Comparison of Exclusion Zone Calculations and Vapor Dispersion Modeling Tools

Overview

Main Objective

This project will compare the various design spill selection methodologies and compare the exclusion zone results for various facility types. The comparison will include a review of the DEGADIS, Phast, and FLACS modeling tools currently approved by DOT PHMSA to perform dispersion modeling to calculate vapor dispersion exclusion zones. The project will also evaluate several design spill selection methodologies and apply them to import, export, peak-shaving, and mid-size truck loading Liquefied Natural Gas (LNG) plants. This project will calculate vapor dispersion exclusion zones with each associated design spill. As a result, this project will help DOT PHMSA better define the approach for determining vapor dispersion exclusion zone distances.

Public Abstract

As detailed in the Department of Transportation's (DOT) regulations in 49 CFR Part 193.2059, each LNG facility under the scope of 49 CFR Part 193 is required to calculate vapor dispersion exclusion zones in accordance with NFPA 59A (2001) as incorporated by reference. Interpretations issued by DOT PHMSA in July, 2010 clarified certain requirements to perform vapor dispersion exclusion zone modeling to satisfy the requirements of 49 CFR 193.2059.

Neither 49 CFR Part 193, NFPA 59A (2001 edition incorporated by reference) nor the DOT PHMSA interpretations provide a quantitative means of defining a "single accidental leakage source" that is required to calculate design spills associated with failures of transfer pipelines associated with impounding areas that serve vaporization, process or LNG transfer areas. Design spills are required to perform vapor dispersion exclusion zone modeling.

As 49 CFR Part 193 can apply to various LNG facility types, a generic design basis for LNG import, export, peak-shaving, and mid-scale truck loading will be created and serve as the basis for LNG facility comparison, This project (a) compares the different methodologies used to determine a "single accidental leakage source" for design spill selection, (b) identifies the "single accidental leakage source" for each methodology for each LNG facility type, (c) calculates the vapor dispersion exclusion zone results for each methodology for each facility type, and (d) provides a comparison of the modeling results for each facility type (Project).

The objective of the Project will be to provide DOT PHMSA with a comparison of the different methodologies used to determine a "single accidental leakage sources" and a comparison of the modeling results for those single accidental leakage sources using the DEGADIS, Phast, and FLACS models approved by DOT PHMSA for calculating vapor dispersion exclusion zones in accordance with 49 CFR Part 193.2059. This comparison will help DOT PHMSA better define the criteria to determine a "single accidental leakage source" to be used for vapor dispersion exclusion zone calculations.

This will provide DOT PHMSA with a decision making tool to assist in the determination of the best methodology for determining a "single accidental leakage source" to serve as the basis for vapor dispersion calculations and will help DOT PHMSA understand the quantitative differences in methodologies used to calculate exclusion zones.

Summary and Conclusions

The results of the vapor dispersion simulations show that the Phast, FLACS, and DEGADIS models generally provided calculated vapor dispersion distances within a factor of 2 across each model. However, comparison across all models did vary from release to release based on the physical properties, plant geometries, locations, and sizes of each release. While all models have been validated against LNG experimental results, each model will predict different distances for the same scenario due to each models' uniqueness.

Mitigation modeling shows that common features included in modern facilities, such as breakaway connections, Emergency Shutdown valves, Powered Emergency Release Coupling valves, and vapor fences can show a significant decrease in vapor dispersion exclusion zones.

A vapor dispersion exclusion zone graph has been created to help developers understand a high level estimate of potential exclusion zones. This graph can help developers to understand during an early phase of development whether a parcel of land will be suitable for the proposed project or if mitigation will be needed to limit vapor dispersion to the property limits.

Relevant Files & Links