ATEX Explosion Protection & Ignition Properties of Hydrogen Gas

Hydrogen technologies are undergoing a significant period of growth as their widespread introduction is planned in the energy generation and production industries. Those of you with the knowledge of the properties and flammability of hydrogen will recognise that hazards due to explosive atmospheres must be assessed more and more frequently as the element is utilised, and, if necessary, appropriate protective measures must be taken. Learn more about dilution and ventilation of hydrogen gas here. 

The avoidance of ignition sources is one of the three basic measures for explosion protection.

  To recap, the three basic methods of protection are

• containment

Explosion containment allows the explosion to occur but confines it to a well-defined area, therefore avoiding the propagation to the surrounding atmosphere. Examples include explosion proof enclosures and motors that contain the explosion in their housing.  

• segregation

Segregation attempts to physically separate or isolate the electrical parts or hot surfaces from the explosive mixture, such as pressurisation or encapsulation.  

• and prevention

Prevention therefore limits the energy, both electrical and thermal, to safe levels under both normal, and fault conditions.   The prevention of an explosion through limiting ignition sources is implemented primarily through the use of special, intrinsically safe, explosion-protected equipment. The design of which reduces the potential for the component to contribute as a source of ignition. Non-electrical equipment such as ATEX industrial fans are important components that must be selected correctly according to accurate hazardous area classifications. Mechanically generated sparks or hot friction surfaces resulting from mechanical impacts caused by the rotating and stationary parts meeting for prolonged time periods, are considered in the prevention concept. This aspect is so important that it is stated in UK and EU ATEX legislation. Directives require the safe and effective construction of components in permissible material pairings to reduce this ignition source before they are placed on the market for use in hazardous environments. Learn more about reduce sparks and friction explosions with permissible pairings here.   [caption id="attachment_17888" align="aligncenter" width="797"] Hydrogen is very sensitive to ignition with a very low MIE of only 0.017mJ, therefore it is assigned to explosion group IIC.[/caption]   Whether ignition of explosive gas mixtures can be caused by mechanical impact depends on many parameters. In addition to the materials involved in the impact, the ignition sensitivity of the fuel gases is important. Hydrogen is very sensitive to ignition with a very low MIE of only 0.017mJ when mixed with air, and even lower when mixed with oxygen at 0.012mJ, when compared with many other fuel gases such as petrol with a MIE of 0.1mJ. Therefore, Hydrogen is assigned to explosion group IIC in accordance with the low minimum ignition energy.

Auto Ignition of Hydrogen

The auto ignition temperature of a substance is the lowest temperature at which it will spontaneously ignite without the presence of a flame or spark. Hydrogen has an auto ignition temperature of over 510°C. This means hydrogen air or hydrogen-oxygen mixtures can ignite at atmospheric pressure. Prolonged contact, low atmospheric pressure and hot air jet ignition temperatures vary. Hydrogens flammability range is very wide when compared to other fuels but under the optimal combustion condition of 29% hydrogen: air ratio, the energy required to initiate hydrogen combustion is much lower than required by other fuels. Prolonged contact, low atmospheric pressure and hot air jet ignition temperatures vary. There are many origins of ignition in many industries, including energy generation. Due to the low MIE, it is often difficult to determine what may have caused hydrogen to ignition but the below sets out to illustrate a few potential sources.   Electrical sources: Electric sparks (e.g. from electrical equipment) Static discharges (e.g. in ungrounded particulate filters) Electric arc (switches, electric motors, portable phones, pagers and radios). Lightning discharge (e.g. lightning strikes near the vent stack) Electrical charge generated by equipment operation (compressors, generators, vehicles and other construction equipment) Electrical short circuits or other electrical equipment Electrified particulates   Mechanical sources: Mechanical sparks (from rapidly closing valves) Mechanical impact and/or friction Metal fracture Mechanical vibration and repeated flexing   Thermal sources: Hot surfaces (e.g. heating equipment) Open flames Hot jets Exhausts (e.g. combustion engines and exhaust stacks) Explosive charges (e.g. charges used in construction, fireworks or pyrotechnic devices) Catalysts, explosives and reactive chemical materials Shock waves and/or fragments Reflected or repeated acoustic and shock waves

 

Conclusion

In addition to hydrogen detectors, the best and experimentally proven method of hydrogen explosion protection is by means of dilution with a mechanical ventilation system using ATEX explosion proof fans to add additional air, therefore reducing the hydrogen - air ratio and thus eliminating the build-up of concentrated hydrogen in a spatially uniform manner. There have been many case studies that show the effects of switched off ventilation systems or failed exhaust fans that have resulted in a build up of hydrogen gas and ultimately explosions. Many systems operate with fans on run and standby, therefore if one fan fails the other picks up the duty while the broken component is repaired or replaced. Hydrogen is a very small molecule with low viscosity, therefore it is prone to leakage. Natural ventilation with vents at high levels in confined spaces will allow hydrogen to escape and quickly disperse into atmosphere safely.   For more information on how our range of IIC T1 hydrogen compatible industrial fans are suitable for explosive atmospheres contact our team on [email protected] or say hello to our technical team on the website chat.   [caption id="attachment_17390" align="alignnone" width="214"] Download Hydrogen Fans Brochure[/caption]