Safety valve

A 125psi pop-safety valve

A safety valve, also known as a pop-safety valve or pop-off valve is a specialty valve installed on a chamber to prevent over-pressurization. They are available in a wide range of pressure ratings, from pressures lower than you'd want to use in a spudgun to pressures higher than a PVC chamber will hold.

These are most commonly installed when you are using a pressure regulator, so that if it malfunctions it won't fill your chamber to the point where it fails, although no such event has been recorded.

Sometimes if you strike up a conversation with the plumbing guy at Home Depot/Lowes about building a combustion cannon, they may attempt to sell you a safety valve, even if the cannon you wish to build is a lowly hairspray inline.

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