NO141483B - INTERIOR THERMAL INSULATION STRUCTURE FOR CONTAINERS WITH LOW TEMPERATURES - Google Patents

Description

The present invention relates to an internal thermal insulation structure for containers with low temperatures containing liquids with low temperatures, comprising several layers of insulation whose outer surface is attached to the container wall, and a reinforcement in the form of a mesh-shaped material.

Typically, rigid polyurethane foams are used to construct thermal insulation layers for low temperature containers. This is due to the material's good thermal insulation properties and the fact that it can be processed on site in many connections. Hard polyurethane foams are known to be virtually unaffected by petroleum hydrocarbons and are impervious to them, unless there is a crack or a leaky joint.

Various methods have previously been proposed for the thermal insulation of substances with a low temperature by a combination of a spray-foaming method for the hard polyurethane foam itself and a reinforcement material, which methods can also be used for the production of a secondary barrier.

It has thus been proposed thermal insulation structures produced from a combination of a spray-produced, hard polyurethane foam f and a reinforcement material, where the last-mentioned e.g. consists of a wire mesh or other network (U.S. Patent 3,757,982, Japanese Laid-Open No. 54,509/1973).

In order to achieve an effective insulation structure, it is in practice necessary to take into account a complicated interplay of properties of the hard polyurethane foam, especially the substance's dependence on temperature, the foaming process, the properties of the network, the placement of the network, etc. However, this interaction has not been clarified.

A thermal insulation layer for a container for freezing temperatures is further proposed, which layer comprises a combination of foam plastic (polyurethane foam) reinforced with a complex, three-dimensional, fibrous material and ..a permeable liner (U.S. Patent 3,814,275, 3,317 .074, Japanese Explanatory Document 47.926/1974). In this case, it has been found insufficient for the thermal insulation structure for freezing temperatures simply to combine foam plastic, which is not reinforced, with a fiber and a lining laminated with a glass network.

It is associated with many difficult problems to carry out thermal insulation of a material with temperatures below -80°C using foam plastic, as the brittleness temperature of the plastic must also be taken into account. Especially when it comes to internal, thermal insulation structures, where the side is determined at ambient (room) temperature, strong thermal stress will be created on the low-temperature side, so that it can easily burst or be damaged not only in a static state, but also as a result of external, mechanical shocks and repeated loading. The problem has therefore not been easy to solve.

A main purpose of the present invention is to provide an internal thermal insulation structure which is very reliable and has a relatively simple construction, is affordable and has a high resistance to low temperatures and thus is not damaged by thermal stresses or external, dynamic influence by repeated loading and mechanical shocks.

Another object of the present invention is to provide an internal thermal insulation structure both for a direct thermal insulation system and a secondary barrier/insulation system using a material and a construction which is resistant to oil and which is liquid tight.

These purposes are achieved by an insulation structure which is characterized by what appears in the requirements.

The present invention has been arrived at on the basis of detailed investigations of the relationship between the properties of a hard polyurethane foam, which is foamed by a spray method, and the reinforcing ability of a net-shaped material, such as netting. It has proven most effective and appropriate for the intended purpose in connection with an internal thermal insulation structure to choose a suitable combination of three factors, namely that the foam is resistant to thermal stresses at low temperatures, that the foaming process is such that the resulting foams do not shows too large property variations and that reinforcement is achieved from a construction point of view.

In connection with this invention, the expression "internal thermal insulation structure" means a structure, where the ambient temperature side of the thermal insulation layer is fixed and the low temperature side is used in such a way that it is almost unlimited, e.g. a thermal structure in a ship, where the thermal insulation layer is arranged on the ship's inner hull surface and where a container for low temperatures in self-supporting or membrane form is arranged in the space within the layer. Use of the thermal insulation layer itself as a container for low temperatures in direct contact with the stored liquid (a direct thermal insulation system) is also covered by the term "internal thermal insulation structure".

The thermal insulation structure according to the present invention comprises at least one layer of a hard polyurethane foam which is placed in place by the spray method and has a safety coefficient as defined below of not less than 1.5 and a fibrous, net-shaped material which fixatively reinforces the inner foam sheet .

The thermal insulation structure for a low temperature container is produced by applying at least one layer of a rigid polyurethane foam with a safety coefficient as defined below of not less than 1.5 by a spray foaming process and connecting the foam surface with a mesh material, made from glass fibres, natural or synthetic fibers using a rubber or clast adhesive.

The invention will be described in more detail below.

According to the invention, a spray-foaming method is used, in which a foam-forming mixture with a relatively high foaming speed is directly applied as a spray on the surface of the object to be given a thermal insulation layer of hard polyurethane foam. The foam rises freely, almost perpendicularly to the object's surface, and the foam thickness thus achieved after a spray treatment is relatively small, usually 10 - 25 mm. Compared to injector foaming, the residual tension after foaming in the spray method is low. With the spray method, a large area can also be treated largely without a joint, so that the quality of the foam material is almost homogeneous across the width. These two properties are particularly advantageous in a thermal insulation structure for low temperature containers which are exposed to strong thermal stress during use. Spray foaming is well known and will not be discussed further.

The invention also deals with polyurethane foam's properties and composition. In general, urethane foams will vary greatly from flexible to hard substances, depending on the composition of the starting material. The "polyurethane foams" referred to here include foams made from polyisocyanates, i.e. isocyanurates, carbodiimides and the like. Until now, it has hardly been clarified which of these materials are suitable for thermal insulation structures for liquids with freezing temperatures, such as natural gas in liquid form. However, it is obvious that the properties of the polyurethane foams themselves are of primary importance, as tests with thermal insulation for low temperatures on different types of foams with different compositions show that the number of breaks in the foams varies greatly with the composition.

The resistance of polyurethane foams to low temperatures is reflected in the tensile strength (TS), elongation at break (EB) or the product of these two factors (TS x EB), as well as their dependence on temperature. In practice, however, it has been shown that this assumption does not necessarily hold true in experiments carried out. After repeated tests, it thus turned out that the coefficient below represents a foam material's effective properties with regard to low temperatures: Safety coefficient =

The tensile strength (1) of a foam material at freezing temperature Thermal stress (1) that occurs when the foam material becomes

cooled from ambient temperature to freezing temperature

where (1) means a force perpendicular to the foaming direction. The reason why the perpendicular force is taken into account in the coefficient is that most fractures occur in the perpendicular direction to a foam surface, so that the ratio between the foam's

tensile strength and the thermal stress in the direction perpendicular to the foaming direction (parallel to the surface of the object) are important. The ratio is thus called the "safety coefficient".

The thermal stress is recorded by measuring the magnitude of a shrinking force that is induced when a piece of foam material, if both ends are at room temperature, is exposed to a low temperature atmosphere. On the other hand, the tensile strength is recorded at low temperature by conventional tension tests with the same device as used above. Fig. 1 shows an example of a device for determining the safety coefficient as defined according to the invention. As shown in the figure, a sample A of 10 mm (width) x 10 mm (thickness) x 100 mm (length in the (1) direction) from a spray-produced layer of rigid polyurethane foam is placed in a thermally insulated chamber R with the upper and lower ends connected to the cross members B and C and thus limited in a longitudinal direction. D is a load detector (a load cell) and E, E' are jigs for connection of the sample. The inside of the chamber R is rapidly cooled from ambient temperature to -192°C by supplying suitable amounts of liquid nitrogen and air. Then, a thermal stress induced in the sample A can be recorded using the load detector D. The thermal stress becomes approximately constant approximately 15 minutes after the temperature in the chamber has reached -192°C. Then the fixing jig E is loosened once to release the sample A and then clamped again. The sample is then loaded while the lower cross-piece G is lowered and the tensile strength at -192°C is measured. The safety coefficient is thus calculated in the following way:

Coefficient of safety =

The tensile strength ( 1) at - 192°C ( kg/ cm2)

Thermal stress (1) induced by cooling

from ambient temperature to -192°C (kg/cm<2>)

The reason why the temperature -192°C is chosen as the basis for the coefficient is that this temperature can be reached relatively easily when using liquid nitrogen (-196°C) and lies in a temperature range that includes the temperature for LNG (liquefied natural gas)

(-162°C) or most cryogenic liquids. Most conventional foams with poor resistance to low temperatures will burst and degrade, even if only cooled to -192°C by this process, if the safety factor is lower than 1.

Repeated tests were carried out at low temperatures under static conditions and under shock for the relationship between the safety coefficient and the resistance to low temperature, as shown in the examples which will be discussed below. On the basis of these experiments, it was demonstrated that the value of the coefficient must not be less than 1.5, preferably not less than 2.0. On the surface of the spray-produced foam material layer, an outer skin is formed with greater density than in the core of the layer.

The outer skin had a density of approx. 2-10 times the size of the core and a thickness of no more than 0.3 mm, usually in the vicinity of 0.1 mm, compared to the 10 - 25 mm inner core,

although the thickness varies depending on the conditions for foam formation. The outer skin is naturally harder and has a lower elongation at break than the inner core. When assessing foam properties, it is therefore important to take into account both the outer skin and the inner core, as a number of layers of spray-produced foam are in practice often arranged on top of each other. Something similar applies to the measurement of the safety coefficient, as mentioned above. In fig. 1 shows the wavy line in sample A, the outer skin. For internal thermal insulation structures, the foam material itself must have suitable compressive strength, usually 3-5 kg/cm 2 and a density of approx. 40 kg/cm 2 or more. It is usually easy to produce the polyurethane foam as such, but most foams produced in this way have a large thermal stress. This makes it difficult to achieve the desired safety coefficient.

In the invention, the hard foam material to be applied should have an elongation at break at break perpendicular to the foaming direction at ambient temperature of at least 8%, preferably more than 10%, compared to 3 - 7% for conventional foam material. Special attention must be paid to the composition of foam components to achieve a desirable foam. Generally, a rigid polyurethane foam is prepared from a polyfunctional polyol with an OH value (mg KOH/g) of 300 - 800 and a functional group of not less than 3.5 and an aromatic polyisocyanate, if necessary together with a foam stabilizer, ca -talysator, a foaming agent, such as halogenated hydrocarbons or water, a fire retardant, a plasticizer, etc. by a semi-pre-polymer method or one-step process. The foam material for low temperatures according to the invention can advantageously be produced from a polyol with a low OH value of no more than 4 50 and a number of functional groups of approx. 4 and a polymeric isocyanate.

The table below shows the physical properties of the foam material for low temperatures with a high safety coefficient, compared to the properties of conventional foam materials.

According to the invention, a satisfactory resistance to low temperatures can advantageously be achieved by a simple construction, where at least the innermost foam material layer is reinforced. It is also possible to produce a foam that has a relatively high safety coefficient by reducing the foam's thermal expansion coefficient, e.g. by depositing glass filament fibers in the foam or by mixing powder-like, short fibers in advance in the liquid component for foaming, the thermal stress being proportional to the product of the modulus of elasticity by the coefficient of thermal expansion, but this method requires a cumbersome and complicated procedure to be applicable in practice.

According to the invention, the inner foam surface is reinforced with a net-shaped material, e.g. a network of natural fibers, such as linen, synthetic fibers, such as rayon, nylon, polyester and the like, or inorganic fibers, such as glass, asbestos or the like.

When choosing a suitable mesh material, the following points must be taken into account. The material itself must have sufficient strength against tension, shock etc. at low temperatures. More importantly, the net-shaped material has low elongation at break in two-dimensional directions, is as far as possible isotropic and easy to handle and place. A typical example that satisfies all these requirements is glass mesh. As the internal thermal insulation structure is cooled with a limited width direction, the reinforcing material must also have a high safety coefficient, as well as great strength, like the foam material. The net-shaped material should also be flexible and fit well with the foam's rough surface, as spray-produced foam does not always have a smooth surface. In this respect, a hard mesh consisting of a single wire, like ordinary wire mesh,

not suitable. The netting material should of course also be an easily accessible material.

If the mesh size of the net-shaped material is too large, the reinforcement will be insufficient, while the net-shaped material will be difficult to conform to the rough foam surface if the mesh size is too small. Against this background, a fiberglass mesh with a mesh size of 2 - 8 mm is preferred. It can e.g. a commercially available fiberglass mesh WG-250 (manufactured by Nitto Boseki Co., Japan) with a mesh size of approx. 3 mm and a thread density of 7/25 mm, produced by ordinary weaving in a grid pattern and where a thread is composed of hundreds of filaments that are twisted to a diameter of approx. 0.35 mm.

The bond between the outer skin of the foam, which is spray-produced on site, and the mesh is created by means of a solution or emulsion type adhesive from a group comprising rubber substances, e.g. chloroprene, SBR, hypalon and plastics, e.g. polyurethane, epoxy, polyester, urea, phenol etc.

As the internal thermal insulation structure is in direct contact with the load, e.g. liquefied natural gas or liquefied propane gas (in the case of a direct thermal insulation system) or may come into contact with the load (in the case of a secondary barrier/insulation system), the adhesive material should be of such a nature that it is not changed or degraded by such a load. On this background, a chloroprene-grummie solution would be suitable as an adhesive.

The bond is created as follows: an adhesive is applied thinly to the foam surface, a mesh is adhered when the adhesive is half-dried and the unit is temporarily held together mechanically, e.g. with cramps.

With the thermal insulation structure according to the present invention, the structure in a direct thermal insulation system also acts as a liquid-tight body, and the liquid tightness is created by the foam material layer itself.

As mentioned above, the mesh is placed exclusively as a reinforcement of the outer skin of the foam material layer so that it does not burst so easily. In this connection, it is necessary that the adhesive film in connection with the netting does not form a liquid-tight membrane for the enclosed liquid. Otherwise, liquefied gas can enter the space between the foam material and the membrane through some channel, whereupon the liquefied gas will quickly evaporate with increasing temperature. Consequently, there is a danger that the reinforcing layer will be destroyed by the resulting back pressure. The used adhesive must therefore be a material that does not form a tough film at low temperature, so that vaporized gas can easily escape. It is certainly not that difficult to choose a satisfactory adhesive, as few adhesives form a tough film at freezing temperatures, e.g. at -162°C, for liquefied gas.

It has also been shown that many adhesives gain remarkably increased resistance to low temperatures in connection with glass fibres. Low temperature tests were carried out using liquid nitrogen, e.g. with chloroprene or urethane adhesives. It was thereby confirmed that the bond between the reinforcement system consisting of glass fiber mesh and chloroprene or urethane adhesive and the foam material is good, while many small cracks are observed in the adhesive film between the mesh threads. The reinforcement system thus does not form a membrane as mentioned, but only reinforces the foam material layer.

Before bonding, it will be necessary to perform sanding only in very rough areas of the foam surface.

The internal thermal insulation structure according to the invention can e.g. is used for a self-supporting, prismatic tank for LNG ships, as "Conch type system", where balsa sheets are also used for the thermal insulation layer.

The drawing shows some embodiments of the invention, in that

fig. 1 schematically represents an example of a device for determining the safety coefficient as defined in connection with the invention,

fig. 2 is a partial section of an example of a thermal insulation structure according to the invention,

fig. 3 shows a fragment on a larger scale of the inner surface of the structure as shown in fig. 2,

fig. 4 is a partial section showing another example of the thermal insulation structure according to the invention,

fig. 5 is a partial section of an embodiment, where the thermal insulation structure according to the invention is also used as a secondary barrier,

fig. 6 is a partial section of another embodiment, where the thermal insulation structure according to the invention is arranged in contact with a tank of the membrane type,

fig. 7 is a partial section of another embodiment, where the thermal insulation structure according to the invention is used for direct thermal insulation, and

fig. 8 is a partial section of a further embodiment, which constitutes a modification of that shown in fig. 5.

In fig. 2 and 3, 1 is an outer tank wall, e.g. made of metal or concrete and similar to the hull of a ship or the outer shell of a double-walled tank placed on land or underground. The outside of this wall (at the bottom of the figure) can be in contact with air, seawater or soil at ambient temperature. The inside of the outer wall must be treated with a primer before spray application of foaming agent for polyurethane to ensure good adhesion between the wall and the foam material. Chloroprene rubber can be used with advantage. The reference numbers 2, 4, 5, 6 and 7 represent insulation layers of hard polyurethane foam with a safety coefficient of not less than 1.5. These layers are spray-applied in turn. In the mentioned example, the average thickness of a foam layer is 20 mm and the combined thickness of five layers is 100 mm. Empirically, it is preferred that the thickness of one layer of the spray-applied foam is close to 10 - 25 mm, and the total thickness required can be suitably regulated by increasing or decreasing the number of layers. A relatively hard, outer skin is formed on the surface of each polyurethane foam layer. An example is shown, as the skin 3 for the foam material layer 2. 10 denotes a thread of a reinforcing mesh with a mesh size of 2-8 mm, although the thread in fig. 3 is apparently excessively large. The wire mesh is attached to the outer skin 8 of the innermost layer 7 with adhesive 9. The space outside the thermal insulation structure is an area 11 with a low temperature. Different embodiments are possible in connection with the use of the area 11 with low temperature: a self-supporting tank for low temperatures arranged at a distance from the room 11 (fig. 5), a membrane tank arranged in contact with the thermal insulation structure (fig. 6) or itself the compartment 11 is a container for liquids with a low temperature (fig. 7).

The thermal insulation structure shown in fig. 4 is a modification of the structure shown in fig. 2. In fig. 4, the two innermost layers of foam material 6, 7 are reinforced with mesh 10, 10' on the outer skin surfaces 8, 8'.

In fig. 5 shows an application form for the thermal insulation structure according to the invention. 21 is a self-supporting tank for liquids with a low temperature, and the tank 21 is arranged at a distance from a room 23.

In fig. 6 another embodiment is shown, where

22, a membrane tank is arranged in contact with the insulation structure.

In the embodiment shown in fig. 7, the thermal insulation structure is used in a direct thermal insulation system, where the innermost surface of the structure is in direct contact with the liquid.

Fig. 8 is a modification of the embodiment according to fig. 5. Here is shown a "Conch type system" of an internal thermal insulation structure for LNG ships, where balsa plates 24 are used in addition to the insulation layer. The self-supporting tank 21 is arranged at a distance from the room 23.

It should be noted that what is discussed above is a basic principle for the invention and that modifications can be carried out without deviating from the scope of the invention. For example the inner surface of a reinforcement layer can be additionally covered with a thin layer of polyurethane foam as an aesthetic finish to the inner surface, or the inner surface can be coated with a material that has nothing to do with thermal insulation, but which provides a smooth surface, which can be beneficial in membrane tanks.

The reinforced layer can be arranged not only on the innermost foam layer, but also on the second or third layer from the inside, as shown in fig. 4. Thereby, safety at low temperatures can be increased to a certain extent. According to the invention, however, a polyurethane foam is chosen with the special safety coefficient, which is discussed above, so that it is not necessary to arrange such additional reinforcements. From an economic point of view, the simplest possible structure would also be preferable.

In the following, the invention will be described in more detail with reference to an example.

Example

A number of bowl-shaped containers were produced by arranging for each a frame of four pieces of plywood with a thickness of 10 mm and a height of 100 mm around four peripheral surfaces of a steel plate with a size of 1200 x 1200 x 5 ( thickness) etc. In the respective containers, a hard polyurethane foam material was spray-applied, as indicated in the following table,

in a thickness of 15 mm per layer, with up to five layers and a total thickness of 75 mm. The above-mentioned commercial fiberglass mesh (WG 250) with a mesh size of 3 mm was placed on the foam surface with the edge attached to the frame. In this way, 90 samples were produced (9 sample types S 10 pieces) which were tested for comparison.

Cooling tests were carried out so that two samples a-e were introduced directly into liquid nitrogen (-196°C) in a space of 25 mm opposite them, while two samples a' - d' were filled with dry ice (approx. -70°C), after which the samples were allowed to stand for at least 2 hours.

Impact tests were carried out by using a steel rod with

a wedge (7 mm wide) at the end was dropped onto the samples. The rod had a diameter of 8 and a length of approx. 1300 mm and a weight

of 600 g. The rod was dropped onto the samples' thermal insulation layer from a height of 1 meter. During the test, liquid nitrogen was present or the dry ice had been removed immediately before the test. In the case where the sample was reinforced with the fiberglass mesh, the bar's wedge cut through the mesh up to approx. 20 mm deep.

The following table shows the results of the experiments.

It was demonstrated that in samples d and e no cold spots appeared on the rear surface of the steel plate, even after the impact test and that the liquid density was perfect according to a color test after the temperature increase.

Repeated, dynamic load tests were also carried out at low temperature to carry out a model test (3.5 x 3.5 m) using the thermal insulation structure according to sample d. Load cycles corresponding to a 20-year lifetime of a ship. The test confirmed that the thermal insulation structure is not damaged, even under vibration, and can be used with advantage in ships.

The above-mentioned results show that when the thermal insulation structure is composed exclusively of foam f layer with a safety coefficient of 2, as in example c, the reactions in the presence of liquid nitrogen at -192°C are not satisfactory, while a structure which is further reinforced with the mesh, as in example d, withstands the hard impact test, as well as the static test at low temperature. In the same way, the purpose of the present invention cannot be achieved if an unsuitable foam material with no more than 1.0 in safety coefficient is reinforced with the same netting, as in b.

It must therefore be concluded that a combination of the low-temperature foam and the mesh is crucial for the structure to withstand the dynamic impact test, even at a moderately low temperature, as in the dry ice test, as is evident from the comparison between b', c1 and d< 1>.

In general, an injection-molded foam material will burst more easily at low temperatures, even if it has the same components as the spray-produced foam material according to the invention. An injection-molded foam will also inevitably have many joints, so that it is completely unsuitable for liquid tightness. According to the invention, a simple, affordable and highly reliable thermal insulation structure is thus provided by an effective combination of the hard polyurethane foam material, the foaming process and a mechanical reinforcement. The structure is therefore very useful for internal thermal insulation of tanks for the storage and transport of liquefied natural gases with a low temperature, such as LPG (-42°), LNG (-162°C) and the like.

Claims (5)

1. Internal thermal insulation structure for low-temperature containers containing low-temperature liquids, comprising several layers of insulation whose outer surface is attached to the container wall, and a reinforcement in the form of a mesh-shaped material, characterized in that each layer of insulation consists exclusively of an in-situ spray-applied rigid polyurethane foam material constituting a continuous layer of polyurethane foam, which rigid polyurethane foam in the continuous layer as a whole has a safety factor as defined below of less than 1.5, and that the continuous layer of polyurethane foam itself serves as a liquid-tight seal, and that the mesh-shaped reinforcement is, in a known manner, a fibrous material which is arranged on the inner surface of the continuous layer, whereby the safety coefficient is: 2. Thermal insulation structure for containers with low temperatures as stated in claim 1, characterized in that the fibrous mesh material is a mesh with a mesh size of 2-8 mm. 3. Thermal insulation structure for containers for low temperatures as stated in claim 1, characterized in that the layer of hard polyurethane foam has a skin layer on the surface and a homogeneous inner foam layer. 4. Thermal insulation structure for containers with low temperatures as specified in claim 3, characterized in that the skin layer for the innermost layer of polyurethane foam is ground down. 5. Thermal insulation structure for containers with low temperatures as specified in claim 1, characterized in that the hard polyurethane foam is produced from a polyol with an OH value of no more than 450 and a number of functional groups of 3 - 5 as well as an aromatic polymeric polyisocyanate.