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

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

3. 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.

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

5. 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

6. GASES

7. 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)

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

9. 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.

10. 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. 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

12. OTHER GASES

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

14. 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.

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

16. 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)

17. 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.

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

19. 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.

20. 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. VALIDATION

22. 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. 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. 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

25. 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

26. 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.

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

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

29. 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. 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

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

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

33. 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.

34. 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

35. 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

36. DEMONSTRATION

37. 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

38. 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. 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.

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