BOILER ACCESSORIES: STEAM SEPARATORS Priming. Steam is said to be wet or to be superheated, according as it has an excess of moisture or an excess of heat. Wet steam not only is uneconomical, because it carries a considerable amount of heat into the engine in the form of water, which cannot do useful work, but, if a considerable amount of water gets into the engine, it is really dangerous, for it may so completely fill the clearances that the piston will strike a blow against the cylinder-head sufficient to break it. The water in the pipes, moreover, may cause a serious hammering, which not only is exceedingly annoying, but may be actually dangerous, for a severe water-hammer may break the joints of the steam pipes, and a considerable quantity of escaping steam at high pressure would be exceedingly dangerous to the lives of the engine-room attendants. This especially would be true on board ship, where the engine-room is small, the supply of air meager, and the means of escape limited.
A considerable amount of water may be deposited in a sag in the pipe line, and would undoubtedly remain there for a considerable length of time if the pressure in the boiler did not fluctuate; but a sudden rise of boiler pressure would likely cause this water to pass bodily through the pipe toward the engine. Moisture is carried directly from the boiler as a result of priming. This is caused by steam bubbles which, instead of bursting, become connected on the surface of the water, forming a foam, half-liquid, half-gaseous, which fills the steam space and passes out of the steam pipe. Priming may be due to fluctuations of boiler pressure or to the presence of dirt, oil, or other foreign matter. The smaller the free surface of the water in the boiler, the more likely the water is to prime. Boilers will frequently prime badly under forced draft, when otherwise there would be little trouble.
Priming may be detected from the unusual behavior of the water in the gauge-glass, or from the hammering in the steam pipes or cylinder. To avoid a breakdown under such conditions, the speed of the engine should be reduced, the drain-cocks of the cylinders and pipes opened, and the fires eased down. Sometimes, by suddenly shutting the main stop-valve, the pressure in the boiler can be increased sufficiently to overcome the difficulty.
Almost any boiler is likely to prime to some extent; and to obtain as dry steam as possible, several devices are employed. On the top of stationary boilers and locomotives, a steam dome is frequently built, from which the steam is drawn, the idea being that less moisture will be found here than if the steam be drawn directly from the main portion of the boiler. In marine work, and sometimes in stationary plants, a dry-pipe is used (see Fig. 50). This is merely a large pipe inside the boiler, from which the steam is drawn. The pipe is
near the top of the boiler, and the upper side of it is perforated with holes through which the steam may pass. A considerable amount of moisture is in this way prevented from leaving the boiler.
The moisture in steam can be reduced by the familiar process of superheating; but if this, for any reason, is impracticable or undesirable, a steam separator may be used for the purpose of extracting the moisture that comes from the priming of the boiler or from condensation in the steam pipe.
Separators. There are several forms of separator; but all are designed on the general principle that if the direction of the steam current is suddenly changed, or if it is diverted upward and then downward, the water will be separated from the steam and will fall to the bottom of a suitable receptacle. The depth of water collected in the bottom of the separator is readily indicated by a gauge-glass, and it may be drawn off as desired. To prevent the possibility of flooding the separator, it is well to connect it with an automatic trap which will empty it without close attention from the engineer. It is needless, of course, to say that the trap from this separator should be connected to the hot well, and the drip should be returned to the boiler with the loss of as little heat as possible.
In the "Stratton" separator, Fig. 59, the steam enters at one side of a cylinder, flows downward, and then upward through a pipe in the middle. Dry steam escapes from a pipe near the top, on the opposite side from which it enters. The separated water is drained at the bottom.
The "Cochrane" steam separator, shown in section in Fig. 60, is of the baffle-plate type. The branches for the entrance. and exit of the steam project from each side of the spherical head. Another branch from the bottom provides for connection with the well. The baffle-plate, which is cast as a part of the head, is ribbed, or corrugated, and has ports at each side for the passage of steam. The area of the ports is large, to prevent loss by friction. A small pipe is inserted in the plate on the outlet side at the bottom of the baffle-plate, to drain any condensation in the outlet chamber. Steam, entering at the lefthand opening, strikes the baffle-plate and passes to the outlet chamber by means of the two side passages, as shown in the plan, Fig. 60.
A form of separator which is fitted to the main steam pipe near the engine, is shown in Fig 61. Steam enters at A and strikes the dash-plate B; any water coming with the steam is separated and falls to the bottom. The steam takes the direction indicated by the arrows, and flows out at D. This separator is fitted with a gauge-glass which is similar to a boiler gauge-glass.