Condensate return line sizing (flash steam)
Enter the steam pressure before the trap, the condensate flow, and the return line pressure to get the flash fraction and flash steam flow, and a recommended return line size based on flash steam velocity. Assumptions, sources, and checks against published examples are listed below.
Verification: comparison with published values
Published Spirax Sarco and Armstrong examples alongside this calculator’s result for the same inputs. Published values are rounded to one decimal. The table is recalculated at every build, and the build fails if a value falls outside tolerance.
| Item | Published value | Calculator value | Source |
|---|---|---|---|
| F1 | |||
| Flash fraction | 13.4 % | 13.40 % | Spirax Sarco "Flash Steam" example: 7 bar g → 0 bar g, condensate 250 kg/h |
| Flash steam flow | 33.5 kg/h | 33.49 kg/h | Spirax Sarco "Flash Steam" example: 7 bar g → 0 bar g, condensate 250 kg/h |
| F2 | |||
| Flash steam flow | 24.5 kg/h | 24.51 kg/h | Spirax Sarco "Flash Steam" example: 7 bar g → 1 bar g, condensate 250 kg/h |
| F3 | |||
| Flash fraction | 10.6 % | 10.63 % | Armstrong Flash Recovery Vessel catalog application example: 100 psig → 10 psig |
Calculation basis
Saturated water before the trap is assumed to expand adiabatically to the return line pressure, and the flash fraction follows from an energy balance. Saturation properties are interpolated from the same IAPWS-IF97 steam table as the steam pipe calculator.
| Item | Equation | Note |
|---|---|---|
| Absolute pressure | p_abs = p_g + 101.325 kPa | Adds standard atmospheric pressure, 101.325 kPa |
| Saturation properties | h_f, h_fg, v_g, v_f = f(p_abs) | IAPWS-IF97 table, 4-point interpolation in ln(p) (same table as the steam pipe calculator) |
| Flash fraction | x = (h_f1 − h_f2) / h_fg2 | The excess sensible heat evaporates part of the water at the return line latent heat (energy balance) |
| Flash steam and remaining liquid | ṁ_f = x · ṁ, ṁ_l = (1 − x) · ṁ | Splits the condensate flow into steam and liquid |
| Flash steam volumetric flow | Q_f = ṁ_f · v_g2 | Saturated steam specific volume at the return line pressure, v_g2 |
| Required bore | D = √(4 Q_f / (π V)) | From the flash steam design velocity; the smallest standard size with at least this bore is recommended |
Design velocity: based on flash steam
A trap discharge line carries flash steam together with condensate. By volume the flash steam dominates (from 7 bar g to atmosphere it occupies over 200 times the liquid volume), so the line is sized on flash steam velocity.
Spirax Sarco, "Sizing Condensate Return Lines", treats trap discharge lines as steam lines carrying very wet steam that should be sized on similarly low velocities, and gives 15 to 20 m/s for wet steam. This calculator defaults to the lower end, 15 m/s, as a practical value and warns above 20 m/s.
An undersized line raises flash steam velocity and back pressure, which can cause waterhammer, reduce trap capacity, and flood the process. Pumped condensate (liquid only), trap selection, and waterhammer analysis are outside this calculator.
Pipe size tables
Pick a pipe schedule to see the outside diameter, wall thickness, and bore for each size.
STPG Sch40 (KS D 3562 / JIS G3454)
KS D 3562 / JIS G 3454 carbon steel pipe for pressure service Sch40. Default in this calculator.
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A | 21.7 | 2.8 | 16.1 |
| 20A | 27.2 | 2.9 | 21.4 |
| 25A | 34.0 | 3.4 | 27.2 |
| 32A | 42.7 | 3.6 | 35.5 |
| 40A | 48.6 | 3.7 | 41.2 |
| 50A | 60.5 | 3.9 | 52.7 |
| 65A | 76.3 | 5.2 | 65.9 |
| 80A | 89.1 | 5.5 | 78.1 |
| 100A | 114.3 | 6.0 | 102.3 |
| 125A | 139.8 | 6.6 | 126.6 |
| 150A | 165.2 | 7.1 | 151.0 |
| 200A | 216.3 | 8.2 | 199.9 |
| 250A | 267.4 | 9.3 | 248.8 |
| 300A | 318.5 | 10.3 | 297.9 |
SGP (KS D 3507 / JIS G3452)
KS D 3507 / JIS G 3452 carbon steel pipe for ordinary piping (SPP/SGP). Only for use below 1 MPa (KDS 31 25 25:2021).
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A | 21.7 | 2.8 | 16.1 |
| 20A | 27.2 | 2.8 | 21.6 |
| 25A | 34.0 | 3.2 | 27.6 |
| 32A | 42.7 | 3.5 | 35.7 |
| 40A | 48.6 | 3.5 | 41.6 |
| 50A | 60.5 | 3.8 | 52.9 |
| 65A | 76.3 | 4.2 | 67.9 |
| 80A | 89.1 | 4.2 | 80.7 |
| 100A | 114.3 | 4.5 | 105.3 |
| 125A | 139.8 | 4.5 | 130.8 |
| 150A | 165.2 | 5.0 | 155.2 |
| 200A | 216.3 | 5.8 | 204.7 |
| 250A | 267.4 | 6.6 | 254.2 |
| 300A | 318.5 | 6.9 | 304.7 |
ASME B36.10M Sch40
ASME B36.10M Sch40 (inch sizes).
| Size | Outside dia. mm | Wall mm | Bore mm |
|---|---|---|---|
| 15A (1/2) | 21.34 | 2.77 | 15.8 |
| 20A (3/4) | 26.67 | 2.87 | 20.93 |
| 25A (1) | 33.4 | 3.38 | 26.65 |
| 32A (1-1/4) | 42.16 | 3.56 | 35.05 |
| 40A (1-1/2) | 48.26 | 3.68 | 40.89 |
| 50A (2) | 60.33 | 3.91 | 52.5 |
| 65A (2-1/2) | 73.03 | 5.16 | 62.71 |
| 80A (3) | 88.9 | 5.49 | 77.93 |
| 100A (4) | 114.3 | 6.02 | 102.26 |
| 125A (5) | 141.3 | 6.55 | 128.19 |
| 150A (6) | 168.28 | 7.11 | 154.05 |
| 200A (8) | 219.08 | 8.18 | 202.72 |
| 250A (10) | 273.05 | 9.27 | 254.51 |
| 300A (12) | 323.85 | 10.31 | 303.23 |
Sources
- Spirax Sarco, Steam Engineering Tutorials, "Flash Steam"
- Spirax Sarco, Steam Engineering Tutorials, "Sizing Condensate Return Lines"
- Armstrong International, Flash Recovery Vessel (catalog)
- IAPWS-IF97 (International Association for the Properties of Water and Steam), same saturated steam table as the steam pipe calculator
Frequently asked questions
How is the flash fraction calculated?
The difference between the saturated water enthalpy before the trap, h_f1, and at the return line pressure, h_f2, evaporates water at the return line latent heat h_fg2, so x = (h_f1 − h_f2) / h_fg2. From 7 bar g to atmosphere this gives about 13.4 %, the same as the Spirax Sarco example.
What return line pressure should I enter?
Enter 0 if the line runs to a vented receiver. If it works against the pressure of a condensate or flash vessel, or a lift, enter that gauge pressure. The higher the return line pressure, the less flash steam forms.
Can I size pumped condensate lines with this calculator?
No. After the pump only liquid flows, with no flash steam, so size it like a water line with the pipe sizing calculator (velocity or friction loss basis). Trap selection and waterhammer checks are also outside this calculator.
Last updated 2026-11-01.