8 Potential Hazards Revealed With a Visual Thread Inspection
What are their causes and how do you spot them?
A visual thread inspection (VTI) is performed to evaluate the condition of the pin and box end threads. During a VTI, protectors are removed, threads are cleaned, dried, and closely examined for potential hazards by a trained professional. The results of the VTI provide the information needed to determine if any repairs or rethreading must the done to bring the connections to running condition. A bad thread does not necessarily condemn the pipe, however, due to the expense and time required for rethreading, it may make a string of pipe impractical for the application being considered. Some of the potential hazards identified during a visual thread inspection are detailed below.
Galling is a form of wear caused by a combination of friction and adhesion between sliding surfaces, especially metals. This results in a visible transfer of material that appears in the form or a raised lump (gall). Galling generally occurs quickly with potential for rapid spreading where there is inadequate lubrication between the metal surfaces in sliding contact with each other.
Heavy galling visible across the front half of this connection.
Scratches can range in severity based on the depth, extent, and location of the scratch on the threaded connection. Slight scratches are often easily repairable and will not compromise the integrity of the connection. More severe scratches on the sealing area may not be repairable as the sealing area is the most critical part of the connection and must be free of scratches, rust, pits, burrs, or any defect.
Scratch visible above white marker line on sealing area.
Burrs are rough edges or ridges resulting from the tools used during the manufacturing or thread cutting process. Burrs typically appear at the end of the pipe or the start of the thread, but can be located further along the threading as well. Depending on the severity of the burr, deburring can be an option to preserve the thread.
Burrs visible on the 7th thread.
4. Mashed Threads
Mashed threads are the result of an external force being applied to the threaded surface causing the metal to reform/reshape. While proper threads are characterized by their precise definition and structure, mashed threads generally have a smoothed or flattened appearance.
Mashed threads visible in the white circled area.
5. Unfinished Threads
Indicated by dark, unfinished sections along the thread crest, unfinished threads are generally caused by a tube end not being completely straight at the time the threads are cut. In this situation, the carbide insert, which cuts the threads, does not make solid/direct contact with the tube all the way around, resulting in unfinished threads on sections of the tube. In addition to providing an inadequate seal, unfinished threads have the potential to cause galling.
Unfinished threads with dark crests visible on threads 7 and beyond.
Rust is the general term for corrosion of irons and steels. Rust on threads is most often associated with poor storage and maintenance of the threaded connections. Thread compounds such as Kendex, when properly applied and maintained, function as a moisture barrier and corrosion inhibitor.
Rust visible in multiple sections of the threading.
Pitting is another form of corrosion which results in cavities or holes (pits) on the surface of the pipe. Pitting is a localized loss of thickness that can act as a stress riser and lead to other imperfections in the threads if not properly handled and remediated. Once initiated, these holes can grow, developing into deep cavities and even complete through-wall pits.
Several pits visible on this connection with pronounced pit on the 2nd thread crest.
8. Copper Flaking
Most premium connections require the threads on boxes, or couplings, to be copper-coated to prevent galling during make-and-break operations. The reddish color of the copper makes it fairly easy to spot imperfections in the coating. If not properly maintained, the copper coating will tarnish to a darker gray/black color, and may lead to the coating flaking off and revealing the silver/steel surface underneath. If the copper coating flakes off on the sealing area, remediation consisting of re-copper plating will be required to maintain the integrity of the threads.
Copper flaking visible at the back of the connection. Tarnishing of threads throughout this connection.
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