Translated from the Russian (Morskoi Sbornik, August, 1896) by Lieut. John B. Bernardou, U. S. N.
In submitting a sketch of the development of the employment of naphtha fuel for war-ships, I do not pretend to say much that is new; but information on this subject is so little spread in our Navy that even an elementary sketch should contribute its share of usefulness, especially in view of the probable employment of naphtha fuel on ships of war in the very near future.
First of all, the naphtha serving to heat steam boilers should not be confounded with raw naphtha or kerosene. Both of these latter substances are easily inflammable, as they contain various more or less volatile hydrocarbons (on account of the presence of which they possess a characteristic odor), while the naphtha which we are now considering, and which is commonly called ''mazut,"[*] is the residue from the distillation of raw naphtha after the kerosene, benzene and other coal derivatives with low ignition points have been expelled there from. Naphtha residue is a yellow brown, thick oily liquid; its specific gravity and ignition temperature depend upon the degree of distillation to which the raw naphtha has been subjected, or upon the temperature at which the process has been conducted. For general purposes of navigation the residue is completely safe when it possesses a specific gravity 0.92 and an ignition temperature of +140° C. Such a residue possesses a very faint odor, and gives generally negative results in all attempts to inflame it. It is not set on fire when burning tow, pine splinters or burning signal lights are thrown upon its surface; and when drops of molten metal are projected into a pail of the residue inflammation only occurs around the stream of metal, and the fire is immediately extinguished when the last drops of metal disappear below the surface of the fluid. In order to consume the whole it is necessary to raise the entire mass to its ignition temperature.[*] Firing into the residue with projectiles and bursting shell does not serve to ignite it. As it consists of a mixture of stable chemical compounds, this substance is not liable to spontaneous combustion, and as far as fire is concerned it is safer in a ship than coal. The superiority of naphtha residue as a fuel compared with coal may be seen from the following:
One pound of coal burned in the furnace of a steam boiler evaporates in practice 7 pounds of water, and one pound of the residue consumed by means of the Petrashevsky and Shtchensnovitch Steam Burner evaporates under the same conditions 13.7 pounds; consequently the heating power of the naphtha residue compared to that of coal is nearly twice as great.
With skilled firemen, absolutely smokeless combustion may be produced if, at the same time, there be not forced through the furnace too large a volume of air, the most favorable amount being only one and one-half times that required theoretically for the complete combustion of the residue; under such conditions the boiler then receives from 78 per cent, to 84 per cent, of all the heat developed.[†] It is possible to determine the exact composition of each different sample of residue empirically, as it consists of a mixture of various hydrocarbons in the series of the form CnH2n+2, in which the fundamental content of hydrocarbons of the various indices n is an indeterminate quantity.
Moreover, naphtha contains a certain quantity (sometimes as high as 5 per cent, of its volume) of water and also earthy impurities.
The greater the specific gravity of the naphtha, the more difficult it is to separate the water from it by allowing it to stand, as is easily understood. This admixture with water is injurious, as it lowers the temperature of the flame upon combustion; and the earthy particles, even when very fine, obstruct, scratch and wear out the naphtha ducts when they fall into the burner, which leads to an increased expenditure of naphtha, injury to the burner, and change in the form of the flame. Hard particles are gotten rid of by settling and careful filtration of the residue.
According to Saint Clair de Ville's analyses:
Variety | Spec. grav. | Composition | 1 lb Naphtha evaporates in lbs. water | Heating power | ||
C | H | O | ||||
Raw Naphtha | 0.882 | 87.4 | 12.5 | 0.1 |
| 11,700 |
Naphtha residue, Baku factories | 0.928 | 87.1 | 11.7 | 1.2 |
| 10,700 |
Light Baku Naphtha | 0.884 | 86.3 | 13.6 | 0.1 | 16.4 | 11,460 |
Heavy Baku Naphtha | 0.938 | 86.6 | 12.3 | 1.1 | 15.5 | 10,800 |
The above table of analyses shows that in point of composition raw naphtha differs but little from naphtha residue, and even surpasses it in heating power. As the residue represents a product of technical industry, it naturally costs the more, and this is the reason why it has hitherto been supplanted by the other material for use in heating steam boilers. To render raw naphtha innocuous in relation to its liability to ignition, it suffices to expose it to the air in flat, open vessels whereby the volatile ingredients gradually pass off by evaporation. Fresh raw naphtha that will ignite at about 40° C. is found after a week's exposure to the air to ignite at +60° C, and after two weeks at +70° C, while simple heating will raise the ignition temperature still higher.
The majority of specimens of raw naphtha are more liquid than the residue, and therefore their consumption will be greater for the same type of burner and the flame larger. It may be that this difficulty will eventually call for a corresponding change in burner and bricklaying in all cases where burners specially designed for use are employed. Calculation shows that for the full combustion of one pound of naphtha residue of composition as shown in the table, 155.5 cubic feet of air are required. This theoretical quantity must be increased in practice to one and one-half times this amount, when the temperature of the flame becomes 1500° C. Notwithstanding such a degree of heat, the holler suffers less (with the corresponding construction of furnace) than with coal, as the flame from the burners does not impinge directly upon the tube sheet, but strikes first the ducts or gates of the fire-brick lining, while the metallic portion of the boiler comes in direct contact only with the heated products of combustion—carbonic acid, steam, carbonic oxide and air.
The tubes do not choke and do not require the customary cleaning with a brush. Every one knows how much labor is required from the firemen when under way under forced draught; all they can do is to throw the coal in the furnaces, while to bring the coal from the coal-bunker into the fire-room almost constantly requires the aid of the crew, which could not be given in time of action or while anticipating it; and it might even happen that the anticipation might be prolonged until the whole force became more or less exhausted, while during the whole of the time heavy masses of thick smoke, objectionable in every way, would continue pouring forth from the smoke-stacks. Under the same circumstances liquid fuel would continue to feed itself into the furnaces; forced draught does not call for the smallest increase of labor on the part of the -fireman. He simply opens wide the steam and naphtha valves of the burners, starts the ventilators, and having regulated the rate of combustion, has nothing to do but watch the level of the water in the boiler. If the burner happens to choke up, it can be cleaned in most cases by blowing steam through it; if the strainer of the naphtha tube chokes, it is easy to remove it and clean it by spraying steam through it from the water-cocks. These accidents are those that happen most frequently when liquid fuel is employed, and are generally caused by fragments of cloth or yarn from around the bungs of the casks in which the naphtha is transported falling into the liquid and eventually finding their way into the burner.
If coal-bunkers were perfectly tight and did not require additional bulkheads for retaining the naphtha within them, then the comparative heating capacity of such reservoirs would be made apparent by the following simple calculations: One ton of coal is able to evaporate 17,360 pounds of water, assuming the volume of one ton as 45 cubic feet; the weight of the same volume of naphtha residue would be 1.143 tons, and this amount would suffice to evaporate 39,406 pounds of water, that is, the amount of heat included in the same volumes of coal and naphtha is in ratio to each other 1 to 2.27. Practically this ratio is less, as the naphtha reservoirs on shipboard require for various reasons numerous bulkheads. Such bulkheads are indispensable to avoid the use of convex walls, to prevent splashing and movement of the fluid due to the roll of the vessel, the considerable loss of the liquid that might be caused by a local injury to the receptacle, etc. If ship's reservoirs were so constructed as to enable even one-half of the advantage of the naphtha as calculated on the basis of weight to be made good, then the radius of action of the ship would be increased by 60 per cent.
The ship that is fully provided with liquid fuel is lifted out of comparison with another in point of quick, clean firing, which, moreover, does not necessitate prolonged labor nor exhaust the strength of the crew. The delivery of the naphtha residue from ship to ship may he perfectly accomplished at sea even in comparatively fresh weather if the naphtha transport, provided with a delivery tube, takes the ship to be provided by it in tow. The control of the amount of fuel delivered and expended may he made perfectly automatic, and, therefore, perfectly exact.
The expenditure of the residue in raising steam to a certain pressure does not exceed 10 to 15 per cent, of the expenditure under the same conditions when under way. To maintain steam at anchor is still more economical; e.g., when carrying from 100 to 120 pounds, extinguish the burners and close fire-room doors and smoke-pipes; then at the end of from 10 to 12 hours there will be yet enough steam in the boilers to operate the burners, and in a quarter of an hour the pressure can be run up to the working limit, after which, if it be not required to get under way, extinguish the burners and re-cover all. In such a case there is effected, first, a very moderate expenditure of fuel and one in proportion to actual needs; and second, the fire-room force have full rest while at anchor. If the vessel be suddenly brought to anchor the burners can be extinguished, and thus at once the expenditure of fuel and the formation of steam are stopped.
If the reservoir be perforated by collision, then, the naphtha being lighter than water, flows out only up to the level of the hole and all above this level remains in the receptacle. A reservoir pierced at the top and filled with water increases in weight about 10 per cent.
Some ships are provided with coal-bunkers that serve to furnish shelter to certain portions of the vessel; in such event the naphtha naturally affords the weaker protection; but it is possible a ship may be compelled to go into action when her supply of fuel is nearly exhausted, when the coal-bunkers would not serve the purpose of armor, and in such a case the naphtha reservoirs, which are more strongly built and are provided with more numerous bulkheads, might prove impenetrable for light projectiles where an empty bunker would be pierced by every shell. In general comparison the naphtha fuel would not be at a disadvantage.
In getting up steam with liquid fuel, primarily, wood is used; if time permits, it may be employed for a single boiler, by the aid of the steam from which the burners of the other boilers may be lit. In large vessels, where steam is constantly maintained, this necessity disappears; while in small ones, such as torpedo-boats, it is a question of the expenditure of a small amount of wood, after which it is so easy and cheap to maintain steam that it is better not to haul fires, except for prolonged anchorages and where a new supply of wood may be obtained. It is better, too, for the preservation of the boilers not to subject them too frequently to considerable changes in temperature, such as arise in the getting-up of steam and the hauling of fires.
Finally, as concerns the preservation of the ship, it is recommended to store the naphtha in those parts of the double bottom and lower surfaces where access for cleaning and painting is most difficult, for naphtha, as experiment shows, proves an excellent preventive to the formation of iron rust.
In firing with naphtha the residue is burned either by igniting it from tubes from which it is projected in a continuous stream, or else by pulverizing it beforehand by means of air or steam. The first method is applied successfully enough to certain shore boilers; the pipe system is evidently unsuitable for marine boilers on account of the rolling of the ship. For the present-day types of high-pressure marine boilers the pulverization of the naphtha becomes necessary, as it leads to the development of a more perfect combustion and a higher temperature of flame. Judging from the quantities of patents and privileges granted inventors, steam burners of very different types have been employed for pulverizing the naphtha. Devices of this kind, however, represent no discovery, but only more or less successful applications of the pulverizer idea to the needs of liquid fuel. The steam enters the burner at a pressure of about 45 pounds and over; it passes through the steam ducts into the furnace, disperses, and, meeting upon its exit the stream of naphtha, pulverizes it; the mixture of steam, naphtha dust and air, artificially directed and impelled by its weight, burns.
The steam and naphtha ducts in the burners are made of various shapes—straight and annular, by which the flame is given a fan-shaped or conical form. As already stated, the flame is made to impinge upon the fire-brick lining in such a direction as to effect the most advantageous admixture of naphtha, steam and air, and to produce complete combustion of the fuel, as is indicated by the color of the flame.
The arrangement of the burners is satisfactory when the flame is bright and brilliant, filling the whole furnace; if it be not so (red spots and dark areas are seen), then, by changing the direction of the burners and their connections, or the form of the brick lining, the combustion may be brought to the desired degree of efficiency.
Experiments for the use of naphtha fuel for marine boilers have been in progress in our Navy for some years, and at length, in 1895, results have been obtained which afford the assurance of the speedy, practical solution of this question.
In the Caspian Sea, on the Volga, steamers have now been using naphtha fuel successfully for the last twenty years, so it may be assumed that sufficient experimental data have been collected to serve as a guide in developing the needs of war-ships. In reality, however, the problem is far from being a simple one, for the practice of ships in the Caspian and on the Volga differs materially from that which would have to be followed on ships of war. On the Volga, and especially in the Caspian, naphtha is cheap, and steam boilers are fed with water taken over the side, so that there is no specially urgent reason for minimizing the expenditure of naphtha and steam in operating the burners; smokeless combustion and successful maintenance of steam is all that is required. In a war-ship it is all-important to possess the capability of steaming the greatest number of miles with the given supply of fuel; therefore very economical burners would have to be employed, and besides, as all present-day types of steam boilers require to be fed with fresh water, the employment of steam from the boiler for the pulverization of naphtha would become a very serious obstacle to the application of liquid fuel on war-ships. The quantity of steam expended in operating the burners and lost by escape into the air through the smokestack amounts, with skilled firemen, to from three to five per cent of the whole steam developed by the boiler; and no present-day type of fresh-water condenser is able to make the loss good. In this way there is presented to experimenters the option either to find a way of pulverizing naphtha besides that requiring the use of steam, or else to design a condenser capable of making good the loss of water caused by the burners. Following the first idea, burners have been proposed in which steam is to be replaced by compressed air, to obtain which ships' air compressors and accumulators are to be used; but by making a liberal allowance, the torpedo-boat Viborg, vv-hen running at mean speed, would require air compressors fifteen times more powerful than she carries, which, while very heavy, would occupy much room and would require additional attendance (all serious matters on a torpedo-boat). The compressed air itself, on its exit from the burners into the furnace, would lower the temperature of the flame through its expansion, which argues against the utility of this type of apparatus.
On the other hand the steam which is used for pulverization impinges at a high temperature upon the fire-brick lining of the furnace, dissociates, and the products of its combustion reunite to produce a flame of high temperature.
In May, 1895, at the commencement of the cruising of the torpedo-boat Viborg, information was received from our naval agent in England concerning an air compressor with uninterrupted action for burning naphtha which a London firm had gotten out. According to information furnished, this device was too clumsy and heavy for torpedo-boat use, and besides it delivered air at a pressure not high enough by one and one-half atmospheres, one which, even with the comparatively economical Petrashevsky and Shtchensnovitch burner, would only serve for mean speed.
The burners with mechanical pulverization make a somewhat better showing. With apparatus of this type the naphtha is preliminarily pumped into a special reservoir in which it is subjected to a pressure of between 75 and 100 pounds by a hydraulic pump; the stream of naphtha is driven at this pressure into the burner, where it strikes a specially shaped edge or rib which disintegrates it into dust. The Svenson burner, one of those constructed on this plan, was tried with some success last year on the Viborg; it gave a good flame and smokeless combustion, but could not burn the requisite amount of naphtha; before expressing a final opinion about it further experiments must be made, and the inventor is now at work on these himself. Fully comprehending that the best burner is one of the steam type, capable of being employed without requiring steam from the boiler to operate it, the officer in charge of experiments with liquid fuel, Captain, senior grade, N. X. Yenish, hit upon the idea of an evaporator in which the pressure of the secondary steam would be sufficiently high to operate the burners, the steam being formed so as not to interfere with the uninterrupted working of the latter. Such a device was designed[*] and developed at the establishment of R. Krug, St. Petersburg, and installed in the starboard fire-room of the torpedo-boat Viborg. This apparatus, called by its constructor a high-pressure evaporator, consists, as shown in the accompanying sketch, of two communicating copper cylindrical vessels, A and B; the first is the evaporator proper, and the second a heater for the feed water, which enters at the side through the tube D and flows freely by the tube D' into the evaporator A. Steam from the boiler enters the evaporator by the tube C and, passing through the coils (in series; not shown in the sketch), heats the water in A; then flows by the pipe C into the feed water heater, where it is cooled and finally passes through C" into the hot well. The pipe E' unites the steam spaces of both parts of the apparatus, in consequence of which the pressure and level of the water is the same in both. The secondary steam passes into the burner from the upper part of the evaporator by the tube E. Water glasses and cocks are provided for indicating the level of the water, while the pressure is indicated by a special gauge.
As installed in the Viborg, the evaporator and mount weighed about 25 poods (1000 pounds), took up very little room and, although not provided with lagging, did not raise the temperature of the fire-room to any appreciable extent. In the following experiments made while under way the apparatus was fed with an artificially prepared solution of sea salt, of the density of sea water, with a boiler pressure between 90 and 95 pounds (250 revolutions, speed about 14 knots); the pressure in the heating coil of the evaporator was about 82 pounds, and of the secondary steam 40 pounds; the expenditure of feed water for operating both boilers (four burners) was about 59.6 gals, per hour, and the amount of naphtha burned was 33 poods (1320 pounds). The heating surface of each boiler was 107 sq. m. (1151 sq, ft.) For the general trial of the evaporator a run was made from Helsingfors and back to St. Petersburg; the results of these trials were reported as follows: "20 September, 1895, steam was gotten up in both boilers of the torpedo-boat, and when at 90 pounds the evaporators were started, commencing at the same time then to feed with artificial sea water (of density by salinometer of 1/32 at 70° R.). The secondary steam from the evaporator was led into four burners, two of which were of the Petrashevsky and Shtchensnovitch and two of the Yanushefif system. The steam valves were opened wide. At first the evaporator showed by its action (as indicated by water glass and drain cock on pressure gauge pipe) that violent ebullition was taking place within it, which was accompanied with a projection of the water into the steam ducts of the burners. The projection of water stopped when the feed valve of the evaporator coil was partly closed. The causes of the violent ebullition of the water in the pulverizer may be taken as (1) the presence of a considerable quantity of dirt in the water, which came from the sea salt as purchased; (2) a too great ratio of heating surface to volume of water in the evaporator.
The salt water carried over by the secondary steam was deposited in the steam ducts of the burners, but was projected into the furnace without clogging them. The quantity of this water was so small that it exercised no injurious effect on the combustion of the naphtha in the furnaces. To form an estimate of the amount of deposit on the surface of the heating coils, the evaporator was blown through about every two hours, when nearly the whole of the water in it was renewed. During the whole time of the trials the pressure of the secondary steam was maintained uniformly, and this pressure preserved a certain relation to that of the steam in the boiler; on increasing the latter the pressure was increased in the evaporator and vice versa. When the steam in the heating coils was maintained at a certain pressure by means of the feed valve, the variation of the pressure of the steam in the boiler did not produce a fluctuation in that of the secondary steam, which constitutes an advantage of this evaporator as an apparatus for effecting the pulverization of naphtha. The necessary regulation is easily effected by feeding in a constant and uniform supply of fresh water.[*] It is to be noted that the variation of pressure in the secondary steam did not depend upon the amount of naphtha consumed; the Petrashevsky and Shtchensnovitch burners did not smoke. During the feeding in of the feed water the pressure fell 3 or 4 pounds, but upon stopping the feed pump, it immediately ran up again.
On blowing out all the water the pressure fell 10 pounds; on extinguishing the burner the feed valve was closed immediately and the formation of the steam stopped at once. The regulation of the supply of steam to the burners may be controlled either by the valves on the burners themselves or else by the feed valves to the hot water heater coils. This capability increases the efficiency of this evaporator as an apparatus for effecting the pulverization of the naphtha.
The torpedo-boat rolled heavily at the time of the trials of the evaporator, but the rolling exercised no influence upon its performance; the level of the water in the gauge glass showed that the foaming did not increase. During the second day of the trial the foaming of the water in the evaporator diminished, and the projection of salt water into the burners ceased nearly altogether; on the third day this projection was hardly noticeable. It should be noted that the foaming stopped when the surface of the heater coils became covered with a layer of scale, that is, when the quantity of heat and its speed of delivery to the water were reduced. After 30 ½ hours' trial the evaporators and the feed water heaters were opened, when it was found that the surface of the heater coils was perfectly clean, while the evaporator coils were covered with a layer of scale of a thickness of 1 ¼ mm. for the upper spirals, of 1/3 mm. for those in the middle, the lower coils showing scarcely a perceptible amount of deposit. This deposit did not impair the evaporative efficiency of the apparatus, and the fires continued to effect a smokeless combustion and to burn the same quantity of naphtha residue as they had burned at the beginning.
The scale can be easily removed from the upper spirals by jarring them with the hand, and from the lower ones by cleaning; to effect this the spirals are removed from the evaporator, which may be done easily and quickly. Speaking in general terms, the Krug evaporator, as an apparatus for effecting the pulverization of naphtha by steam, accomplished its purpose successfully during the whole of the experiments, and delivered to the burners a quantity of steam sufficient for the smokeless combustion of about one and one-half times the quantity of naphtha required by the torpedo-boat for running under forced draught.
Mr. Krug proposes to make certain improvements in later apparatus of this kind, such as (1) constructing the shells of the feed water heater and evaporator of steel instead of copper, as they will then stand greater pressure and, besides, will weigh less; (2) fitting both these vessels with salt water blow cocks, the need of which was felt from the beginning of the experiments; (3) introducing the feed water at the top instead of at the bottom after blowing completely through, so that the hot coils will cool throughout simultaneously and, through their change of form, will effect their own cleaning; (4) constructing the tubes of elliptical instead of circular section, so as to effect maximum deformation on contraction and expansion, etc.
Summarizing what has been stated above, the advantages that have been developed by trial for naphtha residue for fuel for ships of war are its safety as a combustible; its numerous advantages over and its superiority to coal as a fuel; the superiority of pulverization by steam over that effected mechanically or by use of air; and the accomplished development of a successful type of steam pulverizer, etc.
However, certain problems of secondary, yet of great importance remain to be solved, and experiments must be repeated upon a larger scale by way of verification of results already obtained. One of these questions, propounded by Captain, 2nd rank, Gavriloff is worthy of a special mention. It relates to the effect of a submarine explosion upon the walls of a reservoir entirely filled with naphtha in which the pressure of the liquid is distributed equally in all directions. Systematically conducted experiments can alone determine how disadvantageous liquid fuel would prove in such a case and to what extent this difficulty may be overcome.
Further experiments will be made during the present summer with liquid fuel in the Baltic on board the Viborg, which has been definitely assigned to the work, and upon one torpedo-boat supplied with a Yarrow boiler. On the Black Sea a torpedo-boat is being fitted out for the use of naphtha residue (formerly the Novorossisk); and trials may be made on board the torpedo cruiser Kazarski and the ironclad Rostislav.
[*] This substance is hereafter referred to in this translation as "naphtha residue."
[*] Without contact with flame this produces evaporation.
[†] Even as high as 90 per cent., while coal and wood, even under the most favorable circumstances, do not give more than 60 per cent.
[*] With the aid of Shiloff, engineer-mechanician, who has done much work in connection with experiements on liquid fuel.
[*] Unfortunately the Viborg's feed pump was too large, in consequence of which it became necessary to stop it from time to time, so that the evaporator could not be fed uniformly.