Condensate Line Sizing — Two-Phase Flow

Pipe size for a trap discharge, a common return header or a pumped line — with the flash steam counted in.

Unit system

Inputs

Condensate flow

Pressure upstream of the trap (absolute)

Line pressure (absolute)

Duty of the lineAllowed vapour-phase velocity

The selection above sets the default; override it if your plant standard differs.Pipe schedule

Selected size

DN 80 (3")

Flash fraction (by mass)

0.1523

Inside diameter of the selected pipe	77.92 mm
Minimum required inside diameter	77.52 mm
Vapour velocity in the selected size	14.85 m/s
Volume flow of the flash steam	254.9 m³/h
Volume flow of the condensate	0.8845 m³/h
Vapour share of the volume	0.9965
If sized on the liquid alone	DN 15 — 5.3× smaller
Warning	99.7 % of the volume is vapour. This is not a WATER line; sized on the water flow it chokes and back-pressures the trap.

The condensate is taken as SATURATED water ahead of the trap. Sub-cooled condensate flashes less and the line gets smaller.
The size follows the vapour velocity; the water is carried as a film along the bottom and is assumed to take no cross-section.
On long runs the pressure drop must be checked separately — this screen applies the velocity criterion only.

A PDF carrying the inputs, results, assumptions and the source. A one-time code is sent to your address.

Results are for engineering pre-assessment and are not a product selection commitment. Final responsibility rests with the user.