According to the forums, there is a physics problem there causing the crash. For those who are experiencing the problem, here's what you need to do:
Before you turn on the fan, look around the whole room and make absolutely sure that any breakable object (i.e. crate, palette) is destroyed. You can use a crowbar, or whatever you prefer.
If that doesn't work for you, you could also try reloading an earlier save, whether it was a quicksave or whatever.
Unfortunately, I don't have access to the forum I found this info on, but a quick search on google using the following search terms ("black mesa blast pit crash") should pull up any more info you may need.
Hope this helps, and enjoy the game! I know I am.
violinplayer
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