I don’t know if this has happened to anyone else… But at least twice in the past I have gotten so excited restarting my wood gasser that I accidentally introduced engine vacuum to the gasifier without opening the air intake valve. The result was a collapsed Hopper! It was particularly embarrassing when it happened in the Menards parking lot…
The other day I noticed my vacuum gauge needle was buried on the wrong end when I was cranking the starter motor and I knew it was time to finally install a vacuum relief valve.
I looked around in the junk and I found an old Wayne Keith tennis ball valve which I modified with a spring and a spring tension adjustment. I installed my new vacuum breaker in parallel with the air intake valve just before the gasifier.
The spring loaded tennis ball holds the vacuum up to about 40 inches, then opens to protect the hopper. If it opens during normal operation there is no harm done, just lets in a little fresh air for a moment.! In the 1st photo (Tap the blue link) below you can see the vacuum breaker on the left side. It is a freshly painted black 3 in t with the spring tensioner on the lower left end. The upper right hand end of the T is reduced to receive a gauge to use during calibration.