A crack has been found during hydrostatic test at shop of heat exchanger fabrication. The tubes were HYD tested at tubing facilities before supplied to the buyer and the test results were satisfactory. Also, it was passed ECT machine.
The reason of the crack was that tubes had the crack from the beginning however HYD test at tubing facilities had no enough pressure to crack it out. The pressure was acc to standard however it made crack slightly open but not open wide as much as leakage found. And then inspectors thought that the tubes were all find.
When the products were supplied to buyer, buyer welded those tubes to heat exchanger and performed HYD test. A HYD operator applied the same pressure to the tubes. Finally the crack-not yet been through from tube surface to inner diameter-was penetrated and water leaked out.
From the results, I could know that tube manufacturers need to add more pressure during HYD test than the condition mentioned in ASTM/ASME to avoid such a problem. It can’t be found easily if MFG only follow standard.
Also, it’s important to choose a good raw-material. The picture shown below is pockmarked surface. It was happened because MFG sellected poor grade of raw-material. They had polishing on the surface to cover up the marks. Pockmark itself is not a problem actually unless mininum thickness of requirment is kept. Since MFG sellected poor raw-material, and there was hair crack and applied polishing. During the polishing, steel powder filled in the crack and inspector couldn’t find it out in visual inspection. This is another reason of the crack wasn’t found.
MFGs must use right material to avoid problem and save costs. If circumstances were not good, so had to use poor grade, then they must apply tests in severe condition.
Written by Jethro