# Presentation: 504255

## Intrinsically Safe Current Limits for Fuel Tanks 

- Robert Ochs
- FAA/Rutgers University Graduate Fellow

## 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

## 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)*

## 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

## 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

## Previous Test Apparatus

## 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

## 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

## Hydrogen Chamber

## 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

## 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

## 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

## Calculated Spark Voltage and Capacitance

## 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

## Ignition Probability

- Total of 21 ignition checks
- 18 ignitions, 3 non ignitions
- 86% ignition probability with this mixture

## 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

## 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

## 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

## Test Matrix

## Test Configurations

- T.C. 1

- T.C. 2

## Test Configurations

- T.C. 3

- T.C. 4

## Test Configurations

- T.C. 5

## 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

## 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

## Results:  Test Config. #1

## 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

## Slide 27

## 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

## 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

## 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

## 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

## 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

## 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