Selection of Air-Splitting Tee Position in Tunnel Construction Ventilation Systems

Literature Overview

This 2016 paper by Gou Hongsong, Wu Yuanjin, Li Yongsheng, and Zhang Wenxin from China Railway Tunnel Survey and Design Institute Co., Ltd., published in Tunnel Construction, addresses a practical engineering problem in tunnel construction ventilation: the optimal positioning of the air-splitting tee in a parallel duct system supplying two excavation faces simultaneously. The research was funded by a major project from China Railway Tunnel Group Co., Ltd. (Tunyan He 2013-01), reflecting the practical significance of the work within the Chinese railway tunnel construction industry.

Technical Background and Problem Statement

In long tunnel construction, mechanical ventilation is essential for supplying fresh air to the excavation face and removing dust, combustion gases, and heat. When two excavation faces advance simultaneously from opposite ends or from different headings, a parallel duct system with a tee fitting is used to split the airflow. The position of the tee along the main duct directly affects the airflow distribution between the two branches, and an improperly positioned tee can result in inadequate ventilation at one or both faces.

Key Technical Analysis

The authors derived a theoretical relationship showing that the ratio of the main duct diameter to the branch duct diameter is the critical parameter governing the sensitivity of the tee position to the airflow distribution. When this diameter ratio is large, the tee position has a significant effect on the airflow split, and careful positioning is essential. When the diameter ratio is small, the tee position has a lesser effect.

The study validated the theoretical analysis through three engineering calculation examples and one actual engineering case, demonstrating that proper tee positioning can significantly improve the airflow delivery to the excavation faces.

Engineering Practice and Methodology

The practical methodology derived from this study can be summarized as follows:

1. Determine the required airflow at each excavation face based on ventilation standards and the number of workers, equipment, and hazardous gas concentrations.
2. Calculate the system resistance for each branch duct based on duct diameter, length, and fittings.
3. Determine the diameter ratio of the main duct to the branch ducts.
4. Use the theoretical relationship to determine the optimal tee position that achieves the desired airflow split.
5. Verify the solution through a system resistance calculation and, if possible, through field measurement.

Study Insights and Relevance to Fitting Engineering

While this paper focuses on ventilation engineering rather than pressure piping, the underlying principle of flow distribution through tee fittings is directly relevant to the design of piping systems in process industries. The tee fitting acts as a flow divider, and the distribution of flow between the branches depends on the geometry of the tee, the relative resistances of the branch circuits, and the position of the tee within the main duct.

In process piping design, similar considerations arise when designing branch take-offs for instrument air, nitrogen purge, or steam distribution systems. The finding that the diameter ratio is the critical parameter reinforces the importance of proper pipe sizing in branch circuit design. Furthermore, the practical validation through engineering examples demonstrates the value of combining theoretical analysis with field verification, a principle that is universally applicable in piping engineering.

The study also highlights an important aspect of fitting selection: the type and geometry of the tee (welded, threaded, or flanged; standard or reduced) can affect the flow distribution characteristics. In ventilation applications, the tee is typically a fabricated duct fitting, but the same principles apply to manufactured pipe fittings. Engineers designing branch take-offs should consider not only the hydraulic resistance but also the geometric configuration of the tee to ensure balanced flow distribution.

In summary, this paper provides a clear and practical methodology for optimizing tee positioning in tunnel ventilation systems, and its underlying principles are transferable to other flow distribution applications in process piping. The emphasis on the diameter ratio as the governing parameter offers a simple design criterion that can be readily applied in preliminary design stages.