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Watch Out for Cross-Contamination
Commissioning can pose risks unless you plan carefully.
By Dirk Willard, Contributing Editor
It was a close call. An off-hand remark at a morning meeting prompted me to inspect cross-connecting piping between two products. No pancake isolated the two product lines to the tank farm. We dodged a bullet; everyone knew it but no one considered it a near-miss. Exhaustion has a funny way of distorting perspective.
Cross-contamination, the unintentional mixing of two streams, can lead to consequences ranging from ruined product to an accident resulting in a fire or explosion.
Such inadvertent mixing has caused, totally or in part, more than a few major accidents. At a Union Carbide pesticide plant in Bhopal, India, on December 2, 1984, an error in cross-connection initiated the worst accident in the history of the chemical industry ("Bhopal Leaves a Lasting Legacy").
Let's consider some ways to avoid cross-contamination. Obviously, documentation is critical. The site where I spotted the problem relied on an incomplete and inaccurate process and instrumentation drawing (P&ID). We called it the Bateaux Tapestry. It stretched about 50 ft × 6 ft across a wall; most of the new process was added in pencil — just as a caveman would have done it 50,000 years ago if he'd been a process engineer. I found it best to personally walk down the piping, which unfortunately wasn't labeled. It took hours, sometimes days, to trace lines. Ideally, a site should constantly update its P&IDs; at a minimum, it should update them at least a few months before any large project.
Besides P&IDs, other useful documentation includes: 1) isometrics ("Don't Devalue Isometrics"); 2) tie-point lists; 3) piping line lists; 4) commissioning drawings such as ones showing connections for pressure and other tests; 5) marshalling panel terminal drawings; and 6) equipment drawings.
Nothing is more valuable than looking at the equipment in the field. Train your operators, contractors and even the construction foreman to follow a walk-down procedure. Yes, I know it's a bother but teach a course on it! Use marked-up P&IDs prepared in advance by the engineer responsible and then reviewed by safety, environmental and any other stakeholders. Do a quality control check. You don't want to try to tie into a line that was removed two years ago or install two block valves — on two different drawings. Walk down the line with the constructor to update the drawings before making tie-ins. Inspect piping before insulating or burying out of sight — take dimensional photos, if possible. Construction must meet the needs of installation (lugs), testing and purging (taps) as well as operations and maintenance. Always think of pipe, equipment and instruments as temporary. Ask yourself, "How will I remove it to make repairs?" And, "How can I isolate equipment from the process and from the environment to clean it?"
Once you've got a plan, it's time to evaluate how you will use new piping in the future and, particularly, how you will prevent cross-contamination. Isolation works best with pancakes and, for electronics, by pulling wires on both ends but taking care there's a low-resistance ground (fire risk). Hasp-locked cabinets and breakers usually are considered sufficient but you can't turn on what isn't wired.
Double-block-and-bleed (DBB) isolation, the second best choice, only works well if the bleed is open, an impracticality in most chemical plants. Spectacle blinds are a good choice for frequent isolation, if space allows and corrosion isn't a problem. Use bright tags for isolations. I've seen a flagged pancake left behind on a relief valve. Ensure tags are complete, dated and bear contact information and the reason for the isolation; keep a tag list. Don't forget to isolate purges and tubing lines — you don't want them contaminated or corroded.
Frequently, processes are built without considering commissioning: hydrotesting, leak-testing, cleaning, purging and conditioning. Whenever necessary, modify systems for ease of commissioning. Here's something else you might not have thought of: lab tests for cleaning contaminants. When developing commissioning procedures, you probably think about the compatibility, i.e., flammability and health risk. Have you considered the potential chemical reactions from cleaning and conditioning agents? Do you have lab tests for these species? Have you pondered what byproducts they might form with oils, lubricants and other compounds?
A small specialties plant I consulted for switched its packed distillation towers from fish oil to lubrication oil purification but didn't do any lab work in advance. It was surprised when slugs of brown gel fouled the packing and dripped off sample lines. In the end, we had to add filters everywhere. A simple lab experiment could have foretold what the chemistry had in store for us.
There's more to cross-contamination than meets the eye. It's always better to mull over the potential problems before a contamination incident than after when you're in a rush to startup.
DIRK WILLARD is a Chemical Processing Contributing Editor. You can e-mail him at dwillard@putman.net
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