Steam systems carry energy in the form of latent heat — about 970 BTU per pound of saturated steam at atmospheric pressure. When steam gives up that heat to a heat exchanger, radiator, or process load, it condenses back into water. That condensate must be removed continuously, or it accumulates, blocks heat transfer surfaces, and causes water hammer when slugs of liquid get accelerated by following steam. The device that does this job is the steam trap: an automatic valve that opens for condensate (and air) but closes against live steam.

There are three main operating principles:

• Mechanical traps (float-and-thermostatic, inverted bucket) sense density. A float rises with condensate level and opens a discharge valve; live steam, being less dense, can't lift the float. Good for high-condensate-load applications like heat exchangers.
• Thermostatic traps (bellows, bimetallic) sense temperature. They stay open while discharging cooler condensate and close as the fluid approaches saturation temperature. Common on steam tracing and radiators where some sub-cooling is acceptable.
• Thermodynamic traps (disc traps) exploit the flash-steam pressure difference. Cheap, compact, tolerant of superheat and freezing, but cycle audibly and wear out faster. Common on steam mains for drip legs.

Concrete example: a shell-and-tube heater handling 2,000 lb/hr of 100 psig steam needs a float-and-thermostatic trap sized for at least 3× that flow (6,000 lb/hr) to handle startup loads when cold condensate is forming fast and the system has minimal pressure differential. Undersize the trap and the heater "stalls" — condensate backs up, tubes flood, and heat transfer collapses.

Rule of thumb for sizing: use a safety factor of 2 to 3 on calculated condensate load for process equipment, and 2× the warm-up load for steam mains. Always check the differential pressure across the trap at the actual operating point — a trap rated for 100 psi won't discharge properly if downstream return line pressure leaves only 5 psi available.

Failed traps are surprisingly common and expensive. A trap stuck open blows live steam straight into the condensate return — a ½" orifice at 100 psig leaks roughly $5,000–$10,000 of steam per year. Industrial plants typically survey traps annually with ultrasonic or thermal imaging tools; 15–30% failure rates are normal in untended systems.