Presentation on theme: "ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON"— Presentation transcript:
1ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON UKELG 50th ANNIVERSARY MEETING RECENT DELOPMENTS IN AREA CLASSIFICATION FOR GASESROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON
2CONTENTSCurrent standardsPrevious work on gasesQUADVENTNatural ventilation estimationConstraintsExamplesDemonstrationFuture developmentsReferences
3CURRENT STANDARDSBS 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, 2005.IGEM/SR/25, Edition 2, Hazardous area classification of natural gas installations, Institution of Gas Engineers and Managers, 2010.
4CURRENT STANDARDSBS EN :2009, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas.Zone definitionsSource termsVz
5CURRENT STANDARDS Vz Hypothetical gas cloud volume Mean concentration of 50% LEL (for secondary releases)Determines level of ventilationIf Vz is less than 0.1m3, ventilation is regarded as high and zone is classified Negligible Extent (NE) and no precautions against ignition are required.Equations for the calculation of Vz are included in BS EN :2009
7GasesVz estimated from BS EN found to be 100 to 3000 times larger than values obtained from CFDReported at Hazards XIX (Gant et al)
8GASES Vz from BS EN is 2 – 3 orders of magnitude larger In every case Vz from CFD is less than 0.1m3
9GASESBS EN calculation is based on premise that ratios:Actual ventilation rate / Ventilation rate required to dilute gas escape to specified levelAndEnclosure volume / VzAre equal.This has no scientific basis.
10GASES 2006 Steel works sorted Natural gas industry subject to DSEAR Unable to comply with their own code IGEM/SR/25No lower pressure limit for zone 2Unable to justify application of NE zoningUnable to reach agreement with HSE on threshold for zoning low pressure installationsBS EN methodology in doubt
11Natural Gas - Joint Industry Project 2006-2007 GASESNatural Gas - Joint Industry ProjectHSL report RR630 and Hazards XXI paperIGEM/SR/25 revised to include Zone 2 NE, published 2010
13OTHER GASES“QUADVENT”Based on an integral gas jet modelWell established scientific credibilityFull mathematical derivation published 2011, (Webber et al)
14Unchoked flow will result if: QUADVENTUnchoked flow will result if:1.9where P is the gas storage pressure and Pa is atmospheric pressure.
15QUADVENTWhen the flow is choked (sonic) it is necessary to define a pseudo source hole radius rs, where ro is the orifice radius
16QUADVENTVz =ρb (kg/m3) is the density of the background (which normally approximates to that of air)ρs (kg/m3) is the density of the source gasα is the entrainment coefficient (recommended value 0.05)xb (v/v) is the background concentrationxcrit (v/v) is the concentration of interest (50% LEL for secondary releases)
17QUADVENTThe background concentration of flammable gas xb in the enclosure isq1 (m3/s) is the ventilation rateqs (m3/s) is the source gas volume flowrateε is the efficiency of background mixing (see below).The leak rate qs can be derived from standard methods for the estimation of leak flowrates. Appropriate methods are included in BS EN :2009, Annex A.
18QUADVENTOutdoors there is zero background concentration, xb=0, and the background density is that of pure air
19QUADVENTThe axial distance z to a concentration xzone may be derived as an approximation to the zone extent. An appropriate value of xzone should be chosen. BS EN uses 100% LEL for example.
20VALIDATIONQuadvent 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.
22ENCLOSURE 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.
23ENCLOSURE VENTILATION BS5925:1991 contains methods for very simple enclosuresA 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 modelsExperimental measurements, or CFD simulationsQuadvent contains an estimation method
24Local congestion and confinement Safety factor Sub-chambers Pressure QUADVENT CONSTRAINTSHole sizeLocal congestion and confinementVentilation efficiency factor εSafety factorSub-chambersPressureVz value for small enclosuresAvailability of ventilationBackground concentrationValidation limited to methane
25QUADVENT CONSTRAINTS Hole size Vz is a function of the cube of the hole radiusVz must not be under-estimatedMinimum of 0.25mm2 is recommended (except under specified circumstances)Further guidance in Cox Lees and Ang
26Congestion and confinement QUADVENT CONSTRAINTSCongestion and confinementGuidance in IGEM/SR/25 Appendix 6Efficiency of mixing εε = 1 represents an unobstructed releaseε = ½ represents a moderate degree of obstructionε = ⅓ represents a significant obstruction to the ventilation flowOver 100 m3 verify local ventilation effectiveness with smoke tests etc.
27QUADVENT CONSTRAINTS Safety factor To allow for uncertainty Apply a factor of 2 to estimated ventilation rateorEnsure hole size is conservative
28QUADVENT CONSTRAINTS Sub chambers Compute Vz for the volume of the sub-chamber with reduced ventilation rateorSet Vz to sub-chamber volume
29QUADVENT CONSTRAINTS Pressure Limit NE zones to systems at less than 10 bargLimit to 20 barg based on risk assessment taking the consequences of ignition, i.e. the risk of injury, into account
30QUADVENT CONSTRAINTS Vz size criterion For enclosure volumes of less than 10m3 the criterion of 0.1 m3 for Vz should be reduced to 1% of the enclosure volume. This constraint is taken from BS EN :2009
31Ventilation availability QUADVENT CONSTRAINTSVentilation availabilityThe guidance in BS EN :2009 should be observedNE zones not allowed if availability is poor
32Background concentration QUADVENT CONSTRAINTSBackground concentrationSuggested limit 25% LELUnder consideration for inclusion in softwareManual check
33QUADVENT CONSTRAINTS Validation Whilst this methodology is valid for all gases, it should be noted that the validation of the criterion of 0.1 m3 for the value of Vz leading to an NE classification has only been carried out for natural gas.
34QUADVENT EXAMPLES Outdoor butane gas pipework, secondary releases P = 4.5 baraHole size = 0.25mm2Vz, m3ZoneBS EN :20090.63Zone 2QUADVENT0.0021Zone 2 NE
35QUADVENT EXAMPLES Natural gas plant room P = 76 mbarg Hole size = 2.5mm2Vz m3ZoneBS EN :2009183Zone 2IGEM/SR/25N/AZone 2 NEQUADVENT0.025EI 15Zone 1
37CONCLUSIONS BS EN 60079-10-1:2009 QUADVENT Arbitrary results No scientific foundationVz up to 3 orders of magnitude too highQUADVENTScientific basisOften reduces zoning requirementsCapital and maintenance costs of protected equipment can be restricted to genuine risks
38FUTURE DEVELOPMENTS UNDER CONSIDERATION Software – now available for gases (See leaflet)Flashing liquids (LPG, Ammonia) (Project in progress)PlumesImpingementLiquid poolsGas mixturesMist (JIP in progress)
39REFERENCESBS 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, 2005.Cox, A.W., Lees, F. P. and Ang, M. L., Classification of Hazardous Locations, I Chem E, 1990.Area classification for secondary releases from low pressure natural gas systems, HSL Research Report RR630.
40REFERENCES 2Ventilation 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, 2006New 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