Note:  Do not rely on this information. It is very old.

Safety-Valve

Safety-Valve is used to indicate and relieve excessive pressure in steam-boilers or other vessels containing fluids under pressure. It usually consists of a conical plug fitting in a seating, and held in place by a lever and weight. As soon as the pressure exceeds a limit which is determined by the area of the valve, the length of the lever, and the mass of the weight, the plug is lifted, producing an escape of the steam or other fluid, and reducing the pressure. In some cases a spring is used instead of a weight, and in others a weight is placed directly upon the plug without the intervention of a lever.