If you put a sealed can of water on a big flame, you are making a bomb. (one that can scald you badly). I think you get allot more pressure from the car wash than a can will hold, and they are set up to capture oil, as mentioned.

My dad was a safety engineer for California. His work-day was pretty gruesome sometimes. I get nervous around an espresso maker...

Houshold water heaters have those relief valves because if it boils dry, venting back out the inlet, then a few oz's dribble in, Ka Boom! It expands real fast.