A Catechism of the Steam Engine · Chapter 13Steam Room and Priming
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290. Q.--What is the capacity of steam room allowed in boilers per horse power?
A.--The capacity of steam room allowed by Boulton and Watt in their land wagon boilers is 8-3/4 cubic feet per horse power in the two horse power boiler, and 5-3/4 cubic feet in the 20 horse power boiler; and in the larger class of boilers, such as those suitable for 30 and 45 horse power engines, the capacity of the steam room does not fall below this amount, and, indeed, is nearer 6 than 5-3/4 cubic feet per horse power. The content of water is 18-1/2 cubic feet per horse power in the two horse power boiler, and 15 cubic feet per horse power in the 20 horse power boiler.
291. Q.--Is this the proportion Boulton and Watt allow in their marine boilers?
A.--Boulton and Watt in their early steam vessels were in the habit of allowing for the capacity of the steam, space in marine boilers 16 times the content of the cylinder; but as there were two cylinders, this was equivalent to 8 times the content of both cylinders, which is the proportion commonly followed in land engines, and which agrees very nearly with the proportion of between 5 and 6 cubic feet of steam room per horse power already referred to. Taking for example an engine with 23 inches diameter of cylinder and 4 feet stroke, which will be 18.4 horse power--the area of the cylinder will be 415.476 square inches, which, multiplied by 48, the number of inches in the stroke, will give 19942.848 for the capacity of the cylinder in cubic inches; 8 times this is 159542.784 cubic inches, or 92.3 cubic feet; 92.3 divided by 18.4 is rather more than 5 cubic feet per horse power.
292. Q.--Is the production of the steam in the boiler uniform throughout the stroke of the engine?
A.--It varies with the slight variations in the pressure within the boiler throughout the stroke. Usually the larger part of the steam is produced during the first part of the stroke of the engine, for there is then the largest demand for steam, as the steam being commonly cut off somewhat before the end of the stroke, the pressure rises somewhat in the boiler during that period, and little steam is then produced. There is less necessity that the steam space should be large when the flow of steam from the boiler is very uniform, as it will be where there are two engines attached to the boiler at right angles with one another, or where the engines work at a great speed, as in the case of locomotive engines. A high steam chest too, by rendering boiling over into the steam pipes, or priming as it is called, more difficult, obviates the necessity for so large a steam space; as does also a perforated steam pipe stretching through the length of the boiler, so as not to take the steam from one place. The use of steam of a high pressure, worked expansively, has the same operation; so that in modern marine boilers, of the tubular construction, where the whole or most of these modifying circumstances exist, there is no necessity for so large a proportion of steam room as 5 or 6 cubic feet per nominal horse power, and about one, 1-1/2, or 2 cubic feet of steam room per cubic foot of water evaporated, more nearly represents the general practice.
293. Q.--Is this the proportion of steam room adopted in locomotive boilers?
A.--No; in locomotive boilers the proportion of steam room per cubic foot of water evaporated is considerably less even than this. It does not usually exceed 1/5 of a cubic foot per cubic foot of water evaporated; and with clean water, with a steam dome a few feet high set on the barrel of the boiler, or with a perforated pipe stretching from end to end of the barrel, and with the steam room divided about equally between the barrel and the fire box, very little priming is found to occur even with this small proportion of total steam room. About 3/4 the depth of the barrel is usually filled with water, and 1/4 with steam.
294. Q.--What is priming?
A.--Priming is a violent agitation of the water within the boiler, in consequence of which a large quantity of water passes off with the steam in the shape of froth or spray. Such a result is injurious, both as regards the efficacy of the engine, and the safety of the engine and boiler; for the large volume of hot water carried by the steam into the condenser impairs the vacuum, and throws a great load upon the air pump, which diminishes the speed and available power of the engine; and the existence of water within the cylinder, unless there be safety valves upon the cylinder to permit its escape, will very probably cause some part of the machinery to break, by suddenly arresting the motion of the piston when it meets the surface of the water,--the slide valve being closed to the condenser before the termination of the stroke, in all engines with lap upon the valves, so that the water within the cylinder is prevented from escaping in that direction. At the same time the boiler is emptied of its water too rapidly for the feed pump to be able to maintain the supply, and the flues are in danger of being burnt from a deficiency of water above them.
295. Q.--What are the causes of priming?
A.--The causes of priming are an insufficient amount of steam room, an inadequate area of water level, an insufficient width between the flues or tubes for the ascent of the steam and the descent of water to supply the vacuity the steam occasions, and the use of dirty water in the boiler. New boilers prime more than old boilers, and steamers entering rivers from the sea are more addicted to priming than if sea or river water had alone been used in the boilers--probably from the boiling point of salt water being higher than that of fresh, whereby the salt water acts like so much molten metal in raising the fresh water into steam. Opening the safety valve suddenly may make a boiler prime, and if the safety valve be situated near the mouth of the steam pipe, the spray or foam thus created may be mingled with the steam passing into the engine, and materially diminish its effective power; but if the safety valve be situated at a distance from the mouth of the steam pipe, the quantity of foam or spray passing into the engine may be diminished by opening the safety valve; and in locomotives, therefore, it is found beneficial to have a safety valve on the barrel of the boiler at a point remote from the steam chest, by partially opening which, any priming in that part of the boiler adjacent to the steam chest is checked, and a purer steam than before pusses to the engine.
296. Q.--What is the proper remedy for priming?
A.--When a boiler primes, the engineer generally closes the throttle valve partially, turns off the injection water, and opens the furnace doors, whereby the generation of steam is checked, and a less violent ebullition in the boiler suffices. Where the priming arises from an insufficient amount of steam room, it may be mitigated by putting a higher pressure upon the boiler and working more expansively, or by the interposition of a perforated plate between the boiler and the steam chest, which breaks the ascending water and liberates the steam. In some cases, however, it may be necessary to set a second steam chest on the top of the existing one, and it will be preferable to establish a communication with this new chamber by means of a number of small holes, bored through the iron plate of the boiler, rather than by a single large orifice. Where priming arises from the existence of dirty water in the boiler, the evil may be remedied by the use of collecting vessels, or by blowing off largely from the surface; and where it arises from an insufficient area of water level, or an insufficient width between the flues for the free ascent of the steam and the descent of the superincumbent water, the evil may be abated by the addition of circulating pipes in some part of the boiler, which will allow the water to descend freely to the place from whence the steam rises, the width of the water spaces being virtually increased by restricting their function to the transmission of a current of steam and water to the surface. It is desirable to arrange the heating surface in such a way that the feed water entering the boiler at its lowest point is heated gradually as it ascends, until toward the superior part of the flues it is raised gradually into steam; but in all cases there will be currents in the boiler for which it is proper to provide. The steam pipe proceeding to the engine should obviously be attached to the highest point of the steam chest, in boilers of every construction.
297. Q.--Having now stated the proportions proper to be adopted for evaporating any given quantity of water in steam boilers, will you proceed to show how you would proportion a boiler to do a given amount of work? say a locomotive boiler which will propel a train of 100 tons weight at a speed of 50 miles an hour.
A.--According to experiments on the resistance of railway trains at various rates of speed, made by Mr. Gooch, of the Great Western Railway, it appears that a train weighing, with locomotive, tender, and carriages, about 100 tons, experiences, at a speed of 50 miles an hour, a resistance of about 3,000 lbs., or about 30 lbs. per ton; which resistance includes the resistance of the engine as well as that of the train. This, therefore, is the force which must be imparted at the circumference of the driving wheels, except that small part intercepted by the engine itself, and the force exerted by the pistons must be greater than that at the circumference of the driving wheel, in the proportion of their slower motion, or in the proportion of the circumference of the driving wheel to the length of a double stroke of the engine. If the diameter of the driving wheel be 5-1/2 feet, its circumference will be 17.278 feet, and if the length of the stroke be 18 inches, the length of a double stroke will be 3 feet. The pressure on the pistons must therefore be greater than the traction at the circumference of the driving wheel, in the proportion of 17.278 to 3, or, in other words, the mean pressure on the pistons must be 17,278 lbs.; and the area of cylinders, and pressure of steam, must be such as to produce conjointly this total pressure. It thus becomes easy to tell the volume and pressure of steam required, which steam in its turn represents its equivalent of water which is to be evaporated from the boiler, and the boiler must be so proportioned, by the rules already given, as to evaporate this water freely. In the case of a steam vessel, the mode of procedure is the same, and when the resistance and speed are known, it is easy to tell the equivalent value of steam.