396
MODERN STEAM PRACTICE.
packing gland, and the bottom or small end of the plug is merelyfitted, and ground into the seating, which is cast solid at the end;by this means no leakage can occur except at the top, which ismade steam-tight by the packing gland.
The escape or relief valve, fitted to the bottom of the cylindercover and to the front end of the cylinder, is intended to allow theescape of the water which finds its way into the cylinder frompriming and condensation of the steam. It consists of a disc valvewith a spring fitted to the top, screwed down sufüciently tight toresist the steam pressure acting on the internal area of the valve.The valve can only be opened, and the water which is not com-pressible ejected, by the piston striking against the water, andforcibly lifting the valve, compressing the spring, which againreacts when the cylinder is free of water. The valve should beinclosed in a light dorne, with a hole at the bottom side for allowingthe hot water to escape downwardsinto the bilges. In other examples adash or splash plate is cast along withthe valves, when placed horizontallyand vertically; the plate being of acurved shape, the water escaping allround the valve is returned or thrownback again, and so prevented from beingscattered about the engine room, andsealding the engineers or those in at-tendance. The spring is usually screweddown with a crosshead, through whichthe valve spindle passes loosely through a hole bored in the centre.At each end of the crosshead is a column, secured to the valve seat atone end, and the other end screwed, passing through a hole in thecrosshead, and fitted with nuts above and below. The spring is placedaround the spindle, between the valve and the crosshead, which bymeans of the nuts is screwed up carefully, compressing the valve toa little above the working pressure of the steam in the cylinders,which can be easily adjusted when the engines are started, and thenthe nuts below the crosshead can be screwed hard up; in this way thevalve is guided at the top through the crosshead, and at the bottomthe spindle passes through the hole in the central boss cast alongwith the valve seating. Some engineers dispense with the movablecrosshead, using merely a bow or fixed crosshead of wrought iron Fig. 281.—Relief Valve