Bimetallic steam traps contain a valve that is opened and closed by the differential expansion of metallic disks.
Bimetallic steam traps are used in many industrial and commercial applications in the chemical processing, energy production, food processing, HVAC, pulp and paper, mining, petro-chemical and pharmaceutical industries.
Here is a video that illustrates how a bimetallic steam trap works. It shows the thermostatic principal involved and how two bimetallic plates interact and provide physical movement to open and close a valve seat to discharge condensate.