Wednesday, October 6, 2010

TUBE-TO-TUBESHEET JOINT MOCKUPS

For tubular heat exchangers in which tube-to-tubesheet joint tightness and strength are essential tubesheet mockups are useful in establishing appropriate welding and tube expanding procedures. The mockups must replicate the production tubesheets using tube and tubesheet material of the same specifications as will be used in producing the production unit. The drilling, annular grooving, preparation for welding and welding procedure used to produce the mockup must be those used on the production unit.

Once the mockup has been produced, our procurement specifications require the manufacturer to fill the tubes with a plastic that neither shrinks nor expands as it hardens. After the plastic sets, our specification requires cutting the specimen in the axial direction, making sure that the tubes to be examined are sawed on their centerlines. Our specification requires polishing to a mirror finish (#8).

The examination consists of examining the specimens using the VAX digital microscope at a magnifaction of 25X. We can increase the magnification to 150X to examine suspicious regions. We verify that the weld leak path is at least equal to the tube wall thickness and that te unexpanded gap behind the weld root is as specified. We check to see that the metal deformed into grooves bottoms out and that discontinuities where tube metal crosses groove edges is insignificant. We examine the tube-wall contact to make sure it is intimate and without discontinuities. The following photographs are of the VAX digital microscope, a typical specimen marked by the manufacturer, a weld examination, a groove examiation and the examination of an expanded region.









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