Fire Technology

�C 2007 Springer Science + Business Media, LLC. Manufactured in The United States

DOI: 10.1007/s10694-006-0002-4

Effectiveness of Various Concentrations 

of an Inert Gas Mixture for Preventing 

and Suppressing Mining Equipment Cab 

Fires: Development of a Dual Cab Fire 

Inerting System 

Maria I. De Rosa∗ , Industrial Hygienist, Pittsburgh Research Laboratory,
 

National Institute for Occupational Safety and Health, Pittsburgh, PA
 

Charles D. Litton, Research Physicist, Pittsburgh Research Laboratory,
 

National Institute for Occupational Safety and Health, Pittsburgh, PA
 

Abstract. The National Institute for Occupational Safety and Health (NIOSH/PRL) conducted a series of large-scale experiments to evaluate the effectiveness and safety of various 

concentrations of an inert gas mixture (CO2, 8%; N2, 50%; Ar, 42%) for preventing and suppressing cab fires. Comparison of concentrations effectiveness in yielding safe times has led to 

the choice of an optimum gas mixture concentration, discharged in the cab through a muffled 

nozzle system, for the development of a dual cab fire inerting system. Of note is that safety 

training programs, including the synchronization of performed tasks, need to accompany this 

technology to enhance operator’s efficiency and safety during fire emergencies within the safe 

times yielded by the cab fire inerting system. 

Cab fires are caused by the ignition of flammable vapors and mists (ball of fire) that penetrate 

the cab during prolonged hydraulic fluid and fuel fires, and electrical malfunctions involving 

other cab combustible materials. Often, these fires force the operator to exit the cab under 

hazardous conditions during a time needed to perform emergency tasks. Hence, it is important 

to provide the operator, not only with an engine fire suppression system (dry chemical powder), 

but also with a cab fire protection system, effective both in preventing the ignition of flammable 

vapors in the cab, and suppressing cab material fires. 

Keywords: fires, equipment, inert systems, fire suppression, fire prevention 

∗ 

This paper details the results of the experiments, and presents the development of a dual cab fire inert system, 

using an optimum gas mixture concentration discharged in the cab through a muffled nozzle system. Of note is that 

the design of a gas mixture concentration volume according to cab volumes, and system fabrication/installation 

have been undertaken by cooperating industries. 

Disclaimer: This information is distributed solely for the purpose of pre-dissemination peer review under 

applicable information quality guidelines. It has not been formally disseminated by NIOSH. It does not represent 

and should not be construed to represent any agency determination or policy. 
Introduction 

Background 

An analysis of mining equipment fires from 1990–1999 showed that 172 of the 339 mobile 

equipment fires, with 72 injuries and 3 fatalities, were caused by the spraying of pressurized 

hydraulic fluid and fuel onto engine hot surfaces due to ruptured lines [1–3]. On 97 

occasions, these fires grew out of control because of the continuous flow of fluids from 

the pumps due to engine shutoff failure, lack of an emergency line evacuation system and 

fire barriers or lack of effective local fire fighting capabilities. Often, these fires re-ignited 

fueled by the flow of pressurized fluids embedded in the lines. Furthermore, during these 

fires, at least sixteen times, flames rapidly erupted in the cab (ball of fire) due to the ignition 

of flammable vapors and mists that penetrated the cab, forcing the operator to exit under 

hazardous conditions during a time needed to perform emergency tasks. In addition to these 

incidents, at least ten fires were found to originate within the cab itself due to electrical 

malfunctions, involving other cab materials. Hence, it is important to provide the operator, 

not only with an engine fire suppression system (dry chemical powder), but also with a cab 

fire protection system, effective both in preventing and suppressing cab fires. 

One possible solution to this problem is to discharge in the cab various concentrations of 

an inert gas mixture (CO2, 8%, N2, 50%, Ar, 42%; Inergen), through muffled (Photograph 1) 

and un-muffled nozzle systems, to evaluate their effectiveness in inerting the cab volume 

by reducing the oxygen concentrations to levels that inhibit combustion yet are sufficiently 

high to support life. For this purpose, unlit and prelit fuel trays (Photographs 2 and 3) were 

used in the cab (2.5 m3 volume) (Figure 1) to simulate the accumulation of flammable 

Figure 1. Schematic illustration of a large-scale mining equipment 

cab. 

vapors and mists in the cab and cab material fires, respectively. In this study, the gas 

mixture concentrations tested were the 61%, 51%, 45%, 41%, 34%, and 25%, contained 

in pressurized canisters ( ∼ 15 × 106 Pa) bolted to the cab rear wall, at cab open/closed 

vents (cab closed windows), and no forced airflow through the cab (winter conditions). 

For summer conditions (open cab windows or air conditioning), close or shutoff systems 

need to accompany the discharge in the cab of the inert gas mixture concentration. The 

expectations were that the experimental data would lead to the choice of an optimum gas 

mixture concentration and discharge nozzle system for the development of a dual cab fire 

inerting system, effective both in preventing the ignition of flammable vapors and mists in 

the cab, and in suppressing cab fires. Of note is that this technology needs to be accompanied 

by safety training programs, including the synchronization of performed tasks, to enhance 

operator’s efficiency and safety during fire emergencies within the safe times yielded by 

the cab mixture concentrations. Also of note is that the system needs to be recharged after 

either usage: 

1. At engine fire detection time, the operator needs to rapidly perform safe parking/engine 

shutoff and exit the cab. These operations may be preceded and accompanied (double 

engine fire suppression system) by the rapid automatic or manual activation of the engine 

fire suppression system and cab fire inerting system, synchronized with the activation of 

fire barriers to prevent any additional pressurized fluid from being sprayed onto engine 

hot surface. 
2. At cab material fire detection time, the operator needs to rapidly perform safe parking/engine shutoff and exit the cab. These operations may be preceded by the automatic 

or manual activation of the cab fire inerting system. 

The inert gas mixture concentrations, reported above, and their corresponding oxygen 

concentrations are reported in the NFPA Standard for Carbon Dioxide [4]. The mixture 

concentrations are derived by multiplying flooding factors (specific to each gas mixture 

concentration) by the cab volume. The flooding factor values, unit volume of gas mixture 

per unit space volume at 21◦C, and the equation from which they were derived are reported 

in Table 3.5.1 of NFPA 2001 for Clean Agents Fire Extinguishing Systems [5]. Example: 

in order to design the concentration volume for the 45% gas mixture concentration, according to a cab volume of 2.5 m3 at 21◦C temperature, according to the Table reported 

above, one multiplies the given flooding factor (0.598) by the cab volume (2.5 m3). Of 

note is that the design of concentration volumes according to cab volumes, and system 

fabrication/installation, have been undertaken by cooperating industries. 

It has been found that a depletion of oxygen concentrations below certain limits may 

eliminate fire ignition [6, 7], and that most healthy individuals could tolerate a 12% oxygen 

level for a short period of time [6]. Studies have found that subjects exposed to atmospheres 

containing 10% oxygen concentrations and carbon dioxide concentrations up to 5% showed 

normal intellectual functions for a considerable period of time [8]. For inert atmospheric 

gases such as Ar, N2 and CO2, which are not inherently toxic, the first adverse effect 

observed as a result of the hypoxic atmosphere created will be a reduced oxygen supply 

to the brain, compensated in part by the improvement in brain blood flow produced by 

the carbon dioxide components [9–11]. Also it has been found that carbon dioxide, at 

concentrations typical of those obtained in these experiments (2 to 4%), has promptly 

improved tolerance to atmospheres with oxygen concentrations of 10% [12, 13]. This is 

the result of the combined effects of three main physiological mechanisms: stimulation of 

respiration, dilation of brain blood vessels, and shift in the hemoglobin dissociation curve 

which aids unloading of oxygen in all tissues [14]. 

Studies of a commercially available inert gas mixture (Inergen) have found that the agent 

at concentrations ranging between 30 to 50% dilutes the oxygen concentrations to a level 

that does not support combustion [15]. Furthermore, a 40% concentration of the inert gas 

mixture extinguished heptane liquid fuel fires, ranging between 200 kW and 1000 kW, 

within the first 50s of gas mixture discharge, yielding oxygen concentrations ranging 

between 11% and 9.5% and carbon dioxide concentrations of 4.1% and 4.9%, respectively 

[13]. Also, test results of n-heptane pan fires have shown that the fires were extinguished 

with this fire suppression agent at concentration of 31.5% by volume [16]. Of note is that 

a 51% design gas mixture concentration is the highest concentration of agent, for a 5 min 

exposure, resulting in atmospheres containing 10% oxygen and 4–5% carbon dioxide [15]. 

Carbon dioxide concentrations, within these limits, have been found to promptly improve 

tolerance to even severe degrees of hypoxia by preventing a decrease in the normal level 

of carbon dioxide in the lungs and arterial blood [18]. However, tenable O2 levels may not 

be maintained during real fire suppression because the depletion of O2 concentrations is 

dependent on the fire size; hence, O2 concentrations may be much lower than the safe level 

[19]. Persons disabled by degrees of cardiac and pulmonary abnormalities will be able to 
exit during the gas mixture flooding with any transient exposure completely reversing itself 

upon exposure to the external atmosphere [20]. 

Experimental 

In the present study, a total of sixteen experiments (five sets) with unlit and prelit fuel 

trays were conducted in a large-scale mining equipment cab (2.5 m3;1.8 m long × 1.5 m 

high × 0.9 m wide), using various concentrations of an inert gas mixture (Inergen, 61%, 

51%, 45%, 41%, 34%, and 25%). For comparison purposes, each gas mixture concentration 

was discharged into the cab through a muffled and un-muffled nozzle system at closed 

and open cab vents (two vents, 323 cm2 surface area; average cab airflow leakage rates, 

0.008 m3/s and 0.011 m3/s, respectively). For these experiments, the cab windows were 

closed with no forced airflow through the cab (winter conditions). As mentioned earlier, 

for summer conditions (open cab windows or air conditioning), close or shutoff systems 

need to accompany the discharge in the cab of the gas mixture concentration. The average 

airflow leakage rates were calculated according to a mathematical expression (Equation (1)) 

reported in the “Fire Prevention Experiments” section. Of note is that the noise levels of 

discharged nozzles ranged between ∼ 85 db (muffled nozzles), and >115 db (un-muffled 

nozzles). 

The experiments were conducted to evaluate the effectiveness of each gas mixture concentration in preventing the ignition of flammable vapors and mists in the cab (unlit fuel 

trays, simulating the accumulation of flammable vapors and mists in the cab), and in 

suppressing cab fires (prelit fuel trays, simulating cab material fires). The expectations 

were that the muffled nozzle discharge system would reduce the noise level, and slow the 

gas mixture discharge rates into the cab, allowing for a slower displacement of original 

cab air (oxygen) while preserving mixture inerting capabilities and safe cab atmospheres 

(breathable atmospheres). 

The experimental concentrations were obtained by discharging into the 2.5 m3 cab 

volume designed mixture concentration volumes of 2.5 m3 (61% concentration); 1.9 m3 

(51% concentration); 1.5 m3 (45% concentration); 1.4 m3 (41% concentration); 1.1 m3 (34% 

concentration); and, 0.8 m3 (25% concentration) through appropriate size nozzles. Of note 

is that the quoted percentage of the inert gas mixture concentrations are the concentrations 

of the mixtures in the cab at the end of mixture discharge. During the experiments with 

the muffled nozzle system, each mixture concentration was discharged into the cab in 

approximately 120 s; 90% of the gas mixture was discharged within the first 70s, and 

the remaining 10% within the following 50s. During the experiments with the un-muffled 

nozzle system, the mixture concentration was discharged into the cab in approximately 

100s; 90% of the gas mixture was discharged within the first 50s, and the remaining 10% 

within the following 50s. 

For all experiments, a 23 cm diameter fuel tray, containing 250 ml of gasoline and 

250 ml of no. 2 diesel fuel floating on the surface of 250 ml of water (fire size, ∼ 32 kW; 

calculations of fire size are reported in this section), was placed at the center of the cab 

floor and equipped with electrical matches (for remote ignition), positioned 2.54 cm above 

the fuel surface. Three equidistant electrical matches for the unlit fuel tray experiments, 

and one electrical match for the pre-lit fuel tray experiments, were used. A gas sample was 
continuously drawn by a sample line located 28 cm above the center of the tray fuel surface 

and was analyzed for oxygen and carbon dioxide concentrations by MSA Lira Infrared 

Gas Analyzers (Model 3000; accuracy, ± 1%; ranges, 0–25% for oxygen; and, 0–10% for 

carbon dioxide). A thermocouple located 28 cm above the fuel surface was also used to 

measure the flame temperature. The noise measurements were made with a Larson Davis 

Spark Dosimeter with data recorder (maximum range, 130 db; Models 705, 706). Visual 

observations of fuel tray ignition and smoke obscuration were also made. 

All experimental data were acquired with a PC-based acquisition system. 

Fire size calculations: 

Qf (kW) = (As ) (Hs ) (Mf ) 

= (0.041)m2 (40 k J/g (19.5 g/m2 × g) = 32 kW 

Where 

As = fuel surface area 

Hc = Heat of combustion; and 

Mf = fuel loss rate 

Fire Prevention Experiments (Unlit Fuel Trays) 

The experiments with the unlit fuel trays (nine experiments; three sets), simulating the 

accumulation of flammable vapors and mists into the cab were conducted with the 61%, 

51%, 45%, 41%, and 34% gas mixture concentrations to derive the total safe times (total 

time of cab inert volume), and the earliest safe time (earliest time of inert cab volume). 

The total safe times, time from gas mixture discharge-start to time of last failed ignition 

attempt following complete mixture discharge ( ∼120 s), are the critical times available 

for the operator to perform emergency tasks and exit the cab. The earliest safe times, time 

from gas mixture discharge-start to first failed ignition attempt during the earliest time of 

gas mixture discharge-start, are the earliest times at which no ignition of flammable vapors 

and mists in the cab will occur. 

The first set of experiments was conducted with the 51%, 45% and 41% mixture concentrations, discharged into the cab through a muffled nozzle system at closed cab vents 

(0.008 m3/s; noise level 85 db). The second set of experiments was conducted with the 

61%, 51%, 41% and 34% mixture concentrations, discharged into the cab through the unmuffled nozzle system at open cab vents (0.011 m3/s; noise level >115 db); and, the third 

set of experiments was conducted with the 45% and 41% mixture concentrations, using the 

muffled nozzle system at closed cab vents. 

For the first and second sets experiments, three ignition attempts were carried out at 30s 

intervals, following complete gas mixture discharge ( ∼120th s, muffled nozzle system; 

and, 100th s, un-muffled nozzle system). Success was acknowledged if the fuel vapors did 

not ignite during the three ignition attempts, following complete gas mixture discharge, 

while maintaining safe cab atmospheres. For the third set of experiments, success was 

acknowledged if the fuel vapors did not ignite during the earliest stages of gas mixture 

discharge-start. 

The following mathematical expression was used for the calculation of average cab 

airflow leakage rates at closed and open cab vents, using measured minimum oxygen 

concentrations: 

QLEAK(m3

/s) = [Vc(�O2/�t)]/[20.95 − (O2)]MIN (1) 
Vc = cab volume in m3 (2.5 m3); �O2/�t = rate of increase of O2 (percentage of oxygen 

concentration per second, %/s) subsequent to attainment of the minimum concentration, 

(O2,)MIN (%). 

At constant pressure, and assuming uniform mixing, the increase in O2 concentrations 

in the cab due to air leaking in, can be defined as: 

= QAir �O2/(�O2)MAX L �t/Vcab

where QAir is the air leakage rate (ft3/s); Vcab is the volume of the cab (ft3

L ); (�O2) MAX

is the maximum increase that can occur due to air leakage = 20.95% − (O2)MIN; Where 

(O2)MIN is the minimum concentration subsequent to the dispersion of Inergen into the cab 

�O2 = O2(�t) − (O2)MIN 

where �t is the time in seconds 

This can be rearranged to yield Equation (1) in the paper. 

Fire Suppression Experiments (Pre-lit Fuel Trays) 

The prelit fuel tray experiments (seven experiments; two sets), simulating cab fires 

( ∼32 kW) were conducted with the 61%, 51%, 45%, 41%, 34% and 25% gas mixture 

concentrations to derive the earliest fire suppression time. Success was acknowledged if 

the fuel tray fires were suppressed at the earliest time of gas mixture discharge-start while 

maintaining safe cab atmospheres. Of note is that the earliest fire suppression time is the 

time from gas mixture discharge-start to time of fire suppression during the discharge of 

gas mixture. 

The first set of experiments was conducted (twice) with the 45% gas mixture concentration, discharged into the cab through a muffled nozzle system at closed cab vents 

(0.008 m3/s). For comparison purposes, the second set of experiments was conducted with 

the 61%, 51%, 41%, 34%, and 26% gas mixture concentrations, discharged into the cab 

through an un-muffled nozzle discharge system at open cab vents (0.011 m3/s), after 30 s 

fuel preburn time. 

Of note is that the 30 s fuel preburn time experiments were mainly carried out to measure 

oxygen depletion and to observe other fire parameters to advocate the development of rapid 

cab fire detection systems. 

Results and Discussion 

Fire Prevention Experiments 

The experimental results are reported in Figures 2− 5 and Table 1. 

For the first set of unlit fuel tray experiments (prevention of flammable vapors and 

mists ignition into the cab), results show that the gas mixture concentrations tested (51%, 

45%, 41%), discharged in the cab through a muffled nozzle system at closed cab vents 

(0.008 m3/s), with closed cab windows and no forced airflow (winter conditions), were 

effective in preventing the ignition of the fuel vapors while maintaining safe cab atmo­
0 

5 

10 

15 

20 

25 

Oxygen Concentrations, pct 

51% (~2.5 m3) 41% (~1.6 m3) 45% (~1.4 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 

Time, s 

Figure 2. Oxygen concentrations for various concentrations of an 

inert gas mixture with unlit fuel trays (23 cm dia; 500 ml 

diesel/gasoline), at closed cab vents (0.008 m3/s) and muffled 

nozzle discharge system. 

4 

Carbon Dioxide Concentrations, pct 3.5 

3 

2.5 

2 

1.5 

1 

0.5 

0 

51% (~1.9 m3) 41% (~1.6 m3) 45% (~1.4 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 

Time, s 

Figure 3. Carbon dioxide concentrations for various concentrations 

of an inert gas mixture with unlit fuel trays (23 cm dia; 500 ml 

diesel/gasoline), at closed cab vents (0.008 m3/s ) and muffled 

nozzle discharge system. 

spheres. Evidently, the muffled nozzle system, together with lower cab airflow leakage 

rates, slowed the gas mixture discharge rates in the cab and the displacement of cab original 

air (oxygen) while preserving the mixture inerting capabilities and safe cab atmospheres 

(O2, >11%; noise level, 85 db). Of note is that for summer conditions (open cab windows 

or air conditioning), close or shutoff systems need to accompany the discharge in the cab 

of the mixture concentration. 

The oxygen and carbon dioxide concentrations measured for these experiments are 

shown in Figures 2 and 3. The three gas mixture concentrations yielded total safe times 

of 180s and 160s (51%, 45%, and 41% concentrations, respectively), yielding minimum 
0 

5 

10 

15 

20 

25 

Oxygen Concentrations, pct 

61% (~2.5 m3) 51% (~1.9 m3) 41% (~1.4 m3) 34% (~1.1 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Time, s 

Figure 4. Oxygen concentrations for various concentrations of an 

inert gas mixture with unlit fuel trays (23 cm dia; 500 ml 

diesel/gasoline), at open cab vents (0.011 m3/s ) and muffled 

nozzle discharge system. 

3 

Carbon Dioxide Concentrations, pct 2.5 

2 

1.5 

1 

0.5 

0 

61% (~2.5 m3) 51% (~1.9 m3) 41% (~1.4 m3) 34% (~1.1 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Time, s 

Figure 5. Carbon dioxide concentrations for various concentrations 

of an inert gas mixture with unlit fuel trays (23 cm dia; 500 ml 

diesel/gasoline), at open cab vents (0.011 m3/s ) and muffled 

nozzle discharge system. 

O2 concentrations of 11.8%, at the 50th s (maximum CO2, 1.5%). The total safe times, 

reported above, are the critical times available for the operator to perform emergency 

tasks and exit the cab due to the possibility of ignition of newly accumulated flammable 

vapors evolved during a prolonged hydraulic fluid fire. Therefore, safety training programs, 

including the synchronization of performed tasks, need to accompany these technologies 

and methodologies to enhance operator’s efficiency and safety during fire emergency within 

the safe times yielded by the mixture concentrations. 
Table 1 

Cab Fire Prevention Experiments Unlit Fuel Trays 

Muffled nozzle system Un-muffled nozzle system 

Gas mixture (O2)MIN (CO2)MAX Safe time Gas mixture (O2)MIN (CO2)MAX Safe time 

51% 12%∗ 

45% 12%∗ 

41% 11.8%∗ 

Noise level ∼ 85 db 

1.4% 

1.25% 

1.5% 

180 s 

180 s 

160 s 

61% 8.64% 

51% 9.0% 

41% 11.8%∗ 

34% 14.0%∗ 

Noise level > 115 db 

2.6% 

2.6% 

1.75% 

0.8% 

160 s 

150 s 

100 s 

NM 

∗Denotes Breathable Atmosphere. 

Of note is that the above results have led to the choice of an optimum gas mixture 

concentration for the development of a dual cab fire inerting system. The 45% concentration 

(1.5 m3 concentration volume designed for a cab volume of 2.5 m3), discharged into the 

cab through a muffled nozzle system, was effective both in preventing and suppressing cab 

fires (the system needs to be recharged after either usage). Also of note is that the design 

of the concentration volume according to cab volumes, and system fabrication/installation 

have been undertaken by cooperating industries. 

For the second set of unlit fuel tray experiments, as shown in Figures 4 and 5, results show 

that some of the gas mixture concentrations tested (61% and 51%), discharged through the 

un-muffled nozzle system (noise level >115 db) at open cab vents (011 m3/s), although, 

succeeded in preventing the ignition of the fuel vapors (total safe time, 160 s and 150 s), 

yielded minimum oxygen concentrations below 10% (8.64% and 9%, respectively), lasting 

60 s (maximum CO2, 2.6%). The 41% and 34% gas mixture concentrations, instead, failed to 

prevent the ignition of cab fuel vapors at the second and third ignition attempts, respectively, 

yielding minimum oxygen concentrations of 11.8% and 14%, respectively. As shown in 

Figures 6 and 7, for each experiment, the oxygen rapidly decreases and the carbon dioxide 

increases, reaching minimum oxygen (8.64%) and maximum carbon dioxide (2.6%) within 

70 s of gas mixture discharge-start (61% mixture concentration). These changes result 

from the rapid displacement of cab air (oxygen), together with the mixture concentrations, 

brought about by the rapid gas mixture discharge rates and high cab airflow leakage rates. 

Comparing the experimental data, it can be seen that the rate of O2 decrease and the rate 

of CO2 increase are somewhat slower using the muffled nozzle system at closed cab vents. 

In addition, the minimum O2 concentrations are greater and maximum CO2 lower with 

closed cab vents compared to the open cab vents environment. It is also worth noting that 

at a lower cab airflow leakage rate, the O2 and CO2 concentrations change more slowly, 

following complete gas mixture discharge. 

For the third set of unlit fuel tray experiments, results show that the 45% and 41% mixture 

concentrations (the only concentrations tested), discharged in the cab through a muffled 

nozzle system at closed cab vents, were effective in inerting the cab volume at the 10th s and 

20th s of gas mixture discharge-start, respectively, while maintaining safe cab atmospheres 

(minimum O2, ∼12%). Also for the 45% mixture concentration, ignition attempts carried 

out at the 15th s, 20th s, and 25th s also failed to ignite the fuel tray, as expected. For the 
Oxygen and carbon dioxide concentrations, pct

25 

20 

15 

10 

5 

0 

O2 CO2 

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 

Time, s 

Figure 6. Oxygen and carbon dioxide concentrations for the 45% 

inert gas mixture concentration with prelit fuel trays ( ∼32 kW), at 

closed cab vents (0.008 m3/s) and muffled nozzle discharge system. 

25 

Oxygen Concentrations, pct 20 

15 

10 

5 

0 

61% (~2.5 m3) 51% (~1.9 m3) 41% (~1.4 m3) 34% (~1.1 m3) 25% (~0.8 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Time, s 

Figure 7. Oxygen concentrations for various concentrations of an 

inert gas mixture with prelit fuel trays ( ∼32 kW), at open cab vents 

(0.011 m3/s) and muffled nozzle discharge system. 

41% mixture concentration, earlier ignition attempts carried out at the 8th s ignited the fuel 

vapors (O2, 19%), although the flames extinguished themselves within 10 s (O2, ∼17%). 

Fire Suppression Experiments 

The experimental results are shown in Figures 6− 8 and Table 2. 

For the first set of prelit fuel tray experiments (suppression of cab fires), using the 

45% gas mixture concentration discharged in the cab through a muffled nozzle system 

at closed cab vents, results show that the concentration was effective in suppressing the 

fires within the first 20 s of gas mixture discharge-start (Figure 6). Of note is that at fire 

detection time, the operator needs to rapidly perform safe parking/engine shutoff and exit 

the cab; these operations may be preceded by automatic or manual activation of the cab 
0 

0.5 

1 

1.5 

2 

2.5 

3 

3.5 

Carbon Dioxide Concentrations, pct 

61% (~2.5 m3) 51% (~1.9 m3) 41% (~1.4 m3) 34% (~1.1 m3) 25% (~0.8 m3) 

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 

Time, s 

Figure 8. Carbon dioxide concentrations for various concentrations 

of an inert gas mixture with prelit fuel trays ( ∼32 kW), at open cab 

vents (0.011 m3/s) and un-muffled nozzle discharge systems. 

fire inerting system. Minimum oxygen concentrations of approximately 13.8% (maximum 

CO2, 1.65%) were recorded. For the second set of prelit fuel tray experiments, results show 

that the gas mixture concentrations tested (61%, 51%, 41%, 34%, and 25%), discharged 

in the cab through the un-muffled nozzle system at open cab vents, were effective in 

suppressing the fuel vapor fires within the first 20s of gas mixture discharge-start. However, 

as shown in Figures 7 and 8, the 61%, 51% and 41% mixture concentrations yielded 

minimum oxygen concentrations below 10% ( ∼8% and 9%, respectively) at the 50th s, 

lasting 80th s (maximum CO2, 3.1%). The 34% and 25%, instead, yielded minimum oxygen 

concentrations of ∼11% and 13.5%, respectively (maximum CO2, 2.8%). Evidently, the 

low oxygen concentrations are due to fast displacement of cab original air (oxygen), 

aggravated by the depletion of oxygen occurring during the 30 s fuel preburn time (O2, 

<14%; flame temperature, ∼400◦C). As a footnote, it is worth mentioning that at flame 

temperatures of ∼3000◦C, carbon dioxide reverses to carbon dioxide (21). 

Table 2 

Cab Fire Suppression Experiments Prelit Fuel Trays 

Muffled nozzle system Un-muffled nozzle system 

Gas mixture (O2)MIN (CO2)MAX Supp time Gas mixture (O2)MIN (CO2)MAX Supp time 

45% 13.8% 1.65% 20 s 61% 8.0% 3.1% 10 s 

45% 13.9% 1.65% 20 s 51% 8.5% 3.1% 10 s 

(repeat) 

41% 9.0% 2.85% 15 s 

34% 11.0%∗ 2.8% 20 s 

25% 13.5%∗ 2.0% 20 s 

∗Denotes Breathable Atmosphere. 
According to these results, the importance of installing in the cab effective fire protection 

systems needs to be stressed, accompanied by rapid cab fire detection system (optical or 

photoelectric/ionization smoke detectors) for the early detection of cab fires. Similar fire 

detection systems also need to be installed within the engine compartment for the early 

detection of hydraulic fluid/fuel fires before large concentrations of flammable vapors and 

mists penetrate the cab. 

Conclusions 

For the prevention of flammable vapors and mists experiments, the 51%, 45% and 41% 

gas mixture concentrations, contained in pressurized canisters bolted to the cab rear wall, 

were effective in preventing the ignition of flammable vapors and mists in the cab while 

maintaining breathable atmospheres (O2, >11%; noise level, 85 db). Of note is that the 

mixture concentrations were discharged in the cab through a muffled nozzle system at cab 

closed vents (airflow leakage rate, 0.008 m3/s), with closed cab windows and no forced 

airflow (winter conditions). For summer conditions (open cab windows or air conditioning), 

activation of close or shutoff systems need to accompany the discharge in the cab of the 

inert gas mixture concentration. 

Evidently, the muffled discharge nozzle system was effective both in abating the noise 

level, and in slowing the mixture discharge rates in the cab, and, therefore, the displacement 

of cab original air (oxygen) while maintaining mixtures inerting capabilities and safe cab 

atmospheres. For these gas mixture concentrations, the total safe times, during which no 

ignition of flammable vapors occurred, ranged between 180 s (51% and 45% concentrations) 

and 160 s (41% concentration), yielding minimum oxygen concentrations of approximately 

12% (maximum CO2, 1.5%). Also, the 45% and 41% mixture concentrations (the only 

concentrations tested) yielded the earliest safe times at the 10th s and 20th s of concentration 

discharge-start (complete concentration discharge, 120th s). Of note is that the total safe 

times reported above are the critical times available for the operator to perform emergency 

tasks and exit the cab due to the accumulation of additional vapors and mists into the cab 

evolved during prolonged hydraulic fluid and fuel fires. 

In view of the above reported results, the 45% gas mixture concentration (concentration 

volume 1.5 m3 designed for a cab volume of 2.5 m3), discharged into the cab through 

a muffled discharge nozzle system, was chosen as the optimum concentration for the 

development of a dual cab fire inerting system. Of note is that the design of the gas mixture 

concentration volume according to cab volumes, and system fabrication/installation have 

been undertaken by cooperating industries. Also of note is that safety training programs, 

including the synchronization of performed tasks, need to accompany this technology to 

enhance operator’s efficiency and safety during fire emergencies within the safe times 

yielded by the cab fire inerting system. 

For the cab fire suppression experiments (prelit fuel trays), results show that the 45% 

gas mixture concentration (the only concentration tested under these conditions, in view 

of previous results), discharged through a muffled nozzle system at closed cab vents, 

was effective in suppressing the cab fires ( ∼32 kW) within the first 20 s of gas mixture 

discharge-start while maintaining safe cab atmospheres (minimum O2, ∼13.8%). Of note is 

that, at fire detection time, the operator needs to rapidly perform safe parking/engine shutoff 
and exit the cab; these operations may be preceded by the automatic or manual activation 

of the cab fire inerting system. Finally, results of fire parameters obtained during the 30 s 

fuel preburn time such as oxygen depletion (O2, < 14%), and possible evolution of toxic 

gases at flame temperatures of ∼400◦C, imply that any cab fire protection system should 

be accompanied by a rapid cab fire detection system (optical or photoelectric/ionization 

smoke detectors). Similar rapid detection systems may also be installed within the engine 

compartment for the rapid detection of incipient hydraulic fluid/fuel fires before large 

concentrations of flammable vapors penetrate the cab. 

Acknowledgment 

The authors wish to thank Richard A. Thomas, Electronic Technician, NIOSH DPRB/PRL, 

for his contributions in carrying out the experiments. 

References 

[1]	 M.I. De Rosa, “Analyses of Mobile Equipment Fires for all U.S. Surface and Underground Coal 

and Metal/Nonmetal Mining Categories, 1990–1999,” NIOSH IC 9467, 2004, pp. 53. 

[2]	 M.I. De Rosa, “Analysis of Mine Fires for all U.S. Underground and Surface Coal Mining 

Categories, 1990–1999,” NIOSH IC 9470, 2004, pp. 36. 

[3]	 M.I. De Rosa, “Analysis of Mine Fires for all U.S. Metal/Nonmetal Mining Categories, 1990– 

1999,” NIOSH IC 9476, 2004, pp. 52. 

[4]	 NFPA 12, Standard on Carbon Dioxide, National Fire Protection Association, Quincy, MA, 

Appendix A, 1998. 

[5]	 NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, National Fire Protection 

Association, Quincy, MA, 2000. 

[6]	 Laursen, “Overview of Toxicity/Effectiveness Issues,” in Proceedings and Technical Work 

Conference Halon Alternative, Albuquerque, NM, 1993, pp. 357–367. 

[7]	 M.G. Zabetakis, “Flammability Characteristics of Combustible Gases and Vapors,” U.S. Bureau 

of Mines, Bulletin 627, 1965, pp. 120. 

[8]	 F.A. Gibbs, E.L Gibbs, W.G. Lennox, and L.F. Nims, “The Value of Carbon Dioxide in Countering the Effects of Low Oxygen,” Aviation Medical Journal, vol. 14, 1943, pp. 250–261. 

[9]	 C.J. Lambertsen, Hypoxia, Altitude, and Acclimatization, Med. Physiol., (ed.), St. Louis MO, 

1980, pp. 1844–1872. 

[10]	 B. Slobodnik, M.T. Wallick, and J.M. Chimiak, “Effectiveness of Oxygen-Nitrogen Gas Mixtures in Inducing Hypoxia at 1 Atmosphere,” Navy Experimental Division Unit, Panama City, 

Fla., Report 04–91, 1991, pp. 100. 

[11]	 E. Gellhorn, “The effect of O2-lack Variations in the CO2 Content of the Inspired Air, and 

Hypernea on Visual Intensity Discrimination,” American Journal of Physics, vol. 115, 1936, 

pp. 679–684. 

[12]	 F.A. Gibbs, E.L. Gibbs, and W.G. Lennox, “Changes in Human Cerebral Blood Flow Consequent 

on Alterations in Blood Gases,” American Physics Journal, vol. 111, 1943, pp 557–563. 

[13]	 W.G. Lennox and E.L. Gibbs, “The Blood Flow in the Brain and Leg of Man and the Changes 

Induced by Alterations of Blood Gases,” Clinical Investigation Journal, vol. 11, 1932, pp 

1155–1177. 

[14]	 C.J. Lambertsen, “Chemical Control of Respiration at Rest,” Medical Physics, 1980, pp. 1774– 

1827. 
[15]	 J.Z. Su, A.K. Kim, G.P. Crampton, and Z. Liu, “Fire Suppression With Inert Gas Agents,” Fire 

Protection Engineering Journal, vol. 11, 2001, pp. 72–87. 

[16]	 Underwriter Laboratories, “Summary: UL Fire Test Results Inergen Fire Extinguishing Agent,” 

North Brook, IL, Project 93NK29239, File EX-4510, 1994, pp. 30. 

[17]	 C.J. Lambersten, “Inergen: Summary of Relations, Physiological Factors and Fire Protection 

Engineering Design Scenario,” Env. Biom. Res. Data Center, Inst. Env. Med., Univ. Pennsylvania Medical Center, College Park, PA, Report 4-14-94, 1994, pp. 1–21. 

[18]	 G.B. Field, “The Physiological Effects on Humans Exposure to Gas Mixtures of Air and 

Inergen,” Dept. of Resp. Med., The Prince of Wales Hospital, Sidney, Australia, Report and 

Supplementary Report, 1922, pp. 22. 

[19]	 U.S. Environmental Protection Agency, “Questions and Answers on Halon and Their Substitutes,” Revision 14, 1999, 10. 

[20]	 C.J. Lambertsen, “Short Duration Inergen Exposures, Relation to Cardiovascular or Pulmonary 

Abnormalities,” Env. Biom. Stress Data Center, Inst. Env. Med., Univ. of Pennsylvania Medical 

Center, College Park, PA, Report 2-1-93, 1993, pp. 13. 

[21]	 M.W. Chase Jr., C.A. Davies, J.R. Downey Jr., D.J. Frurip, R.A. McDonald, and A.N. Syverud, 

“JANAF Thermochemical Tables, 3rd Ed.,” Journal of Physical and Chemical Reference Data, 

14 supplement, no. 1, 1985, pp. 626–628.