What are the Thread Hazards and What Causes Them
A visual thread inspection (VTI) is a critical process conducted to evaluate the condition of pin and box end threads and identify potential hazards or thread damage. During a VTI, protectors are removed, threads are cleaned, dried, and then examined by a trained professional. This inspection determines whether repairs or rethreading are necessary to ensure thread integrity and prepare the connections for use. While a damaged thread doesn’t automatically condemn the pipe, the time and cost required to rethread may render it impractical for certain applications. Here are eight common hazards identified during a VTI, particularly for premium thread connections and OCTG threading:
Key signs: Heavy galling on the front half of the connection indicates severe damage.
1. Thread Galling
Thread galling is a type of wear caused by friction and adhesion between sliding metal surfaces. It leads to visible material transfer, appearing as raised lumps called “galls.” Galling often occurs rapidly when insufficient lubrication between metal surfaces in contact, particularly during threading pipe in place.
Key signs: Scratch located within the sealing area, visible above the marked white line.
2. Scratches On Threads
Scratches vary in severity depending on their depth, location, and extent. Minor scratches are often repairable without compromising thread integrity. However, deeper scratches, especially in the sealing area, can be critical since this area must remain free of defects like rust, pits, or burns.
Key signs: Burrs visible on the seventh thread.
3. Thread Burrs
Burrs are rough edges or ridges caused during the manufacturing or thread-cutting process. These appear at the pipe ends; the thread starts, or further along the threading. Depending on the severity, deburring techniques may preserve the OCTG threading.
Key signs: Mashed threads identified within the highlighted white circle.
4. Mashed Threads
Mashed threads result from external forces reshaping or flattening the threaded surface. Proper threads have a defined and precise structure, whereas mashed threads appear smooth or flattened.
Key Signs: Dark crests on threads from the seventh thread onward.
5. Unfinished Threads
Unfinished threads are identified by dark, incomplete sections along the thread crest. This happens when the tube isn’t perfectly straight during the threading process, preventing the carbide insert from making full contact with the tube. Unfinished threads fail to provide an adequate seal and can lead to thread galling.
Key Signs: Rust visible across multiple sections of the threading.
6. Rust on Alloy Threads
Rust, a common form of corrosion, typically results from improper storage or maintenance of threaded connections. The correct application of thread compounds, such as Kendex, is a moisture barrier and corrosion inhibitor to protect premium thread connections.
Key Signs: Pronounced pits visible on the second thread crest.
7. Pitting on Threads
Pitting is a type of corrosion that creates cavities or holes (pits) on the pipe surface. These defects act as stress risers, compromising the threading pipe in place if not appropriately handled. Left untreated, pits can grow into deep cavities or through-wall holes, impacting thread integrity.
Key Signs: Copper flaking at the connection’s rear and tarnished threads throughout.
8. Flaking on Copper Alloys
Premium connections often include copper-coated threads on boxes or couplings to prevent galling during repeated use. If not maintained, the copper coating tarnishes and flakes off, exposing the underlying steel surface. Flaking on the sealing area may necessitate re-copper plating to safeguard the connection’s integrity.
Safeguard Your Thread Connections with Proper Maintenance
Understanding and addressing these hazards ensures the longevity and functionality of OCTG threading and other premium thread connections. Regular VTI inspections are pivotal in protecting thread integrity and extending the life of your pipe threading. Always consult professionals for inspections and remediation to maintain reliable connections.