AU676015B2 - Double wall storage tank systems with rigidly attached end cap - Google Patents

Description

.4 -1- This invention relates to storage tank systems. More particularly, the invention relates to underground storage tank systems which have a jacket for secondary containment and which are capable of containing a leak detecting fluid between a dgid inner tank and the jacket.

Single wall storage tanks for the bulk storing of liquids are very prevalent in many industries. Some of these liquids are highly corrosive and/or are flammable. The service life of a storage tank will vary. Eventually, however, the tank is likely to corrode or develop stress cracks which leak.

This can result in a significant danger to the environment and health of nearby residents. Storage tanks which are used for storing gasoline at service stations are of particular concern. Gasoline, of course, is highly flammable and is capable of posing a *significant health and safety hazard if not proprly contained. The potential problem is exacerbated because of the large number of tanks currently in use.

Federal as well as local regulations govern the structure of storage tanks which are intended to store gasoline.

Double wall underground storage tanks are now being built in response to a need. Several different designs have been suggested, with some finding commercial acceptance. All have a cylindrical-shaped 2 main body and either flattened end caps or dome-shaped end caps. Known double wall storage tanks are made of a metal such as steel, fibrous reinforced resinous materials or a combination such as found with a rigid metal inner storage tank and a fibrous reinforced resinous outer jacket. The known tanks have varying manufacturing cost advantages, installation advantages and performance advantages which must all be balanced out by a tank manufacturer and ultimate purchaser.

It has long been known that metal tanks can be made to have requisite strength and corrosion resistance. These tanks commonly have flattened end caps. Forming a double wall tank wherein the advantages of the known single wall metal tank is utilized has been difficult. Simply adding a second metal wall which follows the contour of the inner tank results in a tank system which is unduly heavy and difficult to install. Adding a fibrous reinforced resinous material jacket which also follows the contour 20 of ti-a inner tank lessens the weight. However, the structural integrity of the formed tank system is less than that of steel and, therefore, is questionable.

The flattened end caps on each tank end tend to deflect away from one another to the extent the jacket will 25 develop stresses that may crack during the operational life of the system. The deflection problem is particularly acute when a leak detecting liquid is added by the manufacturer to the closed space between the tank walls. The detecting liquid creates substantial hydrostatic pressure which is the greatest in the lower half of the flattened end cap area. Even those jackets which commonly have a 125-230 mil thickness will rupture because of forces created by the detecting liquid.

U.S. Patent Nos. 4,819,821 and 4,951,844 disclose double wall underground storage tanks which have the advantages of a rigid metal inner tank and a fibrous reinforced resinous material jacket. The tanks are relatively light weight and structurally sound. However, added manufacturing costs due to extra labor steps is a drawback with each of these tank designs. Certain currently produced tank systems which also have an inner steel storage tank with a fibrous reinforced resinous jacket have achieved enhanced strength in the end cap areas. The jacket in these tank systems has been formed in a manner whereby its end caps are joined at the centers to the end caps of the inner tank. However, the overall strength needed to withstand deflection is still considered inadequate. It has been found that the outer areas, especially in the lower half of the end cap area, tend to deflect and are susceptible to cracking over time.

It is the object of the present invention to overcome or substantially ameliorate the above disadvantages.

There has now been developed methods whereby new and used storage tanks with flattened end caps can be provided with a jacket. The jacketed tank is built in a manner whereby it has enhanced strength in the flattened end cap area. Newly manufactured tanks are able to contain detecting fluid in the annular space between the inner storage tank and the jacket because of the enhanced strength. Used storage tanks can be refurbished to a standard equivalent to that possessed by a new tank and then upgraded to have a secondary containment feature by addition of the jacket and end cap attachment.

According to the present invention there is provided a double wall storage tank 20 system for liquids which is capable of holding detecting fluid in a closed space for leak monitoring purposes, comprising: a rigid inner tank having a cylindrical shaped main body and substantially flattened end caps for storing the liquid; S" a jacket having a cylindrical-shaped main body and end caps which are made 25 of a fibrous reinforced fesinous material and which covers a surface area of the inner tank to form the closed space, wherein the end caps of the jacket cover the flattened end caps of the rigid inner tank; and multiple attachment members which substantially rigidly attach the end caps of the jacket directly to the substantially flattened end caps of the inner tank to ensure that the respective end caps are free of substantial deflection.

A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: FIG. 1 is a side view in partial section of a cylindrical-shaped rigid inner storage tank with flattened end caps with a fibrous reinforced resinous material as a jacket completely surrounding the tank and substantially rigidly attached thereto at the inner tank's flattened end cap areas.

FIG. 2 is an end view of the storage tank of FIG. I with a partial cut-away showing the rigid attachment of the jacket to the inner tank's flattened end cap.

[n:\libll00710:CJS -4- FIG. 3 is an end view of the double wall storage tank system of this invention showing alternative attachment members for use on the end caps of the inner storage tank.

FIG. 4 is a partial side view of the double wall storage tank system of FIG. 3.

FIG. 5 is an end view of the double wall storage tank system of this invention showing another attachment member for the end caps of the inner storage tank.

FIG. 6 is a partial top view of the double wall storage tank system of FIG. FIG. 7 is an end view of the double wall storage tank system of this invention showing still other attachment members for the end caps of the inner storage tank.

FIG, 8 is a partial top view of the double wall storage tank system of FIG, 7.

While the description to follow describes the invention in terms of its use with underground storage tanks, it should be understood the invention has applicability for other uses as well. However, the S o*

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*o *o [n:\libl]00710:CJS I I invention lends itself particularly well to underground storage tanks used for storing liquid gasoline and, therefore, this preferred use is described in the following paragraphs.

With reference to FIG. i, there is shown a double wall underground storage tank system 10. The inner storage tank 11 of the type shown in the system of FIG. 1 is well known and widely used, especially in the gasoline service station industry. It is rigid and made of metal, specifically steel. It should be understood, a rigid inner storage tank made of fibrous reinforced resinous material is also feasible. In all instances, the inner tank of this invention has a cylindrical-shaped main body 12 and substantially flattened end caps 13. Sufficient openings are found in the inner storage tank 11 to allow for various access lines to communicate with the interior of the tank. As shown, access lines 14, 15, and 16 are a fill pipe, dispensing line and vent pipe, respectively.

ro.0 20 These access lines fulfill their normal functions.

In building the double wall tank system of the invention, a separating agent is preferably applied to the main body of the inner storage tank extending preferably to the flattened end caps before the jacket 25 is formed. The purpose of the separating agent is to

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ensure that a subsequently applied fibrous reinforcing material and resinous material which form the jacket will not adhere to the inner storage tank or seal closed a space formed between the jacket and inner tank. It is necessary that the cured jacket and the storage tank have the closed space 17 between the two to provide secondary containment capability. Still another function of the closed space 17 is to provide a means to monitor for possible tank or jacket leaks as further discussed below.

A preferred separating agent, shown in FIGS.

1 and 2, is a gas pervious material 20. Such materials

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6 are foraminous or porous and can take on various physical shapes and structures. Examples of such materials are mattings, nets, screens, and meshes.

Specific examples are jute, ?olyurethane foam, polyester foam, fiberglass matting, cotton matting, nylon matting, corrugated cardboard, paper and an absorbent-type material such as asbestos. A heat seal or sealing material, e.g. a polymeric coating or film such as Mylar or a polyethylene, is used on one surface of the gas pervious material when needed to prevent substantial saturation by a subsequently applied o resinous material as discussed in the immediately following paragraphs.

S. Another separating agent is a wax material 15 which is capable of subsequently being separated from the jacket as by heating and optionally removed so as to destroy any adhesion between the jacket and the underlying inner tank. A closed space is consequently formed. Another is a solid material which acts as a 20 separating agent as well as a corrosion inhibiting agent, e.g. grease.

Still another separating agent is a sheet material which may have surface irregularities placed towards the inner tank wall. A wax coated pape 1 is one 25 example. A porous standoff material which is placed on the tank wall and then sealed with a resinous material or wrapped with a solid material such as tape is another example. Sheets or rolls of fiberglass reinforced resin or metal can also be utilized as a separating agent. Paper which is preferably permeable to the liquid stored in the tank system and water, but not to the resinous material used to form the jacket is also usable.

The jacket 21 is made of a fibrous reinforced reainous material. It is formed in one method directly onto the gas pervious material 20 found on the storage tank 11. The fibrous reinforcing material can take on many different physical shapes and structures variously referred to as mattings, nets, screens, meshes, filament winding strands, and chopped strands.

Examples of fibrous materials include fiberglass, nylon, and other synthetic fibrous materials. The fibrous material, if in a sheet form, can be laid onto the storage tank as a continuous matting.

In one embodiment of the invention, though of lesser preference, the fibrous reinforcing material is first applied, next a resinous material is applied to the reinforcing material, and thereafter cured. In this method, the fibrous reinforcing material can serve 9*9* as a separating material and as part of the jacket, provided complete resin penetration is avoided.

Several different resinous materials are known for the purpose of reinforcing fibrous material. Such *a.

materials include polyesters, e.g. vinylesters, isophthalic polyesters, polyethylene, polyurethane, and polyepoxide. The listed resinous materials used in the 20 construction of this jacket are not all inclusive, but only illustrative of some of the resinous materials which can be used.

As an alternative, and in fact preferably, the fibrous material is applied in the form of chopped S 25 strands with the resiious materials described in the s. previous paragraph. That is, the chopped strand and resinous material are sprayed from separate nozzles of the same spray gun and the jacket formed therefrom on the separating agent as the resin cures. Still another method of forming the jacket uses filament windings.

Continuous reinforcing fibrous strands are impregnated with the resinous material and then wrapped around the separating material-covered inner tank in a crossing pattern. Other known methods of forming a fibrous reinforced resin substrate can be used.

The shape of the resultant jacket 21 is such that it encases the main body 12 and flattened end caps 8 13 of the rigid inner storage tank 11 to form the closed space 17. The jacket formed around the cylindrical part of the tank is preferably less than about millimeters (mm) from the inner tank, more preferably from about 1 mm to about 10 mm. The jacket end caps 22 are preferably less than about 25 mm from the inner tank's flattened end caps 13, more preferably from about 1 mm to less than about 10 mm thereby allowing enough space for free flow of a leak detecting fluid.

The jacket itself is capable of containing any liquid which is stored in the storage tank and which has leaked therefrom. The strength of the jacket has sufficient structural integrity to withstand external or internal load forces normally encountered by underground storage 15 tanks without suffering cracking or collapsing.

In accord with this invention, the end caps 22 of the jacket 21 are substantially rigidly attached to the flattened end caps 13 of the inner tank 11.

They are attached so as to keep the jacket end caps and 20 the inner tank end caps in close structural relationships to one another, free of substantial deflection. Further, the respective end caps are attached together at multiple points tP ensure that they retain the same structural relationship throughout 25 and create a structurally strengthened jacket end cap area. The multiple points of attachment are accomplished with multiple attachment members uniformly spaced or randomly spaced with a greater number where greater stress forces are likely. Deflection in any part of the end cap areas of the double wall storage tank system 10 is undesirable because of the likelihood of creating jacket cracks and the likelihood of false leak detecting fluid readings as further discussed below.

The double wall tank system of FIG. 1 has threaded rod studs as attachment members secured to the flattened end caps 13 of the inner storage tank and the flattened end caps 22 of the jacket 21. The rod studs 9 23 are randomly spaced in a manner whereby the jacket's end caps are not able to substantially deflect away from the inner tank end caps in any part thereof once secured together. A greater concentration of the studs 23 are in the lower half of the end cap areas where greater leak detecting fluid forces are experienced.

The randomly spaced threaded rod studs 23 are initially welded to the end caps 13. The studs have a diameter of from about I) mm to about 25 mm and a height of from about 5 mm to about 10 mm. Preferably they are spaced on about 150 mm to about 300 mm centers, with the ocloser on-center spacings in the lower half. Once secured to the flattened end caps 13, a separating o.* agent is applied to the surrounding end cap areas to ensure that the subsequently formed jacket only adheres to the rod studs. The jacket adheres primarily to the threaded side wall area of the rod studs, but also to the end area thereof. In effect, the rod studs are embedded in the jacket end caps.

20 FIGS. 3 and 4 illustrate another set of attachment members used to obtain the substantially rigid attachment. Circular steel bars 25 hvre an about 10 mm to about 25 mm diameter and lengths which extend substantially the width or length of the flattened end 25 caps 13 where welded thereto. As shown, the bars are welded in a staggered generally horizontal manner to the end caps 13. The greater forces from the leak detecting fluid are experienced in the lower half of the end cap area. Accordingly, a greater number of bars are used in this lower area to obtain greater strength where needed. The bars are welded so as to be on about 100 mm to about 400 mm centers, though not necessarily all the same distance. Next, a fibrous reinforcing material 26 is placed over the bars and end caps. Finally, resinous material is applied to form the jacket end caps 27. A mechanical bonding is effected around the circular steel bars to form the

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10 substantially rigid attachment. Areas between the bars are not attached to the inner tank's flattened end caps, thereby allowing a free flow of de"ecting fluid as further discussed below. The bars 25 are solid, though hollow bars are also usable.

FIGS. 5 and 6 show the use of flattened wire mesh lath 30 as the attachment members. As shown, the lath covers substantially the full area of the end cap.

For economy of construction, strips of lath can be used, preferably more in the lower half of the flattened end cap where the greater forces occur which can cause structural damage. The lath is initially welded at maltiple points to the flat end capy 13 of the inner tank 11. Next, fibrous reinforcing material 15 31 is laid up and, finally, resinous material applied.

The fibrous and resinous materials attach to the wire mesh lath, but do not fully penetrate through it. A substantially rigid attachment is secured between the inner tank's end caps 13 and the formed jacket's end 20 caps 32, yet detecting fluid is free to flow between the two.

Still other attachment members used in the invention are illustrated in FIGS. 7 and 8. Angle irons 35 are welded to the end caps 13 of the inner S: 25 tank 11 initially. They are welded such that a flat side is substantially parallel with the end caps. As such a set of flat surfaces are presented on which the jacket's end caps can be formed. A separating material such a paper 36 is positioned over areas of the inner tank's end caps prior to formation of the jacket end caps 37 to ensure such areas remains unattached and permit free flow of the detecting fluid. The angle irons have about 5 mm to about 20 mm legs and are attached on about 100 mm to about 400 mm centers.

A particularly desired benefit of the double wall storage tank system of the invention is that the system can have added at the factory a leak detecting 11 fluid, particularly a liquid, in the closed space between the inner tank and jacket. Known prior art double walled tank systems with flattened end caps are not able to withstand the added forces created by the detecting fluid unless buried in the ground and adequately back-filled. The capability of adding the detecting fluid in the factory provides several benefits. Any manufacturing defect is detected before the tank system leaves the plant and can possibly be repaired. Any damage to the tank system during transportation or installation can also be detected prior to on-site burial and/or use. In all instances, Si. the environment is spared any damage from leaked stored liquid such as the gasoline. Additionally, costs associated with repairing a defective tank is minimized if any defect is detected in tho plant or at least prior to burial at the site.

The space between the jacket 21 and the inner storage tank 11 is monitored. As shown in FIG. 1, an 20 access tube 40 extends through the jacket so as to be in communication with the closed space. Any of well known and commercially available monitor means ale Sused. It is highly preferred that the closed space be filled with a detecting liquid such as a 25 environmentally acceptable viscous liquid. This detecting liquid can be placed in the closed space by the manufacturer of the tank due to the fact the closed space between the storage tank and jacket occupies a small volume, e.g. about 25-100 gallons detecting liquid is sufficient for use with a 20,000 gallon storage tank and the flattened end caps areas have the enhanced strength to withstand the increased forces.

Adding a dye to the detecting liquid further helps to pin-point a defective wall area. Preferably, the 3S viscous detecting liquid has a viscosity of from about 100 centipoises to about 10,000 centipoises. A sight glass can be put at the end of the access tube.

12 Whenever leakage occurs, a change in the level or color of a detecting liquid will occur and will be readily observed L. the sight glass. Instead of the sight glass and visual observation of a change in level or color of detecting liquid, non-visual leak detection means such as pressure transducers or float controls can be used to detect a change in level.

Another embodiment of the detection means utilizes an analyzer capable of detecting the liquid being stored. Thus, the detection means comprises the analyzer which is in direct communication with the closed space. Alternatively, a vacuum means for withdrawing liquid material from the closed spaced is used for the purpose of obtaining a sample. Thus, an 15 analyzer capable of detecting selected liquids is used instead of a pressure change sensor.

Still another detection means utilizes a probe which extends through an access tube so as to monitor for leakage, preferably at or near the bottom 20 of the cl~red space. The probe is capable of detecting pre-selected liquids or gases. In this embodiment, the formed closed space directs any leakage so as to ultimately seep to the bottom of the closed space and be detected. If a solid material which is soluble in 25 the stored liquid-, gasoline-soluble or watersoluble is used as a separating agent, it will ultimately be solubilized by the leaked liquid and the leakage detected by the probe.

Non-atmospheric air pressure can be maintained in the closed space and be monitored by changing pressure. Thus, the detecting fluid is a gas under vacuum or pressure and the leak detection means is a pressure change sensor.

All the leak detection means discussed above can be electronically linked with an alarm system to audibly or visually warn of a pre-set significant change in the closed spaces. The leak detection means 13 and secondary containment means allow for an early warning of a deterioration of either the primary or secondary concainment means thereby permitting the necessary repair work to be done before any significant soil or water contamination has occurred.

Again with reference to FIG. 1, a double wall storage tank system 10 is shown with an expansion chamber 41. The expansion chamber 41 which is positioned over the tank system is in communication with the closed space through the access tube 40. The detecting liquid in the closed space typically experiences temperature variations which cause it to expand and contract. The expansion chamber 41 *oo accommodates this liquid volume change and has the 15 effect the relieving a force build-up in the closed chamber which could cause damage to either the inner tank or jacket. The expansion chamber has sufficient capacity to handle the leak detecting liquid volume change. Generally, an about one to about twenty-five 20 gallon chamber is sufficient.

The added advantage of the double storage tank system of the invention is that the aforesaid reasonably sized expansion chamber is adequate for its intended function. Those systems where substantial end 25 cap area deflections are experienced require larger capacity expansion chambers to be effective. This in and of itself causes an added expense, but also more leak detecting liquid is needed. This in turn creates even greater hydrostatic pressure and more stress in the end cap areas.

The invention herein has been described with particular reference to the drawings. It should be understood other variations of the invention are within the scope of coverage. For example, inner storage tanks with a manway are useful herein. The manway is used to accommodate the various access lines, including a line for leak detection purposes.

Claims (11)

1. A double wall storage tank system for liquids which is ca&able of holding detecting fluid in a closed space for leak monitoring purposes, comprising: a rigid inner tank having a cylindrical shaped main body ad substantially flattened end caps for storing the liquid; a jacket having a cylindrical-shaped main body and end caps which are made of a fibrous reinforced re- sinous material and which covers a surface area of the inner tank to form the closed space., wherein the end caps of the jacket cover the flattened end caps of the rigid inner tank; and multiple attachment members which substantially rigidly attach the end caps of the jacket directly to the rubstantially flattened end caps of the inner tank to ensure that the respective end caps are free of substantial de- flection.

2. The double wall storage tank system of claim 1, wherein the rigid inner tank is a metal tank. S

3. The double wall storage tank system of claim 1, wherein che rigid inner tank is a fibrous reinforced resinous tank.

4. The double wall storage tank system of claim 1, wherein each end cap of the jacket is attached by multiple S attachment members to each flattened end cap of the rigid inner tank. S"

5. The double wall storage tank system of claim 1, further comprising a separating agent on a main wall of the rigid inner tank to make the jacket structurally independent Sthereof.

6. The double wall storage tank system of claim wherein the separating agent is paper.

7. The double wall storage tank system of claim wherein the separating agent is a gas pervious material.

8. The double wall storage tank system of claim 1, further comprising a leak detecting fluid in the closed spaced between the rigid inner storage tank and the jacket. 15

9. The double wall storage tank system of claim 8, wherein the leak detecting fluid is a viscous liquid having a viscosity of from about 100 centipoises to about 1,000 centipoises.

The double wall storage tank system of claim 9, further comprising a leak detection means in communication with the closed space.

11. A double wall storage tank system for liquids which is capable of holding detecting fluid in a closed space for leak monitoring purposes substantially as hereinbefore described with reference to the accompanying drawings. *0 0 0 0 *e *'a *eo o• **o o ooo o [n;\liblll00709CJS Doubl2 Wall Storage Tank Systems with Rigidly Attached End Cap ABSTRACT A double wall storage tank system (10) for liquids comprises a cylindrical-shaped rigid inner tank (11) with substantially flattened end caps (13) and an outer jacket (21) formed from a fibrous reinforced resinous material. The jacket (21) is substantially rigidly attached (23) to the flattened ends (13) of the inner tank (11) in a manner which prevents a substantial deflection of the jacket (21) in the end cap areas (22) yet allows a free flow of leak detecting fluid therebetween. A closed space (17) between the inner 'tank (11) and jacket (21) is capable of being fille' with the leak detecting fluid and being reliably monitored to detect a source of leakage in the Inner tank (11) or jacket (21). Figure 1 *.i 0816T/CMS