Safety-valve, an apparatus the duty of which is to allow the steam to escape from a boiler when it reaches a certain pressure, and thus to prevent undue strain on the boiler-plates. The figure shows in outline the principle of this valve: a, a is a dome on the top of the boiler-shell; b is a conical brass valve resting on a seat, which has been smoothly bored for its reception; e is the fulcrum of a long lever e, f, from which at d there is a projection resting on the top of the valve. The end, f, of the lever is held down against the steam-pressure by the spring h. The pull exerted by this spring can be adjusted by a nut at f, and it is fitted with a brass case, and an index so arranged as always to point to a figure which indicates at what pressure per square inch the steam in the boiler will be able to lift the valve. For most stationary boilers a weight which can be moved along the lever to any desired position is used instead of the spring.
In marine boilers the lever is frequently dispensed with altogether, and weights act directly on the valve. It is also usual, and in steamers compulsory by law, to put two safety-valves on each boiler, one adjustable at will by the engineer, and the other (called the 'government valve') enclosed in a case to which access can only be obtained by means of a key in the possession of the captain. The danger of explosion through overloading the valves is thus obviated. See also STEAM-ENGINE.