Process Puzzler: Guard Against Gas

Readers suggest the source of a safety stumper.

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One of our welders got hurt cutting into a carbon steel header for a chilled brine line to install another chiller in the closed-loop system. He purged the piping but not the room. An invisible gas ignited from his torch. Our plant environment contains chlorine and other corrosives. What was the source of the gas? How can we prevent future accidents and what do we need to change in the system to avoid such safety problems?

Prior to any welding or hot work operation, the room in the affected area should be tested for any hydrocarbons present above the lower explosive limit (LEL). LEL is the lean corner of the flammability envelope. Purging the inside of the line is not sufficient. It is critical that the outside of the line and the adjacent area be free of flammable gases as well.
Kenneth Russell, technical manager – compounding
Solvay Advanced Polymers, Alpharetta, Ga.

Rooms should be reviewed for confined space status. If there is little or no ventilation, then confined space can be an issue. When welding or any other hot work activity must be performed, then the preparations must include testing to determine if the atmosphere is safe. The chemical is probably one of the ones you are concerned about and was there in trace amounts that were sufficient to be flammable. An LEL meter or a specific instrument to sense the potential gases is a necessity.
Jim Becker, instrument reliability engineer
Bayer MaterialScience LLC, Baytown, Texas

What were missing were a job safety analysis and a permit to work. The analysis could have identified the problem.

Hydrogen was the hidden flammable gas. The process of rusting depends on the availability of water and oxygen. Where water is available in an oxygen-poor environment, the following equation is relevant:

2Fe2+ + 4H2O → 2Fe(OH)2 + 4 H+

Normally, the reaction goes to completion:

4e- + 4 H+(aq) +O2(aq) → 2 H2O(l)

But, the reaction won’t go to completion because there is no oxygen. If the pH is low, perhaps because of a heat exchanger leak or poor management of the chemistry of the closed circulating system, hydrogen gas can be produced:

2 H+(aq) + 2 e- → H2(g)

Because there’s no way to prevent iron from rusting and minor leaks can go undetected for months, you’re stuck with monitoring for a low pH and changing some of the process parameters. For example, decrease the circulating pressure on the other side of the heat exchangers so that leaks are into the corrosive fluid instead of into the closed brine system. If the pH of the circulating water is about neutral, it won’t take much of a leak to change the pH. Although the hydrogen reaction is fairly quick, any leak should change the pH enough to forewarn against hydrogen formation.

Another possible safeguard might be a buffering agent. A pH probe could detect the initial dip in pH and allow the buffer to prevent excess generation of hydrogen. The pKa of a sodium carbonate solution is 6.3 at 25°C. When the pH drops below 6.3, that’s because there is enough hydrogen being produced to over-run the buffer. Venting might also help. Maybe this is a solution.

A hazard and operability (Hazop) study is suggested because of the potential for this problem being missed elsewhere in your process.

Now that the source of hydrogen has been suggested it’s time to consider prevention: hot permit welding procedures. The pipe should have been purged with an inert gas before the welding. The work should follow lock-out-tag-out and enclosed-space entry procedures and include a trained sentry to monitor conditions, especially flammable gas concentrations and breathable air. Why not simplify things? Move it outside. The tie-in should be located outside in a well-ventilated place to reduce the risk and the regulatory requirements.

Dirk Willard, senior process engineer
International Steel Services, Inc., New Caledonia

We’re trying to establish the start-up procedure for burners in a natural draught incinerator. As always, there’s the danger that a burner may not light in time to avoid tripping the infrared flame sensors. The standard practice after a trip is to purge any fuel present according to NFPA guidelines. This would be easy if we used natural gas — but this incinerator is in China and uses coke oven gas that contains hydrogen cyanide. How can we modify the purge procedure for this fuel? Do we need to change the burner design and other incinerator components to handle it?

Send us your comments, suggestions or solutions for this question by January 8, 2010. We’ll include as many of them as possible in the February 2010 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.

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