Had a similar situation at my previous job. We would get a rail car full of PETE plastic pellets in about every 2 weeks and unload them with a vacum system that had a 100 horse elec.motor driving a 12-71 roots type blower. I was told by the out going shift all was ready to unload and fired up the system,everything was good for about 15 mins. and was walking back into the plant when I heard a loud bang, looked at the side of the rail car and it was sucked in about a foot. Made a mad dash for the E-Stop and she popped back out. Bad thing is,these rail cars are lined with an FDA approved liner,not cheap to redo. Crawled to the top of the rail car, no vent was ever installed, and no vent was ever installed on the other side of the boot. My bad for not checking .No damage was done, but I was shakin in my boots.