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~a4oooio( ’NRL Memorandum Report 3678

~~ Can Pyr ophor ic Materials Form in Oil Tankerswith Inert Gas Fire Protection Systems?W. A. Arn~is

Combustion and Fuels BranchChemistry Division

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December 1977

D D C

I~ MAR 1 1978of p

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NAVAL RESEARCH LABORATOR YWashington, D.C.

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MAR 1 1978

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r?IIn Pr?IS S U P P L E M E N T A RY NO T E S

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Pyrophoric Igni tion Inert gas systemsOil tankers Flue gas inertingFlammable gasesCrude od

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literature study and analysh was made ~~neeming the potential hazard of pyrophodcignition of flammable vapors in oil tankers with inert gas systems (IGS). Only two pyrophorpossibilities seem to be likely : Formation of ferrous sulfide (FeS) and/or ferrous oxide (FeO).Both FeS and FeO may be formed in an oxyge n limited atmosphere , such as would be the casewith a flue gas IGS. Formation of FeS also requires an atmosphere containing hydrogen sulfide(H~S), as would be the condition with ~ sour crudes, so that the hazard of the formation of

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20. Abstract (Continued)

pyrophoric FeS must be considered for such crudes. Furthermore, if sea water is present in theoil tank, oils which do not contain H~S can become sour by the action of sulfate reducing bac.teria on sulfates in the water. It is concluded that in the event of failure of an IGS system , theullage space should not be exposed to air suddenly.

IIS1I~ URI!Y CLASIIFI CAI’ ION OP TNI S PAGilbilmi Om. ~~~~~~~

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CONTENTS

INTRODUCTION . 1

INERT GAS SYSTEMS (IGS ) 1

PYROPHORIC MATERIALS 2

PYROPHORIC MATERIALS IN OIL TANKS’ 2

PYROPHORIC IRON SULFIDE 3

PYROPHORIC IRON OXIDE 4

PYROPHORIC MATERIALS AND INERT GAS SYSTEMS 4

OTHER CONSIDERATIONS 5

SUMMARY AND CONCLUSIONS 6

REFERENCES 7

TABLE I 10

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CAN PYROPHOR IC MATER IALS FORM IN OIL TANKERS WITHINERT GAS FIRE PROTECTION SYSTEMS?

INTRODUCT ION

The U. S. Coast Guard has under consideration a proposalto require inert gas systems (IGS ) for fire protection oncertain vessels which carry crude oil and similar flammablecargoes (1). Questions have been raised , however , concerningpossible problems which may arise if IGS are used. In thisregard, NRL has been requested to address some of the poten-tial problems. This report is concerned with one of theproblems , namely, the likelihood of formation of pyrophoricmaterials. This is important because pyrophors , under certainconditions, might ignite the flammable vapors , a phenomenawhich the IGS is intended to prevent. NRL has made a literaturestudy and analysis to determine whether the use of IGS on tankvessels might contribute to the formation of pyrophors.

INERT GAS SYSTEMS (IGS)

In order for a fire to occur in a tanker , there must bea ‘proper proportion of fuel vapor and oxidizers (most commonlyoxygen in air), and a suitable ignition source. There are ,therefore , three options for fire prevention on a tanker withflammable liquid cargoes : (a) remove the flammable vapors (“gas-freeing”), (b) remove the oxygen or lower its concentrationbelow that required to propagate a flame (“inerting ”), or,( C ) eliminate all possible sources of ignition . Experiencenas shown that it is not always possible to eliminate orcontrol the wide variety of potential ignition sources(flame s, electric sparks , hot surfaces , static electricity ,etc.) which can be present on tankers. Since gas-freeing isnot usually feasible , and sometimes introduces its own hazards(2), a combination of b and c (above) is frequently use~ forfire prevention purposes on oil tankers.

It is not the purpose of this report to analyze therelative efficiencies of the three fire prevention methods ,nor to go into detail concerning inerting systems except asthey might influence the formation of pyrophoric materials.Rather , the purpose is to address the question : can pyrophoricmaterials form in oil tankers with IGS?Note : Manuscript submitted December 7, 1977.

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IGS have been in widespread use for many years to reducethe risk of explosion in cargo tanks of oil carriers (3-6) .The function of an inert gas, such as nitrogen, carbon dioxide ,etc., is to reduce the oxygen concentration below its limitof about 11% for combustion of hydrocarbon vapors (7). MostIGS use flue gas from the ship ’s boiler for inerting , af terfirst removing most of the undesirable constituents , such asso , by scrubbing through a sea water scrubber. The composi-ti~n of the resulting scrubbed flue gas -varies depending onthe fuel and other factors , but a range of reported composi-tions is shown in Table I (3-6) . The chief constituents arenitrogen , carbon dioxide, and oxygen. Note that the oxygenconcentration is well below the limit of about 11%. Ingeneral , the oxygen concentration is below 6%. The questionnow arises as to whether such an atmosphere (oxygen lean)would enhance the formation of pyrophoric materials.

PYROPHORIC MATERIALS

Pyrophoric is derived from “pyro ” (fire) + “phor ” (carry)and literally means “f i re bearing” (8). It refers to anysubstance which will ignite spontaneously on exposre -to air £(8-il) . Certain metals, such as thorium , cer ium, zirconium ,titanium , magnesium and their alloys art particularly effec-

- tive in emitting sparks as a result of fr iction and arereferred to as “pyrophoric metals ” or “pyrophoric alloys”( 12- 14) . Tlie sparks , which are the result of spontaneousignition of the fragmented particles burning in air , arecapable of igniting a flammable gas mixture, such as is thecase with a cigarette—lighter flint (misch metal). Somepyrophoric materials, however , may ignite spontaneously onexposure to air and do not require fragmentation by abrasionor friction for ignition to occur. These include phosphorous;hydrides of phosphorus , boron , silicon , and lithium ; cacodyl ;zinc dimethyl; and finely divided metals such as iron , magne-sium, aluminim , uran ium , lead , nickel , cobal t, and bismuth(8, 15); and two ferrous compounds : FeO (15, 16, 18) and FeS(15—19)

PYROPHORIC MATERIALS IN OIL TANKS?

The question is now raised : Of the wide variety ofpyrophors which have been mentioned, is it likely that anyof these might be present in an oil tank? The answer tothis question would appear to be negative , with the possibleexceptions of FeS and FeO . Tank linings consist of iron , or

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zinc-coated iron ( 1 4 ) , and various stages of iron rust. Ironas such is not pyrophoric (except in a very finely dividedstate) (15), nor is zinc metal (14). It would not be likelythat either the oil cargo or the flue gas would contain anyof the above mentioned pyrophors. Could there be a reactionbetween the walls of the tank and/or other constituents ofthe oil and/or the flue gas which might contribute to theformation of pyrophors? Since the most likely pyrophor possi-bilities in a tanker would be FeS and/or FeO , could these beformed in the tank by chemical reactions between two or moreof the components which are likely to be present? If so,might the flue gas enhance such a reaction even if it doesnot take part?

PYROPHORIC IRON SULFIDE

Ferrous sulfide (FeS) has been known to be pyrophoricsince it was reported by Berzelius in 1826 (15, 17). Whenpyrophoric FeS is exposed to air it may ignite spontaneouslywith the evolution of considerable heat. If flammable vaporsare present, a fire may occur. This hazard has been known toa wide variety of industries. For example , when Town gas istreated with Fe(OH)2 to remove H,S (“ sweetening”), FeS is formed.It is necessary to take precautions to avoid auto ignitionof the FeS (20), e.g., preventing sudden exposure to air.Similar hazards are known to the chemical industry where FeSmay accumulate in catalyst beds in hydrotreating processes (21).In the petroleum industry, hydrogen sulfide from “sour crudes ”(crude oil containing H.,S) have been known to react with rustscale in pipe lines , drilling equipment , and other machineryto form pyrophoric FeS. Pyrophoric FeS has been known to havecaused ignitions of flammable vapors in oil wells and pipelines (22—24)

Directly related to the Coast Guard problem are studiesby the American Petroleum Institute (API) (25,26) and recentwork at the Shell Thornton Research Centre in Chester, England(17,18). These studies expressed concern with the hazard offormation of pyrophoric FeS in tankers containing sour crudesand the subsequent ignition of flammable vapors.

The Shell work was a detailed labcratory study andanalysis of the problem and included studies of the reacti-vity of FeS as a function of the type of iron oxide fromwhich it is derived . They concluded that it is quite possi-ble for pyrophoric FeS to be formed in the ullage space ofan oil tank containing sour crude , and that ignition fromthis type of material may be the solution to many unexplainedoil tanker explosions .

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The reaction of iron rust with H S (ignoring hydrationeffects) is represented in the Shell tapers (17,18) by thefollowing equation:

Fe203 + 3H2S = 2FeS ÷ 3H20 + 5, t~H = -40 kcal (1)

The oxidation of FeS in air regenerates the Fe203 with con—siderable heat (17 ,18):

4FeS + 302 = 2Fe 2O 3 + 4S, ~H = -151 kcal (2)

In connection with the Coast Guar.d problem , an extensivesearch of , the chemic~ l literature has been made , and it isclear that FeS is the prime candidate for consideration ofall the possible pyrophoric materials which might exist or beformed in an oil tank. The formation of FeS from rust scale,however , does require the presence of H2S, and an atmospherewith a limited supply of oxygen such as would be the casewhen inerting with flue gases (18).

PYROPHORIC IRON OXIDE

Another pyrophoric compound which could be present in anoil tanker under certain conditions is ferrous oxide (or hydroxidewhen moisture is present). Pyrophoric FeO can be prepared inthe laboratory by trea tment of iron with steam at 350° C in alimited oxygen atmosphere , or by other methods (15). It hasbeen cited as-a possible pyrophor which might be formed underthe top surface of iron rust scale in an atmosphere of fluegas (16,18). FeO/Fe(OH), have been shown to be pyrophoricand are know to oxidize readily in air at room temperature (15).On sudden exposure to air they can ignite spontaneously (15,16).The hazard of ignition of flammable vapors by pyrophoric FeOis considered to be a real possibility (16,18) .

As has been mentioned , considerable knowledge has beenaccumulated concerning pyrophoric FeS, but this is not thecase with pyrophoric FeO, particularly as concerned with itspossible formation in oil tanks. The potential hazard ofpyrophoric FeO in inerted oil tanks is based on deductionfrom its known propertj.s, but this hazard has not beeninvestigated specifically . More research is needed on thispyrophoric material.

P?ROPHORIC MATERIALS AND INERT GAS SYSTEMS

In the discussions so far it has been seen that thechief influence of inert gas systems in the formation ofpyrophors is that they maintain a limited oxygen supply .

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This is conducive to the formation of pyrophoric FeS (in thepresence of sour crudes) and FeO or Fe(OH)2. The questionstill remains , however, whether any of the components of theflue gas might contribute to the formation of pyrophors insome other manner. The components of scrubbed flue gas areaerived from the end products of the oxidation (burning) ofships fuels and consist chiefly of nitrogen , Ca2, oxygen , andwater vapor with lesser concentrations of SO.,, SO3, and COas shown in Table I. At temperatures which ~re likely toprevail in an oil tank , it does not appear that there areany probable reactions with the iron or rust scale of thetank walls which might form a pyrophor beyond the FeS orFeO (15) previously mentioned . There are some referencesin the literature to reactions of iron oxides with sulfurdioxide or carbon dioxide which might conceivably lead to theformation of FeS or FeO , but these reactions can only occurat elevated temperatures (15).

OTHER CONSIDERATIONS

There are two other considerations which are related tothis problem. The f irst concerns a potential ignition hazardwhich may result from flue gas inerting which should bementioned , even though it is outside the scope of this reportthat of static electricity . Recent research has shown thatcharged particles and mis t droplets from scrubbed flue gas cangenerate appreciable electrostatic charges which would be morethan su f f i cient to ignite flammable hydrocarbon vapors if dis-charged as an electric spark (27). The hazard of staticelectricity must be considered, even in inert gas systems ,whether the static build up is from the charged droplets inmists or particulate matter , or from tank washing or otherreasons (6, 27—29). In normal operations , of course , theinerting will prevent the forma tion of flammable atmospheres ,but in case of failure of the IGS and the introduction of air,potential hazards due to static electricity may arise (27).

The second consideration concerns a possible source ofH2S from sea water. Oils which do not contain H S can turn“sour ” by the action of microorganisms. The Nav~ hasexperienced problems with fuel contamination in fuel tanksballasted with sea water (30). Sulfate reducing bacteriareduce the sulfate in sea water and form sulfide as the

• reduced product (30). This sulfide , as H2S, turns the fuel“sour ” and makes it corrosive to metals. Iron sulf i de hasbeen shown to form in the linings of fuel tanks by this means(30). Since there is likely to be sea water present in thecargo space of oil tankers, the formation of sour fuel andpyrophoric iron sulfide from the action of sulfate reducingbacteria should not be ignored .

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SUMMARY AND CONCLUSIONS

A literature study and analysis has been made concerningpyrophoric ignition with particular attention to pyrophoricmaterials which might be present in oil tankers inerted byflue gas.

Only two pyrophors are likely to be present in an inertedtanker - FeS (if H2S is present) and FeO .

Based on recent studies at the Thornton Laboratories inChester , England (17 , 18), there does appear to be a possibi-lity that pyrophoric FeS or FeO may be generated in oxygen-limited atmospheres , such as in a tanker inerted by flue gas.Research is needed concerning the probability of formation ofpyrophoric FeO in inerted tank systems.

If for some reason the inerting system should fail, andif the walls of the tank were suddenly exposed to air , it isquite possible that the pyrophoric material might ignitespontaneously and in turn ignite the flammable vapors in thetank. For these reasons , if the ullage space in an inertedtank is to be exposed to air, this should be done graduallyso that the pyrophors might be oxidized slowly and hence theirpyrophoricity eliminated.

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REFERENCES

(1) Federal Register , Monday , May 16, 1977 , Part II , page 24874.

(2) The Secretary of the Treasury ’s Committee on Tanker Hazar ds ,Final Report to the Secretary of the Treasury , U. S. CoastGuard, 14 August 1963.

(3) A. E. Stanford, “What Inert-Gas Systems Mean to a MarineOperator” , Soc. of Naval Architects and Marine Engineers ,Annual Meeting , New York , N. Y . , November 14-15, 1963.

(4) Shipping World ana ~hipb~ ilder, “Explosions in Tankers” ,421—4:4 , March 1970.

(5) W. 0. Willey, “Tank Cleaning” , Safety at Sea International,17—29, September 1972.

(6) R. I. Bass, T. E. Owen , and F. T. Dodge , “Bulk CarrierOperations Safety Enhancement Project” , Phase 1 FinalRepor t, Southwest Research Institute , San Antonio, Texas ,June 1976.

(7) H. F. Coward and G. W. Jones , “Limits of Flammability ofGases and Vapors ” , Bureau of Mines , Bulletin No. 503,1952.

(8) Thorpe ’s Dictionary of Applied Chemi stry , Longmans ,Green and Co., London , 4th Ed., 1950 , Volume X , 327-329.

(9) C. A. Hampel and G. G. Hawley , “Glossary of ChemicalTerms” , Van Nostrand Reinhold, New York , 1976.

(10) L. M. Miall , “A New Dictionary of Chemi stry ” , InterscienceNew York , 1961.

(11) D. H. Hey (Ed.) Kirtgzett’s Chemical Encyclopedia , D. VanNostrand , Princeton, N. J., 9th Ed., 1967.

(12) R. B. Smith , “Pyrophoricity - A Technical Mystery UnderVigorous Attack” , Nucleonics, 14, 28 December 1956.

(13) C. R. Schmitt, “Pyrophoric Materials - A LiteratureReview ” , J. Fire and Flammability , 2, 1957 (1971).

(14) W. A. Affens and B. A. Lange , “Ignition of FlammableGases in Crude-Oil Tankers as a Result of Metal Fracture ” ,NRL Report 8013, June 29 , 1976. (AD A027411)

(15) J. W . Mellor , “A Comprehens ive Trea tise on Inorganicand Theoretical Chemistry ” , Longmans , Green , London , 1922-193/.

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(16) A. Bell, “Corrosion and Fires in Marine Boiler Air Pre—heaters ” , Trans. Inst. Marine Engineers , 72, 309 (1960).

(17) R. I. Hughes, T. D. B. Morgan and R. W. Wilson , “IsPyrophoric Iron Suiphide a Possible Source of Ignition?” .Nature , 248, 670 (1974).

(18) R. I. Hughes, T. D. B. Morgan and R. W. Wilson , “TheGeneration of Pyrophoric Material in the Cargo Tanks ofCrude Oji. Carriers” , Trans . Inst. Mar ine Engineers , ~~153 (1976).

(19) R. H. Griffith and A. R. Morcom , “The Interconversion ofIron Oxides and Sulphides ” , J. Chem. Soc., 148, 786 (1945).

(20) L. Shnidman (Ed.), “Gaseous Fuels” , Amer. Gas Assoc . , NewYork , 2nd Ed. , 1954, pp. 82—84; 234.

(21) L. M. Lovell, “Stabilizing Pyrophoric Materials in aCatal.yst Bed” , U. S.

. Patent 3,838,066 , 24 Sept. 1974;

Chem. Abstracts 82, No. 88374B (1975).

(22) I. G. Zakirov , M. G. Gazi.mov , K. M. Kadeev , 1

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(28) The Motor Ship, “Prevention of Explosions in Tankers -Electrostatic Charge Generation in Supertankers and theFinal Report of I.C.S.” , March 1976, p. 119.

• (29) Ibid , “Reduction of Static Electricity in Cargo orBallast Tanks” , p. 123.

- (30) R. N. Hazlett, “Control of Sulfate Reducing Bacteria inNavy Fuel Systems ,” NRL Memorandum Report 1737, December28, 1966. (AU bOSN3OL)

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TABLE I

COMPOSITION OF SCRUBBED FLUE GAS (3-6)

RANGECONSTITUENT % v/v

N2 75 — 85

CO2 11 — 15

02 2 — 8

H 20 (vapor) 0 .8 - 8

SO2 0 .02 — 0.11

SO3 Trace

CO 0.1

10

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