LECTURE XXVIII.-QUESTIONS,

1. Enumerate the different classes of marine boilers at present in general use, and describe briefly their distinctive features. Give freehand sketches of each of these types, and state the maximum pressures at which they are worked. Why has the rectangular boiler been given up, and what two forms of rectangular boilers were in use 30 years ago?

2. Describe the construction of a marine boiler with four furnaces of modern type for high pressure steam. Sketch a cross and a longitudinal section, showing the water spaces, with a complete index of the various parts. How are the flat surfaces stayed? Enumerate all the principal fittings.

3. Describe the construction of, and sketch both in transverse and longitudinal section, a marine boiler. Mention some of the causes to which a loss of heat may be attributed when the boiler is in operation.

4. Sketch and describe by an index of parts a cylindrical high-pressure marine boiler with two furnaces, showing the mode of construction and staying, and describe the several processes employed in its construction from the commencement until its completion in the shop.

5. Sketch and describe clearly how the furnace tube of a cylindrical marine boiler is constructed, and how it is attached to the combustion chamber and front end plates, and also how expansion is allowed for.

6. Give a freehand sketch of a marine engine boiler, with all the necessary fittings in their relative positions; name them and their respective uses. 7. Sketch and describe by an index of parts Shanks' small vertical marine boiler for steam launches or small tugs.

8. Describe, with sketches, the general construction of a boiler fo generating steam at a high pressure, selecting that one of the three (land marine, or locomotive) with which you are most familiar. (Adv. S. and ▲ Exam., 1887.)

Note. For a Lecture on Water-Tube Boilers, see Appendix V.

LECTURE XXIX.

BOILER CONSTRUCTION.

CONTENTS.-Materials used in Boiler Construction-Wrought-Iron, Steel, Copper-Joints of Boiler Plates, Riveted Joints, Punching and Drilling, Hand and Machine Riveting, Caulking, Welded JointsMethods of Connecting the parts of the Shell, and Flues-Staying of Boilers-Strength of Boiler Shells-Strength of Flues-Strengthening Hoops for Flues-Corrugated Furnaces.

Materials used in Boiler Construction.-The earliest forms of steam boilers were constructed chiefly of cast-iron, but on account of the low tensile strength of this material and its unreliable nature when subjected to the variable temperature and stresses in a steam boiler it has been abandoned for many years, except for certain parts of water-tube boilers, such as in those of Babcock and Wilcox, illustrated in Lecture XXVII.

Wrought-Iron.-Until the recent introduction of mild, soft steel, wrought-iron was the material which was almost exclusively employed for the construction of steam boilers. Wroughtiron possesses great tenacity, combined with the important qualities of toughness and ductility. It is therefore well adapted to resist sudden strains and alterations of temperature, and does not give way suddenly or without warning. Also, its capability of being welded, forged, and flanged, adds to its value as a material for boiler construction; whilst it is a matter of importance that its strength is not influenced to any appreciable extent by a moderately large increase in temperature, such as highpressure boilers are subjected to under ordinary circumstances.

The average ultimate tensile strength of wrought-iron bars may be taken at 22 tons (or about 50,000 lbs.) per square inch; the tensile strength of the best quality of bar iron being about 25 tons (56,000 lbs.) per square inch. The average tensile strength of wrought-iron plates as used for boilers, is—

The plates used in boilermaking should all be of good quality; inferior plates give great trouble and are always unsatisfactory. The plates require to pass through some of the various processes of flanging, dishing, welding, punching and rolling cold and in

dealing with inferior plates the greatest care must be exercised by the workmen to prevent them from injury before they are fitted into the boiler.

Fractured longitudinal joint in a new boiler from the use of a brittle iron plate. Norwich explosion, 25th September, 1866. The plates were of Cleveland iron, and when tested by bending, broke off short. They were quite wanting in ductility.

Only a very good quality of plate will stand flanging with impunity; and where joints have to be welded, satisfactory work cannot be obtained with an inferior plate which is wanting in ductility. Many inferior plates have a high tensile strength, but are brittle and do not possess that toughness and ductility which are essential qualities when much forging has to be done. The plates of the furnace flues are the most important, since these are more severely taxed by variations of temperature (causing sudden expansions and contractions), than any other plates in the boiler. The plates of the furnace crowns are alternately in contact with the fierce hot flames from the fire, and the currents of cold air which rush into the furnace each time the firing door is opened. The constant straining of the plates which is thus induced is very trying, and none but plates of very good quality will stand it for a great length of time. The various brands put upon plates, such as Best, and Best Best, &c., are very misleading, the "treble Best" plates of one maker being no better in some cases than the "Best" plates of another. It is only by the use of efficient testing machines, careful chemical and microscopical analysis, that a thorough knowledge of the capabilities and nature of a given quality of plate can be ascertained.

Steel. This material has come into use very largely for boiler plates within the last few years, owing to the valuable properties it possesses when manufactured in the mild form. Mild steel boiler plates containing about 0.1 per cent. of carbon are now manufactured by the Bessemer, Siemens and basic (ThomasGilchrist) processes, and have an ultimate tensile strength of from 25 to 30 tons per square inch, with an elongation in test strips (8 inches long) of from 20 to 25 per cent. Steel plates with a

higher tensile strength than this, are usually too hard and brittle for boilermaking purposes. Owing to the greater tensile strength of steel, boilers made of that material are much lighter than when made of iron, and the plates being thinner, the joints are more easily made tight. Good mild boiler steel plates also possess a ductility superior to wrought-iron, and are therefore more suitable where flanging has to be done. They can be treated whether cold or hot by experienced workmen with the same freedom and usage as applied to malleable iron plates, except to a small extent in the case of welding, for the steel plates do not weld quite so freely as iron ones, and the welds are not so trustworthy. Steel scrap, however, welds into blooms quite as freely as wrought-iron scrap, and the forgings are generally superior.

There have been a few cases of the failure of boilers constructed of steel, the plates of which had been tested before they were used and found to be of a good quality. These failures have engendered in the minds of some engineers a certain amount of distrust of this material. Steel plates are undoubtedly more severely injured by punching than iron plates, and should always be annealed afterwards. Much of the distrust which has been felt in the use of steel for boilermaking has been caused by the use of plates quite unsuitable for that purpose, and their subsequent failure. Engineers have in many cases been too anxious to avail themselves of the high tensile strength of steel, forgetting that in so doing they are sacrificing the all-important quality of ductility. Steel is undoubtedly superior to iron as a material of construction for steam boilers, but great attention and care must be paid to its special properties and the methods of manipulation most suitable for it. Plates of a very mild nature, possessing moderately high tensile strength but great ductility, should invariably be used. Of all the tests applied to steel plates the bending test is the most valuable.

Copper. Occasionally, copper has been used for boilermaking, although chiefly for small boilers. It is a much better conductor of heat than iron or steel, the ratio of the conductivities of copper and iron being expressed approximately by the numbers 74 to 12. It wears better under the intense heat of the furnace, and gives a higher evaporative efficiency. It has also the advantage

See paper on "Injurious Effects of a Blue heat on Iron and Steel" by Mr. Stromeyer, read before the Institution of Civil Engineers. Vol. lxxxiv. of Proceedings. See also "General Remarks on Steel Boilers" by Thomas Trail, in his Pocket-Book on Boilers, Published by Charles Griffin & Co., London, 1888.