Optimized Plug Stem Design for Large Diameter Pipeline Tee Plugging Operations

Literature Overview

This paper, authored by Zhang Jichang, Yuan Huizan, and Tao Weili from the Maintenance and Emergency Repair Branch of China Petroleum Pipeline Engineering Co., Ltd., addresses a critical practical challenge in oil and gas pipeline maintenance: the design and optimization of plug stems for large-diameter pipeline plugging tee assemblies. Published in Pipeline Technology and Equipment in 2022, the study focuses on pipelines with diameters of 914 mm and above, where the conventional internal bypass structure fails to meet manufacturing and operational requirements. The problem arises because the top shaft diameter of large-diameter pipeline plugging machines is large while the protrusion height is short, making it impossible to push open the sealing steel ball to achieve upper and lower connectivity using the original internal bypass structure. This is a highly relevant topic for pipeline maintenance engineers who routinely deal with large-diameter pipeline emergency repair operations.

Core Technical Problem and Design Approach

The fundamental challenge lies in the geometric constraints of large-diameter plugging machines. For pipelines of 914 mm and above, the top shaft of the plugging machine has a large diameter but a relatively short protrusion height. The original plug stem design relied on an internal bypass structure to push open the sealing steel ball, enabling fluid communication between the upper and lower chambers of the tee. However, this conventional approach could not be achieved through machining processes for the larger diameters, creating a practical bottleneck in emergency repair operations.

The authors proposed a comprehensive redesign of the plug stem structure, incorporating several key modifications. The threaded plug and steel ball sealing structure were redesigned to accommodate the larger diameter constraints. A double sealing arrangement was implemented using an end face pressure cover combined with a sealing ring, providing enhanced sealing reliability. The plug stem connection pipe was manufactured through factory pre-welding, and the connection between the connection pipe and the plug stem was achieved through bolted joints rather than field welding. These design choices reflect a careful balance between manufacturability, field deployability, and sealing integrity.

Key Design Features and Engineering Rationale

The redesigned plug stem incorporates several engineering innovations that merit careful examination. The modification of the threaded plug and steel ball sealing structure represents a fundamental departure from the original design philosophy. Instead of relying on the internal bypass to push the steel ball, the new design integrates the sealing function directly into the plug stem geometry, allowing the steel ball to be actuated through a more mechanically efficient path.

The double sealing system is particularly noteworthy. The end face pressure cover combined with a sealing ring provides two independent sealing barriers, which significantly improves the reliability of the plugging operation. In emergency repair scenarios, where pipeline integrity must be restored quickly and safely, the margin of safety provided by dual sealing is invaluable. This approach is consistent with the defense-in-depth principle commonly applied in pressure boundary design.

The factory pre-welding of the plug stem connection pipe is another critical design decision. Field welding of large-diameter components during emergency repairs introduces significant quality risks, including incomplete fusion, hydrogen-induced cracking, and residual stress concentration. By moving the welding operation to a controlled factory environment, the authors ensure that welding quality can be properly monitored and controlled through standard non-destructive testing procedures. The bolted connection between the pre-welded pipe section and the plug stem provides field adjustability and disassembly capability, which is essential for maintenance operations.

Process Parameters and Quality Control Considerations

The following table summarizes the key design parameters and quality control aspects of the optimized plug stem:

Quality control for this type of component requires attention to several critical aspects. The factory welding of the connection pipe should follow applicable standards such as ASME B31.3 or SY/T standards for pipeline welding, with appropriate preheat, interpass temperature control, and post-weld heat treatment as required by the material specification. Non-destructive testing should include radiographic testing (RT) or ultrasonic testing (UT) for the welds, and magnetic particle testing (MT) or penetrant testing (PT) for surface defect detection. The bolted connection interface requires careful surface finish control and proper torque specification to ensure leak-free assembly.

Application Experience and Practical Implications

The paper reports that the optimized plug stem has been successfully applied in large-diameter pipeline emergency repair projects. This field validation is essential for confirming the practical viability of the design. The application experience demonstrates that the redesigned plug stem can reliably perform its function in real operating conditions, providing a proven solution for the industry.

From a broader engineering perspective, this paper illustrates an important principle in pipeline maintenance engineering: when conventional solutions fail due to geometric or manufacturing constraints, a systematic redesign approach that considers the entire assembly chain can yield effective solutions. The combination of structural redesign, enhanced sealing, and manufacturing process optimization represents a holistic engineering approach rather than a piecemeal fix.

Study Insights and Reflections

This paper provides valuable insights for engineers working on pipeline maintenance and emergency repair operations. The optimization of the plug stem design addresses a real-world problem that affects pipeline operators worldwide. The approach taken by the authors demonstrates the importance of integrating design, manufacturing, and field application considerations in the development of maintenance equipment.

The double sealing concept is particularly instructive. In critical applications where failure of the sealing mechanism could lead to catastrophic consequences, redundant sealing barriers provide a valuable safety margin. This principle is applicable not only to pipeline plugging operations but to various other pressure boundary applications in the oil and gas industry.

The decision to use factory pre-welding for the connection pipe reflects a mature engineering judgment. Field welding of large-diameter components under emergency conditions is inherently risky, and the potential for weld defects is significantly higher than in controlled factory environments. By shifting the welding operation to the factory, the authors ensure that weld quality can be properly controlled and verified, while still maintaining the field deployability of the assembly through the bolted connection interface.

This study underscores the importance of continuous improvement in maintenance equipment design. As pipeline networks expand and operating conditions become more demanding, the maintenance tools and equipment must evolve to meet new challenges. The optimized plug stem design represents a step forward in this evolution, providing a reliable and effective solution for large-diameter pipeline plugging operations.