Inverted Bucket vs Float & Thermostatic Steam Traps
Armstrong manufactures both inverted bucket and float & thermostatic steam traps. Each type excels in different applications. This guide compares their operating principles, specifications, and best-fit applications to help you select the right trap.
Operating Principle
Inverted Bucket
The inverted bucket trap uses a free-floating stainless steel bucket mechanism. When steam enters, the bucket floats and closes the valve. As steam condenses, the bucket sinks and opens the valve to discharge condensate. This produces an intermittent discharge pattern—the trap cycles between open and closed. A unique leverage system multiplies the bucket force to open against system pressure, and a vent hole in the bucket provides continuous air and CO₂ venting.
Float & Thermostatic
The F&T trap uses a ball float connected to a valve. As condensate accumulates, the float rises and opens the valve proportionally. This provides continuous, modulating discharge—condensate drains as fast as it forms. A separate thermostatic air vent element discharges air and non-condensables at temperatures just below steam temperature.
Head-to-Head Comparison
| Feature | Inverted Bucket | Float & Thermostatic |
|---|---|---|
| Discharge | Intermittent (blast discharge) | Continuous (modulating) |
| Air Venting | Continuous via bucket vent hole | Dedicated thermostatic element—faster on startup |
| Water Hammer Resistance | Highly resistant (robust mechanism) | Less resistant (float can be damaged) |
| Dirt Handling | Excellent—orifice at top, self-cleaning purge action | Good, but dirt can accumulate around float |
| Pressure Range | Vacuum to 2,700 psig | Vacuum to 30 psig (B Series) / 465 psig (FT-4000) |
| Material Options | Cast iron, forged steel, stainless, chrome-moly | Cast iron (B/BI Series) |
| Field Repairable | Yes—in-line, without removing from piping | Yes—internal components replaceable |
| Freeze Resistance | Good (SS models excellent) | Less resistant (water-filled body) |
When to Use Each Type
Choose Inverted Bucket When:
- Application involves drip legs, steam mains, or tracer lines
- Water hammer is a concern
- System pressure exceeds 30 psig
- Dirty steam or outdoor/corrosive environments (stainless 1000 Series)
- High-pressure service above 250 psig (300, 400, 5000, 6000 Series)
Choose F&T When:
- Heating and HVAC applications requiring continuous drainage
- Vacuum return systems (B/BI Series operates from 20″ Hg vacuum to 30 psig)
- Heat exchangers where immediate condensate removal maximizes heat transfer
- High air-venting capacity is needed at startup
Armstrong Model Quick Reference
| Application | Recommended IB Series | Recommended F&T Series |
|---|---|---|
| Low-pressure heating (<30 psig) | 800, 880 (cast iron) | B/BI Series (ideal choice) |
| Steam mains / drip legs | 800, 880, 300 (best choice) | Not typical |
| Steam tracing | 1000, 1800 (stainless steel) | Not typical |
| Medium pressure (30–250 psig) | 800, 300, 1000 | FT-4000 Series |
| High pressure (>250 psig) | 300, 400, 5000, 6000 | Not available |