Modern Machine-Shop Practice Part 263

The quant.i.ty of coal burned in marine boiler furnaces is about 15 lbs.

per square foot of fire grate area per hour; hence the quant.i.ty burnt per day with common average engines with 4 furnaces, 3 feet wide and 5 feet long, may be found by multiplying the area of the 4 furnaces (60 feet) by the number of lbs. (15) burned per foot of grate per hour, which will give the total lbs. weight burned per hour, which, divided by 112 lbs., will give the hundredweight burned per hour, and this, multiplied by the number of hours reckoned as const.i.tuting a day, gives the fuel consumption per day, based upon 15 lbs. coal per square foot of fire grate area.

The number of tons of steam coal burnt per day to drive an ordinary steamer of 40 feet beam 10 knots an hour by steam alone (or without sail), will depend upon the kind of engine used. Experience teaches us that with average vessels, the beam squared equals the consumption of coal for 40 days, in the case of an ordinary jet condenser engine; 50 days with a surface condensing engine; and 60 days with a compound engine; hence, in the present example, a.s.suming the engine to be jet condensing, we may calculate the fuel consumption per day, for a vessel 40 feet beam giving 10 knots an hour, as follows:

The beam squared gives 1600 (40 40 = 1600), which divided by 40 (40 days) gives 40 tons per day. For surface condensing the 1600 would be divided by 50, giving 32 tons per day; and for a compound engine the 1600 would be divided by 60, giving 26 tons 13-1/3 cwt. per day.

It is obvious, however, that calculations of this kind, in which the ratio of expansion is not stated, are the merest approximations.

The number of tons of steam coal that will be burnt per day with a pair of average surface condensing engines having cylinders 50 inches in diameter will be, under average conditions, 16 tons per day, the calculations being based upon the common a.s.sumption that the diameter of one cylinder squared and divided by 100 gives the consumption of fuel in tons per day for condensing engines not compounded; thus, 40 40 = 1600 100 = 16 tons of coal burned per day.

Here again, the ratio of expansion not being specified, the calculation has no real practical value.

If at sea and short of coal, bear in mind that the consumption of fuel per mile run is greater for fast than for slow speeds; hence the following points should be attended to:

Reduce the speed of the ship to say half the usual. Regulate the fire so as to keep up full boiler pressure without blowing off. This will allow the expansion or cut off valve to be set to cut off early in the stroke, and thus save steam. If, under these conditions, the steam should sometimes blow off at the safety valve, cover up part of the fire grate area.

Use a thin, rather than a thick, fire, but be careful that it is not so thin as to let currents of cold air pa.s.s through.

TO RELIEVE THE BOILER IN CASE OF EMERGENCY.--Suppose an engine breaks down at a time when the fires are heavy and going full, that the steam gauge shows blowing off pressure, but that the safety valve is stuck, or from some cause or other is prevented from blowing off, and cannot be eased or lifted, and the following is the course to be pursued:

1st. Close the ash pit dampers and open the smoke box door and fire door. If there are no ash pit doors, close the damper in the up take and open the fire and smoke box doors.

2d. Start the donkey engine to feed cold water into the boilers.

3d. Start the steam winches, and any other small engines that take steam from the main boilers.

4th. Slacken the escape valves, and open the drain c.o.c.ks of the cylinders and receivers, and steam will blow through the H.P. cylinder escape valve and drain c.o.c.k at once. The H. P. slide valve may then be worked by hand, back and forth, to let steam pa.s.s into the receiver and blow through its escape valves and drain c.o.c.k.

5th. Open the sc.u.m or brine c.o.c.ks and keep them open, also open all gauge or test c.o.c.ks, etc., about the boiler.[72]

[72] It is not safe to draw the fire at a time when the pressure is at a dangerous point, especially if heavy, as disturbing it may temporarily increase the combustion and the danger of explosion.

[Ill.u.s.tration: Fig. 3410.]

Figs. 3410 and 3411 represent an example of a steel marine boiler, designed for a working pressure of 160 lbs. per square inch, with a margin of safety of 5.

The dimensions are as follows:

Diameter of sh.e.l.l 12 feet 6 inches.

Sh.e.l.l plate 1-1/8 " thick.

Front and back upper plates 31/32 " "

Back rivet plates 7/8 " "

Back lower plates 7/8 " "

Front tube plate 15/16 " "

Front lower plate 13/16 " "

Furnaces 17/32 " "

Inner tube plate 3/4 " "

Combustion chamber back 17/32 " "

Combustion chamber sides 17/32 " "

Outer sides of wing combustion chambers 9/16 " "

And bottom of centre one to be 9/16 " "

Sh.e.l.l of receiver 7/16 " "

Beds of receiver 5/8 " "

Receiver connecting pipe 3/4 " "

The riveted joints have all holes drilled. The longitudinal seams are made with b.u.t.t joints treble riveted, and with double b.u.t.t straps.

The circ.u.mferential seams are lapped and treble riveted.

Fig. 3412 represents the "Martin" boiler for marine engines. In the return flue there are a number of vertical water tubes which are very effective in promoting circulation as well as in generating steam. These boilers are used largely in the United States navy for moderate pressures.

The following upon the testing and examining of a boiler of this cla.s.s is from _Modern Steam Boilers_:

"Every new boiler should, when complete, be tested by water pressure to double the amount of the intended working pressure; for while the wisdom of applying as high a pressure as three times the working pressure, which is sometimes done, may be questionable, experience has shown that a test by hydraulic pressure will reveal defects that would otherwise be apt to pa.s.s unnoticed.

"For instance, when the top plate of a combustion box is stayed against the pressure by girder stays that are not stayed to the boiler sh.e.l.l, the girder stay merely acts to stiffen the top plate, and as a result the whole pressure on the area of the top plate falls on the walls of the combustion box. The back tube plate therefore springs down and transfers part of this pressure to the furnace, causing it to become elliptical, as may generally be found by the application of rod gauges fitted to it before testing and tried while the pressure is on.

[Ill.u.s.tration: Fig. 3411.]

[Ill.u.s.tration: Fig. 3412.]

"This flattening under test naturally drew attention to the defectiveness of girder stays. Another instance may be given with reference to gusset stays, which, if fitted so as to support too large an area of back plate, in proportion to the area of combustion box it supports, may cause the combustion box to distort from its natural shape, pulling the tube sheet back and flattening the furnace. The amount of distortion may be only 1/16 inch in some cases, but that is sufficient to show the existence of unequal strains which require attention in boiler designing.

"This brings us to the important fact that in almost every instance where the furnaces of marine boilers collapse, they come down at the sides, notwithstanding that when collapse occurs from overheating, the crown of the furnace must have been left bare of water first, and should therefore come down first, flattening the furnace at the top.

This points to the conclusion, that the top of the furnace received some extraneous support.

"When a furnace collapses from corrosion, it naturally gives way at the most corroded part. An hydraulic test to twice the working pressures is recommended for new boilers only, unless it be small vertical cylindrical and steam launch boilers, which may always be subjected to the same test as new main boilers.

"In the case of old main boilers, however, and particularly rectangular ones, an hydraulic test of less than twice the working pressure may be employed, the amount being governed by the circ.u.mstances of the case.

If, for instance, a boiler has undergone a thorough repair and received new furnaces, then every part of the boiler should have received proportionate consideration and an hydraulic test depending upon the judgment of the responsible engineer, but not less than one and one-half times the working pressure should be made, while one of one and three-quarter times could scarcely be objected to. This, however, is a subject upon which there is some controversy, especially in the case of old boilers having a good foundation of strength, but patched or local weak spots, such as combustion chamber backs and sides, these patches having been, perhaps, made with a view to a more extensive repair in the near future.

"In such a case as this an hydraulic test sufficient to prove the tightness of the seams and joints may, perhaps, be all that is absolutely essential.

"After a boiler has been tested by hydraulic pressure it should be examined internally, as it sometimes occurs that a stay may break under the test (especially if gusset stays are employed), and the extra strain thrown on the adjacent parts may cause them to fail, and thus cause the destruction of the boiler when under strain.

"When an examination is to be made inside and outside of a boiler, the boiler must be properly prepared for the same, which may be done as follows:

"The tubes should be swept; the furnace cleaned out; the fire bars should be taken out; the bridges in the furnace should be taken down; the up take smoke box and combustion box should be cleaned out and swept; every man hole and hand hole or peep hole door should be removed; the bottom of the boiler should be cleaned out and dried (in damp weather a little heat may be necessary for this purpose); all impediments, if any, should be removed in order to allow the bottom outside to be inspected; at the time of inspection a few mats, good lights, a hand hammer and small chipping hammer should be at hand. In the case of a boiler having any plates weakened by corrosion, a 5/8 inch tapping drill with a drilling brace should also be provided to test the thickness of such plates if considered necessary.

"The safety valves should invariably be taken out for examination, and it is a commendable feature sometimes followed to take out the feed valves, stop valves, blow off and brine c.o.c.ks; at the same time, all the deposits that would prevent a thorough examination of the boiler should be removed. In some cases, however, there may not be time for the scaling before it is necessary for the repairs to be gone on with, and, in that case, a good examination may with care be made by an experienced man.

"To proceed, then, with the examination, the boiler should be entered through the man hole door beneath the furnaces, examining the boiler bottom and the bottom and sides of the furnaces all the way along, and on arriving at the end of the boiler the water s.p.a.ce and stays at the backs of the combustion boxes can be examined as well as the midship combustion box stays and plates. In an old and corroded boiler it may be found necessary to use a chipping hammer very freely about the furnaces, particularly below the lap of the furnace.

"The most corroded part of a furnace will generally be found about on a line with the fire bars, but the furnaces may have suffered from some other cause than the corrosion due to ordinary wear, as, for example, from chemical or galvanic action, and in that case they may be found comparatively good at the sides but with the extreme bottoms in a _dangerously_ corroded state, perhaps in the form of pit holes extending half through the plate and _hidden by a coating of red scale, which requires to be chipped away before the pit holes are brought to light_.

"Corrosion by galvanic action may have produced honey combing or a general attack over the surfaces, which have a dark or _dark and sparkling appearance_, the latter more particularly when corrosive action has been very active.

"Of these various cla.s.ses of corrosion that which is the most deceiving is that which attacks the plates over the largest surface of the plate, leaving at the same time an apparently smooth exterior surface, for in this case the extent of the waste cannot be so clearly detected by the eye, and the only reliable way of testing the thickness is by drilling a hole through the plate.