This week we are re-visiting an article published in Stainless Steel World by By Dr. M J Fletcher, from Delta Consultants. A pre-purge is used to displace air present in the pipework system or dam volume, Dr. Fletcher writes. Numerous factors control the pre-purge time such as pipe diameter, purge volume and maximum permitted oxygen level.
A common misconception is that increasing the purge flow rate will reduce the purge time, however this is incorrect. Increase in flow rate increases turbulence and results in unwanted mixing of purge gas and air and can actually extend the purge time. As a general rule, the pre-purge flow rate and time should allow for about five volume changes in the pipe system or dam volume, but a typical gas flow rate will be in the region of 20 l/min.
Weld joints that require a root gap or which exhibit bad end matching, both of which characteristics provide an unwanted leak path for the purge gas, can be sealed on the outside by taping.
Oxygen and moisture levels in the purge gas should be checked using a weld purge monitor and/or appropriate equipment with checking taking place at the outlet point. Where dam inserts are being used, the outlet point needs to be extended with a flexible pipe to a convenient access position. If this is impractical, a system which has the purge inlet and outlet in the same dam unit should be used.
