A steam trap is an automatic device installed in a steam line to control the flow of steam, air, and condensation. In operation, it opens to expel air and condensation, and closes to prevent the escape of steam. All steam traps operate on the principle that the pressure within the trap at the time of discharge will be slightly in excess of the pressure against which the trap must discharge.

The principal functions of steam traps in steam heating systems include: (1) draining condensation from the piping system, radiators, and steam processing equipment; (2) returning condensation to the boiler; (3) lifting condensation to a higher elevation in the heating system; and (4) handling condensation from one pressure to another.

Steam traps may be classified on the basis of their operating principles as follows:
1. Float traps
2. Bucket traps
3. Thermostatic traps
4. Float and thermostatic traps
5. Flash traps
6. Impulse traps
7. Lifting traps
8. Boiler return traps