A Catechism of the Steam Engine Part 20

383. _Q._--Can any constructive precautions be taken to prevent the furnaces and tube plates of the boiler from being burned by the intensity of the heat?

_A._--The sides of the internal furnaces or flues in all boilers should be so constructed that the steam may readily escape from their surfaces, with which view it is expedient to make the bottom of the flue somewhat wider than the top, or slightly conical in the cross section; and the upper plates should always be overlapped by the plates beneath, so that the steam cannot be retained in the overlap, but will escape as soon as it is generated. If the sides of the furnace be made high and perfectly vertical, they will speedily be buckled and cracked by the heat, as a film of steam in such a case will remain in contact with the iron which will prevent the access of the water, and the iron of the boiler will be injured by the high temperature it must in that case acquire. To moderate the intensity of the heat acting upon the furnace sides, it is expedient to bring the outside fire bars into close contact with the sides of the furnace, so as to prevent the entrance of air through the fire in that situation, by which the intensity of the heat would be increased. The tube plate nearest the furnace in tubular boilers should also be so inclined as to facilitate the escape of the steam; and the short bent plate or f.l.a.n.g.e of the tube plate, connecting the tube plate with the top of the furnace, should be made with a gradual bend, as, if the bend be sudden, the iron will be apt to crack or burn away from the concretion of salt. Where the furnace mouths are contracted by bending in the sides and top of the furnace, as is the general practice, the bends should be gradual, as salt is apt to acc.u.mulate in the pockets made by a sudden bend, and the plates will then burn into holes.

384. _Q._--In what manner is the tubing of boilers performed?

_A._--The tubes of marine boilers are generally iron tubes, three inches in diameter, and between six and seven feet long; but sometimes bra.s.s tubes of similar dimensions are employed. When bra.s.s tubes are employed, the use of ferules driven into the ends of the tubes is sometimes employed to keep them tight; but when the tubes are of malleable iron, of the thickness of Russell's boiler tubes, they may be made tight merely by firmly driving them into the tube plates, and the same may be done with thick bra.s.s tubes.

The holes in the tube plate next the front of the boiler are just sensibly larger in diameter than the holes in the other tube plate, and the holes upon the outer surfaces of both tube plates are very slightly countersunk.

The whole of the tubes are driven through both tube plates from the front of the boiler,--the precaution, however, being taken to drive them in gently at first with a light hand hammer, until the whole of the tubes have been inserted to an equal depth, and then they may be driven up by degrees with a heavy hammer, whereby any distortion of the holes from unequal driving will be prevented. Finally, the ends of the tubes should be riveted up so as to fill the countersink; the tubes should be left a little longer than the distance between the outer surfaces of the tube plates, so that the countersink at the ends may be filled by staving up the end of the tube rather than by riveting it over; and the staving will be best accomplished by means of a mandril with a collar upon it, which is driven into the tube so that the collar rests upon the end of the tube to be riveted; or a tool like a blunt chisel with a recess in its point may be used, as is the more usual practice.

385. _Q._--Should not stays be introduced in subst.i.tution of some of the tubes?

_A._--It appears expedient in all cases that some of the tubes should be screwed at the ends, so as to serve as stays if the riveting at the tube ends happens to be burned away, and also to act as abutments to the riveted tube--or else to introduce very strong rods of about the same diameter as a tube, in subst.i.tution of some of the tubes; and these stays should have nuts at each end both within and without the tube plates, which nuts should be screwed up, with white lead interposed, before the tubes are inserted.

If the tubes are long, their expansion when the boiler is being blown off will be apt to start them at the ends, unless very securely fixed; and it is difficult to prevent bra.s.s tubes of large diameter and proportionate length from being started at the ends, even when secured by ferules; but the bra.s.s tubes commonly employed are so small as to be susceptible of sufficient compression endways by the adhesion due to the ferules to compensate for the expansion, whereby they are prevented from starting at the ends. In some, of the early marine boilers fitted with bra.s.s tubes, a galvanic action at the ends of the tubes was found to take place, and the iron of the tube plates was wasted away in consequence, with rapidity; but further experience proved the injury to be attributable chiefly to imperfect fitting, whereby a leakage was caused that induced oxidation, and when, the tubes were well fitted any injurious action at the ends of the tubes was found to cease.

386. _Q._--What is the best mode of constructing the chimney and the parts in connection therewith?

_A._--In sea-going steamers the funnel plates are usually about nine feet long and 3/16ths thick; and where different flues or boilers have their debouch in the same chimney, it is expedient to run division plates up the chimney for a considerable distance, to keep the draughts distinct. The dampers should not be in the chimney but at the end of the boiler flue, so that they may be available for use if the funnel by accident be carried away. The waste steam pipe should be of the same height as the funnel, so as to carry the waste steam clear of it, for if the waste steam strikes the funnel it will wear the iron into holes; and the waste steam pipes should be made at the bottom with a faucet joint, to prevent the working of the funnel, when the vessel rolls, from breaking the pipe at the neck. There should be two hoops round the funnel, for the attachment of the funnel shrouds, instead of one, so that the funnel may not be carried overboard if one hoop breaks, or if the funnel breaks at the upper hoop from the corrosive action of the waste steam, as sometimes happens. The deck over the steam chest should be formed of an iron plate supported by angle iron beams, and there should be a high angle iron cooming round the hole in the deck through which the chimney ascends, to prevent any water upon the deck from leaking down upon the boiler. Around the lower part of the funnel there should be a sheet iron casing to prevent any inconvenient dispersion of heat in that situation, and another short piece of casing, of a somewhat larger diameter, and riveted to the chimney, should descend over the first casing, so as to prevent the rain or spray which may beat against the chimney from being poured down within the casing upon the top of the boiler. The pipe for conducting away the waste water from the top of the safety valve should lead overboard, and not into the bilge of the s.h.i.+p, as inconvenience arises from the steam occasionally pa.s.sing through it, if it has its termination in the engine room.

387. _Q._--Are not the chimneys of some vessels made so that they may be lowered when required?

_A._--The chimneys of small river vessels which have to pa.s.s under bridges are generally formed with a hinge, so that they may be lowered backward when pa.s.sing under a bridge; and the chimneys of some screw vessels are made so as to shut up like a spygla.s.s when the fires are put out and the vessel is navigated under sails. In smaller vessels, however, two lengths of chimney suffice; and in that case there is a standing piece on deck, which, however, does not project above the bulwarks.

388. _Q._--Will you explain any further details in the construction of marine boilers which occur to you as important?

_A._--The man-hole and mud-hole doors, unless put on from the outside, like a cylinder cover, with a great number of bolts, should be put on from the inside with cross bars on the outside, and the bolts should be strong, and have coa.r.s.e threads and square nuts, so that the threads may not be overrun, nor the nuts become round, by the unskilful manipulations of the firemen, by whom these doors are removed or replaced. It is very expedient that sufficient s.p.a.ce should be left between the furnace and the tubes in all tubular boilers to permit a boy to go in to clear away any scale that may have formed, and to hold on the rivets in the event of repair being wanted; and it is also expedient that a vertical row of tubes should be left out opposite to each water s.p.a.ce to allow the ascent of the steam and descent of the water, as it has been found that the removal of the tubes in that position, even in a boiler with deficient heating surface, has increased the production of steam, and diminished the consumption of fuel.

The tubes should all be kept in the same vertical line, so as to permit the introduction of an instrument to sc.r.a.pe them; but they may be zig-zagged in the horizontal line, whereby a greater strength of metal will be obtained around the holes in the tube plates, and the tubes should not be placed too close together, else their heating efficacy will be impaired.

INCRUSTATION AND CORROSION OF BOILERS.

389. _Q._--What is the cause of the formation of scale in marine boilers?

_A._--Scale is formed in all boilers which contain earthy or saline matters, just in the way in which a scaly deposit, or rock, as it is sometimes termed, is formed in a tea kettle. In sea water the chief ingredient is common salt, which exists in solution: the water admitted to the boiler is taken away in the shape of steam, and the saline matter which is not vaporizable acc.u.mulates in process of time in the boiler, until its amount is so great that the water is saturated, or unable to hold any more in solution; the salt is then precipitated and forms a deposit which hardens by heat. The formation of scale, therefore, is similar to the process of making salt from sea water by evaporation, the boiler being, in fact, a large salt pan.

390. _Q._--But is the scale soluble in fresh water like the salt in a salt pan?

_A._--No, it is not; or if soluble at all, is only so to a very limited extent. The several ingredients in sea water begin to be precipitated from solution at different degrees of concentration; and the sulphate and carbonate of lime, which begin to be precipitated when a certain state of concentration is reached, enter largely into the composition of scale, and give it its insoluble character. Pieces of waste or other similar objects left within a marine boiler appear, when taken out, as if they had been petrified; and the scale deposited upon the flues of a marine boiler resembles layers of stone.

391. _Q/_--Is much inconvenience experienced in marine boilers from these incrustations upon the flues?

_A._--Incrustation in boilers at one time caused much more perplexity than it does at present, as it was supposed that in some seas it was impossible to prevent the boilers of a steamer from becoming salted up; but it has now been satisfactorily ascertained that there is very little difference in the saltness of different seas, and that however salt the water may be, the boiler will be preserved from any injurious amount of incrustation by blowing off, as it is called, very frequently, or by permitting a considerable portion of the supersalted water to escape at short intervals into the sea. If blowing off be sufficiently practised, the scale upon the flues will never be much thicker than a sheet of writing paper, and _no excuse_ should be accepted from engineers for permitting a boiler to be damaged by the acc.u.mulation of calcareous deposit.

392. _Q._--What is the temperature at which sea water boils in a steam boiler?

_A._--Sea water contains about 1/33rd its weight of salt, and in the open air it boils at the temperature of 213.2; if the proportion of salt be increased to 2/33rds of the weight of the water, the boiling point will rise to 214.4; with 3/33rds of salt the boiling point will be 215.5; 4/33rds, 216.7; 5/33rds, 217.9; 6/33rds, 219; 7/33rds, 220.2; 8/33rds, 221.4; 9/33rds, 222.5; 10/33rds, 223.7; 11/33rds, 224.9; and 12/33rds, which is the point of saturation, 226. In a steam boiler the boiling points of water containing these proportions of salt must be higher, as the elevation of temperature due to the pressure of the steam has to be added to that due to the saltness of the water; the temperature of steam at the atmospheric pressure being 212, its temperature, at a pressure of 15 lbs.

per square inch above the atmosphere, will be 250, and adding to this 4.7 as the increased temperature due to the saltness of the water when it contains 4/33rds of salt, we have 254.7 as the temperature of the water in the boiler, when it contains 4/33rds of salt and the pressure of the steam is 15 lbs. on the square inch.

393. _Q._--What degree of concentration of the salt water may be safely permitted in a boiler?

_A._--It is found by experience that when the concentration of the salt water in a boiler is prevented from exceeding that point at which it contains 2/33rds its weight of salt, no injurious incrustation will take place, and as sea water contains only 1/33rd of its weight of salt, it is clear that it must be reduced by evaporation to one half of its bulk before it can contain 2/33rds of salt; or, in other words, a boiler must blow out into the sea one half of the water it receives as feed, in order to prevent the water from rising above 2/33rds of concentration, or 8 ounces of salt to the gallon.

394. _Q._--How do you determine 8 ounces to the gallon to be equivalent to twice the density of salt water, or "two salt waters" as it is sometimes called?

_A._--The density of the water of different seas varies somewhat. A gallon of fresh water weighs 10 lbs.; a gallon of salt water from the Baltic weighs 10.15 lbs.; a gallon of salt water from the Irish Channel weighs 10.28 lbs.; and a gallon of salt water from the Mediterranean 10.29 lbs. If we take an average saltness represented by a weight of 10.25 lbs., then a gallon of water concentrated to twice this saltness will weigh 10.5 lbs., or the salt in it will weigh .5 lbs or 8 oz., which is the proportion of 8 oz. to the gallon. However, the proportion of 2/33rds gives a greater proportion than 8 oz. to the gallon, for 2/33 = 1/16 nearly, and 1/16 of 10 lbs. = 10 oz. By keeping the density of the water in a marine boiler at the proportion of 8 or 10 oz. to the gallon, no inconvenient amount of scale will be deposited on the flues or tubes. The bulk of water, it may be remarked, is not increased by putting salt in it up to the point of saturation, but only its density is increased.

395. _Q._--Is there not a great loss of heat by blowing off so large a proportion of the heated water from the boiler?

_A._--The loss is not very great. Boilers are sometimes worked at a saltness of 4/33rds, and taking this saltness and supposing the latent heat of steam to be at 1000 at the temperature of 212, and reckoning the sum of the latent and sensible heats as forming a constant quant.i.ty, the latent heat of steam at the temperature of 250 will be 962, and the total heat of the steam will be 1212 in the case of fresh water; but as the feed water is sent into the boiler at the temperature of 100, the accession of heat it receives from the fuel will be 1112 in the case of fresh water, or 1112 increased by 3.98 in the case of water containing 4/33ds of salt-- the 3.98 being the 4.7 increase of temperature due to the presence of 4/33rds of salt, multiplied by 0.847 the specific heat of steam. This makes the total accession of heat received by the steam in the boiler equal to 1115.98, or say 1116, which multiplied by 3, as 3 parts of the water are raised into steam, gives us 3348 for the heat in the steam, while the accession of heat received in the boiler by the 1 part of residual brine will be 154.7, multiplied by 0.85, the specific heat of the brine, or 130.495; and 3348 divided by 130.495 is about 1/26th. It appears, therefore, that by blowing off the boiler to such an extent that the saltness shall not rise above what answers to 4/33rds of salt, about 1/25th of the heat is blown into the sea; this is but a small proportion, and as there will be a greater waste of heat, if from the existence of scale upon the flues the heat can be only imperfectly transmitted to the water, there cannot be even an economy of fuel in n.i.g.g.ard blowing off, while it involves the introduction of other evils. The proportion of 4/33rds of saltness, however, or 16 oz. to the gallon, is larger than is advisable, especially as it is difficult to keep the saltness at a perfectly uniform point, and the working point should, therefore, be 2/33rds as before prescribed.

396. _Q._--Have no means been devised for turning to account the heat contained in the brine which is expelled from the boiler?

_A._--To save a part of the heat lost by the operation of blowing off, the hot brine is sometimes pa.s.sed through a number of small tubes surrounded by the feed water; but there is no very great gain from the use of such apparatus, and the tubes are apt to become choked up, whereby the safety of the boiler may be endangered by the injurious concentration of its contents. Pumps, worked by the engine for the extraction of the brine, are generally used in connection with the small tubes for the extraction of the heat from the supersalted water; and if the tubes become choked the pumps will cease to eject the water, while the engineer may consider them to be all the while in operation.

397._Q._--What is the usual mode of blowing off the supersalted water from the boiler?

_A._--The general mode of blowing off the boiler is to allow the water to rise gradually for an hour or two above the lowest Working level, and then to open the c.o.c.k communicating with the sea, and keep it open until the surface of the water within the boiler has fallen several inches; but in some cases a c.o.c.k of smaller size is allowed to run water continuously, and in other cases brine pumps are used as already mentioned. In every case in which the supersalted water is discharged from the boiler in a continuous stream, a hydrometer or salt gauge of some convenient construction should be applied to the boiler, so that the density of the water may at all times be visible, and immediate notice be given of any interruption of the operation. Various contrivances have been devised for this purpose, the most of which operate on the principle of a hydrometer; but perhaps a more satisfactory principle would be that of a differential steam gauge, which would indicate the difference of pressure between the steam in the boiler and the steam of a small quant.i.ty of fresh water enclosed in a suitable vessel, and immerged in the water of the boiler.

398. _Q._--What is the advantage of blowing off from the surface of the water in the boiler?

_A._--Blowing off from a point near the surface of the water is more beneficial than blowing off from the bottom of the boiler. Solid particles of any kind, it is well known, if introduced into boiling water, will lower the boiling point in a slight degree, and the steam will chiefly be generated on the surface of the particles, and indeed will have the appearance of coming out of them; if the particles be small the steam generated beneath and around them will balloon them to the surface of the water, where the steam will be liberated and the particles will descend; and the impalpable particles in a marine boiler, which by their subsidence upon the flues concrete into scale, are carried in the first instance to the surface of the water, so that if they be caught there and ejected from the boiler, the formation of scale will be prevented.

399. _Q._--Are there any plans in operation for taking advantage of this property of particles rising to the surface?

_A._--Advantage is taken of this property in Lamb's Scale Preventer, which is substantially a contrivance for blowing off from the surface of the water that in practice is found to be very effectual; but a float in connection with a valve at the mouth of the discharging pipe is there introduced, so as to regulate the quant.i.ty of water blown out by the height of the water level, or by the extent of opening given to the feed c.o.c.k. The operation, however, of the contrivance would be much the same if the float were dispensed with; but the float acts advantageously in hindering the water from rising too high in the boiler, should too much feed be admitted, and thereby obviates the risk of the water running over into the cylinder.

In some boilers sheet iron vessels, called sediment collectors, are employed, which collect into them the impalpable matter, which in Lamb's apparatus is ejected from the boiler at once. One of these vessels, of about the size and shape of a loaf of sugar, is put into each boiler with the apex of the cone turned downwards into a pipe leading overboard, for conducting the sediment away from the boiler. The base of the cone stands some distance above the water line, and in its sides conical slits are cut, so as to establish a free communication between the water within the conical vessel and the water outside it. The particles of stony matter which are ballooned to the surface by the steam in every other part of the boiler, subside within the cone, where, no steam being generated, the water is consequently tranquil; and the deposit is discharged overboard by means of a pipe communicating with the sea. By blowing off from the surface of the water, the requisite cleansing action is obtained with less waste of heat; and where the water is muddy, the foam upon the surface of the water is ejected from the boiler--thereby removing one of the chief causes of priming.

400. _Q._--What is the cause of the rapid corrosion of marine boilers?

_A._--Marine boilers are corroded externally in the region of the steam chest by the dripping of water from the deck; the bottom of the boiler is corroded by the action of the bilge water, and the ash pits by the practice of quenching the ashes with, salt water. These sources of injury, however, admit of easy remedy; the top of the boiler may be preserved from external corrosion by covering it with felt upon which is laid sheet lead soldered at every joint so as to be impenetrable to water; the ash pits may be s.h.i.+elded by guard plates which are plates fitting into the ash pits and attached to the boiler by a few bolts, so that when worn they may be removed and new ones subst.i.tuted, whereby any wear upon the boiler in that part will be prevented; and there will be very little wear upon the bottom of a boiler if it be imbedded in mastic cement laid upon a suitable platform.

401. _Q._--Are not marine boilers subject to internal corrosion?

_A._--Yes; the greatest part of the corrosion of a boiler takes place in the inside of the steam chest, and the origin of this corrosion is one of the obscurest subjects in the whole range of engineering. It cannot be from the chemical action of the salt water upon the iron, for the flues and other parts of the boiler beneath the water suffer very little from corrosion, and in steam vessels provided with Hall's condensers, which supply the boiler with fresh water, not much increased durability of the boiler has been experienced. Nevertheless, marine boilers seldom last more than for 5 or 6 years, whereas land boilers made of the same quality of iron often last 18 or 20 years, and it does not appear probable that land boilers would last a very much shorter time if salt water were used in them. The thin film of scale spread over the parts of a marine boiler situated beneath the water, effectually protect them from corrosion; and when the other parts are completely worn out the flues generally remain so perfect, that the hammer marks upon them are as conspicuous as at their first formation. The operation of the steam in corroding the interior of the boiler is most capricious--the parts which are most rapidly worn away in one boiler being untouched in another; and in some cases one side of a steam chest will be very much wasted away while the opposite side remains uninjured. Sometimes the iron exfoliates in the shape of a black oxide which comes away in flakes like the leaves of a book, while in other cases the iron appears as if eaten away by a strong acid which had a solvent action upon it. The application of felt to the outside of a boiler, has in several cases been found to accelerate sensibly its internal corrosion; boilers in which there is a large acc.u.mulation of scale appear to be more corroded than where there is no such deposit; and where the funnel pa.s.ses through the steam chest the iron of the steam chest is invariably much more corroded than where the funnel does not pa.s.s through it.

402. _Q._--Can you suggest no reason for the rapid internal corrosion of marine boilers?

_A._--The facts which I have enumerated appear to indicate that the internal corrosion of marine boilers is attributable chiefly to the existence of surcharged steam within them, which is steam to which an additional quant.i.ty of heat has been communicated subsequently to its generation, so that its temperature is greater than is due to its elastic force; and on this hypothesis the observed facts relative to corrosion become to some extent explicable. Felt, applied to the outside of a boiler, may accelerate its internal corrosion by keeping the steam in a surcharged state, when by the dispersion of a part of the heat it would cease to be in that state; boilers in which there is a large acc.u.mulation of scale must have worked with the water very salt, which necessarily produces surcharged steam; for the temperature of steam cannot be less than that of the water from which it is generated, and inasmuch as the boiling point of water, under any given pressure, rises with the saltness of the water, the temperature of the steam must rise with the saltness of the water, the pressure remaining the same; or, in other words, the steam must have a higher temperature than is due to its elastic force, or be in the state of surcharged steam. The circ.u.mstance of the chimney flue pa.s.sing through the steam will manifestly surcharge the steam with heat, so that all the circ.u.mstances which are found to accelerate corrosion, are it appears such as would also induce the formation of surcharged steam.

403. _Q._--Is it the natural effect of surcharged steam to waste away iron?

_A._--It is the natural effect of surcharged steam to oxidate the iron with which it is in contact, as is ill.u.s.trated by the familiar process for making hydrogen gas by sending steam through a red hot tube filled with pieces of iron; and although the action of the surcharged steam in a boiler is necessarily very much weaker than where the iron is red hot, it manifestly must have _some_ oxidizing effect, and the amount of corrosion produced may be very material where the action is perpetual. Boilers with a large extent of heating surface, or with descending flues circulating through the cooler water in the bottom of the boiler before ascending the chimney, will be less corroded internally than boilers in which a large quant.i.ty of the heat pa.s.ses away in the smoke; and the corrosion of the boiler will be diminished if the interior of any flue pa.s.sing through the steam be coated with fire brick, so as to present the transmission of the heat in that situation. The best practice, however, appears to consist in the transmission of the smoke through a suitable pa.s.sage on the outside of the boiler, so as to supersede the necessity of carrying any flue through the steam at all; or a column of water may be carried round the chimney, into which as much of the feed water may be introduced as the heat of the chimney is capable of raising to the boiling point, as under this limitation the presence of feed water around the chimney in the steam chest will fail to condense the steam.

404. _Q._--In steam vessels there are usually several boilers?

_A._--Yes, in steam vessels of considerable power and size.

405. _Q._--Are these boilers generally so constructed, that any one of them may be thrown out of use?