safety valve

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A valve in a pressure container, as in a steam boiler, that automatically opens when pressure reaches a dangerous level.
2. n. An outlet for the release of repressed energy or emotion.

Century Dictionary and Cyclopedia

1. n. A contrivance for obviating or diminishing the risk of explosion in steam-boilers. The form and construction of safety-valves are exceedingly various, but the principle of all is the same—that of opposing the pressure with in the boiler by such a force as will yield before it reaches the point of danger, and permit the steam to escape. The most simple and obvious kind of safety-valve is that in which a weight is placed directly over a steam-tight plate fitted to an aperture in the boiler. When, however, the pressure is high, this form becomes inconvenient, and the lever safety-valve is adopted.
2. n. To be a true safety-valve the area opened by the lift of the valve should be sufficient to carry off all the steam that the boiler can make at that pressure, with the fire at its best and all other outlets closed. It rarely is so ample as this, but the blowing of the safety-valve serves as an alarm, and the pressure rarely rises much above the blowing-off point, because steps are taken at once by the fireman to retard the rapid formation of steam and accumulation of pressure. The force which balances the interior pressure upon the area of the valve may be a weight or a spring. The effort of weight or spring to hold the valve shut may be exerted directly on the back of the valve, or indirectly by means of a lever. The levers should act with knife-edge fulcrums to lessen friction and prevent jamming by side-thrust. With high pressures and large areas of valve the direct-weight system becomes inconvenient. It is still used in England for low pressures, but is little used in the United States. The spring pop safety-valve is the most widely used. The pop-valve was first worked out by Richardson of Troy, New York. The pressure conies on a given area when the valve is shut, and tends to lift it. When the pressure overbalances the weight or spring, and the valve rises from its scat, annular grooves in valve and seat are filled with escaping steam, and, by their reaction upon the larger annular area outside the first area of contact, tend to lift the valve higher and to hold it up from its seat until the pressure has fallen below that at which the valve opened—usually five pounds. Hence the valve opens a large area for discharge, and keeps open in full discharge until the pressure has gone down somewhat. Then it closes suddenly, or with a pop, which gives it its name. Double safety-valves are those which have two separate seats of equal or unequal area, opening a greater area for discharge than either would open alone, or else acting in succession as the pressure rises, the smaller area opening first. If this does not relieve the pressure, the larger area opens later, when the balance is reached. A locked safety-valve is one which is inclosed in a locked case of which only an inspector has the key, or one in which the adjusting device is locked with lock and key, so that when once set to a determined pressure only the possessor of the key can change it.

Wiktionary

1. n. A relief valve set to open at a pressure below that at which a container (such as a boiler) would burst

GNU Webster's 1913

1. a valve which is held shut by a spring or weight and opens automatically to permit the escape of steam, or confined gas, water, etc., from a boiler, or other vessel, when the pressure becomes too great for safety; also, sometimes, a similar valve opening inward to admit air to a vessel in which the pressure is less than that of the atmosphere, to prevent collapse.

WordNet 3.0

1. n. a valve in a container in which pressure can build up (as a steam boiler); it opens automatically when the pressure reaches a dangerous level