Under pressure: the benefits of explosion venting panels
14 September 2021
In industrial, manufacturing or power generation facilities where potentially explosive atmospheres or processes exist, specialised venting can be used as a means of limiting damage in the event of an explosion. Andy Moul, Technical Manager at Construction Specialties considers the role of venting wall panels in protecting the workforce as well as ensuring a building’s structure is not compromised.
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Combustible dust, gas and chemical explosions present a serious risk for owners and operators in a wide range of industries, from pharmaceutical, chemical and food production facilities to energy, recycling and sewage treatment plants and any other facilities storing potentially dangerous substances.
Powders and dust are one of the most common causes of industrial fires and explosions and can include food products, dyes, chemicals, wood and metals which all have the potential to be combustible in dust form. According to a report by Dust Safety Science, 165 fires and 60 explosions were logged globally in 2020 which resulted in 88 injuries and 10 fatalities. On 3 December 2020, four people were killed and one injured at the Wessex Water wastewater plant in Avonmouth near Bristol when a silo that held treated biosolids exploded. Five days later at a chemical plant in Belle, West Virginia, one person was killed and three injured following an explosion after a 1,200-gallon metal dryer became over-pressurised while drying a compound used for sanitation.
Gas and solvent explosions are not uncommon either with 2020 seeing perhaps the largest accidental industrial explosion ever recorded. The result of improperly stored ammonium nitrate, the blast at a warehouse in the port of Beirut killed nearly 200 people, injured more than 6,000 and was heard in Cyprus, 150 miles away. A research team at Sheffield University’s Blast and Impact Engineering Research Group said it was the most powerful non-nuclear explosion of the 21st century. Such was the damage to infrastructure across the city, the economic cost of the explosion on the August 4 is estimated to run to over $10 billion.
Devastating incidents such as these are a stark reminder of both the human and economic cost of industrial accidents and their ever-present risk to society. Fires and explosions are the largest losses for businesses, with figures from global insurance carrier Allianz Global Corporate and Specialty (AGCS) putting the average incident claim at 1.5 million euros. It is clear that it is critical for owners and operators to ensure their facilities are safe, and appropriate measures are in place, to minimise the risk of explosions and their damaging effects.
In industrial or manufacturing facilities, most explosions are caused by deflagration – an explosion that propagates via a heat transfer, at a velocity slower than the speed of sound and may involve dust, gas, chemicals or any combination of these. This is distinct from a detonation caused by high explosive compounds, which propagates faster than the speed of sound.
It is generally accepted that the three components needed to ignite and sustain a fire are fuel, ignition and oxygen and are symbolised by the ‘fire triangle’. In a deflagation, two other factors come into play with dispersion and confinement creating what is termed as the ‘Explosion Pentagon.’
Liquid fuels dispersed in air as a fine mist and solid fuels dispersed in air as dust pose similar deflagration risks as gaseous fuels. Combustible dusts include corn starch, wheat flour, soybean, rice, charcoal or wood; flammable liquids include solvents, emulsions, paints and resins and gases such as oxygen, nitrous oxide and acetylene are all potentially explosive or act as a powerful oxidising source.
As mentioned, deflagration occurs when pressure and temperature builds up and there is an ignition source (which can be a flame, electrical spark, mechanical friction or a hot surface, such as overheating bearings). Deflagration can cause a pressure wave and carry a flame that can disturb and ignite other fuel, causing an even bigger, secondary explosion.
Existing guidance and legislation
There is a raft of legislation covering explosive atmospheres (see box at the end of this article). In the UK, alongside their obligations under the Health and Safety at Work etc Act 1974, employers must comply with EU Directive 99/92 (commonly known as the ATEX Workplace Directive), which is enforced through the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR), to ensure staff are kept safe from harm.
Any equipment and protective measures must meet EU Directive 94/9/EC (ATEX 95 or the ATEX Equipment Directive), which is implemented in the UK through The Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 2016.
Reducing the risk of deflagration
Measures which can be taken to reduce the risk of deflagration and overpressure include dust suppression systems, inerting (the partial or complete substitution of air or flammable atmosphere by an inert gas), avoiding ignition sources, mechanical ventilation and pressure (or explosion) venting.
Suppression systems can be used in equipment such as dust collectors and occasionally in silos, but it is generally not practical to install them in large rooms or manufacturing areas and they are not appropriate for gas stores.
Mechanical venting, using fans and louvres, is an option (and sometimes a requirement) for gas stores. It works by exchanging air with the outside to avoid gas build-up and by maintaining pressure to prevent it increasing. However, these systems are expensive to install, operate and maintain, and are not foolproof. If vents become blocked or there is mechanical failure, pressure or explosive gases can build-up, increasing the risk of deflagration.
Mechanical venting is sometimes used in conjunction with explosion relief venting – deliberate points of weakness built into manufacturing equipment or the building structure that open should pressure rise above a set safe level.
Explosion and pressure relief venting systems
Explosion and pressure-relief venting is an effective standalone measure for relieving overpressure and mitigating the severity of deflagration. Once pressure rises to an unacceptable level, the vent opens quickly, allowing rapidly-expanding gas or dust to be released to the outside, so the equipment or building structure are not subjected to overpressure and the damage and risk to building users is minimised.
While venting of individual pieces of equipment is effective, it is often impossible to prevent the formation of dense dust clouds inside a process room; to eliminate the risk of gas being present in a store or prevent any ignition risks (such as machinery causing a spark) being present.
Venting of a gas or dust deflagration on equipment can cause a tongue of flame to issue from the vent, potentially leading to unburned dust or gas being ignited, creating a fireball and potential overpressure situation. Building venting is an effective solution to mitigate damage.
Depending on the application, explosion venting must comply with European Standards BS EN 14797, BS EN 14491 and BS EN 14994 and should also meet the US National Fire Protection Association (NFPA) 68 Venting of Deflagrations guidelines and FM Global’s 1-44 Damage Limiting Construction document (see box at the end of this article).
Hinged vs blow-out panels
Typical building venting systems take the form of blow-out or hinged wall panels. Blow-out panels are ejected from the building when safe pressures are exceeded; hinged panels open by a top or bottom hinge and then return to a near closed position.
Hinged panels offer a major advantage over blow-out panels principally because they can be non-destructively tested as part of regular maintenance regimes and are reusable because they can be closed and reset.
These panels are factory-calibrated to release at very low design pressures to prevent catastrophic damage. When internal pressures rise rapidly, the panels release quickly and, once pressure has been relieved, return to a near-closed position. A rotating hold open device allows air to return to the room, to protect the structure from implosion forces as super-heated gases begin to cool.
Hinged panels can be made of either insulated aluminium or, where light transfer is a requirement, high-strength translucent polycarbonate, mounted in powder-coated aluminium surround. Panels can be mounted within an extruded aluminium frame and can be manufactured to be installed in portrait or landscape orientation. Smooth and angled surfaces reduce dust accumulation and allow easy cleaning, a key to eliminating secondary dust explosions.
By comparison, the only way to know if a blow-out panel works is if an explosion occurs and the panels have to be replaced after any event, with the associated costs and downtime. Blow-out panels are also typically larger and heavier and use shear bolts and fasteners, making their reliable performance highly dependent on them being installed correctly. Additionally, these panels present a risk as they can act like missiles if blown off a building, so have to be restrained using chains, cables or other methods that still allow them to open fully.
When you consider the devastating impact an explosion can cause to an industrial facility, not just in terms of business interruption and lost productivity, but also the potential for loss of life or injury, the provision of a safe working environment is paramount to owners and operators of all businesses handling and storing potentially combustible materials.
While safe working practices are vital, there is a range of systems available to reduce the risk of overpressure and deflagration. Installing explosion vents in a building structure is a cost-effective way of creating a last line of defence as part of an explosion mitigation strategy.
Explosion venting can help employers meet their legal obligations, protect staff from the risk of injury and death, and ensure buildings remain safe and secure should the worst happen.
Legislation and standards
The Health and Safety at Work etc Act 1974 sets out the responsibilities of employers to ensure they meet the minimum requirements of protecting the health and safety of staff in their place of work.
Directive 99/92/EC (also known as ATEX 137 or the ATEX Workplace Directive) gives minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres. In the UK, this is implemented through the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR).
The Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) requires employers to assess the health and safety risks arising from dangerous substances – materials that “could, if not properly controlled, cause harm to people as a result of a fire or explosion or corrosion of metal,” - and put measures in place to “either remove those risks or, where this is not possible, control them”.
Directive 94/9/EC (also known as ATEX 95 or the ATEX Equipment Directive) allows trade of ATEX equipment and protective systems across the EU, removing the need for separate testing and documentation for each Member State. Certification ensures equipment or protective systems are fit for purpose and that adequate information is supplied to ensure they can be used safely.
The Equipment and Protective Systems Intended for use in Potentially Explosive Atmospheres 2016: Great Britain covers a wide range of equipment, including explosion venting systems. It requires testing by an independent body and equipment must be CE marked.
BS 5908:2012 Fire and explosion precautions at premises handling flammable gases, liquids and dusts
Part 1: Code of practice for precautions against fire and explosion in chemical plants, chemical storage and similar premises
Part 2: Guide to applicable standards and regulations
BS EN 14797: 2006 Explosion venting devices
This European Standard specifies the requirements for venting devices used to protect enclosures against the major effects of internal explosions arising from the rapid burning of suspended dust, vapour or gas.
BS EN 14491:2012 Dust explosion venting protective systems
This European Standard specifies the requirements for dust explosion venting protective systems.
BS EN 14994: 2007 Gas explosion venting protective systems
Andy Moul, Technical Support Manager, Construction Specialties (UK) Ltd
This European Standard specifies the basic design requirements for the selection of a gas explosion venting protective system.
NFPA 68 Standard on Explosion Protection by Deflagration Venting (2018)
The US National Fire Protection Association is recognised as the leading global authority on explosive events. This document provides specific recommendations for the design, location, installation, maintenance and operation of explosion vents.
FM Global FM 4440 Approval Standard for Explosion Venting Systems
The standard sets out the performance requirements for explosion venting systems – examined for their ability to remain in place under normal conditions but to fail at pre-determined pressure levels. It also covers wall fasteners to secure wall panels to buildings, plus latches and magnetic release devices.
FM Global Property Loss Prevention Data Sheet 1-44 Damage-Limiting Construction This provides guidelines for the design and construction of building components including explosion venting panels, based on the pressures, room areas and the type of substances being used or stored in the building.
HTM 02-01 Medical gas pipeline systems – Part B: Operational management
The Department of Health’s Health Technical Memoranda (HTMs) provide best practice engineering standards for healthcare providers. HTM 02-01 Part B covers operational management issues.
British Compressed Gases Association Code of Practice 44: The storage of gas cylinders
This code of practice provides advice and guidance for the safe storage of gas cylinders, including recommendations for construction of stores.
HSG51 Storage of flammable liquids in containers
This guidance is for those responsible for the safe storage of flammable liquids in containers up to 1,000 litre capacity. Covers the fire and explosion hazards and is designed to be used with DSEAR.
About the author:
Andy Moul is Technical Support Manager at Construction Specialties (UK) Ltd, a global manufacturer and supplier of a range of specialist building products. He has worked for the company for over 30 years, and has been in his current position since 2008, leading a Technical Team in providing comprehensive project support from design stages right through to maintenance programmes.
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