A REVIEW OF MARINE ENGINEERING DURING THE PAST DECADE.[1]
By Mr. ALFRED BLECHYNDEN, of Barrow-in-Furness
Steam Pipes.--The failures of copper steam pipes on board the Elbe, Lahn, and other vessels have drawn serious attention both to the material and to the modes of construction of the pipes. The want of elastic strength in copper is an important element in the matter; and the three following remedies have been proposed, while still retaining copper as the material. First, in view of the fact that in the operation of brazing the copper may be seriously injured, to use solid drawn tubes. This appears fairly to meet the main dangers incidental to brazing; but as solid drawn pipes of over 7 inches diameter are difficult to procure, it hardly meets the case sufficiently. Secondly, to use electrically deposited tubes. At first much was promised in this direction; but up to the present time it can hardly be regarded as more than in the experimental stage. Thirdly, to use the ordinary brazed or solid drawn tubes, and to re-enforce them by serving with steel cord or steel or copper wire. This has been tried, and found to answer perfectly. For economical reasons, as well as for insuring the minimum of torsion to the material during manufacture, it is important to make as few bends as possible; but in practice much less difficulty has been experienced in serving bent pipes in a machine than would have been expected. Discarding copper, it has been proposed to substitute steel or iron. In the early days of the higher pressures, Mr. Alexander Taylor adopted wrought iron for steam pipes. One fitted in the Claremont in February, 1882, was recently removed from the vessel for experimental purposes, and was reported upon by Mr. Magnus Sandison in a paper read before the Northeast Coast Institution of Engineers and Shipbuilders.[2] The following is a summary of the facts. The pipe was 5 inches external diameter, and 0.375 inch thick. It was lap welded in the works of Messrs. A. & J. Stewart. The flanges were screwed on and brazed externally. The pipe was not lagged or protected in any manner. After eight and a half years' service the metal measured where cut 0.32 and 0.375 inch in thickness, showing that the wasting during that time had been very slight. The interior surface of the tube exhibited no signs of pitting or corrosion. It was covered by a thin crust of black oxide, the maximum thickness of which did not exceed 1/32 inch. Where the deposit was thickest it was curiously striated by the action of the steam. On the scale being removed, the original bloom on the surface of the metal was exposed. It would thus appear that the danger from corrosion of iron steam pipes is not borne out in their actual use; and hence so much of the way is cleared for a stronger and more reliable material than copper. So far the source of danger seems to be in the weld, which would be inadmissible in larger pipes; but there is no reason why these should not be lapped and riveted. There seems, however, a more promising way out of the difficulty in the Mannesmann steel tubes which are now being "spun" out of solid bars, so as to form weldless tubes.
TABLE I.--TENSILE STRENGTH OF GUN METAL AT HIGH TEMPERATURES.
| Composition of gun metal. | Temperature of oil bath | Tensile strength per square inch. | Elastic limit per square inch. | Elongation in length of 2 inches |
|---|---|---|---|---|
| Per cent. | Fahr. | Tons | Tons | Per cent. |
| Copper 87 Tin 8 Zinc 3½ Lead 1½ | 50° | 12.34 | 8.38 | 14.64 |
| 400° | 10.83 | 6.30 | 11.79 | |
| Copper 87 Tin 8 Zinc 5 | 50° | 13.86 | 8.33 | 20.30 |
| 458° | 10.70 | 7.43 | 12.42 |
Cast steel has been freely used by the writer for bends, junction pieces, etc., of steam pipes, as well as for steam valve chests; and except for the fact that steel makers' promises of delivery are generally better than their performance, the result has thus far been satisfactory in all respects. These were adopted because there existed some doubt as to the strength of gun metal under a high temperature; and as the data respecting its strength appeared of a doubtful character, a series of careful tests were made to determine the tensile strength of gun metal when at atmospheric and higher temperatures. The test bars were all 0.75 in diameter, or 0.4417 square inch sectional area; and those tested at the higher temperatures were broken while immersed in a bath of oil at the temperature here stated, each line being the mean of four experiments. The result of these experiments was to give somewhat greater faith in gun metal as a material to be used under a higher temperature; but as steel is much stronger, it is probably the most advisable material to use, when the time necessary to procure it can be allowed.
Feed Heating.--With the double object of obviating strain on the boiler through the introduction of the feed water at a low temperature, and also of securing a greater economy of fuel, the principle of previously heating the feed water by auxiliary means has received considerable attention, and the ingenious method introduced by Mr. James Weir has been widely adopted. It is founded on the fact that, if the feed water as it is drawn from the hot well be raised in temperature by the heat of a portion of steam introduced into it from one of the steam receivers, the decrease of the coal necessary to generate steam from the water of the higher temperature bears a greater ratio to the coal required without feed heating than the power which would be developed in the cylinder by that portion of steam would bear to the whole power developed when passing all the steam through all the cylinders. The temperature of the feed is of course limited by the temperature of the steam in the receiver from which the supply for heating is drawn. Supposing, for example, a triple expansion engine were working under the following conditions without feed heating: Boiler pressure, 150 lb.;--indicated horse power in high pressure cylinder 398, in intermediate and low pressure cylinders together 790, total, 1,188; and temperature of hot well 100° Fahr. Then with feed heating the same engine might work as follows: The feed might be heated to 220° Fahr., and the percentage of steam from the first receiver required to heat it would be 12.2 per cent.; the indicated horse power in the high pressure cylinder would be as before 398, and in the intermediate and low pressure cylinders it would be 12.2 per cent, less than before, or 694, and the total would be 1,092, or 92 per cent. of the power developed without feed heating. Meanwhile the heat to be added to each pound of the feed water at 220° Fahr. for converting it into steam would be 1,005 units against 1,125 units with feed at 100° Fahr., equivalent to an expenditure of only 89.4 per cent. of the heat required without feed heating. Hence the expenditure of heat in relation to power would be 89.4 + 92.0 = 97.2 per cent., equivalent to a heat economy of 2.8 per cent. If the steam for heating can be taken from the low pressure receiver, the economy is about doubled. Other feed heaters, more or less upon the same principle, have been introduced. Also others which heat the feed in a series of pipes within the boiler, so that it is introduced into the water in the boiler practically at boiling temperature; this is economical, however, only in the sense that wear and tear of the boiler is saved; in principle the plan does not involve economy of fuel.
Auxiliary Supply of Fresh Water.--Intimately associated with the feed is the means adopted for making up the losses of fresh water due to leakage of steam from safety valves, glands, joints, etc., and of water discharged from the air pumps. A few years ago this loss was regularly made up from the sea, with the result that the water in the boilers was gradually increased in density; whence followed deposit on the internal surfaces, and consequent loss of efficiency, and danger of accident through overheating the plates. With the higher pressures now adopted, the danger arising from overheating is much more serious, and the necessity is absolute of maintaining the heating surfaces free from deposit. This can be done only by filling the boiler with fresh water in the first instance, and maintaining it in that condition. To do this two methods are adopted, either separately or in conjunction. Either a reserve supply of fresh water is carried in tanks or the supplementary feed is distilled from sea water by special apparatus provided for the purpose. In the construction of the distilling or evaporating apparatus advantage has been taken of two important physical facts, namely, that, if water be heated to a temperature higher than that corresponding with the pressure on its surface, evaporation will take place; and that the passage of heat from steam at one side of a plate to water at the other is very rapid. In practice the distillation is effected by passing steam, say from the first receiver, through a nest of tubes inside a still or evaporator, of which the steam space is connected either with the second receiver or with the condenser. The temperature of the steam inside the tubes being higher than that of the steam either in the second receiver or in the condenser, the result is that the water inside the still is evaporated, and passes with the rest of the steam into the condenser, where it is condensed, and serves to make up the loss. This plan localizes the trouble of deposit, and frees it from its dangerous character, because an evaporator cannot become overheated like a boiler, even though it be neglected until it salts up solid; and if the same precautions are taken in working the evaporator which used to be adopted with low pressure boilers when they were fed with salt water, no serious trouble should result. When the tubes do become incrusted with deposit, they can be either withdrawn or exposed, as the apparatus is generally so arranged; and they can then be cleaned.
Screw Propeller.--In Mr. Marshall's paper of 1881 it was said that "the screw propeller is still to a great extent an unsolved problem." This was at the time a fairly true remark. It was true the problem had been made the subject of general theoretical investigation by various eminent mathematicians, notably by Professor Rankine and Mr. William Froude, and of special experimental investigation by various engineers. As examples of the latter may be mentioned the extended series of investigations in the French vessel Pelican, and the series made by Mr. Isherwood on a steam launch about 1874. These experiments, however, such as they were, did little to bring out general facts and to reduce the subject to a practical analysis. Since the date of Mr. Marshall's paper, the literature on this subject has grown rapidly, and, has been almost entirely of a practical character. The screw has been made the subject of most careful experiments. One of the earliest extensive series of experiments was made under the writer's direction in 1881, with a large number of models, the primary object being to determine what value there was in a few of the various twists which inventive ingenuity can give to a screw blade. The results led the experimenters to the conclusion that in free water such twists and curves are valueless as serving to augment efficiency. The experiments were then carried further with a view to determine quantitative moduli for the resistance of screws with different ratios of pitch to diameter, or "pitch ratios," and afterward with different ratios of surface to the area of the circle described by the tips of the blades, or "surface ratios." As these results have to some extent been analyzed and published, no further reference need be made to them now.