A nozzle dam automatically latches on an annular ledge surrounding the nozzle opening. A pivotal latch arm on the dam, biased to the locked position is forced toward the unlocked position by a cam on the ledge as the nozzle is pressed on the ledge, and snaps into engagement with the ledge when the dam is fully seated over the opening. A bias arm may be operated by a wrench to pivot the latch arm toward the unlocked position until a detent locks the latch arm in the unlocked position.