WO1996008427A1

WO1996008427A1 - Flexible seal for floating roofs of storage tanks

Info

Publication number: WO1996008427A1
Application number: PCT/US1994/013814
Authority: WO
Inventors: James G. Vana, Jr.
Original assignee: WL Gore and Associates Inc
Current assignee: WL Gore and Associates Inc
Priority date: 1994-09-12
Filing date: 1994-12-02
Publication date: 1996-03-21
Anticipated expiration: 1997-03-12
Also published as: AU1298895A

Abstract

The present invention is an improved seal (10) for use in preventing leakage in a variety of applications where two components move relative to one another in a sliding fashion. The seal (10) of the present invention employs an elastic inner member (12), coated with a chemically resistant fluoropolymer film (14). The film both protects the inner elastic member from chemical attack as well as provide a low coefficient of friction for an improved sliding seal. The seal of the present invention is particularly suitable for use in applications such as floating roofs on chemical storage tanks.

Description

TITLE OF THE INVENTION

FLEXIBLE SEAL FOR FLOATING ROOFS OF STORAGE TANKS

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to sealing devices, and more particularly seals used in sliding junctions between components, such as between an inner tank wall of a storage tank and a floating roof. 2. Description of Related Art Storage tanks of chemicals and petroleum products are often designed with a floating roof that covers the surface of the product in order to reduce the loss of product through evaporation and to prevent product contamination from ambient weather conditions. The floating roof is typically a rigid metal platform with an outer diameter smaller than the inner diameter of the tank resulting in an annular space, or "rim space," between the floating roof and the inner tank wall. It is common practice to provide a seal for the rim space, which otherwise could be a major source of evaporation loss. Evaporation loss is a particular concern in large tanks used to store volatile chemicals and petroleum products. Normally a sealing device is attached to the floating roof and is pushed up against the periphery of the inner tank wall to enclose the rim space completely. Such sealing devices are generally made of polymeric members or a combination of thin metallic and polymeric members since they are flexible and will easily conform to the curved surface of an inner tank wall. The ability of a polymeric member to conform to the inner tank wall to prevent gaps and paths for vapor leaks under harsh chemical and environmental conditions is the primary challenge in the design of the seals. Difficulties inherent in developing an effective seal under these conditions has resulted in a wide variety of seal designs. One existing sealing device includes a flexible metal shield that is mounted to the upper perimeter of the floating roof and extends diagonally upwardly toward the inner wall of the tank, as described in United States Patent 4,308,968. This device employs a yieldable polymeric member, or "wiper blade," mounted on the end of the shield and cantilevered for engaging and pressing against the inner tank wall. The wipers are designed to be relatively free flexing so as to conform to the continuous inner tank wall of a welded tank. This is accomplished by providing breaks in each wiper which are spaced apart along the length thereof and yet staggered relative to the breaks in the other wiper seals to seal the rim space. A disadvantage to this type of seal design occurs when the floating roof changes its vertical direction and the high friction against the tank inner tank wall causes the cantilevered wipers to reverse direction, or "flip." This can result in a vapor path should the staggered seals not flip simultaneously during the transition. This flipping action may cause the seal to buckle on itself, inducing high stresses in the member each time the floating roof reverses its vertical direction. This makes the seals susceptible to fatigue failure and/or stress cracking due to the frequent cycling of the level of chemical products in an active storage tank. Since the introduction of a wide variety of additives in gasoline, there have been reports of premature deterioration of polymeric storage tank sealing members described herein. As a result, storage tank users are exercising more care in selecting a sealing member that is compatible with the materials stored in their tanks. Searching for compatible members, however, can be very difficult since compatibility tests tend to not include the combinational effects of the mixtures of different additives, concentration levels, heat, humidity, and other weathering elements combined with the mechanical stresses induced in the member as a result of the seal design. With so many factors contributing to seal fatigue and failure, it has proven very difficult to study seal deterioration in a laboratory environment. Other concerns include the fact that during the expected lifetime of a sealing member the types of additives and their concentrations may change several times by the refiner or by the tank user who will commonly store different products at different times in the same tank. Accordingly, it is a primary purpose of the present invention to provide a seal for a sliding junction that provides both good mechanical properties and wide-ranging chemical compatibility.

This and other purposes of the present invention will become evident from review of the following specification. SUMMARY OF THE INVENTION

The present invention comprises an improved flexible seal for use in sliding junctions, such as those found in the space between a floating roof of a storage tank and an inner periphery of the tank wall. The seal comprises an inner sheet of a thin, flexible, elastic member that provides the elastic forces necessary for proper sealing between the seal and the inner tank wall. The inner elastic member is covered with, and bonded to, a thin layer of a conductive fluoropolymer member, which provides the chemical resistance to a wide range of chemical additives. The thin layer of conductive polymer may or may not also cover the edges of the inner elastic member. The resulting seal of the present invention is not harmed by buckling stresses or the harsh chemical environments, which can deteriorate current flexible seals. Preferably, the fluoropolymer used in the present invention is conductive so as to dissipate any buildup of static electrical charges caused by friction between the seat and the inner tank wall during normal operation. In addition to the conductive polymer, static drain strips may be provided around the circumference of the roof structure to provide metal-to-metal contact from the floating roof to the electrically grounded tank wall. The seal may be mounted directly to the upper portion of the roof structure and pushed up against the inner periphery of the tank wall in a cantilevered position. The seal may also be mounted to an intermediate spring member which allows greater spring forces to be applied to the seal, as well as allowing a larger rim space between the floating roof and the inner tank wall. The dimensions of the seal are designed such that the seal will remain flexible to conform to the tank inner tank wall yet maintain structural rigidity to provide a satisfactory force against the inner tank wall when in a cantilevered position. Since the frictional forces between the seal of the invention and the inner tank wall are significantly lower than presently available seals, the seal of the present invention is able to contact a larger surface area of the tank wall by increasing the span dimension of the seal relative to the dimension of the rim space. This larger area of surface contact allows the seal to remain cantilevered in one vertical direction, sliding against the inner tank wall independent of the direction of the movement of the floating roof. This eliminates the highly stressful flipping action which occurs with current seals.

DESCRIPTION OF THE DRAWINGS

The operation of the present invention should become apparent from the following description when considered in conjunction with the accompanying drawings, in which:

Figure 1 is an isometric view of a seal of the present invention, showing the outer thin fluoropolymer layer covering two sides of the inner elastic member;

Figure 2 is an isometric view of a seal of the present invention, showing the outer thin fluoropolymer layer covering two sides and one edge of the inner elastic member;

Figure 3 is an isometric view of a seal of the present invention, showing the outer thin fluoropolymer layer covering two sides and two edges of the inner elastic member;

Figure 4 is a cross-sectional view of a seal of the present invention mounted directly to a floating roof by bolting;

Figure 5 is a cross-sectional view of another embodiment of a seal of the present invention mounted to an intermediate spring member; and

Figure 6 is an isometric view of still another embodiment of a seal of the present invention, showing a modified, enlarged outer edge.

DETAILED DESCRIPTION OF THE INVENTION The present invention is an improved seal for mounting in a sliding connection between two components, such as a flexible seal mounted between a floating roof and a side wall in a chemical storage tank.

Figure 1 shows an isometric view of a seal 10 of the present invention. The seal 10 comprises an inner elastic member 12 covered with and bonded to a thin conductive fluoropolymer layer 14 on its first surface 16 and its second surface 18.

The preferred inner elastic member 12 comprises a polyurethane membrane bonded directly to the conductive fluoropolymer film 14 by applying heat and pressure. The composite material so formed provides flexible elastic properties and resistance to chemical attack. For use in a chemical storage tank having a "floating roof," the inner elastic member preferably comprises a sheet of polyester-type polyurethane measuring between 25 and 1 mm or less in thickness (and more preferably between 1 and 3 mm), and between 17 and 38 cm in width. Alternatively, the inner elastic member 12 may comprise any other suitable form of an elastic polymer material, such as natural rubber, polyester elastomer, or foamed polyurethane. A thin sheet of metal with elastic properties (e.g., beryllium copper, spring steel, or stainless steel) may likewise be used. The precise thickness of the elastic member 12 and the fluoropolymer layer 14 are chosen to provide the necessary cantilevered force when deflected against a sealing surface, yet remaining flexible to conform to any curvature or irregularities in the sealing surface in order to provide a tight seal.

While the inner elastic member 12 provides the needed elastic properties for the seal of the present invention, it is the fluoropolymer layer 14 that protects the elastic member 12 and supplies the necessary chemical resistance. A suitable fluoropolymer film 14 is a polytetrafluoroethylene film, such as that made in accordance with United States Patent 3,953,566 to Gore, incorporated by reference. More preferably, the film comprises a densified polytetrafluoroethylene material in accordance with United States Patent Application Serial Number 932,441, filed August 19, 1992, allowed, incorporated by reference. The film should have the following characteristics: a high tensile strength, ideally on the order of about 68.9 MPa or above; tear resistance; fatigue resistance; and low permeability to water and gases. For use with the particular polyurethane elastic member 12 described above, the preferred film 14 comprises a thickness of between 0.1 and 0.5 mm. The film 14 may be mounted to the elastic member 12 in any suitable manner, including bonding under a heat of about 190 to 210°C and a pressure of between 0.680 to 1.7 MPa. Other suitable bonding methods may include use of an adhesive layer. An important additional property of this material is its very low coefficient of friction, allowing the seal 10 to slide freely along its contact surfaces.

It is particularly preferred to make the seal 10 electrically conductive so as to dissipate static electrical charges that can be generated by the sliding movement of the seal within the joint. With conventional chemical storage tanks, the tanks are grounded for the safe dissipation of such charges, but an electrical path is still needed to ground charges generated in the floating roof structure. Ideally, the fluoropolymer film 14 includes electrically conductive elements therein to form this electrical pathway. For instance, a high strength

PTFE film may be form that is carbon filled, as described in co-pending United States Patent Application No. 932,441. This material comprises a redensified in addition to carbon, other electrically conductive fillers may include various metal particles, and metal oxides. Other conductive fluoropolymer materials suitable for use in the present invention may include films of conductive ethylenetetrafluoroethylene, conductive perfluoroalkoxy, or conductive fluorinated ethylene propylene.

Another embodiment of the seal 10 of the present invention is shown in Figure 2. In this embodiment , the inner elastic member 12 is covered with the thin conductive fluoropolymer layer 14 on both sides 16, 18 and along at least one edge 20. This preferred form of the present invention provides a more complete sealing of the elastic member 12 by protecting the elastic member from chemical attack along its edge. Additionally, due to the low coefficient of friction of the fluoropolymer coating of the present invention, the seal of the present invention slides easily along a sealing surface and is not prone to

"flipping" during operation. This significantly reduces the possibility of seal damage during seal operation. This embodiment of the seal 10 of the present invention is shown in a mounted orientation in Figure 4. As is shown, the seal 10 is attached to a floating roof structure 22 by conventional means, such as with a series of bolts 24 placed through openings 26, 28 in the seal. Reinforcement plates 30 and nuts 32 may be used to retain the seal in tight contact with the roof structure

22. Mounted in this manner, the seal 10 is cantilevered against an inner surface 34 of a tank wall 36 in the manner shown as the roof structure 22 moves in a downward direction. As can be seen, the covering of fluoropolymer layer 14 over the seal's edge 20 protects against attack from chemicals and vapors in vapor space 38.

The preferred method of manufacture of this embodiment of the present invention is to fold the film of conductive fluoropolymer 14 in half over the elastic member 12, around the fluoropolymer edge 20. The film 14 is sealed in this position by applying heat and pressure to bond both flat surfaces 16, 18 and the long edge 20 to the elastic member 12.

Most preferably, the seal of the present invention 10 is formed under a vacuum while pressure and the heat is applied so as to remove any outgassing volatiles. This provides a more homogenous inner elastic member. Another preferred construction of the present invention is also illustrated in Figure 2. In this instance, there is provided a gradient thickness of the inner elastic member 12, whereby the thickness decreases approaching outside edge 20. This produces a seal 10 with a thicker section 40 on the bolt side of the seal and a thinner section 42 on the outside edge 20 side of the seal. As is shown in Figure 4, the thicker section 40 attaches to the floating roof 22 to provide a larger cantilever force against the wall 36, while the thinner section 42 provides flexibility to conform against the curved inner tank wall 34.

Yet another embodiment of a seal 10 of the present invention is shown in Figure 3. In this form, the seal 10 comprises an inner elastic member 12 covered with a thin fluoropolymer layers 14a, 14b on surfaces 16, 18. The fluoropolymer layers 14a, 14b extend off the elastic member 12 on at least one side edge 44 to directly attach to each other along section 46, as shown. As was shown in Figure 2, the outer edge 20 is again sealed by wrapping the fluoropolymer layer 14 around edge 20 of the elastic layer 12. The advantage of this construction is that the side edges of the elastic layer 12 are likewise protected from chemical attack. Additionally, the use of fluoropolymer layers 14a, 14b alone in section 46 provides a very flexible element that will readily mate with minimal interference with adjoining seals to form a better seal around the circumference of a tank. Typically both side edges of the seal 10 would be covered with the conductive fluoropolymer in this form of the invention to provide additional protection from chemical attack if necessary. The seal of the present invention is shown in two suggested applications in Figures 4 and 5. As has been explained, the seal shown in

Figure 4 is mounted directly to a floating roof 22. The seal 10 bends into a radius as it is pushed up onto the inner tank wall 34, the elastic forces provided by the inner elastic member 12 allow the seal 10 to adequately press against the inner tank wall 34, forming an effective barrier against evaporating volatiles as well as shielding the fluid in the tank from outside environmental influences, such as moisture. The span of the seal of the invention is defined as the distance from one long edge 20 to the other long edge 48 of the seal 10 when laid flat. The span of the seal 10 can be made sufficiently large so as to allow for large variations in the distance "R" 50 between the inner tank wall 34 and the floating roof 22.

Figure 5 shows another method of mounting the seal 10 of the present invention. The seal 10 is mounted to an intermediate spring member 52. To prevent leakage around a series of spring members 52 mounted around the circumference of a tank, a continuous flexible film 54 is provided on either the interior or exterior of the spring member 52. Spring member 52 is generally a thin gage metal shield that is attached to a periphery floating roof 58 and is bent in a diagonal towards inner tank wall 60 wherein seal 10 will be attached and forced against the tank wall by the spring properties of the member 52. The relatively free flexing of the spring member or members 52, conforming to the continuous inner tank wall 60, is provided by breaks in each spring member spaced apart along the length thereof and yet staggered relative to the breaks in the other spring member components to seal the rim space. Flexible film 54 is typically a thin non-permeable film which is continuous around the rim space to seal the gaps created by the staggered metal spring members 52. This method of mounting of the seal 10 allows even greater distances "R" 62 between the floating roof 58 and the inner tank wall 60. In addition, this method of mounting provides an additional positive force of the seal 10 against the tank wall 60.

The seal 10 shown in Figure 5 may be mounted to spring member component 52 by using any of suitable clamp means known in the art or through the use of high-strength adhesives or similar materials. The spring member 52 and film 54 may be mounted to the floating roof by bolting, clamping, or adhesion as well. Figure 6 is still another embodiment of the seal 10 of the present invention. In this instance, the seal 10 includes an outer edge 64 that is enlarged to form a "bulb" along its length. This outer edge 64 is mounted in contact with the sealing surface so as to more easily move over welded seams and other protrusions along a sealing surface.

With all embodiments of the present invention, the sealing of a tank may be accomplished by using a number of seals of the present invention. It may be desirable to space each seal so as to provide an overlap with the adjacent seals. The complete rim space may then be effectively sealed without danger of the seals flipping and allowing openings for vapors to escape. The fluoropolymer layer gives superior chemical resistance to prevent degradation due to chemical additives, while being conductive to prevent dangerous buildup of electrostatic charges between the inner tank wall and the floating roof.

While the above examples of the present invention are directed to the sealing of a floating roof on a large chemical storage tank against a sealing surface of the tank's inner walls, it should be understood that the seal of the present invention may have applicability to a wide variety of sealing needs, particularly where a dynamic seal must be produced between components parts that move relative to one another. Other suggested uses for the seal of the present invention where the seal mounts in dynamic contact against a sealing surface include: rotary vane pumps, sewage treatment slurry tanks, etc.

While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions. It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.

Claims

The invention claimed is: 1. A flexible fluoropolymer seal comprising: an inner elastic member having a first and a second surface; and an fluoropolymer film bonded to the first and second surfaces of the elastic member, the fluoropolymer film protecting the elastic member from chemical attack; wherein the seal is adapted to be mounted against a sealing surface, the seal sliding along the sealing surface during use to prevent leakage of chemical past the seal and sealing surface. 2. The seal of claim 1 wherein the fluoropolymer film is electrically conductive, providing an electrical path through the seal. 3. The seal of Claim 2 wherein the fluoropolymer film is selected from the group consisting of: conductive polytetrafluoroethylene, conductive ethylenetetrafluoroethylene, conductive perfluoroalkoxy, and conductive fluorinated ethylene propylene. 4. The seal of claim 1 wherein the elastic member is a thin metal selected from the group consisting of: spring steel, beryllium copper, stainless steel. 5. The seal of claim 1 wherein the elastic member varies in thickness along its width, having a thinner edge in contact with the sealing surface. 6. The seal of claim 1 wherein the elastic member comprises of an elastomeric polymer selected from the group consisting of: polyurethane, rubber, foamed polyurethane, and polyester elastomer. 7. The seal of claim 1 wherein the seal is adapted for use in a chemical storage tank having a floating roof, the seal being attached to the floating roof and contacting a sealing surface on an inner wall of the storage tank. 8. The seal of claim 1 wherein the fluoropolymer film wraps around elastic member, sealing the first and second surfaces of the elastic member as well as at least one edge of the seal. 9. The seal of claim 1 wherein the fluoropolymer film on both the first and second surfaces of the elastic member extend beyond the elastic member, the fluoropolymer film being sealed to itself beyond an edge of the elastic member. 10. The seal of claim 1 wherein the seal is adapted to be mounted in cantilever fashion against the sealing surface. 11. The seal of claim 1 wherein the seal includes an outer edge in contact with the sealing surface, the outer edge being enlarged to assist in sliding over protrusions in the sealing surface. 12. A method of sealing a joint between two surfaces that move relative to one another in sliding fashion, that comprises: providing a seal comprising an inner elastic member coated with a fluoropolymer film on at least a first and a second surface; mounting the seal to a first component, the seal extending beyond the first component to contact a sealing surface on a second component; wherein the seal slides along the sealing surface so as to resist leakage between the seal and the sealing surface; and wherein the fluoropolymer film protects the elastic member from chemical attack during use. 13. The method of claim 12 wherein the seal is mounted in cantilever fashion against the sealing surface and the seal resists flipping during operation. 14. The seal of claim 12 wherein the seal is adapted for use in a chemical storage tank having a floating roof, the seal being attached to the floating roof and contacting a sealing surface on an inner wall of the storage tank. 15. The seal of claim 12 that further comprises providing an elastic member with varying thickness along its width, having a thinner edge in contact with the sealing surface. 16. The seal of claim 12 which further comprises providing the seal with an outer edge in contact with the sealing surface, the outer edge being enlarged to assist in sliding over protrusions in the sealing surface.