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Products and solutions, 2006-12-01, 03:59 PM

A sphere of specialization

Over the past years, ThyssenKrupp Xervon has carried out maintenance work on over twenty spherical steel tanks standing at the Wesseling location of Basell Polyolefine GmbH. Among the jobs carried out were the scaffolding of the liquid gas tanks, stripping the existing corrosion-protection coatings, priming, and then applying a fire-protection coat. Work is currently proceeding on the final three tanks one of which is encumbered with a chromium (VI) polluted coating that requires careful and complex removal.

Basell is the world's biggest producer of polyolefins and catalysts. At its largest location in Wesseling (to the south of Cologne), this chemicals company produces chiefly plastic granules. The liquid gases required in the production process are stored on the vast site in a cluster of two dozen futuristically styled spheres. Over the years, the existing corrosion proofing had grown long in the tooth and, moreover, new regulations on the part of the authorities insisted on fire-protection coats and so gradually all 24 tanks are being given a new and shining coat of white.

Complex scaffolding work

The leg-mounted tanks are a good 20 meters tall. Their diameters range between 13 and 18 meters. For the Xervon scaffolders (contractually entrusted with all the scaffolding jobs at Wesseling), the preliminary work is certainly challenging as it includes the dust-proofing of the tanks. In order to access all of the steel surfaces (up to 1,250 m²) it's necessary to erect around 3,000 m³ of spatial scaffolding, a job that, depending on the size of the sphere, can take up to four weeks. Added to this are another three weeks for dust-free encapsulation.

Xervon's Project Manager Wolfgang Oeding, supervising for several years now the tank maintenance work at Basell, is well aware of the complexity of the challenge. "Admittedly, there do exist fastening options lower down at the foot of the tanks and in certain instances we do have the possibility of securing the scaffolds to railings but in actual fact, what we have here are self-supporting structures."

Compounding the complexity on two of the tanks are their "sunroofs." Above equatorial level there is a supporting structure to which is attached a semi-sphere comprising insulating steel that envelops the top half of the tank in the form of a sunshade. On these two tanks, the scaffolders have to work their way inward, step by step. First of all, they install the exterior enveloping scaffold cylinder which is then systematically worked inward until the sunshade can be conveniently removed. Once this is completed, the scaffolding is then again extended inward to fit the spherical shape until the clearance to the sphere surface is no more than 30 cm. Anything in excess would impede the work of the downstream trades.

The next step: all the various items of equipment, the fixtures and fittings that form an integral part of the tank (wiring, switch cubicles, valves and sensors) are carefully wrapped and packed to prevent damage. Then, in preparation for blast-cleaning, the entire scaffolding is meticulously encapsulated.

A tough case

The chromium (VI) contaminated sphere is subjected to the same strict standards as would be a lead-contaminated construction site. During the blasting phase, the sphere itself is encapsulated virtually air tight. For this purpose, the separate tarpaulin sections are welded together. The cleaners switch from their road gear into blast suits with external air supply specially approved for such jobs. As precautionary measure they must first of all undergo all the prescribed medical checks to rule out any possible health risks. In the course of the blasting, some 35,000 m³/h of airflow (fivefold air change) is driven through the encapsulated workstation. This intense exchange of air generates a directed flow that takes in and collects the contaminated mixture of waste blast (slag) and stripped coating.

The blasting work goes on until the steel surfaces are all stripped of the existing corrosion protection and shine metallic bright. In the process, the contaminated sphere is given a special treatment. Whereas the other spheres undergo a morning blast immediately followed by afternoon priming, their contaminated cousin first of all undergoes a complete blast-clean. Then a specialist firm cleans the "workstation" (the encapsulated scaffold) until there are no measurable dust pollutants inside. Then, the entire tank is once again blast-cleaned and primed (coating: 40 to 50 µm) in preparation for the following multi-layer fire-protection skin.

The entire substructure (legs) is also given a fire-protective coating while the stairs, railings, catwalks and all the other add-ons receive a triple-coat of corrosion protection. The Xervon specialists are well aware of where to spray and where to coat. After all, many of them have acquired their routine and expertise from having worked on the already completed tanks.

Wolfgang Oeding is delighted that "apart from a very small number of minor details" the maintenance work has proven so successful. "We really did a good job." Quality checks here are very strict. Each piece of scaffolding, each cleaned and coated surface is not only examined by Xervon's own inspectors but Basell's quality assurance experts are likewise regularly in attendance to give their o.k. before passing on to the next stages. Thickness measurements on the primer and fire-protection coats are regular routine just as compiling and updating the records for the technical inspection authorities.

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