39 §

MODERN STEAM PRACTICE.

on the whole, we think it should be fitted with set screws, to adjustthe pressure against the valve at any time to suit the reduction insteam pressure that may be considered advisable after the boilerhas worked for a lengthened period. Some makers have fitted anadditional valve opening downwards, seated on the main reliefvalve, and opened by hand, while another valve of india rubber isplaced on the top of the main relief one, so that when the additionalvalve is opened, in the event of violent priming at the return strokeof the piston, the disc of india rubber, or metal valve if so fitted,closes, and does not impair the vacuum. This is a complicatedarrangement, not in general use; and the same object is attainedby fitting a plug valve at the bottom of each end of the cylin-der, worked simultaneously from the starting platform, and whichcan be opened when violent priming occurs, or in the act ofblowing through before starting the engine,—in the same way asthe plug valves in the locomotive engine cylinder, which are leftopen for a considerable time to blow out thoroughly all the waterfrom the cylinders. Some first-class engineers consider that greatersafety would be insured by dispensing with the springs for holdingdown the relief valves, and this seems a step in the right direction;for when valves are so arranged that they are held down simplyby the steam, and have the means of blowing all the water backagain into the steam pipes, and collecting it in a suitable Separator,we secure two advantages, namely, that the valves are not so liableto stick or get damaged, and that the hot water does not fly about,but is received into the Separator, which can be run off occasionally.For this purpose, double-beat valves, giving a large circumferentialarea for the water to escape, or common spindle valves, are fitted tothe top of the cylinder, in valve chests, which communicate bypipes with the cylinder on the bottom of the valve and the steampipe on the top of the valve; the steam pressure above the valvebeing greater than that in the cylinder, the valves are of courseheld down by the difference of pressure, and the water is ejectedas in ordinary arrangements, with this difference that it is collectedin a vessel for the purpose, instead of finding its way into thebilges.

Lubricators.-Gro3.se cups are fitted to the steam ports at thefront and back of the cylinder, for lubricating the valve and piston,the lubricant being drawn in with the vacuum. These cups shouldbe fitted in connection with a plug valve having a screwed part at