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Generation of flammable mists from high …

Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2013 Health and Safety ExecutiveGeneration of flammable mists from high flashpoint fluids: Literature reviewRR980 Research ReportSimon Gant Health and Safety LaboratoryHarpur HillBuxtonDerbyshire SK17 9 JNHazardous Area Classification (HAC) for explosive gas atmospheres is well established, with guidance published in various standards and industry codes of practice. However, the same situation is not currently the case for high flashpoint liquid releases that could give rise to an explosive mist atmosphere.

Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2013 Health and Safety Executive Generation of flammable mists from high

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1 Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2013 Health and Safety ExecutiveGeneration of flammable mists from high flashpoint fluids: Literature reviewRR980 Research ReportSimon Gant Health and Safety LaboratoryHarpur HillBuxtonDerbyshire SK17 9 JNHazardous Area Classification (HAC) for explosive gas atmospheres is well established, with guidance published in various standards and industry codes of practice. However, the same situation is not currently the case for high flashpoint liquid releases that could give rise to an explosive mist atmosphere.

2 There is a pressing need for clear guidance on mist hazards to allow operators to determine the extent of areas where flammable mists may be present and to select appropriate equipment for use in those report provides a survey of the recent literature on flammable mists and pulls together information that will be useful in developing a HAC methodology for explosive mist atmospheres. It focuses on the three fundamental issues: mist flammability, mist Generation and mitigation measures. The first of these is discussed with reference to five measurable parameters: the Lower Explosive Limit (LEL), Minimum Ignition Energy (MIE), Maximum Experimental Safe Gap (MESG), Minimum Igniting Current (MIC), and Minimum Hot Surface Ignition Temperature (MHSIT).

3 Measurements of these quantities in mists are analysed and models for their prediction are discussed. The second issue of mist Generation is examined under four categories: mists produced by pressurised sprays, condensation aerosols, agitation/splashing/sloshing and air stripping. Of these, the primary focus is on spray releases and condensation aerosols, which are considered to be the most likely sources of mists . Measurements undertaken in sprays are described and models are discussed. Mitigation measures are surveyed briefly, which include mist detection, use of fire-resistant fluids or anti-misting additives, inerting and control of static charge.

4 Finally, tentative proposals are suggested for developing area classification guidance based on the prediction of the flammable mist cloud report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy. Generation of flammable mists from high flashpoint fluids: Literature reviewHSE BooksHealth and Safety Executive Crown copyright 2013 First published 2013 You may reuse this information (not including logos) free of charge in any format or medium, under the terms of the Open Government Licence.

5 To view the licence visit , write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email images and illustrations may not be owned by the Crown so cannot be reproduced without permission of the copyright owner. Enquiries should be sent to author would like to acknowledge the major contribution from the following people in undertaking this work: Roger Santon, Richard Bettis, Philip Hooker, Darrell Bennett, PeterWalsh, Matthew Ivings, Charles Oakley and Sharon Taylor (Health and Safety Laboratory), Professor Phil Bowen and Dr Peter Kay (Cardiff University) and Ian Buckland (Health andSafety Executive).

6 2 3 CONTENTS 1 INTRODUCTION .. 7 Motivation .. 7 Objectives .. 7 Methodology .. 8 Area classification context .. 9 Previous efforts towards area classification of mists .. 13 Definition of terms .. 16 2 FLAMMABILITY OF mists .. 21 Minimum Ignition Energy (MIE) and Lower Explosive Limit (LEL) .. 21 Physics of spark ignition and flame propagation.

7 21 Effect of droplet size and concentration .. 25 Effect of fuel volatility .. 37 Effect of flow speed .. 41 Other effects .. 43 Maximum Experimental Safe Gap (MESG) .. 43 Minimum Igniting Current (MIC) .. 46 Minimum Hot Surface Ignition Temperature (MHSIT) .. 48 Summary .. 53 3 FORMATION OF flammable mists .. 56 Fundamentals of droplet dynamics .. 56 Droplet forces .. 56 Sedimentation speed and droplet drag coefficient .. 58 Droplet-droplet collisions .. 62 Evaporation .. 63 Droplet mass and heat transfer.

8 65 Pressurized liquid releases .. 66 Mass release rate .. 66 Primary and secondary breakup .. 66 Spray models .. 76 Impinging sprays .. 86 Towards practical models for area classification .. 88 Condensation aerosols .. 91 Agitation and splashing .. 92 Air stripping .. 93 Summary .. 94 4 MITIGATION MEASURES .. 96 Mist detection .. 96 Choice of 97 Fire-resistant hydraulic fluids .. 98 Anti-misting additives .. 100 Inerting .. 100 Control of static charge .. 102 Other.

9 103 4 5 CONCLUSIONS .. 105 Main findings .. 105 Possible future directions for area classification of mists .. 107 Knowledge gaps and potential future work .. 110 6 REFERENCES .. 113 5 EXECUTIVE SUMMARY Hazardous Area Classification (HAC) for explosive gas atmospheres is well established, with guidance published in various standards and industry codes of practice.

10 However, the same situation is not currently the case for high flashpoint liquid releases that could give rise to an explosive mist atmosphere. There is a pressing need for clear guidance on mist hazards to allow operators to determine the extent of areas where flammable mists may be present and to select appropriate equipment for use in those areas. The objective of this report is to survey the recent literature on flammable mists and pull together information that would be useful to develop a HAC methodology for explosive mist atmospheres.


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