Introduction

Equipment, whether it be electrical or non-electrical i.e. mechanical, pneumatic or hydraulics is designed to be used in explosives atmospheres formed by gases and vapours or combustible dusts. These locations where potentially explosive atmospheres may be present are subject to area classification and depending on the outcome we classify these locations as zones based on frequency and duration of any release i.e. zone 0, 1 or 2 for gases and vapours environments and zone 20, 21 and 22 for combustible dusts environments.

But what happens when we have both conditions at the same time in the same location?

Hybrid Mixtures

The term “Hydrid Mixtures” is referenced in a number of documents namely those listed below.

IEC 60079-0:2011 General requirements; NOTE 4 states “Where an explosive gas atmosphere and a combustible dust atmosphere are, or may be, present at the same time, the simultaneous presence of both should be considered and may require additional protective measures”.

With the latest updates to the area classification standards, IEC 60079-10-1 for gases and vapours Annex I and IEC 60079-10-2 for combustible dusts Annex C, references Hybrid Mixtures. These clauses are duplicated below:

General
A hybrid mixture is a combined mixture of a flammable gas or vapour with a combustible dust or combustible flyings. This hybrid mixture may behave differently than the gas/vapour or dust individually. The number of situations that may be encountered in industry will be highly variable and as such it is not practical to provide specific guidance. However this Annex provides guidance on issues that should be considered when hybrid mixtures are found.

Use of ventilation
The use of ventilation as a control measure needs to be carefully considered as it may reduce the gas/vapour hazard but increase the dust hazard or have other varying effects on the different components of the mixture.

Concentration limits
A hybrid mixture may form an explosive atmosphere outside the individual explosive limits of the gas/vapour or explosive concentrations for the dust. It is recommended, unless further data is available, that a hybrid mixture is considered explosive if the concentration of the gas/vapour exceeds 25 % of the LEL or the concentration of the dust exceeds 25 % of the  MEC.

Chemical reactions
Considerations should also be taken to chemical reactions that may occur within the materials or entrapped gas in the dust that may result in evolution of gas in the process.

Energy/Temperature limits
Where a hybrid mixture exists, the minimum ignition parameters such as MIE and auto-ignition temperature for gas/vapour or minimum ignition temperature of a dust cloud could be lower than any component parameter in the mixture. In the absence of other information, the parameter used should be the lowest of any component in the mixture.

Zoning requirements
Consideration should be given to the assignment of both gas and dust zones with the same rating to match the worst case requirement for any component, e.g. zone 21 with zone 2 should be considered as zone 21 with zone 1. It should be identified that the result of ignition of any component will lead to a worst case consequence when considering any EPL assessment.

A hybrid mixture is a location where due to process circumstances you may end up with a gas zone and a combustible dust zone in the same location, two different zones i.e. zone 1 and zone 21, with two very different safety parameters.

 
Design, selection, installation

For the designer, installers e.g. end-users, Hybrid Mixtures are referenced in Annex M of Edition 5 of IEC 60079-14:2013 and this states:

General
A hybrid mixture is a combined mixture of a flammable gas or vapour with a dust or flyings. This hybrid mixture may behave differently than the gas/vapour or dust individually. The number of situations that may be encountered in industry will be highly variable and as such it is not practical to provide specific guidance. However Annex M provides guidance on issues that should be considered when hybrid mixtures are found.

M.2 Concentration limits:
 A hybrid mixture may form an explosive atmosphere outside the individual explosive limits of the gas/vapour or explosive concentrations for the dust. It is recommended, unless further data is available, that a hybrid mixture is considered explosive if the concentration of the gas/vapour exceeds 25 % of the LEL or the concentration of the dust exceeds 25 % of the MEC.

M.3 Energy/temperature limits
Where a hybrid mixture exists the minimum ignition parameters such as MIE and auto-igniting temperature for gas/vapour or minimum ignition temperature of a dust cloud could be lower than any component parameter in the mixture. In the absence of other information the parameters used should be the lowest of any component in the mixture.

M.4 Selection of equipment
 Equipment should be selected that as a minimum requirement meets the criteria for both the gas/vapour and dust components concerned. Care should be taken with assessment of the required temperature class considering that a dust layer may increase the temperature of the equipment above that normally assessed for the gas / vapour condition on its own. This may either be due to an increase in the surface temperature of an enclosure or the internal component temperatures. The gas/vapour temperature class assigned to equipment that has alternative ratings for both gas/vapour and dust hazards is not valid where the enclosure is subject to dust layers.

M.5 Use of flameproof equipment
When using flameproof equipment in a hybrid mixture be aware that the flame transmission is not verified with an external explosive dust atmosphere and the protection technique may also be compromised due to dust in the flame path which may result in the ejection of hot particles.

M.6 Electrostatic hazard
Consideration should be given to equipment that is marked with a warning concerning electrostatic hazards to ensure that the dust conditions do not create electrostatic hazards.
 
M.7 Installation requirements
Cabling, cable glands, electrical protection and other installation factors should meet the requirements for both the gas/vapour and dust components concerned.

Clause M of IEC 60079-14:2013, the installation of electrical equipment in explosive atmospheres is the one that most effects the end-user, therefore we need to look deeper at the various clauses.

General
A hybrid mixture is a combined mixture of a flammable gas or vapour with a dust or flyings. This hybrid mixture may behave differently than the gas/vapour or dust individually. The number of situations that may be encountered in industry will be highly variable and as such it is not practical to provide specific guidance. However Annex M provides guidance on issues that should be considered when hybrid mixtures are found.

They obviously can’t look at every individual circumstance, therefore the end user has to look at every situation on a case by case study. The end user needs to understand the parameters of the hybrid mixture in question. This can be achieved by the use of the following IEC standards:
a)    IEC 60079-20-1:2010 Characteristics of gases and vapours (will be renamed IEC/ISO 80079-20-1 in the future)
b)    IEC/ISO 80079-20-1 Characteristics of Combustible Dusts- to be issued Q4 2015, or.
c)    Getting the gas, solvents or combustible dust independently tested.

M.2 Concentration limits:
 A hybrid mixture may form an explosive atmosphere outside the individual explosive limits of the gas/vapour or explosive concentrations for the dust. It is recommended, unless further data is available, that a hybrid mixture is considered explosive if the concentration of the gas/vapour exceeds 25 % of the LEL (Lower explosive Limit) or the concentration of the dust exceeds 25 % of the MEC (Minimum explosible concentration).
The Hybrid mixture is to be considered explosive, a danger to personnel and plant, unless evidence to the contrary is readily available.

M.3 Energy/temperature limits
Where a hybrid mixture exists the minimum ignition parameters such as MIE and auto-igniting temperature for gas/vapour or minimum ignition temperature of a dust cloud could be lower than any component parameter in the mixture. In the absence of other information the parameters used should be the lowest of any component in the mixture.
The lowest common denominator of each component part is too be considered in the assessment.

M.4 Selection of equipment
Equipment should be selected that as a minimum requirement meets the criteria for both the gas/vapour and dust components concerned. Care should be taken with assessment of the required temperature class considering that a dust layer may increase the temperature of the equipment above that normally assessed for the gas / vapour condition on its own. This may either be due to an increase in the surface temperature of an enclosure or the internal component temperatures. The gas/vapour temperature class assigned to equipment that has alternative ratings for both gas/vapour and dust hazards is not valid where the enclosure is subject to dust layers.
The example being, if an item of equipment has a number of alternative temperature classes e.g. gas equipment can be classified as T4, T5 or T6 this being based on different ambient temperature ranges. The worst case scenario should be considered and then you have to take account of any dust layering that may occur. You have to consider the safety factors as identified in IEC 60079-14:2013 clause 5.6.3.

M.5 Use of flameproof equipment
When using flameproof equipment in a hybrid mixture be aware that the flame transmission is not verified with an external explosive dust atmosphere and the protection technique may also be compromised due to dust in the flame path which may result in the ejection of hot particles.
Many years ago, flameproof enclosures the joints were typically metal to metal, this was to allow the cooling of a flame transmission (Joules Thompson effect). Over the years and especially with this type of equipment being used more and more in combustible dust environments, an IP (ingress protection) rating was applied to equipment either IP5X or IP6X. The equipment became known a DIP (Dust Ignition Proof), now it is known as “protection by enclosure (t)”.

M.6 Electrostatic hazard
Consideration should be given to equipment that is marked with a warning concerning electrostatic hazards to ensure that the dust conditions do not create electrostatic hazards.
Ensure that the assessment considers reducing the electrostatic risk to as low as reasonably practicable (ALARP), earthing, bonding, cleaning regime etc.
 
M.7 Installation requirements
Cabling, cable glands, electrical protection and other installation factors should meet the requirements for both the gas/vapour and dust components concerned.
The risk assessment should cover all items e.g. cables including the method of installation e.g. ladder rack, cable entry devices, blanking plugs etc must be suitable and approved for use in the hydrid mixture.

Selection
The selection of equipment for use in either gas or dust environments is pretty straight forward. Well we hope!
Consequence analysis has been not applied to this table.

By the use of ATEX categories (inside the European Union) or EPL’s outside of Europe, it doesn’t matter if consequence analysis has been applied or not, if the end user if armed with the correct information they should be able to correctly select equipment for use in either a gas or dust environment.
To correctly select equipment we need to be aware of various pieces of information, i.e. zone of use, gas or dust group, temperature class, environmental condition, process conditions etc.
This brings us on to installation, even though equipment may have been chosen correctly for the gas or dust environment, if it is not installed correctly then………….. but wait I digress. Installation is really for another day.

Conclusion

Can equipment be used in Hybrid mixtures, the obvious answer is YES of course it can, otherwise we would be in the position where we have locations were we can’t install equipment and that would be ridiculous.

However the selection criteria may have to be subject to a more onerous analysis of the safety parameters formed by the hybrid mixture e.g. a formal risk assessment should be completed and recorded.  

If in any doubt then the manufacturer of the equipment under consideration or an expert in the field of explosive atmospheres should be consulted.

Ultimately the main question to ask….. is my installation based on the selection criteria and installation practices used,  safe,  and indeed should the selection criteria be reviewed or both!

The answer you’re looking for is YES in both cases!

About the Author

Peter Roberts is the Site Services Manager for ExVeritas Limited, a Notified body for ATEX Directives in Europe, and IECEx Certification provider. The companies remit covers all aspects of explosive atmospheres from the design of products for use in explosive atmosphere, product testing for electrical and non-electrical equipment, implementation of management systems, consultancy, area classification, inspections and a range of training courses (including CompEx) associated with explosive atmospheres.

www.exveritas.com