DNV are proud to announce that the Unified Dispersion Model used in Phast, the world's leading process hazard analysis software tool has been reviewed by the US Department of Transportation's Pipeline and Hazardous Materials Safety Administration and approved for use in LNG siting applications.

Beginning in October 2010, DNV have been working on the approval process for use of the Unified Dispersion Model (UDM) as a vapour dispersion model with the Pipelines and Hazardous Materials Safety Administration (PHMSA). This process consisted of a submission of model theory information and validation against a database of dispersion experimental work as part of PHMSA’s Model Evaluation Protocol.

Following careful evaluation of the model and good results from the validation, PHMSA have now decided that the UDM can be used as a vapour dispersion model for LNG facilities. The scenarios for which it is approved are:

• Dispersion from circular shaped LNG pools
• Dispersion from LNG pools in impoundments with low aspect ratios
• Dispersion from releases in any direction, including releases from flashing, venting, and pressure relief

Previously the only permitted models were DEGADIS or FEM3A.

The decision also acknowledged areas where the UDM may not be an appropriate model. These included dispersion from irregular or high aspect-ratio pools (e.g. trenches), across varying terrain or around large obstacles.

LNG dispersion results from Phast, with additional flammable effects results also available.

Of interest to LNG accidental releases Phast’s capabilities include:

• Releases from vessels
• Vessel Ruptures
• Releases from short pipe-work
• Release from long pipelines
• Releases onto land
• Releases onto water

The UDM, Phast’s dispersion model considers the following aspects of cloud behaviour:

• Continuous, instantaneous or time-varying releases
• Jet, heavy-gas and passive dispersion
• Elevated, touchdown and ground level dispersion
• Droplet dispersion, rainout and vapour added back to cloud
• Dispersion on land or water

As part of Phast’s linked modelling, the dispersion results are passed on to calculated flammable effects including fireballs, jet fires, pool fires and flash fires.

Additionally, accidental release scenarios can be included in a full Quantitative Risk Assessment using Phast Risk.

The PHMSA evaluation is part of a long history of model validation activities including evaluation by Hanna and as part of the EU SMEDIS project.