Background to Current Concerns
A catastrophic failure of a C-steel heat exchanger vessel at Tesoro in 2010 due to HTHA threw the long-standing Nelson curves into the limelight. Since then, regulating authorities have been scrutinising the use of the curves more closely, and plant operators have been reviewing the adequacy of their procedures for the management of HTHA.
The Nelson curves in API 941 are based around data provided largely from industry experience and essentially represent a pressure-temperature limit for operation. Although useful, the black and white picture painted by the curves does not account for the numerous different situations that can develop in practice. Beyond the Nelson curves, there is no widely accepted guideline for assessing the impact of HTHA damage on the structural integrity and remaining life of equipment.
HTHA damage can come in various forms and can be difficult to detect. Furthermore, once the degree of severity has been characterised, there remains a question over how to manage the affected plant. Welds are focal points, where damage is more likely to occur due to the susceptible microstructures, residual stresses and higher likelihood of flaws associated with them. Indeed, the latest API 941 document features a new appendix, with a lower operation limit for non-PWHTed welds.
The treatment of cracks and flaws in components operating in hot hydrogen environments is of particular concern. Pre-existing flaws are considered to be present at welds, of a size corresponding to the detection limit for the NDT procedure employed. Significantly, crack progression due to HTHA can occur before any detectable damage in the bulk material, indicating that the Nelson curves are not a reliable method for assessing safety where crack-like flaws are considered to be present.