Tube leaks are usually caused by under-rolling, over-rolling, improper preparation of tube sheets, and differential thermal expansion. Improperly rolled tube joints can cause several challenges, such as necessary reworking and loss of vessel efficiency.
Under-rolling occurs when the proper amount of wall reduction is not reached. If left uncorrected, this will cause leaks between the tube OD and tube sheet hole ID. Under-rolled tubes can usually be fixed by rolling a second time to obtain the proper amount of wall reduction. Be sure to use a tube gauge to periodically check the amount of wall reduction during rolling.
Over-rolling of tubes occurs when the expansion of the tube surpasses the expansion required for the proper wall reduction. Over-rolling can cause considerable damage to the tube sheet and adjacent tubes. Once a ligament is over-rolled, it will deform the ligaments of the tube joints surrounding it. This deformation can cause adjacent tubes to leak.
In addition to impacting tubes, over-rolling can cause distortion in tube sheets or drums. Over-rolling can potentially cause a tube sheet to bow or warp to the point where the standard length tube could not be used in the vessel until the bowing is returned to normal. This can be corrected in some cases by placing stay rods in the vessel and pulling the tube sheets back to their original position.
Lastly, over-rolling can have a significant impact on expander tool life. It can cause the tube material to flake off, which can get trapped in
between the rolls and mandrel. If this debris is not cleaned from the expander, it can reduce the life of the rolls and mandrel.
Over-rolling tubes is not something that can be easily corrected. Once ligaments become deformed, the entire tube sheet has an increased chance of leaks, higher maintenance costs, and decreased efficiency. As a result, it is important to avoid over-rolling by using an electronic or pneumatic torque control or an assisted rolling system. These systems will ensure you roll to an exact wall reduction every time.