AEEM7063

AVIATION FIRE DYNAMICS

CHALLENGES WITH HYDROGEN FIRES AND THEIR SUPPRESSION

INTRODUCTION

HYDROGEN: BACKGROUND INFORMATION

Hydrogen is the lightest element and its monatomic form (H1) is the most abundant chemical substance.

At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2.

Hydrogen readily forms covalent compounds with most non-metallic elements and is present in the water molecule and in most organic compounds. So, naturally occurring atomic hydrogen is rare on earth.

Hydrogen gas produces water when burned, a property which later gave it its name (in Greek, hydrogen means "water-former“)

Hydrogen poses a number of hazards to human safety, from potential detonations and fires when mixed with air to being an asphyxiant in its pure, oxygen-free form.

In addition, liquid hydrogen is a cryogen and presents dangers (such as frostbite) associated with very cold liquids. 

Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as hydrogen embrittlement leading to cracks and explosions.

Ideally, no oxygen should be present in the liquid hydrogen tanks in a fuel cell vehicle, but trace amounts of air may contaminate the hydrogen supply.

HYDROGEN COMBUSTION

Hydrogen is highly flammable and requires only a small amount of energy to ignite

Hydrogen gas forms explosive mixtures with air and with chlorine.

Hydrogen requires only one 10th as much energy to ignite as gasoline does.

A spark of static electricity from a person's finger is enough to set it off.

EXPOSIVE LIMITS OF COMBUSTIBLE GASES WHEN MIXED WITH AIR

Gas % Gas in Mixture With AirAcetone 2.6 12.8

Benzene 1.4 8.8

Carbon Monoxide 12.5 74.2

Ethanol 3.3 19.0

Ethylene 2.8 28.6

Hydrogen 4.0 74.2

Hydrogen Sulfide 4.3 45.5

Methane 5.0 15.0

Methanol 6.7 36.5

Propylene 2.4 10.3

0 20 40 60 80

With 4%-75% hydrogen, very little energy is required to ignite hydrogen. The speed of a hydrogen fire is ten feet per second to 6750 ft./sec

4% Barely burn and even then only upward 5% Fire starts burning sideways 18% Fire will now burn faster than the speed of sound 45% Burning at 6750 ft./sec -including shock wave 74% Barely burn 75% Will not burn-no air

Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns

If leaking from a pipe at a high enough pressure, hydrogen gas can self-ignite without the aid of an external energy source

Hydrogen has a much wider flammability range and detonability limits compared to most hydrocarbons

A hydrogen flame poses special dangers beyond those posed by hydrocarbon flames because human senses cannot easily detect it.

If you come upon a hydrogen fire, you will not see it – even up close. You might see sparkles, dust particles briefly burning

You won’t feel the heat of the hydrogen fire because very little infrared radiation (which gives us the sensation of heat) is present

The physical flame is stronger than a hydrocarbon flame.

The mixtures spontaneously explode by spark, heat or sunlight.

Hydrogen flames can burn in a strong wind and be stretched out away from its source a number of feet

HYDROGEN COMBUSTION

DISASTERS!!

THE HINDENBURG FIRE

Hindenburg disaster took place on Thursday, May 6, 1937

German passenger airship LZ 29 Hindenburg caught fire and was destroyed during its attempt to dock

Of the 97 people on board (36 passengers, 61 crew), there were 35 fatalities; there was also one death among the ground crew

The actual cause of the fire remains unknown, although a variety of hypotheses have been put forward for both the cause of ignition and the initial fuel for the ensuing fire

SABOTAGE HYPOTHESIS

STATIC SPARK HYPOTHESIS

LIGHTNING HYPOTHESIS

ENGINE FAILURE HYPOTHESIS

INCENDIARY PAINT HYPOTHESIS

PUNCTURE HYPOTHESIS

STRUCTURAL FAILURE

FUEL LEAK

HYDROGEN FIRE HYPOTHESIS

Initial thoughts:

Paint was both the point of ignition and the sole cause of the rapid spread of the fire.

“It did not look like a hydrogen fire”

The hydrogen within the airship is held blameless for both the start of the fire and its subsequent advance

The chemical composition of the Hindenburg paint is known, and it burns too slowly.

Offering support for the hypothesis that there was some sort of hydrogen leak prior to the fire is that the airship remained stern-heavy before landing.

OTHER HYPOTHESIS

Hindenburg's skin was covered with the extremely flammable cellulose nitrate or cellulose acetate, added to help with rigidity and aerodynamics.

The skin was also coated with flecks of aluminum, a component of rocket fuel, to reflect sunlight and keep the hydrogen from heating and expanding.

These substances caught fire from an electric spark that caused the skin to burn.

At this point the hydrogen became the fuel to the already

existing fire

HINDENBURG DISASTER

CHALLENGER DISASTER

The Challenger flew nine successful missions before that fateful day of the disaster in 1986

The Challenger was scheduled to carry some cargo, the Tracking Data Relay Satellite-2, as well as fly the Shuttle-Pointed Tool for Astronomy (SPARTAN-203)

The Challenger had 7 crew members and it was the first flight of a new program called TISP (Teacher In Space Program).

Seventy three seconds into the mission, the Challenger exploded, killing the entire crew.

The commission's report cited the cause of the disaster as a the failure of an “O-ring” seal in the solid-fuel rocket.

The faulty design of the seal coupled with the unusually cold weather, let hot gases to leak through the joint.

Booster rocket flames were able to pass through the failed seal enlarging the small hole. These flames then burned through the Space Shuttle Challenger's external fuel tank and through one of the supports that attached the booster to the side of the tank.

That booster broke loose and collided with the tank, piercing the tank's side.

Liquid hydrogen and liquid oxygen fuels from the tank and booster mixed and ignited, causing the Space Shuttle Challenger to tear apart.

CHALLENGER DISASTER

MORE ON CHALLENGER DISASTER

MORE DISASTERS

WHY HYDROGEN SAFETY RESEARCH?

RESEARCH IN FIRE SAFETY FOR HYDROGEN-FUELED VEHICLES

Hydrogen is currently produced on an industrial scale of about 10 million tonnes annually

By 2040, it is anticipated that the use of hydrogen in fuel cell powered vehicles could replace the consumption of petroleum.

To be economically competitive with the present fossil fuel economy, the cost of fuel cells must be lowered by a factor of 10 or more and the cost of producing hydrogen must be lowered by a factor of 4.

Moreover, the performance and reliability of hydrogen technology for transportation and other uses must be improved dramatically. The role of hydrogen safety to achieve it can not be underestimated.

For automotive applications it is expected that the level of safety at the consumer interface with hydrogen is similar or exceeds that present with natural gas usage.

Hydrogen safety research was initiated decades ago as a result of accidents in the process industries

CFD tools are used to study the phenomena of hydrogen combustion, deflagration-to-detonation transition and hazard and risk assessment

In spite of recent progress there are still some unresolved issues of hydrogen

SLOW SUBSONIC LEAKS

Characteristic for connections and holes in delivery pipes.

The main area of concern is private and maintenance garages, car parks, and other automotive infrastructures

PERMEATION

Diffusion of high pressure hydrogen through storage vessel materials to surrounding space.

Permeation is a slow molecular flow regime through the surface material and must be modeled as diffusion through a solid rather than a flow.

The process of permeation involves the absorption and dissociation of molecular hydrogen to atomic hydrogen on the internal surface

Then atomic hydrogen diffuse through the metal to the opposite surface where atoms recombine and undergo desorption

HIGH-MOMENTUM LARGE-VOLUME RELEASE

Characteristic for accidental breaks of high-pressure tanks or activation of pressure relief device (PRD), e.g. by external fire

HYDROGEN-JET FLAMES

Hydrogen discharges through circular orifices larger than a critical diameter sustain stable lifted flames irrespective of the reservoir pressure driving the release.

At smaller diameters, however, stable burning will only be achieved at operating pressures higher than a particular value.

LIFT-OFF VELOCITY, LIFT-OFF HEIGHT, AND BLOW-OUT VELOCITY

Flame stability is usually characterized by lift-off velocity, lift-off height, and blow-out velocity.

LIFT-OFF VELOCITY

Defined as the mean jet velocity at which the flame becomes lifted above the jet exit rim.

If the jet velocity is further increased, the flame moves downstream to a position where it stabilizes.

LIFT-OFF HEIGHT

Distance between the lifted flame base and the jet exit.

BLOW-OUT VELOCITY

Jet velocity at which the reaction cannot be sustained and the flame is extinguished.

For positions between the jet exit and the flame base, a turbulent mixing region exists.

The stability limits of turbulent jet diffusion flames are important for operation of combustion systems and have safety implication for handling combustible fuels

EFFECTS OF THESE FACTORS ON HYDROGEN

Hydrogen has the much higher turbulent blow-off velocity in comparison to that of hydrocarbons.

This permits turbulent diffusion hydrogen flames to exist at velocities far in excess to those possible for turbulent diffusion hydrocarbon flames in air.

The flame lift-off height of the pure hydrogen jet diffusion flame was found to increase with the jet exit velocity.

The lift-off length and blow-off velocity have strong implications in safety considerations and hazard/risk assessment.

DEFLAGRATIONS AND DETONATIONS

There are two types of explosions:

DEFLAGRATIONS

Characterized by a flame propagation velocity below the speed of sound

Typically far below 100 m/s, and relatively modest overpressures, say below 0.5 bar.

 Subsonic combustion propagating through heat transfer; hot burning material heats the next layer of cold material and ignites it.

Most fire found in daily life, from flames to explosions, is deflagration

DETONATION

 Involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it.

flame propagation velocities, perhaps up to 2000 m/s, and substantial overpressures, up to 20 bars

Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases.

The velocity of detonations in solid and liquid explosives is much higher than that in gaseous ones, which allows the wave system to be observed with greater detail

The ability of a hydrogen-air mixture to detonate is greater than that of hydrocarbons.

For a stoichiometric hydrogen-air mixture, detonation once initiated will propagate as long as the mixture is within the detonability limits.

The direct initiation of hydrogen-air mixture detonation is

possible by just 1.1 g of tetryl.

Only 1.86 g of TNT is needed to initiate detonation in a 34.7% hydrogen-air mixture in the open atmosphere.

For a 20% hydrogen-air mixture, however, the critical charge increases significantly to 190 g of TNT.

DEFLAGRATION-TO-DETONATION TRANSITION (DDT)

Geometrical conditions such as partial confinement and obstacles in the flame path may result in a subsonic flame to accelerate from subsonic speed to supersonic speed, transitioning from deflagration to detonation.

The exact mechanism is not fully understood

Existing theories are able to explain and model both deflagrations and detonations but there are no theories at present which can predict the transition phenomenon

DDT OF HYDROGEN

A specific feature of hydrogen-air deflagrations compared to of the majority of hydrocarbon-air deflagrations is the readiness to DDT

This occurs when mitigating confined deflagrations by the venting technique.

A needle-like structured flame front can be induced by the venting.

The onset of detonation is directly linked to the needle-like structured flame front formed by sudden venting.

HYDROGEN SAFETY ASSESSMENT IN AIRCRAFT AND GROUND VEHICLE

HYDROGEN FUELLED AIRCRAFT

An average airline passenger would opt for the aircraft fueled with Jet A.

Aircraft fueled with liquid hydrogen (LH2) would be declared unsafe.

Mainly because of Hindenburg fire disaster Hazard due to hydrogen leaks : Liquid <-> vapor Greater ignitability of hydrogen relative to jet fuel “Flameless detonations“ : pressure pulses and fire balls

NASA has sponsored a number of studies to explore the potential of LH2, liquid methane (LCH4) and synthetic aviation-grade jet fuel (synjet) as alternatives to petroleum derived Jet A for use in future transport aircraft

It is highly available, less costly, produces high energy and less noise

PASSENGERS WILL BE EXPECTED TO SURVIVE THE IMPACT BECAUSE:

1) LH2 tanks are less apt to be ruptured, thus minimizing spillage potential :

It is important to minimize the surface area-to-volume ratios of these tanks

To minimize the weight of the tank and insulation system To decrease the opportunity for heat to leak into the cold fuel and

cause it to vaporize

Therefore it is preferable to locate the tanks for the cryogenic fuels in the fuselage for a hydrogen-fueled airplane.

This arrangement has been shown to provide lowest direct operating cost, a realistic measure of aircraft operating efficiency as well

In conventional aircrafts, fuel is tanked in the wing between the front and rear spars

LH2 tanks are less susceptible to damage:

a) They are mounted in the fuselage where they present a far smaller dimension to frontal impact.

b) They are protected by a significant amount of structure, both ahead of and beneath, which must be crushed or penetrated before the tanks are exposed.

c) They are designed for higher pressure than the rest of the fuselage and therefore are less apt to be the point of failure in an impact survivable crash.

2) In the event a tank is ruptured and fuel is spilled, LH2 evaporates almost immediately, and it dissipates into the atmosphere so rapidly. So the time and the area exposed to flammable mixtures are both quite small compared to the other candidate fuels

Two parts:

An assessment of the spreading and vaporization of the spilled liquid:

Results show that LH2 spreads the least and evaporates the quickest in all

The behavior of the fuel vapor cloud after vaporization has

occurred:Hydrogen rises rapidly in every case and therefore poses minimum risk to areas surrounding a spill. The other fuels are non-buoyant for extended periods and flammable clouds are blown along the ground

3) If the spilled fuel is ignited, the duration of the hydrogen fire would be so brief that it would not heat the fuselage to the point of collapse, as would be the case with the other fuels, and the heat-affected area would be substantially smaller:

The fuselage should collapse after ~40 s in a methane flame, after ~ 50 s in hydrogen flame and after ~120 s in a JP-4 or Jet A flame.

When these fuselage collapse times are compared with the calculated burning times of the fuel spills for the worst survivable case it is seen that LH2 will pose no problem.

The pool fire would last only 15 s.

For LCH4 the pool fire would last 38 s, uncomfortably close to the 40 s calculated for fuselage collapse.

Although the pool burning times were not calculated for JP-4 and Jet A, it is obvious from accident experience that fuselage collapse would occur in event those fuels were used.

CONCLUSION

LH2 was found to be a safer fuel than any of the other candidates.

The passengers on board, and people and property in the immediate surroundings, would be exposed to less hazard if a crashed aircraft were fueled with LH2.

BLAST WAVES AND FIREBALLS GENERATED BY HYDROGEN FUEL TANK

RUPTURE This paper describes and analyzes the results of Type 4

hydrogen fuel tank fire exposure tests.

Project explored fire safety issues with an on-board hydrogen storage tank

Aimed at replacing compressed natural gas containers with hydrogen fuel tank.

TYPE-4 HYDROGEN CYLINDER RUPTURE

Comprised mainly of a high-density polyethylene inner liner, a carbon fiber structural layer, followed by a fiberglass protective layer

Composite material on the surface of the tank ignited approximately 45 seconds after the bonfire exposure.

After 6 minutes and 27 seconds, the cylinder failed catastrophically through the bottom, launching the 30.9 lb main portion 270 ft. from the test location.

Blast pressures were 300 KPa at 6.3ft. and 41 kPa at 21.3 ft.

CONCLUSIONS

A typical compressed hydrogen tank, when exposed to a bonfire, presents safety challenges.

The consequence of a rupture is catastrophic.

In the test, blast pressures of 6 psi were measured 21 ft. away from the tank, and debris was propelled more than 250 ft.

The test results suggest that bonfire protection and pressure relief sensing for Type 4 hydrogen tank will require some sophistication.

HYDROGEN SAFETY MEASURES

VENTILATION

Ventilation is useful for preventing the formation of explosive atmospheres when hydrogen is dealt with in confined space.

The basic principle of ventilation is to bring fresh outside air inside the room and remove the inside air. The main advantages of using ventilation in hydrogen systems are:

It prevents the accumulation of explosive gas inside the room. It dilutes the explosive gas by bringing fresh air in. It limits the resident time of explosive atmospheres.

Ventilation can be of two types:

Forced (mechanical) Natural.

Selection of appropriate type depends on many factors.

Natural ventilation functions due to temperature differences and winds.

Hydrogen buoyancy and diffusion characteristics should be taken into account while selection the size of opening and their positions.

Although natural ventilation is a cheaper option than the mechanical ventilation, it is less controllable and is affected by weather conditions.

On the other hand mechanical ventilation can give constant and controlled flow.

The sizing of mechanical ventilation is not a big concern. It can be sized based on the previous experiences, based on calculations or based on hydrogen leak detection sensitivity.

The main disadvantage of this type of ventilation is that the gas cloud and air will be more turbulent and in this case if the gas cloud is ignited then the explosions will be more severe.

Using a combination of natural and mechanical ventilation can be a solution in some cases.

AUTOMATIC SHUTDOWN SYSTEM

INERTING :

It is defined as the replacement of a sufficient proportion of oxygen contained in a gaseous atmosphere by an inert gas, to avoid the ignition of atmosphere.

It is an important way to prevent the formation of explosive atmosphere.

However, it can be dangerous for the workers due to the asphyxiating property of inert gas.

The main inerting methods include:

Pressure swing method Volume swing method Flow-through method Displacement method.

SCHEMATIC OF HYDROGEN-AIR-INERT MIXTURES:

The conditions of a hydrogenated atmosphere to be subjected to inerting can be derived from the triangular diagram

The three colored zones in the diagram represent:

Red zone: It corresponds to the explosion area

Orange zone: It corresponds to the mixtures which are not explosive but which may explode if they are mixed with air.

Green zone: It corresponds to mixtures which are not explosive and which will not explode if they are mixed with air.

RECOMBINERS

A recombiner is a device that promotes the recombination of hydrogen and oxygen to form water.

It serves to avoid, remove or at least slow down the formation of flammable mixtures formed due to leakage of hydrogen.

Recombiners can be classified as:

Active devices: Use heat to initiate conversion

Passive devices: Use the property of hydrogen that, in presence of catalysts like

platinum and palladium hydrogen reacts with oxygen at low temperatures in an exothermic reaction.

While using recombiners appropriate measures should be taken to keep the system temperatures below the self-ignition temperature.

CONTROL OF IGNITION SOURCES

Electrical equipments, hot surfaces, open flames, static electricity are the possible ways of ignition of hydrogen and propagation of the flame. By controlling these sources explosions can be avoided

DETECTION MEASURES

DETECTION OF HYDROGEN LEAKS

The major hazards due to an unwanted release of hydrogen are connected with the possibility of building up explosive conditions.

Hydrogen is more dangerous due to its high flammability and low ignition energy. Thus appropriate detection measures should always be taken into consideration.

The main criteria for selecting a hydrogen sensor are listed below.

Response time Detection range Durability Calibration / Maintenance Cross sensitivity Area coverage

DETECTION OF HYDROGEN FLAMES

Hydrogen burns with a pale blue flame and does not emit visible light or smoke. Hydrogen flames emit significantly less heat.

Thus it is difficult to detect the flames by human perception. Therefore hydrogen fire detectors are necessary.

Special imaging systems are required for determining the size and location of the flame. Many times it is difficult to differentiate hydrogen related signals and the parasitic signals. Thus to prevent wrong alarms human analysis may be advantageous.

The hydrogen fire detectors should posses following main characteristics.

Detect hydrogen flame at sufficient distance Detect small flames

SAFETY MEASURES

MITIGATION MEASURES

In case of fires the supply of hydrogen should be shut off as soon as possible since the hydrogen fires are not extinguished due to the danger of re-ignition or explosion.

The adjacent metal surfaces should be cooled by water or other means as they can be the possible source of re-ignition.

Water can be used as a fire extinguisher. Small fires can be extinguished by dry chemical extinguishers or with carbon dioxide, nitrogen, and steam.

ACTIVE INERTING, SUPPRESSION AND ISOLATION SYSTEMS

Inerting as described before is used for dilution of explosive atmosphere to prevent ignition.

In situations where no human activity is involved all air can be replaced by inert gas.

Typical inert gases used are nitrogen, carbon-di-oxide and some special mixtures, for example Inergen (mainly Ar and N2, some CO2), Argonite (Ar, N2).

A chemical suppression or isolation system may also be used in quenching the flame. But they pose more challenges. The flames may propagate faster due to the turbulence created by suppression systems.

In isolation systems, early detection of fire and fast isolation is necessary

WATER BASED PROTECTION SYSTEMS

Water can be used to extinguish the fire. Water spray system can be provided for hydrogen storage containers, grouped piping, and pumps where potential fire hazard exists.

The system should deliver a uniform spray pattern over 100 percent surface of the container, pump and adjacent piping.

EMERGENCY RESPONSE MEASURES

The emergency response plan should be based on systematic identification of hazards, followed by evaluation risk management.

The aim should be minimization of risk to people. Communication is a key element in any emergency response plan.

Effective communication will involve technical measures, organization, procedures and training adapted to each other and to the overall strategy.

If communication fails, effective emergency response is not possible. Effective emergency response will also require an organization intended and prepared for emergency response.

DETERMINING THE SAFETY DISTANCE

The location and distance of hydrogen storage from people and other facilities should be decided in such a way that the effects of fire, explosion and detonation, if occurs, can be reduced to tolerable levels.

While selecting a location the effects of possible combustible cloud drift and thermal radiations should be considered.

The hydrogen storage facilities should be located outdoors or in a building specially built for the purpose.

CONCLUSIONS

Hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas.

One cannot see hydrogen fire even up close and cannot feel the heat of the hydrogen fire but the physical flame is stronger than a hydrocarbon flame.

Hindenburg airship and challenger space shuttle disaster : Hydrogen fire disasters

LH2 was found to be a safer fuel than any of the other candidates.

Type 4 hydrogen tank will require some sophistication.

Prevent the hydrogen from escaping by following good process maintenance practices

In case of leak the area should be well ventilated to prevent hydrogen build up.

Gas detectors and flame detectors can quickly identify a gas leak or the resulting flame.

Combustible gas detection technologies: Infrared catalytic bead

Flame detection technologies: Thermal detectors UV flame detectors Multi-spectrum IR flame detectors

Chemical suppression system and venting method should be handled with care.

REFERENCES

Blast waves and fireballs generated by hydrogen fuel tank rupture during fire exposure” : Robert Zalosh, Firexplo, Wellesley

“Large scale experiments to study fires following the rupture of high pressure pipelines conveying natural gas and natural gas/hydrogen mixtures”: B.J. Lowesmith, G. Hankinson

“An Assessment of the Safety of Hydrogen-Fueled Aircraft”: G. Daniel Brewer

“Research in Fire Safety for Hydrogen-Fueled Vehicles”: Kennerly H. Digges, R. Rhoads Stephenson

“Handling and safety of Hydrogen”: Itsuki Uehara

The Hindenburg Fire: Hydrogen or Incendiary Paint?”: A. J. Dessler, D. E. Overs, and W. H. Appleby

“Hydrogen safety research”: Molkov, V.

“Hydrogen Plant Explosion and Fire”

“Twenty Hydrogen Myths” : Amory Lovins

Analysis of Published Hydrogen Vehicle Safety Research” : U.S. Department of Transportation

“A numerical study of hydrogen or helium release and mixing in partially confined spaces” : K. Prasad, W. M. Pitts and J. C. Yang

http://www.osha.gov/dep/greenjobs/hydrogen_fire.html

http://www.arhab.org/pdfs/h2_safety_fsheet.pdf

http://www.hysafe.org/science/eAcademy/docs/1stesshs/presentations/hyfire1.pdf

http://www.slideshare.net/acadiandivers/h2-safety#

http://en.wikipedia.org/wiki/Hydrogen

http://www.ika.rwth-aachen.de/r2h

http://www.ika.rwth-aachen.de/r2h/Hydrogen_Safety_Measures

QUESTIONS