What is claimed is
1. In a tank for storage of a material at superatmospheric internal pressure, which tank is subject to distortion by such internal pressure and, to substantially prevent such distortion, is anchored about its periphery to foundation means by anchoring means, which anchoring means are subject to failure at a predetermined temperature condition, the improvement comprising:
2. A tank according to claim 1 in which said sensor means is responsive to a temperature condition of less than 300°F.
3. A tank according to claim 1 in which the tank has a flat bottom and cylindrical vertical wall and said anchoring means comprises bolts positioned about the exterior bottom portion of the tank.
4. A tank according to claim 1 in which said sensor means comprises a fusible material.
5. In an enclosed insulated tank having a metal shell with a metal bottom, wall and roof, for storing a flammable liquified gas at superatmospheric internal pressure, said tank being subject to distortion by such internal pressure and, to substantially prevent such distortion, being anchored by metal anchoring means to a supporting foundation, said anchoring means being subject to failure at a predetermined temperature condition, the improvement comprising:
6. A tank according to claim 5 wherein the tank has a flat bottom and said anchoring means comprises a plurality of bolts.
7. A tank according to claim 6 wherein said gas conduit means extends around the tank adjacent to said bolts and includes a plurality of conduit portions of thermally sensitive material adjacent each bolt.
8. A tank according to claim 5 wherein said thermally sensitive material is a metal which melts at above 150°F.
This invention relates to tanks for storing materials. More particularly, this invention is concerned with improvements in storage tanks for liquids which depressurize the tanks to maintain structural integrity of the tanks when heated as by fire.
Many materials are stored in tanks which are often made of metal. Thus, metal tanks are widely used to store fluid products such as gasoline, fuel oil, benzene and alcohol as well as liquefied gases such as methane, propane, butane and ethylene. The storage of some of these materials requires, as a practical matter, that they be stored under superatmospheric pressure. Natural gas, i.e., methane, is commonly stored liquefied at -260°F. and 0.5 to 5 psig.
The type of tank used to store a fluid product depends on the chemical and physical characteristics of the product. It is, of course, desirable to employ the least expensive and least complicated tank construction consistent with safety and usefulness. It is accordingly conventional to employ a tank having a flat bottom, vertical cylindrical wall and a conical or domed roof. Such a tank is comparatively easy to fabricate and erect. It can also be readily insulated to reduce heat transfer to a fluid stored therein. Tanks of this general type are shown in Waugh U.S. Pat. No. 3,338,010 and Sattelberg et al. U.S. Pat No. 3,352,443. In addition, an internal tank consisting of a flat bottom and cylindrical wall can be placed therein to store the fluid therein and insulation placed between the inner and outer tank or shell bottoms and walls. The inner tank or shell can also be provided with a roof and insulation placed between it and the outer roof. Alternatively, an insulated ceiling can be suspended from inside the roof of the outer tank or shell at a level near the top of the inner tank or shell.
Flat bottomed tanks of the generally described types must be anchored to a supporting foundation to store a material at superatmospheric pressure. Suitable anchor means is required because superatmospheric pressure in the tank will cause the bottom to dish outwardly unless restrained and, as a result, the outer part of the bottom and the tank wall will rise. This could lead to fracture or rupture of the shell and any structural elements joined to the tank.
The anchor means is usually located about the periphery of the tank wall lower portion and extends from there to the foundation. Regardless of whether a double shell tank or single shell tank is involved, the anchor means is used to secure that shell to the foundation which is subjected to superatmospheric pressure internally. The anchor means quite commonly used consists of bolts which extend from the foundation to brackets on the tank wall to which they are secured by nuts. The bolts, or whatever other anchoring means is used, are designed to hold the tank firmly in place on the foundation under normal conditions. Fire on the ground or near ground level around a tank is a particular hazard if a flammable liquid is stored in the tank. Flammable liquid can be spilled around the tank due to a leak in a pipe, valve, pump or other equipment. If the anchoring means is subjected to high temperature, such as caused by a fire, it could rupture. Steel bolts, for example, rapidly lose their strength when heated. Above 600° F. they start to lose strength, at 1,000°F. they are seriously weakened and at 1,600°F. they have very little strength. Failure of the anchoring means while the tank is under superatmospheric internal pressure will lead to upward displacement of the outer edge of the tank bottom. Distortion of the tank and related piping can result in fractures and leaking of flammable liquid from the tank. Such leaking could not be stopped so the entire tank contents would drain into the fire area. A safety system is accordingly needed which will prevent the edge of the tank bottom from raising up when and if the anchoring means fails.
According to the present invention, there is provided an improvement in a tank for storage of a product or material at superatmospheric pressure which effects internal pressure relief, and thereby prevents the tank from rising off the foundation, when the anchor means is subjected to heat which causes failure of, or a serious loss of strength in, all or part of the anchor means.
More specifically, the invention provides the improvement in a storage tank for storing a material or product at an internal pressure above atmospheric pressure and which tank is anchored about the periphery thereof to a foundation means, comprising a thermally responsive sensor means positioned in proximity to means anchoring the tank to the foundation, a pressure relief valve mounted on the tank and communicating with the interior thereof, and means actuated by the thermally responsive sensor means upon application of heat to the thermally responsive sensor to open the pressure relief valve and thereby reduce the pressure therein. With the pressure thus reduced, such as to atmospheric pressure or some other suitable pressure, the tank will remain on the foundation even when the anchor means has failed. Of course, before the tank can be again put into normal service, it is essential that the anchor means be repaired or rebuilt.
The invention will be described further in conjunction with the attached drawings, in which:
FIG. 1 is a vertical sectional broken away view of a cylindrical storage tank embodying the improvement of this invention;
FIG. 2 is a vertical sectional view of the bottom edge part of the tank of FIG. 1;
FIG. 3 is a vertical sectional view of the pressure relief valve on the roof of the tank of FIG. 1;
FIG. 4 is an enlarged sectional view showing a fusible plug in the end of a nipple; and
FIG. 5 is a vertical sectional view of another pressure relief valve which can be used in the invention.
So far as is practical, the same numbers will be used in the various figures of the drawings to identify the same elements or parts.
With reference to FIGS. 1 and 2, the tank 10 has an outer metal shell comprising a flat bottom 11, a vertical cylindrical (circular) wall 12 joined to bottom 11 and a domed roof 13 supported on the upper edge of wall 12. The tank is supported about its periphery by concrete foundation 14 which is embedded in the earth.
Positioned around and welded to the exterior lower portion of wall 12 is a series of brackets 15 having a horizontal flange 16 and a vertical web 17 beneath the flange. A plurality of L-bolts 18 is positioned around the tank with the L end of the bolts embedded in concrete foundation 14. The threaded upper end of each of bolts 18 passes through a hole in bracket flange 16 and a nut 19 is threaded thereon. The combination of bolts 18, brackets 15 and nuts 19 comprise one embodiment of an anchor means for holding the tank down on foundation 14.
Tank 10 has an inner shell comprising metal bottom 20 and a cylindrical (circular) wall 21. Bottom 20 is supported by load bearing insulation 22. Insulation 23 is placed between inner wall 21 and outer wall 12. The inner shell is open at the top and insulated ceiling 24 is suspended by rods 25 from ribs 44 reinforcing the inside of roof 13 and is located below the top edge 26, and inside, of inner wall 21. Insulated space 27 is thereby provided in the inner shell for storage of a product such as liquefied natural gas at superatmospheric pressure. Conduit 28 is used to fill and empty the tank. It can be closed by valve 43. Conduit 28 is joined to inner wall 21 in a fluid tight manner but it extends through an enlarged opening in outer wall 12. A fluid tight seal between outer wall 12, around the enlarged opening, and conduit 28 is effected by expansion bellows 29. This permits movement of inner wall 21 relative to outer wall 12 without placing any undue stress on the conduit.
As shown in FIGS. 1 and 3, relief valve 30 is mounted on top of roof 13 and it is joined to conduit 31 which communicates with the inside of the tank. Pallet or membrane 32 presses against the seat ring 33 when the valve is closed and thereby prevents venting of gas or vapor through exit 34. Orifice 35 (FIG. 3) in pallet 32 provides for gas to pass by the pallet and exert a downward pressure on the pallet to close the valve when flow of gas out through conduit 36 is blocked.
Referring to FIG. 2, conduit 36 extends from relief valve 30 down to the lower portion of tank wall 12 to adjacent the anchor means comprising the bolts 18 and their associated elements. Conduit ring 37 extends around the tank wall 12 and is in fluid communication with conduit 36. Conduit ring 37 is supported by brackets 38 on tank wall 12. Positioned periodically in conduit ring 37 are short nipples 39 (FIGS. 2 and 4) which contain fusible metal plugs 42 therein which melt at a comparatively low temperature, but substantially above ambient temperature. The fusible metal plugs 42 are placed in relatively close proximity to, but pointed away from, the bolts 18 so that each will be simultaneously exposed to any source of heat around the base of the tank, such as a fire. By pointing the nipples away from the bolts, heat from burning gas which escapes from the open nipples is not directed to the bolts.
Melting of the fusible metal plugs 42, such as by fire, causes the metal to flow out of the nipples 39 and results in openings through which gas or vapor from the tank will exit. This causes a reduction in the pressure on the top side of pallet 32, the pallet is lifted by the higher pressure applied on its bottom side and the relief valve is thereby opened to permit gas or vapor to escape from the tank to lower the internal pressure. With the internal pressure of the tank lowered to an acceptable pressure, the bottom of the tank will not rise from the foundation even though bolts 18 weaken or fail through heat.
The described system is suitably employed even with a tank used for storing a flammable liquid and even a liquefied gas such as liquefied natural gas. The small amount of flammable vapor or gas which exits through the openings which result from melting of the fusible metal plugs will burn if a fire is present around the tank but this can be readily tolerated and will not affect the structural integrity of the tank so long as the flame and heat are directed away from the bolts.
Conduit 40 communicates with the lower end portion of conduit 36 and valve 41. Valve 41 is placed in a location somewhat remote from tank 10 so that it can be safely manually operated even when a fire is burning around the tank. By opening valve 41, the relief valve 30 is caused to open even if the metal plugs 42 in nipples 39 are not melted. This potential manual operation provides a supplemental or auxiliary means to open the relief valve when desired.
It is also contemplated, and within the scope of the invention to eliminate hole 35 in pallet 32 and to pressurize the top side of the pallet by means of an auxiliary or external fluid supply which for example could be air, nitrogen or some other gas, or even a liquid under pressure. One such system is shown in FIG. 5.
With reference to FIG. 5, the pressure relief valve 50 is similar in many respects to the pressure relief valve 30 and those parts or elements which are the same have the same identifying number. The relief valve 50 in FIG. 5 has pallet 51 which rests on seat ring 33 when the valve is closed. Pallet 51 contains no hole like hole 35 in pallet 32 in the valve 30 in FIG. 3. Pallet 51 is forced downwardly against seat ring 33 by nitrogen under pressure supplied from tank 52 through conduit 53 to the space above the pallet and also through conduit 36 to fusible plugs 42. When plugs 42 melt and create openings in nipples 39, the nitrogen escapes and causes the pressure above pallet 51 to be reduced below the pressure exerted on the lower side of the pallet by the vapor in the storage tank 10. The vapor in the tank thus can escape. By presenting the vapor pressure in the tank from becoming excessive, the tank is prevented from raising at its perimeter off the foundation. Obviously, some other gas than nitrogen can be stored in tank 52 to provide the necessary pressure.
Some fusible alloys which can be used for plugs 42 have the following compositions and melting points, with the percents being by weight:
1. 12.5 percent tin, 50 percent bismuth, 25 percent lead and 12.5 percent cadmium; m.p. 162°F. (Wood's metal)
2. 20 percent tin, 50 percent bismuth and 30 percent lead; m.p. 212°F.
3. 34.5 percent tin, 44.5 percent bismuth and 21 percent cadmium; m.p. 248°F.
4. 48.8 percent tin, 10.2 percent bismuth and 41 percent lead; m.p. 331°F.
5. 14.5 percent tin, 48 percent bismuth, 28.5 percent lead and 9 percent antimony; m.p. 440°F.
In general, a fusible material is advisably used which has a melting point above about 150°F.
In place of the fusible metal plug 42, part or all of conduit ring 37, or nipples 39, can be made of a low melting metal or organic plastic material which will melt, disintegrate or burn to provide the necessary opening to actuate the relief valve as previously described. Other thermally responsive sensor means can also be used if desired. The particular sensor means selected, however, should be one which is reliable and be capable of operating after the tank is in service for a long time. It is necessary, however, that the relief valve and sensor means be capable of operating cooperatively to achieve the desired purposes of the invention.
The safety system provided herewith can be employed with single shell tanks or double shell tanks so long as the sensor means is positioned to be responsive to heat which might affect the tank anchor means. Electrical thermostatic controls which, by means of ancillary equipment, serve to open a relief valve can also be used and this type of system is also intended to be included in the scope of the invention.
Various changes and modifications of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.