Internal Bore Welding of Tubes to Tubesheet Joints in 5G Position

Abstract

Tube to tubesheet welds are very important for functioning of any heat exchanger. Generally, corner or fillet type of joint design is used for these welds. However, both type of joint design is undesirable in certain applications since it leaves “crevice†between tube and tubesheet which acts as potential site for initiation of corrosion. Moreover, volumetric examination of weld is also not feasible. Hence, for very critical applications, tubes are welded to tube sheet using “full penetration – butt joint†which eliminates crevice and makes volumetric examination of weld feasible. Welding of this type of joint is termed as “Internal Bore Welding†since welding is made from inside of tube. Mostly, automatic GTAW process is used. L&T is manufacturing a stainless steel heat exchanger wherein tubes were to be butt welded using internal bore welding technique. This heat exchanger consists of more than 4500 tubes of 15.75 mm inside diameter and 1.65 mm thickness to be welded to tubesheet of thickness greater than 150 mm. 100% Visual Examination from outside and inside, dye penetrant testing from outside surface of weld, radiography examination (PQR) and helium leak testing are required for these joints. Special torch is developed to facilitate IBW for such small diameter of tubes. Welding without filler wire addition is done in 5G position to produce weld metal having acceptable visual appearance. This paper describes selection of welding machine, special considerations for IBW, development of welding procedure.