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Major Hazard Facilities (MHFs) CFD Analysis

​Major hazard facilities (MHFs) are industrial sites such as:

- Military explosive storage facility

- Chemical manufacturing and storage
- Commercial explosive storage depots
- Explosive and munitions manufacturing facilities
- Gas processing plants
- LPG and LNG storage and distribution facilities

- Facilities that store oxidisers, peroxides, toxic solids and liquids materials
- Selected warehouses and transport depots
- Flammable and combustible fuel storage depots

A few examples of major accidents that occurred in a major hazard facility are: 
- Release of toxic material
- Release of flammable or combustible gas with gas/air mixture within LFL and UFL

- Explosion or dispersion of hazardous materials
- Fire and major structural failure
- An incident that leads to environmental damage

- Incidents due to sabotage

The first step in the hazard identification process is a process hazard analysis, which identifies potential major accidents at the major hazard facility and possible initiating events. Common methods used include: 
- Analysing process material properties and process conditions

- Reviewing organisation and industry experience
- Safety checklists
- Conducting what-if analysis on various scenarios
- Developing interaction matrixes
- Hazard and operability studies (HAZOPs)

​​​Our consultants provide specialised modelling and simulation services for cryogenic materials, focusing on consequence analysis. A key area of expertise is liquefied natural gas (LNG), particularly in its storage and distribution phases. Given the hazards associated with LNG, such as its high vapour pressure, we offer comprehensive simulations to assess risks and develop mitigation strategies.

One of the primary services we offer is the simulation of gas pipe explosions. In these scenarios, LNG’s high vapour pressure can cause unchecked pressure build-up, leading to pipe rupture and gas release. This rupture can result in a powerful blast wave that may compromise the surrounding infrastructure, including the structural integrity of the bunker. Our simulations assess the magnitude and impact of the blast wave, providing insights to identify vulnerabilities and implement preventive measures.

In the event of an LNG leak without an explosion, the escaped LNG forms a pool, which rapidly vaporises due to its cryogenic temperature, creating a vapour cloud. We have extensive experience in simulating LNG vapour cloud dispersion. Our analysis takes into account environmental factors such as wind speed, temperature, and humidity to accurately predict how far and at what concentration the vapour cloud will spread. This dispersion analysis is essential for assessing potential risks to nearby personnel and infrastructure, as the cloud could introduce flammable or toxic hazards.

When methane gas from the LNG pool vaporises, and concentrations fall between the lower and upper flammability limits (LFL and UFL), there is a risk of a vapour cloud explosion. Our simulations model these explosions, considering factors like ignition sources, cloud size, and fuel concentration. This helps evaluate the potential consequences for nearby structures and personnel, allowing clients to understand the full scope of the risk and take necessary precautions.

Our in-house chemical engineers are integral to the process, performing the necessary calculations to ensure accurate boundary conditions before the simulations begin. They verify parameters like temperature, pressure and flow rates to ensure realistic conditions. After running the simulations, they carefully analyse the results to confirm that they are consistent with engineering principles and realistic for real-world applications.

By providing these detailed simulations and consequence analyses, we help clients mitigate the risks associated with LNG storage and distribution.

What is An Explosion?

​​​​An explosion can be defined as a very fast chemical reaction involving solid, dust or gas, during which a sudden, rapid and violent release of energy occurs. This phenomenon lasts only several milliseconds and results in the production of very high temperatures and pressures. During detonation, the hot gases that are produced expand to occupy the available space, leading to wave type propagation through space that is transmitted spherically through an unbounded surrounding medium. Along with the produced gases, the air around the blast, particularly for air blasts also expands and its molecules become compacted, resulting in blast wave and shock front. The blast wave contains a large part of the energy that was released during detonation and moves faster than the speed of sound.

When an explosion occurs, the pressure surrounding the element is initially equal to the atmospheric pressure before undergoing an instantaneous increase to a peak pressure at the arrival time when the shock front reaches that point. The peak incident pressure is also known as the peak overpressure. The value of the peak overpressure as well as the velocity of propagation of the shock wave decrease with increasing distance from the detonation centre. After its peak value, the pressure decreases with an exponential rate until it reaches the atmospheric pressure. Above the atmospheric pressure, the pressure-time diagram is called the positive phase duration. After the positive phase of the pressure-time diagram, the pressure becomes smaller (referred to as negative phase) than the atmospheric value, and finally returns to it. During this phase, the structures are subjected to suction forces, which is the reason why sometimes during blast loading glass fragments from failures of facades are found outside a building instead in its interior.

When an explosion occurs, its energy is released in the form of heat. The heat released under adiabatic conditions is called the heat of explosion. The heat of explosion is a fundamental parameter, since it determines the work capacity of the explosive. The violence of the explosion depends on the rate at which energy is released.

There are two (2) kinds of energy which may be released in an explosion, namely: (1) physical energy and (2) chemical energy.

Physical energy may take such forms as pressure energy in gases, strain energy in metals or electrical energy. Examples of the violent release of physical energy are the explosion of a vessel due to high gas pressure and the sudden rupture of a vessel due to brittle fracture. Another important physical form is thermal energy. This is generally important in creating the conditions for an explosion rather than as a source of energy for the explosion itself. In particular, superheat in a liquid under pressure causes flashing off of the liquid if it is let down to atmospheric pressure.

Chemical energy derives from a chemical reaction. Examples of the violent release of chemical energy are explosion of a vessel due to combustion of flammable gas, and explosion of a reactor caused by decomposition of reaction products in a runaway chemical reaction. Chemical explosions are either (1) uniform explosions or (2) propagating explosions. An explosion in a vessel tends to be a uniform explosion, while an explosion in a long pipe gives a propagating explosion.

The magnitude and distribution of blast loads on a structure are governed by several factors, namely:

- Explosive properties: type of material, energy output and quantity of explosive
- Location of the detonation relative to the concrete structure
- Reinforcement of the pressure pulse through its interaction with ground and concrete walls

The reflected pressure and the reflected impulse are the forces to which the concrete walls ultimately respond. These forces vary in time and space over the exposed surfaced of the concrete walls, depending on the location of the detonation in relation to the concrete walls. 

 
The air blast shock wave is the primary damage mechanism in an explosion. The pressure it exerts on concrete wall surfaces may be several orders of magnitude greater than the loads for which the concrete walls are designed. An explosion has the following characteristics:
- The intensity of the pressures acting on the concrete walls can be extremely high, often above 100psi
- Pressures from an explosion decay rapidly with distance from the source hence, direct air-blast damages tend to cause more localised damage

- The duration of the event is very short, measured in thousandths of a second or milliseconds.

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Accidents in Chemical Plant

Accidents in a chemical plant can be divided into fires and explosions (42%), fires (29%), vapour cloud explosions (22%) with the remaining 8% attributed to floods and windstorms. Economic loss is consistently high for accidents that involved explosions. The most damaging type of explosion is an unconfined vapour cloud explosion, where a large cloud of volatile and flammable vapour is released and dispersed through the plant site followed by ignition and explosion of the cloud. Toxic release typically results in little damage to capital equipment, although the resulting personnel injuries, employee losses, legal compensation and clean up liabilities can be significant. The most common root cause for toxic release is mechanical failures such as pipe failures due to corrosion, erosion, high pressure and seal/gasket failures (Source: Crowl & Louvar [2011] Chemical Process Safety, 3rd Edition, Prentice Hall).

What Is an Unconfined Vapour Cloud Explosion and how to mitigate It?

​Unconfined vapour cloud explosions (UVCEs) can pose a severe threat to personnel operating in hazardous environments, resulting in costly damages and casualties if left unchecked. As such, it is of utmost importance for personnel working within these industries to be aware of the basics of UCVEs - what they are, how they occur, and most importantly, how the personnel can protect themselves from UVCEs. This blog post will look into what constitutes an UCVE and several preventative measures that may help mitigate its devastating effects.

An unconfined vapour cloud explosion (UVCE) is a type of combustion that occurs when a large amount of flammable gas or liquid is released into the atmosphere. When these gases mix with oxygen present in the air, they can form an explosive mixture capable of releasing large amounts of energy when ignited. This energy then propels and spreads rapidly, resulting in a powerful explosion that can devastate anything in its blast area. UVCEs are particularly dangerous because explosions can occur far beyond the original point of release due to gases being carried by wind. It means an unconfined vapour cloud explosion has the potential to travel vast distances in search of ignition sources, resulting in a much larger blast radius and increased destruction.

UVCEs pose a serious threat to chemical plants and other industrial installations since they can result in costly damages and casualties. Numerous factors contribute to a plant or facility's unconfined vapour cloud explosion. Some of these include:
- Poorly maintained equipment such as pipelines, tanks, valves, hoses, etc.
- High temperatures and pressure inside a tank or pipeline
- A lack of preventive measures that limit the size and scope of an UCVE, such as fire retardants, flame arrestors, spark detection systems, etc.
- Poor housekeeping practices that increase the likelihood of ignition sources (cigarettes, electrical equipment, etc.) coming into contact with flammable vapours
- The presence of hydrogen or hydrocarbons with gas/oxygen mixture between the lower flammable limit (LFL) and upper flammable limit (UFL)
- Poorly designed ventilation systems that allow combustible vapours to build up without proper dispersal
These are just some factors that can contribute to an unconfined vapour cloud explosion and should be considered when working in hazardous environments.

The consequences of an unconfined vapour cloud explosion can be devastating, resulting in massive destruction and possibly loss of life. Some of the potential effects include:

- Severe property damage caused by the blast wave itself
- Fires are ignited due to the burning gases/liquid mix released during the explosion
- Toxic fumes being released into the atmosphere, posing health risks to those in the vicinity
- Loss of life and serious injury due to flying debris or direct contact with the blast wave
Given these potential consequences, it is essential for personnel operating in hazardous environments to be aware of preventive measures that can help mitigate the effects of a UVCE.

There are several steps that personnel and plant operators can take to help prevent the occurrence and reduce the severity of an unconfined vapour cloud explosion. These include:

- Conducting plant hazard simulation to reveal and identify hidden hazards
- Conducting simulation on various flammable gas leak scenarios to predict the severity and impact to the immediate surrounding
- Implementing risk prevention protocols such as conducting thorough inspections and establishing SOPs before starting a job and while in the process of completing it
- Regularly inspecting pipelines, tanks, valves, etc., for any signs of wear and tear or damage
- Installing flame arrestors, spark detection systems, and other preventive measures to limit the size of a fire/explosion in the event of an UVCE
- Regularly testing safety equipment (fire extinguishers, ventilation systems, etc.) to ensure they function properly
- Implementing strict housekeeping rules, such as no smoking or open flames near combustible materials and using spark-resistant tools whenever possible
- Utilising appropriate safety protocols (e.g., PPE) when handling flammable liquids/gases
- Utilising pressure relief devices to prevent over-pressurization of containers and, in turn, reduce the risk of explosive releases
- Installing fire suppression systems to quickly detect and put out any combustion in the event of an unconfined vapour cloud explosion
By taking the right steps, personnel and plant operators can help protect their plants from the devastating effects of an unconfined vapour cloud explosion. For those working in hazardous environments, it is essential to be aware of the potential consequences of a UVCE and the preventive measures they can take to protect their plants and personnel. By implementing the right protocols and preventive measures, personnel and plant operators can help reduce the risks associated with an unconfined vapour cloud explosion.

If an unconfined vapour cloud explosion occurs, personnel and plant operators should take immediate steps to minimize damage. These include:

- Immediately shutting off all sources of ignition in the affected area
- Activating emergency protocols, such as evacuating personnel and activating fire suppression systems
- Utilising appropriate safety measures, such as wearing protective clothing and respirators when entering the affected area
- Sealing off the affected area to prevent the spreading of toxic gases and vapours
- Working with local fire and rescue personnel to ensure the area is safe before attempting any repairs or clean-up
By taking the right steps, personnel can help minimize the damage caused by an unconfined vapour cloud explosion and protect personnel in the affected area.

The risks associated with an unconfined vapour cloud explosion are real. Nevertheless, a chemical plant can ensure that their personnel and facilities remain protected through implementing proper safety procedures and preventive measures. By understanding the potential consequences of a UVCE and taking the necessary steps to prevent them, personnel can help ensure plants and personnel remain safe from harm. Ultimately, the best way to protect against an unconfined vapour cloud explosion is to be proactive and take the necessary measures to identify and address any potential risks before an incident occurs. By doing so, plant operators can help ensure their personnel and facilities remain safe from the devastating effects of a UVCE.

PLANT HAZARD MITIGATION SPECIALISTS

​Our plant hazard mitigation specialists provide professional plant safety simulation to support plant hazard, risk and consequence assessment in the chemical, petrochemical, hydrogen synthesis, LNG plants and all major hazard facilities. Our plant hazard mitigation specialists provide hazard analysis using both prescriptive and performance-based approaches. Whilst the former relies on relevant standards, the latter employs CFD simulation.

Prescriptive Approach

The prescriptive approach offers the advantage of simplicity and ease of implementation, as it is guided by well-defined standards and regulations. It provides clear guidelines for classifying areas where explosive gas atmospheres may occur, making it straightforward for engineers and safety professionals to apply without the need for advanced analytical tools. This method ensures a basic level of safety by using predefined safety factors and assumptions that cover a broad range of typical scenarios, providing a standardised framework for compliance. This predictability facilitates regulatory approval and simplifies communication among stakeholders, as everyone adheres to the same set of rules. However, the prescriptive approach has limitations. It often does not account for the unique geometries, configurations and specific operational conditions of individual installations. By relying on general assumptions and a ‘broad brush’ strategy, it may overlook critical factors such as the influence of complex airflow patterns, obstacles, or equipment-specific configurations that can significantly impact the behaviour of hazardous gases. As a result, the prescriptive method may not fully capture the intricacies of more complex installations, potentially leading to either over-design or insufficient safety measures, depending on how well the assumptions align with actual site conditions.

Performance-Based Approach

The performance-based approach, utilising CFD simulation, provides a more sophisticated and tailored solution. It enables a customised, site-specific analysis that considers the unique geometry and operating conditions of the installation. By simulating detailed airflow patterns, CFD simulation can pinpoint critical areas such as dead zones, stagnant regions and potential accumulation points for hazardous gases, issues that are often undetectable using the prescriptive method. This allows for a more accurate and thorough assessment of risks, ensuring that the design and mitigation measures are specifically suited to the actual conditions of the facility. Consequently, the performance-based approach can lead to more efficient and targeted safety strategies, potentially reducing costs and improving operational effectiveness. On the downside, the performance-based approach requires specialised expertise and resources, including advanced software and skilled personnel who can interpret complex simulation results. The process can be time-consuming and costly compared to the prescriptive method, making it less accessible for smaller facilities or those with limited budgets.

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