Authors: Yolanda Parras Gomez, process safety engineer and Gary Horgan senior EHS consultant, Chris Mee Group Process and Explosion Safety Management is the proactive and systematic identification, evaluation, and mitigation or prevention of chemical releases, flammable liquids and gases and combustible dust clouds that could occur as a result of failures in process, procedures, or equipment. Unexpected releases of toxic, reactive, or flammable liquids and gases, dust explosions in processes involving highly hazardous chemicals have been reported for many years. Accidents occurred in Seveso in 1976; the Bhopal disaster at the Union Carbide India Limited (UCIL) pesticide plant in 1984, which caused more than 2,000 immediate deaths; the BP Deep Horizon Explosion in 2005; and Imperial Sugar in 2008 are among some of the fatal explosions that have occurred in the industry. In order to have an explosion, a chain of events needs to happen, a single event will not be able to end in such a catastrophic scenario. After carrying out the investigations of those explosions in the industry, there are some recurrent events materialising in an explosion: a)      Lack of awareness of the explosion hazard and consequently insufficient explosion management through explosion protection systems and control of ignition sources; b)      Inadequate fire safety standards and poor emergency procedures; c)      Insufficient training; d)      Management of change not applied correctly; e)      Improper standard operating procedures in place; f)       Disregarding previous near misses and upsets of the process. Nowadays, two ATEX directives apply throughout the European Union; The 94/9/EC directive for manufacturers, also called ATEX 95 and the 99/92/EC user directive or ATEX 137. The commonly used abbreviation ‘ATEX’ derives from the French term ‘Atmosphère Explosible’. Basically, anywhere where an explosive atmosphere can occur, ATEX applies. [caption id="attachment_21218" align="alignright" width="300"]Explosion-2 Explosive atmosphere warning signage[/caption] Directive 99/92/EC on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres, also known as ‘ATEX 137’, has been written for the employer in his capacity as plant user. Main employer’s explosion protection priorities are detailed below: 1. Prevention of the formation of explosive atmospheres; 2. Avoidance of ignition sources; 3. Mitigation of the detrimental effects of an explosion. Within the Irish legislation, Part 8 of the General Application Regulations of Safety, Health & Welfare at Work 2007 called Explosive Atmospheres at Places of Work, re-transposes the (ATEX) Directive 1999/92/EC regarding the risks from fire and explosion arising from flammable substances stored or used in the workplace.

Gas, vapour and dust explosion characteristics

An explosion occurs if a flammable substance is present in mixture with air or another oxidant (i.e. sufficient oxygen), within the explosion limits, together with a source of ignition. In addition, if the vapour, gas or dust cloud is contained, the rapid release of heat and gas causes a pressure rise or over-pressure explosion. [caption id="attachment_21220" align="alignright" width="300"]Explosion-3 Explosion Dust pentagon[/caption] The same quantity of material exploding in an unconfined space will directly cause less damage than it would in a confined space. Examples of flammable substances present at work can be LPG gases, as propane and butane, hydrogen, acetylene, fuels, ethanol, isopropyl alcohol (IPA), toluene, sugar, flour, magnesium and aluminium powders, wide variety of API (paracetamol, ibuprofen) and pharmaceutical excipients.

Flammability and explosive limits

For explosions to occur the fuel to air ratio (%) has to be in a range, between the Lower Explosive Limit or LEL and the Upper Explosive Limit, known as UEL. - Lower Explosive Limit (LEL) – if the fuel to air ratio is too small the mixture will not ignite - Upper Explosive Limit (UEL) – if the fuel to air ratio is too large the mixture will not ignite. [caption id="attachment_21223" align="alignright" width="300"]James-F-4 Flammability and explosive limits diagram[/caption] Flammability limits can change with the oxygen levels, temperature and pressure, as below: -        Oxygen enrichment decreases the LEL and increases the UEL; -        For higher temperature, LEL is lower and UEL is increased; -        Increase of pressure, will increase both LEL and UEL. Another parameter to take into account for the flammability limits is the LOC, which stands for limiting oxygen concentration, also known as the minimum oxygen concentration, (MOC) is defined as the limiting concentration of oxygen below which combustion is not possible, independent of the concentration of fuel.

Gas and vapour explosion characteristics

Some of the key physical terms regarding gas and vapour explosion characteristics are flash point, vapour density, vapour pressure, auto ignition temperature (AIT) and minimum ignition temperature (MIT).

Dust explosion characteristics

For dusts, the important explosion properties are particle size, lower explosive limit (g/m3), minimum ignition temperature, max explosion pressure (bar) , KST max rate of pressure rise (bar m/s), minimum ignition energy (mJ), % moisture and combustibility. Lower Explosive Limit (LEL) for many organic materials is in the range of 10-50 g/m3. Most dusts with a particle size between 0.02 and 0.4mm are considered to be combustible.

Ignition sources

[caption id="attachment_21224" align="aligncenter" width="300"]Jamestable-2a Ignition sources may cause an explosion[/caption]     [caption id="attachment_21225" align="aligncenter" width="300"]Explosion-4 Typical ignition energy required to ignite explosive atmospheres[/caption]     [caption id="attachment_21226" align="aligncenter" width="300"]Explosion-5 When a powder has a low minimum ignition energy (MIE) particular precautions are required in order to avoid ignition. Precautions depending on the MIE levels[/caption]  

Hazardous area classification

[caption id="attachment_21230" align="alignright" width="300"]Jamestable-5 Hazardous area classification (click to enlarge)[/caption] Work areas are classified in to the likelihood of explosive atmosphere is present or expected to be present. There are three different hazardous areas for gases and vapours, zone 0, 1 and 2 and three zones for flammable dusts, zone 20, 21 and 22.

ATEX equipment specifications

[caption id="attachment_21231" align="alignright" width="172"]Explosion-6 Signage of Ex-rated electrical equipment[/caption] As we have seen already, areas in which a hazardous potentially explosive atmosphere can occur are categorised into zones according to the probability of this explosive atmosphere occurring. In addition, depending on the respective zones, operating devices from a corresponding device category (in connection with the atmosphere) must be used. Explosion Groups are classified in Table 6 [caption id="attachment_21232" align="alignright" width="300"]Jamestable-6 Table 6 Explosion groups classification[/caption]

Explosion and process safety risk assessments

The Process Hazard Analysis (PHA) must address the scope of the Process Hazard Analysis techniques [caption id="attachment_21235" align="alignright" width="300"]Jamestable-7 Scope of the Process Hazard Analysis[/caption] In order to control the explosion risk, it can be necessary to carry out some Process Hazard Identification Risk Assessments or also known PHIRA studies, such as, HAZOP, HAZID, What if, Failure Mode & Effect Analysis (FMEA), Fault Tree Analysis (FTA), Layer of Protection Analysis (LOPA) Study, Consequential Modelling Dedicated Explosion Risk Assessment for each part of the plant where an explosive atmosphere can occur are also performed in order to identify the following: Table 8: Typical Explosion Risk Assessment workflow

Administrative and engineering controls for prevention and mitigation of explosions

[caption id="attachment_21238" align="alignright" width="300"]Jamestable-9 Engineering controls for prevention and mitigation of explosions[/caption] [caption id="attachment_21239" align="alignright" width="300"]Jamestable-10 Administrative or organisational controls[/caption] There are a wide variety of organisational and technical controls in order to prevent explosions and mitigate the effects of this possible catastrophic event if it finally takes place including: a)      Technical or engineering controls: b)      Administrative or organisational controls: