Chapter RM:
Prevention and mitigation:
Hydrogen releases, detection and ventilation
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1.1
Control of flammability and explosion hazards
1.1.1
Overview of modes of hazard prevention and mitigation
Definition of a bow tie chart
A bow tie chart is a graphical representation of the safety measures which can be
implemented to control the risks associated to a hazardous event (see Figure 1). Two
kinds of safety measures can be distinguished:

Safety measures can act on the combination of causes of the hazardous
event, so as to prevent the hazardous event from happening - they are then
called prevention measures. Prevention measures are the specific measures
that reduce the likelihood of the event.

Safety measures can be aimed at mitigating (i.e. reducing) the consequences
of the hazardous event – they are then called mitigation measures.
Note: the hazardous event is the same as the “initial event” in the section on Design
for safety.
Mitigation
Prevention
Cause 1
Consequence 1
AND
Cause 2
Consequence 2
Cause 3
AND
Cause 4
OR
hazardous
event
Consequence 3
AND
Cause 5
Consequence 4
Condition 4
= safety measure
Figure 1: Application of safety measures to control risk associated to a hazardous
event
(Source: Air Liquide)
The bow tie chart allows for an exhaustive overview of all safety measures which
can be applied to prevent feared events from happening, or to limit their
consequences.
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Example: bow tie chart of hydrogen release
Hydrogen release is the main risk induced by hydrogen use and the hazardous event
most often feared in hydrogen systems. The bow tie chart of hydrogen release is a
graphical representation of all the safety measures related to hydrogen release (see
Figure 2).
Strong blast
effects
Confined
explosive
atmosphere
H2 release
Unconfined
explosive
atmosphere
Injury /
casualty
Flash fire
Blast effects
Loss of
leaktightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Escalation
Figure 2: Overview of the causes and consequences of a hydrogen release
(Source: Air Liquide)
Hydrogen release can be caused either by a loss of leak-tightness of the system,
or by an equipment failure1. Consequently, different hazardous phenomena may
occur, generally in combination:
Depending on the release properties and the presence of ignition sources, the
hydrogen release may ignite immediately or after a certain delay. In the latter case, if
the release takes place in a confined environment, an explosive atmosphere may build
up by accumulation.
In all cases, depending on the amount of flammable mixture, the mean hydrogen
concentration, and presence of factors accelerating combustion (such as repeated
obstacles, or pre-existing turbulence, such as that induced by the release itself),
ignition of the flammable mixture will result in a flash fire as well as overpressure
effects - i.e. an explosion - ranging from a simple noise to hazardous blast effects.
1
An equipment failure happens when the equipment cannot resist to excessive conditions, such as a too
high internal pressure.
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This explosion is likely to be more pronounced in a confined environment than in an
unconfined one due to the aggravating effect of accumulation. Destruction of the
confining structure (e.g. a building), and the associated kinetic effects2 is in itself likely
to be hazardous.
Once ignited, the release is very likely to produce a jet fire which persists as long as
the release itself.
Independently of the hazards of hydrogen, failure of an equipment containing
hydrogen under pressure is likely to result in kinetics effects.
All these consequences of hydrogen release can impact people - leading to injuries,
or/and impacting equipments. Minor events can turn into catastrophic ones
(escalation of the event) in the absence of appropriate safety measures.
Active and passive prevention measures can be taken, so as to prevent hydrogen
releases:

Equipment validation

Physical protection

Periodic leak test

Periodic equipment inspection.
All these prevention measures will be detailed in other sections of the report.
Passive
Equipment validation
Physical Protection
Strong blast
effects
Active
Periodic leak test
Passive
Equipment validation
Physical protection
Active
Periodic inspection
H2 release
Confined
explosive
atmosphere
Unconfined
explosive
atmosphere
Injury /
casualty
Flash fire
Blast effects
Loss of
leaktightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Escalation
Figure 3: Prevention measures related to hydrogen release
(Source: Air Liquide)
2
Kinetic effects always come along with the blast effects and fire, even if it is not represented on the Figure
2 for a better ease of reading.
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Active and passive mitigation measures can be taken:


Measures to mitigate the consequences of the hydrogen release:
o
Restricting hydrogen flow / using an excess flow valve (see section
1.1.2.4.1 and 1.1.2.4.2)
o
Detecting and isolating hydrogen (see section 1.1.4)
o
Detecting hydrogen and then shutting-down the power (see section
1.1.4)
o
Avoiding unnecessary confinement
o
Using natural ventilation (see section 1.1.5.4)
o
Using active ventilation (see section 1.1.5.5)
o
Designing the system with explosion vents (see section 1.1.6)
o
Avoiding ignition sources (see section 1.1.3)
Mitigation measures to protect people and equipments:
o
Implementing separation distances (see section 1.1.8)
o
Providing emergency response.
All these mitigation measures will be detailed in other sections of the report.
Passive
Avoid unnecessary
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
Passive
Flow restriction
Active
Detection and Isolation
Excess f low valve
H2 release
Confined
explosive
atmosphere
Passive
Passive
Explosion venting
Separation distance
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Escalation
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Passive
No ignition sources
Active
Detection and power
shut-down
Figure 4: Mitgation measures related to hydrogen release
(Source: Air Liquide)
A combination of measures including both prevention and mitigation need to be
applied to control flammability and explosion hazards, considering the reliability of
these measures with regards to the likelihood of the initiating event, in order to
achieve the expected level of safety. A single measure, such as preventing ignition
sources, is in general insufficient to achieve this objective.
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Reference:

European Industrial Gases Association, 2008, “Major Hazards”, IGC
Document 142/08/E
1.1.2
Prevention and mitigation of hydrogen releases
Unignited releases – summary
1.1.2.1
Hydrogen is either released as a pure plume or as a jet, depending on the source
flow rate and diameter. A hydrogen release is characterized by its Richardson
number Ri0:
Ri 0  g.
 a   0 R0
. 2
0
U0
Equation 1
with
U0: release velocity (m/sec)
R0: radius of the releasing orifice (m)
a: ambient density (kg.m-3)
0: released gas density (kg.m-3)
g: the gravitational acceleration (9,81 m.s-2)

When the Richardson number is larger than the unity: the momentum is
negligible compared to buoyancy effects; gravity effects are dominant. The
flow is then a pure plume.

When the Richardson number is lower than the unity: the momentum
dictates the mixing and overturning is induced. The flow is a jet.
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1.1.2.2
Physical extension of flammable mixture
Hydrogen releases might form flammable mixtures. In order to keep people and
equipment out of the physical extension of the flammable mixture, the distance from
the leak source where the hydrogen concentration drops to 4% 3 by volume
should be determined.
In the case of momentum-controlled jets, a similarity law has been developed to
calculate the axial hydrogen concentration decay:
C m x
d
 5.4 noz
x
  amb 


  noz 
1/ 2
Equation 2
Cm-x
fuel mass fraction at location x (% by mass)
dnoz
real nozzle diameter (m)
ρamb
density of the ambient air (kg/m 3)
ρH2
density of hydrogen in the nozzle (kg/m 3)
x
distance from the nozzle along the jet axis (m)
With the aid of nomograms (see Figure 5) based on this similarity law, the axial
distance to chosen hydrogen concentration can be determined once leak
diameter, hydrogen pressure and hydrogen temperature in the nozzle are known. See
additional explanations for the use of this nomogram in section
Error! Reference
source not found..
3
The UK regulator applies the 50% LFL as the lower threshold, which corresponds to
about 2% by volume - instead of 4% by volume.
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Figure 5: Nomogram based on the similarity law
(Saffers and Molkov, 2012)
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Prevention measures
1.1.2.3
Prevention measures related to hydrogen releases aim at preventing any equipment
failure and any loss of leak-tightness of the equipment.
Passive
Equipment validation
Physical Protection
Strong blast
effects
Active
Periodic leak test
Passive
Equipment validation
Physical protection
Confined
explosive
atmosphere
Injury /
casualty
Flash fire
Active
Periodic inspection
H2 release
Unconfined
explosive
atmosphere
Blast effects
Loss of
leaktightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Escalation
Figure 6: Prevention measures related to hydrogen release
(Source: Air Liquide)
1.1.2.3.1
Mechanical integrity
Prevention measures related to the mechanical integrity of hydrogen systems consist
(i) in validating equipment design through performance testing and (ii) in
checking the equipment periodically once it has been put in service.
Furthermore, the equipment may be removed from service at the end of a prespecified lifetime.
Design tests and validation aim at proving that the equipment is suited to the
application for which it is going to be used.
Resistance and endurance tests are examples of tests which can be carried out on the
equipment. Resistance tests are basic tests: all relevant characteristics of a system
are tested (example: hydraulic pressure test). Endurance tests are more elaborate:
the system is subjected to a cyclical loading, and the time until it fails characterizes its
endurance. A factor is applied to establish service life (requirement to withstand 50
000 test cycles for a specified service life of 10 000 cycles)
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Below is a list of equipment subject to mechanical integrity validation testing:

Gaseous hydrogen containers

Container valves

Flexible hoses

Quick connection devices.
1.1.2.3.2
Types of joints and failure modes
Tubing elements are connected either by welded joints or by fittings (elements joining
two pipes together).
Welded joints
Welded joints are permanent. They have a better leak-tightness and reliability than
fittings; their use limits the number of potential leaks. In order to avoid potential leaks,
butt welded pipes having a diameter larger than 15 mm must be tested over their
whole length using x-rays (according to the standard NF EN 13480-5). Only certified
technicians are entitled to weld tubing elements.
Fittings
In the situations where welding is not appropriate, tubing elements can also be
connected by fittings. For use in hydrogen applications, fittings are made of
stainless steel of the appropriate grade. Different kinds of fittings exist. The type of
fittings which should be used depends on the pressure range of the gas flowing
through the piping, and of the nature of the gas itself.
Typical types of connections and fittings are described hereafter.
a. Valve-to-cylinder connection

Tapered thread connection
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Table 1: Tapered thread connection
(Source: Air Liquide)
Examples
Lots of constructors can provide this kind of sealing modes (e.g.
of
Swagelok® and many others). Conical metal-to-metal sealing is a
providers
sealing mode rather than a type of connexion: it can be manufactured
on an element (e.g. valve).
Range of
For medium and high pressure ranges (from about 2 to 700 bar)
applicatio
Example of application: on hydrogen cylinders
ns
Picture
Figure 7: Use of a metal-to-metal sealing conical fitting
(in the yellow round, the sealing area)
(Source: Air Liquide)
Contact to
Leak-tight contact takes place on the thread of the fitting, and is
ensure
achieved due to the conical shape of the fitting and requires tightening
leak-
to a certain level of torque.
tightness
To reduce friction upon mounting and to improve the leak-tightness of
the system, a Teflon tape is applied on the thread. Proper application of
the Teflon tape is required to achieve leak-tightness.
Failure
This kind of sealing mode requires a strong tightening torque of the
modes
fitting to obtain tightness.
and
Failure modes:
associated

safety
Rupture of fragile cylinder neck (e.g. small cylinder, weaker
materials, e.g. aluminum) due to excessive torque
measures

Leaks due to undertightening or to damages on the threaded
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area
Safety measures:

Use additional Teflon of great quality on the threaded surface to
improve tightness of this kind of connection

Limit mounting-dismounting cycles.
Advantage
Advantage: this kind of fittings is robust and unlikely to leak after proper
s/
assembly: if leaktight, will stay leaktight.
drawbacks

Drawback: it is not suitable for frequent dismounting/remounting.
Cylindrical thread connection with O-ring sealing
Leak-tight contact takes place thanks to an O-Ring located between the top surface of
cylinder neck and the valve. This connection is more suitable than the tapered
connection for dismounting / remounting, but it is more prone to un-tightening and
leakage in service.
This connection is widely used in North America.
b. Tubing connections and fittings

Compression fittings (also called double ring fittings)
Table 2: Compression fittings
(Source: Air Liquide and website of Swagelok®)
Examples of
Swagelok® and Rotarex®
providers
Range of
applications
Pressure ranges:

Up to 420 bar with “classical” tube fittings for a 6 mm
external diameter tube (fittings provided by Swagelok ® or
Rotarex® for example)

Up to 1034 bar with Swagelok® medium pressure tube
fittings for a ½’’ external diameter tube
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Example of application: mainly used for tube connections in
hydrogen energy based systems (for the medium and high
pressure sections).
Picture
Nut
Back
ferrule
Front
ferrule
Union
Figure 8: Classical double ring tube fitting (Rotarex® or Swagelok®)
(Source: website of Swagelok®)
Contact to
ensure leaktightness
For a classical double ring tube fitting using conical ferrules:
As the nut is turned, the back ferrule radially applies an effective
tube grip. The tightening of the nut on the main part of the fitting
also creates a force on the back ferrule which axially advances the
front ferrule. This front ferrule then comes into contact with the main
part of the fitting and creates a seal against the fitting body, on the
tubing outside diameter. The leak-tightness takes place mainly on
the conical surfaces of the fitting. The surface where the conical
ring and the tube come into contact reinforces the tightness by
assuring the tube grip.
Failure modes
Failure modes:
and associated

Extraction of tube – due to improper mounting plus traction
safety measures

Leak between Front ring and Union – due to damaged or
unclean surfaces, or leaks due to undertightening
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Safety measures:

Follow the manufacturer’s mounting recommendations

Use of specific gauge in order to check the tightening

Limit mounting-dismounting cycles.
Advantages /
Advantage: this kind of fitting is highly reliable, simple to assemble
drawbacks
and not affected by vibrations.
Drawback: it has a severe failure mode – i.e. extraction of tube - if
not properly assembled (e.g. lack of rings, wrong order, use of
impropriate rings, not sufficiently tightened). This point has been
improved for medium pressure fittings by a pre-assembly cartridge
providing the ferrules in the right way.

Threaded ring tube fittings
Table 3: Threaded ring tube fittings
(Source: Air Liquide)
Examples of
Maximator, NOVA SWISS, SITEC
providers
Range of
Suitable for very high pressure applications (up to 1500 bar for 9/16’’
applications
external diameter tubes for example)
Example of application: high pressure hydrogen buffer systems for
fuelling stations
Picture
Male nut
Tube
Body
Threaded ring
Sealing area
Figure 9: Maximator® threaded ring tube fitting
(Source: Air Liquide)
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Tube end is prepared at time of assembly after tube has been cut to
the right length: threading and shaping of end of tube for leak-tight
surface.
Contact to
The tightening of the nut on the main part of the fitting the cone-
ensure leak-
shaped end of the tube comes into contact with the main part of the
tightness
fitting. This ensures the required leak-tightness.
Failure
Failure modes:
modes and

associated
Leaks – due to improper preparation of tube end, to
undertightening, to an incorrect position of the screwed ring,
safety
to damages on the conical sealing surface by repeated
measures
overtightenings
Safety measures:

Strong tightening torque

Correct position of the screwed ring: the ring must be seen
through the release hole existing on the body of this kind of
fitting

Limit mounting-dismounting cycles.
Note: for hydrogen applications (transport and/or fuel cell), silicone
grease cannot be used to improve sealing.
Advantages /
Advantage: very robust and reliable. As the ring is screwed on the
drawbacks
tube, the risk of wrenching – which is the main hazard in the high
pressure range – is reduced. These fittings can therefore be used for
the high pressure range.
Drawback: in order to ensure a good leak-tightness, the conical
surface should be properly polished. This kind of fittings can be
either directly bought well manufactured or it can be manufactured
on-site. When it is directly bought well manufactured, a high pressure
qualified welder is required. When it is manufactured on-site
(threaded area for screwed ring hosting and conical sealing surface),
a qualified operator is required and this operation is time-consuming.
Besides, leak-tightness may be affected by vibrations.
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
Quick connections
Table 4: Quick-release fittings
(Source: Air Liquide and website of Staubli®)
Provider
Stäubli
Range of
Quick-release couplings are adapted to the high pressure range
applications
(200-400 bar).
They are appropriate fittings for frequent connections and
disconnections.
Picture
O-ring
Figure 10: Staubli® fitting (on the left)
and longitudinal section of a Staubli® fitting (on the right)
(Source: website of Staubli®)
Contact to
The contact which ensures the leak-tightness takes place on the
ensure leak-
gasket surface.
tightness
Failure modes
and associated
Failure modes:

safety measures
Large leaks from extrusion of rings – due to
connection/disconnection with pressure in the connexion
or when the connection is not exactly in the axis of the
fitting, when the O-ring is forgotten

Leaks from aging or damage to o-rings
Safety measures:

Use of a lyre (spiral tube) in order to provide more
flexibility to position the connections correctly.
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Advantages /
Advantage: quick-release fittings are user-friendly for applications
drawbacks
with frequent connections and disconnections.
Drawback: vulnerability of o-rings. Leak hazards may occur when
the fitting is not properly mounted (other types of fittings are
preferred when there is no need for frequent connections and
disconnections).
References:

Website of Swagelok®

Website of Staubli®
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1.1.2.3.3
Limitation of the extension of high pressure part
Consequences of hazardous events occuring in the high pressure part of a hydrogen
system are very severe. Therefore, the extension of high pressure parts inside the
system should be as limited as possible, and highly secure.
This can be achieved by reducing pressure as soon as practicable.
Mitigation measures
1.1.2.4
Once hydrogen has been released, mitigation measures have to be taken. Limiting
the amount of hydrogen released is a mitigation measure of the hydrogen
release.
Passive
Avoid unnecessary
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
Passive
Flow restriction
Active
Detection and Isolation
Excess flow valve
H2 release
Confined
explosive
atmosphere
Passive
Passive
Explosion venting
Separation distance
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Escalation
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Passive
No ignition sources
Active
Detection and power
shut-down
Figure 11: Mitigation measures: passive flow limitation and active flow reduction /
isolation devices
Source: Air Liquide
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1.1.2.4.1
Passive flow limitation (flow restriction)
A small-diameter pipe or orifice can be used downstream of the pressure
regulator which lowers the pressure of the hydrogen flow coming from the storage
tank. When such calibrated pipe is used, even in the case of a hazardous
hydrogen release, the flow of hydrogen is limited.
This maximum flow of hydrogen through the calibrated pipe can be calculated. When
the absolute source vessel pressure is at least 1.7 to 1.9 times as high as the absolute
downstream ambient atmospheric pressure, the maximum4 mass flow rate of
hydrogen through the calibrated orifice is then:
Equation 3
Or:
Equation 4
Where:
Q = mass flow rate (kg/s)
C = discharge coefficient, dimensionless (usually about 0.72)
A = discharge hole area, i.e. section of the calibrated orifice (m²)
κ
= cp/cv of the gas, with cp the specific heat of the gas at constant pressure and c v the specific heat
of the gas at constant volume
ρ
= real gas density at P and T (kg/m³)
P = absolute upstream pressure (Pa)
M = the gas molecular mass (kg/kmol)
R = the Universal Gas Constant = 8,3145 J.mol-1.K-1
T
= absolute upstream gas temperature (K)
Z
= the gas compressibility factor at P and T (dimensionless)
Note: These formulas do not take into account the pressure drop along the piping
length.
4
This flow rate is the initial instantaneous flow rate from a leak in a pressurized gas system or vessel. It is
much higher than the average flow rate during the overall release period because the pressure and flow rate
decrease with time as the system or vessel empties. It is therefore the maximum hydrogen flow rate.
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Knowing the maximum allowed hydrogen flow rate, the section of the calibrated
pipe can be calculated.
Note: a small-diameter pipe could also be used directly downstream of the storage
tank, but it would be less convenient. Indeed, the pressure of the storage tank
decreases when emptying the tank. The diameter of the pipe should then be small (to
limit the hydrogen flow), but yet enough large so that the hydrogen flow rate is
sufficiently high - even when the storage tank pressure is low. It is therefore more
effective to use a small-diameter pipe downstream the relief valve.
1.1.2.4.2
Active flow reduction / isolation devices
Two active mitigation measures aiming at interrupting the hydrogen flow exist:

The excess flow valve is a valve which closed itself as soon as the hydrogen
flow rate is too high. Its major drawback is that the valve might close
inappropriately, as high instantaneous flows within the system may be normal
in certain phases of operation.

A more sophisticated active mitigation device consists in the combination of
a sensor (such as a hydrogen sensor, or a pressure sensor) with a valve.
As soon as an incident is detected, the valve is automatically closed.
Such a measure is applied in hydrogen dispensing applications in case of
hose failure.
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1.1.3
Prevention of ignition sources
Passive
Avoid unnecessary
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
Passive
Flow restriction
Active
Detection and Isolation
Excess f low valve
H2 release
Confined
explosive
atmosphere
Passive
Passive
Explosion venting
Separation distance
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Escalation
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Passive
No ignition sources
Active
Detection and
power shut-down
Figure 12: Mitigation measure: prevention of ignition sources
Prevention of ignition sources only reduces the likelihood of ignition, as hydrogen may
ignite even when efforts have been made to eliminate electrical of mechanical ignition
sources.
1.1.3.1
Introduction
In the United Kingdom, the Health and Safety Executive (HSE) regulates COMAH 5
sites. The COMAH sites are divided into hazardous areas. The hazardous areas are
broken into three classes, which are defined according to the likelihood and duration
of presence of flammable gas or vapour (Health and Safety Executive, 2012a):

Zone 0: An area where an explosive gas atmosphere is present continuously
or for long periods

Zone 1: An area where an explosive gas atmosphere is likely to occur in
normal operation
5
COMAH = Control of Major Accident Hazards; sites are ranked in accordance to how much dangerous
substances are stored on-site.
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
Zone 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

Unclassified zones are referred to as safe areas.
Attempts have been made to try to quantify the duration of presence of an explosive
atmosphere to aid in assigning the correct zone classification for a part of plant.
However, there is no consensus on how many hours constitute long or short period of
presence of an explosive atmosphere.
The HSE website quotes the following
commonly used durations of presence of explosive atmosphere: Zone 0: more than
1000 h, Zone 1: more than 10 h but less than 1000 hr and Zone 2: less than 10 h
(Health and Safety Executive, 2012a).
The Dangerous Substances and Explosive Atmospheres Regulations was introduced
in 2002 (Her Majesty’s Stationary Office, 2002) and made it a legal requirement to
carry out a hazardous area study and report the conclusions.
Any 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. However, equipment might
well be intrinsically safe when it was installed, but correct regular maintenance will
also be required to ensure that the equipment remains intrinsically safe.
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1.1.3.2
Control of ignition sources
There are a number of potential ignition sources. Table 5 lists these sources, in no
particular order (Health and Safety Executive, 2012a). It is not claimed that the list of
ignition sources is exhaustive, but it does cover a great many of the likely sources.
Table 5: Potential sources of ignition
Flames
Direct source of space or process heating
Use of cigarettes/matches
Cutting and welding flames
Hot surfaces
Heated process vessels, like dryers
Hot process vessels
Space heating equipment
Mechanical machinery
Electrical equipment and lights
Spontaneous heating
Friction heating or sparks
Impact sparks
Sparks from electrical equipment
Stray currents from electrical equipment
Electrostatic discharge sparks
Lightning strikes
Electromagnetic radiation of different wavelengths
Vehicles, unless specially designed or modified are likely to contain a range of
potential ignition sources
Daycock and Rew compiled a list of ignition sources and identified their strength,
frequency and density (Daycock and Rew, 2004). The authors stressed that it was a
first attempt, but it complements nicely the list presented by HSE. These two lists
should identify the majority of potential ignition sources and should serve as a good
starting point when looking at a specific plant.
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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, 2012a):

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);

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
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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.
References

British Standards Institution (2009). BS EN 60079-10-1:2009 Explosive
Atmospheres. Classification of areas. Explosive gas atmospheres.

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_Doc
uments_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 of on-site ignition probabilities, Health and Safety Executive
Research Report No. RR226.

Energy Institute (2005). IP15 Area classification code for installations handling
flammable liquids: Model code for safe practice in the petroleum industry, 3 rd
edition.

European Parliament (1994). Directive 94/9/EC on equipment and protective
systems intended for use in potentially explosive atmospheres (ATEX),
25
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
http://ec.europa.eu/enterprise/sectors/mechanical/documents/legislation/atex/
[accessed on 20 June 2012].

European Parliament (1999). Directive 99/92/EC on minimum requirements for
improving safety and health of workers potentially at risk from explosive
atmospheres, http://eurlex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2000:023:0057:0064:en:PDF
[accessed on 20 June 2012].

Health and Safety Executive (2012a). Hazardous Area Classification and Control
of Ignition Sources, http://www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm
[accessed on 20 June 2012].

Health and Safety Executive (2012b). Five steps to risk assessment,
http://www.hse.gov.uk/pubns/indg163.pdf [accessed on 21 June 2012].

Her Majesty’s Stationary Office (1974). Health and Safety at Work etc Act,
http://www.legislation.gov.uk/ukpga/1974/37/contents [accessed on 21 June
2012].

Her Majesty’s Stationary Office (2002). Dangerous Substances and Explosive
Atmospheres Regulations 2002,
http://www.legislation.gov.uk/uksi/2002/2776/contents/made/ [accessed on 21
June 2012].

Institute of Gas Engineers & Managers (2010). IGEM/SR/25 Edition 2 Hazardous
area classification of natural gas installations.

International Electrotechnical Commission (2009). IEC 60079-10-1 Classification
of areas—Explosive gas atmospheres.

Lees, F. P. (1996). Loss Prevention in the Process Industries, Volume 2, 2nd
edition. Butterworth-Heinemann, Oxford, Oxfordshire, United Kingdom.

Leroux, P. (2012). Area Classification. Why? Where? How? Who? When?,
http://www.iecex.com/dubai/speakers/Day%202_08300915_IECEx_Dubai_Area_Classif_final_Leroux_P.pdf , [accessed on 21 June
2012].

Nolan, D. P. (1999). Handbook of fire and explosion protection engineering
principles for oil, gas, chemical, and related facilities, William Andrew Publishing,
Norwich, New York, USA.

Sherwen, S. (2012). The control of ignition sources arising from gas processing
machinery, Scandic Antwerpen Conference, 22-24 February 2012, Antwerpen,
Belgium.

Worsell, N. (1996). Risk of Ignition of Explosive Atmospheres. Health and Safety
Laboratory Report No. RAS/96/13.
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1.1.4
Detection
Passive
Avoid unnecessary
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
Passive
Flow restriction
Active
Detection and Isolation
Excess f low valve
H2 release
Confined
explosive
atmosphere
Passive
Passive
Explosion venting
Separation distance
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Escalation
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Passive
No ignition sources
Active
Detection and
power shut-down
Figure 13: Mitigation measure: hydrogen detection
An important aspect of minimizing the risk of fire and explosion is the use of detection
methods to give advance warning of a potentially hazardous situation, for example
arising from a leak or combustion of fuel. These detectors must be effective and
reliable, because of the safety critical nature of the application, and inexpensive, to
ensure their widespread use. Both fixed location (for gas and fire detectors) and
personal or hand-held monitors (typically for gas detectors only) are necessary for
protection of personnel and plant. Additionally, there may be a requirement to fit gas
sensors on vehicles to warn of leaks (on-board sensors).
Gas and fire detection methods for conventional fuels have been in use for decades
and are well characterized: see for example BSI (1999) and BSI (1996) respectively.
For carbon-containing alternative fuels, most of the conventional methods are
appropriate; their performance in the new fuels are, however, not well known, for
example sensitivity, adverse affects. Hydrogen, however, has very different
characteristics to carbon-containing alternative fuels, which make it more of a
challenge. Both mature and new detection technologies, which are receiving much
attention, are employed.
There may also be environmental pollution issues for carbon-containing fuels but for
hydrogen they are non-existent as only water is produced; although CO and CO 2 are
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generated as by-products from reforming reactions which convert carbon-containing
fuels into hydrogen. However, only detectors for safety applications are considered
here, not those for environmental pollution monitoring.
1.1.4.1
Detection of hydrogen leaks
The applications for hydrogen sensors cover:

The hydrogen generation process from carbon-containing fuel reforming or
electrolysis

Hydrogen storage and distribution, at production sites and filling stations; and

Hydrogen fuel cell/combustion systems. These can be stationary, for example
power production, or mobile, for example automotive.
These systems require sensors for monitoring for the quality of hydrogen feed gas,
that is to say
process control (not considered further here), and the second more
important system for leak detection (Jardine, 2000). The latter category may require
sensors for very low level detection (trace detection in the parts per million, ppm,
concentration range) and for explosion protection, where the concentration range is
around the fractions of % level, around the LEL (4% v/v). Previous reviews have been
reported on hydrogen sensors, for example NASA (1997) and BNFL (2003).
The general characteristics of commercially available sensors for hydrogen are
summarised inTable 6. The most commonly employed are catalytic and thermal
conductivity sensors. These can be used for a range of gases which have different
response factors. Response factors of some sensors, expressed as a percentage of
the methane factor, are shown in Table 7. For the catalytic sensor, hydrogen has a
similar response factor to methane but there is some variation between sensor types.
Table 8 tabulates values of thermal conductivity, relative to air, which indicate the
relative response of the thermal conductivity sensor to these gases in the presence of
air. Hydrogen has a very high value compared to air which makes for high sensitivity
and a detection limit of around 200 ppm, depending on the environment. Typical
performance data from commercially available hydrogen sensors are detailed in Table
9.
None of the above commercially available hydrogen sensors is completely suitable for
its intended purpose. This and the potentially large market, particularly in North
America, for such a sensor has led to much development of novel hydrogen sensors.
A summary of the various new types of hydrogen sensor is given in Table 10.
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Pressure drop in the system: Leaks may be detected by sudden drops in pressure.
However, the smallest fugitive leaks will go undetected, as these may not cause a
large enough pressure drop to raise the alarm.
A number of so-called smart tapes or smart paints have been developed. These
materials reveal a leak of hydrogen by changing color. Thus it is possible to detect
minute leaks at joints and flanges. These leaks fall into the category of foreseeable
leaks so placement of the tap or paint is fairly straightforward.
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1.1.4.2
Detection of flammable mixtures
Table 6: Hydrogen sensors—general properties of commercially available monitors
Type
Operating principle
Hydrogen oxidation at
platinum sensing
Electrochemical
electrode, oxygen
reduction at counter
Advantages

Quite selective

Sensitive to 100

electrode generates
current, ie fuel cell.


Disadvantages

Comments
Some cross-sensitivity to
ppm
CO (for example 1000
Potentially very low cost, compare with domestic CO. But
Very low power
ppm H2  300 ppm CO)
CO interference may require additional protection of
personnel against CO.
consumption –

Narrow temp range
no heating

Short lifetime (2 years)

Affected by total gas
Poison resistant
Wide detection
pressure (for example
range
altitude)
Thin-film
Reversible resistance
hydrogen
increase of heated,
sensor (Robust
temp controlled Pd/Ni

Rapid response
Hydrogen
thin film, used in

Does not require
SensorTM)
Wheatstone bridge.
oxygen

Poisoned by CO, SO2,
H2S

Developed by Sandia Labs (Sandia, 2003). Commercialised
by H2scan (formerly DCH; see ref. H2scan, 2005). Not
inherently expensive technique. As more in use and with
competition, price should drop. Heating introduces intrinsic
safety problems which increase cost. Initial devices had
Heating required to ca
poor reproducibility.
150°C
ChemFETs
Pd/Ni thin film
(forms part of
deposited on the gate
the Robust
of a field effect
Hydrogen
transistor (FET),
SensorTM)
forming an MOS


Does not require
Affected by total gas
Developed by Sandia Labs (Sandia, 2003). Commercialised
oxygen
pressure (for example
by H2scan. Not inherently expensive technique. As more in
Wide detection
altitude)
use and with competition, price should drop. Heating
Poisoned by CO, SO2,
introduces intrinsic safety problems which increase cost.
H2S
Initial devices had poor reproducibility.
range


Low power

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Type
Operating principle
Advantages
capacitor. Hydrogen
consumption
Disadvantages

absorption changes
Comments
Heating required to ca
150°C
charge (capacitance)
on gate which is
detected sensitively.
Catalytic oxidation on
bead heated to 450 °C,
temp increase sensed
with platinum
Catalytic
resistance
thermometry. Detector

Acceptable

Not selective

High power consumption –
Mature technology but intrinsic safety issues leading to
heating
higher cost.
lifetime

and compensator form
Wide temp

Requires 5-10% oxygen
Although adaptation has led to a higher sensitivity and
range

Poisoning by Pb, Si, P, S
specific sensor (RKI, 2001).

High maintenance
two arms of
Wheatstone bridge.
High thermal

Quite selective
conductivity of

Long term

Cross-sensitive to helium
stability

Not as sensitive as
Mature technology but intrinsic safety (IS) issues leading to
electrochemal/chemFETs
higher cost. Still a reliable and proven technique.
Thermal
hydrogen changes
conductivity
relative heat loss of a

Poison resistant
pair of heated

Does not require
elements.
Semiconductor
Surface conductivity
change of metal oxide

Heating required
Commercially

Not selective
Very low cost but reliability and IS issues. Not generally
available with

High power consumption –
favoured for quantitative measurements.
oxygen

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Type
Operating principle
Advantages
semiconductor plus
acceptable
catalytic additives
lifetime
heated to 300°C.

Disadvantages
Comments
heating

Sensitive to humidity and
temperature
Wide temp
range
Mass spectrometry:
molecules ionised and
selected by their
Mass
mass/charge ratio by a
spectrometry
magnetic or quadrupole

Low limit of
detection

expensive

bulky

fragile

needs skilled operator
Analytical tool. Not cost-effective for routine monitoring.

specific

Not susceptible

not specific to hydrogen
Ultrasonic emission
to poisons,

interference from
from gas escaping from
humidity etc only
a high-pressure pipe.
leak properties
field. Ions detected by
sensitive charge
detector.
Ultrasonic

Non-directional?
background noise

only detects high pressure
leaks
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Cannot measure whether a flammable mixture is present.
Table 7: Response factors (% relative to methane*) of various catalytic and infrared flammable gas sensors to hydrogen and carbon-containing fuel fuels
LEL
(%)
Infrared
Catalytic
Drager
City Technology
Drager
City Technology
RaeSystems
IR Ex HC-
IrceL HC
Cat Ex C
MICROpeL 75
LEL
Hydrogen
4.0
nd
nd
100
125
91
Gasoline (unleaded)
1.4
-
157
51
55
48
No.2 Diesel Fuel
1.0
-
-
-
-
-
Methanol
5.5
469
357
63
105
67
Ethanol
3.1
406
257
63
80
59
-
-
51
-
-
MTBE (up to 7% in Gasoline)
n-Butane
1.4
344
257
46
70
50
Propane (LPG)
1.7
313
286
51
60
63
Compressed Natural Gas (Methane)
5.0
100
100
100
100
100
Table 8: Thermal conductivity of gases relative to air
Thermal conductivity
Gas/vapour
 = 0 °C
 = 100 °C
Air
1.00
1.00
Methane
1.25
1.45
Propane
0.58
0.70
Butane
0.55
0.66
Hydrogen
7.0
6.8
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Helium
5.9
5.5
Water
0.66
0.74
Methanol
0.5
0.7
Ethanol
0.5
0.7
Table 9: Hydrogen sensors—typical performance characteristics of commercially available monitors
Type
Electrochemical
Range
Resolution
0-0.2%; 0-2%
2-10 ppm
‘Robust Hydrogen Sensor’ that is to say
Chemresistor/chemFET
0.1-100%
(s)
(yr)
30
1-2
5
?
10 ppm with ChemFET
ChemFET
0-100% LEL
1% LEL
20
3
0-10%
0.5% range
20
10+
50-5000 ppm
50 ppm
30
?
Thermal conductivity
Semiconductor
Lifetime
0.1%
10-1000 ppm with
Catalytic
Response time
Table 10: Hydrogen sensors—general properties of monitors under development6
Type
Operating principle
Advantages
Thick-film hydrogen
Resistance change of a Pd resistor

Potential low cost.
sensor
formed on a ceramic substrate using

Simple method
6
This table will need to be updated for the second draft of this document.
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Disadvantages

Poisoned by CO, SO2, H2S
Comments
Developed by Oak
Ridge National Lab.
Type
Operating principle
Advantages
Disadvantages
Comments
thick film deposition techniques. Four
(ORNL, 2005)
Pd resistors are used – two coated
Cost target is
with a H2-impenetrable film in a
< $50. Promising
Wheatstone bridge.
development close to
commercialisation.
Several types of system including:

Change in optical characteristic of
a mixed metal oxide (eg
V2O5/WO3) on an optical fibre
(Griessen et al., 2004).

Mg-Ni alloy reflective mirror in air,
black absorber in H2 (Butler,
1994).
Optoelectronic

Interferometry – Pd wire

consumption

interferometry.

Surface Plasmon Resonance: a
Immune to
electromagnetic
reversibly stretches in H2.
Deformation is detected using
Very low power
interference

Intrinsically safe in
explosive atmospheres
thin palladium layer is deposited
on the bare core of a multimode
fibre. Modification of the SPR is
due to variation in the complex
permittivity of Pd in contact with
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
Temperature and relative
humidity effects
At prototype stage.
Type
Operating principle
Advantages
Disadvantages
Comments
gaseous hydrogen.

Bragg grating: mechanical stress
induced in a Pd coating when it
absorbs hydrogen. The stress in
the Pd coating stretches and
shifts the Bragg wavelength.

PdAg, PdCr or Pd/InGaP diode
characteristics altered by H2
sorption. For PdAg, the diode
Operating temperature
structure is composed of three
is
layers: a Pd13%Ag catalyst metal

film on a SiO2 oxide layer which
MOS Schottky diodes
is adherent to a n-type Si

Low concentration (15
< 100 °C and < 2% H2.
ppm) can be detected.
PdAg system
Operates in inert or

Oxygen affects response
developed by NASA
substrate. Hydrogen dissociates
oxygen-containing
Lewis and Case
on the surface of the metal and
environments
Western Reserve
migrates to the interface between
University. (NASA
the metal and the oxide. The
Lewis, 2001)
resulting dipole layer changes the
electronic properties of the diode.
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Type
Operating principle

Disadvantages
transmittance changes of a Pd
embraces a wide

unproven

unproven
Surface acoustic wave
(mass)
range of technologies
based on
polymer nano-composite films

Comments
Nanotechnology
Electrical resistance (DiMeo et
al., 2003) and optical
Nanotechnology
Advantages
nanomaterials.
selective sorption onto a layer eg
WO3, carbon nanotubes,
changes the resonance
frequency of the oscillator

Low power

microscale
substrate (eg quartz, LiTaO3).
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
Susceptible to water
vapour
Unlikely to compete
with the above
techniques.
1.1.4.3
Detection of hydrogen flames
Fire detectors are the second step in the detection chain for minimising the risk of
death, injury, damage and loss of production due to the flammability/explosibility of
fuels. Gas detection is the first step but is not totally reliable or cannot, for example
due to lack of 100 % coverage, prevent a fire from starting. It is therefore usual for the
industry to install both types of detector. Fire detectors are typically installed in similar
situations, but not necessarily in the same location, as fixed gas detectors. Certain
types of fire detectors (optical, see below) have the advantage of being able to cover
effectively large areas which would be impossible with a gas detector system, with
either single or multiple detectors.
The fuel industries are constantly in need of reliable and fast response fire detection
systems. Additionally, the system should be able to locate the event quickly and
reliably. The smaller the fire, when detected and located, the easier it is to extinguish.
The fire detection system is typically configured to provide an audible/visual alarm
and/or activate a fire suppression system. There are various approaches to detecting
a fire (see for example Liu & Kim, 2003), based on:

Heat detection

Frangible bulbs (activate directly fire-water deluge valves)

Fusible plugs

Rate compensated heat detectors

Linear heat detection (cables routed through fire risk areas);

Ionisation smoke detection;

Optical smoke (obscuration/line-of-sight) detection;

Gaseous products of combustion detection, eg CO, volatile organic
compounds;

Optical flame detection (IR and UV and combined IR-UV).

High speed and high sensitivity smoke detection - addressable point optical
systems and aspirated optical systems (eg Very Early Smoke Detection
Apparatus VESDA)

CCTV based systems.
Fire detectors use a variety of algorithms to process the sensor data in order to
identify fires and reduce false alarms. Examples of techniques used in the analysis
are:

Flicker frequency between 1-10 Hz

Threshold signal comparison

Correlation between several signals

Comparisons for example ratio, AND/OR gates

Comparison with spectral library.
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This report is concerned principally with optical instrumental methods as they are
becoming the most prevalent, and are the most effective when monitoring over large
areas. Background information on the principles of operation of optical detectors can
be found for example in Liu and Kim (2003) and references therein, and on the
websites of manufacturers such as Spectrex (2005) and Micropack (2005). Again, as
with gas detectors, fire detectors can be conveniently split into those for hydrogen and
those for carbon-containing fuel fires because of the very different characteristics of
hydrogen flames.
The general characteristics of commercially available hydrogen fire detectors are
summarised
in
Table 11. Depending on the choice of wavelength the detectors can be used to detect
not only hydrogen, but also carbon-containing fuels, while the triple IR is selective to
hydrogen, ethanol and methanol but not carbon-containing fuels.
Table 11: Hydrogen fire sensors—general properties of commercially available
monitors
Type
Operating
principle
Advantages
Disadvantages
Comments
False alarms, for
Analysis of
UV
emissions in
Very high
solar blind
speed
region (< 300
High
nm) at high
sensitivity
speed, typically
Low cost
around 200 nm.
example lightning,
Can detect
arc welding,
hydrogen,
radiation, specific
methanol and
solar radiation not
ethanol and
absorbed by the
other alternative
atmosphere
carbon-
Blinded by thick
containing fuel
smoke and
fuel fires.
vapours
UV/IR
Detection based
Moderate
False alarms, for
on a solar blind
speed
example
Can detect
UV sensor
Moderate
combination of
hydrogen,
(around 200 nm)
sensitivity
UV and IR
methanol and
and an IR
Low false
sources.
ethanol and
sensor (over
alarm rate, not
Blinded by thick
other alternative
region 1-3 µm).
blinded by
smoke and
carbon-
The IR sensor
CO2 fire
vapours
containing fuel
reduces false
protection
Moderate cost
fires.
alarms from use
discharges
39
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
of UV only.
Automatic self
test
Spectral and
flame pattern
Triple IR
Detects
analysis from IR
Very high
bands in the
sensitivity
H2O emission
Very high
region (1-4 µm)
speed
hydrogen,
Moderate cost
ethanol and
methanol but not
hydrocarbon fuel
and reference
flames.
regions.
Superimposes
IR/vis
imaging
flame image
Images the
from 2 CCDs in
flame
the near-IR and
Systems in
visible onto a
use at NASA
colour video
Stennis
image.
Developed by
NASA and
Moderate cost
Duncan
Technologies
Inc. (NASA,
2000).
Careful choice of fire detector is required because of the wide variety of types
available. The choice is determined by the type of fire, for example hydrogen, alcohol,
or hydrocarbon fuel, and the application and environment, as these affect the type of
detector (for example point, optical) and potential interferences, especially for optical
detectors.
Guidance on fire detectors can be found in the BS EN 54 series (BSI, 1996). Various
rules of thumb are used to determine the location and coverage of the different types
of fire detector. For example in offshore modules, point heat detectors in open,
naturally ventilated areas are sited at approximately a density of 1 per 25 m 2 and at
spacing of 7 m with a maximum distance from bulkheads of 3.5 m. In enclosed,
mechanically ventilated modules, they are sited at approximately 1 per 37 m 2 and 9 m
apart with a maximum distance from bulkheads of 4.5 m. They are not applied in
areas with high ceilings above 8 m (point heat detectors have poor sensitivity with
height). However they do still need to be located at high level as heat rises. Heat
shields can be used when they are not under a solid ceiling to improve heat build up.
Optical flame detectors are sited such that their vision cone covers areas where fire
may occur. For IR flame detectors around 15 m is considered a reasonable range
because of obscuration by smoke and lack of sensitivity at the periphery of their field
of view. They are generally sited at the corners of an area. CAD tools are used to
optimise their coverage at the design stage and assess the effectiveness of current
installations (Shell Global Solutions, 2005).
40
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Point smoke detectors rely on transport of products of combustion (particulates and
gases) to the detector by convection. The numbers of detectors can be reduced with
increased ceiling height because of more uniform distribution, although the
concentration will be less and the sensitivity of the detectors must be adequate. In
offshore applications, current smoke detectors are located not more that around 7.5 m
apart and not appropriate for high ceilings (>10.5 m). They are also located just below
the ceiling. Open path smoke detectors need to be at high level to avoid obstruction,
eg above 3 m. Smoke detectors in ventilation ducts should be located to avoid the
effects of turbulence, ie in a straight length away from bends.
References:

Bechtold, R. L. (1997) Alternative fuels guidebook. SAE, Pa, USA.

BNFL, (2003) Design Guide for: Hydrogen sampling and measurement. The
sampling & measurement of gaseous hydrogen in active and non-active
applications, BNF.EG.0053_5_A. BNFL Commercial.

Brett, L. (2003) Hydrogen safety sensors and their applications in hydrogen
storage, distribution and use. http://www.jrc.cec.eu.int

BSI (1999) BS EN 50073 : 1999 BSI London.

BSI (2000) BS EN 61779 series. BSI London.

BSI (2002) BS EN 61508 BSI London.

BSI (1996) BS EN 54-1: 1996 Fire detection and fire alarm systems.
Introduction. BSI London.

Butler, M. A. (1994) Micromirror optical-fibre hydrogen sensor. Sensors and
Actuators, B22, 155-163

Concawe (1995) Alternative fuels in the automotive market. Report no. 2/95.
CONCAWE, Brussels.

DiMeo, F. et al (2003) Micro-machined thin film H2 gas sensors.
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/viib5_dimeo.pdf

Ferree, S. (2003) Gas detection for alternate-fuel vehicle facilities.
Occupational Health & Safety. May 2003, 68-75.

Griessen,
R.
et
al.
(2004)
Optical
fibre
hydrogen
sensors.
http://www.nwo.nl/nwohome.nsf/pages/SPES_5RUFNL

H2scan (2005) Details on Robust hydrogen sensor. http://www.h2scan.com

HSE (2004) HSG243 (2004) Fuel cells Understand the hazards; control the
risks, HSE Books.

Jardine, A.P. (2000). Hydrogen sensors for hydrogen fuel cell applications.
http://www.powerpulse.net/powerpulse/archive/pdf/aa_111300a.pdf
41
© HyFacts 2012/13 – CONFIDENTIAL – not for public use

Knight, B. A. et al (2003) Development of sensors for automotive fuel cell
systems.
http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/viib4_knight.pdf

Liu, Z., and Kim, A. (2003) Review of recent developments in fire detection
technologies. J. Fire Protect. Engin. 13, 129-151.

Micropack (2005) http://www.micropack.co.uk/

NASA (1997) Safety standard for hydrogen and hydrogen systems.
Guidelines for hydrogen system design, materials selection, operations,
storage, and transportation. Ch 6. Hydrogen and hydrogen fire detection.
Office of Safety and Mission Assurance, Washington DC.

NASA
(2000)
http://technology.ssc.nasa.gov/suc_sbir_hyd_flame_imaging.html

NASA
Lewis
(2001)
PdAg
Schottky
diode.
http://www.sensorsmag.com/articles/0401/14/main.shtml

ORNL (2005) Low-cost, thick film hydrogen sensors. http://www.ornl.gov

RKI (2001) Hydrogen gas detector for fuel cells. http://www.fuelcellsensor.com

Sandia (2003) New hydrogen sensor is small, rugged, and inexpensive.
http://www.sandia.gov/mstc/technologies/microsensors/hydrogensensor.html

Shell
Global
Solutions
(2005)
http://www.shellglobalsolutions.com/products_services/flame.htm

Spectrex (2005) http://www.flame-detection.com and http://www.spectrexinc.com

Van Well, Murray, S., Hodgkinson, J. et al. (2005) An open-path hand-held
laser system for the detection of methane gas. J. Opt. A. 7, S420-424.

Vesda (2005) http://www.vesda.com/

Zalvidea, D. et al. (2004). Wavelength multiplexed hydrogen sensor based on
palladium-coated fibre-taper and Bragg grating, Electronics Letters 40, 301302.
42
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.5
Prevention of flammable/hazardous mixtures by means of ventilation
Passive
Avoid unnecessary
confinement
Natural ventilation
Active
Active ventilation
Detection and
active ventilation
Confined
explosive
atmosphere
Passive
Flow restriction
Active
Passive
Explosion venting
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Detection and Isolation
Excess f low valve
H2 release
Passive
Separation distance
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Escalation
Passive
No ignition sources
Active
Detection and power
shut-down
Figure 14: Mitigation measure: ventilation
1.1.5.1
Dispersion of hydrogen in enclosures – summary
In the case of a hydrogen release in an enclosed area 7, a flammable mixture might
form. The formation of this hazardous mixture is determined by the hydrogen
dispersion (see details in the section Error!
Reference source not found.).
7
A roofed area with less than 1/3 of the structure perimeter open to the outdoors shall be considered as an
enclosed area.
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Dispersion regimes in a closed unventilated room
Different dispersion regimes occur when hydrogen is released in a closed
unventilated room. Gaseous hydrogen releases are characterized by the Richardson
number Ri0:
Ri 0  g.
 a   0 R0
. 2
0
U0
Equation 5
With:
U0: release velocity (m.s-1)
R0: radius of the releasing orifice (m)
ρa: ambient density (kg.m-3)
ρ0: released gas density (kg.m-3)
g: the gravitational constant (9,81 m.s-2).
If the Richardson number is larger than the unity, gravity effects are dominant,
the hydrogen flow is a pure plume and its dispersion is said to be buoyancydominant. The plume develops vertically up to the roof where it is deflected
horizontally up to the walls. A horizontal interface is formed on the section of the
enclosure. In the upper part of the enclosure, the injected gas build-ups and the
density increases gradually from the interface to the roof.
If the Richardson number is lower than the unity, the momentum dictates the
mixing, overturning is induced and the hydrogen flow is a jet. Several dispersion
regimes can be distinguished, according to the volumetric Richardson number Riv
of the hydrogen jet (see details in section
Error! Reference source not
found.).

If Riv is very small compared to 1, there is a homogeneous concentration in
the enclosure.

If Riv has a very large value compared to 1, a continuous concentration
distribution is obtained in the enclosure.

If Riv has an intermediate value: a stratified profile with a homogeneous
upper layer is observed.
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Dispersion regimes in an enclosure ventilated with a single or two openings
The presence of ventilation in an enclosure has an influence on the dispersion
regimes of hydrogen:

In the case of a buoyant-dominated or momentum-dominated release in an
enclosure with one opening, a well-mixed regime is observed.

When two openings are present in an enclosure (on the lower and upper
parts) in which a buoyant-dominated release is occurring, a ventilation mode
called
displacement
ventilation
takes
place
in
the
volume.
This
displacement ventilation comes along with a homogenous layer in the
upper part of the enclosure and stratification in the lower part. For larger
momentum release, the concentration becomes well mixed in the enclosure.
Note 1: see the figurative schemes in section Error!
Reference source not
found. for the conditions under which the models are valid.
Note 2: external wind conditions, that is to say the direction and speed of the ambient
wind can play a very important role in the venting and dispersion regimes. See details
in section Error! Reference source not found..
1.1.5.2
Safety objectives and criteria
Safety targets are defined, allowing designing the means of protection against hazards
specifically resulting from use of hydrogen in enclosures:
45
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Leak scenario
Leak scenario
Leak scenario
• Hypothesis
• Model
• Consequences
Safety Targets
Safety Targets
Safety Targets
Safety strategy of
the application
Means of protection
Figure 15: Definition of safety targets
Source: HyIndoor project, WP1: Scenario, objectives, and project expected outcomes,
May 2012
Safety objectives (frequency limit for feared event) are translated into practical design
objectives (see section Error!
Reference source not found.):

For expectable leaks, there should be no damage.

For foreseeable leaks, there should be no possible material damage.

For conceivable leaks, mitigation actions should be taken to avoid destruction
of the system.
The formation of a flammable atmosphere can be allowed as long as the
destruction of the system or hazardous effects outside of the system are
avoided. In this objective, the hydrogen concentration should be limited to a
determined value.

No design objectives are set for unlikely feared leaks. There is no specific
measure other than prevention (material choice...) and for emergency
responses.
1.1.5.3
Review of ventilation configurations
To prevent the formation of flammable mixtures and hence meet the safety objectives,
effective ventilation of enclosed spaces should be provided. Several ventilation
configurations exist: one opening, two openings. Enclosures with two openings
allow for better ventilation than enclosures with one opening.
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Design and sizing of natural ventilation (for a given leak size)
1.1.5.4
An enclosed area where hydrogen may be released should be ventilated by
openings, in order to prevent the formation of an explosive atmosphere. If
possible, two openings at least should be provided: one in the upper part of the
enclosure, the other in the lower part - the latter conditions are essential for effective
natural ventilation.
Example of design and sizing of natural ventilation, when two openings can be
provided:
1. The flow rates of the foreseeable hydrogen leaks are assessed.
2. Models described in sections Error! Reference source not found. and
Error! Reference source not found. are used in order to size the openings.
Note: external wind conditions, that is to say the direction and speed of the ambient
wind can play a very important role in the venting and dispersion regimes. See details
in section Error! Reference source not found..
1.1.5.5
Forced ventilation
A forced ventilation system can be included in a hydrogen system, if the natural
ventilation is not sufficient or not reliable. The design of the forced ventilation system
can be based on the model described below.
Two openings are present on the enclosure pictured on the Figure 16. In the upper
part of the enclosure, a ventilation system extracts air at the ventilation flow rate
Qv. This induces a depressurization in the enclosure, which makes air come into the
system through the opening on the lower part of the system.
In this model, the volume below the leak point does not contain any hydrogen.
47
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Ventilation
Qv
xV
z
V
H2 release
QS
Air inlet
Qv - QS
Figure 16: Forced ventilation
(Source: Air Liquide)
The forced ventilation is designed for the steady state. At steady state 8, the hydrogen
concentration is:
xH 2 
QS
QV
Equation 6
Note 1: when designing a system, it might be difficult to assess the volume V as the
location of the future potential leak is unknown. The design of the system should
then be based on a model where the volume V is the smallest (i.e. with the
highest leak point). Indeed, the highest hydrogen concentration is reached in the
smallest volume.
Note 2: when designing forced ventilation, special attention has to be spent on the
location of extraction (dead zones formation) and to the location of the compensating
air vents.
1.1.6
Prevention and mitigation of explosions
Explosion venting is a protective measure preventing unacceptable explosion
pressure build-up inside confined spaces leading to enclosure destruction and
formation of flying fragments.
The vent can either be used:
48
© HyFacts 2012/13 – CONFIDENTIAL – not for public use

Both as a protective measure preventing unacceptable explosion
pressure build-up inside confined spaces AND as a ventilation opening.
In this case, the vent always stays open.

Only as a protective measure preventing unacceptable explosion
pressure build-up. In this case, the vent opens when the opening pressure of
the vent is reached, because of the combustion of the flammable mixture.
Such vent is considered in the system described in the section 1.1.6.2.
Passive
Passive
Avoid unnecessary
Explosion
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
venting
Passive
Flow restriction
Active
Detection and Isolation
Excess f low valve
H2 release
Confined
explosive
atmosphere
Passive
Separation distance
Active
Emergency response
Strong blast
effects
Injury /
casualty
Passive
Flash fire
Separation distance
Active
Emergency response
Unconfined
explosive
atmosphere
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Escalation
Passive
No ignition sources
Active
Detection and power
shut-down
Figure 17: Mitigation measure: explosion venting
(Source: Air Liquide)
1.1.6.1
Deflagration of unconfined hydrogen air mixtures – summary and extension
of hazardous effects
Deflagration of unconfined hydrogen air mixtures: summary
Hydrogen air mixtures are flammable when the proportion of hydrogen in this mixture
ranges from 4 to 78% by volume (these values are for a hydrogen air mixture at 20°C
and 1 bar).
The combustion is characterized by the speed at which the thermal flame front
propagates. The speed of the flame front increases with increasing hydrogen
concentrations up to 40 % in the flammable zone. Different factors may speed up the
flame, such as the turbulence created by obstacles or fans.
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When a hydrogen air mixture is ignited, its temperature rises and an overpressure
wave develops within the flammable zone. As showed in Figure 18, the maximum
overpressure value strongly depends on the hydrogen concentration of in the mixture.
This figure shows the maximal overpressure value which has been experimentally
measured in an enclosure of a few cubic meters after the ignition of a hydrogen-air
mixture.
Figure 18: Influence of the hydrogen concentration in the mixture of the maximal
overpressure value
Source: Air Liquide
Note: the damage threshold for structures is of 150 mbar; but it could be also lower,
possibly 50-100 mbar. The integrity of the structure is highly dependent on the method
of construction, its age and its state of repair.
The pressure wave created by the ignition of the hydrogen air mixture propagates
outside of the flammable zone, while its amplitude decreases. In the case of the
unconfined explosion of a sphere from a mixture characterized by its constant flame
speed, the maximum overpressure decreases as the ratio R/R 0 (with R the distance
from the ignition source and with R0 the initial radius of the sphere).
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Maximal overpressure (hPa)
Figure 19: Wave propagation during the unconfined explosion of a sphere from a
mixture characterised by its constant flame speed Sf
(Source: A. Lannoy L'evaluation des risques d'explosions en phase gazeuse dans
l'industrie de production d'électricité Journal de Physique III 1, 8 (1991) 1359-1376)
Caption: R is the distance from the ignition source and R 0 is the initial radius of the
sphere
Note 1: the higher flame speed, the higher overpressure.
Note 2: 1 hPa = 100 Pa
Ignition of a flammable gas cloud (delayed ignition so that the cloud can form) can
lead to either a deflagration (a.k.a. explosion) or detonation. Deflagration is a term
describing subsonic combustion; the speed of the flame front is smaller than the
speed of sound in the burnt gases. In the case of detonation, the speed of the flame is
supersonic and therefore generates a shock wave. Greater pressures are then
generated by detonations, which are thus more destructive than deflagrations.
A deflagration may become a detonation as the flame front is accelerated by
turbulence (Deflagration to Detonation Transition). A detonation may also be initiated
from the onset if ignition is performed with a high energy source (e.g. explosive).
The
explosions
occurring
in
unconfined
environments
deflagrations since the ignition energies are most often small.
51
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
are
generally
Extension of hazardous effects
Deflagrations of unconfined hydrogen air mixtures have overpressure and thermal
effects. In the case of “small” deflagrations (i.e. when the hydrogen concentration and
the ignition energy are low), thermal effects are generally more severe than
overpressure effects. On the contrary, in the case of large powerful deflagrations, the
extension of hazardous effects depends mostly on the value of the overpressure
generated by the combustion of the flammable hydrogen air mixture. Different
thresholds have been defined, depending on the severity of the overpressure effects.

Extension of overpressure effects
In order to determine the severity of the overpressure effects, the overpressure value
shall be calculated. One shall consider that hydrogen-air mixtures containing less than
10 % of hydrogen approximately contribute to the combustion only in a small extent.
Hydrogen-air mixtures containing more than 10 % of hydrogen approximately have a
more significant contribution to the combustion.
The overpressure value decreases with increasing distance to the ignition point. See
section Error! Reference source not found. for details on the deflagration pressure
decay in the far field.

Extension of fire effects
The area where the combustion occurs is larger than the zone where the flammable
mixture was.
Note: the impulse is also very important. A longer duration of low overpressure can
cause more destruction than a higher overpressure but of short duration. (Source:
Center for Chemical Process Safety (1994). Guidelines for Evaluating the
Characteristics of Vapor Cloud Explosions, Flash Fires, and BLEVEe. American
Institute of Chemical Engineers, New York, USA)
1.1.6.2
Vented deflagration phenomena – summary
A system with an explosion venting is characterized by its vent area A v and the
pressure Pdesign above which the building structure fails. This pressure is defined by
the design of the structure.
In the following section, it is assumed that there is a homogeneous distribution of
hydrogen in the enclosure before ignition of the flammable mixture. Once the
flammable hydrogen-air mixture has been ignited, the flame front expands
52
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
spherically, and then expands to an ellipsoid when it reaches the walls of the
enclosure. In the case of a confined area, the gas cannot expand9: the unburned gas
is compressed and the pressure starts to increase exponentially (phase (a) on the
Figure 20).
The pressure continues to increase due to combustion processes until it reaches the
opening pressure P1 of the vent (see on the Figure 20).
Figure 20: Time dependence of a vented explosion in a near-cubic vessel
with an explosion relief opening at low pressure
(Lautkaski, R., 1997, “Understanding vented gas explosions”, Technical Research
Center of Finland)
Caption:
P1: opening pressure of the vent
P2: pressure of the external explosion
P3: pressure associated with the maximum rate of combustion within the room
P4: oscillatory pressure peak
At this time the unburned gas flammable mixture starts to flow out of the
enclosure. The unburned gas then forms a turbulent cloud outside the vessel. If the
volume production rate of burnt gases exceeds the volume rate of loss of unburnt and
burnt gas through the vent, the pressure rises until the flame reaches the vent
(phase (c) on Figure 20). The flame joins the turbulent cloud which burns, generating
9
When an explosion occurs in open space, the volume of gas expanses.
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an external explosion with a high pressure P2. Highly destructive effects of the
vented deflagration phenomenon may be caused by the combustion and by the
propagation of the overpressure wave outside of the enclosure.
Figure 21: Development of external explosion
(Lautkaski, R., 1997, “Understanding vented gas explosions”, Technical Research
Center of Finland)
The combustion goes on within the enclosure, which still contains unburned
flammable gas mixture. The pressure P3 is associated with the maximum rate of
combustion within the room. It typically occurs when the flame front reaches the walls.
The external explosion due to the venting of unburnt gas can generate a pressure
wave propagating back into the enclosure, contributing to an additional increase of the
internal pressure.
The flame front interacts with the enclosure; the excitation of acoustic resonances
in the gaseous combustion products within the room results in an oscillatory
pressure peak P4.
Note 1: influence of the ignition location
The overpressure effects of the vented deflagration phenomenon depend on the
location of the ignition. When the flammable mixture ignites at the back of the
enclosure (on the side opposite to the vent), the pressure of the external explosion P ext
is higher than when the mixture ignites in the middle of the enclosure. On the other
hand, the acoustic pressure Pac is smaller. Indeed, when the flammable mixture ignites
at the back of the enclosure, a smaller amount of unburned gas is left in the enclosure
after that the external explosion has happened.
When the flammable mixture ignites in a location close to the vent, there is almost no
external explosion as the front flame propagates in the system and the unburnt gases
54
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
are pushed out of the vessel. The only overpressure effect comes from the acoustic
peak.
Note 2: size of the enclosure
Depending on the size of the enclosure, the most destructive pressure can be
either the external pressure or the oscillatory pressure peak. For small
enclosures, the most destructive pressure occurs during the external explosion as the
quantity of unburned gas still in the enclosure is smaller than in large enclosures.
References:

Lautkaski, R., 1997, “Understanding vented gas explosions”, Technical
Research Center of Finland (VTT Tiedotteita)

C. R. Bauwens, J. Chaffee and S. Dorofeev, FM Global, “Experimental and
numerical study of hydrogen air deflagrations in a vented enclosure”,
ISHPMIE 2008

A. Lannoy, L'évaluation des risques d'explosions en phase gazeuse dans
l'industrie de production d'électricité Journal de Physique III 1, 8 (1991) 13591376
1.1.6.3
Review of vented deflagration configurations
Different vented deflagration configurations exist; the location of vents and their size
impact their effectiveness.

The larger the vents, the better.

Vents shall be located where the maximal overpressure is expected. This
is most often in the upper part of the room, because of the buoyancy of
hydrogen. The vent shall be located so that nobody or no equipment would
be reached by the hydrogen cloud.

Vents always staying open shall be preferred over closed vents whose
opening is triggered when the opening pressure is reached. Indeed, in
the case of initially closed vents, overpressures higher than the opening
pressure are reached in the enclosure. A competition between the combustion
and the opening dynamics occur. Besides, the vent effectiveness is not
maximal during its opening, as the free surface of the vent gradually
increases.
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Safety objectives and criteria
1.1.6.4
The safety objective is that the building structure does not fail if an explosion
occurs in the structure.
So as to assess whether this safety objective is met, the maximum pressure
reached during an explosion should be calculated (this pressure being calculated
with the assumption that the structure does not fail and is not even bended).

If this pressure is smaller than the pressure Pdesign above which the building
structure fail, the safety objective is met.

If not, safety measures should be taken, so that the maximum pressure
reached during an explosion gets smaller than the design pressure. Examples
of safety measures (detailed in the following sections) are:
1.1.6.4.1
o
Avoiding elevated hydrogen concentration
o
Avoiding flame accelerating factors
o
Properly designing the venting system.
Avoidance of elevated concentrations – criteria
Elevated concentrations should be avoided thanks to appropriate ventilation, to the
detection and isolation of hydrogen.
Note: the maximum allowed hydrogen concentration is defined by the designer of the
system. The higher this hydrogen concentration is, the more difficult it is to limit the
overpressure effects (see section 1.1.6.4.3).
1.1.6.4.2
Avoidance of flame accelerating factors – criteria
The presence of obstacle in the enclosure will generate more turbulence and then
the overpressure will be higher than in an empty room. Therefore, there should be in
the enclosure as few obstacles as possible.
1.1.6.4.3
Properly designing the venting system – criteria
Vents can be used to avoid any overpressure build-up. In order to be effective, the
vent must be correctly designed to limit the explosion pressure below the
failure pressure of the building structure. See section 1.1.6.5.2 for the calculation
procedure used to design the vent (assessment of its size).
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57
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Mitigation of deflagrations by venting
1.1.6.5
1.1.6.5.1
Principle
Explosion venting is the most wide spread deflagration mitigation technique for
enclosure. It reduces explosion-incurred pressure to an acceptable level by venting
gases out of an enclosure through a vent or number of vents of sufficient area during
the deflagration.
1.1.6.5.2
Design criteria
The size of the vent can be calculated using the Vent Sizing Technology (VST)
(Molkov, 2001; Molkov et al, 1999). The latest procedure of the VST for empty
enclosures or enclosures with insignificant influence of obstacles is as follows (Molkov
et al, 2009):
1. Calculate the value of the dimensionless reduced explosion overpressure
 red  pred / pi
2. Determine
the
Equation 7;
value
of
dimensionless
activation
pressure
3. Then calculate the value of the dimensionless pressure complex
 red /  v2.5
 v  ( pstat  pi ) / pi
static
Equation 8;
Equation 9;
4. Based on the value of
 red /  v2.5 , calculate the value of Br by using the
t
relevant equation:
If
 red

 1 : red
 5.65  Brt  2.5
2.5
2 .5
v
v
If
 red

 1 : red
 7.9  5.8  Brt0.25
2.5
v
 v2.5
Equation 10,
Equation 11;
5. Determine the appropriate values of Sui and Ei for the mixture in the enclosure
using Figure 22.
The correlations were calibrated against experimental data using the
dependence of burning velocity on hydrogen concentrations in air from
Lamoureux et al (2003). For instance, for stoichiometric hydrogen-air mixtures
at an initial pressure of 1 bar and temperature 298 K the burning velocity
Sui=1.96 m/s (Lamoureux et al, 2003; Tse et al, 2000) should be applied for
58
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vent sizing. The corresponding value of expansion coefficient of the
combustion products is Ei=6.90. The influence of the initial temperature on the
laminar
burning
velocity
can
be
estimated
by
the
formula
m
Su i  S u 0  Ti / 298 0 , where Su0 is the laminar burning velocity at 298 K, Ti
4.0
3.8
3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
8.0
7.6
7.2
6.8
6.4
6.0
5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0
Laminar burning velocity
Expansion ratio
0.1
0.2
0.3
0.4
0.5
Hydrogen volume fraction
Figure 22 : Dependence of laminar burning velocity (Lamoureux et al, 2003) and
expansion coefficient of the combustion products on hydrogen concentration in air at
an initial pressure of 1 bar and temperature 298 K (Molkov et al, 2009).
59
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
0.6
Expansion ratio
Laminar burning velocity
(m/s)
is the initial temperature; and m0 =1.7 is temperature index (Babkin, 2003).
6. Determine the vent area by solving numerically the following transcendental
equation (by changing area F until the right hand side of the equation is equal
to the left hand side):
0.8

 F
 
cui
 
F 1   v   1  0.5   2 / 3 
 
S
(
E

1
)
V

ui
i

 


0.94 0.4
(1  2  V )  S u i  ( E i  1)
0.4
0.4
Brt  3 36 0  V 2 / 3
cui  Ei /  u
Equation 12
Where empirical coefficients e=2 and g=0.94, and other parameters are:
Brt
Turbulent Bradley number
cui
Speed of sound, m/s, cui = (γuRTui/Mui)0.5
Ei
Expansion coefficient, Ei = MuiTbi/MbiTui
F
Vent area, m2
M
Molecular mass, kg/mol
pi
Initial pressure, bar abs.
pred
Reduced pressure, bar gauge
pstat
Static activation pressure, bar gauge
R
Universal gas constant, 8.31 J/K/mol
Sui
Initial burning velocity, m/s
V
Volume of enclosure, m3
γu
specific heats ratio

Pi number, 3.14
red
Dimensionless reduced pressure, Pred/Pi;
v
Dimensionless static activation pressure, v = (pstat + pi)/pi
The correlations have been calibrated against experimental data for hydrogen-air
deflagrations for the following range of conditions:

L/D ≤ 5.43;

V ≤ 37.4 m3;

0.005 < F/V2/3 < 0.34;

0 kPa ≤ pstat ≤ 13.5 kPa;

pi =1 bar abs.

0.3 ≤ red ≤ 5.
The maximum experimentally observed overpressures for initially quiescent 4-8%
hydrogen-air mixture deflagrations without turbulisers are below 10 kPa (Saffers et al,
2010). Explosion pressure increases drastically for hydrogen concentrations above
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8% by vol. and could reach an adiabatic explosion maximum in a closed vessel
already at 9% by vol. Theoretical deflagration overpressure at 9% is about 4 bar, i.e.
sufficient to destroy any building or residential structure, as they can withstand
overpressure not greater than 10 kPa. Thus, deflagrations of hydrogen-air mixtures
with concentration above 8% should be mitigated (Molkov et al, 2008).
Let us calculate vent size for the enclosure depending on the concentration of
hydrogen in air in the range 8-10.5% in assumption that the enclosure is a “standard
house construction” that could withstand overpressure 10 kPa (Lees, 1996).
The vent area F was calculated using (HYPER Report, 2009) with an assumption that
the static activation pressure is v= (pstat + pi)/pi =1, i.e. it opens with negligible
overpressure. The expansion coefficient for combustion products Ei was calculated by
Cantera software [http://www.cantera.org/] using the GRI mechanism. Laminar
burning velocity Sui of hydrogen-air mixtures in the range 8-10.5% were taken from
(Tse et al, 2000): 10.8 cm/s for 9% of hydrogen by vol., 13.1 cm/s for 10% by vol. and
14.7 cm/s for 10.5% by vol. Minimum vent area to mitigate hydrogen-air deflagration in
F, m2
the garage to the level 10 kPa in the range 8-10.5% is shown in figure below.
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
8
8.5
9
9.5
10
10.5
Hydrogen concentration, % by vol.
Figure 23: Vent area for the enclosure as a function of hydrogen concentration.
To mitigate a deflagration of a 10.5% by vol. hydrogen-air mixture, the minimum vent
size is 2.8 m2 in the enclosure of 33 m 3 volume. Hence, the venting of deflagration
system designed as described would mitigate explosion of a leaked hydrogen without
construction failure. Indeed, a venting device with such area can be mounted,
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depending on the particular location and design of the enclosure, on any on the four
walls or at the ceiling.
1.1.6.5.3
Extension of hazardous effects of vented deflagrations
In the case of a vented deflagration, the deflagration pressure decay in the far field
can be calculated thanks to the model described in section Error! Reference source
not found..
1.1.6.6
1.1.6.6.1
Mitigation of overpressure effects by separation
Extension of overpressure effects
As explained in section 1.1.6.2, the most destructive effects of the vented deflagration
phenomenon may be caused by the external explosion occurring when the flame front
reaches the vent of the enclosure. Significant overpressure effects might occur
outside of the enclosure, in front of the vent.
Hazards perimeters are defined by different pressure thresholds. These pressure
thresholds slightly differ, according to the considered country. In France, 50 mbar is
the lowest pressure threshold; it corresponds to the first high wounded level as well as
to significant material impacts. A similarity law has been developed to calculate the
overpressure decay (see section Error! Reference source not found.). Thanks to
this law, the distance where the overpressure drops to 50 mbar can be calculated, in
order to determine the appropriate safety distance in front of an enclosure vent.
1.1.6.6.2
Walls
Although this is usually not the most effective approach to ensure safety,
protective barriers can be used to protect environment from blast effects, as a
reduced pressure region is formed behind the wall.
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Figure 24: Schematic of the protective mechanism
(Source: Air Liquide)
Experiments conducted by the Health and Safety Laboratory and by the Sandia
National Laboratory showed that a lower overpressure is induced behind the
barrier as it diffracts the shock wave generated by the explosion. This reduced
pressure region is mainly proportional to the wall height but is also dependant to other
parameters (wall width, flame speed...). After this zone, the shock waves reforms.
In an appropriate design of the wall, when the shock waves reforms, the
corresponding overpressure should not be dangerous anymore for people or
structures.
On the other hand, the presence of a barrier also induces a higher overpressure
front side the barrier (due to shock wave reflexion). The barrier design is
therefore a tradeoff between increase of the pressure front side of the barrier
and decrease of the overpressure behind it.
1.1.7
Mitigation of thermal effects from jet fires
1.1.7.1
Thermal effects from jet fires – summary
Direct or indirect impact of jet fires on structures are listed below:

The increase in temperature and pressure in liquid and gas storage can lead
to a rupture of confinement,
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
Structural elements are weakened and structures or building can fail. The
intensity and duration of exposure depends on the materials under
consideration;

Jet fires can remotely ignite certain type of materials like wood.
There are three basic ways in which exposure of people to hydrogen jet fires, may
lead to incapacitation and death: hyperthermia, respiratory tract burns and body
surface burns (NFPA, 2002).

Hyperthermia or heat stroke involves prolonged exposure (approximately 15
minutes or more) to heated environments at temperatures too low to cause
burns;

Heat damage to the respiratory tract is more severe when the heated air
contains steam and can cause damage down to the deep lung;

The time from the application of heat to the occurrence of burns of various
degrees of severity, depends the heat flux to which the skin is exposed.
1.1.7.2
Extension of hazardous thermal effects
Experimental work conducted by (Schefer et al, 2006; Molina et al, 2007) aimed at
defining spatial and radiative properties of an open-flame hydrogen plume and predict
the heat fluxes at any radial (r) and axial (x) position from underexpanded jet fires (see
Figure 25). The validity of the method presented below has been compared against
experiments and demonstrated good agreement in the range ±10% of the nominal
value (Schefer et al, 2006; Molina et al, 2007).
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
x
r
Figure 25: Coordinate system for jet flame
(Source: University of Ulster)
To determine radiant heat flux, the following steps have to be followed. It is first
necessary to calculate the flame residence time using the following equation:
125.13  W f  L f  f s
2
f 
Equation 13
3  noz  d noz  U noz
2
Where:
f flame residence time (s)
Wf
flame width assuming Wf =0.17·Lf (m)
Lf
flame length (m)
fs
mass fraction of fuel at stoichiometric conditions (0.0283)
noz
density of the flow at the nozzle (kg/m 3)
dnoz
initial jet discharge diameter (m)
Unoz velocity of the flow at the nozzle (m/s)
When knowing the flame residence time, the total emitted radiative power Srad shoud
be calculated using the
Equation 14.
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 noz  H c
S rad  0.0304  ln(  f )  0.0595  m
Equation 14
Where:
Srad
total emitted radiative power (W)
m noz
hydrogen mass flow rate at the nozzle (kg/s)
Hc
heat of combustion of hydrogen (141×106 J/kg)
f
flame residence time (s)
At this point the there are two possibilities:
Determine the heat flux at a chosen location: The coordinates (r, x) correspond to
potential targets of a jet fire: occupants, building, escape routes, other source of
potential hazards like storage of flammable or toxic substance, etc. In that case, x
Equation 17. The
must be inserted in Equation 15 or Equation 16, and r in
Equation 17.
radiative heat flux at the location (x,r) is given by
Determine the coordinates for a chosen heat flux: The coordinates are extracted for a
chosen radiative heat flux and draw a contour around the jet fire.
The radiant power C*, is calculated using one of these two equations.
For Values of x/Lf≤0.65:
 x
C  1.95  
L
 f
*
3


  1.345   x

L

 f
2


  0.777 x

L

 f

  0.254


Equation 15
For values of x/Lf >0.65:
 x
C  0.212  
L
 f
*
5


  2.004   x

L

 f
4


  7.149   x

L

 f
3


  11.64   x

L

 f
2


  7.851   x

L

 f

  1.016


Equation 16
Then the radiant fraction Srad and the non-dimensional radiant power C* have to be
implemented in the
Equation 17 to calculate qrad(x,r) the radiative heat flux
at the location (x,r):
C *  Srad
qrad ( x, r ) 
4   r2
Equation 17
An example of the application of these equations is shown in Figure 26.
66
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Radial distance, r (m.)
16
1577 W/m2
4732 W/m2
25237 W/m2
Jet fire
1.577 kW/m2
12
8
4.732 kW/m2
4
25.237 kW/m2
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Axial distance, x (m)
Figure 26: Radiative heat flux regions around jet fire along jet axis, from a 400 bar
storage pressure through 5 mm nozzle (half domain)
(Source: University of Ulster)
The hot currents downstream of hydrogen jet fire can extend over very large distance,
heating the air to dangerous levels. The limit of 300OC is of interest for separation
distance as it corresponds to the temperature causing third degree burns for a 20
seconds exposure, burns to larynx after a few minutes and would make escape
improbable. This limit extends up to twice the flame length. Another important limit is
115 oC and it corresponds to a level of air temperature at which occupants can
evacuate and withstand more than 5 minutes and is reached at about 3 times the jet
flame length.
2200
2000
1800
Tair-x , K.
1600
1400
1200
1000
800
600
400
200
0
0.5
1
1.5
2
x/Lf
2.5
3
3.5
Imamura et al.
Imamura et al.
Imamura et al.
Imamura et al.
Imamura et al.
Imamura et al.
Imamura et al.
Imamura et al.
Barlow
Sandia 5 s
Sandia 20 s
Sandia 50 s
Sandia 60 s
Sandia 70 s
300 oC limit
115 oC limit
2008,
2008,
2008,
2008,
2008,
2008,
2008,
2008,
Figure 27: Normalized plume temperature from hydrogen jets
(Saffers, 2011)
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1 mm,
1 mm,
1 mm,
2 mm,
2 mm,
3 mm,
3 mm,
3 mm,
12 bar
20 bar
35 bar
3 bar
5 bar
4 bar
7 bar
10 bar
1.1.7.3
Control of direction of flow
The flow shall be directed so that it will not reach equipment or people.
For instance, flanges (which are components where hydrogen leaks might occur)
should be placed and directed in such a way that a possible leak would not cause any
domino effect.
1.1.7.4
Shielding
The basic intent of the various methods of protection is to reduce the rate of heat
transfer to the potential targets in the vicinity of a hydrogen jet fire (NFPA, 2002).
Flame shields are specifically intended to reduce the incident radiant heat flux by
preventing direct flame impingement on equipment.
Flame shields shall be properly designed (choice of the material, thickness...).
68
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.7.5
Thermal insulation
The basic intent of thermal insulation is to reduce the rate of heat transfer to the
potential targets (i.e. hydrogen tanks) in the vicinity of a hydrogen jet fire (NFPA,
2002). Thermal insulation is achieved by surrounding equipment with materials
that preferably have the main following characteristics:

Relatively non-conductive materials;

Non-combustibility and the added attribute of not producing smoke or toxic
gases when subjected to elevated temperatures;

Product reliability giving positive assurance of consistent uniform protection
characteristics;

Availability in a form that permits efficient and uniform application;

Sufficient bond strength and durability;

Resistance to weathering or erosion resulting from atmospheric conditions.
Fire protection coatings providing thermal insulation can be part of the fire protection
strategy of compressed gaseous hydrogen vessels (see in section Error! Reference
source not found.). Such coatings are currently under research.
References for section 3.2.8.:

Babkin V.S., Private communication. Institute of Chemical Kinetics and
Combustion, Siberian Branch, Russian Academy of Science, Novosibirsk,
Russia, 2003.

Lamoureux
N., Djebaili-Chaumeix
N.,
Paillard
C.-E., Flame velocity
determination for H2-air-He-CO2 mixtures using the spherical bomb,
Experimental Thermal and Fluid Science, Volume 27, Issue 4, pp 385-393,
2003.

Molkov V., Journal of Loss Prevention in the Process industries 14, 567-574,
Draft European standard pr14994, “gas explosion venting protective systems,
2001.

Molkov V., Dobashi, R., Suzuki, M., Hirano, T., Modelling of vented hydrogenair deflagrations and correlations for vent sizing, Journal of Loss Prevention in
the Process Industries, volume 12, issue 2, pp 147-156, 1999.

Molkov V., Verbecke F., Saffers J.B., Venting of uniform hydrogen-air
deflagrations in enclosures and tunnels: vent sizing and prediction of
69
© HyFacts 2012/13 – CONFIDENTIAL – not for public use
overpressure, Submitted for presentation at the 7th ISHPMIE, St. Petersburg,
Russia, July 7–11, 2008.

NFPA, Society of Fire Protection Engineers Handbook, Third edition, 2002.

Saffers, J.B, Principles of Hydrogen Safety Engineering, PhD thesis,
University of Ulster, 2011.

Tse S.D., Zhu D.L., Law C.K., Morphology and burning rates of expanding
spherical flames in H2/O2/inert mixtures up to 60 atmospheres, proceedings
of the 28th Symposium (International) on Combustion, pp. 1793–1800,
Pittsburgh, PA: the Combustion Institute, 2000.
1.1.8
Separation distances
Passive
Avoid unnecessary
conf inement
Natural ventilation
Active
Active ventilation
Detection and active
ventilation
Passive
Flow restriction
Active
Detection and Isolation
Excess f low valve
H2 release
Confined
explosive
atmosphere
Passive
Passive
Explosion venting
Separation distance
Active
Emergency
response
Strong blast
effects
Injury /
casualty
Passive
Separation distance
Flash fire
Active
Emergency
response
Unconfined
explosive
atmosphere
Escalation
Blast effects
Loss of
leak
tightness
Flame/
Jet fire
Equipment
failure
Kinetic
effects
Passive
No ignition sources
Active
Detection and power
shut-down
Figure 28: Mitigation measure: separation distances
(Source: Air Liquide)
1.1.8.1
Purpose and objective
Safety distances are a generic means for mitigating the effect of a foreseeable
incident and preventing a minor incident escalating into a larger incident. It can
be in principle applied to any facility using gaseous hydrogen.
All readily applicable prevention and mitigation measures should be applied before
considering mitigation by means of safety distances. These are to be considered as
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
one element of a comprehensive safety approach and are appropriate when
used along with other means. Separation is not always the most appropriate means.
Separation distance requirements are typically specified by means of a table
indicating the separation to be applied between the equipment considered and
elements potentially present in its environment. Different tables are used for
different applications (e.g. fuelling stations, bulk hydrogen storage systems, and
hydrogen installations in non industrial environments).
Distance in meters
Passive hydrogen systems
Category 1
(SP <= 55 MPa)
Safety distances (m)
Cat. 3
(Q > 100 kg)
VS
S
C
VS
S
C
S
C
1,5
4,0
6,0
2,0
5,0
8,0
7,0
10,0
Occupied buildings - bay-windows*
-
5,0
8,0
-
7,0
12,0
9,0
15,0
Unoccupied buildings - openable openings and air intakes
-
2,0
3,0
-
3,0
5,0
4,0
5,0
Buildings of combustible material
1,5
3,0
5,0
2,0
4,0
7,0
8,0
8,0
Flammable liquids above ground <= 4000 L
1,0
2,0
3,0
-
2,5
4,0
8,0
8,0
Flammable liquids above ground > 4000 L
1,5
3,0
5,0
2,0
4,0
7,0
8,0
8,0
5,0
5,0
Occupied buildings - openable openings and air intakes
Exposures or Sources of hazard
Category 2
(55 < SP <= 110 MPa)
Underground flammable liquid storage - vents and fill openings
3,0
-
3,0
-
Stocks of combustible material
1,0
2,0
3,0
-
2,5
4,0
8,0
8,0
Flammable gas storage above ground > 500 Nm3
1,0
2,0
3,0
-
2,5
4,0
8,0
8,0
Facility lot line
-
2,0
3,0
-
3,0
5,0
4,0
5,0
Areas not subjected to restrictions of activity
-
2,0
3,0
-
3,0
5,0
4,0
5,0
Pedestrian and vehicle low-speed passage ways
-
2,0
3,0
-
3,0
5,0
4,0
High voltage lines and trolley or train power line
-
5,0
-
5,0
10,0
Other overhead power lines
-
5,0
-
5,0
5,0
Roadways
-
5,0
-
5,0
5,0
5,0
* non-re-enforced to withstand overpressure effects
Figure 29: Example of separation distances table, for a given application
(Source: Air Liquide)
Note: in the table lines are listed exposures or sources of hazard while system
categories are listed in columns.
Note: safety distances are not intended to provide protection against
catastrophic events or major releases. This is to be achieved by other means such
as prevention, specific means of mitigation, or emergency response, which standards
also need to address.
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
1.1.8.2
Risk informed separation distances
A specific risk informed rationale has been developed to define and
substantiate separation distance requirements for refuelling stations in ISO/DIS
20100 Gaseous hydrogen – refuelling stations.
This approach, which is applicable to all types of hydrogen installations, is based on
the premise that the safety distances can be defined taking into account a criterion
on risk to exposures. The basic steps of the construction of a safety distance are
listed hereafter:

Key system characteristics or parameters that determine actual risk
impact are selected.
For fuelling stations the following principles were adopted:
o
Separation distances should not be determined only by pressure and
internal diameter.
o
They need to integrate fundamental factors such as storage system
size, operating pressure (for small systems only), system
complexity (reflected by the number of components of the system),
and exposure criticality (high in locations where there is a risk of
affecting many people at once).

Through the application of this categorization scheme, system categories
associated to a graduation of risk impact are defined. Systems belonging
to the same category can be considered to have a roughly similar risk impact,
and, therefore, a single set of separation distance requirements can apply
to these. Boundaries can be defined according to equipment types in use
(see Table 12).
Table 12 : Classification of storage systems
(Reference: Air Liquide)
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© HyFacts 2012/13 – CONFIDENTIAL – not for public use
Storage classification for determination of clearance distances
100
2
55
St
3
y
tit
an
qu kg
ed 0
or > 10
Service pressure (MPa)
P > 55 Mpa
P <= 55 MPa
1
1k
g
10
100
1000
10000
3000
100000
Water volume (L)
Figure 30: Storage classification for determination of separation distances
(Source: Air Liquide)

For smaller hydrogen installations the classification below based on the
quantity of stored hydrogen in the system, its power and its maximal
operating pressure, was adopted in the French standard NF M58-003.
Max. operating pressure
Power and mass of
stored hydrogen
< 5 bar
< 50 bar
>50 bar
>500 bar
P < 30 kW and M < 8kg
AA
A
B
C
P < 70 kW and M < 35kg
A
B
C
D
P > 70 kW and M > 35 kg
C
C
D
D
Class of the system
Table 13: Assessment of the class of a hydrogen system
(Source: French standard NF M58-003)
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For each class of the system, leak flow rates are given (by default):
Leak flow rate (g/s)
AA
A
B
C
D
Expectable
0,075
0,24
0,53
0,95
1,48
Foreseeable
0,75
2,23
5,02
8,93
13,95
Table 14: Leak flow rates (by default)
(Source: French standard NF M58-003)
Safety distances are defined, depending on the class of the system (AA, A, B,
C or D) and on the potential targets (opening of the ventilation of a building,
etc).
Class of the system
AA
A
B
C
D
-
-
-
0.6
0.6
1
1.5
2.5
3.5
4
1
2
3
4
5
Openings of non-occupied premises
-
1
2
2.5
3
Depot of combustible materials
1
1.5
2.5
3.5
4
Vehicle parking spots
0
1
2
2.5
3
Public traffic lanes
-
1
2
2.5
3
-
1
2
2.5
3
Buildings where the closest wall has a
fire-resistance rating above one hour
Buildings where the closest wall has a
fire-resistance rating below one hour
Openings
and
bay
windows
of
occupied premises
Extension of perimeter where activities
are limited
Table
15:
Example
of
safety
distances
(Source: NF M58-003)
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(in
meters)

For each system category, a risk model is used to determine the
separation distances, by application of a criterion on estimated residual
risk. Hereafter are listed the main elements integrated in this model:
o
The function providing the cumulated frequency of having a leak
greater than a given size. See in the next section how the whole
system leak frequency distribution can be determined, in function of
components leak frequency distribution.
o
The model also includes an evaluation of the conditional
probability that the leak will produce the feared consequences on
exposed objects, assuming they are close enough to be impacted.
This probability depends on the probability that the leak will generate
dangerous phenomena (probability of ignition), the probability that the
phenomena will impact the exposure (geometric factor) as well as the
probability that the phenomena (flash fire, fire, overpressure) will have
the feared effect on the exposure.
o
The consequence model enables the estimation of the distance at
which a leak of a given size can produce the feared effects if all the
conditions for these effects to materialize are present (e.g. ignition, jet
in the right direction…). The model is based on the interpolation of
flame length and flammable cloud length formulas developed by
SNL (Bill Houf).
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Table 16: Consequence model
(Source: Air Liquide)
With:
SD: separation distance (m);
LD: leak diameter (mm)
SP: service pressure (MPa);
LQ: leak flow (g/s)
LA: leak area (mm²)
o
A residual risk criteria is defined for the exposure considered, translated into
a frequency limit. Target value for the feared event frequency is set at 10-5 /yr
for non-critical exposure, and 4 10-6/yr for critical exposure.
occ./yr
Separation distance
To be applied
Frequency
10-1
1
10
3
30
Separation
distance (m)
Leaks
Cumulated frequency
of feared effects from leaks greater than X g/s
10-2
10-3
10-4
Feared
Effect
Target
10-5
10-6
0,01
0,1
1
10
100
Reference
leak size
Figure 31: Risk model applied
(Source: Air Liquide)
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Leak rate
(g/s)

Then the separation distance table is populated and the result is evaluated.
If the result is not consistent with the separation distances which are used and
accepted for existing applications, it means that the hypothesis of the model
were inadequate and should be changed before re-computing the separation
distances.
Indeed, the risk model does not provide an accurate
evaluation of risk, but allows taking into consideration the main risk
factors consistently.
Note: the separation requirements are said to be risk informed because it takes into
account the risk impact of the particular system and involves the application of a risk
criteria.
1.1.8.3
System parameters determining separation requirements
The whole system leak frequency distribution can be determined in function of
components leak frequency distribution. The main components which contribute to
leaks are fittings, joints, hoses, etc. The main pieces of equipment that are exposed
to leaks are compressors, because they are exposed to vibration, etc. By summation
of contributing component leak frequency data, the cumulated leak frequency in
function of leak size can be estimated for the whole system.
1.1.8.4
Reference hydrogen leak frequencies
Component leak frequencies data input to the risk model is derived from the work of
J. Lachance of Sandia National Laboratories (SNL), which consisted in reviewing
various sources of statistical data on component leak frequencies in order to generate
reference data for components in hydrogen service.
For each type of component, the cumulated leak frequency in function leak size
(% of flow section) can be estimated (see Figure 32).
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Figure 32: Component leak frequency in function of leak size in % of flow section
(Source: Frederic Barth, Air Liquide Hydrogen Energy, 2011,
“Risk informed separation distances for hydrogen refuelling stations”)
Reference:

Frederic Barth, Air Liquide Hydrogen Energy, 2011, “ Risk informed
separation distances for hydrogen refuelling stations”, International
Conference on Hydrogen Safety, San Francisco
1.1.8.5
Use of walls
A possible mitigation strategy to reduce exposure to hydrogen hazards is to build
protective barriers (walls). These protective barriers can be used to reduce the
extent of unacceptable consequences in hydrogen energy applications.
Walls protect from thermal effects in some configurations: with a flame impinging
at the half of the wall height, there is no flame and thermal radiation hazards on the
other side of the wall. While in other configurations, walls create additional
hazards. For instance, in some cases, a jet flame can turn back toward the jet source,
resulting in an additional hazard. Besides, the presence of a barrier induces a
higher heat flux front side the barrier and a lower heat flux behind the barrier.
As explained in the section 1.1.6.6.2, walls protect people and structures from
overpressure effects in a reduced pressure region behind the wall. On the other
hand, it also induces a higher overpressure front side the barrier.
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Besides, walls contribute to the formation of a confined atmosphere, and
prevent the system from being easily accessible.
Thus, walls are a protective mitigation strategy which should not be used
systematically as they can simultaneously protect people and structure right behind
the wall, but can also create additional hazards
1.1.9
1.1.9.1
Coverage by regulations and standards
Regulations
See section Error! Reference source not found. for a focus on European directives
and regulation.
Example of EC directive: the ATEX directive
In 1994, the European Parliament issued a directive, Directive 94/9/EC on the use of
equipment and protective systems in explosive atmospheres, the so-called ATEX
directive (European Parliament, 1994). However, the ATEX regulations did not come
into force until 2003. Equipment in first use prior to June 2003 was allowed to be used
provided that a risk assessment was carried out (Sherwen, 2012). In the Directive, it is
set out that the equipment has to be certified by a notified body. The text also stresses
that national regulations must not be a barrier to free trade within the European
Community, that is to say there must be freedom of movement for the equipment to
which the Directive applies. Directive 99/9/EC is focused on the human being and
complements the ATEX Equipment Directive (European Parliament, 1999).
1.1.9.2
Standards
Below are described the ISO and IEC standards which are the most commonly used
for the control of flammability and explosions hazards.

ISO/TR 15916:2004 provides guidelines for the use of hydrogen in its
gaseous and liquid forms. It identifies the basic safety concerns and risks, and
describes the properties of hydrogen that are relevant to safety.

ISO 26142:2010 defines the performance requirements and test methods of
hydrogen detection apparatus that is designed to measure and monitor
hydrogen concentrations in stationary applications. ISO 26142:2010 sets out
only the requirements applicable to a product standard for hydrogen detection
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apparatus, such as precision, response time, stability, measuring range,
selectivity and poisoning.
ISO 26142:2010 is intended to be used for certification purposes.

IEC 60079-29-2 gives guidance on the selection, installation, safe use and
maintenance of electrically operated group II apparatus intended for use in
industrial and commercial safety applications, for the detection and
measurement of flammable gases complying with the requirements of IEC
60079-29-1.


20100:2008 specifies the characteristics of outdoor public and non-public
fuelling stations that dispense gaseous hydrogen used as fuel onboard land
vehicles of all types.

IEC 600796-10 specifies the requirements for the construction and testing of
oil-immersed electrical equipment, oil-immersed parts of electrical equipment
and Ex components in the type of protection oil immersion "o", intended for
use in explosive gas atmospheres.

IEC 60204-1 applies to the application of electrical, electronic and
programmable electronic equipment and systems to machines not portable by
hand while working, including a group of machines working together in a coordinated manner.

IEC 62282-3-300:2012 provides minimum safety requirements for the
installation of indoor and outdoor stationary fuel cell power systems in
compliance with IEC 62282-3-100. IEC 62282-3-100:2012(E) is applicable to
stationary fuel cell power systems intended for indoor and outdoor
commercial, industrial and residential use in non-hazardous (unclassified)
areas.

ISO 22734-1:2008 and ISO 22734-2:2011 defines the construction, safety and
performance requirements hydrogen generators, using electrochemical
reactions to electrolyse water to produce hydrogen and oxygen gas. defines
See the fields of application of these standards in Table 17.
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Fields of application of the standards
Prevention of
Standard name
Prevention & mitigation
of hydrogen releases
flammable /
Prevention of
ignition
Detection of leaks
sources
hazardous
mixtures by
means of
Prevention and
Mitigation of
mitigation of
thermal effects
explosions
from jet fires
Separation
distances
ventilation
ISO/TR 15916:2004 Basic
considerations for the
safety of hydrogen
systems
 Limited cover in clauses
4.2.3 Fluid delivery lines,
piping, joints and seals and
 Discussion of
7.2.5 Considerations for
ignition sources
piping, joints and
connections
ISO 26142:2010 Hydrogen
detection apparatus -Stationary applications


 Detection limits and
ISO/TS 20100:2008
Gaseous hydrogen -Fuelling stations
activation thresholds
 Presence of isolation
valve (in clause 21.3.
 Sections 21
 Detection limits and
activation thresholds
Safety systems and
15.5. Prevention of
accumulation of
Safety and emergency
ignitable mixtures in
shut-off systems)
enclosures
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 Clause
15.3.1.
 Lay-out
Construction of
requirements
enclosure
and location of
containing
points of
hydrogen
potential leaks
systems

 Stationary
ISO 22734-1 and -2:2008
Hydrogen generators
using water electrolysis
process
fuel cell power

systems Installation
clause
IEC 60079-29-2 Explosive
atmospheres - Gas
detectors
IEC 62282-3-3 Fuel cell
technologies - Stationary
fuel cell power systems Installation
 Selection, installation
 Selection,
and use
installation and use
 Use of detection in
 Clauses 8.3. Fuel shut
fuel cell installations
off and piping and 8.4.
indoors: clause 7.1.2.
Connections to auxiliary
Combustible gas
media supply and media
detection (indoor
disposal
installations only)
 Classification
IEC 60079-6-10 Explosive
of areas for
atmospheres - Equipment
proper selection
protection by oil
of equipment in
immersion "o"
order to prevent
ignition sources
IEC 60204-1 Safety of
machinery - Electrical
equipment of machines
 Grounding
and bonding
Table 17: Standards
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1.2
Venting systems
1.2.1
Description
Venting systems are used to pipe outdoors hydrogen flows from vents and safety
relief equipment. Hydrogen should be piped to a safe location, where it will not generate
any hazard for persons or neighboring structures.
1.2.2
Safety measures and coverage by standards
The design of hydrogen vent stacks should take into account these three main risks:

The risk of explosion due to the delayed ignition of the explosive hydrogen/air
mixture out of the vent stack.

The risk of thermal radiation generated by the jet fire (the flame out of the vent
stack).

The risk of Deflagration to Detonation Transmission (DDT) inside the stack at the
beginning of the venting (see explanations on the DDT in section Error! Reference
source not found..).
Below are listed some examples of design rules for hydrogen venting systems:

Conditions on the vent outlet location:
Explosion venting shall be provided in exterior walls or roof only.
The vent outlet location should be such that the vent may be used without any
limitations and without creating hazardous conditions for operation, maintenance
and emergency responses. The vent outlet should be away from personnel areas,
from electrical lines and from other ignition sources, air intakes, building openings
and overhangs.
The vent outlet location (height, and distance to exposures) should be such that
given limits on maximum hydrogen concentration, on maximum thermal radiation
and on maximum overpressure effects are not exceeded under any foreseeable
venting situation10.
10
The limits applicable to the outlet of a venting system used for the emergency discharge of gaseous hydrogen
storage systems slightly differ from those specified in this section.
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
The maximum pressure drop resulting from the sum of design flows of all the vent
devices discharging into the common vent system should not exceed 10% of the
lowest set pressure of all the relief valves collected.

The vent piping diameter should not be smaller than the diameter of any pressurerelief valve outlet, and large enough to avoid exceeding the maximum allowable
pressure drop previously specified.

Design pressure should be such that the stack should withstand the overpressure
generated by an eventual detonation (in the cases where a Deflagration to
Detonation Transmission would occur).

Cryogenic and non-cryogenic hydrogen should be vented through distinct venting
systems.
Coverage by standards:

ISO/DIS 20100
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