A Catechism of the Steam Engine Part 21

_A._--Marine boilers are now generally supplied with stop valves, whereby one boiler may be thrown out of use without impairing the efficacy of the remainder. These stop valves are usually spindle valves of large size, and they are for the most part set in a pipe which runs across the steam chests, connecting the several boilers together. The spindles of these valves should project through stuffing boxes in the covers of the valve chests, and they should be balanced by a weighted lever, and kept in continual action by the steam. If the valves be lifted up, and be suffered to remain up, as is the usual practice, they will become fixed by corrosion in that position, and it will be impossible after some time to shut them on an emergency. These valves should always be easily accessible from the engine room; and it ought not to be necessary for the coal boxes to be empty to gain access to them.

406. _Q._--Should each boiler have at least one safety valve for itself?

_A._--Yes; it would be quite unsafe without this provision, as the stop valve might possibly jam. Sometimes valves jam from a distortion in the shape of the boiler when a considerable pressure is put upon it.

407. _Q._--How is the admission of the water into the boiler regulated?

_A._--The admission of feed water into the boiler is regulated by hand by the engineer by means of c.o.c.ks, and sometimes by spindle valves raised and lowered by a screw. c.o.c.ks appear to be the preferable expedient, as they are less liable to accident or derangement than screw valves, and in modern steam vessels they are generally employed.

408. _Q._--At what point of the boiler is the feed introduced?

_A._--The feed water is usually conducted from the feed c.o.c.k to a point near the bottom of the boiler by means of an internal pipe, the object of this arrangement being to prevent the rising steam from being condensed by the entering water. By being introduced near the bottom of the boiler, the water comes into contact in the first place with the bottoms of the furnaces and flues, and extracts heat from them which could not be extracted by water of a higher temperature, whereby a saving of fuel is accomplished. In some cases the feed water is introduced into a casing around the chimney, from whence it descends into the boiler. This plan appears to be an expedient one when the boiler is short of heating surface, and more than a usual quant.i.ty of heat ascends the chimney; but in well proportioned boilers a water casing round the chimney is superfluous. When a water casing is used the boiler is generally fed by a head of water, the feed water being forced up into a small tank, from whence it descends into the boiler by the force of gravity, while the surplus runs to waste, as in the feeding apparatus of land engines.

409. _Q._--Suppose that the engineer should shut off the feed water from the boilers while the engine was working, what would be the result?

_A._--The result would be to burst the feed pipes, except for a safety valve placed on the feed pipe between the engine and the boilers, which safety valve opens when any undue pressure comes upon the pipes, and allows the water to escape. There is, however, generally a c.o.c.k on the suction side of the feed pump, which regulates the quant.i.ty of water drawn into the pump. But there must be c.o.c.ks on the boilers also to determine into which boiler the water shall be chiefly discharged, and these c.o.c.ks are sometimes all shut accidentally at the same time.

410. _Q._--Is there no expedient in use in steam vessels for enabling the position of the water level in the boiler to determine the quant.i.ty of feed water admitted?

_A._--In some steam vessels floats have been introduced to regulate the feed, but their action cannot be depended on in agitated water, if applied after the common fas.h.i.+on. Floats would probably answer if placed in a cylinder which communicates with the water in the boiler by means of small holes; and a disc of metal might be attached to the end of a rod extending beneath the water level, so as to resist irregular movements from the motion of the s.h.i.+p at sea, which would otherwise impair the action of the apparatus.

411. _Q._--How is the proper level of the water in the boiler of a steam vessel maintained when, the engine is stopped for some time, and the boiler is blowing off steam?

_A._--By means of a separate pump worked sometimes by hand, but usually by a small separate engine called the Donkey engine. This pump, by the aid of suitable c.o.c.ks, will pump from the sea into the boiler; from the sea upon deck either to wash decks or to extinguish fire; and from the bilge overboard, through a suitable orifice in the side of the s.h.i.+p.

LOCOMOTIVE BOILERS.

412. _Q._--Will you recapitulate the general features of locomotive boilers?

_A._--Locomotive boilers consist of three portions (see fig. 29): the barrel E, E, containing the tubes, the fire box B, and the smoke box F; of which the barrel smoke box, and external fire box are always of iron, but the internal fire box is generally made of copper, though sometimes also it is made of iron. The tubes are sometimes of iron, but generally of bra.s.s fixed in by ferules. The whole of the iron plates of a locomotive boiler Which are subjected to the pressure of steam, should be Lowmoor or Bowling plates of the best quality; and the copper should be coa.r.s.e grained, rather than rich or soft, and be perfectly free from irregularities of structure and lamination.

413. _Q._--What are the usual dimensions of the barrel?

_A._--The thickness of the plates composing the barrel of the boiler varies generally from 5/16ths to 3/8ths of an inch, and the plates should run in the direction of the circ.u.mference, so that the fibres of the iron may be in the direction of the strain. The diameter of the barrel commonly varies from 3 ft. to 3 ft. 6 inches; the diameter of the rivets should be from 11/16ths to 3/4ths of an inch, and the pitch of the rivets or distance between their centres should be from 17/8th to 2 inches.

414. _Q._--How are the fire boxes of a locomotive constructed?

_A._--The s.p.a.ce between the external and internal fire boxes forms a water s.p.a.ce, which must be stayed every 4-1/2 or 5 inches by means of copper or iron stay bolts, screwed through the outer fire box into the metal of the inner fire box, and securely riveted within it: iron stay bolts are as durable as copper, and their superior tenacity gives them an advantage.

Sometimes tubes are employed as stays. The internal and external fire boxes are joined together at the bottom by a N-shaped iron, and round the fire door they are connected by means of a copper ring 1-1/4 in. thick, and 2 in. broad,--the inner fire box being dished sufficiently outward at that point, and the outer fire box sufficiently inward, to enable a circle of rivets 3/4 of an inch in diameter pa.s.sing through the copper ring and the two thicknesses of iron, to make a water-tight joint. The thickness of the plates composing the external fire box is in general 3/8ths of an inch if the fire box is circular, and from 3/8ths to 1/2 inch if the fire box is square; and the thickness of the internal fire box is in most cases 7/16ths if copper, and from 3/8ths to 7/16ths of an inch if of iron. Circular internal fire boxes, if made of iron, should be welded rather than riveted, as the rivet heads are liable to be burnt away by the action of the fire; and when the fire boxes are square each side should consist of a single plate, turned over at the edges with a radius of 3 inches, for the introduction of the rivets.

415. _Q._--Is there any provision for stiffening the crown of the furnace in a locomotive?

_A._--The roof of the internal fire box, whether flat as in Stephenson's engines, or dome shaped as in Bury's, requires to be stiffened with cross stay bars, but the bars require to be stronger and more numerous when applied to a flat surface. The ends of these stay bars rest above the vertical sides of the fire box; and to the stay bars thus extending across the crown, the crown is attached at intervals by means of stay bolts. There are projecting bosses upon the stay bars encircling the bolts at every point where a bolt goes through, but in the other parts they are kept clear of the fire box crown so as to permit the access of water to the metal; and, with the view of facilitating the ascent of the steam, the bottom of each stay bar should be sharpened away in those parts where it does not touch the boiler.

416. _Q._--Is any inconvenience experienced from the intense heat in a locomotive furnace?

_A._--The fire bars in locomotives have always been a source of trouble, as from the intensity of the heat in the furnace they become so hot as to throw off a scale, and to bend under the weight of the fuel. The best alleviation of these evils lies in making the bars deep and thin: 4 or 5 inches deep by five eighths of an inch thick on the upper side, and three eighths of an inch on the under side, are found in practice to be good dimensions. In some locomotives a frame carrying a number of fire bars is made so that it may be dropped suddenly by loosening a catch; but it is found that any such mechanism can rarely be long kept in working order, as the molten clinker by running down between the frame and the boiler will generally glue the frame into its place. It is therefore found preferable to fix the frame, and to lift up the bars by the dart used by the stoker, when any cause requires the fire to be withdrawn. The furnace bars of locomotives are always made of malleable iron, and indeed for every species of boiler malleable iron bars are to be preferred to bars of cast iron, as they are more durable, and may if thin be set closer together, whereby the small coal or c.o.ke is saved that would otherwise fall into the ash pit. The ash box of locomotives is made of plate iron, a quarter thick: it should not be less than 10 in. deep, and its bottom should be about 9 in. above the level of the rails. The chimney of a locomotive is made of plate iron one eighth of an inch thick: it is usually of the same diameter as the cylinder, but is better smaller, and must not stand more than 14 ft. high above the level of the rails.

417. _Q._--Are locomotive boilers provided with a steam chest?

_A._--The upper portion of the external fire box is usually formed into a steam chest, which is sometimes dome shaped, sometimes semicircular, and sometimes of a pyramidical form, and from this steam chest the steam is conducted away by an internal pipe to the cylinders; but in other cases an independent steam chest is set upon the barrel of the boiler, consisting of a plate iron cylinder, 20 inches in diameter, 2 feet high, and three eighths of an inch thick, with a dome shaped top, and with the seam welded and the edge turned over to form a f.l.a.n.g.e of attachment to the boiler. The pyramidical dome, of the form employed in Stephenson's locomotives, presents a considerable extent of flat surface to the pressure of the steam, and this flat surface requires to be very strongly stayed with angle irons and tension rods; whereas the semiglobular dome of the kind employed in Bury's engines requires no staying whatever. Latterly, however, these domes over the fire box have been either much reduced in size or abandoned altogether.

418. _Q._--Is any beneficial use made of the surplus steam of a locomotive?

_A._--To save the steam which is formed when the engine is stationary, a pipe is usually fitted to the boiler, which on a c.o.c.k being turned conducts the steam into the water in the tender, whereby the feed water is heated, and less fuel is subsequently required. This method of disposing of the surplus steam may be adopted when the locomotive is descending inclines, or on any occasion where more steam is produced than the engine can consume.

419._Q._--What means are provided to facilitate the inspection and cleaning of locomotive boilers?

_A._--The man hole, or entrance into the boiler, consists of a circular or oval aperture of about 15 in. diameter, placed in Bury's locomotive at the apex of the dome, and in Stephenson's upon the front of the boiler, a few inches below the level of the rounded part; and the cover of the man hole in Bury's engine contains the safety valve seats. In whatever situation this man hole is placed, the surfaces of the ring encircling the hole, and of the internal part of the door or cover, should be accurately fitted together by sc.r.a.ping or grinding, so that they need only the interposition of a little red lead to make them quite tight when screwed together. Lead or canvas joints, if of any considerable thickness, will not long withstand the action of high pressure steam; and the whole of the joints about a locomotive should be such that they require nothing more than a little paint or putty, or a ring of wire gauze smeared with white or red lead to make them perfectly tight. There must be a mud hole opposite the edge of each water s.p.a.ce, if the fire box be square, to enable the boiler to be easily cleaned out, and these holes are most conveniently closed by screwed plugs made slightly taper. A c.o.c.k for emptying the boiler is usually fixed at the bottom of the fire box, and it should be so placed as to be accessible when the engine is at work, in order that the engine driver may blow off some water if necessary; but it must not be in such a position as to send the water blown off among the machinery, as it might carry sand or grit into the bearings, to their manifest injury.

420. _Q._--Will you state the dimensions of the tube plate, and the means of securing the tubes in it?

_A._--The tube plates are generally made from five eighths to three fourths of an inch thick, but seven eighths of an inch thick appears to be preferable, as when the plate is thick the holes will not be so liable to change their figure during the process of feruling the tubes: the distance between the tubes should never be made less than three fourths of an inch, and the holes should be slightly tapered so as to enable the tubes to hold the tube plates together. The tubes are secured in the tube plates by means of taper ferules driven into the ends of the tubes. The ferules are for the most part made of steel at the fire box end, and of wrought iron at the smoke box end, though ferules of malleable cast iron have in some cases been used with advantage: malleable cast iron ferules are almost as easily expanded when hammered cold upon a mandrel, as the common wrought iron ones are at a working heat. Spring steel, rolled with a feather edge, to facilitate its conversion into ferules, is supplied by some of the steel-makers of Sheffield, and it appears expedient to make use of steel thus prepared when steel ferules are employed. In cases where ferules are not employed, it may be advisable to set out the tube behind the tube plate by means of an expanding mandrel. There are various forms of this instrument. One form is that known as Prosser's expanding mandrel, in which there are six or eight segments, which are forced out by means of a hexagonal or octagonal wedge, which is forced forward by a screw. When the wedge is withdrawn, the segments collapse sufficiently to enable them to enter the tube, and there is an annular protuberance on the exterior circle of the segments, which protuberance, when the mandrel is put into the tube, just comes behind the inner edge of the tube plate. When the wedge is tightened up by the screw, the protuberance on the exterior of the segments composing the mandrel causes a corresponding bulge to take place in the tube, at the back of the tube plate, and the tube is thereby brought into more intimate contact with the tube plate than would otherwise be the case.

There is a steel ring indented into the segments of Prosser's mandrel, to contract the segments when the central wedge is withdrawn. A more convenient form of the instrument, however, is obtained by placing the segments in a circular box, with one end projecting; and supporting each segment in the box by a tenon, which fits into a mortise in the cylindrical box. To expand the segments, a round tapered piece of steel, like a drift, is forced into a central hole, round which the segments are arranged. A piece of steel tube, also slit up to enable a central drift to expand it, answers very well; but the thickness of that part of the tube in which there requires to be spring enough to let the mandrel expand, requires to be sufficiently reduced to prevent the pieces from cracking when the central drift is driven in by a hammer. The drift is better when made with a globular head, so that it may be struck back by the hammer, as well as be driven in. An expanding mandrel, with a central drift, is more rapid in its operation than when the expansion is produced by means of a screw.

421. _Q._--Will you explain the means that are adopted to regulate the admission of steam to the cylinders?

_A._--In locomotives, the admission of the steam from the boiler to the cylinders is regulated by a valve called the regulator, which is generally placed immediately above the internal fire box, and is connected with two copper pipes;--one conducting steam from the highest point of the dome down to it, and the other conducting the steam that has pa.s.sed through it along the boiler to the upper part of the smoke box. Regulators may be divided into two sorts, viz., those with, sliding valves and steam ports, and those with conical valves and seats, of which the latter kind are the best. The former kind have for the most part consisted of a circular valve and face, with radial apertures, the valve resembling the outstretched wings of a b.u.t.terfly, and being made to revolve on its central pivot by connecting links between its outer edges, or by its central spindle. In some of Stephenson's engines the regulator consists of a slide valve covering a port on the top of the valve chests. A rod pa.s.ses from this valve through the smoke box below the boiler, and by means of a lever parallel to the starting lever, is brought up to the engineer's reach. c.o.c.ks were at first used as regulators, but were given up, as they were found liable to stick fast. A gridiron slide valve has been used by Stephenson, which consists of a perforated square moving upon a face with an equal number of holes. This plan of a valve gives, with a small movement, a large area of opening. In Bury's engines a sort of conical plug is used, which is withdrawn by turning the handle in front of the fire box: a spiral grove of a very large pitch is made in the valve spindle, in which fits a pin fixed to the boiler, and by turning the spindle an end motion is given to it, which either shuts or opens the steam pa.s.sage according to the direction in which it is turned. The best regulator would probably be a valve of the equilibrium description, such as is used in the Cornish engine: there would be no friction in such a regulator, and it could be opened or shut with a small amount of force. Such valves, indeed, are now sometimes employed for regulators in locomotives.

CHAPTER VIII.

CONSTRUCTIVE DETAILS OF ENGINES.

PUMPING ENGINES.

422. _Q._--Will you explain the course of procedure in the erection of a pumping engine, such as Boulton and Watt introduced into Cornwall?

_A._--The best instructions on this subject are those of Mr. Watt himself, which are as follows:--Having fixed on the proper situation of the pump in the pit, from its centre measure out the distance to the centre of the cylinder, from which set off all the other dimensions of the house, including the thickness of the walls, and dig out the whole of the included ground to the depth of the bottom of the cellar, so that the bottom of the cylinder may stand on a level with the natural ground of the place, or lower, if convenient, for the less the height of the house above the ground, the firmer it will be. The foundations of the walls must be laid at least two feet lower than the bottom of the cellar, unless the foundation be firm rock; and care must be taken to leave a small drain into the pit quite through the lowest part of the foundation of the lever wall, to let off any water that may be spilt in the engine house, or may naturally come into the cellar. If the foundation at that depth does not prove good, you must either go down to a better if in your reach, or make it good by a platform of wood or piles, or both.

423. _Q._--These directions refer to the foundations?

_A._.--Yes; but I will now proceed to the other parts. Within the house, low walls must be built to carry the cylinder beams, so as to leave sufficient room to come at the holding down bolts, and the ends of these beams must also be lodged in the wall The lever wall must be built in the firmest manner, and run solid, course by course, with thin lime mortar, care being taken that the lime has not been long slaked. If the house be built of stone, let the stones be large and long, and let many headers be laid through the wall: it should also be a rule, that every stone be laid on the broadest bed it has, and never set on its edge. A course or two above the lintel of the door that leads to the condenser, build into the wall two parallel flat thin bars of iron equally distant from each other, and from the outside and inside of the wall, and reaching the whole breadth of the lever wall. About a foot higher in the wall, lay at every four feet of the breadth of the front, other bars of the same kind at right angles to the former course, and reaching quite through the thickness of the wall; and at each front corner lay a long bar in the middle of the side walls, and reaching quite through the front wall; if these bars are 10 feet or 12 feet long it will be sufficient. When the house is built up nearly to the bottom of the opening under the great beam another double course of bars is to be built in, as has been directed. At the level of the upper cylinder beams, holes must be left in the walls for their ends, with room to move them laterally, so that the cylinder may be got in; and smaller holes must be left quite through the walls for the introduction of iron bars, which being firmly fastened to the cylinder beams at one end, and screwed at the other or outer end, will serve, by their going through both the front and back walls, to bind the house more firmly together. The spring beams or iron bars fastened to them must reach quite through the back wall, and be keyed or screwed up tight; and they must be firmly fastened to the lever wall on each side, either by iron bars, firm pieces of wood, or long strong stones, reaching far back into the wall. They must also be bedded solidly, and the residue of the opening must be built up in the firmest manner.

424. _Q._--If there be a deficiency of water for the purpose of condensation, what course should be pursued?

_A._--If there be no water in the neighborhood that can be employed for the purpose of condensation, it will be necessary to make a pond, dug in the earth, for the reception of the water delivered by the air pump, to the end that it may be cooled and used again for the engine. The pond may be three or four feet deep, and lined with turf, puddled, or otherwise made water tight. Throwing up the water into the air in the form of a jet to cool it, has been found detrimental; as the water is then charged with air which vitiates the vacuum.

425. _Q._--How is the piston of a pumping engine packed?

_A._--To pack the piston, take sixty common-sized white or untarred rope-yarns, and with them plait a gasket or flat rope as close and firm as possible, tapering for eighteen inches at each end, and long enough to go round the piston, and overlapped for that length; coil this rope the thin way as hard as possible, and beat it with a sledge hammer until its breadth answers the place; put it in and beat it down with a wooden drift and a hand mallet, pour some melted tallow all around, then pack in a layer of white oak.u.m half an inch thick, so that the whole packing may have the depth of five to six inches, depending on the size of the engine; finally, screw down the junk ring. The packing should be beat solid, but not too hard, otherwise it will create so great a friction as to prevent the easy going of the engine. Abundance of tallow should be allowed, especially at first; the quant.i.ty required will be less as the cylinder grows smooth. In some of the more modern pumping engines, the piston is provided with metallic packing, consisting for the most part of a single ring with a tongue piece to break the joint, and packed behind with hemp. The upper edge of the metallic ring is sharpened away from the inside so as to permit more conveniently the application of hemp packing behind it; and the junk ring is made much the same as if no metallic packing were employed.