On the Construction and Durability of Steam Boilers
Total Page：16
File Type：pdf, Size：1020Kb
217 ON THE CONSTRUCTION AND DURABILITY OF STEAM BOILERS. BY MIL BENJAMIN GOODFELLOW, OF HYDE. The object of this paper is to communicate some circumstances and changes that have been observed by the writer to take place in the size and form of boilers at different temperatures, which affect considerably tlieir strength and durability by causing derangement and wear and tear to a much greater extent than he belicves is generally supposed. His attention was first strongly drawn to this subject some years ago in reference to a large multitubular boiler that he constructed, 23 feet long and 64 feet diameter, with 131 tubes 11 feet long and 3 inches diameter each ; and two similar boilers but of smaller dimensions with 9 feet tubes. A short time after these had been put to work, it was found that several of the tubes began to leak at both ends, although they had previously been proved up to 120 lbs. per square inch with meter pressure, mhen all was good and tight, and the steam pressure they worked at was only froni 50 to 55 lbs. After this leakage had been made good, it took place again in a few weeks; and this was repeated several times, both in the large and small boilers, but not to the same extent in the small ones. This led the writer to conclude that the cause was the elongation of the tubes by their being heated to a greater extent than the casing of the boiler ; and this defect appears to him a serious objection in multitubular boilers with straight tubes of considerable length. In the construction of flued boilers of considerable length, say from 20 to 36 feet long, the mritcr at first adopted the plan of increasing the diameter of the flue, SO ah to increase the heating surface and diminish the quantity of water ; bringing the flues nearer to each other, in the case of the two-flued boiler, and closer to the sides of the boiler, as shown in Figs. 6, 7, and 8, Plate 44, in which the flues are made as large as can be got in. After a nninber of these liad been got to Downloaded from pme.sagepub.com at University of Leeds on June 5, 2016 218 CONSTRUCTION OF STEAY BOILERS. work, several of them gave way transversely about the middle seam at the bottom, especially in cases where the boiler had been blown off for cleaning and cold water then turned in to cool it ; the effect of which was that the bottom of the boiler instantly contracted in length, while the flues retained the same length or nearly so as when working, until the water came in contact with them, thereby necessarily throwing a great and undue strain upon every seam of the boiler, especially on the lower side in consequence of the flues being so near the bottom of the boiler. The writer therefore concluded that it was wrong to increase the diameter of the ends of the flues, as this rendered the ends of the boiler much more rigid and less yielding to the expansion and contraction of the flues and casing, which do not take place in both simultaneously or to the same extent. In a boiler 30 feet long the actual expansion of the barrel of the boiler &ounts to nearly 1 inch in length ; and when fired in the flue, the latter is elongated 4 inch more than the casing, in consequence of its being at so much higher a temperature. The evil effects of this expansion and contraction are further augmented by the ordinary use of gusset stays, by which the ends of the boiler are stiffened and rigidly connected to the barrel, as shown in Fig. 9, PIate 44. The circumstance of the boilers giving way in the middle of their length rather than in any other part was owing in the writer’s opinion to their being supported on a longitudinal centre wall which divided the flues, or on two walls, as shown in Figs. 2 and 3, Plate 43 ; when full of water the boiler would weigh from 38 to 40 tons, and consequently there would be a great friction on the wall when the boiler was contracting ; and the strain thus produced in pulling the two ends of the boiler nearer together is concentrated at the middle of its length, in addition to the strain arising from the resistance occasioned by the rigidity of the flues and gusset stays. In order to obviate these difficulties in flued boilers and to provide €or expansion and contraction taking place without much injury to the material or workmanship, the writer has been led to adopt the construction of boiler shown in Fig. 1, Plate 43, representing the end portions of a two-flued boiler 30 feet long. In this construction the ends of the flues are tapered and reduced in diameter, for the purpose Downloaded from pme.sagepub.com at University of Leeds on June 5, 2016 CONSTRUaTION OF STEAM BOILERS. 219 of giving a greater amount of elasticity to the ends of the boiler ; and with the same view the gusset stays are dispensed with, so that the ends are not connected in any way with the casing except by the angle-iron ring that unites the two together. The same plan may be carried out in a single-flued boiler either by tapering the ends of the flue or placing it nearer the centre of the boiler. The ends of the boiler are strengthened independently by means of T iron or '' fish- back " girders AA, rivetted on each end between the casing and the flues, as shown in Fig. 5 ; and there are no longitudinal stays between the two ends beyond those supplied by the flues and casing, each end plate being treated as an independent transverse girder supported round its edge. In order to strengthen the bottom of the boiler at the middle against the strain produced in contracting by the friction of the longi- tudinal walls on which it is supported, two longitudinal strips of angle iron or T iron BB are rivetted on the inside at about 3 feet apart, extending about two thirds the length of the boiler, as shown in Figs. 1 to 3 ; these strips are rivetted in close contact with the plates, so as to have the same temperature as the plates under all circumstances. The writer has also adopted for some time a plan of strengthening flues of large diameter against collapse by means of rings of T iron or angle iron CC rivetted at suitable intervals round the outside of the flue at the joints, as shown in Fig. 5. In these joints the two ends of the boiler plates are not brought together, but are left with a space between them equal to the thickness of the outer rib of the T iron, as shown in Fig. 4, Plate 43, whereby a joint is obtained having no greater thickness of metal than a double plate at any part. The absence of longitudinal and gusset stays in this construction of boiler does not leave the ends less strong to resist explosion than the other parts of the casing : for taking the whole circumference of both flues and casing, the sectional area of plate resisting the pressure on the ends of the boiler is 42 times greater than that resisting the lateral pressure in the casing j and in the upper half of the ends, where the pressure acts upon the greatest proportionate area, producing the greatest longitudinal tension, the resistance is 34 times that offered to lateral explosion ; while in the lower half of the ends, where there is the least proportionate area for the pressure to act upon, the resistance c2 Downloaded from pme.sagepub.com at University of Leeds on June 5, 2016 220 CONSTRUCTION OF STEAM BOILERS. is 6i times greater than the lateral resistance. The fact that flued boilers generally explode endways by failure of the lower part of the ends or casing, the very part which has been seen to be originally the strongest, proves that the strength of the plates at that part becomes greatly injured by the excessive strains arising from unequal expansion and contraction of the flues and the casing of the boiler. The action of expansion and contraction with this construction of boiler is shown in Fig. 5, Plate 43, the actiial amount being exaggerated for clearness of illustration. The dotted line shows the form assnmed by the ends of the boiler when in full work with a good fire, the flue being then considerably longer than the casing, causing the ends to be bulged outwards. The full lines show the reverse condition, with the ends bulging inwards, in consequence of the flue contracting more than the casing, mhich takes place when the fire is dramn out and the door and damper opened, the flue then cooling more rapidly than the casing of the boiler which is surrounded with hot brickwork. In the ordinary construction of boiler with rigid ends supported by gusset stays this deflection of the ends is prevented from taking place; and consequently when the ends and casing are sufficiently strong, the elongation of the flue under expansion produces a strain of compression which has the effect of slightly corrugating or buckling the plates of the flue, as shown in Figs. 6 and 7, Plate 44, thereby materially assisting in causing the flue to collapse. At the same time a strong tensile strain is thrown npon the casing, tending to tear it across, par- ticularly at the bottom of the boiler, the flues being much nearer the casing at that part than at the top : the same strain also tends to tear the ends away from the gusset stays, and often causes one or two of the riPets of the stays nearest to the flues to begin to leak after the fire has been put in.