A steam trap is an automatic device that removes condensate from a steam system without letting live steam escape. A simple function, but crucial for energy efficiency.

Why condensate is a problem

Steam carries energy as latent heat. When it gives up that heat (in a heat exchanger, radiator, etc.), it turns into condensate — water at the same temperature. Condensate must be removed because:

Reduces heat transfer — a water film insulates the heat exchange surface
Causes water hammer — slugs of water accelerated by steam slam into bends and valves
Promotes corrosion — condensate is corrosive, especially if untreated
Blocks flow — accumulated condensate obstructs steam flow

How steam traps work

All traps exploit a physical difference between steam and condensate:

Temperature — condensate is slightly cooler than saturated steam (thermostatic traps)
Density — condensate is much denser than steam (mechanical traps: float, bucket)
Velocity — flashing condensate creates different flow patterns (thermodynamic traps)

Three families of steam traps

1. Mechanical traps

Use a float or bucket that rises/falls with condensate level.

Float traps (FT)

Continuous discharge, modulates automatically. Ideal for high condensate loads. Sensitive to water hammer.

Inverted bucket (IB)

Robust, tolerates water hammer and dirt. Intermittent discharge. Requires "water seal" after shutdown.

2. Thermostatic traps

A temperature-sensitive element opens when condensate cools below saturation temperature.

Economical, compact. Discharges sub-cooled condensate (5-20°C below saturation). Good for tracing and drip legs.

3. Thermodynamic traps

A free disc lifts with condensate (low velocity) and seats when flash steam arrives (high velocity, low pressure above disc).

Compact, inexpensive, tolerates water hammer. Cyclic operation (audible "click"). Not suitable for low pressures (<1 bar) or high back pressure.

Quick comparison

Type	Discharge	Water hammer	Start-up	Cost
Float	Continuous	⚠️ Sensitive	✅ Fast	€€€
Inverted bucket	Intermittent	✅ Robust	⚠️ Needs prime	€€
Thermostatic	Intermittent	⚠️ Sensitive	✅ Fast	€
Thermodynamic	Cyclic	✅ Robust	✅ Fast	€

Conclusion

The steam trap is the "silent guardian" of steam system efficiency. Correct selection and regular maintenance can save thousands of euros per year in energy.

Roffia distributes steam traps from Spirax Sarco, the world leader in steam management. Contact us for sizing and selection support.

What is a steam trap?