Poly-steel double wall tank

An improved double wall tank has two or more threaded fittings, with a steel outer tank and a thermoplastic inner tank. The inner tank is formed within the outer tank by rotational molding. The inner tank is bonded to the outer tank in an area of the fittings, but is otherwise separate and apart from the outer tank. Before carrying out the rotational molding process, bonding material is applied to the outer tank in the area of the fittings. The thermoplastic material is in powder form and includes mold release material. The thermoplastic material is inserted into the outer tank before rotational molding. The degree of shrinkage of the inner tank can be controlled by air pressure including an area between the fittings.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to an improved double wall tank having an outer tank of steel and an inner tank of thermoplastic material. More particularly, this invention has an outer tank with at least two fittings thereon and the inner tank is separate and apart from the outer tank except that it is bonded to outer tank in the area of the fittings.

2. Description of the Prior Art

It is known to have outer steel tanks with inner tanks made from thermoplastic material. The inner tank is formed by rotation molding within the outer tank and separates from the outer tank when the tank cools (see U.S. Pat. No. 4,625,892). When the inner tank cools, the thermoplastic material shrinks. In tanks that have two or more fittings, as the inner tank shrinks, that portion of the inner tank wall located between the two fittings is placed under enormous stress. The stress can cause the inner tank to rupture or to fail prematurely at the corners where the inner tank bends into the flanges for the fittings.

It is also known to line a tank with an inner layer of polymeric material where the polymeric material is bonded to an inner surface of the tank (see U.S. Pat. No. 5,358,682). An advantage of a double wall tank where there is a space between the inner tank and the outer tank is that a leak detection system can monitor the space between the two walls.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an outer tank of steel having an inner tank of thermoplastic material where the outer tank has at least two fittings and the bonding of the inner tank to the outer tank can be controlled so that the inner tank is bonded to the outer tank in the area of the fittings that is otherwise separate and apart from the outer tank.

A double wall tank has an outer tank of steel and an inner tank of thermoplastic material. The outer tank has an inner surface having first areas and a second area. One of the first areas is located adjacent to each of at least two fittings that are separate from one another. The second area is a remainder of the inner surface that is not the first areas. The thermoplastic material is bonded to the outer tank in the first areas and separate from the outer tank in the second area.

A method of constructing a double wall tank having an outer tank of steel and an inner tank of thermoplastic material, said outer tank having an inner surface with first areas and a second area, one of said first areas being located adjacent to each of at least two fittings that are separate and apart from one another, said outer tank having an opening at each fitting, said second area being a remainder of said inner surface that is not said first areas, said method comprising applying bonding material to said first areas, inserting said thermoplastic material in powder form into said outer tank through an opening for a fitting, said powder including mold release material, inserting a plug into each opening, subjecting the outer tank to heat and rotating the outer tank to mold said inner tank within said outer tank, cooling said tanks, resulting in an inner wall of said thermoplastic material that is bonded to said first areas and separated from said second area, removing each plug and cutting an opening through said thermoplastic material at each opening.

DESCRIPTION OF A PREFERRED EMBODIMENT

InFIG. 1, there is shown a prior art double wall tank2where an outer steel tank4has an inner thermoplastic tank6. The tank2has two flanges8and two flange covers10. One flange cover10covers each tank opening12. The tank2is manufactured by rotational molding to mold the inner tank within the outer tank. The outer tank is heated during the molding process causing the inner tank to form on an inner surface14of the outer tank4. There is a space16between the two tanks4,6where leak detection equipment (not shown) can monitor for leaks. When the two tanks are cooled either as part of or after the molding process, the inner tank6shrinks relative to the outer tank4, thereby creating the space between the two tanks. The space and shrinkage may be controlled by the addition of air pressure into the inner tank during the cooling process. The shrinkage causes stress on the inner tank junctures18,20, often resulting in stress cracking of the inner tank in an area22between the junctures18,20. When threaded fittings are required with the prior art tank, they are supplied by attachment to each of the flange covers10. Air pressure cannot alleviate the stress at junctures18,20. Air pressure cannot be used to exert longitudinal pressure on the inner tank in the area between the flanges because the inner tank contains an opening at each flange.

InFIG. 2there is shown a double wall tank24having an outer tank26and an inner tank28with an interstice30located between the two tanks26,28. The interstice can be used (usually by creating a vacuum) to monitor the inner tank for leaks. The tank24has two fittings32. It can be seen that the inner tank28is affixed to a cover34in two first areas36adjacent to the fittings32. The second area is a remainder of an inner surface38of the outer tank26.

InFIG. 3, there is shown a sectional end view of the tank24. The same reference numerals are used inFIG. 3as those used inFIG. 2for those components that are identical.

InFIG. 4, there is shown a partial sectional view of the tank24in the area of one of the fittings32. The same reference numerals are used inFIG. 4as those used inFIG. 2to describe those components that are identical to one another. Tank24has an opening40that is substantially closed off by cover42, which is welded to the tank by welds44. Similarly, the fittings32are welded to the cover42by welds45. The cover42has a hole (not shown) therein to allow access through a corresponding opening (not shown) in the fitting32. A bonding agent46is applied to an inner surface of the cover42. The thickness of the bonding agent46is exaggerated for purposes of illustration.

It can be seen that the inner tank28is bonded to the cover42in the area where the bonding agent46has been applied (one of the first areas) and separated from the outer tank26by an interstice30in the remaining circumference of the tank24.

InFIG. 5, the cover42has been omitted and the fitting32is welded directly to the outer tank26. The bonding agent46is coated onto an inner surface48of the tank24and the inner tank28is bonded to the inner wall48of the outer tank26in the area of the fitting32, but separated from the inner wall48through the remaining area of the tank24. InFIG. 5, the fitting32covers an opening (not shown) in the tank wall48.

InFIG. 6, there is shown a sectional view of the tank24through the fitting32. It can be seen that the fitting32has screw threads50in an opening52that contains a plug54. Unthreaded flanged fittings may also be used in place of the threaded fittings. An interior surface56of the plug54is coated with bond release material58. The thickness of the bond release material is exaggerated for ease of illustration. Those components ofFIG. 6that are identical to the components ofFIG. 4are described using the same reference numeral. The opening52extends through the cover42.

During manufacture, the opening52of at least one of the fittings32is open and thermoplastic material in powder form is inserted into the outer tank26through the opening52. At the same time, bonding agent46is coated onto an interior surface of the cover42(or, if there is no cover, the bonding agent is coated onto an inner surface60of the tank around the fitting32). A breather pipe is installed into one opening in the tank to vent off gases formed during the rotational molding process. All other openings are then closed off by plugs54preferably containing mold release material58on the inner end56thereof. Preferably, the plugs are made from teflon or are teflon coated.

The outer tank26is then rotationally molded and heated during the rotational molding process to melt the thermoplastic powder and coat the inner surface48of the outer tank26with thermoplastic material.

As a result of the molding process, after the inner tank26and outer tank28are cooled the inner tank28falls away from the outer tank26in all areas except for those areas that were coated with bonding agent. The bonding agent is not affixed to the tank on the interior surface of the plug because of the presence of the bond release material58. The plug is then removed from the opening52and a hole is cut through the thermoplastic material located directly beneath the opening52. The tank24is then ready for use, permitting the attachment of threaded piping directly to the threaded fittings of the tank.

Meanwhile many thermoplastic materials are suitable for the inner tank, the most commonly used materials are polypropylene, nylon (a trade mark) and polyethylene. Usually, the tank openings will be located along a top of the tank, but openings can be located at any convenient location in the outer tank wall. InFIG. 7, there is shown a portal sectional view of an outer tank62having a flanged fitting64. Those components ofFIG. 7that are identical to the components ofFIG. 6are described using the same reference numeral. The flange fitting64is preferably coated with bonding agent46within the fitting64from an outer point66to an inner point68around the flanged fitting64. This causes the inner part of the flanged fitting64to be coated with thermoplastic material70that is part of the inner tank28. It should be noted that the bonding agent46coats an inner corner72of the flanged fitting64to eliminate the corner72as a high stress point when the inner tank cools and separates from the outer tank62. While shrinkage of the inner tank during cooling can be controlled by controlling internal gas pressure within the tank28, shrinkage cannot be easily controlled in the area between two flanged fittings. However, by bonding the inner tank to the outer tank within the flanged fittings and beyond the corner72the risk of the inner tank failing prematurely because of stress caused by shrinkage is greatly reduced.