UKELG 50 th ANNIVERSARY MEETING RECENT DELOPMENTS IN AREA CLASSIFICATION FOR GASES

www.hsl.gov.ukAn Agency of the Health and Safety Executive

www.hsl.gov.ukAn Agency of the Health and Safety Executive

UKELG 50th ANNIVERSARY MEETINGRECENT DELOPMENTS IN AREA

CLASSIFICATION FOR GASES

ROGER SANTON, HEALTH & SAFETY LABORATORY, BUXTON

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


CURRENT STANDARDS

• 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.


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


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


GASES


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)


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 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.


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 60079-10-1 methodology in doubt


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


OTHER GASES


OTHER GASES

• “QUADVENT”

• Based on an integral gas jet model

• Well established scientific credibility

• Full mathematical derivation published 2011, (Webber et al)


QUADVENT

• Unchoked flow will result if:

aP

P1.9

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

9.15.010

as P

Prr


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)

32/33 1

16

9

bcrit

b

s

bs

xx

xr


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 60079-10-1:2009, Annex A.

1q

qx sb


QUADVENT

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

32/33 1

16

9

crits

asZ x

rV


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.

bzone

zones

xx

xrz

1

b

s

2


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

Figure 1 Predictions from CFD (filled symbols) and the integral model QUADVENT (hollow symbols) for Vz plotted against the concentration at the

ventilation outlet.


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


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



• 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– 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.



• Safety factor– To allow for uncertainty– Apply a factor of 2 to estimated ventilation rate

or– Ensure hole size is conservative



• Sub chambers– Compute Vz for the volume of the sub-chamber with

reduced ventilation rate

or

– Set Vz to sub-chamber volume



• 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



• 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 60079-10-1:2009



• Ventilation availability– The guidance in BS EN 60079-10-1:2009

should be observed • NE zones not allowed if availability is poor



• Background concentration– Suggested limit 25% LEL– Under consideration for inclusion in

software– Manual check



• 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 60079-10-1:2009

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 60079-10-1:2009

183 Zone 2

IGEM/SR/25 N/A Zone 2 NE

QUADVENT 0.025 Zone 2 NE

EI 15 N/A Zone 1


DEMONSTRATION


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


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 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.


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, 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


ACKNOWLEDGEMENTS

• Mat Ivings - HSL

• David Webber - HSL

• Adrian Kelsey- HSL

• HSE

• HSL

Documents

UKELG 50 th ANNIVERSARY MEETING RECENT DELOPMENTS IN AREA CLASSIFICATION FOR GASES