A vent flame arrester is a safety device installed in the vent line of a fuel system or storage tank. Its purpose is twofold:

1. Free Venting: It allows gases or vapors to pass through it freely during normal operation. When the system pressure is near atmospheric levels, the vent flame arrester permits unobstructed venting.
2. Flame Protection: In case of external ignition sources (such as lightning strikes or ground fires), the flame arrester prevents flames from propagating beyond it. It acts as a barrier, stopping the transmission of flames into the system.

These devices are commonly used in vertical vent applications, especially at the top of atmospheric vent lines or storage tanks.