Sizing and Testing Steam Traps
A trap is sized for the condensate it must pass at the pressure difference it actually sees, with a safety factor for start-up. Once installed, it fails silently, so every trap needs testing on a schedule. A single trap blowing steam can waste tonnes of fuel a year.
You'll learn
- Size a trap for load, differential pressure and safety factor
- Test traps by sight, sound and temperature
- Estimate the loss from a trap failed open
- Set up a trap survey program that pays
Before this: Types of steam traps
Size by condensate, not by pipe
A trap is chosen for how much condensate it must pass and the pressure difference across it, not to match the pipe it sits in. An oversized trap cycles hard and wears quickly; an undersized one cannot keep up and floods the equipment. The connection size is the last thing to check, not the first.
The pressure difference is the steam pressure before the trap minus the pressure after it — the back pressure of the return line, plus any lift. A trap that discharges against a raised return line has less pressure pushing condensate through it than the steam gauge suggests.
Safety factors
| Application | Factor | Why |
|---|---|---|
| Steam main drainage | 2 | Size on the warm-up load, which is far above running |
| Constant-pressure equipment | 2 | Usual margin over running load |
| Temperature-controlled equipment | 3 | The coil floods on a call for heat and must clear quickly |
- Running condensate 300 kg/h.
- Selection load: 300 × 3 = 900 kg/h, at the lowest differential pressure the trap will see when the control valve is throttled.
Testing, and what a failed trap costs
- Sight: a test tee or sight glass after the trap shows whether it discharges condensate in pulses or continuously, or blows a sharp jet of live steam.
- Sound: an ultrasonic detector hears the steady hiss of steam passing through a closed trap, and the cycling of a working one.
- Temperature: a trap much colder than steam temperature on its inlet is likely blocked; one hot downstream may be passing steam.
- A trap at 10 kg/cm²(g) fails with its 6 mm orifice wide open.
- Coal at an indicative 4,000 kcal/kg, boiler 82% efficient, feedwater 80 °C, 8,000 hours a year.
- Steam lost through the orifice (Napier’s approximation): 187 kg/h.
- Coal to make each kilogram of that steam: 0.178 kg.
- Per year: 187 × 8,000 × 0.178 = 265.7 tonnes of coal.
Key takeaways
- Size traps by condensate load and actual differential pressure, not pipe size.
- Safety factors: about 2 for mains and constant-pressure equipment, 3 for temperature-controlled.
- Test by sight, ultrasound and temperature, on a schedule.
- One 6 mm trap failed open at 10 kg/cm²(g) wastes about 187 kg/h of steam.