PURPOSE OF RELIEF-VALVE ON DRY PIPE.
Should the throttle-valve close so tight that the compressed air from the cylinders can not pass into the boiler, there is danger of bursting the steam-chest or some part of the steam-pipes. The compressed air will lift most of the throttle-valves far enough to prevent any great danger from this source. In some engines a relief-valve is secured in the dry pipe, which provides a passage for this compressed air. When the cylinder-cocks of an engine are opened when the motion is reversed, they form an outlet to the compressed air, and also admit air to the sucking end without letting the piston draw air so freely through the nozzles. Many cylinder-cocks are now made so that they will open automatically to permit the piston to draw air through them. The reversed engine will stop nearly as well with the cylinder-cocks opened as when they are closed, and it is much more easily handled with the cocks opened. Where the cocks are kept closed, the rush of hot air from the smoke-box laps every trace of oil from the valve-seat, and a heavy pressure—frequently above that of the boiler—is present in the steam-chest. When the engine stops under these circumstances, its tendency is to fly back; and an engineer has some difficulty in controlling it with the reverse-lever till a few turns empty the chest and pipes.