GROWTH – DEDICATED CALL – 1/00
TOPIC III.1
Mechanical Ignition Hazards in Potentially Explosive Gas and Dust
Atmospheres
1. CONFORMITY WITH THE WORK PROGRAMME
This topic falls under the Competitive and Sustainable Growth Programme, generic
activity Measurement and Testing. Specifically, it is related to Objective GROW-20006.2.1 Methodologies to support standardisation and Community policies for which
expressions of interest have been called.
Research on mechanical ignition hazards is required by Working Group 2 (Equipment
for use in potentially explosive atmospheres) of CEN TC305 (Potentially explosive
atmospheres-Explosion prevention and protection) working on standards for nonelectrical equipment intended for use in potentially explosive atmospheres. The
working group is preparing a set of harmonised standards to support the essential safety
requirements of the ATEX Directive 94/9/EC
Its aim is the development and validation of a testing method for inclusion in a
European harmonised standard, and the specification of safety criteria needed to assess
the safety of equipment.
2. KEYWORDS
Explosive atmospheres, ignition risk, hazards, ATEX.
3. SUMMARY OF OBJECTIVES AND JUSTIFICATION
Research is needed to provide data that can be used to estimate the probability of
frictional and mechanical ignitions in industrial plant. The guidance currently available
is inadequate . The aims are to:
(a) rank combustible materials in the order of their ignitability by frictional means, by
reference to standard ignitability characters,
(b) measure the energy or power necessary to ignite explosive atmospheres or dust
deposits,
measure the power and energy available to cause ignition in plant where frictional
effects are likely, and produce methods for estimating the danger of an ignition and for
preventing the onset of ignitions.
DC 1/00/Topic III.1/ Pg 2
4. BACKGROUND
Analysis of industrial dust explosion incidents reveals that friction and mechanical
failure and flaming material are, in a substantial percentage of these incidents, the
known source of ignition. The evidence shows that a high fraction of recorded incidents
results from ignitions due to mechanical friction.
The basic European standard on explosion protection and prevention, EN1127-1
‘Explosion prevention and protection - Part 1: Basic concepts and methodology’ lists
ignition sources that manufacturers of equipment must consider. Mechanical ignition
sources are listed in this standard.
Although there is high potential for ignition by mechanical sources, there is no generally
accepted systematic method for estimating the ignition probability of mechanical
sources in specific operations or equipment, or how this probability varies with different
types of explosive environment. There is no over-arching guidance that can generally
be applied throughout industry, or enough data and information available to allow risk
assessments to be applied to industrial plant where frictional effects are a possibility.
Mechanically generated sparks take three forms:
Grinding sparks Friction sparks Impact sparks -
a quick contact (20-50 milliseconds) of two surfaces in
relative motion,
rubbing together over an extended time,
(0.5 – 20.0 seconds) of two surfaces in relative motion
single contact of two surfaces in relative motion.
Tests have demonstrated that mechanically generated sparks are capable of igniting
explosive atmospheres. The likelihood of ignition depends on the atmospheres, the
materials in contact and the relative velocity of the two surfaces.
The important parameters that determine the likelihood of ignition by a hot surface are
the temperature of the surface and its area. The most important hazard as regards hot
surfaces is the ignition of a dust layer. The practical dangers are that a smouldering or
burning layer can act either directly as an ignition source for a dust cloud or produce
burning material that can break away and ignite a dust cloud in another part of the plant.
Dust clouds can be ignited by various burning or smouldering ignition sources.
Currently, there is insufficient data to allow the development of acceptable harmonised
standards for CEN TC 305. The probability of ignition cannot be estimated for the
majority of explosive atmospheres, and this is especially true of dusty environments,
where the number of variables and likely ignition scenarios increase. Without this
fundamental information the ignition hazard of equipment can not be assessed. A
properly researched methodology for specifying accurately the safety requirements of
equipment where frictional effects may occur is an important need for the standards
making process.
DC 1/00/Topic III.1/ Pg 3
5. ECONOMIC AND SOCIAL BENEFITS
Potentially explosive atmospheres occur throughout industry, and explosions can affect
the health and safety of employees, and others beyond the workplace boundary. Apart
from flame and blast effects, explosions produce combustion products which can be
harmful to people and the environment if they are released into the atmosphere.
The essential safety requirements in the ATEX 100a Directive 94/9/EC indicate
strongly that avoidance of ignition sources is a fundamental aspect of establishing a
basis of safety when potentially explosive atmospheres are present. In the absence of a
satisfactory procedure and comprehensive data for estimating the frictional ignition risk,
safety has to be based on explosion protection that precise data on mechanical ignitions
would show to be unnecessary. It is more effective to prevent explosions than to rely on
explosion protection.
The proposed research will produce methods for evaluating the ignition potential of
frictional sources. The economic benefit is that manufacturers will be able to design
equipment that is reliably protected against the ignition hazard without placing undue
reliance on unnecessary protection techniques.
The research should be carried out at a European level because the content of the
proposal covers a number of technical areas, and a range of expertise is required. The
endeavours of several countries will ensure that the resulting guidance and test
specifications would have a wide acceptance for inclusion in standards.
6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES
The aim of the work is to develop criteria and test methods for assessment of ignition
hazards from mechanical ignition sources:
1. To establish the duration, extent and temperature of hot surfaces produced at a point
of contact by impact and rubbing friction, and to relate these to the friction
conditions (eg. surface materials, energy transfer, power loss, velocity at contact and
energy of impact).
2. To establish a test to characterise powder deposit sensitivity to heated surfaces of
the type found in 1. The dusts should be characterised by variation of the
ignitability characteristics, with a view to formulating families of dusts to which
specific sensitivities can apply.
3. To establish a test to characterise the sensitivity of explosive atmospheres to
a)
localised transient heated areas, with the characteristics of those produced in
1, and the ignition of dust deposits as described in 2.
b)
heated metal particles, with the characteristics of those produced in 1.
Explosive gas or vapour atmospheres should be characterised by the existing
classification scheme (EN 50 014: Electrical apparatus for potentially explosive
atmospheres – General Requirements). Dust atmospheres should be characterised
by variation of the ignitability parameters of the dust, with a view to formulating
families of dusts to which certain sensitivities can apply,
4. To establish the conditions in dust handling plant that would ensure avoidance of
ignition by frictional means.
DC 1/00/Topic III.1/ Pg 4
7. TIME SCALE
This research can be completed in four years. The results will then be exploited by
amendments to standards, and inclusion in the update of EN 1127-1.
Result Format
The results will be presented in the form of test methods and assessment methodologies
for inclusion in CEN European Standards.
8. IMPORTANT ADDITIONAL INFORMATION
Regular consultation with Working Group 2 of CEN TC 305 will be necessary
throughout this project, as will contact with manufacturers and users, and notified
bodies. It is recommended that such consultation be specified as part of the work
programme, to ensure that the outcomes are satisfactory to the standards-making bodies.