Using ACFM to inspect high-temperature line welds

M Smith
Eddyfi Technologies – TSC 

The ability to inspect welds at elevated temperatures for fatigue cracking can provide significant cost savings by avoiding plant shutdown. It also avoids any problems of crack closure, which can occur when the structure is cooled down to ambient temperature. Conventional surface inspection methods such as magnetic particle inspection (MPI) or penetrant inspection (PT) have limitations in their upper temperature range in the order of 48-60°C (120-140°F), although some specialist penetrants can be used up to 177°C (350°F) and some specialist dry magnetic powders are rated to 400°C (750°F). There is usually a reduction in sensitivity with these products compared with ambient temperature inspection.

A gas process plant planned to install several weldolets onto an existing pipe run while the line was still in operation at 260°C (500°F) using the hot tap method. Prior to doing this, they needed to ensure that the target pipe surface was free from surface defects and that the welds created in the hot tap process were free from surface flaws.

This article discusses how ACFM probes were made to operate at the required temperature and how they were used to effectively ensure the integrity of the final hot tap welds without the need to implement an expensive line shutdown.