Dateline: July 10, 2007

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Polymer Deal
Arkema plans to buy specialty polymers and additives firm Coatex from Switzerland’s Omva Group for an undisclosed amount.  The deal should close by the end of the third quarter.  The acquisition is part of the French company’s divestments of its non-strategic assets and ties to a new step in its growth plans.  The acquisition offers strong synergies downstream of Arkema’s acrylic monomer production in Bayport, Texas and Carling, France.  Coatex mainly makes acrylic-based polymers used as dispersants and thickeners for paper, paint, water treatment, cosmetics and textile industries.

Technically Speaking

I need advice on changing the duty of an API storage tank?  The service duty is changing and I have to decide on a new relieving set point.

"Low pressure storage tanks can be lethal and dangerous vessels – even in the pressure ranges of 0 to 15 psig and especially in the ranges of negative pressure (partial vacuum or 'external pressure').

If you’re revamping an existing API water tank to include a N₂ blanket system or a fixed desiccant chamber in the breathing pipe, you find yourself in a dilemma because you need to fix the set points and “dead band” for the tank’s instrumentation and don’t know how to obtain these. My reaction to the situation is that one should never attempt to make any modification on a vessel – whether pressure or storage – unless the actual design calculations or specifications are in hand or if one can generate (or calculate) the required mechanical specifications that meet the proposed application.

As I’ve stated in the past, an API 650 or 620 storage tank is usually built to conform to operating pressures of 0 – 5 psig and partial vacuums down to 24 inches of water column (WC). I have found this range to be very appropriate and flexible for application to a very large variety of liquid chemicals – with or without N₂ blanketing. HOWEVER, the ranges I’ve mentioned aren’t necessarily the ones that a specific tank was designed and built for. Tank fabricators will build an API tank to your purchase specifications and order. It has been my experience in inheriting existing field tanks in the past that most (if not all) tanks were purchase with nothing more than the description of “API storage tank”, period. No engineering specification or guidelines were given to the fabricator – especially a requirement to design to a specific MAWP (maximum allowable working pressure) or MAWV (maximum allowable working vacuum) – nor was the requirement for written design calculations and an API stamp noted. Naturally, the fabricator did exactly as the purchaser indicated (or did not indicate) and supplied just that: an API storage tank (probably one that was already partially built and “on the shelf” for immediate application. The sad result is what I and many others (like you) have had to confront out in the field, trying to make a sound, safe, and economic engineering decision.

Some years ago I decided I was not going to jeopardize my health and life as well as other people’s by taking the attitude of 'Just use a setpoint of ____ in W.C. They're all designed to withstand that.' I took a stand with what I thought was sound engineering and demanded that mechanical integrity be proven and documented in all tanks that I applied engineering on. I obtained what I thought was sound logic in the services of an experienced mechanical engineer with whom I worked hand-in-hand in making all vessel revamps. I learned a lot about what can and can’t be done with API storage tanks and the results speak for themselves: no 'suck-ins', stabile and safe operation, credible and reliable documented records, totally reduced emissions, minimal inert gas consumption, and quality product as well as raw material. I’ve done modifications such as umbrella roofs, anchor lugs, double-bottoms, wall re-enforcements, sloped floors, emissions controls, and many others. I would not attempt a revamp on a tank without a minimal of documentation and calculations that confirm the MAWP and MAWV of the vessel. That’s my comment, and I’ll add another directed to your specific first question: No, there is no 'minimum pressure, be it as low as a fraction of an inch of water', that one can believe a mechanically sound ‘atmospheric’ storage tank is capable of withstanding” - at least, not without proving it with checked, accurate, and documented calculations. That’s the very least that any owner can furnish you as an engineer working on their project. You deserve it, and you should have it coming to you. Fair is fair and you can’t compensate for past errors or omissions by taking steps that jeopardize your life and others. It isn’t morally (and now, finally) legally correct.

Paul Ostand has written a lengthy and, in my opinion, a very needed and useful paper on the employment of N₂ blanketing. I dare not speak for such a qualified and experienced individual, but he does state the importance of storage tank mechanical integrity in his paper. Part I, under Methods and Requirements, states:

“When considering safety in relation to the gas blanketing system, bear in mind the following. The tank must be capable of operating at the desired pressure, with an appropriate factor of safety. The tank must be protected from vacuum and pressure that might occur outside of the operating pressure range and within safe limits of tank operation.”

So, even though Paul is not writing about API tank design and fabrication, he specifically identifies a pre-requisite for revamping a storage tank: make sure that your tank can withstand the pressures (or vacuums) you intend to impose on it before attempting any revamp.

Agreeably, our engineering logic and sound judgment is countered in the real-life applications where an owner expects his project engineer to correct a previous mistake or existing safety or environmental problem by simply “converting” a vessel to today’s state-of-the-art through the addition of new equipment on a rusted, non-stamped, undocumented piece of junk. I wouldn’t do it until it was proven that the subject vessel is, in fact, a reliable and mechanically sound piece of equipment with defined pressure ratings. I realize that it’s easy for me to assert my position strongly since I’m a consultant and my paycheck does not depend on any one individual liking me, my under-arm deodorant, or what I say. But nevertheless, I’m doing what’s correct by my engineering license and I can go home knowing that I haven’t harmed or damaged anyone with my decision. And no one can offer a reasonable, logical, and much less legal argument against my decision – at least not to my knowledge.

Thanks to all my prior relations and working with mechanical engineers, I can generate API tank design hand calculations. I have never used this ability to design a tank. I demand a design result from a recognized and experienced engineer. But being able to follow and understand the bases and underpinnings of tank design help out a Chemical Engineer enormously in later making mechanical or process modifications decisions. I highly recommend any Chemical Engineer to follow this principle whenever the opportunity presents itself. Learning and respecting what Mechanical Engineers can do is simply another way to broaden and diversify your capabilities as a professional engineer and dominating your craft.

Art Montemayor via Cheresources.com Forums