Vent Pipe Sizing

The vent of a flash vessel, condensate receiver or deaerator — checked on velocity and on the back pressure it creates.

Unit system

Inputs

Steam to be vented

You can bring this over from the flash steam calculation.Vessel pressure (absolute)

Allowed velocity

15–25 m/s is common for atmospheric vents. Lower it where noise is restricted.Vent pipe length

Equivalent length of bends and fittings

Each 90° bend adds roughly 30 diameters of equivalent length.Pipe schedulePipe roughnessAllowed back pressure

The pressure rise in the vessel must stay under this — that rise is back pressure on the traps.

Selected size

DN 80 (3")

Back pressure produced

0.001845bar a

Velocity in the selected size	14.62 m/s
Inside diameter	77.92 mm
Minimum inside diameter from the velocity criterion	66.62 mm
Volume flow	251 m³/h
Steam density	0.5976 kg/m³
Governing criterion	Set by the velocity limit
Warning	The pressure rise in the vessel is 1.8 mbar — that is the back pressure on the traps, and it is within the limit.

The vented steam is taken as SATURATED and dry; entrained droplets are not counted.
The pressure drop assumes constant density along the vent. Above 10 % of the vessel pressure this assumption weakens.
The visible white plume is condensing flash steam, not a fault — throttling the vent breaks the system.

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.