A pressure relief valve, for pressurised fluid storage vessels such as a rail tank car for volatile liquids, has a flap valve for rotational movement in a valve passage between a closed position seated on a valve seat and blocking the passage and an open position in which the passage is substantially unobstructed. A spring arrangement for keeping the valve closed against a rated pressure includes an externally mounted spring system which, when exposed to fire conditions which might threaten the wall strength of the vessel, loses spring temper so as to reduce the closing pressure of the valve.