WO1996010155A1

WO1996010155A1 - Metallic tanks with inner liner for the storage of liquids under pressure

Info

Publication number: WO1996010155A1
Application number: PCT/GR1995/000018
Authority: WO
Inventors: Christos Fyrogenis
Original assignee: Christos Fyrogenis
Priority date: 1994-09-29
Filing date: 1995-09-28
Publication date: 1996-04-04
Also published as: AU3530995A

Abstract

A tank for the storage of liquids under pressure, comprising an inner liner (1) from polymer material to contain the liquid, and a metallic outer body (2, 3), with the metallic outer body (2, 3) enclosing the inner liner (1), whereby the space between the inner liner and the metallic outer body is filled with fluid to transfer the pressure of the stored liquid, acting in the interior of the inner liner, to the interior of the metallic outer body. These tanks form a new product which can have many applications, and as an example only reference is made to the storage water heaters (solar, electric, etc.). Where the need of the corrosion protection of the tanks is satisfied by the use of the internal liner from polymer material which is resistant to corrosion, while the need for strength to hydraulic pressure is satisfied by the tank itself (usually made out of steel), and this combination has important economical and technical advantages.

Description

Metallic tanks with inner liner for the storage of liquids under pressure.

The invention relates to pressure tanks for heating or/and storing materials in liquid form. Such tanks are used for storage or heating or other treatment (relevant uses) of liquids of any nature, with applications in e.g. storage water heaters, solar domestic hot water systems, etc.

The corrosion protection of storage water heaters is achieved today by applying one of the following methods :

- galvanising or electroplating

- glass enamelling - thermosetting resins or thermoplastic materials or other polymer materials either in liquid phase or in the form of powder (the process of coating takes place in the ambient temperature or in ovens) where the protective polymers adhere on the inner side of the tank.

- metal lining made out of thin sheets of copper or stainless steel or other material.

- copper or stainless steel or other materials which can resist the corrosive action of domestic water.

These methods have been proved to be in some cases expensive, and so they increase the cost of the products, and in some other cases they do no offer complete protection either because of their nature or due to lack of the required technical experience for the specific application (incomplete knowledge, cost, etc.).

In cases of coating with thermoplastic or other materials, the problem of formation of blisters on the surface is well known and it is caused by the infiltration of liquid between the surface and the walls of the tank.

The existence of so many methods of corrosion protection of storage water heaters reflects upon the continuous efforts in searching for new solutions for the corrosion problem which is at the same time technical and economic. - Technical, as far as it concerns the design and the construction of a reliable product, with low cost which covers the needs of the consumers for a long time period.

- Economic, because the usage of storage water heaters is rising continuously (new users, replacements) in all forms of applications (electric, solar, use in combination with heating systems, etc.) with direct effect on the economy of any country.

The corrosion protection of storage water heaters mainly concerns the protection of the internal surface of the corresponding tanks which is exposed to the corrosive action and/or electrolytic phenomena, due to the liquids contained in the tanks, leading to tank failure.

There are known tanks for storing and heating liquids with outer and inner walls, whereby a layer of insulating material is provided between the walls, and whereby the pressure acting within the inner liner is transferred to the outer wall by the layer of appropriate insulating material. The use of such constructions is rather recommended for in house use.

An object of the invention is to provide a tank overcoming the problem related to corrosion, which has sufficient strength when it is subjected to pressure, and which is technical flawless and having a low production cost.

The invention is defined in claim 1.

A new type of product, having economic and technical advantages over the known products, is been created in accordance with this Invention, which satisfies the needs for corrosion protection, as a combination of an internal liner made of polymer material which is resistant to corrosion, a metallic tank (usually made out of steel or other material) which can sustain the hydraulic pressure inside the tank and the special fluid (transfer of pressure) between the internal liner and the metallic parts of the tank.

Water or other corrosive liquid, which is stored or circulates inside the storage water heater and which is manufactured in accordance with this invention, does not come in contact with the internal metallic surfaces of the heater because of the existence of the walls of the internal liner made of polymer material which is resistant to the corrosion. This way, any corrosion problems or electrolytical phenomena on the inner surface of the metallic tank are eliminated and perfect corrosion protection is achieved. The special fluid between the internal liner from polymer material and the metallic parts of the tank has anti-corrosion properties. The internal liner from polymer material which is used for the lining of the tank, is constructed in such shape so that its external surface resembles to the inner surface of the metallic parts of the tank. The walls of the liner are not stressed because of the hydraulic pressure of the liquid inside the tank (internal liner is not necessary to withstand the pressure), as this pressure is transferred to the metallic parts of the tank.

Raw materials utilised for the construction of the internal liners, which are used as inner lining of tanks, are various polymer materials, with low, medium or high density and with low, medium or high molecular weight. The selection of the raw material depends on the desirable properties or characteristics of the lining (anticorrosion or antitoxic, etc.) as well as on the cost. As examples only the materials polyethylene, polypropylene, etc. are mentioned.

The tank (usually made of steel or other material) provides the necessary strength and rigidity to sustain the hydraulic pressure inside the tank, without being under the influence of the corrosive action of the liquid inside, which can destroy it.

The internal liner made of polymer material and used for the lining of storage water heaters can be constructed at any desirable thickness and in this way corrosion protection is achieved for long time period.

The internal liner from polymer material which is used for the inner lining of tanks can be constructed by various methods such as : - Blow moulding, where a tube is formed from polymer material with proper diameter and temperature and which is inflated by compressed air either inside a mould or directly inside the tank in order to take the desirable shape. - Rotational moulding, where the polymer material (as a powder or liquid) is placed in a mould, which is heated while being rotated about two perpendicular axes simultaneously and then cooled taking the shape of the cavity of the mould. The previously described methods are associated with production processes of the internal liners that have cost advantages.

The cost and the technical characteristics of the material for the internal liner are such that they allow any desirable thickness of the lining (indicative thickness of

2mm-3mm is mentioned), providing this way reliability and durability to the liner at the required level and so reducing the danger caused by marginal designs which are obligatory when more expensive materials and methods are utilised for the purpose of corrosion protection.

The rather thick walls of the internal liner of this invention, which are used for satisfactory corrosion protection of the storage water heaters, are not only technologically and economically feasible, but at the same time they provide thermal insulation to the tank and so they reduce the energy losses to the environment, a fact that constitutes another advantage of this invention.

In accordance with dependent claim 2 the fluid is non-toxic with food approval.

Claims 3 and 4 define alternative locations of a layer of insulating material.

Description of embodiments of the invention follows, whereby reference is made to the following figures:

Figure 1 shows a storage tank in accordance with the invention.

Figure 2 shows a typical joint of a metallic tank (details of the walls).

Figure 3 shows the assemblage of a flange to the storage tank top (or bottom).

Figure 4 shows a storage tank in accordance with the invention, with thermal insulating material between the inner liner and the metallic tank.

Figure 5 shows a typical joint is the case of using thermal insulation.

In order to emphasise the advantages of the products and the production methods of this invention, the description of the invention will be based on storage water heaters (i.e. stationary appliances for heating water in a tank intended for long term or temporary storage of the heated water and provided with a device or devices to control and/or limit the water temperature), with internal liner made out of high density, high molecular weight polyethylene. Similar procedures, with no important variations, are applied to all types of tanks as well as to various raw materials used to construct the internal liners. In the case of storage water heaters, the invention covers the need for their corrosion protection, which is the most important item of concern for the design of the tank.

The storage water heaters include electric water heaters, solar water heaters, and tanks with heat exchangers which operate in co-operation with the heating system of a house, etc.

Figure 1 shows in general terms the new tank. Details of the walls are shown in Figure 2. The internal liner from polymer material (1) has the required thickness (approximately 2 mm) and domestic hot water is stored or produced inside it.

The outside surface of the internal liner (1) is constructed to resemble the inside surface of the metallic parts of the tank [parts (2) and (3)].

The external metallic tank consists of the cylindrical part (2) and the two ends (3). The ends (3) have circular openings in order to be possible to insert in the tank, when it is necessary, various components (heat exchangers, electric heater, thermostat, pipes for the inlet and outlet of the domestic water, e.t.c). It is quite possible that only one end (3) has circular opening. The three metallic parts [i.e cylinder (2) and ends (3)] are welded at the two locations (4), so that they form a single rigid body.

A protective strip (5) (Figure 2), from appropriate material (i.e. steel, copper, aluminium, e.t.c), is used in the outside surface of the liner, inside a properly shaped cavity, in order to protect the liner from polymer material during the process of welding the ends (3) with the cylindrical part (2) at locations (4), i.e the strip (5) acts as a shield for the protection of the liner from the heat generated during the welding.

The thickness of the cylinder (2) and the end parts (3) is selected so that they [parts (2) and (3)] can sustain the hydraulic pressure of the liquid inside the tank.

The construction of the metallic tank can be made using any other method.

The circular opening of the end (3) (see Figures 1 and 3) is covered by a metallic flange (8) with the use of a sealing ring (7). The flange (8) is assembled to the end (3) with screws (are not shown in the Figures). Flange (8) is protected against corrosion with a liner (9) (Figure 3) from appropriate material (stainless steel, copper, polymer material, e.t.c).

The pressure of the liquids, stored inside the liner from polymer material (1), is transferred to the walls of the metallic tank [i.e parts (2) and (3)], which is designed to have the required strength. The space (clearence) between the liner (1) and the metallic tank [i.e parts (2) and (3)] is filled with a special fluid (6) (Figure 2), which actually transfers the hydraulic pressure. The fluid (6) has anti- corrosion properties in order to avoid corrosion of the internal surface of the metallic tank, while (when it is necessary) it is non-toxic and it is approved for "food use". A proper method is used in order to fill the space between the liner and the metallic tank with the special fluid. Finally, the special fluid (6) is used to test the welds of the metallic tank for leaks.

A layer of thermal insulation, not shown in Figures 1, 2 and 3, covers the outside surface of the metallic tank [parts (2) & (3)]. The thermal Insulation is protected by a layer of material, which covers its external surface, and which can withstand the ambient conditions, (humidity, sun radiation, pollution e.t.c). This protective cover may be constructed of painted mild steel, stainless steel, aluminium, plastic material e.t.c.

A further embodiment of the invention is presented in Figure 4. In this case the thermal insulation is enclosed between the internal liner from polymer material and the outer metallic wall. Furthermore the tank is shown with only one circular opening.

In Figure 4 the liner from polymer material (1) has the required thickness (approximately 2 mm) and domestic hot water is stored or produced inside it. Heat losses are reduced with the use of the thermal insulation (10) which has the required mechanical and thermal insulation properties. The thermal insulation (10) is immersed in the special fluid (6), between the liner and the metallic pressure vessel and it is waterproof, so that its mechanical and the thermal insulation properties are not influenced by the presence of the special fluid (6). The inner surface of the insulation (10) is constructed to fit the external surface of the liner (1) and furthermore the external surface of the thermal insulation (10) is constructed to fit the inner surface of the pressure vessel [parts (2) and (3)]. In Figure 4, the pressure vessel is metallic and consists of the cylindrical part (2) and the two ends (3). Only one end (3) is shown to have a circular opening in order to be possible to insert in the liner various components (heat exchanger, electric heater, thermostat, pipes for the inlet and outlet of the domestic water, e.t.c). It is possible that both ends (3) have circular opening. The three metallic parts [i.e. cylinder (2) and ends (3)] are welded at the two locations (4), so that they form a single rigid body, the pressure vessel.

A protective strip (5) (Figure 5) from appropriate material is used in the outside surface of the insulation, inside a properly shaped cavity, in order to protect the insulation during the process of welding the ends (3) with the cylindrical part (2) at locations (4).

In Figure 4 and 5, the pressure of the liquid stored inside the liner (1) is transferred to the insulation (10) and finally from the insulation to the walls of the metallic pressure vessel [parts (2) and (3)], which is designed to have the required strength. The space between the liner (1) and the metallic tank [i.e. parts (2) and (3)] is filled with a special fluid (6) (Figure 5), which actually transfers the hydraulic pressure, especially at the points where there is no good contact (imperfections) among the surfaces, reducing this way the load on the liner (1). The fluid (6) has anti-corrosion properties in order to avoid corrosion of the internal surface of the metallic pressure vessel, while it is non-toxic and it is approved for "food use". A proper method is used in order to fill initially the space between the chamber and the metallic pressure vessel with the special fluid.

The embodiments described are only examples for carrying out the invention and they do not limit it.

Claims

C L A I M S

1. A tank for the storage of liquids under pressure, comprising an inner liner from polymer material to contain the liquid, and a metallic outer body, with the metallic outer body enclosing the inner liner, characterised in that the space between the inner liner and the metallic outer body is filled with fluid to transfer the pressure of the stored liquid, acting in the interior of the inner liner, to the interior of the metallic outer body.

2. A storage pressure tank as in claim 1 whereby the fluid is non-toxic with food approval.

3. A storage pressure tank in accordance with claim 1 or 2, whereby a layer of thermal insulation covers the outside surface of the metallic tank, and whereby a layer of protective material covers the layer of insulating material.

4. A storage pressure tank as in claim 1 or 2 whereby a layer of insulating material is provided in the space between the inner liner and the metallic outer tank.