“We were doing some cleaning at the end of yesterday. I boiled some water in the HLT to pump through the heat exchanger. At the end of the day, there were 40-50 gal left in our 3bbl HLT. I turned off the heat and went home for the night. The next morning, I came into the brewery to find an imploded HLT. I had clamped the lid on to keep humidity down in the building, and the cooling air/water in the HLT caused the tank to implode.”
Source: www.probrewer.com