a suitable mixture.

The base charge had been loaded under a pres-.

sure of about 5,000 lbs. per square inch and extended slightly beyond both faces of the wall.

' The charges were brought to explosion by means of a. fuse. A close-fitting junction, impervious to high pressures, resulted from the expansion of the bolt metal.

Example No. 2

Similarly, a threaded 78" metal bolt was screwed into another /2" steel plate. The borehole in the bolt end was of /5" depth and 0.235"

' in diameter. A cylindrical metal container was Example No. 3 Additional joints were made between staybolts and metal plates in which the tightness of the junctures were tested. A length of 4" diameter extra heavy steel pipe was first taken and closed at both ends by welding thereon pieces of boiler plate. A tube was likewise welded onto the pipe to allow the application of hydraulic pressure and its measurement by a gauge. Two different bolts made of the above mentioned nickelcopper alloy were used with such a pressure container, being screwed into opposite openings in the two plates. Longitudinal holes were drilled into the bolts, 0.235" in diameter to depths varying from to /2". ritol tetranitrate were used, varying from 5.75 to 9.0 grains. In some cases the explosive charge was co-extensive with the thickness of the plate, while in others it extended slightly beyond the edge or just to said edge. It was found, generally, that the tightest joints resulted when the explosive charge overlapped both edges of the plate slightly. In these cases it was found that the metal of the staybolt surrounding the bore not only had expanded tightly into contact with. the wall of the perforation in the sheet but that it had even bulged slightly on either side of the sheet, resulting in a very strong pressuretight joint. Somewhat weaker joints were obtained when the explosive charge did not ex tend past the edge although these joints were also satisfactorily pressure-tight. Under'conditions where the charge extended'slightly beyond both edges of the plate, tests indicated that no leaking occurred even under a pressure of 4800 Charges of pentaerythlbs. per square inch. In no.case did even a slight leak occur at a pressure less than .2200 lbs. per

square inch. This appears very striking and significant when it is realized thatsteam boilers ordinarily operate at pressures around 225 lbs.

to about 250 lbs. per square inch.

Using a similar test and an explosive charge of 10.5 grains of pentaerythritol tetranitrate, an unthreaded nickel-copper alloy bolt was used. After the explosive charge had been detonated,

the joint was found to withstand a pressure of 3200 lbs. per square inch without any leakiness, which was the maximum pressure that could be exerted on this particular assembly.

An additional test of an annealed 18-8 chromium-nickel stainless steel bolt under similar conditions showed that leakiness occurred in no case at a pressure below 4600 lbs. per square inch.

In the immediately foregoing examples, bolts have been used consisting of stainless steel arid the nickel-copper alloy sold under the trademark Monel. It will'be understood that various,

Example No. 4 i

Heat exchange apparatus was constructed by' means of explosive staybolts. A cylindrical catalyst chamber was spaced within a cylindrical steel case by means of steel staybolts 73;" in diameter.v The bolts were expanded explosively within the steel plate forming the steel case, and within the steel walls defining the catalyst chamber. The result was a heat exchange apparatus wherein the staybolt connections were pressure-tight both with respect to the heat exchange medium and the chemical fluid being treated in the catalyst chamber. The

connections between the bolts and the walls were effected by forming boreholes 0.235" in diameter in each end of the bolt, and about in depth. The metal defining these holes was expanded into contact with the walls to be spaced apart, by means of an explosive capsule loaded with a 10 grain base charge of tetryl and conventional primer and ignition compositions. Explosion was brought about by means of a fuse to cause the pressure-tight joints already described. I

It is not essential in all cases that the staybolts and perforations be threaded as pressuretightjoints can be" produced by the process of the present invention between unthreaded bolts and smooth walled perforations or between staybolts and perforations which are threaded for only a portion of the thickness of the plate. Nor is it essential that the base charge of the explosive extend beyond both faces of the fire or wrapper sheet. Satisfactory fluid-tight joints are also obtained when the bolt terminates at the plane of the exposed face of the sheet, as illustrated at 21 in Fig. 1. This has the advantage of providing a smooth surface which is not cut away to any appreciable extent by-the flow of the cinders therealong. The bolt may it a lustrated in staybolt l2 of Fig. 1

The present invention provides an extremely simple and rapid method of fitting staybolts so tightly in the boiler plates that the joints are fluid tight at pressures 1000% to morej'than 2000% greater than the ordinary working steam pressure. In the United States the boilers ordinarily operate at a pressure between the ranges ,of about 200 to 300 lbs. per square inch, with an average of about 225 to about 250 lbs. per square inch. The present invention provides a high safety factor even for boilers operating at very much higher working pressures such as have been contemplated and to a certain extent used abroad with working pressures in excess of 1000 lbs. per square inch. These joints are not only fluid-tight initially but they will remain fluid-tight in service. As those skilled in the art are aware,. staybolts are exposed in service to severe stresses tending not only to loosen the joint between the staybolts and the boiler plates but even to fracture the bolt. Many boilers operate at temperatures up to 550 F. and even higher. These high temperatures impose stresses on the staybolt and the joint due to thermal expansions of the metals of the staybolts andboiler plates. Moreover, at least once each month the locomotive must be washed out by emptying the hot water and refilling with'fresh water which sometimes is cold. This invariably results in uneven cooling of the boiler producing very high stresses known in the art as wash-out stresses. The staybolts and oiler plates are also under static stress due to the steam pressure. Moreover, particularly in boilers of locomotives and marine vessels, the staybolts also are exposed to dynamic stresses, mechanical shocks, and vibrations resulting from the movement of the boiler. The movement of the boiler may also impose sheer stresses from warping or wrenching of the boiler assembly. Thus, in

locomotive boilers the weaving and warping of the locomotive frame is transmitted to the boiler assembly and causes it also to weave and warp. The Association of American Railroads and various engineering societies, including the American Society of Mechanical Engineers and the American Society for Testing Materials, as a result of many years of experience and research, have developed codes and specifications designed to reduce staybolt breakage and leakage 'to a minimum. See for example, the A. S. M. E. Boiler Construction Code, 1940 edition, and the specifications of the Association of American Railroads, 1939 revision. Some of the more important provisions relating to staybolts appearing in these codes and specifications are as follows:

Physical and mechanical properties Tensile strength: 47,000 lbs: per sq. inch minimum.

Yield point: 60% of the tensile strength as a minimum.

Elongation in'8 inches: 28% minimum.

Reduction in area: 48% minimum.

Maximum allowable stress on staybolts: 7,500 lbs. per sq. inch and 8,000 lbs. per square inch, depending upon the staybolt. length and composition.

Permissible variation in size: The staybolt must boilers.

' be truly round within 0.01 inch and the diameter shall not be less than 0.005 inch over nor more than 0.020 inch over the specified diameter.

Tests: Staybolts shall be tested at least every month and after every hydrostatic test.

The hydrostatic test .shall be made at least every 12 months and it must be carried out at greater pressure than the working steam pressure. In this connection it is. significant that the staybolts fitted in accordance with the present invention will stand over pressures from 8 to 16 times that specified for the hydrostatic test.

Even with the precautions taken in these specifications and codes breakage of the staybolt and leakage between the staybolts and the boiler plates have occurred in normal operation of The present invention makes an important contribution to the boiler art in'greatly reducing such breakage and leakage. The ends of the bolts that are explosively expanded by the method embodying the present invention contact the walls of the holes in the boiler sheets so tightly. thatthe joints will remain fluid tight in service despite thermal stresses due to heating,

cooling, wash-outs, etc., and mechanical stresses resulting from shocks, vibration, warping and the like.

Although the invention has been described and illustrated in connection with certain specific embodiments. it will be appreciated that many modifications may. be made. therein without departing from the scope of the invention as de fined in the following claims.

We claim:

1. A boiler comprising a fire sheet and a-wrapper sheet provided with pairs of aligned threaded holes, staybolts securing said fire sheet and wrapper sheet in spaced relation to each other, each staybolt being threaded at /its ends and being screwed into a pair of said aligned holes, each end of the staybolt extending beyond the plane of the outer face of said sheets and being provided with a bore extending inwardly beyond the plane of the inner face of said sheets, the metal surrounding each of said bores being in blasted interfacial contact capable of withstanding fluid pressures of at least 2000 pounds 'per square inch throughout the adjoining wall area and the sheet such perforations until it extends at least to the outer planes of the sheets, said ends being each provided with a central bore extending at least slightly beyond the inner faces of said sheets to provide for expansion of the staybolt beyond the perforations, inserting a charge of high explosive in a capsule into each of said bores, said charges extending from said ends to the full depth of said bores, and detonating said charges to blast the metal surrounding the bores tightly into contact with the threads of said perforationsand with theperiphery of said perforations on at least said inner faces.

FREDERICK P. HUSTON.

LAWTON A. BURROWS. WALTER E. LAWSON.

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