The risk of a fire or explosion at major industrial facilities, such as those found in the hydrocarbon (oil, gas and coal) industry, is ever present. The highly volatile nature of the materials mined and processed at sites around the world requires significant process safety measures to mitigate against the risk of disaster (Kerin 2017).
The management of hazards at these facilities is designed to prevent what is known as “low probability, high consequence” incidents. These incidents are usually very rare (the result of significant and multiple system failures) but with enormous casualties, both in personnel and equipment (Kerin 2017).
In a report by Marsh (2016), detailing the 100 largest property damage losses, between 1974-2015 fire and explosion events were responsible for over 26 billion dollars (US) in insured property losses.
Assael and Kakosimos (2010) report the top 15 industrial explosions between 1976-2006 were responsible for almost 9,000 deaths and over 51,000 further casualties.
The fallout of these events, and the need to mitigate against them, is so great the discipline of ‘process safety’ has been developed into its own specialist field. With a specific focus on the prevention of loss of control of hazardous materials or energies, process safety is most often associated with major hazard facilities (Kerin 2017).
Examples of typical hydrocarbon industry facilities:
Offshore oil platforms
Chemical reactions are the cause of many of the explosions within major hazard facilities. The underlying energy is thermal energy as a result of either endothermic or exothermic reactions.
Exothermic reactions: give off thermal energy, in the form of heat, as a by product of the reaction. Chemical bonds are created.
Endothermic reactions: Require energy to complete the reaction. The result is a decrease in temperature in the surroundings of the reaction. Chemical bonds are broken (Science Made Simple 2014).
Chemical reactions are the result of energy transfer at a molecular level involving matter in one of three states.
Kerin (2017) lists the below underpinning science of matter, energy and chemical reactions as critical to understanding actions and consequences related to process hazards within major facilities:
- Consists of molecules. Individual atoms held together by bonds.
- Occurs in three states – solid, liquid and gas.
- Gas expands spontaneously to fill its container and is highly compressible.
- The volume of a gas is inversely related to the pressure at a given temperature.
- At a constant pressure, the volume of a gas is directly proportional to its temperature.
- The pressure of a gas at constant volume is proportional to the temperature.
“If the temperature changes then either the pressure or volume or both will change in proportion to the temperature.
(P1V1)/T1=(P2V2)/T2 (Combined Gas Law)
Most importantly in process safety, as the volume decreases the pressure increases, and as the pressure for a fixed volume of gas increases, the temperature will also increase.”
- Cannot be created or destroyed.
- Can be transferred or transformed into other forms of energy.
- Heat transfer occurs along a gradient from hot to cold. Occurs via conduction, convection and/or radiation.
Involve the breaking and/or making of molecular bonds with associated changes in energy level
Reactivity level determines a substance’s ability to undergo chemical reaction.
- The rate of a chemical reaction is affected by:
- concentration and physical state of the reactants;
- surface area of the reactants;
- presence of solvents and catalysts; and
- pressure (for gases).
- Runaway reactions occur when the heat generated by a reaction exceeds the maximum rate of cooling.
“A runaway reaction can occur where the reaction speed continues to accelerate until reactants are used up or the vessel containing it overpressures and loses containment, frequently with high risk of injury and equipment damage.”
- A catalyst speeds up the rate of reaction by lowering the energy required to make the reaction happen.
Flammable Substances and the Fire Tetrahedron
A flammable substance is any form of matter that is able to be ignited when certain conditions are met.
- Fuel – the flammable substance
- Oxygen – the presence of oxygen required at a level sufficient to sustain the chemical reaction (normal level of oxygen in air is usually sufficient)
- Ignition source – a source of energy sufficient capable of igniting the fuel
- Uninhibited chemical chain reaction – sustained reaction that continually feeds energy or fuel back into the reaction to maintain it (Kerin 2017).
Measurement and Evaluation Information
As all flammable substances require the above four conditions for ignition, each substance has a series of values that define its flammability range.
Gas and vapour
Lower flammability/explosive limit (LFL/LEL):
- Below this limit the concentration of gas in air is insufficient for ignition
- measured as a percentage (%)
Upper flammability/explosive limit (UFL/UEL):
- Above this limit the concentration is too great and the mixture cannot ignite
- measured as a percentage (%)
- Flammability measured by the term flash point.
- Flash point is the lowest temperature at which the liquid can be ignited.
- Measured in either degrees Celsius or Fahrenheit.
Autoignition temperature (AT):
- The lowest temperature a substance will spontaneously ignite.
- Measured in either degrees Celsius or Fahrenheit.
Explosive materials: Some chemicals are so volatile they can explode accidentally during the production process. For example, nitroglycerin (dynamite) or ammonium nitrate.
Corrosion: Chemicals adversely reacting with the materials used to construct the plant and machinery involved in the production process leading to chemicals leaking from the plant, toxic gas releases or explosions and fire as the corrosion leads unexpected chain reactions.
- Gases transported and stored under pressure contain large amounts of potential energy. Failure of the storage vessel to contain the pressure can lead to catastrophic damage as the gas escapes the vessel.
- Steam as part of the production process can create significant hazards. The unexpected escape from a faulty valve or standing in the vicinity of a discharge point could result in serious burns from direct contact or by touching objects the steam is exhausting on to.
Mixing incompatible chemicals: Allowing chemicals that react exothermically to, inadvertently, come into with each other can lead to unwanted chemical reactions. If the mixture is large and volatile enough, a fire, explosion or toxic release can be the result.
Combustible dust explosions: Any carbon based material when suspended in air, in sufficient quantities, with the right oxygen amount and an ignition source is capable of creating a dust explosion. Commonly associated with processes that produce fine dust as a by-product. For example, sugar refining, flour mills and underground coal mines.
(Pasman 2015; Kerin 2017)
Work Health and Safety Regulation 2011 (Qld), ch. 7 regulates the manufacture, supply and handling of hazardous chemicals.
The regulation requires manufacturers, suppliers and importers of chemicals to determine if they are hazardous and to classify the chemicals in line with the Globally Harmonised System (GHS) of Classification and Labelling of Chemicals (Workplace Health and Safety (2017). This ensures all persons involved in the chemical life-cycle are supplied with relevant information related to the chemical, regardless of country of origin.
In process safety, Kerin (2017) states many redundant controls are designed into the production process to achieve:
- elimination of ignition sources;
- prevention of loss of control; and
- event mitigation including managing an emergency if one occurs.
The highest control is elimination of hazards. By designing the process and associated plant inherently safer, removal of the hazard can occur (kerin 2017).
Principles of inherently safer design include:
- Minimisation – Reducing the size of the process, amount of chemicals used to minimise the hazardous energy.
- Substitution – Substituting a less-hazardous process to reduce the overall hazard. For example designing the process to use water as a solvent instead of an alcohol-based solvent.
- Moderation – Moderating the conditions under which the process occurs or the material is stored to reduce the hazardous energy and prevent loss of control. For example, storing liquefied natural gas under refrigeration to reduce the need for pressurisation.
- Simplification – Designing the process to be as simple as possible. Removing complex mechanisms reduces the need for greater and more complex safety systems to control the process. For example, Using stronger equipment to contain the process, removing unused piping and designing the process with human factors in mind – ensuring the equipment operates as the user expects it to (Kerin 2017).
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Kerin, T 2017, ‘Process hazards (chemical)’, in, The Core Body of Knowledge for Generalist OHS Professionals, e-book, Safety Institute of Australia, Tullamarine, available at http://www.ohsbok.org.au/
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