Christopher Tidy wrote:

Am Freitag, 6. Oktober 2017 02:00:24 UTC+2 schrieb David Billington:

Items that are hot dip galvanised have to be vented in order to prevent
dangerous pressure build-up, I've had many items dipped and it is a good
process but the sections need to be vented to relieve the pressure
build up.

Makes sense. Not what I guessed, though. I was guessing it was to prevent
stagnant water from building up inside the tubes and causing corrosion.
If a closed segment had some water left in it by accident, and then you
welded on it, it could explode. Some people have apparently been killed in
such explosions, or horribly burned by flying molten metal.

Theoretically, this could happen just from trapped air, too.

Jon