Steam traps defined by devices or special automatic valves for discharging condensate and non condensable gases from live steam with less possible loss or consumption of live steam. Furthermore not only steam traps are covered but also all kind of apparatus or valves and component parts and accessories thereof for draining-off liquids from enclosures, gas lines, steam lines or containers containing gases or vapours. Said valves are controlled thermally (thermostatically, bi-metallically driven valves), by excess or release of pressure (thermodynamically driven valves), by floats (mechanically driven valves) or controlled without moving parts (Venturi-type or labyrinth-type).

Valves in general are classified in F16K. Steam engine plants where engine aspects predominate are classified in F01K, domestic central heating systems using steam are classified in F24D, heat exchange or heat transfer in general is classified in F28 and the generation of vapour in cores of nuclear reactors is classified in G21, steam boilers in general in F22B and devices for superheating steam in F22G.

Attention is drawn to the definition of "steam" and "vapour". In cases where a specific entry for vapour is missing, documents related to special vapours are classified in groups where only "steam" is explicitly mentioned.