What Is a Steam Trap? Condensate Removal
Key Specifications
| Feature | Details |
|---|---|
| Function | Remove condensate and air; retain live steam |
| Main types | Mechanical (float, inverted bucket), thermostatic (bellows, bimetallic), thermodynamic (disc) |
| Operation | Fully automatic (self-actuating) |
| Connections | Threaded (NPT/BSP), socket weld, flanged |
| Sizes | 1/2” to 2” (typical); up to 4” for high-capacity |
| Pressure rating | Up to Class 600 (varies by type) |
| Body materials | Carbon steel, 316 SS, ductile iron |
| Standards | ISO 6552 (terminology), ISO 6553 (marking), BS 6023 |
| Testing | ISO 7841 (determination of steam loss), ISO 7842 (operating characteristics) |
Steam Trap Types
| Type | Operating Principle | Response | Condensate Discharge | Air Venting |
|---|---|---|---|---|
| Float (mechanical) | Ball float rises with condensate level | Continuous | Continuous and modulated | Good (with thermostatic air vent) |
| Inverted bucket (mechanical) | Bucket floats on steam, sinks on condensate | Intermittent | Intermittent (slug discharge) | Fair (limited vent hole) |
| Thermostatic (bellows) | Bellows contracts when temperature drops below steam temp | Intermittent | Near steam temperature | Excellent |
| Bimetallic (thermostatic) | Bimetallic element deflects with temperature | Intermittent | Sub-cooled condensate | Good |
| Thermodynamic (disc) | Disc lifts on condensate, seats on flash steam | Intermittent (cyclic) | Flash-based discharge | Fair |
How Each Type Works
Float trap: a hollow ball float rises as condensate fills the trap body. A lever connected to the float opens a valve at the bottom, discharging condensate continuously. When the condensate level drops, the float lowers and the valve closes. A separate thermostatic element vents air during startup.
Inverted bucket: an inverted cup (bucket) inside the trap body floats when steam enters (buoyancy from steam inside the bucket). This closes the discharge valve. When condensate fills the bucket, it sinks, opening the valve and discharging the condensate.
Thermostatic: a temperature-sensing element (liquid-filled bellows or bimetallic strip) opens the valve when the temperature drops below saturation. Condensate, which is cooler than live steam, causes the element to contract and open the discharge.
Thermodynamic: a flat disc inside a chamber lifts when condensate pressure pushes it up. The condensate flashes to steam in the control chamber above the disc, creating pressure that pushes the disc down (closed). The cycle repeats as the flash steam condenses.
Selection Guide
| Application | Recommended Type | Reason |
|---|---|---|
| Process heat exchangers | Float trap | Continuous discharge prevents condensate flooding |
| Steam main drip legs | Inverted bucket or thermodynamic | Handles varying loads, durable |
| Tracing lines | Thermostatic (bellows) | Compact, drains sub-cooled condensate |
| Superheated steam lines | Bimetallic or thermodynamic | Resistant to superheat damage |
| High-pressure mains (40+ bar) | Thermodynamic or inverted bucket | reliable at high differential pressure |
Energy Impact
A single 1/2” steam trap stuck open at 10 bar can waste 25-50 kg/h of live steam; roughly $15,000-$30,000 per year in fuel cost. Regular steam trap surveys (annual minimum) using ultrasonic and temperature testing identify failed traps. Industry data shows 15-30% of steam traps in a typical plant are malfunctioning at any given time.
Leave a Comment
Have a question or feedback? Send us a message.