Modelling and Predicting the Deformed Geometry of Thick-Walled Pipes Subjected to Induction Bending

Abstract

Induction bending offers a rapid, cost-effective method of producing complex convoluted pipework. The resultant bends, however, typically show unwanted geometric deformations which include wall thinning at the extrados, wall thickening at the intrados, awkward transitions on going from tangent to bend, and wrinkling at the intrados surface. All forms of geometric deformation are worse depending on the tightness of the bend and the thickness of the original pipe. Predicting the final geometry of the bend is a non-trivial problem.This article first considers the use of simple analytical models to predict the final deformed geometry of induction bends in thick-walled pipe. It then goes on to compare the predicted geometry obtained from those analytical models with the outputs from empirically derived charts and computational models. The article concludes that empirically derived charts do not offer accurate predictions of wall thinning, are not currently available for intrados wall thickening, and cannot, with confidence, be extrapolated to produce accurate results for very tight bends. Numerical models are developed which predict wall thickening and thinning for any combination of outside diameter/wall thickness/bend radius combination; however, it is concluded that their limited accuracy makes their use questionable. Elastic-plastic FEA is developed which is shown to provide the most consistently accurate means of predicting both wall thickening and wall thinning, and, while not providing a comprehensive prediction of the post-bend geometry did, nevertheless, to go some way towards predicting the overall deformed geometry including transition ramps and intrados surface wrinkling.

Hot Tube-Forming

“…Tube (pipe) bending using local induction heating is an advanced process that is used to bend thick-walled pipes with small bending radii and large diameters (74)(75)(76)(77). It is cost effective and has been widely used in many fields such as power, transportation, and chemical industries since it was invented by Dai-ichi High Frequency Co. (DHF0 of Japan); Figure 40 shows the bending principle of induction tube bending.…”

“…As the pipe moves through the induction coil, the position at where bending occurs travels down the pipe. Thus, a continuous bend can be produced (77).…”

“…The thinning and thickening ratio of the pipe wall thickness, the pushing force, the oval shape of the cross section of the pipe, and the springback after unloading were discussed. The literature (76,77) reported the challenges associated with induction bending in the production of tight bends in thick-walled pipe, and concluded that the existing charts are deficient when considering extrados wall thinning. The analytical models predicted the final deformed geometry of induction bends in thick-walled pipe, which was compared with the results of empirically derived charts and computational models.…”