To enhance heat transfer on the tube side of the heat exchanger, some manufacturers use inserts in the tubes.  The inserts are thin gauge metal strips that have been twisted and inserted into the tubes.  The twist in the metal strip will cause the gas to swirl down the tubes increasing turbulence near the tube wall and increasing the heat transfer coefficient.  This allows the exchanger size to be reduced with a corresponding reduction in cost.

In one design, the metal strips are inserted into the tubes loose and secured at the inlet end by a single weld.  The outlet end is left loose so that the metal strip is free to move and expand.  In one heat exchanger, the weld holding the metal strip in place broke allowing the inserts to be blown out of the tube and into the outlet duct from the heat exchanger.  The inserts eventually got caught in a downstream damper causing it to eventually fail as the actuator was trying to close the damper fully but was not able to due to the large number of inserts that had wedged themselves in and around the damper.

The mode of failure appeared to be a broken weld caused by excessive vibration of the metal strip which weakened the weld.

The problem was eventually discovered during a shutdown and when an investigation was done to see why the damper was not closing fully.  The missing inserts were replaced and to ensure that the inserts would not break free again a second weld was placed on the metal strip at the inlet end of the exchanger.  This would hopefully prevent the leading edge of the insert from vibrating at the tube inlet.  As well, a screen was place at the outlet of the exchanger to catch any inserts should they break away.