Achieving Proper Roll By Controlling The Amount Of Tube Expansion
Although there are disagreements on how much to roll a tube, all operators understand there are undesirable effects to over-rolling, and these are more serious than those resulting from under-rolling. An under-rolled tube can be given sufficient additional roll to properly seal it, but re-rolling an over-rolled tube simply contributes more of the undesirable effects. Controlling the amount of roll and eliminating over-rolled tubes is vital since a seal might be formed with over-rolling, however, the life of the tube is shortened.
The basic purpose in rolling a tube is to obtain a good hydraulic and mechanical joint. A secondary, but equally important, purpose is to obtain a seal that is durable, resistant to crevice-corrosion and free of longitudinal tube stress.
When rolling a tube, it is expanded to the point of metal-to-metal contact, then cold-worked between the tube and sheet, to give the desired seal. This cold-working proceeds until the yield point of the tube is reached. Beyond that point, any additional expansion becomes extrusion (the metal has to go somewhere), with the tube extruding in both directions from the seal. Tests by various authorities, while not agreeing on the precise percentages, agree that approximately 60% to 75% of this extrusion goes into the bundle, with the remaining 25% to 40% extending out of the bundle. The result of this tube travel is to shear the sealing surface between the tube and sheet. The ideal seal is one that produces the desired radial stress between tube and tube sheet, but does not exceed this point sufficiently to cause too much extrusion. This point occurs just beyond the yield point of the tube.
Although over-rolling the tube in the first tube sheet will not cause longitudinal stress, since the tube is free to move through the other tube sheet, the internal extrusion from over-rolling at the other sheet will cause stress in the tube. This compressive stress leads to stress corrosion in the bundle, frequently a major factor in shortening tube life. Since it is impossible to roll a tube such that no extrusion takes place, this compressive stress exists in all tube bundles. However, this stress and its undesirable effects can be minimized by limiting and controlling the amount of roll.
Over-rolling also causes distortion of the tube sheet ligaments, weakens the bridge between tubes and causes egg-shaped tube sheet holes, which further contributes to a poor joint. Consider a group of tubes in which several adjacent tubes have been over-rolled. Then, as each adjacent tube is rolled, the resulting distortion through the tube sheet ligament further loosens the earlier rolls and could cause the tube sheet to warp or bow.
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