Abstract
Two successive leakages were reported in the heat exchanger composed of 8 rows of finned tubes of the convection section of the condensate stripper re-boiler (U-H8300), the 3rd and the 4th rows used in Qatar. A comprehensive study (Failure Analysis) has been conducted to reveal the form of failure, by finding out the root cause of that failure, then suggest and implement the suggested practical solution.
The failure was recognized as erosion and cavitation damage. All 8 rows of the re-boiler tubes, except the 1st and the 2nd rows, were made carbon steel as per ASTM 106 Grade B. The most severe working conditions were encountered in the early stages of the stripping process, i.e., in the 1st and 2nd rows, where the amount of the gas carried, temperature and pressure were higher. However no failure occurred in the tubes of the 1st and 2nd rows, simply because they are made from higher corrosion resistance alloys, AISI 316 stainless steel. Moreover, no failure has been yet reported in the 5th-8th row tubes. This was due to the working conditions were less severe in the 5th-8th row tubes, although they were made from same carbon steel (ASTM 106 Grade B).
The recommended measures to avoid the re-occurrence of this failure, is to replace those tubes with another carbon steel tubes of comparable cost but of higher erosion corrosion resistance. The material chosen was ASTM A 423 steel. This alternative material was found to resist the erosion-corrosion and cavitation failure more than the ASTM 106 Grade B. It had been put in service for more than three years ago and no failure was detected since.
