Steam Traps are devices which are used to release condensate which may form in steam lines. Steam traps allow only the condensate and prevent the useful steam from escaping.
Condensate should be removed from steam lines. Condensates can cause hammering in the pipelines and corrosion.
In addition to Steam, Steam traps also release air and other gases.
There are many different types of steam traps. The most simple can be a nipple in a pipeline. Since condensate (water) is heavier than steam, it will collect at the lowest point. The trap opens once sufficient amount of condensate has collected.