Preservation of Boilers.-When boilers are not required for use they are best preserved by keeping them quite full of water at a slight pressure, say 5 lbs. per square inch; they should be pumped up directly after steaming, so that any air that may be contained in the feed-water will become heated and will escape from the boiler through the air cock at the top of the steam collectors. In all cases the water in the boilers should be maintained in a slightly alkaline condition, and lime in solution should, therefore, be supplied with the feed-water when filling up the boilers in the same manner as explained previously in this chapter.

If it is desired to keep the boilers full of water and out of use for a considerable time, it should be ascertained at least twice a week that the water has not leaked away from the boilers; the air cocks should then again be opened, and the feed pumps applied until it is certain that the boilers are again quite full.

If it is not practicable to keep the boilers quite full of water, for instance if they are required to be kept empty for repairs or examinations, they should be thoroughly dried out as soon as possible after being opened and cleaned.

Boilers.-Boilers should never be allowed to remain for any length of time only partly full of water; such treatment will leave a portion of the tubes exposed to damp and corrosion in their interiors. Airing stoves, placed in the furnaces underneath the generator tubes, should be used for the purpose of drying the boilers and also for keeping them dry afterwards.

Packing. A special kind of packing, composed of granular white metal, mixed with mineral grease and plumbago, is used for the glands of the spindles of the automatic feed valves, and also for those of the steam stop valves. It is known as "Belleville anti-friction packing," and is supplied in two varieties-viz., fine and coarse-either in tins, or enclosed in a skin of woven cotton, in which latter case it is known as sausage packing" from its shape. The sausage packing is supplied in various sizes and diameters to suit any glands, and is composed either of the fine or the coarse varieties, or of a mixture of them.

Vulcanite Rings. Special vulcanite rings of various sizes are also supplied with this packing; the object of the vulcanite ring being to fit tightly around the rods at the top and bottom of the stuffing-boxes of the glands, and so prevent the particles of Belleville packing from being squeezed through. In order to ensure successful working, too much care cannot be given to the accuracy of fit of these vulcanite rings around rods or spindles. In the piston-rod and slide-rod glands of small engines and in steam stop-valve glands, if vulcanite rings are not available, a

tightly-fitting turn of asbestos packing at the top and bottom of the glands is an efficient substitute for the vulcanite rings, but the latter appear to be absolutely necessary in the stuffing-boxes of the automatic feed valves, also in those of the reducing valve spindles of the main engines.

Belleville Packing Substitutes.-Of late years a substitute for Belleville anti-friction packing has been manufactured by the Admiralty Authorities and by Naval Engineer Officers themselves, which gives as good results as the Belleville packing.

This packing is manufactured as follows:-Lead is melted and poured through a wire-gauze sieve of 40 mesh held 1 inch or so above a pan of water. The globules so formed are sifted to separate those above inch in diameter, the larger of the globules left are separated by rocking in a trough containing water, and are then mixed half and half with those above inch in diameter. A greasy paste is made, consisting of I part black lead to 1 part of mineral oil by bulk, and this paste is mixed with the lead globules in the proportion of 10 parts of metal to 1 part of greasy paste. The packing so formed is then filled into convenient sized cylindrical casings made of canvas.

Gauge Glasses.-These fittings gave considerable trouble in the earlier types of Belleville boilers, as the glass tubes used broke frequently, until an improvement was made in the method of packing them, which has practically overcome the original difficulties. They are now packed with specially made hexagonal washers composed of a mixture of asbestos fibre, mineral grease, and black lead. A closely-fitting india-rubber ring is enclosed at each end of the gauge-glass tube between two of these hexagonal washers, and the whole is screwed up by the nuts at the top and bottom of the gauge mounting, the nuts being just "hand taut" when screwed down to their final position.

Gauges made of talc were tried as a substitute for the glass tubes, but the talc became laminated and opaque, and was discarded. It is now the usual practice to fit one of the water gauges with a glass tube, and the other water gauge on each boiler with a window-frame mounting, and this arrangement has proved satisfactory.

With the view of affording assistance to officers concerned in the management and working of Babcock & Wilcox, Niclausse, and Yarrow boilers, my Lords Commissioners of the Admiralty are pleased to direct the following instructions to be issued:

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Babcock & Wilcox Boilers*—Raising Steam.—The boilers should always be filled with clean fresh water, or distilled water, and steam should be raised slowly when the brickwork is new; at other times, if the service require it, steam can be raised as quickly as the fires can be got to work.

Whilst the boilers are being heated, attention should be devoted to all the boiler fittings and feed arrangements, to see that they are in good working order, and special care should be exercised in tightening up again, with the special spanner provided for the purpose, any handhole fittings that may have been taken off for examination or cleaning.

After steam is raised, the boilers should be connected up with care, and the main stop valves opened very slowly; the opening of the stop valves to be just to that extent which shall give a small amount of wire drawing to the steam, but not to contract the area of opening sufficiently to cause any considerable loss of pressure between the boiler and the engines.

Firing. To maintain a steady steam pressure and ensure economical working, it is necessary to fire at equal intervals of time, the method usually adopted being to number the fire-doors as per sketch and to fire through each opening in order of numbers at equal intervals.

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Fig. 110.

As a rough guide of what should be done, it has been found that for maximum continuous steaming with good Welsh coal, from three to four good shovelsful at a time should be thrown through one fire-door opening on each boiler every two and a half minutes. The number "One" fires of all the boilers to be fired in rapid succession one after the other, one door only to be open at a time-two and a half minutes afterwards the number "Two" fires in all the boilers to be fired in rapid succession; two and a half minutes after that, the number "Three " openings,

See the Admiralty circular letter of 25th July, 1904.

and so on. This may be conveniently arranged with a clock in each firing-room-each clock set a little in advance of the other, so as to have as few fire-doors as possible open at one time.

If a higher rate of steaming is required, it is preferable to decrease the interval between firing, rather than add largely to the quantity of coal put on at a time.

For ordinary service the number of boilers in use should be arranged for at a rate of combustion of from about 15 to 18 lbs. of coal per square foot of fire-grate per hour, with fires from 4 to 5 inches thick, and no air-pressure in the stokehold. With an air-pressure in the stokehold of half-an-inch of water, the fires should be from 6 to 7 inches thick.

The fires should be kept free of the dead-plates, so that the furnace door baffles shall not become over-heated. The amount of opening of the ashpit doors should be kept as small as possible, and great attention should be paid to this point, as the best results are obtained with an excess draught checked at the ashpit.

The coal must be well broken up and thrown on the fires quickly, the largest lump being not greater than a ball of 4 inches diameter. The furnace doors should not be left open longer than is necessary, so as to prevent too much cold air getting into the furnaces. Two men should be employed for the operation of firing; one to put the coal on, and the other to open and close the furnace door in the intervals between separate shovelsful. When a furnace door is opened for firing, the stoker should see at a glance where coal is required, for the flame and smoke caused by the first shovelful, as soon as it is thrown in, prevents the exact condition of the fire from being afterwards seen. A sharp look-out should be kept for any thinning of the fires at the sides and between the furnace doors, and, in order to prevent this, the fire near the brickwork should be kept rather thicker than that of the remaining area. stoker should learn to distribute the coal over the grate so uniformly with the shovel that it will be unnecessary to level the surface with the rake or slice. A rake has only to be used when a fire has become too thick on one part of the grate and too thin on another.

The

Under natural-draught conditions of steaming, the same amount of coal should be burnt in each furnace, and, as it frequently happens that some furnaces have a better draught than others, the draught plates should be regulated accordingly. In order to maintain fires of uniform thickness under forced draught, fires which get greater draught should be supplied with more coal in order to keep them of even thickness and free

from holes; the maintenance of a steady output of steam under forced draught should be effected by the speed of the fans and not by the ashpit draught plates.

The ashes in the ashpits should be removed as often as convenient, as they interfere with the draught and injure the fire-bars.

It is important for the most effective combustion and economy that the fires should be kept light and level, thus enabling the combustion to take place in the fire-box itself, preventing as far as possible the sooting-up of the tubes and flaming in the uptakes and funnels.

Cleaning the Fires.-When steaming at powers exceeding 60 per cent. of the full power, only half the grate surface of a boiler should be cleaned at a time, the fires of the other boilers being forced more heavily if necessary. The fire on half a grate should be burnt down, clinkered, and re-charged. When the fire on this half has again become bright, the other half should be cleaned in the same way.

For the development of powers below 60 per cent. of the full power, the number of boilers in use should be sufficient, with an average rate of combustion of about 20 lbs. of coal per square foot of fire-grate, to allow of one boiler at a time being burnt down for cleaning purposes.

At all rates of combustion the fire-bars must be kept free from clinker by the frequent use of the slice in the intervals between the cleanings of the fires.

Feeding. The feeding must be done uniformly, and not irregularly.

When getting under way, commence by opening all the feed check valves a little and to the same extent, also try through all the water gauges.

Should it so happen that the feed-water ceases to flow for a short while owing to the pump ceasing to feed, the combustion should at once be checked by closing the ashpit doors and immediate steps taken to put on the auxiliary supply.

Steps should also be taken to ascertain by working the float lever that the regulator is not at fault.

Water Level. The water level should be kept between 1 and glass when under way; when the boiler has the ashpit doors shut, and is doing no work, about 2 inches in the glass is sufficient. The handle of the automatic feed regulator should be moved about once a day so as to ascertain whether the float is working freely, and, at least every three months, the float lever and piston of the automatic feed apparatus should be taken out, examined, and cleaned.