Mercury Risk

One of the toxic-liquid in Oil and Gas Exploration and Production is the Mercury. You may hear about this during performing the Hazard Identification (HAZID) study. Some potential sources of mercury substance are specified in ISO 17776 such as Electrical switches, gas filter, and etc.

However, some hydrocarbon reservoirs, especially those containing sour gas or heavy crude oil, may naturally contain elevated concentrations of mercury. Mercury can accumulate in reservoir fluids through geological processes, such as volcanic activity or the presence of mercury-bearing minerals. As a results, offshore oil and gas production facilities located in regions with mercury-containing reservoirs may encounter mercury contamination in produced fluids.

Overview Hazards of mercury risk

The following are the overview of the hazards of mercury that normally is analyzed during Hazard Identification (HAZID) study.

Corrosion and Erosion: Mercury present in produced fluids can pose a risk of corrosion to equipment since mercury has a high affinity for metals. This will lead to increase maintenance requirement and reduced equipment lifespan.

Environmental impacts: Mercury discharged from offshore production platform can pose the environmental risk to marine ecosystems and human health. This is because of mercury can contamination in produced water, drilling muds, and wastewater which can bioaccumulate in aquatic organisms, such as fish, and shellfish.

Offshore platforms may also release mercury vapor into the atmosphere during routine operation, contributing to air pollution and decomposition in coastal areas.

Health and Safety Risks: Occupational exposure to mercury vapor poses health risks to offshore worker involved in drilling, production, and maintenance activities. Mercury vapor can be releases during well testing, sampling, and handling of contaminated equipment.

Chronic exposure to mercury vapor can cause neurological and respiratory disorder.

How to treat the mercury

For sure, during the Hazard Identification (HAZID) review meeting, the provision of reduce or remove measures need to be discussed. The following information is the general technology that use to reduce or remove the mercury hazards.

Treating mercury contamination involves various techniques and technologies aimed at removing or reducing mercury concentrations in different media, such as air, water, soil, and hydrocarbon fluids.

Mercury Removal Technologies:

  • Adsorption: Adsorption involves the attachment of mercury ions or molecules to the surface of a solid adsorbent meterial. Activated carbon, zeolites, and other specialized absorbents can effectively remove mercury from gas streams, aqueous solution, and hydrocarbon fluids. Adsorption is widely used in air and water treatments to capture mercury contaminants.
  • Chemical Precipitation: Chemical precipitation involves adding chemical agents, such as sulfur-based compound (e.g. sodium sulfide), to wastewater or aqueous solutions containing mercury to convert soluble mercury species into insoluble mercury sulfide (HgS) precipitates. Precipitated mercury can then be separated and removed through filtration or sedimentation processes.
  • Ion Exchange: Ion exchange involves the exchange of mercury ions in solution with other ions on the surface of a solid ion exchange resin. Ion exchange resins with specific functional groups, such as thiol (-SH) or sulfide (-S) groups, can effectively capture mercury ions from aqueous solutions, facilitating their removal from contaminated water sources.
  • Membrane Filtration: Membrane filtration technologies, such as reverse osmosis and ultrafiltration, can remover mercury ions and particulate mercury contaminants from water by physically separating them from the solution. Membrane filtraton systems are used in wasterwater treatment plants and industrial processes to treat mercury-contaminated.

Mercury Vapor Control:

  • Activated Carbon Adsorption: Activated carbon filters can effectively adsorb mercury vapor from air streams by trapping mercury molecules on their porous surfaces.
  • Gas Scrubbing: Gas scrubbing or absorption involves passing mercury-containing air streams through a liquid scrubbing solution, such as aqueous sulfuric acid or potassium permanganate, to chemically react with and capture mercury vapor.

Mercury Recovery and Recycling:

  • Thermal Desorption: Thermal desorption involves heating mercury-contaminated material, such as soil, slude, or industrial waste, to high temperature to volatilize and recover mercury vapor. The recovered mercury vapor can then be condensed, captured, and recycle for reused or proper disposal.
  • Electrochemical Treatment: Electrochemical treatment methods, such as electrolysis and electrocoagulation, can be used to extract mercury ions from aqueous solutions by applying an electric current to induce chemical reactions that convert dissolved mercury species into solid or gaseous from separation and recovery.

Physical Properties of Mercury

  • Physical State: Liquid
  • Color: Silver
  • Odor: Odorless
  • Vapor Pressure: 0.002 mmHg @25deg.C
  • Vapor Density: 7
  • Viscosity: 15.5 mP @25deg.C
  • Specific Gravity: 13.59 (water = 1)
  • Boiling point: 356.72 deg.C
  • Freezing/Melting Point: -38.87 deg.C
  • Flash Point: Not applicable
  • Auto Ignition temperature (AIT): Not applicable
  • Explosion Limit (Lower/Upper): Not applicable

Toxicity

NFPA Rating: Health = 3, Flammability = 0, and Instability = 0.

OSHA-PEL-TWAs = 0.05 mg/m3, OSHA-PEL-Ceiling = 0.1 mg/m3

ACGIH-TWA: 0.025 mg/m3

NIOSH-TWA (Vapor phase) = 0.05 mg/m3, NIOSH-IDLH (Vapor phases) = 10 mg/m3

Subsea Pipeline Major Accident Event (MAE) and Safeguards Protection

Major Accident Event (MAE) from the pipeline

Major Accident Event (MAE) is an accident with significant consequences or catastrophic incident in terms of human safety, environmental impact, and property damage.

The example MAE from subsea pipeline such as;

  • Pipeline leak, spill, or rupture;
  • Subsea infrastructure damage;
  • Subsea blowout or overpressure; and
  • Anchor dragging or vessel impact

Any damage and leakage of subsea pipeline can lead to the release of large quantities of oil, gas, or other hazardous substance into the marine environment. This can result in environmental pollution, damage to marine ecosystems, and impacts on the marine life and coastal communities.

Example the cause and safeguards protection

Subsea pipeline crashing during laydown operation

  • Site survey and route selection to check the potential of steep slopes, geological faults, or unstable sediment.
  • Maintain the minimum distance of existing pipeline (if any)
  • Design the suitable pipe wall thickness by considering the hydrostatic pressure, currents, seabed movement, potential impacts from marine crash, potential impacts from the drop objects.
  • If necessary, install the concrete mattress, or rock berm to prevent the pipeline movement.
  • If necessary to crossover the existing pipeline, provide the suitable sleeper.
  • Periodic General Structure Inspection (GSI) and Geotechnical Test & Inspection (GTI)
  • Stop production of the nearby pipeline during installation.
  • Utilize the ROV as supportive monitoring system during installation.

Pipeline Aging

  • Consider the cathodic protection;
  • Extent the pipeline life cycle by considering the interior lining and exterior coating;
  • Periodic Inline Inspection (ILT) by smart pigging

Pipeline Support Aging

  • Install the strain gauge, tiltmeter, or displacement sensor;
  • Preventive Maintenance Program;
  • Geotechnical monitoring such as soil moisture monitoring; slop stability analysis, or ground movement monitoring.

Rigging failure during transferring the pipe from barge to pipe laydown vessel

  • Training & Qualification of operation person;
  • Load calculation or engineering calculation;
  • Load inspection;
  • Equipment inspection such as hook, rigging gear, shackle, sling, anchor system;
  • Securing the load by tag line to control movement; and
  • Environmental considerations.

Anchor pilling of vessel damages.

  • Design and select the proper anchor (material, dimension, and configuration);
  • Installation of coating system or cathodic protection system;
  • Periodic geotechnical investigation i.e. soil composition, bearing capacity, potential of scour or erosion.
  • Selection the vessel that equipped with Dynamic Positioning (DP) system; and
  • Mooring analysis and mooring system.

Hazardous Area Classification (HAC) IEC – Class, Zone Scheme

Hazardous Area Classification (HAC) is a process used to identify and classify areas where flammable gases, vapors, liquids, or combustible dusts may be present in sufficient quantities to create a risk of explosion of fire.

The classification process involves assessing the likelihood and extent of the presence of hazardous material, as well as the frequency and duration of their presence. Based on this assessment, area are categorized into different zones or classes according to the probability of the presence of explosive atmospheres.

Proper hazardous area classification ensures that appropriate safety measures, such as the selection and installation of explosion-proof equipment are implemented to mitigate the risk of ignition and explosion in these area.

Common used HAC system

  1. NEC – Class/Division System (North America): This system divided areas into classes based on the type of hazardous material present (Class I for gases and vapors, Class II for combustible dusts, and Class III for ignitable fibers) and divisions based on the likelihood of the hazard being present (Division 1 for continuous hazard, Division 2 for intermittent hazard)
  2. IEC – Zone System (International): This system categories area into zones based on the likelihood and duration of the presence of hazardous material. Zone are number from 0 to 2 (for gases and vapors) or 20 to 22 (for combustible dusts).

Hazardous Area Condition and Subdivision (IEC – Zone)

To start determining A “Zone” there are some set of questions that need to be answered such as What is the emission level of gas/vapor? Does the process contain more than a specified minimum of flammable material? Can it be released/leakage to outside the containment? If the results are “Yes”. the hazardous area of flammable material class shall be defined. In the IEC – Zone, it can be classified as below.

Flammable materials – Gases & Vapors

  • Zone 0 (Continuous): Is present continuously or for long periods or frequently.
  • Zone 1 (Primary): Arises in normal operation occasionally.
  • Zone 2 (Secondary): Is not likely to arise in normal operation, or if it does, will persist for a short time only.

Flammable materials – Dust

  • Zone 20 (Continuous): Is present in the form of a cloud continuously, or for long periods or frequently.
  • Zone 21 (Primary): Occasionally develops into a cloud during normal operation.
  • Zone 22 (Secondary): Is not likely to develop into a cloud during normal operation, or if it does, for a short time only.

The supportive criteria of justification for the “grade of release source” are below.

  • Continuous: x>1,000 hours per year or 10% of time.
  • Primary: 10<x< 1,000 hours per year or 0.1% – 10% of time.
  • Secondary: 1<x<10 hours per year or 0.01% – 0.1% of time.

Working Steps of the developing the Hazardous Area Classification (HAC)

In summary, the working steps of developing the Hazardous Area Classification are the following;

  1. Identify the characteristics of gases and vapors;
  2. Identify the type of hazard, Explosion, or Gas Group
  3. Specify the source of release
  4. Determine the grade of release;
  5. Analyze the hazard extension.
    • Determine the release rate, velocity, etc;
    • Determine of the type of area; and
    • Verify the degree of availability of ventilation.

Characteristic of gases and vapors

Basically, all gaseous and vapors require oxygen to make them flammable, hence, the properties of the mixture below are required to analyze the potential of ignition.

  • Flashpoint;
    • Flashpoint is used as a basis for categorizing the volatility of flammable liquids. NFPA30 categories flammable material into 3 classes based on their flashpoint and boiling point vapor pressure
  • Auto Ignition Temperature;
  • Explosion Limit (Lower & Upper Explosion Limit);
NFPA 30 Classification

Apparatus Group

In the IEC system, electrical equipment is divided into several apparatus groups based on the type of explosive atmosphere it can safely operate in. The groups are typically denoted as follows.

Group I: Equipment intended for use in underground mines or other environments where methane or other flammable gases may be presents.

Group II: Equipment intended for use in above ground locations where flammable gases or vapors are likely to be present.

Group III: Equipment intended for use in environments where combustible dusts or fibers are present, such as grain handling facilities, flour mills, or textile factories.

To assign of the vapor of gas group followed IEC, it can be determined by using the Maximum Experimental Safe Gap (MESG), and Minimum Ignition Current (MIC) ratio.

Minimum Ignition Current (MIC) is the minimum current that, in a specified spark test apparatus and under specific condition, is capable of igniting the most easily mixture.

Maximum Experimental Safe Gap (MESG) is the maximum gap of the joint which prevent the ignition of the surrounding gas by the escaping gas which release from the test chamber through a gap length of 25 mm. (between the cover and the chamber).

Temperature Class of suitable apparatus group

The temperature class of suitable apparatus group refer to the maximum surface temperature that electrical equipment can reach under normal operating conditions without igniting the surrounding flammable atmosphere.

Source of Release

The following equipment or sources are recommended for determining the degree and extent of classified locations commonly specified in production facilities.

  • Hydrocarbon storage tank
  • Hydrocarbon pressure vessel
  • Fired equipment
  • Gas compressor or pump handling volatile, flammable fluid
  • Instrument such as Breather Valve, Pressure Safety Valve, Pressure Relief Valve
  • Sumps, Drains, and
  • Connection, flange, screw connection.

Grade of Source of Release

Grades of sources of release shall be defined in each item of process equipment if the item cannot contain flammable material. By means of the HAC study, the grade of source of release will be graded as continuous, primary, or secondary.

The definition of the grade of release source of the release has been defined already in the above section. However, this section will provide the example.

  • Continuous: Surface of a flammable liquid in the storage tank, or reservoir
  • Primary: Sealing of pump, compressor, sample point, relief valve, liquid drainage that contain flammable liquid
  • Secondary: Flange, connection of piping

Extent of Zone

The next influence factors that shall be checked during determining the area classification is What is ventilation? What is level of ventilation?

Degree of ventilation

There are three (3) degrees of ventilation are recognized in IEC 60079-10-1;

  • High: It can reduce the concentration at the source of release instantaneously resulting in concentration below the LEL. It will result to a zone of Negligible Extent (NE).
  • Medium: It can control the concentration.
  • Low: It cannot control the concentration while releasing.

How to estimate the gas leakage

Gas leakage involves the release of gas from a containment system, whether it’s a pipe, or vessel. From a thermodynamic perspective, the process of gas leak can be understood through principles such as ideal gas law, thermodynamic equilibrium, and entropy (s).

Gas Thermodynamic Process

Isentropic (Adiabatic) Process

  • An isentropic process is one in which the entropy (s) of a system remains constant. In fact, gas leakage trends to increase entropy as the gas expands into a larger volume, resulting in a more disordered state since the entropy (s) is a measure or the disorder or randomness of a system.
  • In practical terms, an isentropic process is often an idealization of a process that occurs without any heat exchange with the surrounding environment. (Q = 0)
  • For an ideal gas undergoing an isentropic process, the relationship between pressure (P) and Volume (V) can be described as following;
  • For example, the isentropic process is the expansion of gas in a piston-cylinder.

Isothermal Process

  • An isothermal process is one that occurs at constant temperature.
  • In an isothermal process, the internal energy of the system remains constant (dU = 0), meaning that any heat added to or removed from the system is entirely converted into work (Q = -W)
  • For an ideal gas undergoing an isothermal process, the relationship between pressure (P) and volume (V) is described by the ideal gas law.
  • The example process of isothermal is a gas confined within a cylinder fitted with a movable piston. And to maintain the temperature, this cylinder shall be placed in a large water bath with provided temperature by heat sink.

And the gas leakage formula is presented in below.

When apply the discharge coefficient (CD)

  • Sharp-edged orifice and Reynold number > 30,000: CD = 0.61
  • Short section of pipe attached to a vessel and L/D ratio > 3: CD = 0.81
  • Conservative approach: CD = 1

How to estimate the liquid leakage from piping flow

Estimating the leakage flow rate allows the designer to understand the amount of fluid being lost from the system since the leakage of certain fluids, such as hazardous chemicals which can pose safety risks to personnel and the surrounding environment. Hence, quantifying the leakage flow rate can help in the mitigation of those hazardous impacts.

For the liquid discharge, the density remains constant during the discharge, then the mechanical energy balance which is derived from the Open System Steady-State Energy Balance can be used to determine the discharge rate model.

How to ensure the leakage liquid will not flash

Flashing will be considered only for the liquids stored under the pressure above their normal boiling point.

Example: The boiling point of n-Hexane and propane are 68.72oC (155.73oF) and -42oC (-43.73oF) respectively at atmospheric pressure. If stored pressure 600 psia, 100oF leakage occurs, the n-Hexane is still in the liquid form but propane will flash to two-phase form.

Pipe leakage calculation formula

If the leakage size 10mm. occurs on the 10inch pipe (25.4mm), the calculated r/d is 0.394, the kf of entering the hole is 0.24 and kf of exiting the hole is 1.

When apply flow coefficient (CD) term

The following are suggested of the discharge coefficient (CD)

  • Sharp-edged orifice and Reynold number > 30,000: CD = 0.61
  • Short section of pipe attached to a vessel and L/D ratio > 3: CD = 0.81
  • Conservative approach: CD = 1

How to estimate the liquid leakage from ATM tank

Estimating the leakage flow rate allows the designer to understand the amount of fluid being lost from the system since the leakage of certain fluids, such as hazardous chemicals can pose safety risks to personnel and the surrounding environment. Hence, quantifying the leakage flow rate can help in the mitigation of those hazardous impacts.

For the liquid discharge, the density remains constant during the discharge, then the mechanical energy balance which is derived from the Open System Steady-State Energy Balance can be used to determine the discharge rate model.

The energy loss is represented by the friction loss occurs during transferring by using the below equation. The kf is the excess head loss from pipe, fitting, and other which are dimension less and can be determined by 2K methods.

In the turbulence flow (high Reynold number), the first term can be negligible, and kf of liquid entering the hole is 0.5 and exits the hole is 1.

Since the storage tank is atmospheric pressure, then the first term, differential pressure and initial velocity (V1) can be eliminated, then from the below equation, the exit velocity (V2) can be calculated by the reducing of the liquid level in storage tank.

The 2-K method presented above is a much more general approach. However, the alternative method which is also applicable to flow through an orifice plate can also be applied. This method will apply the discharge coefficient (Cd) which is equal to 0.61 for a sharp-edge orifice for the Reynolds number greater 30,000.

Below are the leakage rates from 10, 25, 50, and 100 mm diameters from the ATM storage tank which contains the hydrocarbon liquid density are 450, 800, 1000, and 1300 kg/m3 respectively.

How to estimate the explosion pressure

Understanding the potential explosion pressure allows for the implementation of appropriate safety measures to protection personnel, equipment, and facilities. This may include designing blast-resistant structures, implementing evacuation plans, and providing personal protective equipment.

Blast effects on structure and equipment

TNT Equivalency

TNT equivalency is a simple method for equating a known energy of a combustion fuel to an equivalent mass of TNT and uses an overpressure curve to apply a point source detonation of TNT.

How to estimate the liquid velocity in the bore pipe

The estimating liquid velocity in the piping offers several benefits such as optimized system performance, prevention of erosion & corrosion, and avoidance of cavitation.

In safety point of view, fluid velocity is one of critical parameters to determine the erosion and corrosive effects on the pipe material. Excessive fluid velocity can lead to the cavitation and can cause damage to the pump when apply the pump series connection or boosting concept.

Erosion Velocity

Erosion velocity refers to the fluid velocity at which erosion of the pipe material begins to occur. One common used empirical correlation is API RP 14E equation, which provides guidelines for estimating erosion velocity in hydrocarbon service.

Example

Estimate the fluid velocity in the 4-inch pipe for carrying the fluid flowrate 400 gallons/min and liquid density is 65 lb/ft3.

From the above figure shown, the result velocity is 8 ft/s which is lower than the erosion velocity at 12.4 ft/s. [100/(65^0.5)]

How to estimate the oil spill volume on water

Estimating the potential size and impact of an oil spill is crucial for several reasons such as Emergency Response Planning, Risk Assessment, Environment Protection, Public Safety & Health, and Resource Allocation.

A rough estimate of spill volume can be generated from observation of the oil slick’s size and thickness (1st figure) and the appearance, and light condition (2nd figure).

Example

Estimate the area impact of oil 100-gallon leakage from the offshore platform to the sea surface and spreading with dull colors.

Since the oil slick thickness is unknown, the area covered after 24 hours is between 0.0004 to 0.8 sq. miles. The more conservative area that needs to be investigated shall be as big as possible.

From the table, interpolate 2,500,000 + (100-61.7)*2,500,000/(123.4-61.7) = 4,051,863 sq. ft. or 0.145 sq. miles.

Blast Explosion Study

Blast explosions are typically associated with flammable cloud when it is formed during the leakage of flammable gases. If its direct ignition once released lead to a flash fire. If, however, its ignition is for some delayed (5-10 mins), then a Vapor Cloud Explosion (VCE) is the probable outcome.

The study of blast explosions involves understanding the dynamics, effects, and mitigations of these explosions.

Key aspects of blast explosion studies include:

Shockwave Propagation: Blast explosions generate shockwaves the travel through the air, causing damage to structures and injuring people. Understanding how these shockwaves propagate is crucial for assessing the potential impact of an explosion.

Blast Effects: The study can demonstrate the effects of blast waves on structures, infrastructure, and the human body. This includes the evaluation of overpressure (peak pressure), impulse (the total pressure applied over time), and the duration of the blast.

Structural Response: Buildings and other structures react differently to blast loads depending on their design and construction. The design aims to develop methods to design structures that can better withstand blast impacts.

Human Injury and Protection: Blast explosions can cause injuries to humans, including primary injuries from the blast wave, secondary injuries from flying debris, and tertiary injuries from being thrown or crushed. Studies focus on protective measures and strategies to minimize these injuries.

Two (2) main important parameters output from Blast Explosion Study

  1. Peak Pressure (Overpressure): The peak pressure, also known as overpressure, refers, to the maximum pressure level reached by the blast wave during its propagation. It is usually measured in (psi) or (Pa). Peak pressure is a critical factor in assessing the potential damage caused by a blast. Higher peak pressures are associated with greater destructive potential.
  2. Impulse: Impulse is the cumulative effect of the blast wave over time. It is the integral of the pressure-time curve and represents the total momentum imparted by the blast wave. Impluse is calculated by integrating the pressure-time curve over a specified time interval and is expressed in (psi.s) or (N.s/m2)

Damage estimated based on overpressure published by V.J Clancey “Diagnostic Features of Explosion Damage” are given in below table.

psigkPaDamage
0.32.07“Safe distance” probability 0.95 of no serious damage below this value. Projectile limit. Some damages to house ceiling. 10% window glass broken.
213.8Partial collapse of walls and roofs of house
320.7Heavy machines (3,000 lbs) in industrial buildings suffer little damage. Stell frame building distort and pull away from foundation.

Parameters Significantly Affecting the Behavior of Explosions

  • Ambient temperature
  • Ambient pressure
  • Composition of explosive material
  • Physical properties of explosive material (material reactivity)
  • Nature of ignition source i.e. type, energy, and duration
  • Geometry of surrounding i.e. confined or unconfined space
  • Degree of confinement
  • Amount of combustible material
  • Turbulence of combustible material
  • Time before ignition

Deflagration and Detonation

The damage effects from explosion depend highly on whether the explosion results from a detonation or a deflagration. Deflagration and detonation are both processes of rapid combustion, but they differ in terms of the speed of the combustion wave and the mechanisms involved. The below table is a summarize of the difference between deflagration and detonation.

DeflagrationDetonation
SpeedSubsonic combustion processSupersonic combustion process
PropagationCombustion wave moves through the substance relatively slowly. Combustion wave moves through the substance very rapidly.
Shock WaveNo strong shock wave produced. Shock wave is the formation of Detonation.
ExampleBurning of a piece of paper.Explosion of dynamite.

In the detonation mode, the flame front travels as a shock wave and exceeds the velocity of sound (330 m/s) typically of the order of 2,000 – 3,000 m/s. A detonation generates greater pressure than a deflagration explosion flame.

A deflagration can also evolve into a detonation. This is called a deflagration to detonation transition (DDT). The transition is particularly in pipes but unlikely in vessels or open spaces since in the piping system the energy from a deflagration can convert to pressure wave, resulting in an increase in the adiabatic pressure rise.

Key parameters

Ignition of the flammable gas, dust or mist cloud will result in the propagation of a frame front, or deflagration wave. The effect of a deflagration is to increase the pressure-volume product due to a large rise in temperature and a change in mole amount of gas present. Hence, based on this explanation, the relationship of ideal gas equation of state can be applicable.

For the explosion to take place, three conditions must be presented.

  1. Sufficient quantities of flammable fuel, air, and ignition source or cloud dimension;
  2. Flame speed (Sf); and
  3. System geometry.

Cloud dimension, R (m), is derive from the volume, V (m3) of the vapor cloud, which is composed from flammable gas and air on the surface. This can be calculated from the reaction’s stoichiometry. The geometry of the cloud dimension could be considered as a hemisphere, as

Burning velocity is defined as the speed at which the flame front propagates through the flammable mixture relative to the unburned mixture ahead of the flame. However, burning velocity does not take into account the flame expansion relative to stationary objects. Hence, when such expansion is taken into account, the term flame speed is used.

As a minimum of flame speed,

However, flame speed (Sf)can become high, particularly in the tube situation where displacement of the gas ahead of the flame creates pipe flow turbulence. Flame speed without turbulence for hydrogen without stationary objects would be about 24 m/s (laminar) but turbulence could lead to flame speed of 240 m/s or more. Hence, the system geometry is more contribute to the pressure of explosion than the flame speed significantly.

The value of the estimate the burning velocity (Su) for a number of gases in air can be referred to NFPA 68.

System geometry

Open area

If the explosion occurs in the unconfined space, it can be called as unconfined vapor or dust cloud deflagration which can presents the hazards of an expanding fireball rather than the detonation since it does not produce large localized overpressure.

The size of fireball generated can be estimated by assuming that a fixed amount of fuel burns with a stoichiometric equivalent amount of air to yield a burned volume at the flame temperature. The increase in volume in volume of the burned mass is estimated by using the relationship of ideal gas equation of state.

For example, the maximum theoretical hydrocarbon’s pressure, with initial pressure and temperature between 1 and 40 bar and 0-300oC.

Shepherd et al. (1997) developed a relatively simple and sufficient accurate model on the basis of energy conservation law applied to a constant volume adiabatic system.

Partially confined

A partially confined deflagration is represented by combustion of a vapor or dust cloud in a small volume of a larger enclosure which the total pressure rise is proportional to the volume of gas burned. The example case are;

  • Explosion inside the building;
  • Explosion in vessels and pipes

In case of building with some opening, A simple method to determine the peak pressure has been described by Weibull (1968) as follows.

Unconfined Vapor Cloud Explosion (UVCE) Methods

UVCEs are among the most serious scenarios in QRA consequence assessments, due to the potential huge and large impact on people and assets. The main model and summary of each model are presented as below.

Equivalent TNT Mass Method

  • Compare heat of combustion with the energy release of TNT which is equivalent to 4.69×106 J/kg

Multi-Energy Model (MEM)

  • Blast pressure is determined by charts which is defined by scaled pressure and scaled duration against a scaled distance;

Baker Strehlow Tang (BST)

  • BST methodology requires selection of the maximum flame speed, on the basis of the combined effects of congestion, fuel reactivity, and confinement.
  • Congestion levels for BST is segregated by area blockage ratio per plane, number of obstacle layers, and pitch.
  • Reactivity of fuels are defined by burning velocity (Su);
  • Then, the flame speed correlations are determined by congestion and reactivity.
  • Blast pressure is defined by above factor including the applying of the chart scaled pressure and scaled duration against a scaled distance same as MEM method.