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immediate effect of stopping ebullition is to lower the water level Sudden variations of water level are sometimes caused by the drain pipe from the superheater having been fitted below the water level of the boiler and not being closed on occasions when this was necessary; and more recently similar effects have
A been produced by negligence in not closing the feed-pipe suctions from the boiler bottoms, which are only intended to be open while in use, when steam is being raised.
In marine boilers shortness of water is not necessarily as dangerous as in land boilers, because the furnace crowns are many feet below
B the ordinary water level, and the upper heating surfaces are not exposed to a very high temperature; besides, in rough weather they are being constantly moistened by water splashing on them. But the case is different in calm weather, if the
F steamer has a strong list; for then, even if there is a reasonable height of water in the glass, one of the wing combustion chamber tops may be laid dry for a long period and get overheated. Furnace crown plates would grow red-hot in about five minutes after being laid bare, while combustion chamber top plates would
D require twice or four times as much time. Obviously this danger is intensified if the gauge glass is fitted to the boiler side, and even the trim of a vessel may expose parts of the heating surface to this influence if the boiler is a very long one. In such cases it is advisable to fit fluid levels in the engine room, which will indicate how much water ought to be in the glass. They are very simple instruments, and can easily be made with the help of a few glass tubes (see figs. 32, 33).
Fusible Plugs.- To guard against the troubles arising from shortness of water, some engineers fit fusible
Fig. 31 plugs to the combustion chamber tops, and in some countries this practice is compulsory; but there is
3 4 5 6
this serious objection, that when a plug is once fused the boiler cannot be used again for some hours. On a ship with only one
boiler such a mishap might lead to her total loss, and almost under any circumstances which are likely to occur at sea, it would be
better to have the use of Fig. 32
boilers which can be worked
even if only at a very low pressure, than not to be able to use them at all till a new plug has been inserted. (See p. 19.)
Low Water Alarms.—The majority of land boilers are fitted with safety valves, which blow off when the minimum permissible water level is reached; they are inapplicable to marine boilers, and if modified to suit the altered circumstances would, though to a less degree, be open to the objections urged against fusible plugs. Water alarms, especially such as are arranged to ring an electric bell, situated if necessary in the engineer's berth, may in some steamers be found useful. These fittings seem to be reliable, but it would be well if they could be fitted with a cock or valve so as to produce artificially the conditions of low water in the boiler, and thus ascertain whether they are in working condition. In this form they would be a convenient means of summoning the chief engineer in case of emergency. Water alarms in which a float actuates a whistle work well, but would not combine the above advantages.
Automatic Feeding Arrangements are as yet only being fitted to water-tube boilers, and are hardly likely to come into use in the boilers under consideration.
Safety Valves.-Another very important boiler mounting is the safety valve. Its action and design are so simple that little need be said about it, but being practically always out of use, there is a great danger that it will be found out of working order when it should act, and to ensure that this shall not happen, all the working parts should be lined with brass, and should be loose fits, and their working condition should often be tested by lifting the valves from their seats or turning them round. As it has sometimes happened that the valve seats have lifted with the valves, thus preventing any escape of steam, they should always be securely bolted or pinned down.
As long as low pressures were customary, weighted safety valves were common and worked satisfactorily; but now, since high pressures are almost universal, dead weights have been replaced by springs, for otherwise the working pressure would have to be reduced about 30 % in rough weather, on account of weights being thrown up under the combined action of the steam and the pitching of the vessel. (Wilson, ‘Marine E.,' 1892.)
Formerly spring loaded valves were very inefficient, allowing the steam pressure to accumulate considerably, unless the springs were made exceptionally long. This difficulty has been more than overcome by utilising the reaction of the escaping steam, and, instead of allowing the pressure to rise after steam has commenced to blow off, there are many valves which will not close until the pressure has been seriously reduced.
The principles on which such safety valves can be constructed are shown in figs. 34, 35, 36. In the first of these the valve diameter is d, but when the steam is escaping it acts on the diameter D, and keeps the valve well open until the pressure has dropped considerably.
In fig. 35 the escaping steam reacts on the surrounding lip, while in fig. 36 it reacts, as shown, by striking against the valve top.
The object of these various designs is that the valve should open fully when it does so at all, that it should not be closed till the pressure has dropped a pound or two below the working pressure, and that this should be effected with as short a spring as possible. Obviously all these points are
FIG. 36 affected by the angle of the seat and the curvature and size of the lip; but, besides that, the diameter and length of the waste steam pipe are influential factors, and it often happens that safety valves which worked satisfactorily when tested at the works, will not do so when fitted on board. The chattering noise which they make is evidently due to the length of the pipe as well as to the weight of the valve and the elasticity of the spring. Every time that the valve comes down and stops the flow of steam, a partial vacuum is formed by the uprushing column, and the valve is lifted up again. The possible remedies are, to have an opening at the lower end of the waste pipe, through which air will flow whenever a vacuum is formed, or to fit frictional appliances in the lower part of the pipe, such as a series of diaphragms, wire brushes, or a box of pebbles.
Funnel Vibrations. A related phenomenon is the buzzing noise made by boilers when all the tubes are clean and the fires are alight. The action is evidently the same as that which takes place in an organ tube, only the periods of vibration are slower ; but they evidently depend on the height of the funnel.
Alternate Heating and Cooling of Plates.- Very exhaustive and interesting experiments on this subject were made by E. Wehrenfennig (Organ,' 1884, vol. xxi. p. 216, &c.) He found that heating various metals and cooling them had certain effects, which were reproduced on repeating the experiment, and which were intensified the longer the heating lasted and the higher the temperatures. His limits were boiling water and red heat. (See p. 35.)
His results are that steel and iron bars and plates shorten, but grow thicker if heated and cooled, about 20 to 1 % for one red heat.
Cast-iron and copper bars and plates lengthen, but grow thinner if heated, say, 20 % and } % respectively. Col. H. Clerk (*Proc.,' 1863, vol. xii. p. 452) and H. Caron (* Comp. Rend.,' 1863, vol. Ivi. p. 828) arrive at conclusions opposed to the above, but in their cases the cooling was effected suddenly.
E. Wehrenfennig also refers to experiences in connection with locomotives—for instance, that certain fittings grow tighter after a short use if exposed to heat; that iron nuts cannot be unscrewed from copper stays after heating, but brass nuts can; that blisters crack on the fire side, being exposed to greater changes of temperature; that rivet holes of seams in iron or steel furnaces crack, in copper plates they do not do so. Possibly his most interesting fact
is that a land boiler contracted so much in length that this led to various troubles about the fittings.
The same thing evidently happens with the through combustion chambers of double-ended boilers, and cases have repeatedly occurred where, on removing the rivets in the circumferential furnace seams, the holes were found to be somewhat blind, although
drilled in place, and then, having been FIG. 37
rimered fair and re-riveted, they soon
grew as blind as before. Even flanged furnace saddle seams show the same wandering of rivet holes, but, for fear that this statement might be used to shield bad workmanship, it is as well to point out that in all such cases
there is a regularity in the blindness of the holes which does not exist if the holes were drilled carelessly out of place.
It has been suggested that the cracking of rivet holes is caused by the carbonising of the iron and steel, due to their contact with
fuel or hydrocarbons of the gases; but as an addition of carbon increases the volume of the metal, it is difficult to see how this can set up any tension stresses. The four accompanying sketches show cracks as they occur under palm stays (fig. 37), in the flat parts of combustion chamber backs when covered with scale (fig. 39), in combustion chamber back plate seams (fig. 38), and in furnace saddle seams (fig. 40).
Deterioration of Materials. That this cracking of rivet holes is due more to the heat than to the impinging action of the flame, is shown by the fact that the joint of the tube plate with the furnace saddle does not give trouble if kept out of the fire, even though, as in