The Unified Dispersion Model (UDM) used in Phast, Phast Risk and Neptune has been extensively verified and validated against a large number of field experiments. These include continuous, elevated two phase and vapour releases, ground level liquid spills and unpressurised instantaneous releases.

A subset of these experiments were used in the European Community (EC) funded Scientific Model Evaluation of Dense Gas Dispersion Models (SMEDIS )project. The SMEDIS project is an independent review of both the theory and the performance of many dispersion models. The UDM has excelled in both aspects of the review.

The UDM models the dispersion following a ground-level or elevated two-phase pressurised release. It effectively consists of the following linked modules:

• jet dispersion.
• droplet evaporation and rainout, touchdown.
• pool spread and vaporisation.
• heavy gas dispersion.
• passive dispersion.

The UDM includes the effects of droplet vaporisation. Rainout produces a pool which spreads and vaporises. Vapour is added back into the plume and allowance is made for this additional vapour flow to vary with time. In addition to the standard droplet model, the UDM also includes a specific model for Hydrogen Fluoride (HF) that includes the effects of polymerisation.

The UDM allows for continuous, instantaneous, constant finite-duration, and time-varying releases from a variety of sources, such as short and long pipes, relief valves, pressurised and unpressurised vessels and disc ruptures.

The UDM has been extensively verified and has been validated against a large number of field experiments. These include continuous, elevated two phase and vapour releases, ground level liquid spills and unpressurised instantaneous releases. A subset of these experiments were used in the EC funded SMEDIS project. The SMEDIS project is an independent review of both the theory and the performance of many dispersion models. The UDM has excelled in both aspects of the review.
Extracts from the UDM SMEDIS report:

• Section 4.1.2: Verification Undertaken
  "The authors have gone to considerable lengths, using whatever means and techniques available, to verify the coding of their model."
• Section 5.1: Validation Already Performed
  "An extensive validation document has been assembled for UDM. The document includes a description of each validation experiment, the details of the assumptions made for the UDM simulation plus a detailed discussion of the results obtained from a statistical and graphical comparison against the field data. The validation comparisons are made exclusively with field experiments. The experiments selected are wisely chosen to cover a wide spread of phenomena."