Safety Guideline C-2-0-2
IDENTIFICATION AND PREVENTION OF EXPLOSION HAZARDS
Abstract
This Safety Guideline is intended for use with all activities involving flammable substances which may give
rise to explosive atmospheres and hence explosion hazards. It describes the method to be used for the
assessment which must be recorded using the Safety Form C-2-0-1, Explosion Risk Assessment.
DOCUMENTATION
Reference documents:
[1] General Safety Instruction, GSI-C2, Explosive atmospheres (EDMS No. 1113408).
[2] Safety Guideline C-0-0-1, Identification of hazardous chemical agents (EDMS No. 1050101).
[3] Safety Guideline C-2-0-1, Explosion protection measures (EDMS No. 1097065).
[4] Safety Guideline C-2-0-3, Classification of hazardous areas (EDMS No. 1113399).
[5] Safety Form C-2-0-1, Explosion risk assessment (EDMS No. 1106970).
[6] Safety Form C-2-0-2, Classification of hazardous areas (EDMS No. 1113456).
Further reading:
-
Non-binding guide to good practice for implementing Directive 1999/92/EC “ATEX” (explosive atmospheres), European
Commission 2003.
CENELEC Report R-044-001 “Guidance and recommendations for the avoidance of hazards due to static electricity”.
Full flammability test of gases and gas mixtures in air, an industrial training report from CERN, S. Besnard, 1996.
Hazard and Operability Study of the design of the ATLAS RPC Gas System, TNO, TNO-MEP-R 98/436, November 1998.
Hazards and Operability Study of Thin Gap Chambers Gas System, ATLAS Muon Spectrometer, Part 2 Final Report, Hazmat Ltd,
September 2000.
TRACEABILITY
Reference No.: Safety Guideline C-2-0-2
EDMS No.: 1106529 ver. 1
Prepared by: J. Gulley, DGS/SEE
Date: 06/09/2010
Verified by: F. Angerand, C. Laverrière, DGS/SEE
Date: 24/01/2011
Approved by: I. Bejar Alonso, DG
Date: 01/02/2011
Distribution: Public document published in the Safety rules web site.
Rev. No.
Page 1 of 5
Date
Description of Changes
EDMS No. 1106529
1
INTRODUCTION
This Safety Guideline is intended for use with all activities involving flammable substances which may
give rise to explosive atmospheres and hence explosion hazards. It describes the method to be used
for the assessment which must be recorded using the Safety Form C-2-0-1, Explosion risk assessment
[5].
Are flammable substances present?
No
No explosion
protection measures
necessary
No
No explosion
protection measures
necessary
Yes
Can sufficient dispersal in air give rise to an
explosive mixture?
Yes
Where can explosive atmospheres occur?
Is the formation of a hazardous explosive
atmosphere possible?
No
No explosion
protection measures
necessary
Yes
Apply adequate
explosion protection
measures
Yes
Explosion protection measures necessary
Is the formation of hazardous explosive
atmospheres reliably prevented?
See Safety Guideline C-2-0-1
No
Further explosion protection measures necessary
Complete Safety Form,
C-2-0-2 – Classification
of hazardous areas
To what zones can the hazardous areas be
assigned?
Is the ignition of hazardous explosive
atmospheres reliably prevented?
See Safety Guideline C-2-0-1
Yes
Apply adequate and
proportionate mitigation
measures
For these two questions the answer
"Yes" can be given only if the
technical and organisational
measures already in place are such
that there is no need to take into
account the risk of occurrence of an
explosion, taking into account all
operating conditions and
reasonably foreseeable cases of
malfunction.
No
Further explosion protection measures necessary
Mitigate the detrimental effects of an
explosion
Record assessment using
Safety Form, C-2-0-1 –
Explosion risk assessment
See Safety Guideline C-2-0-1
Figure 1- Assessment flowchart for identification and prevention of explosion hazards
2
ARE FLAMMABLE SUBSTANCES PRESENT? (see also Safety Guideline C-0-0-1 [2])
Flammable substances that are present or used at the workplace must be identified. Examples are:
Flammable gases and gas mixtures (e.g. isobutane, methane, acetylene, carbon monoxide). See
also Annex 1;
N.B.: non-standard gas mixtures must be characterized by a certified, independent laboratory;
Flammable liquids (e.g. solvents, fuels, paints);
Dusts of combustible solids (e.g. wood, metals). For the assessment of dust explosion hazards a
competent outside firm must be used.
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EDMS No. 1106529
Other substances which are not readily combustible or flammable under normal conditions, but
which are explosive if the particle size is particularly small or the ignition energy is particularly
high (e.g. some aerosols).
Flammable substances include those labelled as:
Flammable (R10)
Highly Flammable (F and R11/R15/R17)
Extremely flammable (F+ and R12)
3
CAN SUFFICIENT DISPERSAL IN AIR GIVE RISE TO AN EXPLOSIVE ATMOSPHERE?
If the necessary degree of dispersion is attained and if the concentration of the flammable
substances in air lies within their explosion limits, an explosive atmosphere is present. By their very
nature, gases and vapours have a sufficient degree of dispersion.
4
WHERE CAN EXPLOSIVE ATMOSPHERES OCCUR?
The properties of the flammable substances and the environmental conditions must both be
considered to determine where explosive atmospheres can occur. The following must be considered:
Possible sources of leak (valves, pipework connections, gas mixing racks, etc.);
Density of gas or vapour compared to air (e.g. dense gases will sink and spread out and may travel
long distances before reaching an ignition source);
Ventilation, or lack of ventilation.
5
IS THE FORMATION OF A HAZARDOUS EXPLOSIVE ATMOSPHERE POSSIBLE?
If an explosive atmosphere may occur in such quantities as to require special protective measures, it
is described as a hazardous explosive atmosphere and the places in question are termed hazardous
areas.
Where the potential for an explosive atmosphere has been identified, it must be determined
whether this represents a hazardous explosive atmosphere. This will depend on the volume of the
flammable gas or vapour mixture and the harmful consequences of any ignition. In general, it can be
assumed that an explosion will cause substantial harm and that a hazardous explosive atmosphere is
present.
Exceptions to this rule may apply to work with very small quantities, for example in laboratories. In
such cases, it has to be decided whether the anticipated volume of explosive atmosphere is
hazardous.
Examples are:
A volume or more than 10 litres of explosive atmosphere in a congested area must always be
regarded as a hazardous explosive atmosphere, irrespective of the size of the room.
Explosive atmospheres should be regarded as potentially hazardous if they occupy more than one
ten thousandth of the room volume, e.g. only 5 litres in a room of 50 m3. This does not mean that
the whole room is to be classified into ATEX zones, but only the part in which the hazardous
explosive atmosphere can arise (N.B.: the consequences of any explosion could be inside or
outside the ATEX zone).
Evaporation of even small quantities of pressurized flammable liquids (e.g. liquefied propane) may
give rise to large quantities of flammable vapours. 1 litre of liquid propane when transformed into
gas and diluted in air to the lower explosive limit would give a 13,000 litre explosive atmosphere.
Page 3 of 5
EDMS No. 1106529
6
IS THE FORMATION OF HAZARDOUS EXPLOSIVE ATMOSPHERES RELIABLY PREVENTED?
If it is possible for a hazardous explosive atmosphere to be formed, explosion protection measures
are necessary. Safety Guideline C-2-0-1, Explosion protection measures [3] outlines the possible
technical prevention measures that must first be considered together with organisational measures
which are also described. The effectiveness of the measures adopted must be assessed and recorded
in the Safety Form C-2-0-1, Explosion risk assessment [5].
7
TO WHAT ZONES CAN THE HAZARDOUS AREAS BE ASSIGNED?
The places where a hazardous explosive atmosphere may form, the hazardous areas,
must be classified in terms of ATEX zones. Refer to General Safety Instruction,
GSI-C2, Explosive atmospheres [1], Safety Form C-2-0-2, Classification of hazardous
areas [6], and associated Safety Guideline C-2-0-3, Classification of hazardous areas [4].
8
IS THE IGNITION OF HAZARDOUS EXPLOSIVE ATMOSPHERES RELIABLY PREVENTED?
If the formation of hazardous explosive atmospheres is not preventable, measures must be taken to
remove, or if this is not possible, to control ignition sources inside the hazardous area. Safety
Guideline C-2-0-1, Explosion protection measures [3] describes some of the possible technical
measures for the avoidance of ignition sources, to be considered together with the organizational
measures which are also described.
Unless it is discounted that hazardous explosive atmospheres and sources of ignition will occur
simultaneously, explosion mitigation measures must also be taken as described in Safety Guideline
C-2-0-1, Explosion protection measures [3]. Otherwise proportionate mitigation measures must be
taken. The effectiveness of the measures adopted must be demonstrated and recorded in the Safety
Form C-2-0-1, Explosion risk assessment [5].
N.B.: for large scale installations or activities involving flammable gases which have major Safety
implications, more complex assessments may be required (e.g. HAZOP, FMEA, Fault Trees, Event
Trees) to assess the risk of accidents. For examples, refer to the two HAZOP Studies listed under
‘Further Reading’ at the front page of this document.
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EDMS No. 1106529
ANNEX 1
Table 1 - Tci values for some commonly used mixtures
Inert gas
Nitrogen (45)
Methane
(35)
Flammable gas
Ethane
Propane
(24)
(35)
Iso-butane
(25)
9.9%
4.94%
4.25%
4.1%
Carbon dioxide (30)
22.45%
9.09%
7.95%
7.95%
Helium (46)
11.86%
5.45%
4.41%
4.15%
Neon (40)
9.2%
4.37%
3.45%
3.26%
Argon (46)
6.15%
3.05%
2.76%
2.4%
Sulphur hexafluoride, SF6 (30)
50.4%
20.4%
20.4%
19.43%
CF4 (27)
33.4%
13.0%
11,76%
9.5%
R134A (27)
11.98%
7.14%
6.7%
5.75%
Tci = The concentration of a flammable gas in a mixture with an inert gas for which the mixture, when
released to air, is just not flammable.
Numbers in brackets after the name of gas (e.g. 35) refer to the purity of the gas used in the flammability
tests.
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EDMS No. 1106529