Safety Valve

a device calculated to prevent steam boilers from exploding. It consists essentially of an opening or short tube connected with the boiler and closed with a movable but tightly fitting valve. In the case of stationary low-pressure engines this valve is held in place by a weight that may be blown off and permit the escape of steam before the internal pressure reaches the danger point. For high-pressure boilers, the valve is held down usually by a lever which passes over the valve and is weighted at its free end according to the pressure which the boiler is designed to carry. In the case of a locomotive which bumps and jerks along over a rough track, it is evident that neither the dead weight nor lever valve would be suitable. For this purpose the pot valve has been invented. The lid is held in place by a spiral spring which permits the escape of steam when the pressure becomes too great, but returns the lid to its place as soon as the pressure has diminished. In exaggerated accounts of steamboat races the captain is facetiously described as sitting on the safety valve. Laughter is sometimes called the safety valve of the human heart. See Steam Engine