All About Engines

96 All About Engines engine, performing some special office, or a steam pump, may be supplied by a branch of the main steam pipe, and may require steam at a lower pressure than that demanded by the main engine. In this case a reducing valve is employed. An examina- tion of Fig. 52 will explain how this operates. The valve is held up from its seating by a lever and spring, and the valve rod is attached near the bottom to a rubber dia- phragm covered with water. If the steam pressure on the surface of the water exceeds a certain amount the diaphragm is depressed, the rod is drawn downwards against the effort of the spring, and the valve closes. Such a valve, therefore, only permits steam of a certain maximum pressure to pass. Fig. 52 .—Reducing Again, it is very necessary valve that the engine should be sup- plied with dry steam — that is, with steam free from drops of water. Many boilers are subject to “ priming ” : the water foams and splashes up from the surface, and is carried to the engine by the rush of steam. And as water expands and contracts very little with change of temperature, it is capable of doing no work, and takes up heat from the cylinder during the latter part of the stroke, when the cylinder walls are warmer than the expanding steam. For the purpose of removing this water a steam dryer or separator is employed, and a simple form consists