Preventing bacterial-generated gas explosions
by Dave Vachavake

(Excerpted from the January/February 2012 issue of Paper360° magazine.)

On July 29, 2008, my friend Don died in an explosion, along with our co-workers Randy and Steve. The pain of that day was made worse by learning too late that the accident was completely preventable.

The two days following the accident were the worst of my life as our mill simultaneously tried to cope with tragedy and understand the cause of the accident. Walking into a control room I saw an article from a trade journal entitled, "Bacteria Cause Fatal Explosion at Corrugating Mill."

I wondered how our story got published so quickly and how anyone speculated that bacteria played a role.

As I read the article I saw that it wasn't written about our explosion, but about one that happened at another mill nearly 20 years earlier. Change a few names and details and this article could have been a near-perfect description of our accident. My heart sank. How could such knowledge about the cause and prevention of this type of explosion have existed, and yet be unknown to all of us involved in this tragedy?

THE ACCIDENT
Millwrights Randy, Steve and Alex went to the top of an 80' high OCC high density storage tank to weld on a temporary clamp to stabilize a flange connection. After lunch, paper mill superintendent, Don, climbed to the top of the tank to check on how the job was going. At some point the hot work caused a spark to enter the top of the storage tank triggering a hydrogen gas explosion that threw Don and Randy to the ground below. Alex had been observing the work from another catwalk and survived with minor injuries. Steve was badly injured but was still at the top of the tank. Risking their own lives, mill rescue workers were able to reach him, but his traumatic injuries were too great to overcome.

INVESTIGATIONS
Subsequent investigations by OSHA, The Chemical Safety Board (CSB), and indepent studies supported the nearly 20 year-old trade journal article. Naturally occurring bacteria had acclimated to normal mill conditions and then thrived under ideal conditions in whitewater and pulp storage tanks. These tanks carry significant amounts of dissolved solids, including starches and simple sugars that are a preferred food source for acidifying hydrogen-producing bacteria in the spore-forming Clostridium family. Starches are applied during the corrugating process and introduced to the paper mill with the OCC used to supplement virgin fiber for board production. Pulping liberates simple sugars (glucose and others).

Ideal growing conditions for this type of bacteria include the following: acidic (4.5 to 5.5 pH), anaerobic (low oxygen levels) caused by high temperatures and stagnant conditions, and high temperature (55o C). These conditions are more common in mills with relatively closed white water systems. Such systems are present in nearly all mills today due to rising environmental and cost concerns and are not unique to board mills.

Ideal conditions can quickly develop in stock or whitewater tanks that are idle because of shutdown, or have poor agitation. Mill studies have shown gases can reach Lower Explosive Limits (LEL) in as little as two hours after a shutdown, and gas production can be sustained for days. Some gas production can even occur in 170o F virgin stock tanks before high density storage.

Lab studies show that under certain conditions, explosive gases can be produced at a rate of 0.02 liters of H2/liter of whitewater/hour for 35 hours. Mill conditions produce about 50% of lab conditions. Acetic and Lactic acids are also produced by bacterial metabolism, causing relatively harmless but telling odors.

Combining food sources, ideal growing conditions, and limited venting can be deadly when an ignition source is introduced.

According to the CSB there have been over 60 deaths in the U.S. since 1990, related to explosions caused by hot work. The CSB has issued a safety bulletin entitled Seven Key Lessons to Prevent Worker Deaths During Hot Work in and Around Tanks.

As recently as January 19, 2011, a 47 year-old pipefitter at Nogentais Emin Leydier papermill in France was killed by explosive gases. I read a comment online that said, "All experts consulted after the event could not understand how this event could happen, and all speak of an isolated and exceptional event." The lack of awareness is still there in our industry!

My coworkers did not die due to negligence or lack of caring. Yet those involved did not have awareness of the serious risk of bacterial-generated hydrogen gas explosions. How can we ensure that the awareness of these dangerous conditions does not fade, so that 20 years from now, someone else doesn't again suffer such tragedy?

I believe it is critical to get our pulp and paper universities to add this topic to the curriculum. Our trade organizations need to raise awareness and keep it high. Our suppliers and mills need to build it into internal training programs. We all need to raise awareness and communicate the message that these accidents are possible, but completely preventable.

Dave Vachavake has worked for fine paper and packaging companies in Operations, Technical, R&D and Product Development and is currently semi-retired. He would like to thank all those who helped with this article. Contact him at dvachavake@gmail.com.

NOTE: Read the complete article about this disaster, including recommendations by the Chemical Safety Board as well as methods proven effective at reducing bacterial activity and hydrogen gas generation, in the January/February issue of Paper360° magazine.

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