Purge time for root shielding

Pipework, based on DVS guideline 0937 (2023), section 6

Input

mm
m
l/min

Result

Purge time
2 min 59 s
per metre of purge length
1 min 30 s
Pipe volume
3.93 l
Gas needed for pre-purging
29.85 l

That is 7.6 times the pipe volume (factor k).

A starting point, not an approval value: only an oxygen analyser close to the weld shows whether the residual oxygen has been reached.

How it is calculated

t [s] = 6 · π · di² · l / (400 · Q) · k

DVS 0937 gives this formula as a first guide for pipework. The fraction is the time in which the gas would fill the pipe volume exactly once. The factor k accounts for gas and air mixing: for 100 ppm, about 7.6 pipe volumes have to flow through.

Residual oxygen k
1,000 ppm 5.3
100 ppm 7.6
50 ppm 8.3
30 ppm 8.6
10 ppm 10.0
What the calculator does not know
  • The formula applies to pipework. There is no simple calculation for vessels and complex parts.
  • Doubling the flow halves the purge time only on paper. In practice too much gas stirs up the air and purging takes longer. Less gas without turbulence often works better.
  • Leaking joints, damp hoses and air drawn in through joints not yet welded extend the time.
  • Besides the formula, DVS 0937 gives a chart that yields shorter times. The calculator uses the formula because it takes the residual oxygen into account and is on the safe side.
Measure, do not guess

A lighter flame at the gas outlet only shows that gas is coming out, not how much oxygen is at the root. DVS 0937 recommends an oxygen analyser that measures below 10 ppm, with the probe as close to the weld as possible.

Based on DVS guideline 0937:2023-05 on root shielding in gas-shielded welding, section 6.