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Apr-2011

Main fractionator revamp

Space restrictions on site called for an innovative solution to replace a refinery’s main fractionator mid-section

John Payne and Dan Darby
Foster Wheeler
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Article Summary
The Coryton refinery is a Petroplus-owned oil refinery in Essex, England. It has a crude throughput capacity of 172 000 b/d and can run up to an additional 70 000 b/d of other feedstocks. The refinery’s major units include atmospheric and vacuum distillation units, a catalytic reformer, a fluid catalytic cracking unit and several hydrotreating units. Since its start-up in the early 1980s, Foster Wheeler has been involved in seven of its eight major turnarounds.

In January 2009, Foster Wheeler was approached by Petroplus and tasked with replacing a 2m band of shell midway up the 5.2m-diameter main fractionator column during a planned October 2009 turnaround (see Figure 1).

The normal method of replacement would involve the removal of the top half of the column, but this was ruled out due to a large quantity of pipework on one side of the vessel, crane availability and available plot location.089 Replacement of the mid-section in situ was, therefore, the only option.

The novel solution was to replace the section in eight petal pieces. A skid track was devised, which allowed the petals to be landed on one side of the column and then skidded round to the congested side (see Figure 2). Working in parallel on opposite sides of the column, a petal of old shell was removed and replaced with a new petal complete with tray supports and downcomers. The sequence in which the petals were replaced was driven by the design of the stiffeners required to reinforce each of the openings left by the removed petal.

The work was completed safely and on schedule, proving that this unique method is a feasible option for the replacement of column sections.

The challenge
Due to a change in duty, high levels of corrosion had occurred on a 2m-high section of the main fractionator column, just above the clad lower section of shell. Non-destructive testing (NDT) during previous turnarounds had found certain corroded sections to be thinner than required. These sections had been overlaid as a temporary measure. As further corrosion was expected, Petroplus and Foster Wheeler decided that replacement of this section with a new band, complete with cladding, was required.

Finding the right solution
There are several methods usually considered when a section of a column has to be replaced. These include replacement of the entire column top with new, or replacement of the section through removal of the top half to grade, insertion of a new band, and then reinstatement of the old top half. The short time frame meant that it was not deemed feasible to purchase an entire new vessel upper-half due to the significant engineering and fabrication time required. Even to just replace the corroded band of column using the conventional “lift down top half” approach would require the use of a large crane. Since this work was added to the turnaround scope at such short notice, the crane availability and plot space required for such a lift were not guaranteed.

Added to this, a large amount of pipework is present on the north side of the column, including a 42in main overhead line, three 18in lines, four 8in lines and several small bore lines. Each of these lines would require cutting and bracing and rework should the top half be removed. This could add significantly to the planned turnaround duration. Replacement of the mid-section in situ was, therefore, the most feasible option.

Ideally, the section would be replaced as a prefabricated band, complete with internals that could be slotted into place. For this to be possible, the existing band would have to be removed while the entire tower top remained above. The total weight of shell, internals and piping above the section in question was an estimated 200 tonnes. Supporting this would require a substantial framework.

One option would be a framework linking the lower half to the top. This was prevented by the metallurgy of the column, the lower half being 1¼ Cr with a 304L cladding. Although welding onto 1¼ Cr is possible, it would require local post-weld heat treatment (PWHT). It was felt that this was an option not worth pursuing since it could have led to further problems and complications during the turnaround. The other option would be to create a framework linking the upper half to grade. A big advantage of this approach would be that the framework could be erected pre-turnaround, with the new section ready to be lifted and slotted into position. However, this would require a huge amount of steelwork, which, although possible, would be fairly cumbersome and might impede other work taking place during the turnaround. Having evaluated the options for replacement of the band in one single section, Petroplus and Foster Wheeler then considered methods for replacing the band in pieces.

The petal approach
Cutting windows into shells is common practice to provide access while replacing cyclones in fluidised catalytic cracking units. The team conceived the idea that this methodology could be adapted to replace the 2m-high band of fractionator by cutting away the existing shell to leave a window and installing a new petal piece in its place. Each petal piece would be prefabricated with its required tray support rings and downcomers. Significant design work was required to confirm the right number of petals and the work that would be involved.

When a window is cut in any shell, the structure must be appropriately reinforced to ensure the vessel will not fail (eg, buckle) due to the weakening through removal of the shell plate. This is usually done using stiffening ribs either side of the window and replacing the section modulus of the area removed. It follows that where a larger window is cut, the section modulus removed is larger, and therefore more substantial reinforcement is required. With large arcs, a circumferential stiffener is also required above and below the window. The vertical stiffeners tie into these circumferential stiffeners and this effectively forms a bridge around the cut.

Since the entire circumference of the vessel was to be replaced, a 360-degree ring would be required as the circumferential stiffening piece (see Figure 3).
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