Industrial Effluent : Knowing What You Don't Know and How Much You Should Care

I still work as an engineer, even though a lot of the things I write on here are about my side-lines as a part-time academic and author. Earlier this week I had a discussion with a chemical engineer in a different sector which reminded me of the difference between academic and professional rigour, and between taking and dodging professional responsibility.

Without giving away too many details, it concerned the zoning of a confined space in an industrial effluent treatment collection system. Apparently the American NFPA 820 standard states that sewer gas is non-flammable. On the strength of this statement, an engineer thought that the confined space presented no risk of explosion hazard.

There are a number of standards in play in a case like this, though I have not seen an American standard used in this context the UK before. IEC 60079, BS EN 50281 and BS EN 60079 part 10 are the more commonly used guidance. The key word here is guidance. Codes and standards are not to be applied blindly. Professional engineers are employed to exercise professional judgement. Applying an inappropriate standard, or applying an appropriate one inappropriately is less than competent.

Thinking that reference to a standard gets us out of thinking about whether its recommendation is sensible is an abdication of professional responsibility. Not considering whether the question we are being asked falls outside our area of expertise is also a failing of professionalism. Luckily, someone on the team knows me, and knew that this falls in my area of expertise. I already knew that the idea that "sewer gas is non-flammable" was not universally true. I am confined space trained, and we wear gas monitors with (amongst other things) LEL detection when accessing chambers such as this. It also transpired upon investigation that this was not what I would call normal sewer contents, as it was mostly hot, high strength trade effluent rather than domestic sewage. Organic solvents are handled at the site, and it was uncertain as to whether there were any routes by which these solvents, or their heavier than air vapours could make it to the sump in question.

At this point I knew enough to say that there were grounds to have some concern about whether flammable atmospheres might occur in the sump, and to rule out the idea that this was an inconceivable condition. If I were looking to milk the client, I might have insisted that only a rigorous program of sampling and analysis could tell us how the chamber should be regarded, but I applied some common sense. What were the financial implications of making the conservative assumption, (zoning it as likely to contain an explosive atmosphere), compared with the costs of the investigation?

It turned out that there was no plan to put electrical equipment in the sump, so the zoning was a paper exercise. A study to "rigorously" establish the "proper" zoning would have been a waste of money. We are not scientists. Our investigations are intended to increase safety and robustness, or decrease cost. Sometimes the answer to an engineering question is "who cares"? Not that I'm immune to caring about things no-one else does myself. I'm an engineer too...

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