All About Engines

Raising Steam 81 soluble sodium salts remain in solution. In districts where the water is very “ hard ” this process has to be employed. The presence of oil in the boiler water is produc- tive of greater trouble. It causes the water to froth so that the level cannot be ascertained from the water-gauge, and if it settles on the metal surface it is liable to char, and to lead to over-heating. It can often be partially got rid of by opening a blow-off cock fixed near the water level, but it should, if possible, be removed before it enters, and this is not easy. Granted, however, that the boiler is well designed, and well looked after, the two precautions necessary to avoid accidents are to prevent the pressure rising to beyond a certain limit, and the provision of an ample supply of pure water. In the first chapter an illustration was given of a simple type of lever safety valve. Suppose the area of the valve is 4 square inches, and it is required to blow off at 60 lb. per square inch. Then the total pressure on the valve is 4 x 60 = 240 lb. If the distance of the weight from the fulcrum is ten times the distance of the rod which holds down the valve, the weight on the end of the lever will have to be 10 = 24 b‘ Thls ls an excellent safety valve for stationary boilers at low pressures. If the lever be dispensed with, and the weight placed over the valve, as in Fig. 45, we get what is called a dead-weight safety valve. It will be seen that the valve is held down on its seating by a sort of bell-shaped coven