CH635182A5 - METHOD FOR FORMING INSULATING LAYERS ON A TUBE, AND TUBE THUS OBTAINED. - Google Patents

Description

The present invention relates to a method for thermally insulating a tube by means of two or more layers of a foam plastic insulation having for example different apparent densities, and preferably different thermal stabilities, the foam of lower density being adjacent to the surface of the tube and the highest density adjacent to the external surface of the foam insulation.

It has so far been proposed to insulate a tube by spraying it with one or more layers of a foam plastic insulator such as polyurethane and then hardening and coating or surrounding the insulating layer with a protective layer. or forming a barrier as described, for example, in US Patent No. 3,480,493 in the name of Bauer et al. The foam insulation layer resulting from such a process normally has an apparent density of an intermediate value, therefore it has properties which are a compromise between those required for maximum thermal insulation characteristics and those required for resistance. mechanical, maximum crush resistance and abrasion resistance. In US Patent No. 4094715 in the name of Henderson et al., It has also been proposed to spray the tube with a foaming liquid composition, then to surround the foam in the formation of a protective sheet before this foam has fully set, in order to increase the density of the foam near its external surface. The method has a major drawback because it is restricted to an outer layer in a protective sheet wound in a spiral, applied under tension to obtain an apparent increase in the density of the outer layer of the foam. Such a process is difficult to control in commercial operations, because the extent of apparent density increase and the thickness of the high density layer depend on several critical and different factors, including the temperature of the plastic material. 5 that foam, the rate of foaming of the plastic composition, the rate of hardening of the foam plastic, the time elapsed before application of the protective sheet, and the tension applied to this sheet during its application. All these factors influence the extent to which the plastic or foam hardens or sets before application of the sheet. Furthermore, the thickness of the layer of increased density depends on the thickness and the compressibility of the mass of low density foam already present.

The method according to the invention consists in applying, to a tube, at least one foamable plastic composition, in foaming the composition in order to form an insulating inner layer of foam plastic having an apparent density of between 0.024 and 0.096 g / cm3, applying to the surface of the inner layer a foamable plastic composition and foaming and curing the composition to form an outer layer into a rigid and foamable plastic composition having an apparent density which is at least 25% greater than that of the internal layer, and the thickness of which represents 5 to 30% of the total thickness of the layers. As the thickness and density of each layer 25 are independent of those of the other and can be adjusted and controlled individually, the method according to the invention makes it possible to use an internal layer having a high value of thermal insulation and low price, with a stronger and denser outer layer, and therefore more expensive. The two layers, together, offer combined properties which are optimal for insulating the tube and protecting it at a minimum total price.

The extent of the density difference between the inner and outer insulating layers for the best results varies according to the diameter of the tube, the relative thickness of the layers, the desired minimum mechanical strength, and the temperature difference between the tube and its environment, among others. However, in general, the bulk density of the outer layer should be at least 25% greater than that of the inner layer, and the thickness of the outer layer should be 5 to 30% of the total thickness of the layers. foam insulation. 40 If desired, the inner layer can be subdivided into two or more layers applied in sequence and which differ from each other in their physical properties.

The invention will be better understood and the characteristics, details and advantages thereof will appear more clearly during the explanatory description which follows, made with reference to the appended schematic drawings given solely by way of example illustrating several modes of realization of the invention and in which:

- fig. 1 is a cross section showing an embodiment of the invention; and

- fig. 2 is a cross section showing another embodiment.

In the embodiment of FIG. 1, the metal tube 10 is provided with an internal layer 12 of a rigid poly-55 urethane foam insulation of low density (0.032 to 0.096 g / cm3), having a thickness of 2.54 to 15.24 cm , and a second layer 14 made of a rigid polyurethane foam insulation having a density which is at least 25% greater than that of layer 12, with a thickness which is 5 to 30% of the total thickness of the layers 12 and 14 in-6o seems.

In a preferred embodiment of the invention, the internal layer of foam insulation differs from the external layer or from the external layers, not only by its lower apparent density and consequently its higher thermal insulation capacity, but also 65 by its greater resistance to heat, that is to say that it is more stable at high temperatures, up to 177 ° C., than is the outer layer or the outer layers in foam insulation, which should only be stable up to a temperature of 93.5 "C.

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Fig. 2 illustrates a second embodiment where the tube 10, studied for transporting hot fluids at temperatures from 149 to HT C, is provided with an internal layer of insulator subdivided into a first portion or layer 20 into an insulator in low density polyurethane foam formulated to have stability at temperatures up to 177 ° C, and a second portion or layer 22 of about the same low density polyurethane foam, but formulated to be stable only at temperatures up to 93, 5 ° C. The outer layer 24 of high density foam polyurethane has a density which is at least 5% greater than that of each layer 20 or 22, and its thickness is 5 to 30% of the total thickness of the layers 20, 22 and 24. In another embodiment where the tube 10 is studied for the transport of cryogenic fluids at very low temperatures of the order of - 129 ° C., the layer 20 is formulated to be semi-rigid instead to be rigid, in order to avoid a e embrittlement and possible cracking during use, while the layers 22 and 24 are formulated to be traditionally rigid in nature.

In the practice of the method according to the invention, the tube, which is usually made of steel, is first heated to remove moisture condensation and is cleaned to remove dust, encrustations and rust. Any traditional cleaning process can be used as a sand blast or a shot of sand, brushing or the like. If desired, two of these processes can be used in combination.

In some cases it may be desirable to form a corrosion protective coating on the surface of the tube before applying the foam plastic, although this is not essential. Any traditional corrosion protection coating can be used as a rolled polyethylene / butyl tape, epoxy coating, heat activated adhesive, or asphalt. The foamable thermoplastic may be applied directly to the bare tube or to the surface of any corrosion protection coating previously applied to the tube. It is usually desirable for the temperature of the tube to be raised above room temperature, the precise temperature depending on the nature and composition of the plastic material which may foam. The latter is usually applied in the form of a jet from a tube while the tube is rotated and advances beyond the various tubes. A wide variety of liquid polyurethane foam plastic formulas can be used, depending on the specific characteristics desired for the final product. A particularly desirable composition is formed from a liquid mixture of ingredients which can react to form a polyurethane, and containing a traditional blowing agent in an amount sufficient to produce a foam having the desired bulk density, which may be from 0.024 to 0.096 g / cm3. The first layer of foamable liquid is allowed to rise or swell by the action of the blowing agent. The thickness of the first layer of foam can vary over a wide range according to the same factors as those indicated above in relation to the difference in density of the insulating layers, such as the diameter of the tube and the temperature at which the tube, with its content must be used and, in general, it can vary between 2.54 and 15.24 cm.

The second layer or external layer 14 or 24 of foamable liquid plastic composition is then applied to the external surface of the first foam insulating layer, by spraying in the usual manner on the surface of the tube which rotates and advances. The second layer can be applied to the first before or after the latter has hardened, and even before its foaming is complete. Preferably, it is applied before the first layer is cured. In one embodiment, the formula of the second foam plastic composition is different from the first, only by the amount of the blowing agent used, and this amount is adjusted to form an insulating layer of foam plastic having a bulk density which is at least 25% greater than that of the first layer, and a thickness which is 5 to 30% of the total thickness of the two layers. In another embodiment, the outer layer 14 or 24 may have lower heat resistance than the inner layer, or higher stiffness, or both.

The total thickness of the foam insulation, as well as the thickness of the internal layer (approximately 2.54 to 15.24 cm) can be modified according to the temperature difference between the contents of the tube and the environment of the tube, the diameter of the tube and the apparent density of the foam, among others. For a temperature difference of 41 to 150 ° C, a thickness of 2.54 to 15.24 cm is normally sufficient. In general, the greater the temperature difference between the pipeline and the ground, the thicker the insulation. For example, the cryogenic industries normally use 7.6 and 15.24 cm of polyurethane foam because the temperature differences are greater. For hot oil tubes, the difference is usually 55 to 88 ° C. For hot oil pipelines, the thickness will normally be 2.54 to 6.4 cm. A tube for the conduction of molten sulfur will be provided with a foam insulation of 6.3 to 7.65 cm, the internal layer 12 or 20 being formulated to have a high resistance to heat, that is to say for be stable at temperatures up to 149 ° C. A tube for hot oil, when buried in the ground, will normally require a foam thickness of 5.1 cm or less. Polyurethane foam has an extremely low K factor, around 6.94 x 10 ~ 6 g-cal / cm2 / s / ° C / cm, unlike a K factor of 19.9 for glass foam . For example, in the case of a polyurethane foam having an apparent density of 0.048 g / cm3, a layer of 5.1 cm thick on a tube of 15.24 cm in diameter used at a temperature difference of 69 ° C produces a heat loss (or gain) of the order of 43.7 x 10 ~ 6 g-cal / sec / cm2.

An appropriate formula will be given below to obtain a satisfactory rigid foam:

Ingredients Parts by weight

Crude methyl diisocyanate 115

PolyoIA 100

Silicone oil 1

Triethylene diamine 0.5

Dibutyl tin dilaurate 0.1

Trichloromonofluoromethane 35

Tris (2-chloroethyl) phosphate 10

All the ingredients, with the exception of the diisocyanate, are premixed, the diisocyanate being mixed immediately before foaming and spraying in conventional equipment to form a rigid foam having an apparent density of 0.029 g / cm3. A decrease in the proportion of trichloromonofluoromethane as a swelling agent leads to an increase in density. An increase in solids in the above formula, for example by adding 30 parts of methylcellulose without increasing the blowing agent, also increases the bulk density. The increase in heat resistance of the foam is obtained by traditional formulation techniques and the choice of ingredients, for example by reacting compounds containing an aromatic amine with the remaining ingredients as described in US Patent N ° 3909465 in the name of Wiedermann; increased resistance to friability, also desirable for the outer layer of foam, can also be obtained by choosing the polyol ingredients as described, for example in US Patent No. 3,928,257 in the name of Fuzesi and in US Patent No. ° 3928258 in the name of Alexander.

Any outer layer or coating can be applied over the outer foam insulating layer. For example, the outer foam insulating layer may be provided with a moisture barrier or rolled up or extruded protective layer, made of paper or plastic, such as polyethylene or polypropylene.

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ene, a wrapped envelope or layer of heat shrinkable plastic, shrunk in place to conform to foam insulation, a coating of a hardened liquid epoxy resin on the spot or a coating of mastic bituminous, or a layer of any traditional protective material. However, thanks to the significant crushing resistance and the abrasion resistance of the external layer of the foam polyurethane plastic used in the process according to the invention, any additional external protective layer must, in most cases , simply form a barrier against puncture and ingress of moisture.

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1 sheet of drawings

Claims (8)

635182 1. A method of forming, on a tube, at least one internal layer and an external layer of a foam plastic insulator, characterized in that it consists in applying, to said tube, at least one composition of plastic material foamable, to foam the composition to form an insulating inner layer of foam plastic having an apparent density between 0.024 and 0.096 g / cm3, to apply to the surface of the inner layer, a foamable plastic composition and foaming and curing the composition to form an outer layer of a rigid, foam plastic composition having an apparent density which is at least 25% greater than that of the inner layer, and whose thickness is 5 to 30% of the total thickness of the layers. 2. Method according to claim 1, characterized in that it consists in applying, previously, to the tube, one or more other internal layer forming compositions in order to form one or more other internal layers. 2 3. Method according to one of the preceding claims, characterized in that the tube is a metal tube like a steel tube, and. what is additionally applied, initially, to the tube, a protective coating against corrosion. 4. Method according to one of the preceding claims, characterized in that the foamable plastic compositions are polyurethane compositions. 5. Method according to one of the preceding claims, characterized in that the innermost layer of foamable plastic material is stable at a temperature up to 177 ° C and in that the outer layer (s) are stable only at temperatures up to 93.5 ° C. 6. Method according to one of the preceding claims, characterized in that the innermost layer is rigid. 7. Method according to one of claims 1 to 5, characterized in that the innermost layer is semi-rigid. 8. Insulated tube obtained by the method according to claim 1.