Mechanical Technology February 2016

⎪ Proactive maintenance, lubrication and contamination management ⎪

the column are of different materials to compensate for different process condi- tions, and a difference in the mean wall thickness between the two would have been expected had the material loss been due to process corrosion. This suggests that the loss of material thickness is due to external corrosion due to poor maintenance of the lagging vapour barrier on the external surface. Conclusions That the column has suffered damage as the result of exposure the fire is proven beyond doubt. The wrinkle observed in the shell close to the base is the result of localised heating, which could only have been the result of the fire. The most serious damage to the col- umn, however, is the deformation, which presented as both longitudinal bowing and localised ovalling, and the loss of mechanical strength as indicated by the reduction in hardness. The effects of the fire cannot adequately explain either of these phenomena, but both can be explained by incorrect post-weld stress- relief heat treatment during manufacture. If it is accepted that both these fea-

occurred during thermal stress-relief of the column during manufacture. The possibility of an incorrect mate- rial, such as ASTM A285 Grade A – with similar properties to those found – having been used in error was discounted fol- lowing examination of the manufacturing records. This, together with the uniformity of loss of properties, tends to suggest that the effect had occurred during stress-relief heat treatment. Post-weld stress-relief heat treatments, however, are usually carried out at around 600 °C for short periods, and do not generally degrade the properties to any extent. This in turn sug- gests that stress-relief heat treatment at a higher than normal temperature is prob- ably the cause, rather than exposure to elevated temperatures during the fire. The column would probably have collapsed under its own weight had the reduction of properties been caused by heating to stress-relief temperature whilst erect. The thickness measurements taken on the column show a uniform loss of wall thickness, but examination of the vessel interior and internal structures do not show any major corrosive attack. In addition, the upper and lower vessels in

tures originated during manufacture, it may be argued that, despite their being of a magnitude greater than that which the design code regards as permissible, the fact that the column has performed satisfactorily for twenty years could be taken as an indication that their presence would not compromise the integrity of the column, and on these grounds it could be returned to service. In the final analysis, however, the thick- ness of the column shell was considered too thin for extended service, and it would have had to be replaced within a short time, necessitating a second major pro- duction interruption. A decision to replace the column was therefore taken, based on economic rather than technical grounds. q

References 1 ASTM A516. 2 BS PD 2647. 3 Mechanical Properties of The BS En Steels: Woolman & Mottram; Pergamon; 1964. 4 Author: unpublished work. 5 ASTM A370. 6 Boeing Aircraft Co. Specification: BAC 5617. 7 Auerkari P: On the correlation of hardness with tensile and yield strength; Technical Research Centre of Finland; Research Report 416; Espoo, Finland; 1986. 8 Author: unpublished work.

Mechanical Technology — February 2016

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