A steam trap is a type of automatic valve used to open and close based on the presence of steam.
Steam traps are used to remove condensate out of a system by opening when in contact with water and closing when contacting steam.
Steam traps are commonly used in Sterilisation In Place systems to remove condensate and keep Dry steam within a system to sterilise.
There are a few types of steam traps but the most common in sterilisation is a thermostatic steam trap where a bi-metalic disc deforms once heated, this deformation closes the valve trapping steam in, once that bi-metalic disc is cooled through contact with condensate, the valve opens and drains until re-heated with steam again.