Simulating tank pit fire in tank farm
- 15 Oct 2024
- 7 Minutes to read
Simulating tank pit fire in tank farm
- Updated on 15 Oct 2024
- 7 Minutes to read
Article summary
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Tank pit fire
A tank pit fire occurs when a flammable liquid, typically stored in large tanks within a tank farm, is released into a confined area (a bund or dike) and ignites. This type of fire is a specific kind of pool fire, where the liquid pools within a designated containment area before catching fire. Tank pit fires represent a significant risk in industrial settings where large quantities of flammable liquids are stored. Proper containment, regular maintenance, effective firefighting strategies, and advanced simulation tools are essential in managing and mitigating the risks associated with these fires.
Understanding the dynamics of tank pit fires and preparing for such events can significantly enhance safety and reduce potential damage to facilities and the environment. Here is a detailed explanation of the elements and dynamics involved in a tank pit fire:
Key characteristics
The key characteristics of a tank pit fire are as follows:
Containment Area (Bund or Dike)
Bund/Dike definition: A bund or dike is an embankment or wall that surrounds storage tanks. It is designed to contain spills or leaks, preventing the spread of flammable liquids and minimising the environmental impact.
Purpose: The containment area helps to confine the liquid, making it easier to manage and control in the event of a spill.
Cause of Fire
Release incident: Tank pit fires often result from a catastrophic failure of a storage tank, such as a rupture or leak, releasing its contents into the bunded area.
Ignition sources: The flammable liquid can be ignited by various sources, such as static electricity, sparks, hot surfaces, or other nearby fires.
Pool Fire dynamics
Formation of a pool: The released liquid spreads out and forms a pool within the containment area.
Burning characteristics: The fire burns across the surface of the pooled liquid, creating intense heat and large flames. The burning rate and flame height depend on the properties of the liquid and environmental conditions like wind speed and temperature.
Heat radiation
Radiative heat transfer: Tank pit fires emit significant amounts of heat through radiation, which can affect nearby structures, equipment, and personnel.
Thermal impact: The heat can cause secondary fires, structural damage, and even explosions if it affects other tanks or flammable materials.
Smoke and emissions
Smoke production: The combustion of the flammable liquid produces smoke, which can contain toxic substances, contributing to air pollution and health hazards.
Emission control: Managing and controlling emissions is crucial to mitigate the environmental and health impacts of the fire.
Scenario
Tank pit fire |
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The scenario considers a storage vessel in a tank farm that suffers a catastrophic rupture, releasing all its contents into a bund (as depicted in the figure below). The flammable liquid ignites, resulting in a tank pit fire.
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We can assess the possible loss of containment scenarios using the liquid event tree for instantaneous releases. In case of direct ignition, we can expect a pool fire which will spread in bund around the tanks.
Liquid LOC event tree
Modelling approach
The following steps need to be followed to simulate a tank pit fire in EFFECTS:
Add a pool fire model
Right-click on your equipment in EFFECTS
Navigate to Add model - Fire - Pool fire
For more information about other ways of adding a model in EFFECTS refer to the article Adding a model in EFFECTS
Input main parameters
First, the chemical of interest needs to be found and selected.
Then, the “Type of pool fire calculation” must be chosen. The “Two zone model” originally described by Rew & Hulbert is recommended as it separates the pool into two parts. First, the bottom or clear part of the pool fire and the second, the top or sooty part of the pool fire.
Enter all the remaining key input parameters such as the amount of type of pool fire source, mass involved in the release, storage temperature, ambient temperature, wind speed, and relative humidity, of which information can be found in the scenario description.
A demonstration of adding a model and the main parameters for a tank pit fire scenario.
Define the tank area
Ensure that the "Show expert parameters" is selected to allow for more detailed input.
Since the fire occurs within a bunded area, select the "Polygon" option for the "Type of pool shape (pool fire)". This can be found in the "Process Dimensions" section of the required inputs.
Click the "Edit" button for the "Shape of pool" and define the bunded area by clicking points along its boundary. The area will be highlighted in purple (see figure below).
Once the tank pit has been correctly defined, click on the "Done" button.
Then the user may choose to “Include shielding at bottomside flame” and possibly define the “Height of shielding at bottomside flame”. The bund wall reduces the risk of the fire spreading and acts as a shield against radiant heat emitted by the fire. Including shielding at the bottomside of the flame reduces the heat exposure to nearby tanks and infrastructure. Thus, this input plays an essential part in modelling the behaviour of the fire, and the extent of the damage.
A demonstration of how to select expert mode and add a polygonal-shaped pool.
Account for non-burning areas
Assuming that the other tanks in the bund remain intact, the area where those tanks are placed will not burn. Thus, the corresponding non-burning area within our defined tank pit (bunded area) will need to be calculated. This non-burning area will correspond to the footprint of the non-failing tanks, which will result in a more realistic flame dimension.
Measure these non-burning areas using the ruler functionality in EFFECTS.
Click the ruler icon in the toolbar of EFFECTS to select your desired option.
Option 1: "Measure path" - Use this to measure tank diameter or radius. Click on opposite sides of the tank, creating a line that dissects the tank. Then calculate their area with A=π.r2.
Option 2: "Measure area" - Directly measure the area of tanks by clicking around their circumference.
Multiply the number by the number of tanks within the pool if there are more than one. These measurements will already be up to scale when using a tile server or a georeferenced image.
Enter the calculated non-burning area into the "Non burning area within pool" as part of the required input parameters.
A demonstration of how to use the ruler function in EFFECTS to measure distance and / or area.
Remaining inputs
Enter all the remaining inputs that are required.
Selecting “Defaults” can be used to populate any additional information that is required and has not been provided.
Calculate the scenario
Click "Calculate" to compute the Tank Pit Fire scenario. This will generate heat radiation and lethality contours.
A demonstration showing how to add the remaining inputs and calculate the scenario.
Results
Analyse and export results
The heat radiation and lethality contours of a tank pit fire can be viewed in a few quick steps. The pool fire model (or any fire model in EFFECTS) also allows you to report a top view or side view heat radiation grid. Thus, you can read out values from the grid, which is a straightforward and quick way of determining the heat radiation value at a selected coordinate in the map.so
A demonstration of how quickly the heat radiation grid and sideview grid can be viewed in EFFECTS.
A summary of all the results can be viewed on the “Results” tab.
In the graphs you can see the results for first-degree, second degree and third-degree burns and heat radiation sideview, to mention just a few. There is also the “Graph expect” window, accessed but clicking on the “…” button in the bottom right-hand corner. The graph expert allows you to view multiple graphs on the same set of the axis for enhanced reporting.
A demonstration of how to view the results and graphs in EFFECTS.
You can also export your contours to Google Earth, which steps are detailed in the article, How to export consequence results into Google Earth.
Image of the heat radiation contours exported to Google Earth.
By following these steps, you can simulate a tank pit fire in EFFECTS, providing valuable insights into the potential impact and helping to develop strategies for risk mitigation and emergency response.
Conclusion
The tank pit fire occurs when a flammable liquid, typically stored in large tanks within a tank farm, is released into a confined area (a bund or dike) and ignites. These fires are hazardous in industrial settings with large quantities of flammable substances, as they can spread heat and flames, affecting nearby structures and equipment. The significant heat radiation can cause structural damage and increase the risk of secondary fires or explosions, while the resulting smoke may contain toxic substances. Proper containment and shielding are essential to mitigating the fire’s spread and impact, emphasizing the need for accurate modelling and preparedness to minimize damage and enhance safety.
Download the project file
Access the project file simulating the tank pit fire and compare views under different weather conditions. Analyse the heat radiation at various distances to determine how close a fire brigade can safely approach. Export pool fire contours into Google Earth for a more immersive visual experience.
To view the project file, please open it using the EFFECTS software. If you don’t have the software, you can download and use the free viewing demo version of EFFECTS via the link below.
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