ISO/PAS 15594:2004

PUBLICLY ISO/PAS
AVAILABLE 15594
SPECIFICATION
First edition
2004-10-01

Airport hydrogen fuelling facility
operations
Exploitation d'installation aéroportuaire d'avitaillement en hydrogène




Reference number
ISO/PAS 15594:2004(E)
©
ISO 2004

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ISO/PAS 15594:2004(E)
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ISO/PAS 15594:2004(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references. 1
3 Terms and definitions. 1
4 Symbols and abbreviated terms. 2
5 Fuelling procedures. 2
5.1 General requirements. 2
5.2 Bonding and grounding procedures. 2
5.3 Refuelling of a cold system. 3
5.4 Defuelling. 3
5.5 Refuelling of a warm system. 3
5.6 Monitoring of fuelling parameters. 4
5.7 Monitoring of the safety parameters. 4
6 Hydrogen boil-off management. 5
7 Storage of hydrogen . 5
7.1 Storage capacity. 5
7.2 Storage means. 5
7.3 Properties of hydrogen stored at the airport . 6
8 Ground support equipment. 7
8.1 General requirements. 7
8.2 Stationary storage tanks . 7
8.3 Portable tank containers . 7
8.4 User system for boil-off gas. 7
8.5 LH pipelines . 8
2
8.6 Refuelling and boil-off coupling units . 8
8.7 Filter on airport side . 9
Annex A (informative) Example of a hydrogen aircraft fuel system layout and aircraft refuel/defuel
interface point . 10
Annex B (informative) LH requirements for different types of aircraft. 12
2
Annex C (informative) Hydrogen boil-off in onboard LH tanks. 13
2
Annex D (informative) Considerations for the selection of storage means . 14
Annex E (informative) Selection of LH refuelling pressure and temperature. 15
2
Annex F (informative) User systems for boil-off gas . 16
Bibliography . 17

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ISO/PAS 15594:2004(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
In other circumstances, particularly when there is an urgent market requirement for such documents, a
technical committee may decide to publish other types of normative document:
 an ISO Publicly Available Specification (ISO/PAS) represents an agreement between technical experts in
an ISO working group and is accepted for publication if it is approved by more than 50 % of the members
of the parent committee casting a vote;
 an ISO Technical Specification (ISO/TS) represents an agreement between the members of a technical
committee and is accepted for publication if it is approved by 2/3 of the members of the committee casting
a vote.
An ISO/PAS or ISO/TS is reviewed after three years in order to decide whether it will be confirmed for a
further three years, revised to become an International Standard, or withdrawn. If the ISO/PAS or ISO/TS is
confirmed, it is reviewed again after a further three years, at which time it must either be transformed into an
International Standard or be withdrawn.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/PAS 15594 was prepared by Technical Committee ISO/TC 197, Hydrogen technologies.
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ISO/PAS 15594:2004(E)
Introduction
When this document was introduced in the ISO/TC 197 programme of work, all aircraft- and airport-relevant
procedures, systems and components concerning hydrogen technologies were in an early development state,
and the technical solutions that would enable the future use of hydrogen as a fuel for aviation were not fully
developed.
The development of this document within ISO/TC 197 depended on the progress achieved within the
European Aeronautic Defence and Space Company (EADS)-Airbus Cryoplane project. However, this project
is no longer planned to start in the near future, and there are no other relevant practical projects underway.
ISO/TC 197 experts are convinced that the subject of using liquid hydrogen in commercial aviation is of great
importance and will gain new momentum in the next decade. As a result, the latest results are presented in
this Publicly Available Specification to make the information available to all interested parties.
This document is not to be regarded as an International Standard. It records the latest results of ISO/TC 197
experts until the subject of using liquid hydrogen in commercial aviation gains interest.
It is understood that this document is far from complete and that it represents the knowledge available at the
time of publication. Should work on this subject resume in the next years, the primary objective of the
standardization work will be to ensure safety at all phases of handling while taking into account the conditions
prevailing at civil airports and the results of risk assessment studies.

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PUBLICLY AVAILABLE SPECIFICATION ISO/PAS 15594:2004(E)

Airport hydrogen fuelling facility operations
1 Scope
This Publicly Available Specification specifies the fuelling procedures, hydrogen boil-off management
procedures, storage requirements of hydrogen, and characteristics of the ground support equipment required
to operate an airport hydrogen fuelling facility.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 14687, Hydrogen fuel — Product specification
ISO 20421-1, Cryogenic vessels — Large transportable vacuum insulated vessels — Part 1: Design, fabrication,
inspection and testing
ASME 1998, Boiler and Pressure Vessel Code
1)
KSC -STD-Z-0009C, Cryogenic Ground Support Equipment, Design of, Standard for
KSC-STD-Z-0005B, Pneumatic Ground Support Equipment, Design of, Standard for
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
block fuel
quantity of fuel to be used for refuelling prior to each flight
3.2
refuelling block time
time needed to refuel the aircraft, measured between connection and disconnection of the couplings
3.3
inert gas
nonflammable and nonreactive gas
EXAMPLES Helium, nitrogen, carbon dioxide.

1) Kennedy Space Center.
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ISO/PAS 15594:2004(E)
4 Symbols and abbreviated terms
AP auxiliary power unit
CO carbon monoxide
CO carbon dioxide
2
C H hydrocarbon containing n carbon atoms and m hydrogen atoms
n m
EADS-Airbus European Aeronautic Defence and Space Company
GH gaseous hydrogen
2
LH liquid hydrogen
2
N nitrogen
2
O oxygen
2
5 Fuelling procedures
5.1 General requirements
At the airport, the following situations shall be considered:
 normal refuelling during ground turnaround, with an onboard system in cold condition up to the
refuelling/boil-off coupling;
 defuelling of the system on the ground due to planned maintenance activities and applicable
troubleshooting cases;
 refuelling of a warm, air-floated onboard system before putting into service and after planned
maintenance activities and applicable troubleshooting cases.
The airport infrastructure shall provide the ground support equipment required for performing the above-
mentioned refuelling and defuelling operations, including the aircraft tank warm-up and precooling, the
necessary purification and purging processes, and the evacuation and GH /LH recovery that is required for
2 2
defuelling operations and refuelling of a warm onboard system. Purge, precooling and warm-up procedures
for the onboard fuel system shall be required only for putting into service, maintenance and troubleshooting
activities.
The connection point between the aircraft and the ground support equipment shall consist of two couplings
(similar but mistake-proof), a refuelling coupling for providing the tanks with LH , and a boil-off coupling for the
2
discharge of GH . In Annex A, Figure A.1 provides an example of an aircraft refuelling and defuelling interface
2
point and Figure A.2 an example of a hydrogen aircraft-fuel system layout.
5.2 Bonding and grounding procedures
Airport personnel shall apply appropriate bonding and grounding procedures prior to performing any refuelling
or defuelling operations on an aircraft.
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ISO/PAS 15594:2004(E)
5.3 Refuelling of a cold system
For normal refuelling during ground turnaround, the airport personnel shall ensure that the tanks are cold and
still contain a small quantity of fuel. The time required for the refuelling of a cold system shall be minimized
and shall be such that the aircraft-refuel block time requirements can be met (see Annex B). The time required
for the refuelling of a cold system shall include an acceptable time for connection and disconnection, including
time for cleaning of inner cavities from air at connection and from hydrogen at disconnection, and time for
warming before disconnection.
In order to perform the refuelling of a cold system, airport personnel shall use the following refuelling
procedure.
a) Establish the connection of the refuelling and boil-off couplings between the aircraft and ground support
equipment. Purging and precooling of the refuelling connecting hose and coupling need not be performed.
b) After the refuelling system is in “ready” mode, open the tank refuelling and boil-off valves to start the
refuelling operation.
c) Monitor the fuel level of the tanks and control it using the refuelling and boil-off valves.
d) After filling the tanks, close the refuelling and boil-off valves and separate the couplings and auxiliary
connections.
NOTE 1 The renunciation of purge, purification, evacuation and precooling at the coupling can be justified by existing
advanced coupling designs.
NOTE 2 During the refuelling of a cold system, no boil-off gas is expected due to recondensation within the onboard
tank.
5.4 Defuelling
For yearly maintenance checks or any troubleshooting, airport personnel may need to defuel the aircraft tanks.
Defuelling of the aircraft fuel system shall be done using the refuelling and boil-off couplings.
After coupling the aircraft with the refuelling and boil-off connectors of the ground support equipment, an
overpressure pipe on the gas side within the onboard tank should deplete the tanks back to the airport
stationary storage tank or portable tank container. The onboard pumps could assist the depletion.
If warming up and purging of the aircraft tanks and piping are required, they shall be done using temperature-
conditioned inert gases.
5.5 Refuelling of a warm system
Refuelling of a warm, air-floated onboard system shall be carried out before putting an aircraft into service and
after planned maintenance activities and applicable troubleshooting cases. Perform the refuelling of a warm
system as follows.
a) Purge the tank and piping system with an inert gas (evacuated, if possible) to remove the air or other
foreign gases from the system. Decrease the foreign gas concentration within the system to an
acceptable level that is yet to be determined, and measure at the boil-off coupling.
b) Purge the tank and piping system and precool with conditioned hydrogen to remove the inert gas from the
system. Decrease the inert gas concentration within the system to an acceptable level that is yet to be
determined, and measure at the boil-off coupling.
NOTE At the time of the publication of this PAS, no detailed procedure for refuelling a warm onboard fuel system
could be given, because the initial state of the system before purge could differ and was not really known. The same
applied to the required end-state of the system after purge and precooling. The requested procedure may vary due to the
design of the onboard tank and piping system. The development task is to define a procedure which is optimized with
respect to cost, time required, careful material handling, and safety aspects.
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ISO/PAS 15594:2004(E)
5.6 Monitoring of fuelling parameters
5.6.1 Monitoring during refuelling of a cold system
Control and monitoring during the refuelling of a cold system shall be implemented at one master logic point.
A refuelling and monitoring panel shall provide the necessary monitoring indication and enable the selection of
all possible automatic/manual procedures, including the preselection of the fuel quantity from the information
provided by the aircraft automatic fuel-level control. Shut-off valves in connection with level indication shall
execute the refuelling procedure. To avoid tank overfilling, the airport personnel shall monitor onboard-tank
liquid level, pressure, temperature and valve positions.
The recommended position for the refuelling control and monitoring-panel is near the refuelling and boil-off
couplings integrated in the aircraft structure. The possibility of monitoring the procedure from the aircraft
cockpit and ground supply equipment should also be considered.
5.6.2 Monitoring during defuelling and refuelling of a warm system
Airport personnel shall perform control and monitoring during defuelling and refuelling of a warm system.
Control and monitoring provisions shall be available from the airport ground infrastructure.
5.7 Monitoring of the safety parameters
5.7.1 General requirements for monitoring devices
Monitoring of the safety parameters is aimed at decreasing the risk associated with handling flammable and
cryogenic fuel. Monitoring devices shall not interfere with the refuelling operations, and they shall not be an
ignition source.
As much as possible, monitoring devices should be independent of external power supplies, and instead have
their own internal power supplies. Devices that take their energy from the fuel (its pressure, flow, or low
temperature) or are an integral part of the fuelling system should be given preference.
When a faulty condition is detected, monitoring devices shall trigger an interruption of the fuel flow if the line is
already open, or the locking of the main valve if the line is still closed. An audible and/or visible alarm shall be
activated indicating the kind of failure and where it occurred.
5.7.2 Monitoring of interface leakage
Interface leakage shall be monitored during the fuelling and defuelling operations. Interface-leakage
monitoring shall enable the detection of
 leaks from the lines or the connections,
 open or not fully or faulty closed connections,
 other abnormal conditions that might be dangerous.
The tightness of the connection shall be verified by measuring the pressure decrease or increase in a suitably
2)
selected volume or by measuring the pressure difference between such a volume and atmosphere.
When a leak is detected, personnel shall verify the suspected area with foaming agents, leak detectors or
equivalent methods.

2) Measuring pressure decrease or volume increase for tightness of a connection is not very responsive. However, no
alternative technology was available at the time of the publication of this PAS. By the time any serious construction of
large fuelling facilities for aircraft materializes, the field of hydrogen detection/situational awareness systems should be
more mature and could be the preferred method of control and monitoring.
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ISO/PAS 15594:2004(E)
5.7.3 Overpressure
A mechanical contact manometer or an equivalent device shall be used to monitor overpressure in the
onboard fuel system and shall be used to stop the fuelling operation immediately when an overpressure
condition is detected.
5.7.4 Heat insulation deterioration
Airport personnel shall monitor signs of deterioration of the vacuum insulation. Suitable criteria shall be
established to determine the amount of insulation deterioration that is acceptable, and that which requires
repair or replacement. Deterioration of insulation can easily be detected in an early stage by observing the
outer surface of the vacuum space becoming cold, and water condensing or even freezing on it.
A spot on which such effects can be detected shall be in the view of the airport personnel responsible for the
refuelling operations. Means to detect temperatures that are too low can also be used.
In the open air or in a hangar, the air humidity is high enough for water vapour to undergo condensation. In
areas where air humidity is too low, means other than visual inspection should be used to detect signs of
deterioration of the vacuum insulation.
NOTE A metal rod that contracts when it cools and breaks a contact when its length falls under a certain threshold
can be used to monitor deterioration of the insulation.
6 Hydrogen boil-off management
Airports that service hydrogen aircraft shall be equipped with boil-off gas user systems. These systems shall
be designed to collect hydrogen boil-off gas generated in the onboard LH tank (see Annex C for a description
2
of the boil-off gas problem) during the following aircraft modes:
a) ground overnight parking (approximately 12 h);
b) long-time overhaul with cold tanks;
c) applicable failure cases.
The connection point between the aircraft and ground support equipment shall be the boil-off coupling, which
allows a gas feed to the airport boil-off gas user system for utilization.
7 Storage of hydrogen
7.1 Storage capacity
The storage capacity for LH shall be established based on the demand for hydrogen at the airport. LH
2 2
requirements for airports range from a few tons per day to 6 000 000 kg per day for a large airport.
7.2 Storage means
LH shall be stored at the airport either in portable tank containers or in stationary storage tanks.
2
Considerations for the selection of storage means are provided in Annex D.
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ISO/PAS 15594:2004(E)
7.3 Properties of hydrogen stored at the airport
7.3.1 LH state condition at aircraft refuelling interface point
2
LH at the aircraft refuelling interface point shall have a pressure equal to or higher than 700 kPa and a
2
temperature equal to or less than 20 K, as shown in Figure 1. The reasons that lead to the selection of the
refuelling pressure and temperature requirements are provided in Annex E.

Key
X fuel temperature, kelvin
Y fuel pressure, kPa
1 solid
2 liquid
3 triple point
4 vapour
5 fluid
6 critical point
7 required fuel condition at aircraft interface
8 hydrogen equilibrium state
Figure 1 — Hydrogen refuelling condition
7.3.2 Hydrogen quality
LH at the aircraft interface shall meet the requirements set forth in ISO 14687 for type II hydrogen fuel, with
2
the exceptions stated below:
 LH purity W 99,999 9 % (volume fraction);
2
 O content u 0,000 02 % (volume fraction);
2
 N content u 0,000 02 % (volume fraction);
2
 H O content u 0,000 05 % (volume fraction);
2
 C H content u 0,000 01 % (volume fraction);
n m
 CO content u 0,000 01 % (volume fraction);
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ISO/PAS 15594:2004(E)
 CO content u 0,000 01 % (volume fraction);
2
 LH shall not contain particles > 5 µm.
2
The hydrogen shall not contain impurities in the form of particles large enough to cause component or aircraft
fuel system malfunction. The maximum size of 5 µm given above needs to be confirmed, and the requirement
specified in 8.7 adjusted accordingly.
In order to define the requirement for hydrogen purity, studies of impurities accumulation in aircraft tanks are
needed. These studies should be carried out on tanks with near-real structural dimensions. At the time of
publication of this PAS, there was no stable opinion about quantities and composition of dangerous impurities
(oxygen, nitrogen, their mixtures).
NOTE It is a design aim for future serial aircraft to keep the onboard fuel system in cold condition for approximately
1,5 years. During this period, hundreds of refuellings will be carried out on a single aircraft, and O and N could
2 2
accumulate within the aircraft storage system, jeopardizing the safety of the aircraft. In order to prevent such solid nitrogen
and oxygen accumulation within the onboard fuel system, a high level of refuelled hydrogen purity and a control device
within the aircraft storage system is foreseen. If procedures are developed and are proven efficient in avoiding this
accumulation, either at the airport or on the aircraft, the hydrogen purity requirement could be decreased accordingly.
8 Ground support equipment
8.1 General requirements
Ground support equipment shall meet the requirements set forth in KSC-STD-Z-0009C and
KSC-STD-Z-0005B and the additional requirements described in the following subclauses. If discrepancies
are found between the requirements of KSC-STD-Z-0009C and KSC-STD-Z-0005B and the requirements set
forth in the following subclauses, the most recent requirements shall prevail.
8.2 Stationary storage tanks
The storage tank shall consist of a suitably supported inner liquid vessel, enclosed within an outer shell with
insulation between the inner liquid vessel and outer shell. The inner liquid vessel shall be made of appropriate
stainless steel, and designed and constructed in accordance with, and fulfilling the requirements of, the ASME
Boiler and Pressure Vessel Code in all respects. The outer shell of the tank shall be made of carbon steel with
good weld-ability. Piping between inner and outer shells shall be made of appropriate stainless steel.
Appropriate instrumentation for measuring temperature, pressure and fluid level shall be provided.
The boil-off rate of the storage tank shall be designed to match the proposed withdrawal rate of product from
the tank, in order to minimize product losses. The annular space between the inner liquid vessel and the outer
shell shall be evacuated and include an adequate insulating material. The insulating material shall not be
subject to damage by hydrogen, in either its gaseous or liquid state. The insulating material shall not attack the
material of the inner liquid vessel or the outer shell. The insulation shall be designed with a vapour-tight seal in
the outer covering to prevent the condensation of air and subsequent oxygen enrichment within the insulation.
The insulating material and outer shell shall be of adequate design to prevent insulation attrition under normal
operating conditions. Means to monitor the vacuum level of the annular space between the inner liquid vessel
and the outer shell shall be provided.
8.3 Portable tank containers
Portable tank containers shall be designed and constructed in accordance with and fulfill the requirements of
ISO 20421-1 in all respects.
8.4 User system for boil-off gas
Different types of boil-off gas user systems can be used at the airports. Annex F gives an overview of the
systems that can be implemented.
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