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Fig. 12.—Section of a water-gauge.

Most gauges on high-pressure boilers have a thick glass screen in front, so that in the event of the tube breaking, the steam and water may not blow directly on to the attendants. A further precaution is to include two ball-valves near the ends of the gauge-glass. Under ordinary conditions the balls lie in depressions clear of the ways; but when a rush of steam or water occurs they are sucked into their seatings and block all egress.

On many boilers two water-gauges are fitted, since any gauge may work badly at times. The glasses are tested to a pressure of 3,000 lbs. or more to the square inch before use.

THE STEAM-GAUGE.

It is of the utmost importance that a person in charge of a boiler should know what pressure the steam has reached. Every boiler is therefore fitted with one steam-gauge; many with two, lest one might be unreliable. There are two principal types of steam-gauge:—(1) The Bourdon; (2) the Schäffer-Budenberg. The principle of the Bourdon is illustrated by Fig. 13, in which A is a piece of rubber tubing closed at one end, and at the other drawn over the nozzle of a cycle tyre inflator. If bent in a curve, as shown, the section of the tube is an oval. When air is pumped in, the rubber walls endeavour to assume a circular section, because this shape encloses a larger area than an oval of equal circumference, and therefore makes room for a larger volume of air. In doing so the tube straightens itself, and assumes the position indicated by the dotted lines. Hang an empty "inner tube" of a pneumatic tyre over a nail and inflate it, and you will get a good illustration of the principle.

Fig. 13.—Showing the principle of the steam-gauge.

Fig. 14.—Bourdon steam-gauge. Part of dial removed to show mechanism.

In Fig. 14 we have a Bourdon gauge, with part of the dial face broken away to show the internal mechanism. T is a flattened metal tube soldered at one end into a hollow casting, into which screws a tap connected with the boiler. The other end (closed) is attached to a link, L, which works an arm of a quadrant rack, R, engaging with a small pinion, P, actuating the pointer. As the steam pressure rises, the tube T moves its free end outwards towards the position shown by the dotted lines, and traverses the arm of the rack, so shifting the pointer round the scale. As the pressure falls, the tube gradually returns to its zero position.

The Schäffer-Budenberg gauge depends for its action on the elasticity of a thin corrugated metal plate, on one side of which steam presses. As the plate bulges upwards it pushes up a small rod resting on it, which operates a quadrant and rack similar to that of the Bourdon gauge. The principle is employed in another form for the aneroid barometer (p. 329).

THE WATER SUPPLY TO A BOILER.

The water inside a boiler is kept at a proper level by (1) pumps or (2) injectors. The former are most commonly used on stationary and marine boilers. As their mechanism is much the same as that of ordinary force pumps, which will be described in a later chapter, we may pass at once to the injector, now almost universally used on locomotive, and sometimes on stationary boilers. At first sight the injector is a mechanical paradox, since it employs the steam from a boiler to blow water into the boiler. In Fig. 15 we have an illustration of the principle of an injector. Steam is led from the boiler through pipe A, which terminates in a nozzle surrounded by a cone, E, connected by the pipe B with the water tank. When steam is turned on it rushes with immense velocity from the nozzle, and creates a partial vacuum in cone E, which soon fills with water. On meeting the water the steam condenses, but not before it has imparted some of its velocity to the water, which thus gains sufficient momentum to force down the valve and find its way to the boiler. The overflow space O O between E and C allows steam and water to escape until the water has gathered the requisite momentum.

Fig. 15.—Diagram illustrating the principle of a steam-injector.

Fig. 16.—The Giffard injector.

A form of injector very commonly used is Giffard's (Fig. 16). Steam is allowed to enter by screwing up the valve V. As it rushes through the nozzle of the cone A it takes up water and projects it into the "mixing cone" B, which can be raised or lowered by the pinion D (worked by the hand-wheel wheel shown) so as to regulate the amount of water admitted to B. At the centre of B is an aperture, O, communicating with the overflow. The water passes to the boiler through the valve on the left. It will be noticed that the cone A and the part of B above the orifice O contract downward. This is to convert the pressure of the steam into velocity. Below O is a cone, the diameter of which increases downwards. Here the velocity of the water is converted back into pressure in obedience to a well-known hydromechanic law.

An injector does not work well if the feed-water be too hot to condense the steam quickly; and it may be taken as a rule that the warmer the water, the smaller is the amount of it injected by a given weight of steam.[2] Some injectors have flap-valves covering the overflow orifice, to prevent air being sucked in and carried to the boiler.

When an injector receives a sudden shock, such as that produced by the passing of a locomotive over points, it is liable to "fly off"—that is, stop momentarily—and then send the steam and water through the overflow. If this happens, both steam and water must be turned off, and the injector be restarted; unless it be of the self-starting variety, which automatically controls the admission of water to the "mixing-cone," and allows the injector to "pick up" of itself.

For economy's sake part of the steam expelled from the cylinders of a locomotive is sometimes used to work an injector, which passes the water on, at a pressure of 70 lbs. to the square inch, to a second injector operated by high-pressure steam coming direct from the boiler, which increases its velocity sufficiently to overcome the boiler pressure. In this case only a fraction of the weight of high-pressure steam is required to inject a given weight of water, as compared with that used in a single-stage injector.

[1] "The Steam-Engine," p. 3.

[2] By "weight of steam" is meant the steam produced by boiling a certain weight of water. A pound of steam, if condensed, would form a pound of water.