Differentiating Original Equipment Defects From In-Service Damage

Damage of carbon steel pressure vessels due to various in-service damage mechanisms continues to be a serious concern in the refining and petrochemical industries. A survey conducted in 1990 by the NACE T-8-16 Work Group to determine the nature and extent of cracking problems in wet H2S refinery environments showed that there was insufficient information reported about the type of cracking found to correlate cracking incidence with cracking mechanisms. Most of the inspections for cracking reported were detected during internal inspection using Wet Fluorescent Magnetic Particle Testing (WFMPT). As a result, it was concluded that in addition to the service related cracks reported, a number of "cracks" detected were the result of original fabrication, repair or alteration of the pressure vessels.

The survey responses reported inspection results for almost 5,000 pressure vessels. Overall, cracking was reported in 26% of the inspected vessels. For these vessels with reported "cracks", little or no correlation was found between the incidence of cracking and the operating conditions (such as temperature), post weld heat treatment (PWHT), or extent of weld repairs. For example, throughout the range of operating temperatures, the incidence of cracking only varied between 23% and 37%. These inspection results are not consistent with more recent results reported in the refining industry.