NO161208B - PROCEDURE FOR INSULATING PIPES BY MULTI-STEP APPLICATION OF FOUNDABLE PLASTIC. - Google Patents

Description

The present invention relates to a method for placing at least one inner and one outer layer of foamed plastic insulation on pipes of the type specified in the introduction to the claim.

Until now it has been proposed to insulate pipes by spraying them with one or more layers of a foamed plastic insulation such as polyurethane with subsequent curing and covering or wrapping the insulating layer with a protective or barrier layer as described e.g. in US Patent No. 3,480,493. The foam layer insulation from such a process usually has an apparent density of an intermediate value, so that it has properties that are a compromise between requirements for maximum heat insulating characteristics and requirements for maximum strength, crush resistance and wear resistance. It has also been proposed in U.S. Patent No. 4,094,715 to spray the tubes with a foaming liquid composition and then wrap the resulting foam with a protective sheet material before the resulting foam has fully settled so as to increase the density of the foam near its outermost surface . A main disadvantage of the process is that it is limited to a spirally wound protective outer sheet layer, which is applied under tension to achieve an apparent increase in the density of the outer layer of the room. Such a process is difficult to control in commercial operation due to the extent of the increase in apparent density and the thickness of the high density layer depending on several different critical factors including the temperature of the foaming plastic, the rate of foaming of the plastic mixture, the cure rate of the plastic foam, the time course before the application of the protective layer and the voltage applied to the protective layer during the transfer. All of these factors influence the extent to which the foamed plastic has solidified or settled prior to the application of the protective layer. The thickness of the layer of increased density also depends on the thickness and compressibility of the mass of the low density foam already present.

The present invention solves the above-mentioned problems by means of a method of the nature mentioned at the outset, the characteristic features of which appear in the claim.

Due to the fact that the thickness and density of each layer is independent of the other and can be individually adjusted and controlled, the method according to the present invention makes it possible to use an inner layer, which has a high heat insulating value and a low cost together with an outer layer of high strength and high density and therefore a layer with a higher cost. The two layers together provide combined properties that maximize pipe insulation and protection at a minimal total cost.

The extent of the difference in density between the inner and outer insulating layers for optimal results varies depending on the diameter of the pipe, the relative thickness of the layer, its minimum desired strength and the temperature difference between the pipe and the environment among other things. As mentioned, the apparent density of the outer layer must be at least 25% greater than the density of the inner and the thickness of the outer layer from 5 - 30% of the total thickness of the layers of foam insulation. If desired, the inner layer can be divided into two or more layers added in sequences and which have different physical properties.

The invention will now be described in more detail using embodiments and reference to the drawing, where: Fig. 1 shows a cross-section of an embodiment of the invention. Fig. 2 shows a cross-section of another embodiment. The embodiment in fig. 1 shows a metal pipe 10 with an inner layer 12 of low density of 0.032 to 0.096 g/cm3, where the rigid polyurethane foam insulation has a thickness of about 2.54 to 15.24 cm, and a second layer 14 of rigid polyurethane foam insulation which has a density which is at least 25% greater than that of layer 12 and which has a thickness of 5 - 30% of the total thickness of layers 12 and 14 together.

In a preferred embodiment of the invention, the inner layer of foamed insulation differs from the outer layer or layers in that they not only have a lower apparent density and therefore a higher thermal insulation capacity, but also have a higher heat resistance, i.e. they are stable at higher temperatures up to 177°C than the outer layer or layers of foam insulation which only need to be stable up to a temperature of 93.5°C. Another embodiment is shown in fig. 2 where the pipe 10 is designed to carry hot liquids at temperatures from 149 - 177°C, and which is made with an inner insulating layer divided into a first layer or part 20 of foamed polyurethane insulation of low density formed to be stable at a temperature as high as 177°C and a second part or layer 22 of foamed polyurethane of approximately the same layer density, but manufactured to have stability only at temperatures up to 99.5°. The outer layer 24 of high density foamed polyurethane has a density that is at least 25% greater than that of either layer 20 or 22 and has a thickness of 5 to 30% of the total thickness of layers 20, 22 and 24. In another embodiment where the pipe 10 is designed to transport cryogenic mixing fluids at very low temperatures on the order of -129°C, the layer 20 is made to be semi-rigid rather than rigid to avoid embrittlement and possible cracking during use, while layers 22 and 24 are made rigid.

When carrying out the method according to the present invention, the pipe, which is usually made of steel, is first heated to remove condensation moisture and cleaned to remove dirt, scale and rust. Any usual cleaning method can be used, such as sandblasting or sling cleaning, brushing or the like. If desired, two or more such methods can be used in combination.

In some cases, it may be desirable to place a corrosion-protective coating on the pipe surface before applying the foam plastic, although this is not essential. Any common corrosion protection coating can be used, such as wrapped polyethylene/butyl adhesive tape, epoxy coating, heat-affected adhesive or asphalt. Foam thermoplastic material can be applied directly to the pipe itself or to the surface of any corrosion protection coating previously applied to the pipe. It is usually desirable that the temperature of the pipe be raised above room temperature, the exact temperature depending on the nature and composition of this foamed plastic mixture. The latter is usually applied in the form of a spray from a spray nozzle, while the tube is rotated and guided past the various nozzles. Any of the many different versions of the foamable polyurethane-plastic mixture can be used depending on which characteristics are desired in the final product. A particularly desirable composition is a liquid mixture of ingredients reactable to form a polyurethane and which includes a common foaming agent in an amount sufficient to produce a foam having the desired apparent density, which, as mentioned, is from about 0.024 to 0.096 g/cm<3 >. The first layer of foamable liquid is allowed to rise under the influence of the foaming agent. The thickness of the first layer of foam can vary over a wide range depending on the same factors described above in connection with the difference in density of the insulating layers, such as the pipe diameter and the temperature at which the pipe and its contents are intended to be operated, and in generally it can vary from about 2.54 to 15.24 cm.

The second or outer layer 14 or 24 of foamable plastic compound is then applied to the outer surface of the first foamed insulation layer by spraying it in the usual manner onto the surface of the rotary advancing pipe. The second layer is applied to the first before or after the latter has set, and also before the foaming of the latter has been completed. Preferably, it is applied before the first layer has hardened. In one embodiment, the preparation of the second foamable plastic composition differs from the first only in the amount of foaming agent used and is adjusted so as to produce a foamed insulating plastic layer having an apparent density that is at least 25% higher than that of the first layer and has a thickness from 5-30$ of the total thickness of both layers together. In another embodiment, the outer layer 14 or 24 can have a lower heat resistance than the inner layer and greater stiffness or both.

The total thickness of the foamed insulation as well as the thickness of the inner layer can be varied depending on the temperature difference between the pipe's contents and the pipe's surroundings, the pipe's diameter, and the apparent density of the foam i.a. For a temperature difference of 41.5" - 150°C, a thickness of about 2.54 to 15.24 cm is usually sufficient. The greater the temperature difference between the pipeline and the substrate, the greater the thickness of the insulation should generally be. F .eg the cryogenics industry typically uses 3" to 6" of polyurethane foam due to the large temperature difference. For hot oil lines the temperature difference is typically from 55.5° - 89°C. For hot oil lines the thickness would typically be from 2 .54 to 6.5 cm. A pipe for conducting molten sulfur will be made with 6.3 to 7.65 cm of foam insulation, where the inner layer 12 or 20 is made to have high heat resistance, i.e. is stable at temperatures up to 149°C. A hot oil pipe when buried will normally require a foam thickness of 2 inches or less. Polyurethane foam has an extremely low K-factor on the order of about 6.94 x 10"^ g/cal /cm^/sec/°C/cm, in contrast to a K-factor of about 0, 39 for glass foam. In the case of a polyurethane foam which has an apparent density of 0.048 g/cm<3>, e.g. a 5.1 cm layer on a 15.24 cm diameter pipe operated at a temperature difference of 69.7°C results in a heat loss (or gain) of about 43.7 x 10"^ g/cal/ sec/cm<2>.

The following is a suitable recipe for producing satisfactory rigid foam:

All these ingredients except 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/cm<3>. The increase in the ratio with trichloromonofluoromethane - the blowing agent leads to an increase in density. The increase in the solids in the above-mentioned statement, when adding 30 parts of methylcellulose without an increase in the emulsifier, also increases the apparent density. Increased heat resistance of the foam is achieved by usual execution technique and choice of ingredients, such as e.g. by reacting aromatic amine-containing mixtures with the remaining ingredients as described in US Patent No. 3,909,465. Increased resistance to brittleness, which is also desirable for the outer foam layer, can be achieved by choosing polyol ingredients as described e.g. in US Patent No. 3,928,257 and US Patent No. 3,928,258.

Any desired outer layer or coating may be applied over the outer foamed insulating layer. The outer foamed insulating layer can e.g. be made with a wrapped or extruded protective or moisture barrier layer of paper or plastic, i.e. polyethylene or polypropylene, a wrap or wrapped layer of heat shrinkable plastic that is shrunk in place to conform to the foamed insulation, a coating of liquid-resin cured in place or a coating of bituminous mastic, or a layer of any other common protective material. Due to the solid crush resistance and wear resistance built into the outermost foam layer of polyurethane plastic in the present invention, in most cases a further outer protective layer must only be performed as a barrier against punctures and moisture penetration.

Claims (1)

Method for applying at least one inner and one outer layer of foamed plastic insulation to pipe, wherein at least one inner layer of a foamable plastic mixture is applied to the pipe, the mixture or each mixture being foamed to form respective layers of foamed plastic insulation having an apparent density of 0.024 to 0.096 g/cm<3>, and then applying to the surface of the last applied layer an outer layer of a foamable plastic composition, characterized in that the outer layer is foamed and cured without boundaries to form a separate outer layer of rigid foam plastic with an apparent density that is at least 25# greater than any inner layer and with a thickness of 5-30% of the layers' total thickness.