Expanded, and Welded-and-Expanded Tube-to-Tubesheet Joints

This paper discusses some practical aspects of expanded and welded-and-expanded tube-to-tubesheet joints. It reviews elastic-plastic tube expanding theory, which it uses as the basis for setting pressures for uniform-pressure expanding and for estimating residual interfacial stress at the tube-hole interface. It addresses uniform-pressure-expanded tube joint strength and the problem of correlating of roller-expanded joint strength with wall reduction and rolling torque. It advocates two-stage expanding: 1) applying just sufficient pressure or torque to create firm tube-hole contact over substantially the tubesheet thickness; and 2) re-expanding at full pressure or torque. It points out the advantages of segregating heats of tubing and mapping the tube-heat locations. It recommends non-TEMA Standard (and non-HEI Power Plant Standard) grooves for grooved-hole joints made by roller-expanding, when the tubes are thin-walled, high-strength, low-elastic-modulus, and for joints made by uniform-pressure expanding [1, 2]. It states what to examine when considering grooves for small-diameter tubes. It reviews tube-to-tubesheet welding requirements and discusses welding before and after expanding. It suggests TEMA revise its standards to define strength and seal welds and urges the ASME Boiler and Pressure Vessel Code Committee (the Code Committee) to incorporate the TEMA definition in Section VIII of the ASME Boiler and Pressure Vessel Code (the Code) [3]. It makes recommendations for pressure and leak-testing. The work shows why welded-and-expanded joints should be full-strength expanded and why expansion length should equal the tubesheet thickness minus 1/8 in. (approximately 3 mm) rather than the lengths the TEMA and HEI Standards permit.