Blog categories

View all categories

Search in blog

Latest posts

Valve Materials and Elastomers

Posted in: Engineering guides

06/14/2026

Valve material selection should not be based only on nominal pressure or valve size. Operating pressure, temperature,...

View all latest posts

Blog RSS

Calculating Flash Steam

TECHNICAL DATA

Flash Steam Generated from Condensate Pressure Reduction

Flash steam is generated when hot condensate at a higher pressure is released to a lower pressure. The pressure reduction lowers the saturation temperature, and part of the heat contained in the condensate is converted into steam.

This phenomenon commonly occurs downstream of steam traps, pressure reducing points, condensate return lines and flash vessels. Correct estimation of flash steam quantity is important for sizing condensate lines, flash vessels, vents and energy recovery systems.

Basic Calculation Principle

When condensate is close to the boiling point at its initial pressure and is released to a lower pressure, the flash steam percentage can be calculated from the difference in saturated water enthalpy before and after pressure reduction.

Flash steam fraction = (h_f1 - h_f2) / h_fg2

h_f1: saturated water enthalpy at the initial pressure
h_f2: saturated water enthalpy at the lower pressure
h_fg2: latent heat of evaporation at the lower pressure
Flash steam fraction: kg of flash steam per kg of condensate

To convert the result into a percentage:

Flash steam (%) = Flash steam fraction × 100

Calculation Example

Indicative example for condensate released downstream of a steam trap:

Operating Data	Value
Gauge pressure upstream of the steam trap	10 bar
Gauge pressure downstream of the steam trap	0 bar
Flash steam quantity	0.162 kg/kg
Flash steam percentage	16.2%

This means that for every 1 kg of condensate released from 10 barg to approximately atmospheric pressure, about 0.162 kg of flash steam may be generated, provided the condensate is close to its boiling point.

Why Flash Steam Matters

Technical Issue	Effect on the System	Design Note
Condensate line sizing	Flash steam significantly increases the volume flow.	The line should not be sized as a water-only line.
Steam trap back pressure	Flash steam may increase condensate return pressure.	Back pressure affects steam trap capacity and operation.
Noise and vibration	Two-phase water-steam flow may create unstable conditions.	Correct pipe slope, drainage and diameter selection are required.
Energy recovery	Flash steam can be reused in lower-pressure steam applications.	A flash vessel can be used to separate and recover usable steam.
Installation safety	Incorrect estimation may cause overpressure or insufficient venting.	Safety valves, vents and pipework should be checked.

Applications in Steam and Condensate Systems

Condensate return lines downstream of steam traps.
Flash vessels for recovery of low-pressure steam.
Feedwater preheating systems.
Low-pressure steam networks for auxiliary services.
Back pressure assessment in condensate headers.
Sizing of vents, drains and condensate return pipework.

Related Equipment

Philippopoulos S.A. supplies equipment for steam and condensate systems, including steam traps, flash vessels, separators, strainers, isolation valves, check valves, pressure reducing valves, safety valves, condensate pumps and boiler house equipment.