Fire main dust cap blown away by pressure from the line

What happened?

A fire main dust cap was blown off by pressure from the line and hit the wall 10m away.  The incident occurred when a fire team were checking the firefighting water line filling point connection.  The filling point had a camlock fitting and the blank end steel dust cap was in position.  The fire team proceeded to release the dust cap by disconnecting the clasps.  When the clasp was released, pressure from behind the dust cap blew it off, causing it to hit the wall approximately 10 meters away.  This caused minor damage to the wall.  There were no injuries to personnel.

What actions were taken?  What lessons were learned?

Work was stopped immediately.  A thorough inspection of the site was made which identified a few fire ring main lines where the same scenario could occur; these were carefully checked, and any trapped pressure was released.  Consideration may be given to the fitting of a drain cock which would allow the release of any residual line pressure prior to cap removal.