Corrosion Surveys of NPS 6 and Smaller Lines with High Production Digital Radiography

Introduction

Piping failures account "for 60% of mechanical integrity losses [1]." Traditionally, we have monitored piping corrosion losses with ultrasonic thickness testing (UTT) based corrosion surveys. While these surveys are valuable, their information for small diameter piping can be challenging to interpret. In contrast, for NPS 6 and smaller lines, digital radiography (DR) based piping corrosion surveys give clearer and much broader information: an image showing wall loss, weld condition, threaded connection integrity, potential problems with insulation, internal product buildup, and often much more.

Using profile radiographic techniques (PRT) for small diameter lines is not a new concept. The API 570 "Piping Inspection Code: In-service Inspection, Rating, Repair and Alteration of Piping Systems" has clear guidelines for PRT of small-bore piping. This article illustrates the significant value of digital radiography as an efficient tool to visualize corrosion and other small bore piping anomalies without disturbing insulation. It also highlights the importance of properly interpreting radiography images and using PRT in conjunction with UTT to survey small diameter lines.

Some Key Benefits of Profile Digital Radiography

Profile radiography is described in detail in API RP 583 "Corrosion Under Insulation and Fireproofing." As stated in the RP, "The exposure source is usually iridium-192, with cobalt-60 being used for heavier wall piping … can also be done using X-ray sources." A typical exposure setup is illustrated in Figure 1. All DR digital detector array (DDA) images shown in this article were obtained using iridium-192.