Inadequate heat treatment in the head and head-to-shell weld areas of a low-pressure separator vessel was determined to have led to conditions which allowed for the development of sulfide stress cracks, and ultimately resulted in rupture of the vessel. This determination was made through the use of destructive testing and failure analysis of the ruptured vessel. Generally, verification of vessel heat treatment is done through review of temperature recordings from manufacturing documentation or third party verification at the time of fabrication. Unfortunately, heat treatment records of pressure vessels are often misplaced, lost during acquisitions/mergers, or simply never existed.
Under the assumption that these vessels were adequately heat treated, the vessel’s owners do not normally take into consideration the high residual stresses which create favorable conditions for sulfide stress cracking (SSC) and other stress corrosion cracking type damage mechanisms. In this case, cold formation of the head resulted in high residual stresses in the flange and knuckle regions. The welds, which had not been properly post-weld heat treated, similarly had high residual stresses. These high stresses resulted in favorable conditions for the SSC to occur when exposed to a corrosive environment such as oil and gas operations.
In an effort to prevent similar events from occurring in the future, it was determined to be necessary to evaluate other vessels for inadequate heat treatment that may result in SSC. A non-destructive approach was investigated to evaluate vessels, especially in the absence of the aforementioned heat treatment records, to determine if the formed heads and heat affected zones after welding had adequate heat treatment. A method was developed to identify vessels without adequate heat treatment utilizing standard non-destructive testing techniques. There is the potential that many other low pressure vessels in lethal service could have received inadequate heat treatment. This methodology can therefore be used to determine the heat treatment state for these pressure vessels without destructive testing.