Cracks in Structural Steel - Investigations & Mitigations

Abstract

Abstract Vibratory Screens of Sulphur Granulation Units are typically mounted on framed steel structures. The supporting structure requires special design considerations as rotary equipment transmit significant dynamic loads. Appearance of frequent cracks in the steel structure and equipment resulted in shut down of unit in one of the GASCO plants. This paper presents investigations undertaken to ascertain the underlying causes of cracks in structural steel and methods employed to enhance the integrity of support structure thereby ensuring continuous availability of equipment and extended asset life. A detailed study with OEM intervention was carried out which included mapping of cracks, study of crack patterns, vibration measurements, study of equipment maintenance history & cyclic stress effect on structure. Frequency analysis of structure was carried out to verify if the natural frequency of structure is coinciding with excitation frequency of equipment. Time history analysis of structure was performed to examine if velocity amplitudes are exceeding tolerable limits. Investigation findings, OEM feedbacks and dynamic analysis outcomes succeeded in revealing the root causes of cracks in structural steel and consequently, remedial methods were finalized to avoid recurrence of cracks. Analytical results of the structure indicated that natural frequency of the structure was very close to the critical speed of resonance, implying fundamental deficiency in the stiffness of the structure contributing to high vibrations and structural damage. Time history analysis corroborated the same, as vibration amplitudes were found far exceeding vendor specified limits. About 40% of vibration measurements on the equipment were found outside the ‘acceptable range’ set forth in ISO standards. Investigations carried out to ascertain the cause of high vibration attributed to unapproved field modifications made on the equipment. Inspection of equipment assembly revealed that the inlet chute arrangement & its thickness were changed, an inlet cone was added to equipment and its mass was ignored in weight balancing calculation. Remedial works to reduce vibrations and to avoid cracks have included strengthening of structural members, addition of bracings, modifications of inlet chute box, adjustment of counterbalance weight & rectification of installation errors. The said remedial works were implemented three years ago and monitoring of structure demonstrated successful results (no cracks and vibrations are within allowable limit). Application of this sound engineering method has eliminated frequent maintenance / shutdown costs and also contributed to asset life extension.