The 2012 Memorial Day explosion and fire at a Minnesota paper mill that killed an employee and injured several more was caused by an overheated air compressor that ignited oil vapors in the compressor and an air receiver.  The report states the cooling water supply to the compressor was severely reduced in the hour before the explosion and that two other compressors had already shut off because of overheating. The report indicated that a maintenance issue or a mechanical failure possibly caused the one compressor to exceed its thermal protection shutdown controls.  The MN State Fire Marshal lead investigator estimated the damage from the explosion and resulting fire at $50 million.

The mill had been shut down earlier in the day to repair a leaking water pipe and that the mill was just being brought back online when the explosion happened.  The shutdown caused the mill to switch its source of cooling water for its air compressors and the report indicated that cooling water flow to the air compressors had dropped from about 215 gallons per minute at 7:54 a.m. that day to a fluctuating rate of between 19 and 3 gallons per minute between 10:05 a.m. and 10:14 a.m. until the time of the incident at 11:17 a.m. 

If you have never read a fatality report from a Fire Marshal, here is your chance.  The detail in these reports really describes the scene to a level where many can visualize it like we are standing there.  I do CAUTION everyone though, this was a fatality and it may be upsetting to some to read how a fatality investigation takes place.  Pay particular attention to the "little things" the investigator(s) pick up on when surveying the scene.

"At the north end of the compressor room (with mezzanine above) is the location of two (2) vertical 7,500-gallon receiver tanks, one for mill air and the other instrument air. The mill air receiver tank is along the west wall of the compressor room and the instrument air receiver tank is along the east wall of the compressor room. The mill air receiver tank is 84” in diameter and has exploded and/or ruptured. The ½” thick steel receiver tank is ripped apart, and the welded dome and base of the tank have blown off. The ruptured tank is lying atop compressor #1, which has been displaced from its elevated concrete pad to the south. The interior of the ruptured mill air receiver tank is coated with a thick layer of oil and black soot. In a bottom section of the receiver tank I observed a large pool of oil. Examination of the dislocated 6” in diameter stainless steel compressed air piping from the ruptured receiver tank also revealed heavy black oily soot deposits within the piping."

Lastly, OSHA's Air Receiver standard 1910.169 is often overlooked, yet almost every industrial facility will have an air compressor and reciever.  Do you recall reading that their was a lot of oil present at the scene (after a fire that burned for a couple of days!) and burned oil inside the receiver that catastrophically failed?  This is NOT suppose to be the case if we are strictly compling with 1910.169(b)(2), which states...

1910.169(b)(2) Drains and traps. A drain pipe and valve shall be installed at the lowest point of every air receiver to provide for the removal of accumulated oil and water. Adequate automatic traps may be installed in addition to drain valves. The drain valve on the air receiver shall be opened and the receiver completely drained frequently and at such intervals as to prevent the accumulation of excessive amounts of liquid in the receiver.

These days many receivers are equipped with automatic drain valves that do the required draining for us, but this is an after market addition and must be installed by the user.  I would say that less than half of the receivers we come across in our audits have these auto-drains and when we ask if the receiver is on any type of maintenance schedule we tend to get the deer in the headlight look!

Allowing the oil to build up inside this pressure vessel and being exposed to the kinds of temperatures this compressor was running basically vaporized the oil inside the receiver and when flames entered the piping and then into the receiver it was like a bomb going off! 

Examination of thermal patterns to the compressor room clearly show the fire originated at compressor #3.  Compressor #3 overheated internally and igniting compressor oil vapors on fire, which communicated through compressed air piping to the mill air receiver tank where explosive compressor oil vapors were ignited, causing the mill air receiver tank to explode. Flames from the mill air tank explosion communicated to the southwest quadrant of the paper warehouse, igniting rolls of stored paper on fire. The explanation for why compressor #3 didn’t shut down due to overheating is unknown, but possible scenarios are failure of a thermal protection device and/or devices at the compressor and/or in-house maintenance issues. In my opinion, severe reduction of the water flow to compressors for cooling the morning of the incident is a contributing factor to compressor #3 overheating, which resulted in a fire and explosion occurring at the mill air receiver tank. 

So although the FM report does not determine the root cause of this explosion, I encourage you to read the report carefully and let's discuss what we think the root cause and the contributing factors of this fatal explosion are.

Click Here (pdf) to read the text portion of the report.