Ex installations in the airplane tank

check out Guidance on “EASA Fuel Tank Safety letter”

it says (cut outs):

In recent years the aviation industry has experienced a number of incidents or accidents involving fuel tank explosions. These experiences suggest that on some aircraft types, the fuel tank system installation does not provide as high a level of protection against explosion as had been expected.

Any components located in or adjacent to a fuel tank must be qualified to meet standards that assure, during both normal and failure conditions, ignition of flammable fluid vapours will not occur. This is typically done by a combination of design standards, component testing and analysis.
Testing of components to meet explosion proof requirements is carried out for various single and combinations of failures to show that arcing, sparking, auto ignition or flame propagation from the component will not occur. Testing for components has been accomplished using standards and
component qualification tests. The standards include for example Eurocae / RTCA DO160 and BS 3G 100 that defines explosion proof requirements for electrical equipment and analysis of potential electrical arc and friction sparks.

Minimizing electrical components hazards within fuel tanks

One of the lessons learned listed above is the undesirable presence of electrical components within fuel tanks. Power wiring has been routed in conduits when crossing fuel tanks, however, chaffing has occurred within conduits. It is therefore suggested that such wiring should be routed outside of fuel tank to the maximum extent possible. At the equipment level, connectors and adjacent area should be taken into account during the
explosion proofness qualification of the equipment (typically, pumps).

However, for some wiring, such FQIS or sensor wiring, it might be unavoidable to route them inside of tanks, and therefore they should be qualified as intrinsically safe. The Safety Assessment section below indicates how any residual fuel tank wiring may be shown to meet the required Safety Objectives.

Component Qualification Review

Qualification of components such as fuel pumps, using the specifications has not always accounted for unforeseen failures, wear, or inappropriate overhaul or maintenance. Service experience indicates that the explosion proofness demonstration needs to remain effective under all of the continued operating conditions likely to be encountered in service.
Therefore an extensive evaluation of the qualification of components may be required if qualitative assessment does not limit the component as a potential ignition source.

there is another report about the same – “Intrinsically Safe Current Limit Study for Aircraft Fuel Tank Electronics”

it says (cut outs):

A flammable mixture of fuel vapor and air can exist at times in a partially filled aircraft fuel tank containing jet fuel. Research has been done to develop methods to eliminate or reduce the risk of having an explosive condition in the fuel tank. There are a few different approaches to preventing fuel tank explosions. Explosions need three conditions to occur simultaneously: a flammable fuel source, sufficient oxygen to react with fuel molecules, and an ignition source to start the chemical chain reactions. Eliminating any one of these conditions will prevent a fuel tank explosion.

Recently, attention has been focused on developing a low-cost, low weight, high-efficiency fuel tank inerting system for use in large transport airplanes [1]. This system uses high temperature bleed air from the engines to create nitrogen-enriched air (NEA) with as high as 98% nitrogen concentration. The NEA is plumbed into the ullage space above the liquid fuel in the fuel tank, forcing air out the vents and creating an atmosphere with a maximum oxygen concentration of 12%. This value has been shown to be the lowest oxygen concentration that will support ignition of jet fuel vapors [2]. This approach eliminates one of the key ingredients required to have a fuel tank explosion (sufficient oxygen).

Spontaneous ignition of flammable vapors can also occur due to heat transfer from a hot surface to fuel molecules. A standard test method has been developed to measure the autoignition temperature of a liquid fuel by dropping a small amount of fuel onto a flat, heated surface and noting the temperature at which a flame is observed [5]. It has been accepted that the autoignition temperature of jet fuel is around 450°F [6], although this is not an exact figure. Many factors can affect the ignition of the fuel vapors and the propagation of a flame front from the hot spot. The design of the test apparatus will determine the type of combustion that will occur. Cool flames can develop and propagate through a flammable mixture without creating an explosion as long as the rate of heat generated is not much greater than the rate of heat lost; explosions can only occur if significantly more heat is generated than lost.

The purpose of this study was to determine the lowest electrical current required to ignite a flammable fuel vapor mixture. It was proposed to determine if the burning/thermal sparking created by steel wool is significant enough to cause an explosion in a flammable mixture, and what currents would be required to cause steel wool to ignite the mixture. Several different methods of creating a short circuit with steel wool were used to explore various fault possibilities. Also, various materials were used for comparison to the steel wool, such as aluminum wool, bronze wool, and wire from FQIS probes.

The current study employed a mixture of hydrogen (H), oxygen (O), and argon (Ar) that allows for repeatable ignition and can be calibrated with a standard voltage spark at a low energy. Tests were performed in a small chamber that was filled with the ignitable mixture. Experimentation was performed per a test matrix, and voltages and currents were recorded to determine the lowest currents that will cause ignition of the hydrogen mixture.

The gas mixture used in the device consisted of hydrogen, oxygen, and the inert gas argon. Research has shown that the lower-flammability limit for this mixture is about 5% H2, 5% O2, and 90% inert gas (% by volume) [3]. The oxygen concentration can be increased to 12%, and the hydrogen concentration can be slightly increased to give a gas mixture with a desired minimum ignition energy and ignition probability. Increasing the oxygen and hydrogen concentrations further results in overpressures higher than the maximum pressures recommended for the explosion chamber [4]. It should be noted that although ignitions can be achieved at oxygen concentrations below 12% with hydrogen gas as the fuel, this oxygen concentration is insufficient to sustain flame propagation in a hydrocarbon/air mixture such as Jet A, as the lower-flammability limit for hydrocarbon fuels has been experimentally determined to be near 12% O2.

Note: This technical note describes research performed to determine the ignition hazard presented by small fragments of superfine steel wool that contact energized direct current wires in aircraft fuel tanks. Several different methods of shorting a circuit with steel wool were explored. An ignitable mixture of hydrogen, oxygen, and argon, calibrated to have a minimum ignition energy of 200 micro Joules, was used as an ignition detection technique. The electrical currents at the ignition threshold were recorded to determine safe maximum allowable current limits for fuel tank electronics. The lowest current found to ignite the flammable mixture was 99 milliamps (mA); the lowest current found to ignite a steel wool wad in air only was 45 mA.

So here (aircraft) Ex is another technical protection layer?

Keep up the good work!

Arpad
veress@exprofessional.com
exprofessional.com