A blow off tube would be a great idea! When you take out the airlock, do it very slowly, or you'll have a beer fountain. Then gently take off the lid to reduce some of the pressure. Install a blow-off tube, and you should be all set.
You'll use a piece of tubing, sanitized, and replace your airlock with it. If you're using a carboy, I think it's a 1" size. If it's a bucket, the siphon tubing can fit in the hole. Put one end in the top of the fermenter, above the krausen line, and the other end in a pitcher of water/sanitizer or water/vodka mix so that the tube is covered by liquid. That should help! (Oh, and put the pitcher on the floor, not on top of the fermenter).
One other thing- check the temperature of the fermenting beer. It's great if you have a stick-on thermometer on the outside, but if you don't sanitize a thermometer and check the temperature. Violent fermentations like you describe tend to get "hot" and you may be a bit too warm for optimum fermentation conditions. If it's too warm, we can give you tips on how to get it lowered.
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