Quite a bit of information can be found on the web about backfire or blowback explosions related to forge blowers and bellows. I encountered it when I was doing a demonstration using a great bellows. It was a windy day and I think the air pressure going into the hooded forge forced coal gas back into the bellows where it exploded with a muffled thud. No damage was done to the leathers but I built a blow back valve to prevent it happening again. I came across a yellow foam practice golf ball in my hardware carousel and it reminded me of that project.
I used a piece of thin wall tube about 4” long and cut it into top and bottom halves which I hinged. I put a lid on the bottom half which had a center hole slightly smaller than the golf ball. This lid created a bottom compartment so air could only enter from the bellows side by lifting the ball. Air entering from the forge side was obstructed by the ball sitting in the hole. Four nail posts were welded so the ball could only move up and down. The distance between the top of the ball and the top chamber wall is small enough that the ball can’t escape over the ends of the posts.
After everything was sealed, I tested it and it seemed to work fine. However, I stopped using the bellows for demonstrations shortly thereafter so the system never really saw expended use.
Good instructions for making a great bellows.