1 INTRODUCTION
1.1 For the carriage of liquefied gases in bulk by ships, the ships should
comply with the relevant requirements in the IGC Code, as amended by resolution MSC.370(93) ("the Code"). The scope of the Code provided in
paragraph 1.1.1 is:
-
"The Code applies to ships regardless of their size, including those
of less than 500 gross tonnage, engaged in the carriage of liquefied gases
having a vapour pressure exceeding 0.28 MPa absolute at a temperature of 37.8°C,
and other products, as shown in chapter 19, when carried in bulk".
1.2 A ship carrying liquefied hydrogen in bulk (hereinafter called
"liquefied hydrogen carrier") should comply with the Code.
1.3 The Code requires that a gas carrier should comply with the minimum
requirements for the cargo listed in chapter 19. However, the requirements for liquefied
hydrogen are not specified in the Code.
1.4 This annex provides the interim recommendations, as referred to in
paragraph 5 of the preamble of the Code, for the carriage of liquefied hydrogen in bulk,
which are intended to provide the basis for the future minimum requirements for the
carriage of this cargo.
1.5 These recommendations have been developed under the assumption that a
liquefied hydrogen carrier does not carry liquefied gases other than liquefied hydrogen.
These recommendations, therefore, are not applicable to liquefied hydrogen carriers
carrying gases other than liquefied hydrogen.
1.6 In the Code, reference is made to paragraph 5 of the Preamble; paragraph
1.1.6.1; and Note No.8 on completion of certificate in "model form of international
certificate of fitness for the carriage of liquefied gases in bulk" in appendix 2 to the
Code.
2 INTERIM RECOMMENDATIONS FOR CARRIAGE OF LIQUEFIED HYDROGEN IN BULK
2.1 The Interim Recommendations for the carriage of liquefied hydrogen in
bulk have been developed based on the results of a comparison study of similar cargoes
listed in chapter 19 of the Code, e.g. liquefied natural gas.
2.2 In the Code, chapter 19 governs the application of general requirements
for respective cargoes. Selections of the general requirements for respective cargoes
are expressed in columns 'c' to 'g'. In addition to general requirements, special
requirements may apply to specific cargoes depending on the properties/hazards of the
cargoes.
2.3 Tables 1 and 2 specify the proposed selection of the general
requirements and the special requirements, respectively, for liquefied hydrogen.
Table 1: Interim Recommendations
for carriage of liquefied hydrogen in bulk
a
|
b
|
c
|
d
|
e
|
f
|
g
|
h
|
i
|
Product
name
|
|
Ship
type
|
Independent tank
type C required
|
Control of vapour
space within cargo tanks
|
Vapour
detection
|
Gauging
|
|
Special
requirements
|
Hydrogen
|
|
2G
|
-
|
-
|
F
|
C
|
|
See table 2
|
Table 2: Special Requirements for
carriage of liquefied hydrogen in bulk
No.
|
Special Requirement
|
Related hazard
|
1
|
Requirements for materials whose design temperature is
lower than -165°C should be agreed with the Administration, paying
attention to appropriate standards. Where minimum design temperature is
lower than -196°C, property testing for insulation materials should be
carried out with the appropriate medium, over a range of temperatures
expected in service.
|
Low temperature
(see 4.2.1)
|
2
|
Materials of construction and ancillary equipment such as
insulation should be resistant to the effects of high oxygen
concentrations caused by condensation and enrichment at the low
temperatures attained in parts of the cargo system (refer to the
requirement for nitrogen).
|
Low temperature
(see 4.2.2)
|
3
|
For cargo pipes containing liquid hydrogen and cold
hydrogen vapour, measures should be taken to prevent the exposed
surfaces from reaching -183°C. For places where preventive measures
against low temperature are not sufficiently effective, such as cargo
manifolds, other appropriate measures such as ventilation which avoids
the formation of highly enriched oxygen and the installation of trays
recovering liquid air may be permitted in lieu of the preventive
measures. Insulation on liquid hydrogen piping systems exposing to air
should be of non-combustible material and should be designed to have a
seal in the outer covering to prevent the condensation of air and
subsequent oxygen enrichment within the insulation.
|
Low temperature
(see 4.2.2)
|
4
|
Appropriate means, e.g. filtering, should be provided in
cargo piping systems to remove impure substances condensed at low
temperature.
|
Low temperature
(see 4.2.3)
|
5
|
Pressure relief systems should be suitably designed and
constructed to prevent blockage due to formation of water or
ice.
|
Low temperature
(see 4.2.4)
|
6
|
At places where contact with hydrogen is anticipated,
suitable materials should be used to prevent any deterioration owing to
hydrogen embrittlement, as necessary.
|
Hydrogen embrittlement (see 4.3)
|
7
|
All welded joints of the shells of cargo tanks should be
of the in-plane butt weld full penetration type. For dome-to-shell
connections only, tee welds of the full penetration type may be used
depending on the results of the tests carried out at the approval of the
welding procedure.
|
Permeability
(see 4.4.1)
|
8
|
Double tube structures ensuring no leakage, or fixed
hydrogen detectors being capable of detecting a hydrogen leak, should be
provided for places where leakage of hydrogen may occur, such as cargo
valves, flanges, and seals.
|
Permeability
(see 4.4.2)
|
9
|
Helium or a mixture of 5% hydrogen and 95% nitrogen
should be used as the tightness test medium for cargo tank and cargo
piping.
|
Permeability
(see 4.4.3)
|
10
|
The amount of carbon dioxide carried for a carbon
dioxide fire-extinguishing system should be sufficient to provide a
quantity of free gas equal to 75% or more of the gross volume of the
cargo compressor and pump rooms in all cases.
|
Fire by Hydrogen
(see 4.7.3)
Wide
range of flammability limits
(see 4.10)
|
11
|
When deterioration of insulation capability by single
damage is possible, appropriate safety measures should be adopted taking
into account the deterioration.
|
High pressure
(see 4.8)
|
12
|
When vacuum insulation is used for a cargo containment
system, the insulation performance should be evaluated to the
satisfaction of the Administration based on experiments, as
necessary.
|
General
(see 4.1)
|
13
|
Appropriate measures should be provided to prevent vents
becoming blocked by accumulations of ice formed from moisture in the
air.
|
Low temperature
(see 4.2.2)
|
14
|
Due consideration should be given to means for handling
boil-off gas.
|
High pressure
(see 4.8)
|
15
|
Due consideration should be given to static electricity
associated with rotating or reciprocating machinery including the
installation of conductive machinery belts and precautionary measures
incorporated in operating and maintenance procedures. Anti-static
clothing and footwear, and a portable hydrogen detector should be
provided for each crew member working in the cargo area.
|
|
16
|
An operation manual for a liquefied hydrogen carrier
should include limitations of various operations in relation to
environmental conditions.
|
Wide range of flammability limits
(see
4.10)
|
17
|
An appropriate procedure should be established for
warm-up, inert gas purge, gas-free, hydrogen purge and pre-cooling. The
procedure should include:
-
.1 selection of inert gas in relation to
temperature limit;
-
.2 measurement of gas concentration;
-
.3 measurement of temperature;
-
.4 rates of supply of gases;
-
.5 conditions for commencement, suspension,
resuming and termination of each operation;
-
.6 treatment of return gases; and
-
.7 discharge of gases.
|
Prevention of dangerous purging operation
(see 4.11)
|
18
|
Only almost pure para-hydrogen (i.e. more than 95%)
should be loaded in order to avoid excessive heating by ortho- to
para-hydrogen conversion.
|
General
(see 4.1)
|
19
|
Fire detectors for detecting hydrogen fire should be
selected after due deliberation, taking into account the features of
hydrogen fire, to the satisfaction of the Administration.
|
Features of hydrogen fire
(see
4.7.4)
|
20
|
At the design stage, dispersion of hydrogen from vent
outlets should be analysed in order to minimize risk of ingress of
flammable gas into accommodation spaces, service spaces, machinery
spaces and control stations. Extension of hazardous areas should be
considered based on the results of the analysis.
|
Low density and high diffusivity
(see
4.5)
|
21
|
Due consideration should be given to appropriate safety
measures to prevent formation of explosive mixture in the case of a
leakage of hydrogen, including:
-
.1 installation of hydrogen detectors in order to
detect a possible ground-level travel of low temperature
hydrogen gas, and at high points in spaces where warm
hydrogen gas can be trapped; and
-
.2 application of "best practice" for land-based
liquid hydrogen storage taking into account appropriate
guidance such as "Cryogenics Safety Manual – Fourth Edition
(1998)"8).
|
General
(see 4.1)
|
22
|
In the case that fusible elements are used as a means of
fire detection required by paragraph 18.10.3.2 of the Code, flame
detectors suitable for hydrogen flames should be provided in addition at
the same locations. Appropriate means should be adopted to prevent the
activation of ESD system owing to false alarm of flame detectors, e.g.
avoiding activation of ESD system by single sensor (voting
method).
|
Fire hazard
(see 4.7.4)
|
23
|
Consideration should be given to enhance the ventilation
capacity of the enclosed spaces subject to liquefied hydrogen leakage,
taking into account the latent heat of vapourization, specific heat and
the volume of hydrogen gas in relation to temperature and heat capacity
of adjacent spaces.
|
Low density and high diffusivity
(see
4.5)
|
24
|
Liquid and gas hydrogen pipes should not pass through
enclosed spaces other than those referred to in paragraph 5.2.2.1.2 of
the Code, unless:
-
.1.1 the spaces are equipped with gas detection
systems which activate the alarm at not more than 30% LFL
and shut down the isolation valves, as appropriate, at not
more than 60% LFL (see sections 16.4.2 and 16.4.8 of the
Code); and
-
.1.2 the spaces are adequately ventilated;
or
-
.2 the spaces are maintained in an inert
condition.
This requirement is not applicable to spaces
constituting a part of a cargo containment system using vacuum
insulation where the degree of vacuum is monitored.
|
Permeability
(see 4.4)
|
25
|
A risk assessment should be conducted to ensure that
risks arising from liquefied hydrogen cargo affecting persons on board,
the environment, the structural strength or the integrity of the ship
are addressed. Consideration should be given to the hazards associated
with properties of liquefied hydrogen and hydrogen gas, physical layout,
operation and maintenance, following any reasonably foreseeable failure.
For the risk assessment, appropriate methods, e.g. HAZID, HAZOP,
FMEA/FMECA, what-if analysis, etc., should be adopted taking into
account IEC/ISO 31010:2009 "Risk management – Risk assessment
techniques"7) and SAE ARP 5580-2001 "Recommended failure
modes and effects analysis (FMEA) practices for non-automobile
applications"9).
|
General
(see 4.1)
|
26
|
Relief valve sizing should be undertaken for the most
onerous scenario. Whether this scenario is brought into existence due to
fire or by loss of vacuum from the overall insulation system should be
assessed and the resulting magnitude of the heat flux on the containment
system considered in each case.
|
High pressure hazard
(see
4.8)
|
27
|
A filling limit exceeding 98% at reference temperature
should not be permitted.
|
High pressure hazard
(see
4.8)
|
28
|
Bolted flange connections of hydrogen piping should be
avoided where welded connections are feasible.
|
Permeability
(see 4.4.2)
|
29
|
Due consideration should be given to the invisible
nature of hydrogen fire.
|
Fire hazard
(see 4.7.1)
|
3 EXPLANATION ON GENERAL REQUIREMENTS
3.1 Properties of liquefied hydrogen
The application of general requirements in the Code for liquefied hydrogen
has been considered based on a comparison study on the physical properties of liquefied
hydrogen and LNG. LNG and liquefied hydrogen are cryogenic liquids, non-toxic, and
generate flammable high pressure gas. For reference, table 3 shows the comparison of
physical properties of hydrogen and methane, the major component of LNG.
Table 3: Comparison of physical
properties of Hydrogen and Methane
|
Hydrogen
|
Methane
|
References
|
Boiling temperature
(K)*
|
20.3
|
111.6
|
ISO1), Annex A,
Table A.3
|
Liquid density
(kg/m3)*
|
70.8
|
422.5
|
ISO1), Annex A,
Table A.3
|
Gas density
(kg/m3)** (Air: 1.198)
|
0.084
|
0.668
|
NIST
RefProp10)
|
Viscosity (g/cm•s x
10-6)
Gas
Liquid
|
8.8
13.49
|
10.91
116.79
|
NIST RefProp10)
NIST RefProp10)
|
Flame temperature in air
(°C)
|
2396
|
2230
|
Calculated using Cantera
and GRI 3.0 mechanism
|
Maximum burning velocity
(m/s)
|
3.15
|
0.385
|
Calculated using Cantera
and GRI 3.0 mechanism
|
Heat of vapourization
(J/g)*
|
448.7
|
510.4
|
ISO1), Annex A,
Table A.3
|
Lower flammability limit
(% vol. fraction)***
|
4.0
|
5.3
|
ISO1), Annex B,
Table B.2
|
Upper flammability limit
(% vol. fraction)***
|
75.0
|
17.0
|
ISO1), Annex B,
Table B.2
|
Lower detonation limit
(%vol. fraction)***
|
18.3
|
6.3
|
ISO1), Annex B,
Table B.2
|
Upper detonation limit (%
vol. fraction) ***
|
59.0
|
13.5
|
ISO1), Annex B,
Table B.2
|
Minimum ignition energy
(mJ)***
|
0.017
|
0.274
|
ISO1), Annex B,
Table B.2
|
Auto-ignition temp.
(°C)***
|
585
|
537
|
ISO1), Annex B,
Table B.2
|
Toxicity
|
Non
|
Non
|
Orange
book5)
|
Temperature at critical
point (K)
|
33.19****
|
190.55
|
Hydrogen: ISO1),
Annex A, Table A.1
Methane: The Japan Society of
Mechanical Engineers, Data Book, Thermophysical Properties of Fluids
(1983)
|
Pressure at critical point
(kPaA)
|
1297****
|
4595
|
Hydrogen: ISO1),
Annex A, Table A.1
Methane: The Japan Society of Mechanical
Engineers, Data Book, Thermophysical Properties of Fluids
(1983)
|
Remarks:
* At their normal boiling points for comparison purpose.
** At normal temperature and pressure.
*** Ignition and combustion properties for air mixtures at 25°C and 101.3
kPaA.
**** Normal Hydrogen.
3.2 Explanation on respective requirements
3.2.1 Ship type (column 'c')
3.2.1.1 As a result of the studies, the following points were noted in
relation to ship type allocated in the Code:
-
.1 type 1G is allocated only to dangerous goods of class 2.3footnote in the International Maritime Dangerous Goods
Code, but not to class 2.2 and class 2.1;
-
.2 type 2G and type 2PG are allocated mainly to non-toxic flammable
gases of class 2.1; and
-
.3 type 3G is allocated only to non-flammable and non-toxic gases of
class 2.2.
3.2.1.2 "Type 2PG" is not applicable to liquefied hydrogen for the reason
that the design temperature is lower than -55°C. Taking into account that liquefied
hydrogen is a class 2.1 dangerous good, it is appropriate to allocate "type 2G" to
liquefied hydrogen.
3.2.2 Independent tank type C required (column 'd')
Independent tank type C is allocated only to dangerous goods of class 2.3
whose vapour density is heavier than air. Independent tank type C is considered not to
be required for liquefied hydrogen.
3.2.3 Control of vapour space within cargo tank (column 'e')
Special environment controls such as drying and inerting are generally
required for liquid chemical products in consideration of the reactivity of cargo vapour
and air. As is the case for LNG, it is considered not to be necessary to apply such
requirements for liquefied hydrogen.
3.2.4 Vapour detection (column 'f')
Because hydrogen is flammable and non-toxic, it is appropriate to require
Flammable (F) as vapour detection for liquefied hydrogen.
3.2.5 Gauging (column 'g')
On the grounds that Closed (C) gauging is required, in principle, for
flammable or toxic cargoes, such as methane, it is considered to be appropriate to
require Closed (C) gauging for hydrogen, taking into account that hydrogen has high
ignitability and a wide flammable range in air and that closed gauging is effective to
prevent leakage of gases into air.
4 SPECIAL REQUIREMENTS AGAINST HAZARDS OF LIQUEFIED HYDROGEN
4.1 Hazards of liquefied hydrogen to be considered
4.1.1 The hazards related to liquefied hydrogen are low ignition energy, a
wide range of flammability limits, low visibility of flames in case of fire, high flame
velocity which may lead to the detonation with shockwave, low temperature and
liquefaction/solidification of inert gas and constituents of air which may result in an
oxygen-enriched atmosphere, high permeability, low viscosity, and hydrogen embrittlement
including weld metals. Where vacuum insulation is adopted, due consideration should be
given to the possibility of untimely deterioration of insulation properties at the
envisaged carriage temperatures of liquid hydrogen. The vacuum insulation evaluation
should be specified for the normal range or upper limit of cold vacuum pressure (CVP),
and loss of vacuum should be defined with respect to this value. Accordingly, effect of
vacuum pressure should be taken into account at the time of design and testing of cargo
containment systems and piping. Supporting structure and adjacent hull structure should
be designed taking into account the cooling owing to loss of vacuum insulation.
4.1.2 Hydrogen is essentially a mixture of ortho- and para-hydrogen, with an
equilibrium concentration of 75% ortho-hydrogen and 25% para-hydrogen at ambient
temperature. When liquefied at 20K, there is a slow but continuous transformation of
ortho-hydrogen to para-hydrogen. The exothermic conversion of the nuclear spin isomers
of hydrogen (ortho- to para-hydrogen) may take place and the effect of the conversion
may have an impact on the cooling capacity and relief valve capacity of the vessel's
equipment.
4.1.3 For consideration on the special requirements for liquefied hydrogen
carriers, bibliographic studies were conducted using the references at the end of this
document, in particular, ISO/TR 15916, "High Pressure Gas Safety Act"1)
(Japanese law), "Safety standard for hydrogen and hydrogen system" by AIAA2)
and NFPA 2 "Hydrogen Technologies Code"6). The majority of special
requirements for liquefied hydrogen carriers are provided based on ISO/TR 15916. This
standard refers to liquefied hydrogen tank storage facilities on shore, tank trucks and
so on, and includes basic viewpoints when discussing the properties of liquefied
hydrogen.
4.1.4 Trace amounts of air will condense or solidify in an environment with
liquid hydrogen possibly resulting in an unstable and explosive mixture. Precautions
should be taken to assure that the possibility of condensed air is accounted within
properly secured hazard areas.
4.2 Low temperature hazard
4.2.1 Selection of appropriate material
4.2.1.1 Tables 6.3 and 6.4 in the Code prescribe material selection for
piping or cargo tanks whose design temperature is -165°C or higher. According to Note 2
of table 6.3 and Note 3 of table 6.4 of the Code, the requirements for materials whose
design temperatures are lower than -165°C should be specially agreed with the
Administration. In this regard, the publication by AIAA2) introduces some
appropriate materials corresponding to the design temperature and the Administration
should take into account such references for the material selection.
4.2.1.2 Although paragraph 4.19.3 in the Code requires testing of materials
used for thermal insulation for various properties adequate for the intended service
temperature, the minimum test temperature is -196°C. The requirements in the Code do not
refer to the normal boiling point of hydrogen, being -253°C. In case of carriage of
liquefied hydrogen, special requirements should be provided to consider the lower design
temperature.
4.2.2 Measures for condensed air
4.2.2.1 In the case of nitrogen whose normal boiling point is -196°C, for
which air condensation and oxygen enrichment are concerns, the following special
requirement has already been included in paragraph 17.17 in the Code:
A similar special requirement is applicable to hydrogen.
4.2.2.2 A vent may be blocked by accumulation of ice formed from moisture in
the air, which may result in excessive pressure leading to rupture of the vent and
relevant piping (see paragraph 4.2.4).
4.2.3 Removal of impure substances condensed
The removal of impure substances, such as those contained in condensate in
pipes, should be separately considered. Installation of filters can be an appropriate
measure and should be stipulated as a special requirement.
4.2.4 Prevention of blockage due to formation of water or ice
Pressure relief systems may become blocked due to formation of water or ice,
depending on the temperature and humidity of air, resulting from the low temperature of
the cargo and its vapour (see paragraph 4.2.2). Appropriate means should be provided to
prevent such phenomena.
4.3 Hydrogen embrittlement
4.3.1 Selection of appropriate materials should be required to prevent
failures owing to hydrogen embrittlement. The publication by AIAA2)
introduces some appropriate materials resistant to hydrogen embrittlement, and concludes
that aluminium is the material least affected.
4.3.2 International or national standards should be followed for the
selection of materials for the design of liquefied and gaseous hydrogen installations in
a marine environment.
4.4 Permeability
4.4.1 Prevention of leakage from cargo tanks
To mitigate leakage of hydrogen, it is deemed appropriate to require "butt
weld full penetration" type welds, regardless of tank types, taking into account the
high permeability of hydrogen. Furthermore, dome-to-shell connections welds and nozzle
welds should be designed with full penetration regardless of tank types, taking into
account paragraphs 4.20.1.1 and 4.20.1.2 of the Code.
4.4.2 Prevention of leakage from pipes
To mitigate undetected accumulation of hydrogen in a confined space,
effective measures should be employed to reduce the possibility of leakage of hydrogen,
taking its high permeability into account. Effective measures can be double tube
structures, or fixed hydrogen leak detectors in areas assessed as being highly hazardous
with regard to hydrogen leakage. Hydrogen leakage through welds, joints and seals is an
important consideration for the design of hydrogen systems and an important operational
issue.
4.4.3 Implementation of effective tightness test
4.4.3.1 Tightness tests for cargo tanks and cargo pipes/valves are required
by paragraphs 4.20.3.2, 5.13.1 and 5.13.2.3 in the Code respectively. Helium or a
mixture of 5% hydrogen and 95% nitrogen should be used as the medium for tightness
tests, instead of air, because the permeability of hydrogen is high.
4.4.3.2 For a hydrogen installation, the pipework should be pressure-tested
at its design pressure. Consideration should be given to using oxygen-free nitrogen with
a small molecule tracer gas, such as helium as the test medium and an electronic leak
detector for identifying leaks.
4.4.4 Confirmation of appropriate operating procedure
Instructions/manuals containing the operating procedures for the prevention
of leakage during transport, methods for early detection in case of leakage, and
appropriate measures after such events, should be provided. For this, paragraph 18.3 of
the Code requires that the information shall be on board and available to all concerned,
giving the necessary data for the safe carriage of cargo. In detail, the Code requires
such information on action to be taken in the event of spills or leak, countermeasures
against accidental personal contact, procedures for cargo transfer, and emergency
procedures to be on board. With regard to the manuals on procedures for liquefied
hydrogen during carriage and transfer operations, the requirements in the Code are
applicable and no special requirement is necessary.
4.5 Low density and high diffusivity
Though low density and high diffusivity of hydrogen may reduce the
possibility of formation of a flammable atmosphere in open spaces, adequate ventilation
is necessary for enclosed spaces in cargo areas where formation of hydrogen-oxygen/air
mixture may occur. Paragraph 12.2 of the Code requires fixed ventilation systems or
portable mechanical ventilation for such enclosed spaces. These requirements in the Code
are applicable to liquefied hydrogen carriers and no special requirement is necessary in
this regard.
4.6 Ignitability
4.6.1 The Code requires electrical bonds of the piping and the cargo tanks
in paragraph 5.7.4, exclusion of all sources of ignition in paragraph 11.1.2, electrical
installations to minimize the risk of fire and explosion from flammable products in
paragraph 10.2.1 and so on, in order to prevent ignition of flammable cargoes.
4.6.2 The Code requires compliance with the relevant standards issued by the
International Electrotechnical Commission (IEC) and the IEC standards specify the
details of such safety measures depending on the respective properties of flammable
gases including hydrogen. No special requirement is necessary with regard to
ignitability of hydrogenfootnote.
4.7 Fire hazard
4.7.1 Safety of personnel in case of fire
To avoid the effects of flame and UV radiation produced by a hydrogen fire,
it is effective to use firefighter's outfits and protective equipment. The Code already
requires firefighter's outfits for ships carrying flammable products in paragraph 11.6.1
and safety equipment in paragraph 14.3. This issue should be considered as the matter of
cargo information required by paragraph 18.3 of the Code. Due consideration should be
given to the invisible nature of hydrogen fire.
4.7.2 Compatibility of fire-extinguishing systems
Dry chemical powder fire-extinguishing or carbon dioxide fire-extinguishing
systems are considered to be effective in case of hydrogen fire and such
fire-extinguishing systems are already required by paragraphs 11.4 and 11.5 of the Code.
Special requirements for installation of other types of fire-extinguishing systems are
considered unnecessary, except with regard to the increased of amount of carbon dioxide
required, as mentioned in the next paragraph in this document.
4.7.3 Increase of the amount of gas for carbon dioxide fire-extinguishing
systems
4.7.3.1 Paragraph 11.5.1 of the Code requires as follows:
-
"Enclosed spaces meeting the criteria of cargo machinery spaces in
1.2.10, and the cargo motor room within the cargo area of any ship, shall be
provided with a fixed fire-extinguishing system complying with the provisions of
the FSS
Code and taking into account the necessary concentrations/application
rate required for extinguishing gas fires."
4.7.3.2 Chapter 5 of the FSS Code,
i.e. Fixed gas fire-extinguishing systems, requires that the quantity of carbon dioxide
for cargo spaces, unless otherwise provided, shall be sufficient to give a minimum
volume of free gas equal to 30% of the gross volume of the largest cargo space to be
protected in the ship, in paragraph 2.2.1.1.
4.7.3.3 On the other hand, NFPA 123) requires that the design
quantity of carbon dioxide for hydrogen fire should be 75% or more of the gross volume
of the protected space. The special requirement for an increased amount of carbon
dioxide should be provided for carbon dioxide fire-extinguishing systems.
4.7.4 Features of hydrogen fire
Hydrogen burns at high temperature, but generally gives off less radiant
heat than propane or other hydrocarbons (e.g. only about 10% of that radiated by an
equal-sized propane flame). Although the heat radiated by a hydrogen flame is also
relatively low compared to hydrocarbons, it is important to take into account the
differences in heats of combustion, burning rate and flame size. Hydrogen flames are
colourless or nearly colourless. Both of these characteristics make it more difficult to
detect a hydrogen fire. Even relatively small hydrogen fires are very difficult to
extinguish. The only reliable approach to extinguish a fire is to shut off the source of
hydrogen supply.
4.8 High pressure hazard
4.8.1 High pressure is a hazard common to hydrogen and other flammable gases
listed in the Code. To prevent overpressure, the Code requires various measures such as
pressure control and pressure design. Specifically, paragraph 8.2, in regard to the
provision of pressure control of cargo tanks, requires fittings of pressure relief
valves to the cargo tanks. Furthermore, paragraph 7.1.1 requires temperature control by
the use of mechanical refrigeration and/or design to withstand possible increases of
temperature and pressure. In addition, paragraph 15.2 specifies the filling limit of
cargo tanks taking into account cargo volume increase by its thermal expansion. These
requirements are applicable for hydrogen and no special requirement is considered
necessary in this regard.
4.8.2 Vacuum insulation systems are likely to be used for liquefied hydrogen
containment systems and the insulation capability of such systems may be adversely
affected by damage to the system, depending on the design of the system. If a rapid
deterioration of the insulation system took place, rapid increase of temperature in the
cargo tank would occur and/or the rate of vapourization of liquefied hydrogen might
exceed the capacity of pressure relief valves. To prevent such dangerous deterioration
of insulation, appropriate safety measures should be taken.
4.8.3 Boil-off may be a bigger problem for hydrogen than for LNG in
particular when insulation properties have deteriorated. Means of handling boil-off gas
should be carefully considered taking into account the following issues:
-
.1 Re-liquefaction of hydrogen involves very specific and costly
equipment. Cargo cooling in order to avoid boil-off shows the same kind of
issues; and
-
.2 Notwithstanding the provision in paragraph 7.4.1 of the Code,
thermal oxidation of hydrogen may be permitted in accordance with paragraph 1.3
of the Code.
4.8.4 The special requirements in these aspects are considered necessary.
4.9 Health hazard
4.9.1 Human safety concern under low temperature
With regard to the influences of cold hydrogen on persons' bodies, suitable
protective equipment is effective. In this aspect, paragraph 14.1 of the Code requires
suitable protective equipment taking into account the character of the products,
therefore, no special requirement is considered necessary.
4.9.2 Static electricity
Hydrogen ignition energy is very low and hydrogen can be easily ignitable by
static electricity and due consideration should be given to this issue, in accordance
with the requirement in the Code on suitable protective equipment.
4.9.3 Oxygen depletion and asphyxiation
Leakage of hydrogen may cause low level of oxygen and associated
asphyxiation.
4.10 Wide range of flammable limits
4.10.1 Extinguishing hydrogen fire
4.10.1.1 As mentioned in paragraph 4.6, for flammable products the Code
already requires elimination of sources of ignition, including use of electrical
installations of appropriate types in order to minimize the risk of fire and explosion.
No special requirement is considered necessary with regard to ignitability of hydrogen.
4.10.1.2 Furthermore, with regard to the wide range of flammable limits of
hydrogen, the increased quantities of carbon dioxide as a fire-extinguishing medium
should be specified as mentioned in paragraph 4.7. No additional special requirement is
considered to be necessary with regard to the wide range of flammable limits of
hydrogen.
4.10.2 Disposal of cold hydrogen gas
The wide flammability range makes disposal of cold hydrogen gas a major
hazard. Cold plumes downwind and inadequate dilution to below 4% provide possibilities
for flash-back to the vent from distant ignition sources outside safety-controlled
areas. The low ignition energy and wide flammable range may present significant
challenges.
4.11 Prevention of dangerous purging operation
4.11.1 During cargo operations for maintenance, pipes and tanks should be
purged with an inert gas or inert gases as illustrated in the figure below. For safety,
due consideration should be given to temperature and boiling points of the inert gases.
Residual pockets of hydrogen or the purge gas will remain in the enclosure if the
purging rate, duration, or extent of mixing is too low. Therefore, reliable gas
concentration measurements should be obtained at a number of different locations within
the system for suitable purges. Temperature should also be measured at a number of
locations. Oxidizing agents may exist in a hydrogen containing equipment, specifically:
air, cold box atmospheres containing air diluted with nitrogen, or oxygen-enriched air
that can be condensed on process pipe work within the cold box in special circumstances.
4.11.2 There are special measures that may need to be put in place in order
to mitigate the hazards, e.g. air should be eliminated by nitrogen purge prior to
introduction of hydrogen into cargo piping or processing equipment. Nitrogen should then
be eliminated by hydrogen purge, where there is a possibility of its solidification in
the subsequent process.
References
1) ISO/TR 15916, Basic consideration for the safety of hydrogen systems
(ISO)
2) American Institute of Aeronautics and Astronautics, "Safety Standard for
Hydrogen and Hydrogen Systems (Guide to Safety of Hydrogen and Hydrogen Systems)", 2005
(AIAA)
3) NFPA 12: Standard on Carbon Dioxide Extinguishing Systems 2005 Edition
(NFPA)
4) IEC 60079-20-1 Ed. 1.0:2010 (b) Explosive atmospheres – Part 20-1:
Material characteristics for gas and vapour classification – Test methods and data
5) UN Recommendations on the Transport of Dangerous Goods – Model
Regulations, Nineteenth revised edition
6) NFPA 2: Hydrogen Technologies Code 2016 Edition (NFPA)
7) IEC/ISO 31010:2009 Risk management – Risk assessment techniques
8) Cryogenics Safety Manual – Fourth Edition (1998)
9) SAE ARP 5580-2001 "Recommended failure modes and effects analysis (FMEA)
practices for non-automobile applications"
10) National Institute of Standards and Technology (NIST) RefProp
database