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Www.hsl. gov.uk An Agency of the Health and Safety Executive www.hsl. gov.uk An Agency of the Health and Safety Executive UKELG 50 th ANNIVERSARY MEETING.

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Presentation on theme: "Www.hsl. gov.uk An Agency of the Health and Safety Executive www.hsl. gov.uk An Agency of the Health and Safety Executive UKELG 50 th ANNIVERSARY MEETING."— Presentation transcript:

1 gov.uk An Agency of the Health and Safety Executive gov.uk An Agency of the Health and Safety Executive UKELG 50 th ANNIVERSARY MEETING RECENT DELOPMENTS IN AREA CLASSIFICATION FOR GASES ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON

2 An Agency of the Health and Safety Executive CONTENTS Current standards Previous work on gases QUADVENT Natural ventilation estimation Constraints Examples Demonstration Future developments References

3 An Agency of the Health and Safety Executive CURRENT STANDARDS BS EN :2009, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas. Area classification code for installations handling flammable fluids, Model Code of safe practice, IP 15 3rd edition, The Energy Institute, IGEM/SR/25, Edition 2, Hazardous area classification of natural gas installations, Institution of Gas Engineers and Managers, 2010.

4 An Agency of the Health and Safety Executive CURRENT STANDARDS BS EN :2009, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas. –Zone definitions –Source terms –V z

5 An Agency of the Health and Safety Executive CURRENT STANDARDS V z –Hypothetical gas cloud volume –Mean concentration of 50% LEL (for secondary releases) –Determines level of ventilation –If V z is less than 0.1m 3, ventilation is regarded as high and zone is classified Negligible Extent (NE) and no precautions against ignition are required. –Equations for the calculation of V z are included in BS EN :2009

6 An Agency of the Health and Safety Executive GASES

7 An Agency of the Health and Safety Executive Gases V z estimated from BS EN found to be 100 to 3000 times larger than values obtained from CFD Reported at Hazards XIX (Gant et al)

8 An Agency of the Health and Safety Executive GASES V z from BS EN is 2 – 3 orders of magnitude larger In every case V z from CFD is less than 0.1m 3

9 An Agency of the Health and Safety Executive GASES BS EN calculation is based on premise that ratios: Actual ventilation rate / Ventilation rate required to dilute gas escape to specified level And Enclosure volume / V z Are equal. This has no scientific basis.

10 An Agency of the Health and Safety Executive GASES 2006 Steel works sorted Natural gas industry subject to DSEAR –Unable to comply with their own code IGEM/SR/25 –No lower pressure limit for zone 2 Unable to justify application of NE zoning Unable to reach agreement with HSE on threshold for zoning low pressure installations BS EN methodology in doubt

11 An Agency of the Health and Safety Executive GASES Natural Gas - Joint Industry Project –HSL report RR630 and Hazards XXI paper IGEM/SR/25 revised to include Zone 2 NE, published 2010

12 An Agency of the Health and Safety Executive OTHER GASES

13 An Agency of the Health and Safety Executive OTHER GASES QUADVENT Based on an integral gas jet model Well established scientific credibility Full mathematical derivation published 2011, (Webber et al)

14 An Agency of the Health and Safety Executive QUADVENT Unchoked flow will result if: 1.9 where P is the gas storage pressure and P a is atmospheric pressure.

15 An Agency of the Health and Safety Executive QUADVENT When the flow is choked (sonic) it is necessary to define a pseudo source hole radius r s, where r o is the orifice radius

16 An Agency of the Health and Safety Executive QUADVENT V z = ρ b (kg/m 3 ) is the density of the background (which normally approximates to that of air) ρ s (kg/m 3 ) is the density of the source gas α is the entrainment coefficient (recommended value 0.05) x b (v/v) is the background concentration x crit (v/v) is the concentration of interest (50% LEL for secondary releases)

17 An Agency of the Health and Safety Executive QUADVENT The background concentration of flammable gas x b in the enclosure is q 1 (m 3 /s) is the ventilation rate q s (m 3 /s) is the source gas volume flowrate ε is the efficiency of background mixing (see below). The leak rate q s can be derived from standard methods for the estimation of leak flowrates. Appropriate methods are included in BS EN :2009, Annex A.

18 An Agency of the Health and Safety Executive QUADVENT Outdoors there is zero background concentration, x b =0, and the background density is that of pure air

19 An Agency of the Health and Safety Executive QUADVENT The axial distance z to a concentration x zone may be derived as an approximation to the zone extent. An appropriate value of x zone should be chosen. BS EN uses 100% LEL for example.

20 An Agency of the Health and Safety Executive VALIDATION Quadvent has been validated against detailed CFD simulations which themselves have been validated against experimental data. The validation data includes simulations of a range of flammable gas release rates in enclosures of various sizes at a range of different ventilation rates. All of the simulations are for unobstructed releases of methane in a ventilation controlled chamber. The agreement between the QUADVENT model and the CFD simulations is surprisingly good considering how simple the QUADVENT calculation is.

21 An Agency of the Health and Safety Executive VALIDATION

22 An Agency of the Health and Safety Executive ENCLOSURE VENTILATION The ventilation rate of an enclosure is a key input to an area classification assessment. Forced ventilation rates can be established from design or equipment specifications. The natural ventilation rate will vary through time as it is strongly influenced by the weather conditions. Simple approaches for the estimation of ventilation rates, suitable for use as part of HAC methodologies, are therefore required.

23 An Agency of the Health and Safety Executive ENCLOSURE VENTILATION BS5925:1991 contains methods for very simple enclosures A spreadsheet containing a simple model of wind and buoyancy driven ventilation is available from the Chartered Institution of Building Services Engineers (CIBSE) COMIS or CONTAM multizone models Experimental measurements, or CFD simulations Quadvent contains an estimation method

24 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Hole size Local congestion and confinement –Ventilation efficiency factor ε Safety factor Sub-chambers Pressure V z value for small enclosures Availability of ventilation Background concentration Validation limited to methane

25 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Hole size –V z is a function of the cube of the hole radius –V z must not be under-estimated –Minimum of 0.25mm 2 is recommended (except under specified circumstances) –Further guidance in Cox Lees and Ang

26 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Congestion and confinement –Guidance in IGEM/SR/25 Appendix 6 –Efficiency of mixing ε ε = 1 represents an unobstructed release ε = ½ represents a moderate degree of obstruction ε = represents a significant obstruction to the ventilation flow –Over 100 m 3 verify local ventilation effectiveness with smoke tests etc.

27 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Safety factor –To allow for uncertainty –Apply a factor of 2 to estimated ventilation rate or –Ensure hole size is conservative

28 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Sub chambers –Compute V z for the volume of the sub-chamber with reduced ventilation rate or –Set V z to sub-chamber volume

29 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Pressure –Limit NE zones to systems at less than 10 barg –Limit to 20 barg based on risk assessment taking the consequences of ignition, i.e. the risk of injury, into account

30 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS V z size criterion –For enclosure volumes of less than 10m 3 the criterion of 0.1 m 3 for V z should be reduced to 1% of the enclosure volume. This constraint is taken from BS EN :2009

31 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Ventilation availability –The guidance in BS EN :2009 should be observed NE zones not allowed if availability is poor

32 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Background concentration –Suggested limit 25% LEL –Under consideration for inclusion in software –Manual check

33 An Agency of the Health and Safety Executive QUADVENT CONSTRAINTS Validation –Whilst this methodology is valid for all gases, it should be noted that the validation of the criterion of 0.1 m 3 for the value of V z leading to an NE classification has only been carried out for natural gas.

34 An Agency of the Health and Safety Executive QUADVENT EXAMPLES Outdoor butane gas pipework, secondary releases P = 4.5 bara Hole size = 0.25mm2 V z, m 3 Zone BS EN : Zone 2 QUADVENT0.0021Zone 2 NE

35 An Agency of the Health and Safety Executive QUADVENT EXAMPLES Natural gas plant room P = 76 mbarg Hole size = 2.5mm 2 V z m 3 Zone BS EN : Zone 2 IGEM/SR/25N/AZone 2 NE QUADVENT0.025Zone 2 NE EI 15N/AZone 1

36 An Agency of the Health and Safety Executive DEMONSTRATION

37 An Agency of the Health and Safety Executive CONCLUSIONS BS EN :2009 –Arbitrary results –No scientific foundation –V z up to 3 orders of magnitude too high QUADVENT –Scientific basis –Often reduces zoning requirements –Capital and maintenance costs of protected equipment can be restricted to genuine risks

38 An Agency of the Health and Safety Executive FUTURE DEVELOPMENTS UNDER CONSIDERATION Software – now available for gases (See leaflet) Flashing liquids (LPG, Ammonia) (Project in progress) Plumes Impingement Liquid pools Gas mixtures Mist (JIP in progress)

39 An Agency of the Health and Safety Executive REFERENCES BS EN :2009, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas. Area classification code for installations handling flammable fluids, Model Code of safe practice, IP 15 3 rd edition, The Energy Institute, Cox, A.W., Lees, F. P. and Ang, M. L., Classification of Hazardous Locations, I Chem E, Area classification for secondary releases from low pressure natural gas systems, HSL Research Report RR630.

40 An Agency of the Health and Safety Executive REFERENCES 2 Ventilation theory and dispersion modelling applied to hazardous area classification, D.M. Webber, M.J. Ivings and R.C. Santon, Journal of Loss Prevention in the Process Industries 24 (5) September 2011, Gant, S.E., Ivings, M.J., Jones, A., and Santon, R., Hazardous Area Classification of Low Pressure Natural Gas Systems using CFD Predictions. Hazards XIX, Manchester, 2006 New Methods for Hazardous Area Classification for Explosive Gas Atmospheres, R.C. Santon, M.J.Ivings, D.M. Webber and A Kelsey, Hazards XXIII, Southport 2012

41 An Agency of the Health and Safety Executive ACKNOWLEDGEMENTS Mat Ivings - HSL David Webber - HSL Adrian Kelsey- HSL HSE HSL


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