Modern Machine-Shop Practice Part 262

Outside the gauge casing the steam pipe is bent into a loop forming an inverted syphon which is to contain the water.

At the lowest point in the bend of the syphon a small c.o.c.k is inserted, which lets the water out of the leg of the syphon nearest to the boiler, because water in that leg would, from its weight, cause the gauge to show a pressure higher than that in the boiler.

The pressure shown by a steam gauge is that above atmosphere,[69] and not that above vacuum.

[69] See page 367, Vol. II., for remarks on total pressure and pressure by gauge.

The stop valve of a marine boiler is a valve that is opened to let the steam into the main steam pipe.

A blow off c.o.c.k is a c.o.c.k employed to blow off, or let all, or a part of, the water out of a boiler. There are generally two, one on the bottom of the boiler, and the other at the ship's side, so that if the pipe was to break or get damaged, the c.o.c.k at the vessel's side can be closed to keep the sea water out, while that on the boiler may be closed to keep the water and steam in the boiler. These two ends cannot obviously be obtained if one blow off c.o.c.k only was used.

Blow off c.o.c.ks are opened and closed by a spanner or key that is removable from the c.o.c.k, and to prevent the possibility of taking off the spanner or key, before the blow off c.o.c.k is closed, a spanner guard is employed.

A spanner guard is a cap having a lug or tongue, which projects into the hole in the spanner guard, through which the spanner or key must pa.s.s before it can fit on the head of the blow off c.o.c.k, and the key or spanner has a corresponding recess, so that the spanner or key can only be put on or taken off when the c.o.c.k is closed.

Blowing off a boiler is emptying it entirely, as for examining the whole interior of the boiler.

Blowing down a boiler is letting out a portion of the water, so as to carry off the loose scale, mud, or sludge that may acc.u.mulate on the bottom of the boiler. The mud or sludge would form into scale if allowed to remain.

A sc.u.m c.o.c.k is a c.o.c.k employed to blow off a portion of the surface water in a boiler, and thus remove the sc.u.m, salt, and impurities which float or are thrown up to the surface.

Two sc.u.m c.o.c.ks are employed, one on the side of the boiler, and one on the side of the ship. These two c.o.c.ks are connected by a pipe. That on the boiler is placed a little below the working level, which is supposed to be (and is kept as nearly as possible) about 9 inches above the top row of tubes.

Sluice valves are doors sliding, water tight, in ways at the entrance to the bulkheads on both sides of the ship. They should be worked from above, in order that they may be shut when the depth of water in the bulk heads might prevent them from being worked from below. These valves should be operated occasionally to ensure that they slide easily and are in working order.

Scale in marine boilers using salt water is composed of sulphate of lime. It is most objectionable on the furnace tops, on the sides and tops of the combustion chamber, on the tubes and on the tube plates. It may be prevented to some extent from forming by a rapid circulation of the water in the boiler, by blowing down the boiler through the sc.u.m c.o.c.ks, by the suspension in the boiler of zinc plates in contact with iron ones, by impregnating the water with chemical antidotes, which maintain the impurities in the form of mud or sludge, and by purifying the feed water. If surface condensers are used, scaling is obviously diminished by feeding as little salt water as possible, which may be done by not getting up a steam pressure high enough to cause the safety valve to blow off, and by preserving the water from the exhausts of the donkey or other engines about the ship.

A thin coating of scale, as say 1/32 inch thick, may serve as a protection against the chemical action of water that would act to corrode the surfaces, as in the case of harbors receiving the waste waters from chemical works or other impure waters. A thick coating of scale causes the plates to burn on the side receiving the furnace heat, and causes blisters to rise, while at the same time it decreases the value of the heating surface.

Scale on the tubes causes them to expand more, and therefore leak in the tube sheets.

This extra expansion sometimes breaks away the scale at the neck of the tube in the tube sheet and gives access to the water there, and the chemical action of water will in some cases cause the tube to be eaten through close to the tube plate.

Scale is removed mechanically by chisels, sc.r.a.pers and chipping hammers, which are applied to all the surfaces that can be got at from the inside of the boiler (the man hole affording access to the boiler). After the scale has thus as far as possible been removed, it is washed out of the boiler. The efficiency with which scale may be removed from the tube sheets and tubes depends, to a great extent, upon the facilities the arrangement of the rows of tubes affords in giving access to the scaling chisels.

The salinometer. Salt water is heavier than fresh water, hence the amount of saltiness of water may be known from its density or weight. A salinometer is an instrument that determines from the density of the water the amount of salt contained in the water. It consists of a graduated stem at whose extremity is a weighted bulb which partially sinks the tube in the water; the depth to which the bulb sinks shows the density of the water.

The reading of a salinometer is taken at the water level, and is read on the tube, which is graduated as follows: The mark furthest from the bulb or highest up the stem is marked O, and if the zero line is level with the surface of the valve in which the salinometer floats, it indicates fresh water. If salt be added to the fresh water, the salinometer will rise in the water, and when the water contains 1 lb. of salt to 32 lbs.

of water (which is the average degree of saltiness of sea water), the line marked 1/32 on the salinometer tube will be level with the surface of the water. If the saltiness of the water be increased, the salinometer will rise in the water until, at 2 lbs. of salt to 32 lbs.

of water, a line (on the tube) marked 2/32 will be level with the surface of the water. The s.p.a.ce between the 1/32 and 2/32 is divided into halves and quarters.

As the density of the water varies with its temperature, therefore the readings on the salinometer must agree with some specific temperature, which is usually 200 Fahrenheit, and the reading of the salinometer is correct only when the water is at that temperature. If, however, the water varies a few degrees from the standard of temperature for which the salinometer is marked, a correction of the reading may be made by adding 1/8 of 1/32 for each 10 degrees, that the water is hotter, or subtracting the same for each 10 degrees that it is cooler than the temperature at which the salinometer is correct.

The density or specific gravity of ordinary sea water is 1.027 (that of distilled water being unity or 1), and it contains about 4 oz. of salt per imperial gallon.

Tallow is sometimes forced into a boiler fed with salt water to stop priming, by means of a syringe that is screwed into a tallow c.o.c.k provided upon the boiler below the water level. If the boiler is fed with fresh water, tallow is apt to cause priming.

Angle irons are used in boiler construction to be riveted to plates that require supporting or strengthening, or for gusset stays to be riveted to. f.l.a.n.g.ed plates are used in the construction of the furnaces, flame, boxes or combustion chambers, boiler ends and tube plates or tube sheets.

Division plates are fitted in some boilers to prevent the water from pa.s.sing from one side of the boiler to the other when the vessel rolls heavily. This prevents some of the tubes from being left uncovered by water, and thereby getting injured from undue heat.

These division plates are neither steam nor water tight, and stand fore and aft of the ship. Similar division plates are sometimes used, however, to prevent the tops of the combustion boxes from getting overheated from the motion of the ship leaving them uncovered with water, their location being subserved to this end and varying with the position of the boiler.

The superheater of a marine boiler is provided with a safety valve, and sometimes with a pressure gauge to enable the comparing of the steam pressure with that in the boiler, and should also be provided with a gauge gla.s.s, to show when heavy priming is going on.

The main stop valve is upon the superheater, as is also the blast pipe.

Priming is a lifting, into the steam s.p.a.ce of the boiler, of a part of the water, and may arise from heavy firing, from the safety valve blowing off, from too little steam s.p.a.ce, and from other causes.

Priming[70] often occurs when the boiler feed is changed from salt water to fresh water, or from fresh to salt water.

[70] See page 370 Vol. II., on priming.

A separator or interceptor is a device fitted to either the superheater or to the steam receiver, for separating entrained water from the steam.

It consists of a rectangular box or chamber with a part.i.tion plate extending from the top half down into the box.

The entering steam strikes the face of the part.i.tion plate against which the water collects, and from which it drops to the bottom of the box, while the steam pa.s.ses under the part.i.tion and out at the other side to the engine.

The draught of a boiler is caused by the heat expanding the air and lightening it, thus causing it to ascend. It can be checked by stopping the exit of heated air up the funnel by means of a damper, by checking the flow of cold air into the furnace, by closing the dampers, by opening the furnace doors and letting cold air in the furnaces above the fires.[71]

[71] See page 368, Vol. II.

A blast pipe is a small pipe leading from the superheater to the funnel, and provided with a stop c.o.c.k.

It is used for letting a jet of steam up the funnel to promote the draught.

Flame seen at the top of the funnel is caused by the combustion of gases that would have been consumed in the furnace had there been sufficient air or sufficient room for complete combustion. It may be caused in a variety of ways, as insufficient openings between the fire bars, too narrow a s.p.a.ce between the bridge wall and the boiler, or too deep a fire upon the bars. It is detrimental, because it obviously wastes fuel.

Dampers are used to regulate the draught in the furnace; they are fitted to the ash-pits or to the funnel, and should be fitted to both, because closing a damper in the funnel sets up a certain amount of pressure in the furnace by holding the heat, whereas dampers at the ash pit doors and none in the funnel lets the heat out and prevents cold air from getting in to promote combustion.

When there are no dampers the furnace doors are open instead, to check the draught; this is, however, highly injurious to the boilers.

The most rapid wasting of the plates of a marine boiler occurs alongside the fire bars, on the furnace tops, at the back of the flame box or combustion chamber, and in those plates generally that receive the most intense heat, and especially when they are heavily coated with scale and are not covered with water.

The scale that forms on the face of the tube sheet keeps the water away from contact with the plate, which, with an undue thickness of scale, will crack between the tube holes.

A tube that is split or that cannot be made steam tight by being re-mandrelled or expanded is plugged up at each end by means of either wooden or iron plugs. The best plan, however, is to use iron discs having a stepped diameter, so that one end will fit the bore of the tube, and the other will form a shoulder that will cover the end of the tube.

Each disc has a hole through its centre, so that a wrought iron rod or bolt may be pa.s.sed through the hole and receive a nut at each end.

Beneath the f.l.a.n.g.e of each disc, a grummet of spun yarn and white lead is placed, so as to make a steam tight joint when the nuts are screwed home. This stays the tube plates as well as stopping the leaky tube.

If wooden plugs are used, they are made a driving fit in the tube bore, and driven through until they have pa.s.sed the split, and a second wooden plug is driven tightly from the same end of the tube.

Black smoke is an evidence of incomplete or imperfect combustion, and may be, to a great extent, prevented by careful firing, as by feeding gradually and evenly, by the admission of the proper quant.i.ty of air, or by a jet of steam admitted above the dead plates.

The furnace bars are ordinarily of cast iron about 1-1/4 inches thick at the top, tapered towards the bottom, and with an air s.p.a.ce of from 1/2 to 3/4 inch between them.

They require less air s.p.a.ce for Welsh than for Newcastle coal, as the latter is the flaming or gaseous coal, and burns the fastest.