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# ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON

## Presentation on theme: "ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON"— Presentation transcript:

ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON
UKELG 50th ANNIVERSARY MEETING RECENT DELOPMENTS IN AREA CLASSIFICATION FOR GASES ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON

CONTENTS Current standards Previous work on gases QUADVENT Natural ventilation estimation Constraints Examples Demonstration Future developments References

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, 2005. IGEM/SR/25, Edition 2, Hazardous area classification of natural gas installations, Institution of Gas Engineers and Managers, 2010.

CURRENT STANDARDS BS EN :2009, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas. Zone definitions Source terms Vz

CURRENT STANDARDS Vz Hypothetical gas cloud volume
Mean concentration of 50% LEL (for secondary releases) Determines level of ventilation If 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

GASES

Gases Vz estimated from BS EN found to be 100 to 3000 times larger than values obtained from CFD Reported at Hazards XIX (Gant et al)

GASES Vz from BS EN is 2 – 3 orders of magnitude larger
In every case Vz from CFD is less than 0.1m3

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 / Vz Are equal. This has no scientific basis.

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

Natural Gas - Joint Industry Project 2006-2007
GASES Natural Gas - Joint Industry Project HSL report RR630 and Hazards XXI paper IGEM/SR/25 revised to include Zone 2 NE, published 2010

OTHER GASES

OTHER GASES “QUADVENT” Based on an integral gas jet model Well established scientific credibility Full mathematical derivation published 2011, (Webber et al)

Unchoked flow will result if:
QUADVENT Unchoked flow will result if: 1.9 where P is the gas storage pressure and Pa is atmospheric pressure.

QUADVENT When the flow is choked (sonic) it is necessary to define a pseudo source hole radius rs, where ro is the orifice radius

QUADVENT Vz = 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 gas α is the entrainment coefficient (recommended value 0.05) xb (v/v) is the background concentration xcrit (v/v) is the concentration of interest (50% LEL for secondary releases)

QUADVENT The background concentration of flammable gas xb in the enclosure is q1 (m3/s) is the ventilation rate qs (m3/s) is the source gas volume flowrate ε 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.

QUADVENT Outdoors there is zero background concentration, xb=0, and the background density is that of pure air

QUADVENT The 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.

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.

VALIDATION

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.

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

Local congestion and confinement Safety factor Sub-chambers Pressure
QUADVENT CONSTRAINTS Hole size Local congestion and confinement Ventilation efficiency factor ε Safety factor Sub-chambers Pressure Vz value for small enclosures Availability of ventilation Background concentration Validation limited to methane

QUADVENT CONSTRAINTS Hole size
Vz is a function of the cube of the hole radius Vz must not be under-estimated Minimum of 0.25mm2 is recommended (except under specified circumstances) Further guidance in Cox Lees and Ang

Congestion and confinement
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 = ⅓ represents a significant obstruction to the ventilation flow Over 100 m3 verify local ventilation effectiveness with smoke tests etc.

QUADVENT CONSTRAINTS Safety factor To allow for uncertainty
Apply a factor of 2 to estimated ventilation rate or Ensure hole size is conservative

QUADVENT CONSTRAINTS Sub chambers
Compute Vz for the volume of the sub-chamber with reduced ventilation rate or Set Vz to sub-chamber volume

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

QUADVENT 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

Ventilation availability
QUADVENT CONSTRAINTS Ventilation availability The guidance in BS EN :2009 should be observed NE zones not allowed if availability is poor

Background concentration
QUADVENT CONSTRAINTS Background concentration Suggested limit 25% LEL Under consideration for inclusion in software Manual check

QUADVENT 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.

QUADVENT EXAMPLES Outdoor butane gas pipework, secondary releases
P = 4.5 bara Hole size = 0.25mm2 Vz, m3 Zone BS EN :2009 0.63 Zone 2 QUADVENT 0.0021 Zone 2 NE

QUADVENT EXAMPLES Natural gas plant room P = 76 mbarg
Hole size = 2.5mm2 Vz m3 Zone BS EN :2009 183 Zone 2 IGEM/SR/25 N/A Zone 2 NE QUADVENT 0.025 EI 15 Zone 1

DEMONSTRATION

CONCLUSIONS BS EN 60079-10-1:2009 QUADVENT Arbitrary results
No scientific foundation Vz 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

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)

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 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.

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

ACKNOWLEDGEMENTS Mat Ivings - HSL David Webber - HSL Adrian Kelsey- HSL HSE HSL

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