512
THE NEW ZEALAND GAZETTE.
No. 16

Probably more than half the steamers afloat are fitted with water-gauges as shown in Figs. 3 and 4, and it is therefore specially important that engineer candidates should thoroughly understand their construction, the principle on which they act, and the steps which must be taken to keep them in an efficient condition.

When fitting a gauge-glass into its place it is specially important that it should not be placed so high as to prevent a clearing-rod being inserted at G, Figs. 1, 2, 3, 4, and 5. This defect, especially if it occurs in a water-gauge attached to a boiler subject to priming, permits a rapid accumulation of scum around the top of the glass, and results in the choking of the orifice leading from cock B to the gauge-glass in each of the figures.

When a gauge-glass is too short, or is placed either too high or too low in the fittings, it is also liable to become choked by the packing-material being forced over its ends by the glands whilst being screwed up.

The use of unsuitable or insecure internal pipes in connection with either the ordinary glass gauge-cocks of the description shown in Fig. 1, or with test-cocks which are jointed to the boiler itself, should also be carefully guarded against.

Boiler casualties have resulted from the cocks B and D having the parts wrongly placed, as shown in Fig. 6, Plate II. In one case of this kind, which forms the subject of Report No. 208 under the Boiler Explosions Acts, the engineer in testing the water-gauge omitted to see that the passages in the cocks B and D were clear when the handles were in their proper working-position. This defect could easily have been discovered if proper attention had been paid to the condition of the cocks. A defect of this nature may be due to faulty construction originally, or to the handle of the cock having been overstrained and the neck twisted. Whether the passages in the plugs are fair and clear can, however, be verified in a few minutes. As an illustration, the water-cock D, Fig. 6, Plate II, can be verified by blowing through E with B shut, and then removing the handle of D to one side until it is just closed, and then to the other side until it is again just closed. The proper working-position of the handle is about equally distant from each of the above positions. The other cocks can be verified in the same manner.

Another serious casualty occurred through the handle of the cock A, Fig. 3, having been twisted from its original position relatively to the orifice of the cock, resulting in the cock being shut when apparently open.

When a water-gauge that is clear in all its parts has been thoroughly blown through, the water in the glass rises above the level at which it formerly stood immediately the drain-cock E is closed, but if left undisturbed for a time it gradually falls to its former position. The amount of rise which occurs on these occasions depends chiefly on the temperature of the contents of the boiler and on the length of the pipes by which column Y is connected top and bottom to the boiler; but in cases where the gauge is of the description illustrated in Figs. 3, 4, and 5 it amounts in high-pressure boilers to about 4 in., while the time occupied by the water in returning to its former level ranges from thirty to forty minutes. The cause of this rise is twofold—namely, (a) the displacement of the comparatively cold water in pipe H by hotter and proportionately lighter water from the boiler, and (b) a slight condensation of the steam and a corresponding fractional reduction of pressure in pipe I. The cause of the gradual subsidence of the water in the glass to its former level is also of a dual character—namely, (a) the cooling of the water in pipe H, and (b) the diminution in the condensation of steam in pipe I owing to the collection therein of air released from the steam condensed.

These results will, however, be somewhat modified if the water in the boiler is of higher density than in pipe H, and this will nearly always be the case owing to the condensation of the steam in the glass and upper fittings of the water-gauge causing the water in the lower part to be fresher than that in the boiler.

The Examiner should impress upon candidates the necessity for periodically blowing through the water-gauge on each boiler (no matter what the form may be) in a systematic and thorough manner, and in cases where a boiler is fitted with two water-gauges of keeping both in constant use. Finally, he should further impress upon them the necessity for keeping the water-gauges well lighted, clean, and in all respects efficient.