s-lg.gif (1089 bytes) 



Safety survey for critical Equipments like pressure vessels, Converters, Absorption towers, furnaces, silos, etc. are usually done using experience, and certain basic data like Ultrasonographic tests on the shell plate. All these data / methods show the thickness at different regions of the shell plate only. They donot give an insight into the actual condition of the equipment. In order to know the kind of stress developed in the shell plates, a stress analysis using the finite element method will be required. This method, being highly time taking was usually was usually avoided in the past. However, with the advent of the computer, and particularly the highly reliable softwares, the detailed deformation, and stress distribution for the entire equipment under operating loads is available in hard copies of coloured displays known as stress contours. These softwares require the inputs of the entire 3-Dimensional geometry of the equipment, like shell thickness, details of openings, etc, along with all loads like temperature, pressure, liveloads, etc. After a stress analysis with the given loads has been performed, the software would depict the effect the effect of these loads on the structure, including the deflected shape, showing the deflection at each region, and the stress contour which shows the stress at each and every point. This stress contour, and the deformation diagram in 3-Dimensions will aid in visualisation of the stress conditions, and the deformation geometry in totality for the entire equipment.

The degree of corrosion under chemically reactive atmosphere is directly proportional to the stress intensity. The corrosion will be more pronounced at the high stressed zones of the structure. In thermal conditions, (other than cryogenic conditions) rise in temperature induces higher stress that accelerates corrosion. Once the highly stressed conditions in an equipment are determined through a finite element analysis, it is obvious that this would be the region subjected to the worst corrosion in future, and result in further over stressing. The maintenance and safety departments of the plant could then take adequate measures to relieve these regions of the stress, so that all the regions are equally stressed, thereby avoiding heavy corrosion to any one particular area.

Also, with this data the residual life of the equipment, and the time for repair could be accurately determined. This technology could be applied to the long time planning of safety requirements & preventive maintenance, primarily to counteract the aggressive chemical corrosion, thermal stresses, fatigue conditions due to dynamic loads, etc, assuring a longer service life for the equipment.

It would eliminate crisis management of maintenance, arising out of non-availability of data to predict distress. The maintenance department would not need to repair an equipment after the damage is already done, but instead, each equipment would be subjected to preventive maintenance at predetermined areas as predicted by the stress contours, much before the equipment could fail / leak and create disruption of production.