At work at the fertilizer plant I work at they have air operated bin vibrators on the two load out bins to shake loose any fertilizer that sticks to the walls. They have maybe a 3 hp unit supplying the air. The chap I work under is fairly lazy and doesn't do the proper maintenance on the compressor he should be, namely draining the water out. Twice in the 5 years the plant has been in operation they vibrators have quit working because they became plugged up with water and the bit of oil from the cylinder walls in the head that makes it by the rings. When the repairman opened the pet cock you wouldn't believe how much gunk came out. I think it may have been a couple of 4 liter pails worth.