US3102972A - Fuel storage system - Google Patents

Description

Sept. 3, 1963 G. J. c. ANDREsEN FUEL STORAGE SYSTEM 2 Sheets-Sheet 1 Filed Jan. 22, 1959 \o/\QI\I\O o ollJo o v 3N o 9 8 l o o o o 0 R 0 \\\\\\\\\A\^\/\\ O O O O O O O O W o o o o E o o o o V o o o o N o o o o I 9 o o o o l o o o o o o o o o o o o o o o o o o o o O O O O o o o o o o o o ,f o o o o o o o o .f /l o o o o o o o O Y ,f 1 5 o o o o 4 f o C O. O C o f o o o o .f .PN o o o o o G f O O O I F o o o o F Sept. 3, 1963 G. J. c. ANDRESEN 3,102,972

FUEL. STORAGE SYSTEM Filed Jan. 22, 1959 2 Sheets-Sheet 2 FIG. IO

FIG. 8

INVENTOR. GILBERT J. C. ANDRESEN ATTORNEY HG. 1; i

United States Patent This invention relates to the storage and transportation `of fuels `and in particular to the safek storage and transportation of fuels which tend to build up dangerous- A ly high static charges. f

VThe handlingv of fuel for jetv aircraft and vsimilar fluids has presented many difficulties.

cedures, an acute problem has arisen since, under certain conditions, jet fuels tend to build up high static charges with the concomitant probability of an electrical discharge of 'suflicient energy t-o cause 'an explosion. nation of the mechanism of the build-up and discharge of static charges in such fuels and the provision of a means by which this obviously dangerous condition may be avoided has presented a major challenge to the field.

It is a primary object of the present invention to provide method and apparatus for the safe storage and transpor-tof fuels for jet aircraft-tand the like. It is an additional object of the present-invention to provide method and apparatus which effectively provide for the dissipation of static charges accumulating in jet fuels randsimi-v 'v fuels and the like which may be utilized in a fuel storage system without interfering :with the normal use of vthe system. These and 'other objects of thepresent invention will bec-ome lappa-rent from the following description and drawings in which: i

` FIG. 1 is a View of a rolling'uid'storage and transportation device;

`FIG. 2 is a cross section ofthe axle of the device of FIG. 3 is a cross section of one form'v of, an antistatic element'according to the present invention;

. FIG. 4 is a planview of one form of the elements of FIG; 3 as it maybe used inthe fuel storage system;

FIG. 5 is another form ofl an antistatic element; V FIG. 6 is a view along line- 6 6 of FIG. 2;' FIG. 7 is a detail of the apparatus illustrated in as applied to an axle such .as shown i-n FIG. 2;

FIG. 8 is a still further form of antistatic element; FIG. 9 is a cross sectional View` of an aircraftfuel tank having antistatic elements accordingto the present invention applied thereto; `and l f FIG. l0 illustrates a fuel storage tank-such as used in a barge or similar device hav-ing"v antistatic Velements FIG. 4

mounted therein.

Aspreviously mentioned, the build up of static charges in jet vfuels poses a serious problem in the `military and civilian air transport field. Because jet fuel is used in large quantities and ltransported substantial distances under extreme conditions, the possibility that a suflicient 'static charge may build up within the .fuel and arc to cause an explosion must be `seriously considered. It has been discovered that la staticxcharge build up is most likely to iocur in jet fuel storage and transportation systems when the fuel is severely agitated, a temperature differential exists between the fuel and the container or between different parts of the fuel, and the fuel is of high electri- With the development of special fueling techniques fand fuelv transportation pro- The determilevel.

'ice

cal resistivity so that the static `discharge cannot dissipate as rapidly 4as accumulated. With a fuel' of a resistivity of 10'12 ohm-cm. or higher under conditions of substantial agitation, a' sufficiently high charge has-been found-k to build up within the fuel so that an arc may occur acrossatheV fuel surface, usually to a metall-ic surface with- 4in or adjacent to the fuel container. However, it should be` noted that it is not essential that a metallic surface be present in order 'for an arc to occur but that it is possible for sufficiently high charges to be built up so that a spark may occur between one portion of the fuel andanother. For an explosion to occur it is necessary-that the fuelvapor be mixed with lair or somev other source of oxygen .and that the static discharge in the container have la `minimum energy ofapproximately 290- microjcules.. Under these circumstances, it has been found possible to causez explosive conditions in a fuel merely through the staticcharge -built up while the fuel is being The present invention has been tested under a variety of operational conditions similar to those mentioned and it has'been found to successfully prevent the accumulation of. high static vcharges and eliminate discharges which may be suiliciently high in energy` to attain a detonaticn While details of the present invention will be made clear in the subsequent explanation, the invention in general comprises providing, within the fuel or betweenthe fuel and any higl'trlyv conductive surface present which might catalyze a static discharge, a conductive path of a resistivitysubstantially below that of the fuel* but substantially above that of a goodconductor and as resistance sufciently high to limit the' maximumy energy levels of any discharge to a value below that which would detonate any fuel vapor present. i

FIG. l illustrates arolling fluid transporter designated; generallyl by the numeral 1 which contains la quantity of` jet fuel .The transporter 1 comprises a'flexible in.- flatable elastomeric tank 3 having a metallic axle 4 extendingy therethrough. A towing frame 5 is coupledy in conventional manner to axle 4 in order to facilitate overland movement of the tank. The axle 4 is provided'with aplurality of coneelike antistatic elements 6 which will be 'described in detail subsequently. As the transporter 1 isr rolled over .the ground the axle 4 rotates causing the.

the tanker Surrounding the metallic axle 4., l

- The axle 4 is shown yin detail in FIG. Zand comprises ahorizontal preferably hollow tube 7 having hub plates S--mountedzat either end thereof and Iadapted to receive and retain the exible tank 3. in much thesame fashion asa tire is mounted on a wheel rim. 'The axle 4 is providedvwith :a plurality of `cone-like projections `6 covering the horizontal tube Tand the interior face 49 yand outer circumferential yedge ofy the hub plates 8. The elements 6, which may be seen detail in FIG.. 3, comprise a generally ,conical body lll extending upwardly from a flat unitary base section 11. tion as :antistatic elements and satisfy certain electrical requirements which twill be discussed later, theprojeetions may be :formedv of a variety 4of materials, for example conductive rubber.

The' individual 'conical bodies 10 of the .antistatic elements may tbe cast or rnolded in large numbers in sheet form `as illustrated in FIG. 4, .and the individual sheets i 12 provi-ded with serrated vend portions 13 which are adapted to lit together so that when a plurality lof the.

.FlGS.-3 and 4.

ment 27 having an enlarged base 28 is `attached to a,

cular -pad l5 "having a plurality of conical projections 16` j `may be utilized on the face 9 of the hub plates 8 as illustra-ted in FIG. 6, the serrations on the molded base exvtending outwardly to serve :as antistatic projections to protect'the periphery of the hub. `Of course, as will later appear, theponical antistatic elements may be manufactured asy individual units' and mounted on the desired surface in-a 4variety of ways although :the mode illustrated in FIGS. 2,` 3, and 4 is preferable.

An alternative form of the antistatic element may be seen in FIG. 5 and comprises a generally flat sheet 17 having a plurality of downwardly extending substantially cylindricalprojections 18 and adapted to be mounted on the surface of an axle 19 in such a manner that the projections 18 `function as spacers to position the sheets 17 outwardly of the axle i9. The upper surface of sheets 'illustrated -in FIG. 4 to a surface such :as .an axle is illus-tratedin FIG. 7. The sheets 20 and 21 are wrapped about the axle-and positioned so that theserra-ted 'ends .2.2k and23 fit intimately together. This assures that no substantialportion of the axle is left exposed. In order i vto minimizeftfhe possibility that .a charge might arc to theaxle beneath the crack or space at the-conjunction of the serrated portions, the sheets may be manufactured with an antistatic projection such as 24 or 25 located at each poi-nt in the serrated area.V As will later become clear, the sizeffand location of these projectionsv is such y that the likelihood of an arc to the `space betweeny is extremely slight. Obviously the serrations may be omitted and a straight edge utilized if desired. The sheets may he removably attached to a surface by means of yclips 26v such as. illustrated or -by means ofscre'ws, rivets, or

semi-permanently mounted with any of a variety of adhesivesil HG. 8 illustrates another form of the antistatic `device made .according tothe present invention. iFor simplicity,

the operation of therpresent invention will be discussed withfrespect -t-o the element illustrated in FIG.` 8, it being understood that multiple elements may be utilized com- 'prising'either a plurality of those illustrated in -F-IG. 8

or a sheet of elements llas illustratedffor example in In FllG.V 8 a substantiallyconical ele- Imetallic orv highlyconductive surface Z9 by means of ay screw tlembedded in the base Z8 of element 27. The

' baseZS is proyided'tvith :a raised tapered ridge 31 for reasons which -Will subsequently be explained. 'I'lhe element 27 together with its ibase 28 may be made of .a number .of materials, one `of which is conductive rubber comt Sub-f pounded to,l meet the desired electrical conditions. sequent explanation of theelectrical phenomena involved and the conditions to beV satisfied will clearly show .that other materials may lbe used although conductive rubber has certain particular advantages Ifor this purpose.

` A Referring to FIG.v l, assume thatthe rolling fluid 'transporter Villustrated contains a quantityfof a jet fuel, for example that usually designated lP-4, and that the tanker is rolled, `as might occur in operation, over rough terrain s"o that considerable sloshing :or agitation occurs in the fuel. Consider also that the fuel may have been transfer-red'irito the tanker from asubterraneanstorage facility soithat the fuel is at a different temperature .than the tanker.

f cess of 500 microjoules, the danger of an explosion is fof the fuel vapors, i.e. 200 microjoules.

This 4condition will, after a time, give rise to temperature gradients Within the fuel. `It is important also to note that fuels such as LIP-4 have a resistivity of f t aboutl'tlr13 ohm-cm. and `as suc-h represent bad conducv tors, or conversely, .goodiinsulators The build up or accugreater or lesser tendencies to give up electrons or ionize. :Those portions lhaving higher ener-gies will tend to lose their electrons 4to adjacent portions or droplets of the fuel. Since the fuel is a very poor conductor, and thus the charge cannot easily migrate to neutralize the ionized areas, a net charge will thus be built up upon the surface of or within .an isolated portion of the fluid. Referring to FIG. 8, in the'usual case fwhere a highly conductive surface such asy the axle 29 is present, ya negative charge 32 in the fuel fis mirrored at the conductive surface by an opposite positive charge 33. lIf. the `gradient becomes sufficiently high an arc will occur between charged areas 32 and 33 as illustrated iby'thearrows. If the axle 29' is exposed to the vaporof the fuel 'and the mixture is approments utilizing a transporter of this type it has been pos-l I sibleto cause static discharges measured at levels in exsubstantial.

According to the present invention, a conductive path` formed, -forfexample byl the elementl27 in FIGQ- 8 is provided between the highly Vconductive axle and the fuel i has beenfound that it is not suflicient merely to provide a path hav-ing a sufficiently high total resistance to limit the D C.- current because of what may be ltermed the capacitance effect of highlyg conductive surfaces such-as axle 29. Without entering'into a detailed theoretical discussion, it has been discovered that, in thek presence i of highly conductive material, the total D.C. resistance of the elementk v27 `will. not limit the sparking current level inthe fashion of a simple DC. circuit but that the "presence of highly conductive materialin vany substantial amount will act as a low value reactance or a virtual short.

rent. For this reason highly conductivematerials, even though covered y:with an insulator of some thickness,ex`

avoid suchanoccurrence 'ltlhas been `found that in order to provide'a safe non-arcing path. itis necessary that the resistivity of the Iantistatic path ybeV of intermediatev i value, or between and l0? ohm-cm; Broadly speaking, the element must bejmade of material havin'gga resistivity substantially below the resistivity ofV the fuel,V i

whichV is 1012V ohm-cm. orv higher in fuels capable of building substantial charges, but substantially-above that of the axle or other highly conductive surface present,

the commonly encountered good conductors having re-' sistivities in the range of a micro ohm-cm. to ajmilli n ohm-cm. -In order to meetthis condition, 4the element 27 may be made ofa substantial number of what might be termed.semi-conductive'materials such Ias loaded plastics, resins, or elastomers ,Howeven preferred among them is conductive rubberwhich, due to its flexibility, may be easily'installed on curved surfaces and easily manufactured by casting or molding. lt is also preferred that materials such as conductive nitrile or polyurethane rubber be utilized since they aresubstantially unaffected by the fuels 'with which they come in Contact.

Referring once again to FIG. v8, underl the circumstances previously postulated if a net negative charge occurs at. 32 and, as is required, the fuel as a Lwhole has a zero net charge, then a net positive charge will occur in another portion of the fuel, for example at 34. Ordi- I-Iowever, it

4One explanation for, this is that the dischargel or arc occurs over such a brief period of time land has ,Y such a steep wave front that it possesses the characteristics n of a pulse of extremely high-frequency alternating cur- Hence, the con-y ythe fuel.

. narily this positive chargewould be mirrored at the axle 29 and, once again, the possibility of dangerous arcing would be present. However, the element 27 presents a path of intermediate conductivity extending from the conductive axle 29 into the fuel container. The positive charge 34 .is then mirrored by ia negative charge 3S `at the tip of the element Z7 and. current Iflow to the charge 34 due to local ionization is effectively limited by the total resistance of the element 2.7 to a value below 200 microjoules. As previously mentioned, the element 27 is made substantiallyA wholly of material having a resistivity of intermediate value to avoid arcing due to the so-called capacitance effectA of highly conductive rnaterials. For this reason, it is essential that no substantial mass of highly conductive material be included in element Z7 in any portion exposed to or closely adjacent The element 27 is also preferably generally conical with the base of the cone mounted on the axle 29. The tapered character of the element increases partial ionization about the tip which results in a constant bleed of charge in la silent discharge to neutralize static charges building -up in the fuel. rPhe elements could, of course, be elongated, generally cylindrical members of rather small diameter, if desired, and substantially the same effect could be obtained. It should be noted that the polarity of the local charge in the fuel adjacent any conductive path provided by the elem-ents 27 is unimportant since a charge of opposite sign will at once be supplied in limited rates from the clement to, reduce the discharge energy level.

The lelement 27 is illustrated as attached to axle 29` by kmeans of threaded screw 30 embeddedin the base thereof. In orderto minimize any capacitance discharge effect caused by the screw and to reduce the possibility l of arcing to the kbare axle area between two adjacent elements, the enlarged base 23 may beprovided with a tapered ridge or lip 3l which promotes partial ionization 36 in the area and reduces any local space `charge 317 to below the danger point. iIt is understood, of course, that the device illustrated in FIG. l8 should be utilized in large numbers to substantially blanket any conductive surface to effectively shield it electrically from any charge in the fuel container. It has also been found that in order to avoid arcing to exposed metal conductor between adjacent elements the optimum condition is that the length of the elements 27 Yincluding the base 28 should be at least equal to or greater than the spacing between adjacent elements.

As mentioned, it has been lfound that the danger of explosive discharges due to static-charge accumulation exists primarily in fuels having a resistivity of at least 1012 ohm-cm. Where a highly conductive surface such as a metal axle is present in or adjacent the fuel, the intermediate conductive path should be provided having a resistivity in the range of 1105 to 109 ohm-cm. and preferably between l()3 and 109 ohm-cm. In addition, the current'limiting resistance of the element should be in the range of 50 to 500 megohms and preferably between 50 and 100 megohms. Slightly 4lower or higher values may be tolerated but only with a loss in safety factor. However, it is essential that the resistivity of the conductive path be substantially below that of the fuel and above that of any metallic conductor present.

Even though no highly con-ductive surface is present in the system, there fis always the possibility that an arc can occur between differently charged areas of the fuel itself, and hence it is desirable to provide at least one conductive path, and preferably a plurality thereof, from one part of the fuel to the other to minimize the danger of arcing. In such cases theresistivity of the conductive path may be somewhat lowerthan the stated range so long as it does not substantially approach that of a good conductor. Nonetheless currentv limiting resistance of the path should, ofcourse, be in the range `mentioned to limit arcing energies to below the200 microjoule level.

' While the operation of the invention has 'been described in regard to the type of element illustrated `in FIG. 8, the form illustrated in FIGS. 3 and 4 is preferred since it is simpler to mold sheets havin-g a pluriality of elements thereon. Such a form eliminates the need for met-al mounting screws and hence reduces the possibility of a high energy arc due to the capacitance effect of highly conductive material. In addition, sheets of elements such as illustnated are easier to install and require less time and laborious tapping of the conductive surface in order to insert the screws.

FIG. 9 illustrates a typical fuel cell 38 having a filler neck 39 or similar fitting mounted thereon. In 'certain crash refueling procedures the fuel is forced into the cell and considerable lagitation of the fuel occurs. In addition, the fuel may Ibe Iseverely agitated during rough iiying conditions. In either case, the opportunity for 1a static charge accumulation is substantial and, hence, an explosion due to larcing is a danger. According to the present invention, .a plurality of elements 40' similar to those illustrated `and previously described lare mounted on the highly conductive surfaces in and around the fitting 39 to minimize the possibility of an explosion. It is clear that the elements may extend into the container and orifice fitting without 4substantially interfering with the normal use of the cell. i

FIG. 10 illustrates` \a fuel storage and transportation device such as -a barge or a tank therein. The tank walls 42 may Ibe made of metal or a metal coated on the interior with @another less conductive material. In any case, because of the possibility of an arc due to capacitance effect previously mentioned, it is desirable to minimize the opportunity `of the occurrence of a dangerous spark \by providing la plurality of conductive paths of intermediate resistivity and limiting resistance by means of elements 43 similar to those previously described mounted on and shielding the wail-ls of the container.

While certain representative embodiments and details have been shown for the purpose of illustnatin-g the invention, it will be ,apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or `scope of the invention.

I claim:

l. A system for storing and transporting high resistivity fuels with minimum risk of the occurrence of static discharges of energies exceeding the explosion level of vapors of said fuels comprising a rollable fluid container having fuel therein and having a highly conductive metallic axle therethrough, and a plurality of conductive elements mounted on said conductive axle and projecting into the interior of said container, said elements having resistivities substantially [above that of said conductive axle and substantially below that ofthe fuel land resistances suicient to limit the flow of electrical energy therethrough to a value below' the explosion level of the vapor of said fuel.

2.. A system as claimed in claim l in which the fuel a resistivity of at least 1012 ohm-cm. and the elements have resistivities -in the range of l05 to 109 ohm-cm. and resistances in the range of 50 to 500 megohms.

3. A. system as claimed in claim `2. in which said elements are elongated in the directionsubstantilally normal to said axle and are located thereon in spaced apart relation so that the length of said elements is at least equal tothe space between adjacent elements.

4. A .system as claimed in-claim 3 in which -said elementsare substantially conical in shape with the base of the cone mounted on said axle.

5. A system as claimed in claim 4 in which said elements are composed substantially of conductive rubber.

6. A system for storing high resistivity fuels with minimum risk of the occurrence of static discharges of energies exceeding the explosion level of vapors of said fuels comprising a container having fuel therein and 'a highly cone ymounted on said conductive surface and having a `resistivity substantially below that of the fuel tand substantially above that of the conductive surface and a resistance sufficient to imitthe :ow of electrical energy therethrough to a value lbelow the explosion ylevel of the Vapor of said fuel. f

, 7. A system las `claimed in claim 6 in which said conical elements are |located so that thelength of the elements is Iat least equal to the space between adjacent elements on said surface.

References Cited in the tile of this patent UNITED STATES PATENTS French Apr. 24, 1917 Chapman Jan. 27, y1925 Smith Ian. 22, 1935 Bulgin Feb, 8, 1944 Crise Dec. 7, 1948 t Mlapes July 15, 1952 Cain Oct. 4, 1955 Fino Feb. 21, 1956` Buckman Mar. 27, 1956 Cunningham June 11,1957. ShortA Oct.4 6, 1959 Fino Nov. 24, 1959

Claims (1)

1. A SYSTEM FOR STORING AND TRANSPORTING HIGH RESISTIVITY FUELS WITH MINIMUM RISK OF THE OCCURRENCE OF STATIC DISCHARGES OF ENERGIES EXCEEDING THE EXPLOSION LEVEL OF VAPORS OF SAID FUELS COMPRISING A ROLLABLE FLUID CONTAINER HAVING FUEL THEREIN AND HAVING A HIGHLY CONDUCTIVE METALLIC AXLE THERETHROUGH, AND A PLURALITY OF CONDUCTIVE ELEMENTS MOUNTED ON SAID CONDUCTIVE AXLE AND PROJECTING INTO THE INTERIOR OF SAID CONTAINER, SAID ELEMENTS HAVING RESISTIVITIES SUBSTANTIALLY ABOVE THAT OF SAID CONDUCTIVE AXLE AND SUBSTANTIALLY BELOW THAT OF THE FUEL AND RESISTANCES SUFFICIENT TO LIMIT THE FLOW OF ELECTRICAL ENERGY THERETHROUGH TO A VALUE BELOW THE EXPLOSION LEVEL OF THE VAPOR OF SAID FUEL.

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