High precision distortion control and quality improvement in the fabrication of large size quenched and tempered low alloy steel shells

Abstract

The manufacture of Pressure Vessels and Heat Exchangers involves the circumferential seam welding of individual shell courses to buildup the assembly. In one of the Heat Exchangers, Bharat Heavy Electricals Limited, Tiruchirappalli faced a major challenge in the fabrication of a shell assembly. The assembly was to be made out of two sub-assemblies; one had a length of 10 000 mm and weighed 25 000 kg, while the other had a length of 4 250 mm and weighed 18 500 kg. The latter sub-assembly consisted of a tubesheet of 400 mm thickness, resulting in eccentric loading. At the joint location, the shells were of a diameter of 1880 mm. The welding between these two sub-assemblies had to be completed ensuring an axis tilt within 0.034 degrees. Since the activity was falling on the critical path and the completion timing determined a large and significant cash inflow to the organisation, the cycle time was a significant factor. As the consumables used in the welding were very expensive, the demand on weld quality was high to achieve competitive cost of production. These challenges were met by a systematic and scientific approach. As a result, the role of preplanned welding sequence and effect of zero root gap on high precision distortion control, could be understood. Also, the role of positive root gap, uniformity in preheating, alignment of positioner setup, continuous welding with minimum interruptions, supports during post weld heat treatment in controlling axis tilt of less than 0.012°, weld defect level less than 0.1% and cycle time of manufacture less than 50% could be vividly brought out. This has also successfully demonstrated the capability of Bharat Heavy Electricals Limited. Tiruchirappalli in developing indigenous technology to achieve such high standards of performance, for critical equipments.