Pipe Inert Purge Volume and Time
Inert (nitrogen) purge volume and time for a pipe run while brazing or purging a gas / process line: pipe volume = pi/4 x ID^2 x length; purge volume = pipe volume x air changes; time = purge volume / flow. 100 ft of 2 in pipe holds 2.33 ft^3, so five volume changes is 11.65 ft^3 and about 12 minutes at 60 scfh; a 4 in line holds 8.84 ft^3 and takes 44 min. The volume changes and the acceptable oxygen or dew point come from the spec; a meter confirms the endpoint. Distinct from the room confined-space-purge.
Formula and source
pipe_volume_ft3 = (PI/4) x (pipe_id_in/12)^2 x length_ft; purge_volume_ft3 = pipe_volume_ft3 x air_changes; purge_min = purge_volume_ft3 / flow_scfh x 60.
Inert-purge identity by name (pipe volume from the bore; purge volume over the air changes; time over the flow); first-principles arithmetic.
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