Guidance for Non-Intrusive Inspection of Pressure Vessels

Pressure vessels must undergo periodic inspections to help ensure their mechanical integrity and continued safe operation. Such inspections are usually mandated by regulations or prescribed in the integrity management programs of prudent operators. Traditionally, internal visual inspections have been employed. These can be costly because of the need to shut down the vessel, isolate it, prepare it for entry, and follow requirements for confined-space entry. Furthermore, vessel entry may even have an adverse effect on its future performance. For these reasons, it is desirable to utilize non-intrusive inspection methods where a vessel can be non-invasively inspected from its exterior. However, the use of non-intrusive inspections must not compromise safe and reliable vessel operation. Compared with traditional intrusive internal inspection, non-intrusive inspection is relatively new and there are a wide variety of inspection techniques available. Each technique has its strengths and weaknesses, and many engineers are not fully acquainted with the capabilities and limitations of the various non-intrusive inspection techniques. To address this issue, Recommended Practice DNV-RP-G103 on Non-Intrusive Inspection (NII) was developed [1]. This paper reviews the recommended practice and discusses example applications of the recommended practice. The recommended practice provides guidance on the following key aspects of non-intrusive pressure vessel inspection: (1) determining when its use is appropriate, (2) information that is needed for inspection planning, (3) defining requirements for inspection methods, (4) selecting inspection methods based on requirements, (5) evaluating inspection results, and (6) requirements for proper documentation of inspection results. The essential elements of the procedures covered in the recommended practice are performing a mechanical integrity review, deciding if non-intrusive inspection is possible, planning for the inspection, performing the inspection, and evaluating the results of the inspection. Finally, the inspection interval is evaluated.