Methods for Detection, Characterization & Quantification of High Temperature Hydrogen Attack

Nelson Curve changes in the late 1980's provided cause for Shell Oil Company to look at more reliable NDE nondestructive evaluation (NDE) methods for assessment of materials/equipment in high temperature hydrogen service. The primary change motivating Shell was the lowering of C-0.5 Mo steel Nelson Curve to the carbon steel level. Prior to this change, C 1/2 Mo equipment was considered more resistant to high temperature hydrogen attack (HTHA) than carbon steel. Shell, like other refining and chemical companies, has equipment constructed of this material in this type of service.

The costs to metallurgy upgrade all equipment was substantial. Shell proactively chose to develop NDE methods to more accurately and reliably determine and monitor the condition of its equipment while methodically upgrading the metallurgy of existing C 1/2 Mo pressure equipment.

This NDE research commenced at Shell's Westhollow Technology Center in 1990, often undergoing field trials along the way, culminating in the development of the advanced ultrasonic backscattering techniques (AUBT) described in this article. Scanning is performed on the external surface to detect damage in early (fissuring) stages during outages. Inspection is used for both stainless steel lined and non-lined equipment.

Prior to the advent of these advanced methods, the most popular inspection techniques were field metallography and replication (FMR) and ultrasonic techniques including "attenuation", velocity ratio (VR), and amplitude-based backscattering methods. FMR can only be applied on non-lined equipment when the internal surface is accessible. The attenuation technique cannot discriminate HTHA from abnormal grain size, inclusions, surface roughness, or geometry. Velocity ratio lacks in sensitivity. Amplitude-based backscattering has difficulty discriminating between actual attack and "dirty" steel or inclusions, which often leads to false positive results.

In the initial stages of development, Shell saw the value in considering the interdependence of high-frequency short wavelength signals to discrimination of real damage versus inclusions and voids. This departure from the voltage/amplitude treatment of backscatter was a breakthrough. Shell developed and patented (US Patent Number: 5,404,754; Date 4/11/95) advanced backscattering techniques/procedures, which resulted in significant improvements in reliability of equipment condition assessment. Shell took them to the field for company-wide use in 1991.

The key steps in the AUBT procedure are as follows: