LNG Exclusion Zones-Radiant Heat Flux & Vapor Dispersion Modeling

Exclusion zones are established to protect the public from the potential consequences of an unintended release of LNG. Operators are required to legally control all activities in the area surrounding an LNG facility which may be impacted by radiant heat flux from a fire or from vapor dispersion from initial release through formation of a cloud or pool to final dispersion. The endpoint thresholds are prescribed by regulations in 193.2057 and 193.2059.

Operators use hazard analysis software based on failure release scenarios to predict the extent of an exclusion zone. For vapor dispersion modeling purposes, the physical process is typically divided into two phases: source term and atmospheric dispersion. The source term phase describes what occurs immediately after a release where the fluid behavior (LNG and its vapor) is mainly controlled by release conditions. After the source term phase, the atmosphere dominates the vapor dispersion behavior.

PHSMA approves which radiant heat flux and atmospheric dispersion hazard analysis software can be used in determining the extent of an exclusion zone. PHMSA does not prescribe or approve source term software. Atmospheric dispersion software petitioners must demonstrate that their model has been validated by experimental test data by using a three-stage process described in the model evaluation protocol (MEP). The MEP is based on three distinct phases: scientific assessment, model verification and model validation. The petitioner submits a Model Evaluation Report (MER) which PHMSA evaluates and issues a Final Decision. The Final Decision provides PHMSA's summary of findings, limitations and conclusions. The Final Decision also provides additional clarifications and information that must be included with model submissions for use in exclusion zone calculations.

PHMSA issued advisory bulletin ADB-10-07 to provide guidance on the requirements for obtaining approval for alternate vapor dispersion software. PHMSA is currently updating the MEP and validation database. If you are interested in petitioning for alternative radiant heat flux or vapor dispersion software, contact the Office of Pipeline Safety at 202-366-4595.

The following software is approved for use:

Radiant Heat Flux Models

• LNGFIRE3: A Thermal Radiation Model for LNG Fires

Vapor Dispersion Models and Links to Final Decisions

• LNG Vapor Dispersion Prediction with the DEGADIS 2.1 Dense Gas Dispersion Model
• Evaluation of Mitigation Methods for Accidental LNG Releases. Volume 5: Using FEM3A for LNG Accident Consequence Analyses
• Gexcon's FLACS v9.1r2 https://www.regulations.gov/search?filter=phmsa-2011-0101
• DNV's Process Hazard Analysis Software Tool (Phast) version 6.6/6.7 https://www.regulations.gov/search?filter=phmsa-2011-0075
• DNV's Process Hazard Analysis Software Tool (Phast) version 8.4 https://www.regulations.gov/search?filter=phmsa-2021-0041 and https://www.phmsa.dot.gov/regulatory-compliance/phmsa-guidance/approval-phast-v84-alternate-model-liquefied-natural-gas-lng

Model Evaluation Protocol Documents

• LNG Model Evaluation Protocol and Validation database update at: https://www.nfpa.org/News-and-Research/Data-research-and-tools/Hazardous-Materials/LNG-model-evaluation-protocol-and-validation-database-update
• Guide to the LNG Model Validation Database at: https://www.nfpa.org/-/media/Files/News-and-Research/Fire-statistics-and-reports/Hazardous-materials/RFLNGDatabaseGuidev12.ashx
• LNG Source Term Models for Hazard Analysis: A Review of the State-of-the-Art and an Approach to Model Assessment at: http://www.nfpa.org/news-and-research/fire-statistics-and-reports/research-reports/hazardous-materials/gases/lng-source-term-models-for-hazard-analysis