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Project Profile

Computational Fluid Dynamics: Assessing Fire and Gas Maintenance Systems

Learn how we applied Computational Fluid Dynamics (CFD) modeling scenarios to an onshore Fire and Gas System (FGS) mapping study to assess the effectiveness of our client's proposed outdoor flammable and toxic gas detector layout.

Project Objectives

Computational Fluid Dynamics: Measuring the Effectiveness of Fire and Gas Maintenance Systems

Project Overview

Gas detection systems in processing facilities ensure that risk mitigation procedures such as isolation, blowdown and active fire protection are activated if there is a hazardous event. A well-designed system provides an appropriate level of redundancy to guard against false trips and detector faults. Gas detector mapping studies provide an objective analysis of detector layouts to support the design process and optimize the number of detectors needed to meet coverage targets.

Gas detection modeling following a leak is a difficult task since gas dispersion depends on many variables such as process conditions, hole size, release position and direction, ventilation conditions, impingements and more. Furthermore, dispersion studies using Computational Fluid Dynamics (CFD) can determine the cloud formation across a process area.

It was found that the originally proposed layout could be improved so that the leak detection rate could be increased to greater than the minimum specified by the client. ABS Group achieved this by increasing a small percentage of the total number of gas detectors and relocating some of the originally proposed gas detectors.


Our Solution

The approach to this work was to use the CFD software FLame ACceleration Simulator (FLACS) to predict the spread of flammable and toxic gas dispersions for various leak scenarios. A ventilation simulation of the wind flow through the site using prevailing meteorological conditions was analyzed. This was performed prior to the dispersion analyses to predict the wind conditions local to each leak source.

 

Dispersion analysis was performed for the release of gas from specified hole sizes at discrete leak locations. The FLACS program analyzed the momentum-driven part of the flow that was local to the release point; then it analyzed the passive-driven flow of gas carried by the surrounding wind conditions.

Critical leaks that could defeat the detector layout were identified. Although small, these leaks were noted as they could produce flammable gas clouds that if ignited, would produce overpressures of negligible consequence. The large leaks were detected by the automatic shutdown safety features.

Typical CFD contour plot of flammable gas concentration

Typical CFD Contour Plot of Flammable Gas Concentration

 

Assessment procedure for flammable gas detectors
Assessment Procedure for Flammable Gas Detectors

Several detection criteria were specified; for example, detectors are required to sense flammable gas at 20% Lower Flammability Level (LFL).

The assessment determined where the most efficient detectors should be located and hence whether proposed detectors should be relocated, additional detectors installed, or a combination of the two.  The efficiency was determined based on the ability to detect and time to detection.

Scenarios detected and average time to detection for an example accident event

Scenarios detected and average time to detection
for an example accident event

 

Value Delivered

ABS Group's Extreme Loads and Structural Risk division helped the client clearly understand the facility risks and quantify consequences. Our knowledge and experience with hazard analysis tools, including Detailed Fire Modeling, allows us to select and apply the most appropriate techniques for each situation. Our Facility Siting services cover a wide range of explosion, fire and toxic release hazards that pose a threat to refining, petrochemical and specialty chemical process installations.

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