US2936925A - Electrically insulated floating roof - Google Patents

Description

May 17, 1960 F. D. MOYER ETAL 2,936,925

ELECTRICALLY INSULATED' FLOATING ROOF Filed July 5. 1957 sheets-sheet 1 May 17, 1960 F, D, MOYER ETAL 2,936,925

ELECTRICALLY INSULATED FLOATING ROOF 3 Sheets-Sheet 2 Filed July 5. 1957 nmnunnuunl lll/lili' IIIIIHIIIIIMM May 17, 1960 F. D. MoYER ETAL ELECTRICALLY INSULATED FLOATING RooF 3 Sheets-Sheet 3 Filed July 5, 1957 'g y by.

nited States Patent 2,936,925 ELECTRICALLY INSULATED FLoATmG noon Frederick D. Moyer and Ivan L. Wissmiller, Chicago, Ill., assignors to Chicago Bridge'and Iron Company, 'a 'corporation 'of Illinois Application July 5, 1957, Serial No. 670,289

8 Claims. (Cl. 220-2'6) This invention relates to liquid storage tanks having heating roofs. More particularly, this invention is concerned with means to prevent lires in floating roofs caused by high electrical charges ysuch as are induced by lightning. y Y

As is well known in the art, as shown by United States Patent Nos. 2,554,497 and 2,587,508, oating roofs are constructed with a diameter less than the inside diameter of the tank. Accordingly, a seal must be provided between the edge of thek floating roof and the inner side walls of the tank to seal that space against liquid loss through evaporation. Such a seal must, of course, not interfere with the up-and-down movement of the roof as the liquid contents of the tank are increased or def creased. The most successful devices for effecting the seal use a sealing ring or shoes which run around the inside of the tank between the tank shell and the edge of the roof pressing against the tank shell. The ring `is f usually of relatively thin metal plate and of a height suitable for use with the tank and roof. A flexible vapor' tight material is positioned to extendv yfrom the roof to the shoes to cover the open area and provide a vapor barrier. Hanger and pusher means are provided to support the shoes and keep them pressed firmly against the inside tank surface. The pusher and hanger means are generally located below the vapor barrier and come in contact with the roof and also the tank through the shoes.

At the time of an electrical storm, an electrical charge is induced on the floating roof in the tank by the electric field of a thunder cloud. When the field is suddenly removed by thedischarge of the cloud by a lightning stroke, the charge on the roof is released rapidly and passes to the earth by wayof the grounded tank shell. This electrical charge must accordingly pass over or go. through the rim space between the roof .edge and tank wall. In the case of hangers and pushers located in the vapor space below the vapor barrier, the charge can ignite the vapor with sparks generated at points of imperfect or inadequate electrical contact such as Vwhere the hangers land pushers contact the roof or shoes or at the pinconnections between the hanger and pusher members. This the tloating roof, on the tank shell or elsewhereV near- According to the present invention it has been-found that the `danger of oating roof res ignited by lightning can be largely eliminated by barring electrical discharge through the hanger and pusher mechanism by properly insulating the mechanism and providing electrical shunts between the roof and shoes.

The invention will now be described in con'unction with the attached drawings in which:

Fig. 1 is Ia vertical section through a liquid storage tank showing portions of the sidewalls and 'a floating roof with shoes and insulated hanger and pusher mechanism in place iloating on liquid stored in the tank;

. angle and an inwardly extending portion 26. A-

2 Fig. 2 is similar to Fig. 1 but shows a-'dfferent type insulated hanger and pusher mechanism;

Fig. 3 is a vertical section along the .line 3--3 of Fig. 2.

Fig. 4 shows another embodiment used to insulate a hanger and pusher mechanism; and Fig. 5 Vis a section of Fig. 4 lalong the line 5-5 As to' lall the figures, 10 indicates a liquid storageftank having a side wall 11 and a oating roof 12 oatng upon the surface of the liquid 13 contained in the tank. The oating roof has a top 14, a side 15 and a bottom y16. 'An annularlseal 17 of flexible material seals the kvapor space 18 between theside 15 of the tank and a Y shoe 19 which slidably contacts the side Wall of the tank.

Ythe upper end of a lever 23 to provide a; pivotal coni nection` of the lever to the 'upper portion of the roof. The levei 23 has a downwardly extending portion 24, an

weight 27j is providedon the inwardly extending portion to exert a downward force upon thelever. Pivotally connected at 28 to the upper portion of the shoe is a shoe supporting member 29. The other end of the i shoe supporting member isconnected to the lo'wer portion of the side 15 by means of a pin and slot arrangement. A bracket 30 is attached to the lower portion of thereof having a vertical slot 31, the sides of the slot being engaged bya pin 32 attached to the lower end of the member 29. The shoe supporting member is .pivotally connected to the lever 23 at 33.

The roof and tank shell Iare insulated from each other in `the `vaporspace 18 by eliminating direct metal contact between them through the hanger and pusher members.. Thus, supporting member 29 is formed in two parts. 34 and 35 separated from each other by lgap 36; the parts however -are connected by insulating material 37 fastened securely to each part. The downwardly extending portion 24 is also provided with insulating material 37 to prevent metal contact with the shoe. OneV or more shunts 38 are secured at one end to the edge ofthe roof and at the other end 'to the shoes.y f

A lightning stroke may terminate on the heating roof, on the tank shell or Iat some location near ythe tank, and as a resultthe bjou'nd charge on the oating roof is released/This4 suddenly creates a voltagediiierence Ysparking can occur if the lighning stroke terminates on4 between the floating roof and the tank shell. The voltager drop between the floating roof and the tank shell depends upon the number of shunts provided. A larger number of shunts will result ina smaller voltage drop. The changes of voltage occur over extremely short in.

' tervals of time resulting in rapidly increasing surges of current which the shunts must accommodate.l Hence it s important that the shunts be designed to have lowin-v ductance. Low induct'ance is accomplished. by shunts across the shortest iand most direct path and-by shunts having large surface area. The p-referred form of shunts is therefore a flat strip of metal attached 'between -the' edge of thefroof and the shoes; abrupt changes in di.

Hrection or alignment of the shunts is to be avoided. The

shunts may be attached by bolting at each end without materially increasing the inductance. the bolted connections or between the shoes and the tank shell is relatively unimportant because sparks can occur` at these locations without danger of ignition.

With an adequate number of low inductance shunts,

the maximum voltage drop between the floating roof and., the shoes or tank shell is reduced. It is then Ypractical'v to insulate the hanger and pusher mechanism to accor'nf. modate that maximum voltage without Spark-over. Hence none of the currentwill be carried through thehanger:

High resistance atl and pusher mechanism and no spark can occur inY of a lever 39 having its upper end pivoted at 40 to the side of the floating roof, and extending downwardly from the point of attachment to the roof to curved area 41 and a further portion 42 extending inwardly from the curved area. Shoe supporting member 43 is pivoted to the lever 39 at 44 and to the upper portion of the shoe at 45. It is preferred to construct the members 39 and 43 of parallel bars of steel to give increased strength and stability as shown in Fig. 3. Cross-braces 46 help to keep the unit rigid. insulating material 37 is used in supporting member 43 as Well as on lever :39 at the angle 41 where it pushes the shoe against the shell to prevent electrical conductance through the roof and tank shell. Shunt 47 'communicates with the metal of the roof and shoe to ground electrical charges on the roof; such shunts are disposed around the roof in spaced relationship to each other. y

Figs. 4 and 5 illustrate the upper part of a shoe supporting member 50 which may be used instead of member 29 in Fig. l or member `43 in Figs. 2 and 3. The shoes are insulated from the roof with member 50 by insulating barrier 51 which is a sleeve that lits over rod 52 connected as by welding to the upper end of the lower part 53 ofthe shoe supporting member 5t). Tube or pipe 54 fits over the insulating sleeve 51. 'The lower end of the insulating sleeve extends outside of the lower end of the pipe 54 to deter electrical charges from jumping from rod 52 to the pipe. yInside pipe 54 is inserted plug 55 of insulating material. The plug bears against the end of rod 52 and contacts the end of insulating sleeve 51 to seal the rod 52 from pipe 54. Metal disc 56 is inserted in the pipe 54 so that it bears against plug 55. Disc 56 and pipe 54 are secured together by welding or screw means to prevent independent movement of each with respect to the other. The upper part 57 of the shoe supporting member 50 is connected as by welding to the disc 56, and hole 58 is provided for pivotally joining it to a shoe, such as at 28 in Fig. l.

Although any suitable insulating material may be used, it is generally advisable to select a material for use in a specific insulated shoe supporting structure so that the structure will remain stable and not lose its supporting ability in case of a lire.

Repair of a ire damaged floating roof is more readily achieved with the shoe in place than where the supporting members collapse and the shoe sinks to the bottom of the tank. Furthermore, experience has shown that as long as the shoes maintain their normal position against the tank shell, the destruction of the seal material 17 will generally be localized, and the fire will also be local and relatively easily controlled. If the shoes were permitted to sink or pull away from the shell, the entire rim space would' be opened and the fire would spread around the entire periphery of the oating roof.

Accordingly, with the structures shown in Figs. l-3, it is advisable that at least insulating material 37 be fire resistant and not melt so that if a re results from any cause the shoe will continue to be supported in its proper position and not sink to the bottom of, or pull away from, the tank.

In the structure shown in Figs. 4 and 5, electricalinsulating material may be used that is either fire resistant or not lire resistant. Destruction of the insulating material in this form of insulated shoe hanger would not destroy its shoe supporting capacity because rod 52 would bear against pipe 54 and disc 56 since it is a compression joint as used in the floating roof tank. Of course, the insulating material, if destroyed, wouldhave to' be replaced after a fire to again provide the desired electrical insulation.

While there are shown and described certain embodiments of the invention, it is to be understood that it is capable of many modications. Changes, therefore, in the construction Iand arrangement may be made without departing Yfrom the spirit and scope of the invention as disclosed in the appended claims.

What is claimed is:

1. A liquid storage tank having a floating roof, a plurality of rigid metal shoes adapted to slidably contact the inner side walls of the tank, metal members for supporting and pressing each shoe against the side walls, said members being connected to and in contact with the roof and shoes, a flexible seal above the shoe supporting members and covering the space between the roof and tank side walls, said members having electrical insulators to. prevent ow of electrical current from the roof to the tank'side walls through the members, and a plurality of low inductance electrical shunts positioned above the iiexible 'seal connected to the roof and shoe as an electrical bond to permit ow of lightning induced electric current to the tank side walls and thus to the ground.

l2. The tank of claim 1 in which the insulators are lire resistant.

3. The tank of claim 1 in which the insulators are not re resist-ant but the members are compositely constructed with insulators separating the metal components of the members, and the members can support the shoes without the insulators.

4. In a liquid storage tank having a oating roof, one or more rigid metal shoes adapted Ito slidably contact the inner side walls of the tank, meansl for supporting the shoes against the side walls including a lever swingably attached to the roof, said lever having a curved portion, a. portion extending downwardly in the space between the roof and the tank from the point of attachment of the roof to the curved portion and a portion extending inwardly from said curved portion, and a rigid metallic shoe supporting member having one end pivotally connected to the shoe and having the other end connected to the roof, said shoe supporting member being pivotally connected intermediate its ends to the downwardly eX- tending portion of the lever, and a flexible seal above the shoe supporting means covering the space between the roof and shoes, the improvement which comprises electrical insulation interposed on the lever and shoe supporting member between the roof and shoes to prevent metal to metal contact, and thereby the flow of electrical current, between the roof and shoes through the lever and shoe supporting member, and a plurality of low inductance electrical shunts connecting the roof and shoes above the flexible seal thereby providing an electrical bond between the roof and shoes.

i 5. In a liquid storage tank having a oating roof, rigid metal shoes adapted to slidably contact the inner side wall of the tank', metal levers attached to the roof which eX- tend downwardly between the roof 1and side walls and curve inwardly beneath the roof, curved portions of the levers pressing the shoesagainst the side wall and a flexible seal above the space and levers between the roof and shoes, the improvement which comprises electrical insulation on the levers positioned to sever the metal-tometal contact between the shoes and roof to prevent the flow of electrical current from the roof through the levers Vto the shoes and side wall, and a plurality of electrical shunts of'low inductance above the yflexible seal connecting the roof and shoes to provide an electrical bond between the lfloating roof and the tank wall.

, 6. In a liquid storage tank having aoating roof, shoes adaptedY to slidably contact the inner side walls of they ductance above the flexible seal connected to the roof and shoes to provide an electrical bond therebetween to earnl an electrical current produced by lightning, and electrical insulators in the support members to prevent the flow of lightning-produced current from the roof through the members and then to the shoes and tank side walls.

7. In a liquid storage tank having a oating roof, rigid metal shoes adapted to slidably contact the inner side walls of the tank, metal members for supporting and pressing the shoes against the side walls, said members being connected to and in contact with the roof and shoes, and a exible seal above the members and covering the space between the roof and tank side walls, the improvement which comprises a plurality of electrical shunts of low inductance comprising flat flexible metal strips of large surface area electrically bonded at opposing ends to the roof and shoes, said shunts being disposed around the roof in spaced relationship to each other thereby facilitating flow of lightning-induced electriecurrent from the roof to the shoes and thus to the tank side walls and then to the ground. v

8. In a liquid storage tank having Ia floating roof, rigid metal shoes adapted to slidably contact the inner side walls of the tank, metal members for supporting and pressing the shoes against the side walls, said members being connected to and in contact with the roof and shoes, and a exible seal above the members coveringr the space between the roof and tank side walls, the improvement which comprises insulation interposed in the members to prevent ilow of electrical current from the roof to the tank side walls through the members, and a. plurality of electrical shunts of low inductance comprising ilat ilexible metal strips of large surface area electrically bonded above the flexible seal at opposing ends to the roof and shoes, said shunts being disposed around the roof in spaced relationship to each other, thereby facilitating How of lightning-induced electric current from the roof to the shoes and thus to the tank side walls and then to theground.

References Cited in the le of this patent UNITED STATES PATENTS 2,554,497 Moyer May 29, 1951 2,735,573 Fino Feb. 21, 195s 2,790,575 Wiggins Apr. 3o, 1957 2,801,763 Ulm Aug. 6, 1957 UNITED STATES PATENT oFFICE CERTIFCATION OF CORRECTION Patent, N0 2336925 May 17, 1960 Frederick D., Moyer et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column lY line 3l, for Neating" read floating --;l column 6Y list of references cited, after line 25, insert:

OTHER REFERENCES "The Water Tower" Volo 43, No. l, published Sept Y 1956 by the Chicago Bridge and Iron Co., Chicago1 111 the article on page 7 dealing with insulated hanger for floating roofs.

Signed and sealed this 2nd day of May 1961. (SEAL) Attest: v

ERNEST W., SWIDER DAVID Lo LDD Atesting @fficer Commissioner of Patents

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