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1.1 About HGSYSTEM

The HGSYSTEM package is a PC based set of atmospheric dispersion models developed by Shell Research Ltd. It contains a wide range of models (or modules) to simulate source terms, near-field and far-field dispersion. HGSYSTEM can be used to study one particular aspect of a dispersion situation, using one of the available HGSYSTEM models. But by using several modules in sequence, HGSYSTEM can also be used to simulate a complete scenario starting from a release source, then modelling the near-field dispersion and finally calculate the far-field dispersion.

The thermodynamical descriptions available in HGSYSTEM are either a reactive hydrogen fluoride (HF) chemistry and thermodynamics model or a non-reactive two-phase multi-compound thermodynamics model.

The first publicly available release of HGSYSTEM version 1.0 was in November 1990 and this version is also denoted as the NOV90 version. Some later versions of HGSYSTEM have been developed but were not made publicly available.
Version 3.0 is the second publicly available version of HGSYSTEM. It is made available, both in executable form and in Fortran source code, to all interested parties, under copyright of Shell Internationale Research Maatschappij B.V., The Hague.

Please note that HGSYSTEM has a complete package has a version number, but the individual HGSYSTEM modlues all have their own version number as well. The module version numbers are usually different from the package version number.

1.2. About this User's Manual

This User's Manual is intended as a concise guide to all the HGSYSTEM modules, giving per chapter a short description of each model and more detailed information for all input parameters.
It can be used for HGSYSTEM version 3.0 and later.

New users of HGSYSTEM are advised to read the first 4 chapters of this Manual before running HGSYSTEM models. Then they can read the more detailed information for a specific HGSYSTEM module in the corresponding chapter of the Manual.
Experienced HGSYSTEM users can read the chapter for the HGSYSTEM model they want to use.
Every user is strongly advised to study the information for the HGSYSTEM module to be used, as the description of the input parameters will also make clear what the program capabilities and limitations are.

1.3. HGSYSTEM modules

The HGSYSTEM package consists of several modules which can all be used as independent simulation models for a specific atmospheric dispersion situation. Some of the modules calculate release (or source) terms, others simulate different dispersion scenarios like evaporating liquid pools, jet dispersion and heavy gas dispersion.

Although all models can be run individually, it is often quite useful when simulating a complete dispersion scenario (source term calculation, near-field dispersion simulation and far-field dispersion simulation) to run models consecutively. HGSYSTEM offers several combinations of consecutive model runs. Each model is run separately, but information generated by one model can be communicated to a following model using so-called link files. See Chapter 4 on running HGSYSTEM scenarios.

The following models (or modules) are available in HGSYSTEM.

Database program
DATAPROP generates physical properties used in other HGSYSTEM models

Source term models

SPILL transient liquid release from a pressurised vessel
HFSPILL SPILL version specifically for hydrogen fluoride (HF)
LPOOL evaporating multi-compound liquid pool model ('unpressurised release')

Near-field dispersion models
AEROPLUME high-momentum jet model
HFPLUME AEROPLUME version specifically for hydrogen fluoride (HF)
HEGABOX dispersion of instantaneous heavy gas releases

Far-field dispersion models
HEGADAS heavy gas dispersion (steady-state and transient version)
PGPLUME passive Gaussian dispersion

Utility programs
HFFLASH flashing of hydrogen fluoride (HF) from pressurised vessel
POSTHS/POSTHT post-processing of HEGADAS results (steady state and time dependent version)
PROFILE post-processor for concentration contours of air borne plumes
GET2COL utility for data retrieval

Most of these models can be used as individual models (see Chapter 3 of the User's Manual). However, running them consecutively, as described in Chapter 4 of the User's Manual, adds considerably to the value of the models.

Many of the HGSYSTEM modules are linked; For example, the database program DATAPROP generates physical properties for the compounds used in a mixture and communicates these via a link file to AEROPLUME, SPILL, LPOOL, HEGABOX and HEGADAS. In another example, first SPILL is used to calculate discharge rate from a pressurised vessel. Using this information an AEROPLUME run is made and this model makes a transition to HEGADAS. A complete scenario is thus simulated, starting from a source term description (SPILL), calculating near-field jet dispersion (AEROPLUME) and simulating far-field heavy gas dispersion (HEGADAS). For more details, see Chapter 4 on running HGSYSTEM scenarios.

1.4. Parameters describing atmospheric conditions

Most HGSYSTEM modules require specification of parameters describing the state of the ambient atmosphere. As not all users may be familiar with these a short description is given of the most important of these parameters.

Wind speed and ambient temperature
The speed of the ambient wind and the atmospheric temperature are measured at a given reference height (usually 10 m). These important atmospheric parameters can be obtained from meteorological data.
Both the reference wind speed and temperature are used in HGSYSTEM models to calculate the full boundary profiles for wind speed and temperature.

Atmospheric stability class.
In order to define meteorological conditions in a relative simple way, a system of stability classes has been introduced. These are called Pasquill-Gifford classes. They are given by the single characters A, B, C, D, E and F ranging from very unstable to stable conditions.
Table 1 gives an overview of the Pasquill stability classes and Table 2 shows how the stability class depends on the meteorological conditions.

Surface roughness length
The surface roughness parameter is a measure for the ground surface roughness which determines the turbulence characteristics of the ambient boundary layer. It is taken to be about 1/10 to 1/30 of the height of a typical roughness element as present on the ground surface.
It is about 1 m for cities, forests and industrial sites, 10 cm for agricultural crops, 1 cm for grass and 1 mm for water or paved surfaces.

Monin-Obukhov length
This is a turbulent length scale depending on meteorological conditions. This parameter never has to be specified by the user when running HGSYSTEM models. If needed, it is calculated within the model itself.

Table 1. Pasquill-Gifford stability classes

A: Extremely unstable conditions D: Neutral conditions
B: Moderately unstable conditions E: Slightly stable conditions
C: Slightly unstable conditions F: Moderately stable conditions

Table 2. Meteorological conditions defining Pasquill stability classes.

Daytime insolation Night-time conditions
Surface wind speed (m/s) Strong Moderate Slight Thin overcast or > 4/8 low cloud <= 4/8 cloudiness
< 2 A A - B B    
2 - 3 A - B B C E F
3 - 4 B B - C C D E
4 - 6 C C - D D D D
> 6 C D D D D

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