Chapter IM:
Hydrogen ignition mechanisms.
Prevention and mitigation of ignition
Compiled by Prof Vladimir Molkov (University of Ulster) and Dr Stuart
Hawksworth (HSL) from the free eBook “Fundamentals of hydrogen safety
engineering” (www.bookboon.com), and the HyFacts project Deliverable D2.2
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Contents
1
Introduction ........................................................................................................... 3
2
Minimum ignition energy of hydrogen and other fuels ........................................ 3
3
Ignition energy as a function of concentration ..................................................... 4
4
Ex-zone classification............................................................................................. 5
5
Electrostatic discharge ignition ............................................................................. 5
5.1
Types of electrostatic discharge..................................................................... 5
5.2
Spark from isolated conductor....................................................................... 5
5.2.1
Hydrocarbon-air mixtures ...................................................................... 5
5.2.2
Hydrogen-air mixtures ........................................................................... 6
5.3
Electrostatic spark ignition prevention .......................................................... 6
6
Mechanical ignition ............................................................................................... 6
7
Hot surface ignition ............................................................................................... 7
8
Adiabatic compression .......................................................................................... 7
9
Diffusion mechanism of spontaneous ignition ...................................................... 7
9.1
Spontaneous ignition of sudden release........................................................ 7
9.2
Physical mechanism ....................................................................................... 8
9.2.1
Instantaneous opening........................................................................... 8
9.2.2
Effect of burst disk finite opening time .................................................. 9
9.3
Spontaneous ignition in a T-shape Pressure Relief Device (PRD) ................ 10
9.3.1
Experiment ........................................................................................... 10
9.3.2
Simulation results for 1.35 MPa (no spontaneous ignition) ................ 10
9.3.3
Simulation results for 2.9 MPa (ignition) and 2.43 MPa ...................... 11
10
Control of ignition sources .................................................................................. 13
11
References ........................................................................................................... 14
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1 Introduction
In their 2007 paper Astbury and Hawksworth reported an analysis of hydrogen
ignitions and mechanisms, and highlighted that over the last century, there have been
reports of high pressure hydrogen leaks igniting for no apparent reason. They
underlined that although many leaks have ignited, there are also reported leaks where
no ignition has occurred. They stated that investigations of ignition often simply do not
stand up to rigorous scientific analysis, and that there are gaps in the knowledge of
the exact ignition mechanism for releases of hydrogen.
Some potential mechanisms will be considered in this lecture: electrostatic discharge,
mechanical ignition, hot surface ignition, adiabatic compression, spontaneous ignition
of sudden release (diffusion ignition), etc.
The paper by Astbury and Hawksworth used data from the Major Hazard Incident
Database Service of the Health and Safety Executive (UK), and revealed 81 incidents
involving releases of hydrogen with ignition. Of those, a delay between release and
ignition was reported for only 4 releases. In 11 cases, the source of ignition was
identified, but in the remainder, i.e. 86.3% of incidents, the source was not identified.
This contrasts with data on the non-hydrogen releases, where 1.5% did not ignite, and
only 65.5% of ignition sources were not identified.
It is worth noting that since this is a Major Hazard Incident Database, releases of
hydrogen which simply dispersed and did not involve fire, explosion, are not recorded,
so the non-ignition being reported as zero is not necessarily an indication that all
hydrogen releases ignited (Astbury and Hawksworth, 2007).
Potential ignition sources/mechanisms include:
 Electric sparks (static charges, short circuiting, fuse tripping, contactors)
 Adiabatic compression (pressure increase)
 Mechanical sparks (grinding, impact)
 Ionizing radiation (radioactivity)
 Electromagnetic radiation
 Ultrasonic radiation
 Light (laser / flash)
 Chemical reactions
 Metallic particles
 Hot surfaces
 Explosives
 Flames
2 Minimum ignition energy of hydrogen and other fuels
Minimum ignition energy (MIE) of flammable gases and vapours is the minimum value
of the electric energy, stored in the discharge circuit with as small a loss in the leads
as possible, which (upon discharge across a spark gap) just ignites the quiescent
mixture in the most ignitable composition.
For a given mixture composition the following parameters of the discharge circuit must
be varied to get the optimum conditions: capacitance, inductivity, charging voltage, as
well as shape and dimensions of the electrodes and the distance between electrodes.
MIE of hydrogen in air is as low as 0.017 mJ, and MIE of hydrogen in oxygen is more
than an order of magnitude lower – only 0.0012 mJ, i.e. 1.2 J (!).Hydrogen has such
low MIE that it is often difficult to determine the exact mechanism and the cause of an
ignition when it occurs.
The minimum ignition energy of hydrogen is smaller than other motor fuels:
 Hydrogen
0.017 mJ
 Methane
0.28 mJ
 Propane
0.25 mJ
 Gasoline
0.23-0.46 mJ
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
Methanol
0.14 mJ
A weak spark caused by the discharge of static electricity from a human body may
ignite these fuels in air.
3 Ignition energy as a function of concentration
Ignition energy is a function of hydrogen concentration in air (Figure IM1).
Figure IM1. Ignition energy as a function of hydrogen concentration in air
(Schmidtchen, 2009).
Figure IM1 demonstrates that an ignition source with energy 20 J = 0.020 mJ (red
horizontal line) will ignite hydrogen-air mixtures with hydrogen concentration in the
range 15%-35% by volume; and an ignition source with energy 1000 J = 1 mJ energy
(blue line) – in the range of concentrations about 6%-64% by volume.
At the limits of flammability the ignition energy is somewhat similar for these gases
(Figure IM2). It is quite high compared to the minimum ignition energy (MIE) yet many
sources would provide this level of energy.
Figure IM2. Ignition energy as a function of flammable gas concentration in air
(Schmidtchen, 2009).
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Source with ignition energy 0.24 mJ (green horizontal dash line) will not ignite
methane- and propane-air mixtures at any composition yet this source will ignite
hydrogen-air mixtures in the range of about 6.5%-58% by volume of hydrogen.
4 Ex-zone classification
Description of ex-zone classification and frequency of leaks (in hours per annum) are
shown in Table IM1.
Table IM1. Ex-zone description. (Health and Safety Executive 2012)
Zone
Description
Frequency
0
An area where an explosive gas atmosphere is
present continuously or for long periods
>1000 h/a
1
An area where an explosive gas atmosphere is
likely to occur in normal operation
>10 h/a but <1000 h/a
2
An area where an explosive gas atmosphere is
not likely to occur during normal operation, or if
it occurs will only exist for a short period
<10 h/a
No zone
Safe area
No
Ex-Atmosphere
present at any time
Equipment used in a zone 0 area has to be intrinsically safe. The equipment has to be
certified by a notified body in order to get it marking. Regular maintenance will also be
required to ensure that the equipment remains intrinsically safe.
5 Electrostatic discharge ignition
5.1
Types of electrostatic discharge
Three main types of electrostatic discharge are: spark, brush and corona.
Spark discharge is single plasma channel between the high potential conductor and
an earthed conductor.
Brush discharge is a discharge between a charged insulator and a conducting
earthed point. They are characterised by many separate plasma channels, combining
at the conductor. As the charged surface is a non-conductor, a capacitance, hence
energy cannot be determined.
Corona discharges are silent, usually continuous discharges with a current but no
plasma channel. A corona discharge is able to ignite a hydrogen–air mixture without
there being a discrete spark. The voltage required for a corona depends on tip-radius.
The prevention of ignition of hydrogen deliberately vented to atmosphere has been
applied before by using a polished toroidal ring at the end of the vent (tip radius is
large). In practice, corrosion and dirt deposits on such rings are effectively small
radius protrusions.
5.2
Spark from isolated conductor
5.2.1
Hydrocarbon-air mixtures
The energy of a spark discharge is E=CV2/2, where C is the capacitance of the item,
and V is the potential (voltage). A person has a capacitance of about 100 pF. Taking a
typical MIE of 0.29 mJ for a hydrocarbon-air mixtures, then the voltage required to
produce a spark of sufficient energy to ignite is V=(2E/C)1/2, i.e. 2408 V. For air, the
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dielectric strength is about 30 kV/cm. So, the gap required between the charged
conductor and the earthed point for breakdown to occur would be 2400/30=0.08 cm or
0.8 mm. People typically cannot feel an electrostatic shock of less than about 1 mJ, so
would be unaware of the potential to ignite a hydrocarbon–air mixture (and hydrogenair mixtures – see next section).
5.2.2
Hydrogen-air mixtures
If hydrogen is now considered, the corresponding voltages and gaps are greatly
reduced. The dielectric strength of hydrogen atmospheres is only 17.5 kV/cm. Its
quenching gap is only 0.64 mm (not 2-3 mm as for hydrocarbons). The variation of
dielectric strength with concentration of hydrogen in air mixtures is unknown, but as a
simple case, it could be assumed to be linear within a specified narrow concentration
range, so the theoretical dielectric strength of an about stoichiometric mixture of 30%
of hydrogen in air would be (0.3x17.5 + 0.7x30) = 26.25 kV.
With a quenching distance of 0.64 mm and a dielectric strength of 26.25 kV/cm the
breakdown potential would correspond to a voltage of 26.25x0.064=1.68 kV (2 kV can
be generated easily without people being aware of it), which corresponds to a stored
energy of 0.5x[100x10−12]x[1.68x103]2, i.e. 0.141 mJ. This is more than sufficient to
ignite the stoichiometric hydrogen–air mixture (requires 0.017 mJ).
5.3
Electrostatic spark ignition prevention
Hydrogen belongs to the flammable gas Group IIC. European Committee for
Electrotechnical Standardization (CENELEC, 2003) restrictions on widths of narrow
materials depending on zones and gas categories are given in Table IM2.
Table IM2. CENELEC (2003) restrictions on widths of narrow materials.
Zone
0
1
2
Group IIA
Maximum area, cm
Group IIB
Group IIC
0.3
3.0
No limit
0.3
3.0
No limit
0.1
2.0
No limit
CENELEC (2003) restrictions on chargeable surface depending on zones and gas
categories are shown in Table IM3. This document restricts areas of insulating
materials that may become charged, and limits the maximum charge that can be
transferred from the surface in the form of a brush discharge (maximum tolerable
charge transferred for hydrogen is 10 nC).
Table IM3. CENELEC (2003) restrictions on chargeable surface depending on zones
and gas categories.
Zone
0
1
2
Group IIA
Maximum area, cm2
Group IIB
Group IIC
50
100
No limit
25
100
No limit
4
20
No limit
6 Mechanical ignition
The key properties of burning metal particles or sparks that are relevant to their ability
to cause ignition of a flammable atmospheres are:
 Size
 Material
 Velocity
 Temperature
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

Number
Combustion rate and time
There is a metal-to-metal contact pressure and relative velocity threshold for spark
production during impact, rubbing or grinding. Above the threshold metal particles are
lost from the weaker of the two materials. Generally, particles are only produced when
the relative velocity between the two surfaces exceeds 1 m/s.
7 Hot surface ignition
The lowest temperatures for ignition are associated with large volumes and surface
areas. Hot surface ignition temperature is a function of characteristic size of hot
surface (Figure IM3). Hydrogen is represented by the curve “Group IIC”.
Figure IM3. Hot surface ignition temperature as a function of characteristic size of hot
surface.
Catalytic surface, e.g. platinum, has a dramatic effect on the ignition temperature:
ignitions are reported at temperatures as low as 70°C (standard AIT is 510°C).
8 Adiabatic compression
The ideal gas, when compressing it at constant entropy, would increase the pressure
due to the compression in accordance with the relationship PV =const. It can be
shown with use of the equation of state for the ideal gas that the following relationship
is valid as well TV -1=const. For example, for compression ratio V1/V2=28 at the initial
temperature of 293.15 K (=1.39) this theoretical relationship gives an increase of
temperature to T2=T1(V1/V2) -1=1075.2 K, i.e. the temperature rise is 782 K.
However, in experiments conducted by Pan et al. (1995), the actual measured
temperature realised by a compression ratio of 28 times was only 149 K. Based on
this, Astbury and Hawksworth (2007) concluded that isentropic compression ignition is
unlikely to occur in practice.
9 Diffusion mechanism of spontaneous ignition
9.1
Spontaneous ignition of sudden release
Figure IM4 shows hydrogen spontaneous ignition pressure as a function of pipe length
in three series of tests with flat burst disks. In these experiments hydrogen at elevated
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pressure was separated from air in a pipe at atmospheric pressure by a flat burst disk.
In experiments performed by Golub et al. (2007) hydrogen and air had the same initial
temperature. In tests by Pinto et al. (2007) hydrogen was compressed just before the
burst disk rupture by a piston. This pre-heating by compression explains why
spontaneous ignition happens at a shorter pipe length. Contrary, in experiments by
Mogi et al. (2008) longer pipe was required for ignition at fixed hydrogen storage
pressure. Explanation of this pipe increase for ignition at particular pressure is in the
use of aqueous Na2CO3 solution on internal pipe wall to visualise flame in this series
of tests.
Figure IM4. Storage pressure, which leads to spontaneous ignition of hydrogen during
sudden rupture of burst disk separating hydrogen from air in a pipe, as a function of a
pipe length.
Lowest pressure for spontaneous ignition by diffusion mechanism observed in
experiments by Dryer et al. (2007) is 2.04 MPa. A convex burst disk was applied in
this experiment that provides shock wave reflection from the pipe wall and thus higher
temperature and better mixing conditions compared to the flat burst disk shock wave.
9.2
Physical mechanism
9.2.1
Instantaneous opening
The mechanism of spontaneous ignition by the diffusion mechanism is demonstrated
in Figure IM5. For simplicity, the simulations were performed in this case for
instantaneous opening of a burst disk separating hydrogen and air at the same initial
temperature but different initial pressure (hydrogen - 9.6 MPa; air – 0.1 MPa).
Temperature
H2
Air
Cold H2
Contact
surface
Flame
front
Heated
air
Shock
front
Igniti
on
Hydrogen
Rupture disk
location (x=0)
Figure IM5. Dynamics of spontaneous ignition (Bragin and Molkov, 2011):
temperature (left) and hydrogen mole fraction (right).
Figure IM5 shows the dynamics of temperature (left) and hydrogen mole fraction
(right) during the first 56 s after the burst disk instant opening (non-inertial rupture
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disk). At the initial moment t=0 the “burst disk” boundary separating high-pressure
hydrogen and atmospheric air, located at x=0, is instantaneously removed and a
shock wave propagates into air heating it up due to compression. The shock wave is
followed by a contact surface separating cold hydrogen and heated air. It is seen that
the thickness of the heated air layer between the shock front and the contact surface
increases in time (Molkov, 2012).
The shock wave propagates along the tube as a plane wave without any curvature
outside the boundary layer. The shape of the contact surface changes from plane to
slightly convex in the beginning due to non-slip conditions at the pipe walls. The
highest temperature is observed in the boundary layer due to velocity decrease
(stagnation conditions). Owing to counter diffusion of hydrogen and air the contact
surface, and heat diffusion from hot air to cold hydrogen through the contact surface
the spontaneous ignition occurs when critical conditions for chemical reaction are
reached. The chemical reaction propagates in the direction from the wall to the tube
axis as the contact surface propagates downstream. After 45 s from the start of the
process at distance 20 mm from the non-inertial “burst disk” location the combustion
occupies the whole cross-section area of the pipe. Initiation of the chemical reactions
in the boundary layer accelerates propagation of the reaction front along the wall
compared to the axial part leading to change of the convex shape of the contact
surface in the most of the tube cross-section to a slightly concave form.
9.2.2
Effect of burst disk finite opening time
Figure IM6 shows dynamics of temperature and hydrogen mole fraction at an initial
stage of hydrogen release into air in the mock up PRD with instant opening (left) and
inertial opening (right) of the rupture disk for initial pressure 1.35 MPa in the highpressure chamber with hydrogen (Bragin et al., 2011). The inertial burst disk opening
is accompanied by formation of an under-expanded jet from increasing with time open
area of the burst disk. A barrel-like structure can be seen for the case of inertial
membrane opening (Figure IM6, right).
Figure IM6. Dynamics of temperature and hydrogen mole fraction at the initial stage
following an instant rupture disk opening (two left graphs: temperature T, and mole
fraction of hydrogen H2) and inertial ten-step rupture disk opening (two right graphs)
for the 1.35 MPa high-pressure chamber case.
For the case of instant opening of an imaginary non-inertial membrane case the shock
heated zone in air (red colour on temperature graph) is almost three times thicker
compared to the inertial membrane rupture case at the same moment of time. It is
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obvious, that the inertial opening of the membrane generates more intense mixing of
hydrogen and air, including in the boundary layer of the axial channel walls. Maximum
temperature in the shock-heated air is 50 K lower for the inertial burst disk opening
case. Thus, modelling of the inertial burst disk opening is an important part of
predictive simulations of spontaneous ignition in complex geometries.
9.3
9.3.1
Spontaneous ignition in a T-shape Pressure Relief Device (PRD)
Experiment
The geometry of the experimental set up that includes a T-shaped channel mimicking
PRD is taken from Golub et al. (2010) and is shown in Figure IM7. The high pressure
system consists of a 210 mm long tube with 16 mm internal diameter (ID) followed by
a 280 mm long tube with 10 mm ID at the end of which a flat rupture disk, made of
copper or aluminium with cuts facilitating failure, is located followed by a mock-up
PRD (T-shaped channel) open to the atmosphere.
The PRD has a 48 mm long axial channel of 6.5 mm ID with a flat end, and two radial
channels on opposite sides of the axial channel to vent hydrogen to atmosphere. Each
radial channel has 6.25 mm length and 4 mm ID (distance from the end connected to
the atmosphere of one radial channel to the end of another is 19 mm). Radial
channels are flushed with the axial channel end wall and positioned so that the edge
of each side channel touches tangentially the flat end wall of the axial channel. The
burst disk opening was estimated by Golub et al. (2010) as about 10 microseconds.
To register spontaneous ignition a light sensor was installed along the axis of radial
channels of the PRD. Temperature behind the shock wave reflected from the flat end
wall was estimated by authors as more than twice higher than the temperature behind
the incident shock wave. Authors stated that when the initial pressure in high pressure
chamber did not exceed 1.2 MPa the light sensor did not record any signal, and when
pressure was 2.9 MPa the ignition was registered.
Figure IM7. The geometry and computational domain: 1 and 2 – high pressure tubes,
3 – mock-up PRD (T-shaped channel), 4 – burst disk, 5 – external computational
domain.
9.3.2
Simulation results for 1.35 MPa (no spontaneous ignition)
Figure IM8 is convenient to analyse a flow pattern and to get an insight into the
process of the spontaneous ignition in T-shaped channel.
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Figure IM8. Temperature dynamics of hydrogen release into the T-shaped channel at
storage pressure 1.35 MPa – no ignition (your ellow co– heated by shock wave air;
red – heated by reflected shock wave air; light blue in the pipe – flowing hydrogen;
dark blue – cooled during expansion to atmosphere hydrogen).
The first snapshot denoted “60 s” shows a contact surface between hydrogen at
close to room temperature (in light blue colour) and heated by pressure wave air (in
green-yellow) within the horizontal axial channel. The contact surface is inside the
axial channel at this moment and it is disturbed by Rayleigh-Taylor instability. Close to
the flat end of the axial channel there is a volume of air that is heated further by the
reflected shock (in red). New areas of heated air (in red) in radial channels are seen in
the next snapshot “63 s” along with the increasing volume of hot air at the axial
channel end. These new areas are probably due to the second reflection from a wall
of the radial channel that is opposite to the flan end of the axial channel. At time 67 s
“cold” hydrogen starts to exit the axial channel and flows along one side of the radial
channel. Hot air temperature in front of hydrogen reduces (snapshots from “67” to
“77”). There is a drop in heated air temperature when hot air exits the radial channel to
the atmosphere. Reflected shock is weak at storage pressure of 1.35 MPa and hot air
in flow following the reflected pressure wave cannot enter the axial channel (only to
small extent at the axial channel axis). At time 77 s hydrogen starts exit to
atmosphere and its temperature drops further down during this expansion (dark blue).
9.3.3
Simulation results for 2.9 MPa (ignition) and 2.43 MPa
Temperature and hydroxyl mole fraction profiles in the axial cross-section for a case
with initial hydrogen pressure of 2.9 MPa in the high-pressure chamber are presented
in Figure IM9. Figure IM10 shows temperature and hydroxyl mole fraction profiles in
the axial cross-section for a case with initial hydrogen pressure of 2.43 MPa in the
high-pressure chamber.
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Figure IM9. Temperature (top) and hydroxyl mole fraction (bottom) for 2.9 MPa
hydrogen storage pressure.
Figure IM10. Temperature (top) and hydroxyl mole fraction (bottom) for 2.43 MPa
hydrogen storage pressure.
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It is clear that ignition is not possible at location of the leading shock reflection on the
flat end of the axial channel since hydrogen is not supplied to this area yet. Once the
hydrogen flow diffracts around the edge from the axial into radial channels, it starts
mixing with heated by shocks air. This provides the necessary “fire triangle” conditions
(fuel, oxydiser, high temperature) for ignition of the hydrogen-air mixture. It can be
seen that the chemical reaction is initiated in both cases in the radial channel in a
location close to the upstream wall.
Larger high temperature regions can be observed for storage pressure 2.9 MPa
(Figure IM9) compared to pressure 2.43 MPa (Figure IM10). The difference becomes
obvious when combusting mixture is pushed outside. At pressure 2.9 MPa a semispherical cocoon of a heated to critical conditions flammable hydrogen-air mixture is
formed outside the PRD being followed by expanding hydrogen. Combustion is
initiated in a number of spots within the cocoon (see snapshots of temperature and
OH mole fraction at 79 µs). The concentration of hydrogen in these spots outside the
PRD just before the ignition is in the range 29-36% by volume. Thus, a conclusion can
be drawn that the ignition by the diffusion mechanism takes place in the most reacting
near stoichiometric hydrogen-air mixtures.
At storage pressure 2.43 MPa (Figure IM10) the combustion is rather weak, compared
to the case of pressure 2.9 MPa, and the reacting mixture is dragged with the adjacent
hydrogen flow, which leads to the elongation of combusting regions. Upon exit from
the radial channels of the PRD, the flow undergoes an expansion and hightemperature regions disappear thus ceasing reaction and hydroxyl (OH)
concentration, including the cooling role of expanding pure hydrogen jet on the
flammable mixture.
The whole process from combustion initiation to self-extinction (disappearance of high
temperature and OH mole fractions region, Figure IM10) takes a bit more than 10 μs.
This is sufficient to register the chemical reaction by a light sensor in the experiments.
This explains the uncertainty in interpretation of experimental observations by Golub
et al. (2010). Indeed, at pressure 2.9 MPa the combustion quite pronounced but at
pressure 2.43 MPa the spot of spontaneous ignition is weak and disappears due to
self-extinction. However, the light sensors still registered a signal during the
spontaneous ignition. Thus, in full compliance with experimental observations the
simulations confirmed the spontaneous ignition at storage pressure of 2.43 MPa
(which is not sustained).
Figures IM9 and IM10 demonstrate that the process is asymmetrical, i.e. the ignition in
the upper radial channel is weaker than in the lower channel. At pressure 2.9 MPa the
ignition survives the expansion from only the lower radial channel, while the upper part
is extinguished. This is sought due to the asymmetrical opening of the rupture disk in
the simulations. This result supports previous conclusion about the sensitivity of
spontaneous ignition to the burst disk rupture process that should be investigated
further along with effects of other factors such as air humidity, etc.
More details on spontaneous ignition in the T-shaped PRD could be found in (Molkov,
2012).
10 Control of ignition sources
There are a number of measures that can be undertaken in order to eliminate or at
less reduce the risk of ignition; the list below has been compiled by the Health and
Safety Executive (Health and Safety Executive, 2012):
 Using electrical equipment and instrumentation classified for the zone in which
it is located. New mechanical equipment will need to be selected in the same
way;
 Earthing of all plant/equipment;
 Elimination of surfaces above auto-ignition temperatures of flammable
materials being handled/stored (see above);
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







Provision of lightning protection;
Correct selection of vehicles/internal combustion engines that have to work in
the zoned areas;
Correct selection of equipment to avoid high intensity electromagnetic
radiation sources, e.g. limitations on the power input to fibre optic systems,
avoidance of high intensity lasers or sources of infrared radiation;
Prohibition of smoking/use of matches/lighters;
Controls over the use of normal vehicles;
Controls over activities that create intermittent hazardous areas, e.g. tanker
loading/unloading;
Control of maintenance activities that may cause sparks/hot surfaces/naked
flames through a Permit to Work System;
Precautions to control the risk from pyrophoric scale, usually associated with
formation of ferrous sulphide inside process equipment.
Control of static electricity might require change of material in workers’ overalls and
adoption of antistatic footwear. Physical separation of ignition sources, such as
welding, flames or hot working, and the explosive atmosphere is desirable (Nolan,
1999). However, the ambient wind conditions and layout of the plant greatly influences
the dispersion of a flammable gas cloud and hence physical separation might not
always be possible. In addition, battery powered devices, for example cameras and
mobile phones, and radio transmitters have also been identified as potential ignition
sources (Commonwealth of Australia, 2006).
What is not covered in the list above is to do with human factors and training and
supervision of personnel. The training should not just entail safe use of equipment, but
also an appreciation of the safety climate. The Health and Safety at Work etc Act of
1974 places responsibility for the health and safety on both the employer and the
employee/contractor (Her Majesty’s Government, 1974).
The permit to work system is the main control measure during commissioning or
maintenance, when the hazardous area classification might not be valid (Leroux,
2012). A Permit to Work system is not fool proof, if it not adhered to. There may also
be issues at the change of shift—is all relevant information passed on the on-going
shift team? An exhaustive assessment of the benefits and limitations of the permit-towork system have been considered (Daycock and Rew, 2004). Some of the issues
with the permit-to-work systems were: i) lack of planning of the operation, ii)
inadequate or permit-to-work system in place, and iii) permit-to-work system in places
but not followed correctly (Lees, 1996; Worsell, 1996).
A number of potential reasons for failure in controlling ignitions sources have been
presented (Daycock and Rew, 2004). In the case of electrical equipment, the
following reasons were presented: (Daycock and Rew, 2004): i) lack of or insufficient
maintenance so that the protection of the equipment fails, ii) temporary use of
equipment with the wrong rating for the zone where it is being used, and iii) changes
to the plant without updating the hazardous area classification.
Any measures put in place to control ignition have to be checked at regular intervals (if
appropriate), maintained correctly and replaced if faulty.
11 References
Astbury, GR and Hawksworth, SJ (2007) Spontaneous ignition of hydrogen leaks: A
review of postulated mechanisms. International Journal of Hydrogen Energy, vol. 32,
pp. 2178-2185.
Bragin, MV and Molkov, VV (2011) Physics of spontaneous ignition of high-pressure
hydrogen release and transition to jet fire, International Journal of Hydrogen Energy,
vol. 36, pp. 2589-2596.
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Bragin, MV, Makarov, DV and Molkov, VV (2011) Pressure limit of hydrogen
spontaneous ignition in a T-shaped channel. Proceedings of the 4th International
Conference on Hydrogen Safety, 12-14 September 2011, San Francisco, USA.
CENELEC (2003) Electrostatics – Code of practice for the avoidance of hazards due
to static electricity, PD CLC/TR 50404:2003.
Commonwealth of Australia (2006). Draft National Code of Practice For the Control of
Workplace Hazardous Chemicals,
http://www2.unitar.org/cwm/publications/cbl/ghs/Documents_2ed/D_National_Docume
nts_and_Legislation/267_Australia_ASCC_Code.pdf [accessed on 20 June 2012].
Daycock, J. H., and Rew, P. J. (2004). Development of a method for the determination
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