Plugging Tee Ball Jamming Analysis and Plugging Equipment Improvement Design

Literature Overview

This paper by Liu Jianjun, Yang Tao, Cai Haisheng, Liu Guojian, and Zhao Haining, published in Pipeline Technology and Equipment (2017, No. 3, pp. 25-27), addresses a critical operational issue encountered during in-line inspection (ILI) of a natural gas pipeline between Station A and Station B. The study analyzes the root cause of a pig jamming incident at a plugged tee fitting and proposes an optimized design for the plugging device to resolve the problem.

Problem Description and Root Cause Analysis

During the pigging operation for internal inspection of the natural gas pipeline, the inspection pig became jammed at a plugged tee fitting. This is a well-known challenge in pipeline operations where branch lines are taken from the main pipeline. When a branch is no longer in service, it must be plugged to maintain the integrity of the main line during pigging operations. The plugging device, referred to as a saddle plate or plugging insert, must be securely installed in the tee branch opening without obstructing the main flow path.

The root cause analysis identified several contributing factors:

The original plugging design lacked a rotational limit mechanism, allowing the plug to rotate within the tee bore under fluid dynamic forces.
The plug geometry did not provide adequate sealing against the tee wall, creating gaps that allowed the pig to catch on the plug edge.
The plug material and surface finish were not optimized for long-term service in high-pressure natural gas conditions.

Plugging Equipment Improvement Design

The authors proposed an optimized plugging design based on theoretical calculations and engineering practice. The key improvements include:

Design Parameter	Original Design	Improved Design
Rotational Limit	None	Positive mechanical stop mechanism
Sealing Method	Simple compression	Enhanced seal with controlled interference fit
Surface Treatment	Standard	Polished to reduce friction with passing pigs
Material Selection	Standard carbon steel	Improved grade for pressure and impact resistance
Installation Method	Basic bolting	Precision alignment with torque-controlled fasteners

The rotational limit mechanism is particularly important because, without it, the plug can rotate under the asymmetric loading from the flowing gas and passing pigs. Once rotated, the plug may no longer align with the intended seal geometry, creating a step or edge that can snag the pig. The improved design incorporates a mechanical stop that prevents rotation while still allowing thermal expansion accommodation.

Engineering Practice Implications

This case study has significant implications for pipeline engineering practice, particularly in the following areas:

Pre-installation inspection: Before installing plugging devices, the tee geometry and internal surface condition must be thoroughly inspected to ensure proper fit.
Post-installation verification: After installation, the plugging device should be verified for correct alignment, seal integrity, and rotational restraint.
Pigging procedure: The pigging procedure should account for the presence of plugged tees, with appropriate launch and receive pressures to avoid excessive impact on the plug.
Design review: Future tee plugging designs should incorporate rotational limits as a standard requirement, not an optional feature.

From a welding perspective, the installation of saddle plates and plugging devices often involves field welding of the attachment hardware to the pipeline. The quality of these welds, including proper preheat, interpass temperature control, and post-weld heat treatment where required, directly affects the structural integrity of the plugging arrangement. Any weld distortion or residual stress concentration at the tee branch connection can compromise the seal and lead to operational issues.

The paper also highlights the importance of theoretical calculation in validating design improvements. By performing pressure and stress calculations on the improved plug design, the authors demonstrated that the new design maintains adequate safety margins while providing the necessary functional improvements. This approach aligns with modern engineering practice where analytical methods complement empirical knowledge to ensure reliable designs.

Study Insights

This paper is a valuable example of how a specific operational problem can lead to meaningful design improvements. The systematic approach of identifying the problem, analyzing the root cause, proposing a solution, validating it through calculation, and planning for practical implementation is a model for engineering problem-solving. The emphasis on resolving the rotational limit issue demonstrates that sometimes the simplest design oversight can have the most significant operational consequences.

For pipeline engineers, this case reinforces the importance of considering all potential failure modes during the design phase. A plugging device that appears adequate under static conditions may fail under dynamic loading from fluid flow and pig passage. The lesson here is that operational scenarios must be fully understood and incorporated into the design basis, and that post-installation verification is not optional but essential for ensuring long-term reliability.