Steam trap sizing
A trap does not pass condensate — it passes a flashing mixture, because the pressure drops across the orifice and part of the condensate re-evaporates on the way through. Sizing on the liquid flow alone undersizes the trap, and it shows up as water logging rather than as a calculation error. The return line has to carry the flash, not the condensate.
| Quantity | Value | Note |
|---|---|---|
| Condensate load | 500 kg/h | as entered |
| Safety factor | 2.0× | start-up and fouling |
| Design capacity | 1,000 kg/h | what the trap must pass |
| Steam pressure | 8.0 bar | upstream |
| Return pressure | 0.50 bar | downstream |
| Orifice | 3.80 mm | liquid orifice equation |
| Flash fraction | 16.50% | (h_f1 − h_f2)/h_fg2 |
| Flash flow | 83 kg/h | vapour formed |
| Flash volume | 267 m³/h | at the return pressure |
| Return bore | 79 mm | at 15 m/s |
| Nominal return | DN 80 | next standard size |
| Trap type | float | suits this duty |
The trap passes 1000 kg/h of condensate, of which 83 kg/h flashes to steam at the return pressure — 267.4 m³/h of vapour that the return line has to carry. That is why the return line is sized on the flash, not the condensate.
ṁ = 0.65 A √(2ρΔP) · Liquid discharge.
(h_f1 − h_f2)/h_fg2 · Energy balance.
A = V̇/v · At 15 m/s.
load × safety factor · 2× is typical.
A clear path from inputs to results.
A trap does not pass condensate — it passes a flashing mixture, because the pressure drops across the orifice and part of the condensate re-evaporates on the way through. Sizing on the liquid flow alone undersizes the trap.
How to use
- Enter the condensate load.
- Set the steam and return pressures.
- Choose a safety factor and a trap type.
- Read the orifice, the flash fraction and the return line size.
The formula
Reference: Liquid orifice discharge for trap capacity; flash from the saturated steam table (IAPWS-IF97).
Worked example
500 kg/h at 8 bar to 0.5 bar
- 500 kg/h condensate, 8 bar steam, 0.5 bar return.
- Safety factor 2, float trap.
- 1
The design capacity is 1,000 kg/h.
- 2
Across 7.5 bar the orifice works out at 3.8 mm.
- 3
The flash fraction is (721.0 − 340.5)/2305.4 = 16.5%.
- 4
That is 82.5 kg/h of flash, occupying 267 m³/h at the return pressure.
- 5
At 15 m/s the return line needs a 79 mm bore, so DN 80.
3.8 mm orifice · 16.5% flash · DN 80 return.
Interpreting the result
The return line is the part people get wrong. The flash is only a sixth of the mass but it is most of the volume, so a return line sized on the condensate flow will be far too small and will run with a two-phase mixture at high velocity.
Assumptions & limitations
- Saturated condensate at the trap inlet.
- A discharge coefficient of 0.65.
- No heat loss between the trap and the return line.
- A single trap, not a group.
Frequently asked questions
Why the safety factor?
Because a trap must pass the start-up load, which is often two or three times the running load, and because the orifice fouls over time. 2× is conventional.
Why does the flash matter for the return line?
Because vapour occupies hundreds of times the volume of the liquid. A return line sized on condensate will be two-phase and noisy, and will back up the traps.
Does back pressure affect the trap?
Enormously. A thermodynamic trap stops cycling if the back pressure approaches the inlet pressure, and a float trap loses capacity as the differential falls.
Which trap type should I use?
Float-thermostatic for a modulating load, thermodynamic for high pressure and superheat, thermostatic for small loads where live steam loss matters.
Can I recover the flash?
Yes — a flash vessel takes the vapour off at an intermediate pressure and returns it to a low-pressure main. The flash-steam page values it.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.