Study Note on Shock Wave Entrainment of Aluminum Powder Causing Secondary Explosion in Tee Pipe and Explosion Suppression

Literature Overview

The paper by Bi Haipu and colleagues from Changzhou University presents an experimental investigation into the phenomenon of shock wave entrainment of aluminum powder causing secondary explosion in tee pipe geometries, along with explosion suppression strategies. Published in Fire Science and Technology (2019, Vol. 38, No. 5, pp. 607-610), this work addresses a critical safety issue in industrial dust handling systems: the propagation of primary explosions through tee junctions and the subsequent entrainment of accumulated dust leading to secondary explosions of potentially greater severity. The research was funded by the Jiangsu Provincial Natural Science Research Project for Higher Education Institutions (Project No. 16KJB620001) and the Jiangsu Graduate Research Innovation Project (Project No. KYCX182625).

Technical Context and Hazard Analysis

Dust explosions are a major industrial hazard, particularly in facilities handling combustible metal powders such as aluminum. The severity of dust explosions is governed by several factors:

• Dust concentration in the air-dust mixture
• Particle size distribution
• Ignition energy and source
• Confinement geometry and volume
• Presence of accumulated dust in system components

Tee junctions in industrial piping systems are particularly hazardous locations because:

1. Flow separation at the tee can cause dust deposition in the branch pipe
2. The sudden change in flow direction creates turbulence that can entrain deposited dust
3. The geometry can amplify or attenuate shock wave propagation depending on the direction of travel

Aluminum Powder Explosion Characteristics

Experimental Setup and Methodology

Dust Explosion Test Platform

The researchers designed a comprehensive dust explosion test platform that includes:

• Primary explosion initiation system
• Dust suspension and dispersion mechanism
• Pressure and flame propagation measurement system
• Tee pipe test section with instrumented sensors
• Explosion suppression agent injection system

Tee Pipe Test Configuration

The laboratory horizontal tee pipe explosion suppression system was designed based on the characteristics of industrial dust collection systems. The tee configuration includes:

• Main pipe section for primary explosion propagation
• Branch pipe section for studying dust entrainment and secondary explosion
• Instrumentation for measuring explosion pressure and flame propagation velocity in both directions

Key Experimental Findings

Effect of Dust Concentration on Explosion Severity

The study found that aluminum powder with a median particle size of 35 μm exhibited the highest maximum explosion pressure and other explosion characteristic parameters at a mass concentration of 500 g/m³. This concentration represents the optimal fuel-air mixture for maximum explosion severity, as it provides sufficient fuel for complete combustion while maintaining adequate oxygen availability.

Effect of Tee Geometry on Explosion Propagation

This finding is particularly significant for industrial piping design. The tee junction acts as an amplifier for explosions propagating through the main pipe, potentially increasing the severity of the explosion as it passes through the junction. Conversely, the branch pipe experiences reduced explosion severity due to flow divergence and energy dissipation at the junction.

Explosion Suppression Effectiveness

The study investigated the effectiveness of ammonium dihydrogen phosphate (ADP) as an explosion suppression agent:

The finding that 10% ADP mass fraction can completely suppress aluminum powder explosion is a valuable design parameter for explosion suppression system design. This concentration provides a safety margin that can be used to size suppression agent storage and injection systems.

Engineering Practice Implications

Industrial Piping Design Considerations

The findings have direct implications for the design of industrial dust handling and collection systems:

1. Tee junction placement: Tee junctions should be designed to minimize dust accumulation in branch pipes, as accumulated dust can be entrained by passing shock waves to form secondary explosions
2. Flow direction optimization: The main flow direction should be aligned to minimize the amplification effect of tee junctions on explosion propagation
3. Branch pipe design: Branch pipes should be designed with adequate diameter and smooth transitions to minimize dust deposition
4. Isolation systems: Explosion isolation devices should be installed at tee junctions to prevent explosion propagation between connected systems

Explosion Suppression System Design

Based on the findings, explosion suppression systems should be designed with the following parameters:

• Agent concentration: Minimum 10% ADP mass fraction for complete suppression of aluminum powder explosions
• Injection timing: Suppression agents must be injected within milliseconds of explosion detection to be effective
• Coverage: Suppression agents must be distributed uniformly throughout the protected volume
• Redundancy: Multiple injection points should be provided to ensure reliable coverage

Maintenance and Inspection Requirements

To prevent secondary explosions caused by dust accumulation:

1. Regular inspection of tee junctions and other low-flow areas for dust accumulation
2. Implementation of dust collection and removal systems
3. Monitoring of dust concentration in system components
4. Documentation of inspection and maintenance activities

Study Insights and Independent Reflection

This research provides critical insights into the behavior of dust explosions in tee pipe geometries, which are ubiquitous in industrial piping systems. The finding that tee junctions can amplify explosion severity in the main pipe direction is particularly important for piping designers and safety engineers.

From a piping engineering perspective, the tee junction represents a complex flow geometry that can significantly affect explosion dynamics. The flow separation, reattachment, and turbulence generation at the tee junction create conditions that can either amplify or attenuate explosion propagation, depending on the direction of travel. This finding underscores the importance of considering explosion dynamics in the design of industrial piping systems that handle combustible dusts.

The complete suppression of aluminum powder explosions at 10% ADP mass fraction is a valuable finding, but it must be interpreted with caution. The laboratory conditions may not fully replicate the complexity of industrial systems, where factors such as pipe geometry variations, flow turbulence, and dust distribution non-uniformity can affect suppression effectiveness. Therefore, field testing and validation are essential before relying on these findings for industrial safety system design.

The research also highlights the importance of understanding dust behavior in piping systems. Dust accumulation in tee junctions, branch pipes, and other low-flow areas creates a reservoir of fuel that can be entrained by passing shock waves to form secondary explosions of potentially greater severity than the primary explosion. This emphasizes the need for regular maintenance and inspection of industrial dust handling systems.

Reference Value and Outlook

This paper provides valuable experimental data on the behavior of dust explosions in tee pipe geometries and the effectiveness of explosion suppression agents. The findings have direct implications for the design of industrial dust handling systems, explosion protection systems, and safety regulations for facilities handling combustible metal powders. Future research should focus on extending these findings to other dust types, larger-scale systems, and more complex piping geometries.