Modern Machine-Shop Practice Part 261

In the United States the grades of iron used for boilers are C H No. 1 S, or charcoal hammered No. 1 sh.e.l.l iron, for the sh.e.l.l, and C H No. 1 F, or charcoal No. 1 f.l.a.n.g.e iron, which is used for the furnaces and such parts as require flanging.

In both countries steel is also used for boilers, except for the tubes, for which it is not entirely reliable if very high pressures are to be used.

Both the iron and steel plates are tested for tensile strength and ductility.

The breaking strain is that which is sufficient to cause rupture, while the _proof strain_ is that which the metal is required to withstand with safety.

The safe working strain, or working pressure, W P, is the strain under which it is considered safe to work the boiler.

The strength of a boiler of a given diameter and thickness of plate varies according to the construction of the riveted seams or joints.

Boiler stays or braces are rods, ribs, or plates for supporting the weaker parts of the boiler. Thus the tube plates may be stayed by rods pa.s.sing through both plates and screwed into them, or nuts and washers may be used on the stays one on each side of each tube plate.

Gusset stays are iron plates which are riveted to [T] irons or in some cases to [L] irons, which are riveted on the surfaces to be stayed.

Stay tubes are thick tubes (usually about 3/16 inch thick), which screw into the tube sheets and are riveted over at the ends. A superior construction, however, is to provide nuts and washers to the ends of the stay tubes, one on each side of each tube plate.

Boiler stays are usually made of such diameters that when new they will sustain a tensile strain of not more than 5,000 lbs. per square inch of cross section, this being the rule of the Board of Trade.

Stays are sometimes screwed into the tube plates and then riveted over at the outside ends. A better method, however, is to let the ends of the stays receive a nut on each side of each tube plate.

Boiler tubes are secured in their tube plates by being _expanded_ in.

This may be done by driving in a taper steel mandrel, and then clinching them over, or by using a tube expander. There are two princ.i.p.al kinds of tube expanders, in one of which small rolls travel around the bore of the tube and expand it, while in the other a number of segments, held together by a spring, are forced outwards by a mandrel driven in by hammer blows.

Too much expanding is apt to weaken the tube close to the inside face of the tube sheet.

Boiler tubes leak first at the end which receives the greatest heat from the fire, the leakage being caused by the expansion and contraction of the tube, which is obviously hotter than the water which causes the tube to expand more than the boiler sh.e.l.l. The remedy is to re-mandrel or expand the tube.

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.

[Ill.u.s.tration: Fig. 3409.]

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.

The up take of a marine boiler is the casing or pa.s.sage way through which the heat and gases pa.s.s after leaving the boiler. A dry up take is one which is outside of the boiler, as in Fig. 3409, which represents an outside view of a boiler such as shown in Figs. 3410 and 3411.

A wet up take is one which pa.s.ses through the boiler, and therefore has fire on one side and steam on the other. It is therefore under a collapsing pressure.

The furnace of a marine boiler extends from the fire door to the combustion chamber (_i. e._, the box in which the heat of the furnace pa.s.ses before returning through the tubes).

The superheater of a marine boiler is a cylindrical vessel receiving the steam from the boiler, and delivering it to the main steam pipe, whence the steam is delivered to the engines, etc. When it has no connection with the up take, it may, however, be more properly termed a steam driver, since it serves to separate the steam from entrained water, and does not superheat the steam.

In some cases, however, the superheater takes the form of a spherical ended cylinder standing in the up take.

The receiver of a marine boiler is a drum or cylinder that receives the steam from the boiler and from which the steam pa.s.ses through the steam pipe to the engine. The receiver is by some called the _steam chest_ of the boiler.

The fittings essential for a marine boiler are: The safety valves; the test c.o.c.ks (or gauge c.o.c.ks, as they are sometimes termed); the water gauge gla.s.s; the stop valves; the check valve for the boiler feed pipe, and the valves for letting on steam to the main engine and such other engine or engines as may take steam from the main boiler; the sc.u.m c.o.c.ks; the blow off c.o.c.ks; and a small c.o.c.k to enable the drawing of water from the boiler to test its degree of saltness.

There are two kinds of safety valves, the dead weight and the spring loaded.

A dead weight safety valve is one in which the valve is held to its seat by dead weight, the objection to which is, that when the vessel rolls the effect of the weight or weights upon the valve is diminished; hence under heavy rolling the steam may blow off at a less pressure than the valve is set for.

A lock up safety valve is a dead weight safety valve, the top of whose spindle is provided with a cast iron cap or bonnet with two handles on.

This cap is keyed to the spindle, and the keyway is so disposed that no extra weight can be added to the valve, while at the same time the valve can be lifted from its seat and turned around.

A spring loaded safety valve is one in which the valve is held down by the pressure of a spiral spring, and this pressure will obviously not vary, no matter how much the ship rolls.

In proportion as the valve lifts and the spring compresses, its resistance increases, and this tends to impair the accuracy of the valve. This, however, is offset from the fact that when the valve rises from its seat it presents a greater area for the steam to act against.

The area of safety valve required by the English Board of Trade is about 1/2 square inch of valve area per square foot of fire grate area.[67]

[67] See page 409, Vol. II., for safety valve calculations.

There are three test c.o.c.ks, which are sometimes placed in a diagonal row on the front of the boiler, and sometimes on the fitting for the gauge gla.s.s. The top test c.o.c.k shows highest level to which the water should rise in the boiler, and the lowest one the lowest level, the middle c.o.c.k indicating the average. There is usually a vertical distance of about 4 inches between the test c.o.c.ks, which gives a permissible range of 8 inches in the level of the water in the boiler.

Test c.o.c.ks are prevented from choking with scale by pa.s.sing a wire through the c.o.c.k and clear into the boiler, a plug being provided, which, when removed by uns.c.r.e.w.i.n.g, permits the insertion of the wire.

This cleaning must obviously be performed when there is no steam on the boiler.

A gauge gla.s.s is a gla.s.s tube whose bore is open to the boiler. It is fitted at each end to a bra.s.s socket that is screwed into the boiler, each socket having a c.o.c.k that permits communication between the gauge gla.s.s and the boiler to be shut off in case the gla.s.s should break. The bottom socket is also fitted with a c.o.c.k, which, on being opened, permits the water and steam to blow through the gauge gla.s.s and clean it of sc.u.m or dirt.

The gauge gla.s.s must be plainly in sight, and placed at such a height that when the desired quant.i.ty of water is in the boiler it will half fill the gauge gla.s.s.

Gla.s.s water gauges, instead of attaching to the boiler, are sometimes fitted to a fitting that connects to the top and bottom of the boiler, with the object of attaining, for the gauge gla.s.s, water free from the sc.u.m and impurities which collect at and near the surface of the water in the boiler. This fitting should have c.o.c.ks in each pipe leading to the boiler, so that in case the gauge gla.s.s breaks, steam can be shut off from the boiler.

In some cases the test c.o.c.ks are also attached to this fitting, and in this case the construction should be such that shutting off communication between the gauge gla.s.s and the boiler will not at the same time shut off communication between the test c.o.c.ks and the boiler.

When the boiler is priming or steaming very fast, the gauge gla.s.s may show a false water level, hence reading should be compared with that of the test c.o.c.ks.

If the water gets too low, the first parts of the boiler to be injured will be the top of the flame box, or the combustion chamber, and the top row of tubes, because they are the first surfaces that the water will fall below and leave exposed to the heat without having water on the other side.[68]

[68] See page 370, Vol. II., on low water in boilers.

The pressure in the boiler is shown by a steam gauge, pressure gauge, or dial gauge as it is promiscuously called.

A Bourdon dial gauge or pressure gauge consists of a dial casing, containing a hollow thin bra.s.s hoop, oval in cross section, which receives steam from the boiler.

This hoop is fixed at one end, while the other end is closed and free to move. The free end is connected by a small link to a toothed sector, which gears or engages with a small pinion fast upon the spindle of the pointer or needle. When the steam is admitted into the hoop, it straightens out or expands in diameter to an amount that is proportionate to the amount of the pressure within the hoop, and thus causes the needle or index pointer to revolve, and denote from the markings or readings of a dial plate the amount of pressure within the hoop.

If the pressure within the hoop is released, it will move to its normal or zero position. In the course of time, however, the hoop is apt to get a slight permanent set and not indicate correctly. It may, however, be approximately tested for accuracy by testing its readings with that of the safety valve.

The working parts of the gauge, and its casings also, are made of bra.s.s, so that they shall not corrode, and to prevent the heat of the steam from permeating the gauge and impairing the action from expanding the parts, a small quant.i.ty of water interposes between the gauge and the steam, the construction being as follows: