Bjorn’s Corner: The challenges of Hydrogen. Part 12. Safety

By Bjorn Fehrm

October 23, 2020, ©. Leeham News: In our series on Hydrogen as an energy store for airliners we look deeper at the safety of a hydrogen airliner.

Do the safety rules for the aircraft or the airport need to be written new or can the existing ones be used with changes?

How do you certify hydrogen aircraft and its infrastructure?

We have discussed different technical challenges that confront Airbus and others that embark on hydrogen-fueled aircraft projects.

But technical solutions are only part of the areas that require work. An equally important area is the safety aspects and what rules shall a new hydrogen airliner be designed and certified to?

What are the rules needed for the storage of hydrogen at the airport and the refueling of the aircraft?

A lot can be learned from the car and space launcher industry but it must all be adapted to the air transport situation and the safety level achieved there.

The Airbus Cryoplane study spent considerable effort and time to go through the safety aspects of the aircraft and the infrastructure around it. The goal was to identify any areas that could be showstoppers. Must the certification rules be written new?

After three years of work, where existing certification procedures and airworthiness regulations were assessed, it found the existing rules cover the needed areas.

No new rules need to be written, but the existing ones must be amended/changed as many safety aspects are specific to hydrogen as a fuel and its cryogenic (-253°C) storage state.

There are burn and explosion risks but also the inverse, where the low temperatures of the liquid fuel pose risks as does its heat absorption when it goes from liquid to gas, should a leak develop.

The tanks must be protected from engine disc bursts as a ruptured cryogenic tank can take in oxygen, creating an explosive mixture.

Also, all components of the aircraft that can come in contact with liquid hydrogen must be designed to withstand the low temperatures of the cryogenic fuel.

The rules for the airport storage and filling of fuel must be changed, adapted from the rules from the car and space launcher industry.

Not less safe than a carbon fueled aircraft

The aircraft safety assessment demonstrated that the existing paragraphs of the safety and airworthiness regulations cover the needed areas. Technical solutions and regulations can be adapted to meet at least the same level of safety as for carbon fuels.

Overall, the conclusion was:

Hydrogen poses its specific safety aspects to be considered in design and operation. However, the overall safety level will not be worse than for kerosene aircraft.

9 Comments on “Bjorn’s Corner: The challenges of Hydrogen. Part 12. Safety

  1. Bjorn, another consideration is if the tank is vacuum insulated and the vacuum is broken for any reason, the LH2 will almost immediately boil-off at a high rate (the technical report estimated 200 seconds to reach relief pressure, 20 seconds if fire also occurs). That poses risks for overpressure, vent overflow and fuel loss. And of course if the inner tank is also ruptured, a cryo-spill would occur in the fuselage during flight, so outer containment would be needed.

    I would think safety regulations will need to be substantially rewritten for LH2. More a case of the risks being different than being greater.

    • Hi Rob,

      Whenever you store a lot of energy in a confined space, and the integrity of the enclosing shell of the storage container is compromised, you have a highly dangerous situation. An example, China Airlines flight 120, resuling in the loss of four lives.

      • I agree, as I said the risks are different but not necessarily beyond management. One way to think of it is in terms of stored energy.

        With kerosene, you have the chemical energy of the fuel, uncontrolled release of which is a hazard. With LH2, you have that same hazard, but additionally you have the energy of liquefaction. Although that is an uncontrolled absorption rather than release, it has about equal potential for harm, just in a different manner.

        So the total energy hazard is larger for LH2, and that will need to be accounted for in the safety regulations.

        • Its a good idea to keep things in perspective.

          Anyone that owns a home or bossiness that if Natural Gas heated, has a major issue in safety terms .

          A leak or a break (think flex lines to stoves and dryers) and then its a matter of are you home and do you act fast enough.

          Or in the case of the main line pressure regulators, some someone do in a whole area as happened not too long ago.

          Those are not dual contained line and they have a lot of joints from the meter on into the house or business (think threads, sealing, leaks)

          Its how a hazard is handled. Private homes have very low gas pressure after the meter, leaks are far less likely and a break is lower volume.

          I worked in a facility that their first reaction was to dump the power, sorry guys, we have full generator backup, its just coming back on.

          No one has found a solution to a crash landing and fuel in the tanks.

    • For pressure vessels you always have a number of safety valves, it will be similar here if they allow an overpressure in the tank. If not, the tank will be vented at the top of the tail and probably be nitrogen purged as well. Cryo fuel rockets have problems with ice buildup on its tank outside that add mass and loose ice as it shredds. The low temperatures makes for very small critical crack lengths of cooled and stressed materials making the production and MRO NDT inspections harder. Hence any rotating pump like the Engine fuel pumps must be designed and inspected accordingly. Shut-off valves will be of another class as hydrogen leaks very easily. The LH2 Aircrafts most likely will need to be defueled at Airports with special trucks and dumped into the Airport LH2 processing plant.

    • Of course new failure cases will have to be considered, assessed and addressed by manufacturers. And the authorities will have to validate new aspects of certification dossiers.
      But it does not mean that completely new safety regulation chapters have to be written.

      On top of existing fuel safety requirements, part of these for O2 and N2 pressurized tanks could be considered for LH2 ones for example.

      • Alban, this is only a guess, but I suspect there will need to be new & separate regulations for LH2, as opposed to kerosene.

        Some parts will overlap, such as combustion hazards and safeguards, but some parts will be new to cover differences in handling, storage, and breach scenarios.

        But I agree with the overall view that this is perfectly possible, it wouldn’t be a reason to not pursue LH2. Like I said, different but not necessarily worse.

  2. can the tanks not be protected from engine disc bursts by placing them in front of the engine?

    • Yes, tank and engine placements are key, but other considerations also prevail. Wing-mounted engines are preferred for structural and center-of-gravity reasons. Tank mounting in the aft fuselage is preferred for practical cabin and safety reasons. But other combinations are possible.

      Existing kerosene fuel tanks have liners that are self-sealing or leak-limiting for small punctures. That may be difficult for LH2 tanks due to the cryogenic temperatures. Also LH2 has the capacity to locally liquefy the components of air (N2, O2, Co2) if insulation is lost.

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