Types of Steam Traps: A Complete Guide

What Steam Traps Do

Every steam system produces condensate as steam gives up its latent heat. If condensate is not removed promptly, it reduces heat transfer efficiency, causes water hammer, and can damage equipment. A steam trap sits at every drainage point in the system and performs three critical functions:

• Discharge condensate as soon as it forms, keeping equipment filled with live steam
• Vent air and CO₂ that enter the system during startup and operation
• Conserve live steam by preventing it from escaping through the trap

Steam traps achieve this through three operating principles: mechanical (density difference), thermostatic (temperature difference), and thermodynamic (velocity and pressure changes).

Mechanical — Inverted Bucket Steam Traps

The inverted bucket is widely regarded as the most reliable steam trap operating principle. Armstrong inverted bucket traps use a free-floating stainless steel mechanism with no fixed pivots to create wear or friction.

How It Works

A unique leverage system multiplies the force provided by the bucket to open the valve against system pressure. Because the mechanism is located at the top of the trap, no dirt can collect on the orifice. Small particles are held in suspension until discharged by full differential purging action when the bucket sinks, pulling the valve off the seat.

The discharge orifice is surrounded by a water seal, preventing live steam loss. A small vent hole in the bucket provides continuous automatic air and CO₂ venting at steam temperature. Inverted bucket traps drain continuously while discharging intermittently, allowing no condensate backup. They are also resistant to water hammer.

Armstrong Inverted Bucket Series

• Cast Iron (200, 800, 880 Series) — Pressures to 250 psig, capacities to 20,000 lb/hr. The 880 Series includes an integral strainer.
• Forged Steel (300, 521 Series) — Pressures to 650 psig for higher-pressure industrial applications.
• Forged Chrome-Moly (400, 5000, 6000 Series) — Pressures to 2,700 psig for power generation and high-pressure process.
• Stainless Steel (1000, 1800, 2000 Series) — ASTM A240 Grade 304L construction typically lasts three to four times longer than conventional traps. Three-year guarantee.

Browse all Armstrong inverted bucket steam traps →

Mechanical — Float & Thermostatic Steam Traps

Float & Thermostatic (F&T) traps combine a ball float for continuous condensate drainage with a balanced-pressure thermostatic air vent for rapid air and CO₂ removal.

How It Works

The ball float rises and falls with the condensate level, continuously modulating the valve to drain condensate as fast as it forms. A separate thermostatic element opens to discharge air and CO₂ at temperatures slightly below steam temperature.

Armstrong B and BI Series F&T traps are ideal for heating service where continuous drainage with high air-venting capacity is required. The thermostatic air vent is charged for compensated response to the pressure-temperature curve of steam from less than 20″ Hg vacuum to 30 psig gauge.

Key Advantage: Continuous Discharge

Unlike inverted bucket traps that discharge intermittently, F&T traps provide truly continuous condensate drainage. This makes them especially effective for heat exchangers, HVAC coils, and other applications where condensate must be removed immediately to maintain heat transfer.

Browse all Armstrong F&T steam traps →

Thermodynamic — Controlled Disc Steam Traps

Armstrong’s CD-33 Series controlled disc traps use a thermodynamic principle—the disc responds to differences in velocity and pressure between steam and condensate.

How It Works

The CD-33 is designed with three discharge ports, which offer stable disc operation to extend operating life. By controlling the cycle rate, the CD-33 extends its service life well beyond that of typical disc traps, minimizing maintenance and reducing steam costs.

Built from ASTM A743 GR. CA40 stainless steel with a maximum allowable pressure of 915 psig at 752°F and maximum operating pressure of 600 psig, these compact traps are engineered for steam main drips, process equipment, and HVAC heating on constant pressure.

CD-33L for Low-Capacity Applications

The CD-33L (low capacity) is designed specifically for steam main drips and tracing lines. By matching capacity to application—rather than oversizing—the CD-33 Series lasts longer than disc traps with excessive capacity ratings.

Browse all Armstrong disc steam traps →

Steam Trap Comparison: Inverted Bucket vs F&T vs Disc

Which Type for Which Application?

• Drip legs and steam mains: Inverted bucket traps are the standard choice. Dirt handling and water hammer resistance make them ideal for main line drainage.
• Steam tracing: IB traps, especially the stainless steel 1000 Series for corrosion resistance and long life in outdoor installations.
• Heating and HVAC: F&T traps (B/BI Series) are ideal due to continuous drainage, high air-venting capacity, and vacuum-to-30-psig operating range.
• Process equipment: IB or F&T depending on whether intermittent or continuous discharge is preferred and the pressure/capacity requirements.
• High pressure (400+ psig): IB traps in forged steel (300 to 650 psig), chrome-moly (400 to 1,000 psig; 5000 to 1,800 psig; 6000 to 2,700 psig).
• Compact installations: CD-33 disc traps where space is limited and pressures are within 600 psig.