Steam traps
Steam traps stop the flow of steam but pass water.
Their purpose to to let the water that condenses in steam lines
escape. Even with good insulation, hot steam lines lose heat
to the surroundings. Cooling of the steam causes condensation.
As lines fill with water, they can get quite cool. Draining
the water keeps the lines hot and the steam dry. One type of
trap is shown in this sketch:
The spring when expanded holds the plug tight against the outlet.
When the trap collects water, the spring cools, contracts to
open the plug, and water flows out. The steam that replaces the
water expands the spring and pushes the plug tight.
Another type of steam trap has the same principle as a flush toilet. A float controls a plug in the outlet system. The trap fills with water that lifts the float and opens the outlet. There is enough inertia to let most of the water pass before the plug seats and closes the outlet.