Zhuojin Pipe Fitting Co., LtdPlug-in Tee Ball Jamming Analysis and Plugging Equipment Improvement Design
Literature Overview
This 2017 paper by Liu Jianjun, Yang Tao, Cai Haisheng, Liu Guojian, and Zhao Haining, published in "Pipeline Technology and Equipment" (Issue 3, pages 25-27), addresses a critical operational challenge in natural gas pipeline integrity management: the jamming of inspection pigs at plug-in tee locations during in-line inspection (ILI) pigging operations. The authors are from Shandong Natural Gas Pipeline Co., Ltd. and Sinopec Zhongyuan Oilfield Natural Gas Production and Sales Plant. The paper presents a root cause analysis of the ball jamming problem and proposes a redesigned plug-in plugging device with rotational limit functionality.
Root Cause Analysis of Ball Jamming
The paper describes a specific incident at the A Station to B Station segment of a natural gas pipeline where an inspection pig became lodged at a plug-in tee fitting during a pigging operation. The authors conducted a thorough analysis using a combination of engineering investigation and theoretical calculation. The key findings are summarized below:
| Failure Mode | Root Cause | Consequence |
|---|---|---|
| Ball jamming at tee | Plug-in plugging device design defect | Pipeline blockage, reduced gas transmission efficiency |
| Lack of rotational limit | Original design did not constrain pig rotation | Pig enters tee branch at unfavorable angle |
| Insufficient sealing clearance | Saddle plate geometry did not accommodate tee bore tolerance | Sealing failure or excessive friction |
The analysis revealed that the original plug-in plugging device had two critical design deficiencies:
- No rotational limit mechanism — The plugging device allowed the inspection pig to rotate freely as it approached the tee junction, causing the pig to enter the tee branch at an angle that resulted in jamming.
- Improper saddle plate design — The saddle plate (the sealing component that interfaces with the tee bore) did not adequately account for manufacturing tolerances and wear, leading to either insufficient sealing or excessive friction that contributed to the jamming.
Optimization Design Proposal
The authors proposed an optimized plugging device design based on theoretical calculations. The key improvements include:
- Rotational limit mechanism — A mechanical constraint that controls the orientation of the inspection pig as it passes through the tee junction, ensuring the pig enters the branch at a favorable angle.
- Improved saddle plate geometry — Redesigned to accommodate manufacturing tolerances and provide reliable sealing without excessive friction.
- Enhanced sealing performance — The new design provides better gas-tight sealing during the plugging operation, which is critical for maintaining pipeline pressure integrity during hot-tapping and plugging procedures.
Engineering Practice Considerations
For pipe fitting and pipeline engineering, this case study highlights several important lessons:
- In-line inspection planning must account for all fittings — Tee fittings, reducers, and other non-uniform-diameter fittings require special consideration in pigging route planning.
- Plug-in plugging devices must be designed for specific fitting geometries — Generic designs may not accommodate the geometric variations present in different tee configurations.
- Hot-tapping and plugging operations require rigorous pre-job analysis — Including geometric analysis of the target fitting, pressure rating verification, and simulation of pig passage through the fitting.
The paper also raises important considerations for the design and fabrication of plug-in tees (also known as stub-in tees or inserted tees). These fittings are commonly used in natural gas pipelines for branch connections and must be designed to accommodate both normal operational flow and maintenance activities such as pigging. The saddle plate design, in particular, must balance sealing requirements with the need to allow inspection tools to pass through.
Key Reflections
This paper exemplifies the importance of integrating operational requirements into the design phase of pipeline infrastructure. The jamming incident was not caused by a manufacturing defect or material failure but by a design deficiency in the plugging equipment that failed to account for the geometric constraints of the tee fitting. For engineers involved in pipe fitting design, this reinforces the principle that fittings must be designed not only for their primary structural function but also for the full range of operational and maintenance activities they will encounter during their service life. The proposed optimization design, with its rotational limit mechanism, represents a practical engineering solution that addresses the root cause of the problem rather than merely treating its symptoms.