US1617788A - Device for preventing electrical ignition of stored inflammable fluids - Google Patents

Description

BEST AVAILABLE CCP;

I 1,617,788 Feb, 15 1927i L, BALDWIN M- DEvlc'EfoR PREvEyTING ELECTRICAL IGNITIoNf o? s'ro'nED INI'LQMMABLSFLUIM t Fiied nay 25, 1925 4 sheets-shut 1 Feb. 15, 1921.

51:6 l AVAILABLE CUF" l.. A. BADwlN ET AL DEVICE FOR' PREVENTING ELECTRICAL IGNITION OF STORED 'INFL-AJIHABLE TTJUIDS Filed May 25A. 1925 14 sneetssnqet :s

"n 'E f@ BEST AVMUBLE COP\ I 1,611,788 Feb l5 1927' l.. A. BALDWIN ET AL navlcs son Pnsvsx'rms. ELECTRICAL IGNITION oF sToREn :NFL/mumps Fwzv v Filed May 25', 192s'. 4 sh'ets-snet 4 am, Kwam/ABLE COPi Patented Feb. 15,1921.

'juNiTEosTAriis l ...-...w AwoLABLE-COF ori-ien LESILIE A. BALDYV'HV, 'UNIVERSITY CITY, HISISOUBI, AND ELMEB E. SCAEFZFEB, Ol'

CBESTWOOD,

YOBX, AS8IGNOBS TO JOHNSMANV`ILLE INCORPORATED, OF

NEW YORIB', N. Y., A CORPORATION OF NEW YORK.

IJEvICE Eon'lPnEvEn'rINo ELECTRICAL IGNITION or s'roEEn man lApplication led Hay 25, 1925. Serial No. 32,738.

This invention isl concerned withthe preven'tion ofpreventable fire and .explosion risks from electrical ignition of naphthas,

oils,- hydrocarbon 'gases and other inflam- -iiiiiiiiiy i0 .explosions @rares iniaaieaby lating coverings are not'easily penetrated by .spark discharges, they are not wholly inielectrical disehang'es: Petroleum. its deriva.

tives, va'pors and. associated gases are good insulators; the tanks used for storage and handling are 'generally metallic. necessarily `open"to`thfatmosphere at one point at least usually also',"-fatf1ma'ny accidental `vp'o'ints)`: "and Athe structure of these tanks dictated by-'custom or by intelligent econ- 'muy does not avoid providing` ideal opport/unitylfor electricVV spark discharges between parts of'the structure through combustiblessuch as inxeda'ir and vapor or gases. Most of the oilf'ields are in regions of high probable occurrence of isolated, lightning discharges and of thunderstorms, high winds` large diurnal variations of temperature, and other meteorological conditions lending to high surface elei-trifications aswell as'to ex-A posiire to the vast suddenfdiii'erences of potential incident to lightning discharges. pre- Vlightning stroke electritications. and the in' f duced fiows and discharges following nearby lightning flashes.

In modern practice heat insulation of stor` .age tanks l'iyproltective coverings is more and more resortedio. In Vthe best practice. suc'h structures arecharacteristically also of dielectric siibstiiinceili. Welt adapted to increase the mutual electrostatic apacity'ot metallic o'r,A moisture-filrn` surfaces between which they.l may intervene. lVliile such insumune to puncture by discharges fi'onrsurfacejtn lsurface of the Leyden-jar type of condenser which they often constitute, and they do not' oppose any adequate obstruci tion,v of course. t discharges` either direct or secondary, of lightning stroke intensities.

High static potentials'on the outer surfaces of insulating covers, paint or other coatings shouldbe avoidedbecause they connote induced rpotentials on surfaces at or near which sparking would ,be ,`d is as,trous, andindicate the possibility of penetrating sparks or disruptive .or sudden discharge, or sudden augmentation, of the 'exterior charge. It is desirable to prevent, therefore, the

building-up .to high. potentials of electro-` staticl charges of Oopposite sign across the insulation vcovers lor fabrics of such tanks.

j It is desirable to avoid every poible opportunity for` a spark discharge through or across-spaces likelyto contain air and vapor mixtures, as well'as to provide, in optimum relation'to the containers to'be protected, sound metallic :conducting paths to earth capable of dealing with current surges of great 'quantity land 'high inten/sity. Provisions-to these ends should be of sufficient effective reliability and durability to defy being put vinto an inoperative state by ignorant handling-` but they should nottake up undue space or be' too costlyior general adoption in and about oil-Wells, tank farms and refineries. K

Since the times of Franklin and -Faraday lightning prevention devices have been provided rather as the effect of iiigenuous faith than as the result of exact engineering knowledge. Lightningstroke conditions remain in part 'a mystery' of physical science, but nevertheless a sufficient basis of established fact is in hand to indicate and predict suc-- cess or failure of preventive devices upon exposure to lightning .risk plieric electrification phenomena of normal characters and intensities.'A This invention seeks to provide optimum protection against normal risks for such exposed objects' as oil tanks within a practicable minimum 4of coniplexity and cost.

tion of high static potentials between the .exterior-'surfaces and interior structures of heatinsulated tanks, to

avoid differences of potential between internal parts of tanks,`

gradients abnormally steep in comparison with the potential gradients at V'nearbyobjects, buildings and-surfaces of the earth or' earthed surfaces. The Ainvention will be explained in conneet-i011 with typical and recommended constructions illustrative .of the various' arrangement by which it may be carried out.

In the accompanying drawings of the lform-shown forpurposes of illustration,

Figgl isa diagrammatic. plan view of a 'quadrant Vof a tank showing a system of conductors; I

Fig. 2 isa diagrammatic vertical. sectional view on' a.. radius of the tank;

Fig. 3lis a side elevation and Fig. 3 a

I plan ofk acentral'psupport. for.. certaincon.-`

Fig. 4-is` a side` elevation of vone of the peripheral supports in position on the side of a tank;J A

Figs. 5- arid 6 are respectively side elevationsand plan views of a holding clamp;

Fig. 7 isl a ,sectional view onl theline 7- -7 of Elg. 4; Y

Fig. 8 is fa sectional view on the line 8,8 ofFig. 4;

Fig. 9 :s adiagram illustrating the dis-v tribution of equi-potential surfaces in a typical vertical plane of a tank between .earth and an elevated atmospheric layer,

such as a cloud;

F ig. 10 is a similar diagram of a detail.

of Fig. 9 showin equi-potential surfaces between and near idle protcctivesy'stem and the tank surfaces; and

Fig. 11 is a diagram in radial section illustrating the distribution ot induced static charges between the tank top. a heat-insulating cover, and the protective system.

Referring now to the drawings, .let it be assumed that the structure. to be protected is a tank, which has a peripheral wall 2 built up of courses of Steel platesand a nonametallic .heatin'sulating top 4 making a sealed joint at 5, Fig. 4, with the Nall 2; for example, of the improved construction described and claimed in the application for providing BESTA i VAlL-fgpv patent of Leslie-A. Baldwin, Serial No. 675,146, tiled November 16, 1923.

As shown, upon the Centex` of the tank. roof a suitable support is erected, which may comprise a rectan ular wooden ost 6, suitably braced by mem ers 7 and ma ing a water-tight -oint with the cover 4 with the aid of suita le fiashings Siembedded in a waterproof cement. At its u per end the post 6 is provided with a snug y fitting galvanized metal cap 9' and this is surrounded by and supports a two-part ring member 10 to which the central portions of a protective conductor'system may be attached.

At the periphery of the tank there may be arranged a spaced series of erect peripheral conductor and supporting members 12, which members may be made up of a pair of angle-irons 13 of comparatively light section bent, as indicated at 14 in Fig. 4, at

their lower ends and fastened together suitably at points 34. At the lower extremities of the members 12 the angle-irons 13 may embrace and' be attachedi to a plate 1omounted between a pair of vertical angleirons 16 of slightly heavier section to which th; plate 15 may be riveted or otherwise xed. At a substantial distance above their lower ends the members 12 are secured as by means o rivets and plates 17, to a pair of horizontally extending anfrle-irons 19 and the latter at their inner endsembrace at opposite sides a plate 20 in turn fastened between the -upper ends of the angle-irons 16. The structure so formed is adapted to be attached to the tank 1 in any suitable way a good mechanical and electrical Contact with the vertical tank wall. example, at the cxtreme upper and lower ends of the, members 16 T-seetions Z1 andlZ respectively are arranged, these being riveted to'the members 16 to provide relatively flat surfaces :23 and 24 against which tension cables 29, 30. may bear. The members 23 and 24 may be drilled vas indicated at 25, Fig. 8, to permit thepa5sage through them of the threaded extremities 26 of U-holts 27, for example, which provide a fairly close tit for the heavy wire tension cables 2S), 30 surrounding the tank, and adapted to be stretched taut by turn buckles 31 and 32, Fig. 2. The bearing surfaces' between the parts 1z l1nd the trink wall 1 may be scraped and cleaned before erection and protected by applied paint or cement after erection, in order to preserve dependable areas of actual conta-ct, und any other fastening means for the erect conductorsl 12 capable ofi convenient construction and safely holding the con ductors 12 against wz-ithing and expansion motions of the tank and maintaining electrical contncti'may be resorted to. Suitable lclamp connections 35, for 'example having portions forming a loop 36 and provided tl AVAILALE COPi with ti hteninen 88 in .t e mem the latter, as shown-four of these-connecl.tions being provided on each of the members 18, the first directly op oeite the periphery 'of the tank cover-and t e second and third `43 of,which upper conductor may be a strongI wire cable-43 spiaced substantially above thej top ofthe tan for example at a l :height of 'about 7 feet, and defining a periphery somewhatgreater than that of the tank. Between ths'member 43 andthe ring member there may extcndfany sutiicient number of substantially radial conductor ele ments 44, the latter at their outer ends en-y gaging the'conductor 43. lAt points 4Z approximately one-third-of the way from the rin Y 10 to the meinber.43 there may be ati 'tac ed to each ofthe elements 44 a pair ofwire conductor elements 45, which are con-V 1 galvanizedv wire cable.

nected to the peripheral .conductor 43at equal distances ong-fo posite sides of the terminal portions of t e conductors 44. 'lhe vv-wirea 4,5{may' be twisted around the wires 44 andthe-joint may beheld by any suitable type Aoffpoeitive clam' The n1embera 45 may be twisted aroun the member 43, so1 dered orclamped; referably all wiring' eonnectionajaresaitab y. recuredbyhindngscicw clamps of any suitableconstruction.

' Ah forf tie. various cables and conductor elements; it is satisfactory to provide galvanized steel cables 29, to hold the members l2 in fixed position upon the' tank, and ordinary galvanized iron wire of sullccnt size may he used-for all the other conductors. 'lhe lxfripheralconductor' 43 is desirably a The lower ends of the members 16 may advantageously be allowed to rest against. the top edge of the next to the top course of plates in the construction of the tank wall.

The tank wall 2 is, as usual, in reliable .electrical contact with' the earth.. When this is -not assured by pipe line connections or standing foundation water. pnoper earthin'g` conductors are provided. Unnecessary project ions above the tanktopare rcconuncndwl to he avoided,

ats 37 are' placed in holes re 13 at various pointe in suitable material may be .employed i' fined by them from the roof surface. These recommendations -low .from the conditions under which the addition of the protective conductor system to the tank resultsy in a redistribution of the equipotential surfacesl 70 -related to induced charges, for the followlng `reasons:

Assuming an elevated electrified area,such as a-charged cloud C, Fig. 9, directly-overhead, the normally dielectric air between'l earth and cloud is in a state of stress between the oppostepotentials of earth and cloud. This condition results in a gradient of potential differences, of. which equal values will define equipotential surface?1 c1, ege, SQ

etc., the distribution of whichI fol ws in the main the depressions and elevations of the surface of the earth and conductive objects on the earth. Potential gradient intensities of the order of 35,000 t-o 150,000 volts per Q foot of elevation 'are 'of common occurrence during,r thunderstorms, and the potential gradient to upper-strata'may approach-a substantial fraction of these values during Aweather conditions not delinitely of the na. 90

ture of a thunderstorm. l.

In the vcase of a cloud or charged stratum .net overlwzul, the equi'poten-tial surfaces are vconcave toward the c tuin, and the lLradlent intensities may be of iai-ged 4elevated straany lesser values-dependent ou. the lateral distance. of thecharged ftratlun. In either casetthel lilies Uf force representing Athe `path 'of discharge and indicating;r the -norma s to tlu` equipotential surfaces are not necessarily vertical. .()n the contrary, in the case of a shari terrestial elevation, such us a tree, a luiildinpy or a tank. there is a compression to: gelber of the equipotential sui-faces at th tips` edges and -projections of these objects;-

this is inert-,ly another way of saying that at these places thepotential gradient is steeper, and that the directions ol the lines of force are incliped 'toward the center of Aligure of the terrestrial projection as indicated at f1,

f2, f" in Fig. t). Wlien there is normal uniformity of the atnrosphere, it may he predictedthat a lightning stroke discharge will strike along the steepest gradient 'and through one lof these compressions of the 315 equipotential surfaces. When 'thereI is no stroke, thel induced electrostatid vcharges are of greatest-intensity' at there places.

The linesof force end at. any most. elevated part, as-measured along the. direction of the nes ofA force, of the terrestrial projection, when that part is competently conductive'to earth. ln the case'ota suspended earthed' conductor, such as the wires 43, 44, 45 of .the described apparatus, the place of coiu- .1

pression together of the equipotentialsur# faces is elevated away from the tcrrestial projection. (the tank in this case) and if of any substantial electrostatic capacity, or'if the c'onductor bearsV a moving charge or eur- 'series of cquipotentialsurfaces of reversed rent, will then be "the" starting point of a .rent corresponding to static charges induced upon any such systeuias a whole are localized in the surfaces represented by an intervening earthed conductive networ so spaced away'from the earthed Vprojecting We have determined by new research' the proportional protective etiect of the descri `ed svstem ofspaced radial and periph.- eral conductors under high potcntialgradients. The distribution of equpotential surfaces (altered by the additions to the tank as shown in Fig'. 9 in full lines) in one plane is'indicated by the traces p.' 7i?. 7i etc. in' Fig. 10,without attempt toindicate the. ahsolute intensities from plotted surface to surface. When the mean lateral spacing` from Vconductor to conductor is equal to -or less than the verticalspacing from the tank roof to the conductors, the induced flux of current (a. direct measure 'of the static charge induced charge s uch isolated surfaces by direct by theeirciting charged stratum) was found -to be proportionally distributed between the tankroof and the conductor system in a proportion between extremes represented bythe observedextreme'values 3.2 to 96.8 and 1.6. to 98.4. When the number ot radial conductors wasreduced 'so that the mean separation of the conductors was greater than their elevation abovejthe tank4 roof, thcn'the inean values of'tlie. proportional charges were as-14.3 isto 85.7. In'this ease-e iupotentialv surfaces between .the suspen ed conductors and the rootl intersected the roof .at the iiiore separated places between the. conductors.

. The protective system, in the recommended spacing. -eiirries an average value of' the total induced charge of 97.6%. AThis may be taken as a probableniininium evaluation oitlie proportion of the current of a light- `ning holt discharge likely to be carried by the 'protectivesystem an( the tank roof rcspect-ively in caso of a stroke to a protected tank. The relativel high conductivity and even distribution o the conductoi' paths to earth from the protective covering warrants belief that a -far greater pro oi-tion of a heavy discharge'than the induced charge ratio indicates would in that case tlo'iv in the protective devices, resulting in a practically complete exclusion of the ischarge currents from the tank top androof proper.

Referring now to Fig. 11, the recoin-- mended proteetive conductor system is also Aand top surfaces, separate BEST AVAiLABLE-COP- 'charge from part to part of the tank. The

probability of inter-part sparks increases as the potential or intensity 6i mutually-in duce( or bound charges is permitted te increase. (and to a lesser degree paint or other coitings) will provide exterior surfaces t interior surfaces R which, because conductire or'anade conductive by condensed mois'- ture, residual -sulphuric acid fumes or other thin tiliiis, 'may provide a path for a-static charge built up' to discharging intensity.

Tivo such surfaces, especiall the large roof by a felt, as-

'lhe heat insulation I of the tank.

plialt or paint. dielectric I arei'an ideal Leyden-jar condenser capable of energetic d1s` charges 'amply able to fire the inflammables. lVorking )oints in the surface l, for example, under expansion or filling writhings of the tank, might, if this surface (and the sui-face t by induction) carried heavy in- 'duced charges. permit local sparks from part to part: :is illustrated .at '1-1, sections'of the i tank, especially the eaves portion, may re resent a resistance to ground (due to o structcd joints,-openings at the eaves, etc.) and the moisture film at t may "well be electrically discontinuous or highly resistant, as at i. Y c .l Y

There is no good way to reach and discontact wiring system could be made. lVe

therefore. prefer to rel),r for discharge upon the overhead conductor system 12, 44, spaced from the surface t by an air-gap dielectric resistance. The overhead conductors coinpletc a system in which the typical assumed 'charges at preferentially discharge to earth through the conductors 44, 39, 40, 1 2 and through the resistance r2. rather than by penetrationiot l. Such charges at t, when that surface is conductive, arc of the saine sign as the earth; hut as`indicatcd. when the surface is not uniformly conductive oi' is conductive in isolated areas only, these charges may he negative or positive. oi negative in one place and positive in another. or iii a state ot' oscillation troni one to another relative sign ot potential. 'l`he' discharge of such charges to thc overhead conductors is gradual rather than disruptive. Dangerously high potentials therefore do not build up on the isolated surface or surfaces t, and hv induction on the opposite surface R; and,

it and when a dangerous potential in sip-- preached. this is discharged' without heavy ioo llti

rio

-eesmiiiiieaetecow flow to the conductors 39, t07 41 or 44. It will be observed that the spacing of theconf ductors mutuallyand from the insulated .surface of the top of the tank is of optimum '5 'fnnctio l value when ythe relations are such *as both to discharge' surface static charges and to elevate regions .of high potential vgradient away from this surface.

During times of' great lmeteorologi(al lec- Il trical disturbance, "v ettectofthe protective rconductors is aided by'- the' corona discharge ionization "phenomena at the conductors, which then carry-'hea currents at gizeat potentials and u liigl'iifosc'l freqiiencies.- When it lis b'or'ne inl-.mind that theelfect of.l the pro- -tective gridis to lessen the charges on the tank proper to only from 1.6 to 3.2 per centuin. of the .quantum otherwise induced ,l

.flammable fluids having u vapor and beat in- :o by the charged cloud o r elevated stratum, it will be perceived thatthe protective sysv tem specified may be reliedvupon to-reduce tlieprobabilityof induced interior sparking from condenser action 'of .lms borne by heat z insulating or paint coverings to a minimum, while providing@ good probable protection for lightning-stroke disaster as it is f possible to desi l 1. The combination with a tank for inflammable fluids exposed to meteorological electrical influences and having an insulat ing layer above a portion of its extent of a sys moi earthed conductors spaced from .Il -each other and spaced from the surface of insulating layer bya greater distance than theirmean separation from each other.

2. The combination with a tank for in' flammable fluids exposedto meteorological i electrical influences and having an insulating, layer above a rtion of its extent of a System ofearthedgonductors, spaced from each other and spaced from the surface of said iii-'- sulating layer, si. l conductors being principally concentrated by closer spacin at the regions of maximum potential gradient ./between the tank top and an elevated charged atnios heric stratum. l

3. lllie combinationwvith a tank for. inflammable fluids exposed to meteorological potential gradient lying about and above the p periphery of the tank top and between the tank top and an elevated charged atmospheric stratum. f

4: The combination with a tank for mflammable 'luids exposed to meteorological electrical influences and having, a dielectric laver superimposed upon' its top of a system of earthed conductors extending beyond the the .surface-charge dising layer above a portion of its extent con- I-prisin electrical intluences of a system of earthed 'i periphery of the tank and spaced from'cach other and from the top portion of vthe tank by erect earthed conductors, said conductors being principally concentrated in the regions of maximumpotei. '.ial gradient between the tank top and an eievated chargedl atmospheric stratum.' i 5. The combination with a tank for inlariimable'ifluids exposed to meteorological electrical `influences and having an insulat stitiitiiig fi' dielectric separating outer andv inner snrfaces, of vmeans for dissipating inl duced s'tatic chai-ges held by mutual attraction at opposite sides of said dielectric coma system of eartlied conductors' space by an air-gap from tlie'exterior of the insulating layer.

6. The combination with a tank forinsulating top exposed to meteorological electrical 'influences of an interconnected. sys-- tem-of conductors spacedfrom each other and from the top portion of the tank and com rising a series of erect supporting and con uctingniembers in-earthed contact with metallic sidewalls of the said tank, one or more peripheral conductors connecting said erect members and encircling the region of the eaves ofA the tank, and a 'series of con- 95 -ductors extending from the region of the center of the to .of the tank toone of said peripheral con uctors and sup rted above the tank top at a distance greati); than their mean separation apart. -f` 10 7. The combination witha tank for inlflammable fluids exposed to meteorological electrical influences andliavingan insulating layer above a ortion of its extent constitutl ing a dielectric separating outer and inner 106 surfaces, of` means for dissipating induced static charges held b mutual attraction at op site sides of sai .dielectric comprising a system of eartbed conductors space by an Aair-gap from the exterior .of the insulating. 110 layer, at' a distance at which the air-gap dielectric resistance is not eomtantly lgreater at times of atmospheric electric: disturbance than the dielectric resistance of the said insulatn layer. 116

8; T e" combination with ametallic stora e tank, and a roof therefor having low eactrical conductivit of a protective system 'comprising con uctors overlying said tank and supporting means therefor includ- 120 ing supporting elements projecting beyond and above the periphery of said tank and means for holding the snpportingelements in position including-flexi e members holding their lower ends in mechanical 'and elec- 125 trical contact with the walls of said tank below sajd roof,

'9. The combination A with a metallic storage tank, and a roof therefor'having lowr electrical conductivity, of a protective sys- 13,0

, ductors overlying said tank, and supporting point well'below the eaves.

BEST AVAILABLE COP.

B Y A 1,617,788

temlcomprising a series of connected eon-y electrical Contact with the tank Walls'at a Y i0 means therefor including a,` plurulityof erectl vSigned by me at St. Louisliissorl1i, rliis conductive elements projecting upwardly fifteenth day of May 1925. and outwardly from the walls of said tank,- LESLIE A. BALDVv'lN.

'and holding means therefor including a plui Signed by me at Boston, Massachusetts,

rality 'of tension elements `adapted to hold this 22nd day of May, 1925. the said erect conductors in`meehanieal and ELMER R. SCHAEFFER.

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