Intrinsically Safe Current Limits for Fuel Tanks

1. Intrinsically Safe Current Limits for Fuel Tanks Robert Ochs FAA/Rutgers University Graduate Fellow

2. Motivation • Small fragments of cleaning debris, i.e. steel wool, when contacted between two charged electrodes, can spark, glow and burn • If this occurred in an environment filled with flammable vapors, as in a fuel tank, an explosion could occur, resulting in catastrophic damage

3. Definition of Intrinsically Safe Any instrument, equipment, or wiring that is incapable of releasing sufficient electrical or thermal energy under normal operating or anticipated failure conditions to cause ignition of a specific hazardous atmospheric mixture in the most easily ignited concentration ( AC 25.981-1X)

4. Objective To determine the minimum electrical current that could cause a flammable fuel/air mixture to ignite using a minimum ignition energy test apparatus with a calibrated hydrogen-oxygen mixture and spark source that can reliably ignite the mixture at a determined spark energy

5. Previous Work • Previous work was done at the FAA Tech. Center using an open cup flash point tester to generate flammable vapors • Electrodes with either AC or DC current were placed over the cup, and wads of dry and fuel soaked steel wool were dropped onto the electrodes • Single filaments of steel wool, aluminum drill shavings, and aluminum or brass wool were short circuited between the electrodes

6. Previous Test Apparatus

7. Difficulties With Open Cup • The mixture above the open cup flash point tester was not reliably consistent • Different mixtures require different ignition energies • An apparatus was needed that can deliver repeatable mixtures consistently

8. Current Test Apparatus • Hydrogen-Oxygen-Argon minimum ignition energy chamber developed by Lightning Technologies, Inc. • System can consistently and reliably ignite the mixture at 200 microjoules • Designed to test ignition capabilities of electrical failures of system components • Similar testing with the same materials to be tested in this apparatus

9. Hydrogen Chamber

10. Hydrogen-Oxygen Mixture • Using hydrogen as an ignition detection technique • Relatively low overpressures • High probability of ignition at low spark energy levels • Mass flow controllers regulate the amount of each gas flowing into chamber

11. Standard Voltage Spark Ignition Source • High voltage DC power supply • Variable-vacuum capacitor • Adjustable spark gap • Corona source used to initiate breakdown at specified voltage

12. Calibrating the Spark • Set spark gap for 2 mm • Fill chamber with non-ignitable mixture • Record breakdown voltage for 10 spark events • Spark voltage was selected at one std. dev. below the mean • This spark voltage was determined to reliably breakdown the gap using the corona source • The capacitance was calculated for 200 microjoule spark energy

13. Calculated Spark Voltage and Capacitance

14. Calibrating the Mixture • A mixture is desired that will “just” cause ignition at the given spark energy so that the minimum energy that will ignite this mixture will be no less than 200 microjoules • Fill tank w/5 VTE to get ~99% purity • Start with 5% H2, 12% O2, and 83% Ar, increase H2 and decrease Ar until 85-95% ignition probability is achieved

15. Ignition Probability • Total of 21 ignition checks • 18 ignitions, 3 non ignitions • 86% ignition probability with this mixture

16. Experimental Setup • Agilent model 6554A microprocessor-controlled DC power supply • In-line, thin film, non-inductive resistors used to dampen transient current overshoot • Tektronix P5205 voltage probe and Tektronix TCP202 current probe, combined with Tektronix TDS3014B digital phosphor oscilloscope were used to measure and record voltage and current traces

17. Experimental Procedure • Pre-testing Checklist • Spark gap checked w/feeler gauge for 2 mm • Electrodes, glass insulators wiped with isopropyl alcohol to avoid current leakage • Electric space heater used on humid days to eliminate water vapor near gap and capacitor • Mass flow controllers turned on to allow to warm up

18. Experimental Procedure • Verification of ignitable mixture • Chamber filled with proposed test mixture • Up to four ignition attempts were made at 200 microjoules • If ignition was not achieved, a new chamber fill was attempted, if again no ignition, increase H2% and try again • When an ignitable mixture is found, testing can begin

19. Test Matrix

20. Wire or filament fixed to electrodes Electrode Wire or filament initiating contact to electrode Wire or filament fixed to electrode Electrode Test Configurations T.C. 1 T.C. 2

21. Wire or shield initiating contact to flat electrode Wire or shield fixed to electrode Wire or filament initiating contact to flat electrode Electrode Flat aluminum electrode Wire or filament fixed to electrode Electrode Flat aluminum electrode Test Configurations T.C. 3 T.C. 4

22. Clump of wool initiating contact to electrode Electrode Test Configurations T.C. 5

23. Overall Results • Lowest ignition currents (A) • Non-inductive resistors used to regulate current with 28 VDC • Steel wool is the only material that caused ignition below 100 mA

24. Results: Test Config. #1 • Heating of filament was found insufficient to cause hot-surface ignition of the gas • Ignition was only achieved when filament failed and burned • Steel wool has machine oil coating on it, inherent in the manufacturing process, may initiate or sustain filament burning • Several brands of steel wool were used in testing; no significant difference found between brands

25. Results: Test Config. #1

26. Results: Test Config #1 • The length and thickness of a single filament of steel wool had a strong effect on the current at which the filament would fail and burn • Non-uniformity of cross sectional area of a single filament made it difficult to quantify the thickness of the strand • Measuring the resistance of a filament with an ohmmeter gave a good approximation of the relative thickness of a filament when compared to filaments of the same length • Filaments of various length and thickness were tested to find the lowest failure current

28. Results: Test Config #1 • Several tests were conducted with only air in the chamber • Filament failure/burning was found at currents as low as 53 mA in air • When filaments of similar length/thickness were tested with flammable gas, no filament burning or ignition was witnessed, only filament failure • %O2 in air ~21%, %O2 in mixture ~12% • Reduced O2 concentration in gas mixture may not be sufficient for filament burning

29. Results: Test Configs. #2 & #3 • It was found that when a single filament of steel wool makes/breaks contact with ground, small thermal sparking can be observed • Thermal sparks were insufficient to ignite the mixture • However, if the filament was left in contact with electrode for several seconds, a situation similar to test config. #1 was witnessed, and the filament would fail and burn

30. Results: Test Config #5 • When a wad of steel wool is brought into contact with a charged electrode pair, burning of the sample was witnessed • The wad would glow orange and leave behind a skeleton of steel wool that still conducts current; possibly low temperature burning of the oil coating • The burning of the wad was found to ignite the flammable mixture at currents as low as 99 mA

31. Results: Test Config. #5 • As before, several tests were conducted in air only • It was again found that a wad of steel wool could burn with an input current of only 45 mA • When tested in the flammable mixture, no burning of steel wool wads was found below 99 mA • Again, the oxygen deficiency in the flammable mixture inhibits combustion at low currents

32. Conclusions and Recommendations • The lowest current found to ignite the flammable mixture was 99 mA with a wad of steel wool • Other materials were found to ignite the mixture at higher currents, out of the scope of this research • The lowest current that caused failure and burning of steel wool was 45 mA in air

33. Conclusions and Recommendations • It can be assumed that if the same burning occurred in air as in the flammable mixture, ignition would be achieved • Future testing should use known calibrated flammable mixtures with oxygen concentrations similar to that of air