Explosion on a cold box

An explosion occurred in a nitrogen wash cold box (outside the equipment) after a several day cold shutdown followed by a derime with nitrogen. It is assumed that ice may have split tubes permitting carbon monoxide and hydrogen to leak into the perlite installation. It is likewise assumed that the insulation had channelled and had many voids.
However, the insulation was continuously purged with nitrogen and the mechanism of the explosion is still unknown.

The INITIATING CAUSE were cracks in aluminium welds which leaked hydrogen cumulating in the cold box.

A CONTRIBUTING FACTOR was the cold weather which could have caused the formation of ice. Since the cold box thermal insulation, consisting of perlite, was purged with nitrogen, it is unclear why hydrogen and CO could form an explosive atmosphere.

The ROOT CAUSE is not provided. It could nevertheless plausibly attributed to design and/or manufacturing (welding) error.

cold box

DECRITION OF THE PROCESS
A "derime" is a maintenance procedure used to remove frozen impurities from a cryogenic system, such as a nitrogen or air separation unit. This is done by warming up the equipment and flowing a warm, dry gas through it to melt and purge any accumulated moisture, carbon dioxide, or other contaminants that would otherwise freeze and block the system. The terms "derime," "defrost," "de-ice," and "thaw" are often used interchangeably for this process.