UKELG 50th ANNIVERSARY MEETING
RECENT DELOPMENTS IN AREA
CLASSIFICATION FOR GASES
ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON
www.hsl.gov.uk
www.hsl.gov.uk
AnAn
Agency
of theof
Health
and Safety
Executive
Agency
the Health
and
Safety
Executive
CONTENTS
• Current standards
• Previous work on gases
• QUADVENT
• Natural ventilation estimation
• Constraints
• Examples
• Demonstration
• Future developments
• References
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CURRENT STANDARDS
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•
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BS EN 60079-10-1:2009, 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.
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CURRENT STANDARDS
• BS EN 60079-10-1:2009, Electrical
apparatus for explosive gas
atmospheres – Part 10: Classification of
hazardous areas.
– Zone definitions
– Source terms
– Vz
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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 60079-10-1:2009
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GASES
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Gases
• Vz estimated from BS EN 60079-10-1
•
found to be 100 to 3000 times larger
than values obtained from CFD
Reported at Hazards XIX (Gant et al)
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GASES
Vz from BS EN is 2 – 3 orders of
magnitude larger
In every case Vz from CFD is less
than 0.1m3
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GASES
• BS EN 60079-10-1 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.
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GASES
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•
•
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 60079-10-1 methodology in doubt
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GASES
• Natural Gas - Joint Industry Project
2006-2007
– HSL report RR630 and Hazards XXI paper
• IGEM/SR/25 revised to include Zone 2
NE, published 2010
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OTHER GASES
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OTHER GASES
• “QUADVENT”
• Based on an integral gas jet model
• Well established scientific credibility
• Full mathematical derivation published
2011, (Webber et al)
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QUADVENT
• Unchoked flow will result if:
P
Pa
1.9
where P is the gas storage pressure and
Pa is atmospheric pressure.
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QUADVENT
• When the flow is choked (sonic) it is
necessary to define a pseudo source
hole radius rs, where ro is the orifice
radius
P

rs  r0 1  0.5  1.9 
 Pa

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QUADVENT
Vz =
•
•
•
•
•
9 r
16
3
s
 b

 s



3/ 2
 1  xb

 xcrit  xb



3
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)
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QUADVENT
The background concentration of flammable gas
xb in the enclosure is
xb

qs
q1
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 60079-10-1:2009, Annex A.
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QUADVENT
Outdoors there is zero background
concentration, xb=0, and the background
density is that of pure air
9 r
VZ 
16
3
s
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 a

 s



3/ 2
 1

 xcrit



3
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
60079-10-1 uses 100% LEL for example.
z
rs

1  x zone 
x zone  xb 
s
  2
b
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VALIDATION
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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.
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VALIDATION
Figure 1 Predictions from CFD (filled symbols) and the integral model
QUADVENT (hollow symbols) for Vz plotted against the concentration at the
ventilation outlet.
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ENCLOSURE VENTILATION
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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.
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ENCLOSURE VENTILATION
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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
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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
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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
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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.
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QUADVENT CONSTRAINTS
• Safety factor
– To allow for uncertainty
– Apply a factor of 2 to estimated ventilation rate
or
– Ensure hole size is conservative
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QUADVENT CONSTRAINTS
• Sub chambers
– Compute Vz for the volume of the sub-chamber with
reduced ventilation rate
or
– Set Vz to sub-chamber volume
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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
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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 6007910-1:2009
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QUADVENT CONSTRAINTS
• Ventilation availability
– The guidance in BS EN 60079-10-1:2009
should be observed
• NE zones not allowed if availability is poor
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QUADVENT CONSTRAINTS
• Background concentration
– Suggested limit 25% LEL
– Under consideration for inclusion in
software
– Manual check
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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.
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QUADVENT EXAMPLES
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•
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Outdoor butane gas pipework, secondary
releases
P = 4.5 bara
Hole size = 0.25mm2
Vz, m3
Zone
BS EN 60079- 0.63
10-1:2009
Zone 2
QUADVENT
Zone 2 NE
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0.0021
QUADVENT EXAMPLES
• Natural gas plant room
• P = 76 mbarg
• Hole size = 2.5mm2
Vz m3
Zone
BS EN
183
60079-101:2009
IGEM/SR/25 N/A
Zone 2
QUADVENT 0.025
Zone 2 NE
EI 15
Zone 1
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N/A
Zone 2 NE
DEMONSTRATION
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CONCLUSIONS
• BS EN 60079-10-1:2009
– 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
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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)
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REFERENCES
•
•
•
•
BS EN 60079-10-1:2009, 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.
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REFERENCES 2
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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, 612-621
http://dx.doi.org/10.1016/j/jlp.2011.04.002
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
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ACKNOWLEDGEMENTS
• Mat Ivings - HSL
• David Webber - HSL
• Adrian Kelsey- HSL
• HSE
• HSL
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