Source : Webster's Revised Unabridged Dictionary (1913)

   {Safety tube} (Chem.), a tube to prevent explosion, or to
      control delivery of gases by an automatic valvular
      connection with the outer air; especially, a bent funnel
      tube with bulbs for adding those reagents which produce
      unpleasant fumes or violent effervescence.

   {Safety valve}, 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.

Source : WordNet®

safety valve
     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 [syn: {relief valve}, {escape
         valve}, {escape cock}, {escape}]