Performance of RETIMET Metal Foam Vents 

on Explosion-Proof Enclosures 

By Lawrence W. Scott and Arthur J. Hudson 
Mission: As the Nation's principal conservation 

agency, the Department of the Interior has responsibility for most of our nationally-owned public 

lands and natural and cultural resources. This 

includesfostering wise use of our land and water 

resources, protecting our fish and wildlife, preserving the environmental and cultural values of 

our national parks and historical places, and providing for the enjoyment of life through outdoor 

recreation. The Department assesses our energy 

and mineral resources and works to assure that 

their development is in the best interests of all 

our people. The Department also promotes the 

goals of the Take Pride in America campaign by 

encouraging stewardship and citizen responsibility for the public lands and promoting citizen participation in their care. The Department also has 

a major responsibility.for American Indian reservation communities and for people who live in 

Island Territories under U.S. Administration. 
Report of Investigations 941 0 

Performance of RETIMET Metal Foam Vents 

on Explosion-Proof Enclosures 

By Lawrence W. Scott and Arthur J. Hudson 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 

T S Ary, Director 
CONTENTS 

Page 

Abstract ........................................................................... Introduction ........................................................................ Acknowledgments .................................................................... Arrester material .................................................................... Laboratorytests ..................................................................... Experimental apparatus and procedure .................................................. Experimentalresults ................................................................ Large-scaletests ..................................................................... Experimental apparatus and procedure .................................................. Experimentalresults ................................................................ Conclusions and recommendations ....................................................... 

ILLUSTRATIONS 

1 . Prototype pressure vent ........... .............................................. 3 

2 . Schematic of test vessel ........................................................... 4 

3 . Dynamic gas-mixing system and high-speed recorder ....................................... 4 

4 . Gaschromatograph ............................................................... 4 

5 . RETIMET metal foam material used in arrester assembly .................................. 5 

6 . Multicompartmented variable vent area enclosure in MSHA's 500-ft3 explosion gallery in 

Triadelphia, WV ................................................................ 6 

7 . Vent area enclosure showing 100-in2 vent. 36-in2 vent with protective cover open. and 64-in2 vent ...... 7 

8 . Pressure versus vent-area-to-enclosure-volume ratio using 6.8% CH,. air ........................ 7 

9 . Failureof36-in2vent .............................................................. 7 

10 . Pressure versus methane concentration using 150-in2 vent ................................... 8 

11 . Spaced-plate heat absorber ......................................................... 8 

12 . Heat absorber attached to enclosure cover .............................................. 8 

13 . RETIMET metal foam after test with coal dust. oil. and grease on its external surface .............. 9 

TABLES 

1 . Properties of grade 45 NC-13 RETIMET metal foam ...................................... 2 . Summary of test results for RETIMET metal foam in 9.75% CH,. air .......................... 3 . Summary of test results for RETIMET metal foam in methane-hydrogen-air mixtures .............. 4 . Test results for RETIMET metal foam at three ignition points ............................... 
Library of Congress Cataloging in Publication Data: 

Scott, Lawrence W. 

Performance of RETIMET metal foam vents on explosion-proof enclosures / by 

Lawrence W. Scott and Arthur J. Hudson. 

p. cm. - (Report of investigations; 9410) 

Supt. of Docs. no.: I 28.23:9410. 

1. Mine explosions-Prevention. 2. Foamed materials. I. Hudson,Arthur J. 11. 

Title. 111. Series: Report of investigations (United States. Bureau of Mines); 9410. 

TN23.U43 [TN313] 622 s--dc20 [622'.82] 91-43015 CIP 
gram 

hertz 

inch 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

Btu/(in* ft2*"F) British thermal unit in2 square inch 

per inch per square foot 

per degree Fahrenheit in3 cubic inch 

Btu/(lbm OF) British thermal unit per pound square inch per cubic foot 

per degree Fahrenheit 

inch of water (pressure) 

OC degree Celsius 

inch per inch per degree 

cal calorie Fahrenheit 

cal/(ge "C) calorie per gram per kilocalorie 

degree Celsius 

kilocalorie per mole 

OF degree Fahrenheit 

pound per cubic inch 

ft3 cubic foot 

millimeter 

ft lbf/in2 foot pound (force) per square 

inch pound (force) per square 

inch 

ft/s foot per second 

pound (force) per square 

g inch, gauge 

Hz second 

in 

kcal 

kcal/mol 

lb/in3 

mm 

psi 
PERFORMANCE OF RETIMET METAL FOAM VENTS 

ON EXPLOSION-PROOF ENCLOSURES 

By Lawrence W. ~cott' and Arthur J. ~udson* 

ABSTRACT 

The performance of RETIMET metal foam as a flame arrester on explosion-proof enclosures was 

investigated by the U.S. Bureau of Mines both in laboratory tests and at the U.S. Mine Safety and 

Health Administration's (MSHA's) Approval and Certification Center, Triadelphia, WV. The objective 

of this research was to develop a permissible pressure vent for use on lightweight, vented, explosionproof enclosures. In laboratory tests, four grades of RETIMET, a stainless steel foam material, were 

evaluated: (1) 45 NC-7, (2) 45 NC-13, (3) 80 NC-7, and (4) 80 NC-13. 

Explosive gas mixtures were prepared by a dynamic flow system. Ignition was by a low-voltage arc. 

Each grade of RETIMET metal foam successfully arrested the flame front in all methane-air tests. 

To evaluate RETIMET metal foam on large, commercial size enclosures, a multicompartmented 

enclosure was designed and tested in MSHA's explosion gallery in Triadelphia, WV. Extensive explosion 

testing revealed that a minimum vent-area-to-enclosure-volume ratio of 11.33 in2/ft3 is required to keep 

internal pressure rises below 3 psig. The RETIMET metal foam functioned satisfactorily in all tests as 

evidenced by the absence of external ignitions. 

'~lectrical engineer. 

2~lectronic technician. 

Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

A flame arrester is a device that prevents the passage 

of a flame from one location to another through absorption of heat energy. Items commonly used as flame arresters remove sufficient heat to quench the combustion front. 

In comparison with solid materials, gases are ordinarily 

quite poor heat conductors. Solid surfaces can extract 

substantial amounts of heat only from those portions of 

the gas that are close to the surface. Quenching distances 

represent the maximum diametrical separation of surfaces 

at which heat extraction by the surfaces destroys the selfpropagating reaction front. Thus, flame arresters must 

comprise some configuration adequate to break the flame 

front into flamelets of quenchable size and then to quench 

those flamelets. To extinguish the combustion front, each 

flamelet must be quenched; in addition, if hot combustion 

gases pass through the arrester, the gases must be cooled 

sufficiently so that reignition does not occur beyond the 

arrester. 

Arresters usually consist either of an aggregation of 

parallel small channels or a maze of small channels and 

are intended to offer minimal resistance to gas flow. 

Common types of flame arresters consist of crimped metal 

ribbon, wire gauze, compressed wire mesh, perforated 

plates, and sintered metal arresters. Some of these have 

fairly general application, whereas others are more 

specialized. 

A commercially available material called RETIMET) 

metal foam (made by Dunlop Aviation in the United 

Kingdom) has been developed that has characteristics indicating its suitability for use as a flame arrester. This 

material is a metallic foam that can be made from various 

metals and alloys with co~ecting cavities, having a very 

high void volume. The pore sizes of the material may be 

varied. Such a structure mieht ". ~erform well as a flame 

arrester for use on explosion-proof enclosures and be used 

to reduce the pressure generated by an internal methaneair explosion. 

Since internal pressures may exceed 100 psig, explosionproof enclosures are characterized by heavy wall construction, tight flange gaps, and numerous cover bolts. If an 

enclosure could be designed to vent the pressure in a safe 

manner, lighter and less expensive enclosures would be 

possible. Lightweight construction would facilitate handling, thus reducing associated injuries, and improve the 

maintenance of large enclosures that often require 

mechanized equipment or several workers. 

Under contra~t.~*~ the U.S. Bureau of Mines investigated numerous concepts designed to reduce the internal 

-- -- -- - - 

'~eference to specific products does not imply endorsement by the 

U.S. Bureau of Mines. 

pressure generated in explosion-proof enclosures during 

internal methane-air explosions. From this study, it was 

determined that any venting mechanism must1. Quench both methane and coal dust flame fronts, 

2. Be highly permeable to gas flow to minimize vent 

size, 

3. Be capable of being readily cleaned to reduce the 

possibility of clogging during underground use, and 

4. Have sufficient corrosion and mechanical shock 

resistance to be compatible with the mine environment. 

In light of these requirements, RETIMET metal foam 

was judged to offer the best combination of flamearresting and mechanical properties. For a prototype vent, 

a 112-in-thick piece of grade 45 NC-13 RETIMET metal 

foam was set in a metallic frame and mounted in an enclosure cover. A cross-sectional view is shown in figure 1. 

The RETIMET metal foam was protected against mechanical damage by a hinged metal cover that swung open 

when the internal pressure exceeded 2 psig. Normally, this 

cover is held in place by a small permanent magnet. 

To have a significant impact upon enclosure design, it 

was determined that a vent must limit internal explosion 

pressure to 12 psig maximum. To determine the relationship between vent area, pressure, and enclosure volume, 

a vent-area-to-enclosure-volume ratio was established 

through laboratory tests. Results showed that any enclosure with a vent-area-to-enclosure-volume ratio larger 

than 4 in2/ft3 would meet the 12-psig criterion. 

To evaluate the practical application of the vent assembly, the Bureau entered into an informal agreement 

with BergbauVersuchsstrecke (BVS), Dortmund, German~.~ The objective of this work was to determine 

if RETIMET metal foam could withstand repeated explosions without failure or extensive damage. The results of 

this work are summarized below: 

1. The pressure vent assembly withstood explosions of 

9.8% CH,-air. 

2. The resistance to bending of the RETIMET metal 

foam was not significantly affected by the explosion. 

3. An initially nonmagnetic piece of RETIMET metal 

foam became magnetizable through the thermal loadings 

of explosions. 

4. The RETIMET metal foam, which previously fitted 

the vent structure well, underwent a shrinkage in length of 

4~underman, R J. Innovations for Explosionproof Electrical Enclosures (contract H0357107, Dresser Ind., Inc.). BuMines OFR 121-81, 6~mmission of the European Community. Investigation on the 

1980, 209 pp.; NTIS PB 82104936. Operational Safety of Housing for Electrical Equipment Which Are 

S~valuation and Acceptance Criteria for Innovations in Explosion- Vented Through Large Area Flame Barriers. Rep. 7258/07/01/77, lW, 

proof Electrical Enclosures (contract H0357107, Dresser Ind., Inc.). '' PP' 

BuMines OFR 127-83,1982,141 pp.; NTIS PB 83-233379. 
0.7% after a single explosion. However, total shrinkage 

remained about 1% after repeated explosions. 

Enclosure cover 5. If the pressure vent is installed without the protective cover, a "howling" noise is heard following a test 

explosion. The frequency ranged from 140 to 557 Hz. 

6. Using 12% mixed gas (58% CH,, 42% H, mixed 

with air), a standard test according to European Standard 

EN 50018 (Electrical Apparatus for Potentially Explosive 

Steel ring vent Atmospheres, Flameproof Enclosure "d," March 1977), 

ignition flashthrough occurred. 

Thus, BVS concluded that pressure venting of electrical 

tal equipment to be used in areas where mine gas explosion 

hazards exist is not acceptable in light of present safety 

requirements. Results showed that, following an explosion 

inside the test housing, flames continue to burn for several 

minutes if the test housing is located in a large area filled 

with an explosive mixture. The source of these flames are 

the small metal pores of the RETIMET metal foam. 

This Bureau study was initiated to address concerns 

raised in BVS's work and to obtain experimental results 

that would be useful in evaluating RETIMET metal foam 

Tapped blind hole as a flame arrester for possible use on lightweight, vented, 

explosion-proof enclosures. This study was part of the 

Figure 1.-Prototype pressure vent. Bureau's program to improve mining safety. 

ACKNOWLEDGMENTS 

The authors wish to thank John W. Conn, physicist, technicians, MSHA, for their assistance in conducting the 

MSHA, Triadelphia, WV, for his theoretical insight and explosion tests required for the successful completion of 

valuable suggestions in conducting this work; and this project. 

Steve Murtaugh and Bill McCord, electrical engineering 

ARRESTER MATERIAL 

The RETIMET metal foam material tested is of a 

cellular nature and has been described as a skeletal threedimensional network consisting of a large number of interconnecting open cells. An important property of this 

structure is that only about 5% of the total volume is 

occupied by the metallic network and about 95% is void. 

In the present work, four nominal grades, as quoted by the 

manufacturer (Dunlop Aviation), were evaluated: (1) 

45 NC-7 (45 pores per linear inch, 7 mm (114 in) thick), 

(2) 45 NC-13 (45 pores per linear inch, 13 mm (112 in) 

thick), (3) 80 NC-7 (80 pores per linear inch, 7 mm 

(114 in) thick), and (4) 80 NC-13 (80 pores per linear 

inch, 13 mm (112 in) thick). To determine the thermal, 

mechanical, and physical properties of grade 45 NC-13, 

samples were submitted to material-testing laboratories. 

Typical properties are shown in table 1. 

Table 1.-Properties of grade 45 NC-13 RETIMET metal foam 

Thermal: 

Expansion coefficient ............. in/(in* OF). . Conductivity. ................... ~tu/(in*ft~*'~). . Specific heat ...................... Btu/(lbeaF). . Melting temperature ....................... 'F. . Mechanical: 

Tensile strength .......................... psi. . Elongation at break (2 in) .................... 96. . Compressive strength 

(40% of thickness) ...................... psi. . Flexural strength ......................... psi. . lzod impact strength .................. ft*lbf/in2. . Physical: 

Density .............................. lb/in3.. 

Pressure drop at 350-ft/min 

air velocity ......................... inH,O. . 
LABORATORY TESTS 

EXPERIMENTAL APPARATUS AND PROCEDURE 

The test vessel consisted of a commercially available 

explosion-proof enclosure (volume approximately 2.0 ft3) 

connected to a 0.18-ft3 circular enclosure (schematically 

shown in figure 2). The reason for this configuration was 

to obtain a 10:l volume ratio to determine if, when ignition occurred in the 0.18-ft3 chamber, fresh gas would be 

drawn in from the 2.0-ft3 chamber and reignited. However, this did not occur. 

The test vessel was fitted with two strain-gage pressure 

transducers, whose output was continuously recorded on a 

high-speed chart recorder. Explosive methane-hydrogenair mixtures were obtained using Teledyne Hastings dynamic gas-mixing system (fig. 3). These mixtures were 

verified by infrared and chromatographic analysis before 

each test (fig. 4). Combustion was initiated by a lowvoltage arc located at one end of the 0.18-ft3 chamber. An 

exhaust solenoid valve was provided to vent the products 

of combustion or, in the case of a misfire, the explosive 

mixture to the outside atmosphere. 

RETIMET metal foam flame arrester assemblies were 

fabricated using 3-in-diameter circular pieces of material 

shown in figure 5. These pieces of RETIMET metal foam 

were flange-fitted in a 3-112-in-diameter, 5-in-long plexiglass tube (for video observation of the flame front), which 

was attached to one end of the 0.18-fP chamber (see 

figure 2). The assembly will accommodate up to three 

pieces of 112-in RETIMET metal foam in the event that 

the thickness needed to be increased. A window mounted 

in one side of the 2.0-ft3 enclosure permitted visualphotographic observations to be made as the flame front 

approached the arrester. 

Flame velocity measurements were made using two ' 

LS-400 photodiodes spaced 6 in apart in the 0.18-ft3 

enclosure (see figure 2). Light from the approaching 

flame front was sensed by the photodiodes, which in turn 

(-Gas inlet Observation 

window 

! 

\ 2.0-ft3 enclosure 

Figure 2.-Schematic of test vessel (top view). 

triggered an electronic counter and an oscilloscope. The 

distance traveled divided by the elapsed time is a measure 

of flame velocity. 

EXPERIMENTAL RESULTS 

Using a 9.75% CH,-air mixture, a series of 30 tests was 

conducted on four grades of RETIMET metal foam: (1) 

45 NC-7, (2) 45 NC-13, (3) 80 NC-7, and (4) 80 NC-13. 

Video observations of the flame front in contact with the 

arrester showed that the arrester became visibly hot for 

about half its thickness in each case in the direction of 

flame travel. Also, a low-frequency howling noise (similar 

to BVS's observations) was observed for about 5 s during 

the explosion. This noise was attributed to resonance in 

Flgure 3.4ynamlc gas-mixlng system and hlgh-speed recorder. 

Figure 4.--Gas chromatograph. 
. . . . . . highest pressure occurred with grade 80 NC-13. It appears 

that, for a given grade, the restriction due to thickness is 

the controlling factor. 

To obtain higher flame speeds, a series of tests was 

conducted using various mixtures of methane-hydrogenair. Hydrogen flame speeds are considerably higher than 

methane flame speeds. The ratio of the velocity in 

methane-hydrogen-air to the velocity in methane-air should 

present a reasonable safety margin relative to RETIMET 

metal foam's use in methane-air atmospheres. 

To determine the stoichiometric mixtures of methanehydrogen-air to be used, Le Chatelier's law was used, 

which states that: 

%CH4/Cst + %H2/Cst = 1, (1) 

where C,t = stoichiometric mixture of methane-air 

and hydrogen-air, respectively? 

the 2.0-ft3 enclosure. Periodic examination of the 

RETIMET metal foam showed a very slight weight loss (a 

few milligrams); however, there was no evidence of any 

observable shrinkage or significant thermal damage, although there was a slight change in color. 

A statistical analysis was conducted on the results of 

tests for all four grades of RETIMET metal foam using 

the Statistical Package for the Social Sciences computer 

program. The RETIMET metal foam successfully arrested the flame front in all cases. These results are summarized in table 2. If flame had not been arrested, the peak 

pressure on the ignition side of RETIMET metal foam 

(upstream) (P,) and the peak pressure (downstream of 

RETIMET metal foam) (P,) would have been 70 to 

90 psig. 

Table 2.-Summary of test results for RETIMET metal foam 

in 9.75% CH,-air 

Grade Velocity, ft/s P,, psig p,, psig 

Mean SD Mean SD Mean SD 

P, Peak pressure on the ignition side of RETIMET metal 

foam (upstream). 

P, Peak pressure after arresting flame (downstream of 

RETIMET metal foam). 

SD Standard deviation. 

The highest flame speed and the lowest pressure 

occurred with grade 45 NC-7. This grade has the highest 

porosity and lowest thickness. The lowest flame speed and 

Again, a series of 30 tests was conducted on each grade 

of RETIMET metal foam using the following stoichiometric concentrations: 

Hydrogen, % Methane, % 

The results are summarized in table 3. 

Table 3.Summary of test results for RETIMET 

metal foam in methane-hydrogen-alr mixtures 

Maximum con- Mean Safety 

Grade centration, % velocity, factor 

'42 '34 ft /s 

45 NG7 . . . 10 6.33 255.51 1.44 

45 NC-13 . . 15 4.75 385.73 2.60 

80 NC-7 . . . 20 3.17 449.69 3.43 

80 NG13 . . 25 1.58 198.1 1 1.91 

Ignition of the external mixture occurred in all tests 

where the hydrogen concentration was higher than the indicated maximum. Video tape observations and strip chart 

recordings revealed that these ignitions were thermally 

induced, caused by the high surface temperature of the 

RETIMET metal foam, not by flamelets passing through 

the RETIMET metal foam. The RETIMET metal foam 

did not dissipate the heat sufficiently to prevent ignition 

'~urgess, D. S., A. L. Furno, J. M. Kuchta, and K. E. Mura. 

Flammability of Mixed Gases. BuMines RI 8709,1982,20 pp. 
even though it arrested the initial flame front. There was 

a delay of approximately 2.5 s between the arrival of the 

flame front at the arrester face and ignition of the external 

mixture, indicative of a thermal ignition. Further increases 

in hydrogen concentrations beyond the point at which thermal ignitions occurred resulted in ignition flashthrough, 

which indicated that the flame velocity exceeded the 

maximum quenchable velocity. 

Assuming that all of the heat from the explosion is 

transferred to the RETIMET metal foam (the metal foam 

assembly is plexiglass), an estimate of the RETIMET 

metal foam temperature can be made for a 25% 

H2-1.58% CH,-air mixture using grade 45 NC-13: 

where 

and 

Q = v1 (H, of H2) + V2 (H, of CH4), (2) 

Q = the total heat, 

= 0.05 (57.8 kcal/mol) + 0.00314 (212.79 

kcal/mol) = 3.56 kcal of total energy, 

V1 = volume of hydrogen, 

H, = heat of combustion, 

V2 = volume of methane. 

Assuming the RETIMET metal foam absorbed all of 

this energy and solving for temperature yieldswhere 

and 

m = mass of RETIMET metal foam, 

c = specific heat of RETIMET metal foam, 

T2 = temperature of RETIMET metal foam, 

T1 = ambient temperature. 

Assuming T, to be 25" C, then T2 is 564.39' C. This 

temperature is above the autoignition temperature of 

hydrogen (400" C). This accounts for the thermal ignitions 

as opposed to ignition flashthrough. Although velocity 

ratios were used to define a safety factor here, autoignition 

temperature might be a more viable parameter when 

assessing the relative merits of RETIMET metal foam in 

flame-arresting applications, especially when used in gases 

other than methane. This is indicated by the fact that 

grade 80 NC-13 appears less safe than grades 80 NC-7 

and 45 NC-13; however, it must be remembered that grade 

80 NC-13 has the lowest porosity but can withstand higher 

maximum concentrations. 

LARGE-SCALE TESTS 

In general, the RETIMET metal foam performed well arrangement was to determine the minimum vent-area-toas a flame arrester in laboratory experiments. It success- enclosure-volume ratio that will reduce the explosion presfully arrested the flame front in all tests using methane-air sure to a prescribed level and keep the external surface 

mixtures. No significant thermal damage occurred and no 

observable shrinkage was evident. However, the question " 

arose as to whether or not RETIMET metal foam could % 

withstand the large quantity of heat generated by "large- t- ' 

volume" methane-air explosions. These concerns led to an . 

agreement with MSHA's Approval and Certification Center to test RETIMET metal foam on a large, commercial- I .. size enclosure. 

EXPERIMENTAL APPARATUS AND PROCEDURE a 

The test enclosure consisted of a 54.48-ft3 multicompartmented, variable vent area enclosure designed 

jointly by the Bureau and MSHA. The enclosure is 

partitioned into four volumes of 13.31, 13.63, 13.63, and 

13.91 ft3 (fig. 6). There are four covers with vent areas 

of 36, 64,100, and 150 in2. (Three are shown in figure 7; Figure 6.-Multlcompartmented variable vent area enclosure 

the 150-in2 vent is not shown.) Again, the purpose of this In MSHA's 500-ft3 explosion gallery In Trladelphla, WV. 
Figure 7.-Vent area enclosure showing 100-in2 vent (left), 

36-in2 vent with protectlve cover open (center), and 64-in2 vent 

(right). 

temperature of the RETIMET metal foam below the ignition temperature of methane-air and coal dust. 

As before, this test enclosure was fitted with a pressure 

transducer whose output was recorded on a high-speed 

recorder. Explosive methane-air mixtures were provided 

using MSHA's dynamic gas-mixing system and verified by 

infrared analysis. Explosions were initiated by a highvoltage spark. Visual observations and video tapes were 

made of all tests. 

EXPERIMENTAL RESULTS 

The first task was to determine the proper vent-area-toenclosure-volume ratio to be used in the experiments. 

Using the 13.31-ft3 section of the enclosure, vent areas of 

36, 64, 100, and 150 in2 were tested using a 6.8% CH,-air 

mixture. This mixture was chosen so that the basic curve , 

could be generated before higher concentrations destroyed 

the RETIMET metal foam samples (for example, 9.8% 

destroyed the 36-in2 vent but not the 150-inz vent in a 

previous test). Twelve tests each were conducted on grade 

45 NC-13 and grade 80 NC-13. The results are shown in 

figure 8. After obtaining this basic curve (for later application to similar materials), explosion tests were conducted 

on all four vent areas using 9.6% CH,-air mixtures. The 

36-, 64-, and 100-in2 vents all ignited the external mixture. 

Figure 9 shows the catastrophic failure of the 36-in2 vent. 

The 150-in2 vents (both grades 45 NC-13 and 80 NC-13) 

did not permit ignition of the external mixture. Thus, the 

150-in2 vents, corresponding to a vent-area-to-enclosurevolume ratio of 11.33 in2/ft3, were used in subsequent 

tests. 

To compare the performance of grades 45 NC-13 and 

80 NC-13, tests were conducted in methane concentrations 

ranging from 5.5% to 9.6% using the 150-in2 vents. The 

results are shown in figure 10. Grade 80 NC-13 showed 

extensive thermal damage and produced a higher pressure 

Figure 8.--Pressure versus vent-area-to-enclosure-volume 

ratio usin$ 6.8% CHCair. (P, = maximum pressure.) 

Figure 9.--Failure of 36-111' vent 

drop. Grade 45 NC-13 showed less discoloration and produced a lower pressure drop; however, there was some 

bowing at the middle of the vents. Grade 45 NC-13 was 

used in subsequent tests. 

To reduce thermal degradation of the RETIMET metal 

foam, a heat absorber was designed and incorporated into 

the vent structure. Computer analysis showed that a series 

of spaced plates, placed in series with the RETIMET metal foam, would reduce the temperature of the metal foam 

significantly. The heat absorber consisted of 60 aluminum 

plates 10 x 6 x 0.125 in, spaced 0.125 in apart and mounted in a rectangular aluminum frame (fig. 11). The objective was to obtain a large surface-area-to-volume ratio to 
Figure 10.-Pressure versus methane concentration using 

1 50-in2 vent (P, = maximum pressure.) 

Figure 11,-Spaced-plate heat absorber. 

maximize heat transfer from the gas. The aluminum 

frame, along with the RETIMET metal foam, is then 

mounted on the enclosure cover (fig. 12). 

A total of 36 tests were conducted in methane concentrations ranging from 6.0% to 10.5%. No external ignitions occurred. The results using the 45 NC-13, 150-in2 

vent are summarized in table 4. 

Table 4.-Test results for RETIMET metal 

foam at three Ignition points 

6.0 . . . 0.08 0.16 0.08 

.08 -08 .16 

.12 .08 .08 

.64 .40 .40 

.60 .40 .32 

.60 .52 -40 

-80 2.40 1.20 

2.80 1.60 1.20 

1.20 1.20 1.20 

.92 .64 .56 

.96 .76 .56 

.64 -80 .56 

B Ignition at back of enclosure. 

M Central ignition. 

P, Maximum pressure. 

V Ignition near vent. 

To address the concern of possible coal dust ignition 

and clogging, a series of tests was conducted with minus 

200-mesh coal dust, oil, and grease on the external surface 

of the RETIMET metal foam and coal dust on the interior 

plates of the heat absorber. A 9.6% CH,-air concentration 

was used and ignition occurred close to the vent. No external ignitions occurred. The maximum pressures (Pa, 

Figure 12.--Heat absorber attached to enclosure cover. 

RETIMET metal foam fits on underside of cover. 
in pounds per square inch, gauge, generated using the 

45 NC-13, 150-in2 vent are summarized below 

Coal dust Oil Grease 

0.80 1.20 1.04 

1.04 1.20 1.12 

1.04 1.28 .% 

Figure 13 shows the RETIMET metal foam material after " 

being tested with coal dust, oil, and grease on its external ' 

surface. It is slightly bowed but is intact. 

Although the vent structure did not cause any ignitions 

in explosion testing, the question arose as to what actual 

external temperature the RETIMET metal foam attained. : 

Using type K (Chromel-Alumel) thermocouples, the external temperature was measured at five locations on the surface as a function of methane concentration. The maximum temperatures attained were 75" C, using 6.0% CH,; 

140" C, using 7.5% CH,; 190" C, using 9.6% CH,; and Figure 13.-RETIMET metal foam after test with coal dust, oil, 

180" C, using 10.5% CH,. These temperatures were and grease on its external surfacereached in approximately 50 s. 

CONCLUSIONS AND RECOMMENDATIONS 

In general, the RETIMET metal foam vent structure 

functioned satisfactorily as a flame arrester in the present 

experiments. The RETIMET metal foam successfully arrested the flame front in all methane-air tests. 

In laboratory tests using mixtures of methane-hydrogenair, two failure modes were observed: (1) thermal and 

(2) ignition flashthrough. As the hydrogen concentration 

increased, the temperature of the RETIMET metal foam 

increased as well as the flame velocity. Above a maximum. 

hydrogen concentration, as indicated in table 3, thermal 

ignitions occurred. 

Further increases in hydrogen resulted in ignition 

flashthrough. The RETIMET metal foam simply could 

not stop these high-speed flame fronts. 

In large-scale tests, again the RETIMET metal foam 

functioned satisfactorily. Both grades 45 NC-13 and 80 

NC-13 arrested the flame front in methane-air concentrations ranging from 6.0% to 10.5%. However, grade 80 

NC-13 produced a higher pressure differential and showed 

more thermal damage than grade 45 NC-13 and was thus 

abandoned. 

Although grade 45 NC-13 did not cause any ignitions, 

some method of precooling the' combustion gases before 

contact with the RETIMET metal foam is recommended. 

If a spaced-plate arrangement is used, the minimum 

thickness of the plates should be 0.125 in (for mechanical 

strength) and the width of the plates should be 6 in minimum. The length and number of the plates are determined by the vent size (major dimension). The spacing 

between the plates should be 0.125 in maximum. Experimental results showed that a vent-area-to-enclosurevolume ratio of 11.33 in2/ft3 will function satisfactorily; 

therefore, this value is recommended. Lower ratio values 

will keep the internal pressure below 12 psig but are 

unsafe in methane-air atmospheres. 

For enclosure volumes requiring vent areas of more 

than 150 in2 (volumes greater than 13.31 W), multiple vent 

structures are recommended. This is to ensure that the 

mechanical integrity of the RETIMET metal foam is not 

compromised. Larger areas would require some form of 

mechanical support at the center of the vent to prevent 

excessive deformation (bowing). 

The present experiments revealed much about the 

performance of RETIMET metal foam on explosion-proof 

enclosures. However, the recommendations presented 

here apply only to a RETIMET metal foam vent structure 

that is constructed with a spaced-plate-type heat absorber. 

There are numerous other techniques for.extracting heat 

from gases that should be considered for similar 

implementation. 

1NT.BU.OF MINES,PGH.,PA 29535 

U.S.S.P.O.: 5-26-92/611-007/60034/1900