CHAPTER XXX

THE SAFETY-VALVE

Q. What is to prevent the boiler blowing up, in case steam is made faster than used?

A. Up to a certain point, the evaporation of a greater weight of water than is passed out as steam, causes increase of pressure; this would continue until all the water was evaporated, or the pressure got too great for the boiler to stand. To prevent the boiler bursting or exploding, there is a large valve, opening from the steam-space and held down by a spring, the tension of which is adjustable so that the valve will lift when the pressure upon it from below reaches a certain point, very much below the safe working-pressure of the boiler. When the steam-pressure reaches the point at which the valve is set to blow, there is discharge of steam; and if the valve has sufficient area to let through all the steam that the boiler can make, there will be no explosion. In order to diminish the chances of explosion there are often two of these valves side by side, set to blow at the same or about the same pressure.

Q. What is to prevent the engineman screwing down the safety-valve so as to give more steam pressure than he would otherwise have; or what is to prevent some malicious person rendering the boiler liable to explosion by doing the same thing unknown to the engine-runner?

A. One of the valves is usually arranged so that the spring which holds it down cannot be readily got at to change the pressure at which the valve will blow.

Q. What precaution should be taken as to that safetyvalve which is held down by a lever and not locked?

A. It should be raised daily, to insure that the disk is not corroded on the seat, or otherwise inefficient.

Q. How may the pressure in the boiler be relieved if necessary, before the safety-valve blows?

A. By lifting the safety-valve by the relief-lever.

Q. What is the advantage of the ordinary safety-valve with long lever?

A. Without leaving the cab it may be readily adjusted to blow at any desired pressure.

Q. What is the advantage of the "pop" safety-valve? A. It gives larger discharging-area than the ordinary valve.

Q. How is the Crosby pop safety-valve constructed? A. The valve rests on two flat ring-shaped seats lying in the same plane and forming part of the shell, which is in two parts, an inner and an outer cylindrical chamber, connected by hollow horizontal radial arms between which the steam passes, acting on that part of the valve

which shows above and between the two valve-seats. (See Fig. 107.)

Q. How is the noise of steam which escapes from the safety-valve lessened, to prevent frightening horses when trains are standing at stations, and from being a general nuisance?

A. By a muffler, one form of which consists of a wire coil through the interstices of which the steam escapes, making much less noise than where it has to pour through a more contracted area. Other mufflers are made of boxes full of glass beads or of similar substances offering an immense amount of friction with large discharging-area. Some, again, have a central vertical pipe with a large number of L-shaped tubular branches pointing upward. In all, the principle is the same; to give the steam a very large area of escape, divided up into as many jets or sheets as possible. (Fig. 108.)

LAGGING

Q. How is radiation from the boiler lessened?

A. By lagging the boiler and dome with a non-conductor of heat, as wood strips, and covering these with a Russia-iron jacket; sometimes by covering with wool felt, then with wood strips and Russia iron; sometimes by asbestos cloth or some plastic material, as magnesia cement and Russia iron.

Q. What are the advantages of magnesia lagging?

A. It does not char, as do felt and wood, nor get hot, as does asbestos, and may be removed for inspection purposes.

Q. How are fire-boxes lagged in the L. S. & M. S. Railway?

A. A sheet of asbestos is placed next the hot surface, and over that placed a covering of hair felt one inch thick, the whole kept in place by a sheeting of kalamein or planished iron; the boiler-heads being done the same way.

Q. What is the disadvantage of hair felt as a non-conducting, lagging or jacketing material?

A. Although an admirable non-conductor of heat, it is readily disintegrated by high temperatures.

Q. Is asbestos in itself a good non-conductor?

A. It is not; the natural stone in a mass is much less so than the fibers into which it separated for weaving, but even then the value of all asbestos coverings lies in the resistance to charring (it being practically fireproof) and in the air in the spaces among the fibers.

Q. What is the disadvantage of plastic coverings? A. They do not show, as a rule, where leaks are, but indicate a spot far away, so that search often calls for skinning a comparatively large area, usually higher than where the leak apparently is indicated.

Q. Should a boiler have a lagging thickness proportionate to its diameter?

A. Yes; there being a greater mass of hot steam and water behind the lagging.

CHAPTER XXXII

THE WHISTLE AND MUFFLER

Q. How does the steam-whistle act?

A. There is an inverted cylinder, cup or bell of thin metal, with a sharp circular edge, against which an annular sheet of steam is discharged from an annular orifice; the force of the escaping steam causes the bell or cup to vibrate and give out a musical tone, the pitch of which depends on the diameter and the depth of the cup. (Fig. 109.)

Q. What is the advantage of the chime whistle?

A. Its sound is less disagreeable than that of one giving only a simple tone, as it produces a pleasing chord of three tones.

Q. How is this triple effect produced?

A. By the bell being divided lengthwise into three compartments of different lengths. The shortest gives the highest tone.

Q. What is a muffler?

A. A device by which the steam which escapes from the safety-valve is quieted; necessary where the shrill noise of the escaping steam would produce prejudice