A steam trap is an automatic valve that opens to expel air and condensation from steam lines and closes to prevent the flow of steam. The functions of a steam trap are:
• Remove (vent) air from the system so that steam can enter. (Air in the pipes will block the flow of steam into the radiators.)
• Prevent steam from leaving the system until all of its latent heat is removed.
• Remove (drain) the condensate from the system after the latent heat has been removed. (Draining the condensate from the system prevents corrosion and water hammer.)
All steam traps operate on the fundamental 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. This includes the friction head, the velocity head, and the static head on the discharge side of the trap. The steam trap cannot operate unless the excess pressure of discharge is greater than the total back pressure.
Each steam trap used in steam heating is designed for a specific range of applications that its operating characteristics best suit. Although there is no universal steam trap per se, the many different types can be grouped into the following three classes on the basis of their operating characteristics:
• Separating traps
• Return traps
• Air traps
Separating traps are designed to release condensation but close against steam. They are float-operated, thermostatically operated, or float-and-thermostatically operated. Thermostatic traps are designed to release air and condensation but close against them.
Return traps may be operated to receive condensation under a vacuum and return it to atmosphere or a higher pressure. Air traps are generally operated by a float.