Before welding, after plant modifications, before commissioning a gas line: purging is required. But for how long? The Purge Time Calculator for Gas Lines replaces the rule of thumb with the dilution model – saving gas or rescuing your gas quality.
The dilution model
t = V ÷ Q × ln(C0 ÷ C) – purge time depends on system volume V, purge gas flow Q and the ratio of start to target concentration. The logarithm is the core: from 209,000 ppm O₂ (air) down to 10 ppm takes ln(20,900) ≈ 10 so-called volume exchanges – every further decade costs 2.3 more exchanges.
Step by step
- Determine the volume: enter pipe length and inner diameter – the calculator derives the volume; for vessels, enter the volume directly
- Select the purge flow: realistic flow in Nl/min at the flow meter
- Set the concentrations: start 209,000 ppm (air), target 100, 10 or 1 ppm depending on the application
- Read the result: purge time, number of volume exchanges and gas consumption
Worked example
50 m of DN10 stainless line (10 mm inner diameter): volume ≈ 3.9 l. At 2 Nl/min from air down to 10 ppm O₂: 12.3 volume exchanges → purge time ≈ 24 minutes, gas consumption ≈ 48 Nl. Purging a flat hour instead wastes over 70 Nl of nitrogen – with helium the same mistake gets properly expensive.
Limits of the model
The model assumes ideal mixing. Dead volumes – branches, valve cavities, blind holes – exchange their gas only by diffusion and need a multiple of the calculated time. In practice, pressure-cycle purging (repeatedly pressurising and venting) beats long continuous flow against dead volumes by far. The binding criterion remains the measurement at the outlet: the calculation tells you when measuring becomes worthwhile.
