Steam Trap
General / GeneralDefinition
A steam trap is an automatic valve that removes condensate and non-condensable gases from steam systems while preventing live steam from escaping. Every steam-using system — from a simple heater to a complex turbine cycle — needs traps at condensate collection points. Without them, condensate backs up into equipment, reducing heat transfer and causing water hammer. With the wrong trap type, live steam blows through, wasting energy and raising operating costs dramatically.
Working Principle
Steam traps sense the difference between steam and condensate using one of three principles: mechanical (density difference — condensate is denser), thermostatic (temperature difference — condensate is cooler), or thermodynamic (flow velocity difference — steam moves faster). When condensate accumulates at the trap, the trap opens and discharges it. When steam reaches the trap, the trap closes. The specific mechanism varies by trap type, and choosing the wrong type for the application leads to either condensate flooding or live-steam loss.
Key Components
| Trap body | Housing for the internal mechanism; inlet and outlet connections |
| Internal mechanism | Varies by type: float, bimetallic disc, orifice, etc. |
| Inlet connection | Typically threaded, flanged, or socket-weld |
| Outlet connection | Same options as inlet; may include a check valve feature |
| Strainer (optional) | Removes pipe scale and debris before the mechanism |
| Test valve | Allows manual check of trap operation without disconnecting |
Types & Variants
| Type | Operating Principle | Discharge | Best Application |
|---|---|---|---|
| Float & thermostatic (F&T) | Float rises with condensate level | Continuous | Low- and medium-pressure heating, temperature control |
| Inverted bucket | Bucket sinks in condensate, rises in steam | Intermittent | High-pressure steam mains, process heaters |
| Thermodynamic (disc) | Steam velocity creates pressure drop under disc | Intermittent | Steam tracing, distribution, outdoor/remote locations |
| Bimetallic | Bimetal strips bend with temperature change | Intermittent/continuous | Energy conservation, variable-pressure systems |
| Orifice trap | Fixed orifice sized for condensate flow | Continuous (no moving parts) | Steam tracing, constant-load applications |
Materials
Cast iron bodies for low-pressure heating systems (up to 250 psi). Forged steel or cast steel for medium and high pressure (up to 600 psi and above). Stainless steel internals for corrosion resistance. The F&T trap uses a float (usually stainless) and a thermostatic element (bellows or bimetal). Inverted bucket traps use a stainless bucket. Thermodynamic disc traps have a single moving part — the disc — typically 316 stainless steel. Connections: NPT threaded is most common, socket-weld for high-pressure, flanged for large sizes.
Applicable Standards
ASME PTC 39 (steam trap performance testing), ISO 7841 (steam traps — determination of steam loss), ISO 7842 (steam traps — determination of drainage capacity). EN 14535 covers thermostatic traps. ASHRAE Handbook provides steam trap selection guidance. No single design standard covers all trap types — each manufacturer's catalog is the primary reference.
Applications
Steam distribution mains: inverted bucket or thermodynamic traps at drip legs remove condensate before it reaches equipment. Heat exchangers: float and thermostatic traps discharge condensate continuously to maintain heat transfer. Steam tracing: thermodynamic disc traps are standard on instrument and pipe tracing because they work in any orientation and tolerate freezing. Building heating: F&T traps on radiators and unit heaters. Process reactors: inverted bucket or F&T traps depending on pressure stability. Turbine extraction: specialized high-capacity traps d