IE49291B1 - Insulation of pipe by multi-stage application of foam - Google Patents

Abstract

Metal e.g. steel pipe insulated with foamed polyurethane plastic is made in a multi-stage continuous process in which an inner layer of polyurethane plastic is foamed on the pipe and at least partially cured to form low-density foam having high heat insulating capacity, and an outer layer of polyurethane plastic is foamed on the inner layer to provide a foam layer of higher density, the outer layer having a density at least 25% greater than that of the adjacent inner layer, the thickness of the outer layer being 5 to 30% of the total thickness of the foamed insulation. The inner layer may be subdivided into two or more layers having different physical properties. A common protective layer e.g. polythene/butyl adhesive tape, epoxy, heat actuated adhesive or asphalt may be first applied to the pipe. An outer protective layer of paper, butuminous inactive or plastic e.g. polythene, polypropylene or epoxy may be applied. The plastic may be heat shrinkable.

Description

This invention relates to a method of thermally insulating metal pipe and pertains more specifically to a method of providing a metal pipe with two or more layers of foamed plastic insulation of differing apparent densities and preferably different thermal stabilities, the lowest density foam being adjacent to the surface of the pipe and the highest adjacent to the outer surface of the foamed insulation. Apparent density is the Ί0 density calculated on the basis that the volume of the foam includes the volume of the cells.

It has hitherto been proposed to insulate pipe by'spraying thereon one or more layers of foaming plastic insulation such as polyurethane followed by curing and coating or wrapping the insulation layer with a protective or barrier layer as described for example in Bauer et al., U.S. Patent 5,460,495. The layer of foam insulation resulting from such a process has normally possessed an apparent density of intermediate value such that it has properties which are a compromise between those required for maximum heat insulation characteristics and those required for maximum strength, crush resistance and abrasion resistance. It has also been proposed in Henderson et al., U.S. Patent 4,094,715 to spray pipe with a foaming liquid composition, then wrap the rising foam with a protective sheet material before the rising foam has completely set so as to increase the density of the foam near its outer49291 - 3 most surface. A major disadvantage of the process is that it is restricted to a spirally wound protective sheet outer layer, which is applied under tension to achieve 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 increase in apparent density and the thickness of the high density layer depend upon several different critical factors including temperature of the foaming plastic, go speed of foaming of the plastic composition, cure rate of the foaming plastic, time lapse before application of the protective sheet, and the tension applied to the protective sheet during application. All of these factors influence the extent to which the foamed plastic has cured or set before application of the sheet.

Moreover, the thickness of the layer of increased density depends upon the thickness and compressibility of the mass of low density foam already present.

The present invention provides a process for applying to a pipe a first layer of foamable polyurethane plastic composition and foaming it in place to form a first layer of foamed plastic having a low apparent density, from 1.5 to 6 lb/cu.ft.(about 0.024 to 0.096 grams/cu.cm.), then applying over said first layer a second separate layer of foamable plastic and foaming it in place to form a second separate layer of foamed plastic having a high apparent density. - 4 Because the thickness and density of each layer is independent of the other and can be individually adjusted and controlled, the process of the present invention makes it possible to employ an inner layer having high heat insul5, ation value and low cost together with an outer higherstrength and higher-density, therefore higher-cost layer. The two layers together provide combined properties which are optimal for pipe insulation and protection at minimum total cost. -10 The extent of difference in density between inner and outer insulating layers for optimum results varies depending upon the diameter of the pipe, the relative thickness of the layers, the minimum strength desired, and the temperature differential between the pipe and the surroundings, among other things. However, in general, the apparent density of the outer layer should, be at least 25% greater than that of the inner, and the thickness of the outer layer should be from 5 to 30% of the total thickness of the foam insulation layers. If desired, the inner layer can be subdivided into two or more layers applied in sequence and differing in physical properties between thsmselves.

In the drawing: Figure 1 is a view in cross-section showing one embodiment of the invention; and Figure 2 is a view in cross-section showing another embodiment.

In the embodiment of Figure 1, metal pipe 10 is provided with an inner layer 12 of low density (2 to 6 lb/cu.ft. or 0.032 to Ο.Ο96 grm/cu.cm.) rigid polyurethane foamed insulation having a thickness from about 1 to about 6 inches (2.54 to 15.24cm.), and a second layer 14 of rigid polyurethane foamed insulation having a density at least 25% greater than that of layer 12 and having a thickness from 5 to 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 not only in having a lower apparent density and hence a higher heat insulation capacity, but also in having higher heat resistance, i.e., being stable at higher temperatures, up to 35O°F O77°C)» than is the outer layer or layers of foamed insulation, which need be stable only up to a temperature of 200°F (93·5°θ)· A second embodiment is illustrated in Figure 2 in which pipe 10, designed to carry hot fluids at temperatures of 300° to 35O°F (149° to 177°C), is provided with an inner insulation layer subdivided into a first portion or layer 20 of low density foamed polyurethane insulation formulated to have stability at a temperature as high as - 6 49 291 55O°F (177°C) and a second portion or layer 22 of approximately the same low density foamed polyurethane but formulated to have stability only at temperatures up to 200°F (93-5°C). Outer layer 24 of high density foamed polyure5 thane has a density at least 25% greater than that of either layer 20 or 22 and has a thickness from 5 to 30% of the total thickness of layers 20, 22 and 24. In another embodiment in which pipe 10 is designed to cnnvey cryogenic fluids at very low temperatures of the order of -200°F (-129°C), layer 20 is formulated to be semi-rigid instead of rigid to avoid embrittlement and possible cracking during use, while layers 22 and 24 are formulated to be conventionally rigid in nature.

In practicing the method of the present inven15 tion, the pipe, which is usually steel, is first heated to remove moisture condensation and cleaned to remove dirt, scale and rust. Any of the usual procedures for cleaning may be employed such as sand blasting or shot blasting, brushing or the like. If desired, two or more of such procedures may he employed in combination.

In some cases, it may he desirable to provide a corrosion protective coating on the pipe surface before applying the foaming plastic to it, although this is not essential. Any conventional corrosion protective coating may he employed such as wrapped polyethylene/butyl adhesive - 7 tape, epoxy coating, heat activated adhesive, or asphalt.

The foamable thermoplastic material may be applied directly to the bare pipe or to the surface of any corrosion protective coating previously applied to the pipe. It is usually desirable that the temperature of the pipe be elevated above room temperature, the precise temperature depending upon the nature and composition of the foamable plastic composition. The latter is usually applied in the form of a spray delivered from a spray nozzle as the pipe is rotated and advanced past the various nozzles. Any of a wide variety of formulations for the foamable liquid polyurethane plastic composition may be used depending upon the precise characteristics desired in the end. product.

A particularly desirable composition is a liquid mixture of ingredients reactable to form a polyurethane and including a conventional foaming agent in an amount sufficient to produce a foam having the desired apparent density, which may be from about 7.5 to about 6 pounds per cubic foot (0.024 to 0.096 grams/cu.cm.). The first layer of foamable liquid is allowed to rise by action of the foaming agent.

The thickness of the first layer of foam may vary over a wide range depending upon the same factors discussed above in connection with difference in density of the insulating layers, such as the pipe diameter and the temperature at which the pipe with its contents is designed toqaerate, 4929 1 and in general may vary from about 1 to about 6 inches (2.5^ to ^5.24 cm).

The second or outer layer 14 or 24 of foamable liquid plastic composition is then applied to the outer surface of the first foamed insulation layer by spraying in the usual manner on the surface of the rotating and advancing pipe. The second layer may be applied to the first before or after the latter has been cured, and even before foaming of the latter has been completed. Preferably it is applied before the first layer is cured. In one embodiment, the formulation of the second foamable plastic composition is different from the first only in the amount of foaming agent used and is adjusted so as to provide a foamed plastic insulation layer having an apparent density at least 25% higher than that of the first layer and a thickness from 5 to $0% of the total thickness of both layers together. In another embodiment, the outer layer nn or 24 may have a lower heat resistance than the inner or greater rigidity or both.

The total thickness of foamed insulation as well as the thickness of the inner layer (about 1 to about 6 inches or about 2.5^ to 15·24 cm.) can be varied depending upon the temperature differential between the contents of the pipe and the surroundings of the pipe, the pipe diameter, and the apparent density of the foam, among other’s. 48291 For a temperature difference of 75° to 270°F (41.5° to 15O°C ), a thickness of about 1 to about 6 inches (2.54 to 15.24 cm.) normally suffices. In general, the greater the temperature difference between the pipeline and the ground, the thicker the insulation should he. For example, cryogenic industries normally use 3 to 6 inches (7.6 to 15.24cm.) of polyurethane foam because of larger temperature differentials. For hot oil lines, tie differential is usually from 100 to 160°F (55.5° to 89°C ). For hot oil pipelines, the thickness would normally he from 1 to 2.5 inches (2.54 to 6.4cm.). A pipe for conducting molten sulphurwill be provided with 2 1/2-3 inches (6.3 to 7.65cm.) of foamed insulation, the inner layer 12 or 20 of which is formulated to have high heat resistance, i.e., to he stable at temperatures up to 300°F (149°C). A pipe for hot oil will when buried in the ground normally require a foam thickness of 2 inches (5·1cm.) or less. Polyurethane foam has an extremely low K factor, of the order of 0.13 Btu/sq. ft/hr/°F/in. (about 6.94 x 10_6grm-cal/sq.cm/sec/°C/cm.), in contrast to a K factor of about 0.39 for glass foam.

For example, in the case of a polyurethane foam having an apparent density of 3 lh/cu.ft. (0.048grm-cu.cm.), a layer 2 inches (5.1cm.) thick on a pipe 6 inches (15.24cm.) in diameter operating at a temperature differential of 125°F (69.4°C) results in a heat loss (or gain) of 5·θ Btu/hr/sq.ft - 10 »> 4 9 2 9 1 (shout 4-3.7 χ 10 grm-cal/sec/sq.cm.).

There follows a suitable recipe for making satisfactory rigid foam: Ingredient Parts By Weight 5 Crude MDI 115 Polyol A 100 Silicone oil 1 Triethylene diamine 0.5 Dibutyl tin dilaurate 0.1 in Trichloromonofluoromethane 35. Tris(2-chloroethyl) phosphate 10. All of the ingredients except the diisocyanate are pre· blended, the diisocyanate being mixed in immediately prior to foaming and spraying in conventional equipment to form a rigid foam having an apparent density of 1.8 lh/cu. ft. (0.029 grm/cu.cm.). Decreasing the proportion of trichloro monofluoromethane blowing agent leads to increase in density. Increase in solids in the foregoing recipe, as hy adding 30 parts of methyl cellulose without increase in blowing agent also increases apparent density. Increased heat resistance tf the foam is achieved hy conventional formulating techniques and selection of ingredients, as for example hy reacting aromatic amine-containing compounds with the remaining ingredients as described in Wiedermann et al. U.S. Patent 3,909,465; increased resistance to friability, also desirable for the outer layer of foam, - 11 49291 can also be achieved by selection of polyol ingredients as described for example in Fuzesi et al. U.S. Patent 5,928,257 and in Alexander U.S. Patent 5,928,258.

Any desired outer layers or coatings may be 5 applied over the outer foamed insulation layer. For example, the outer foamed insulation layer can be provided with a wrapped or extruded protective or moisture barrier layer of paper or plastic, e.g., polyethylene or polypropylene, an envelope or wrapped layer of heat10 shrinkable plastic which is shrunk in place to conform to the foamed insulation, a coating of liquid epoxy resin cured in place or a coating of bituminous mastic, or a layer of any other conventional protective material. However, because of the tough crush resistant and abrasion resistant nature built into the outermost layer of foamed polyurethane plastic in the present invention, any supplemental outer protective layer need in most cases provide only a barrier against puncture and moisture penetration. 4S291

Claims (9)

1. A method of providing pipe with at least one inner layer and an outer layer of foamed plastics insulation, which comprises applying to said pipe at least 5 one inner layer-forming foamable plastics composition, foaming the or each composition to form a respective separate layer of foamed plastics insulation having an apparent density of from 1.5 to 6.0 lbs/cu.ft (0.024 to 0.096 g/cc), applying to the surface of the last applied. 10 inner layer an outer layer-forming foamable plastics composition and foaming and curing the outer-layer forming composition to form a separate outer layer of rigid foamed plastics composition having an apparent density at least 25% greater than that of the, or any, 15 inner layer and having a thickness of from 5 to 30% of the total thickness of the layers.

2. A method as claimed in Claim 1 and further comprising previously applying to the pipe one or more further inner layer-forming compositions to form one or 20 more further inner layers.

3. A method as claimed in Claim 1 or 2, wherein the pipe is a metal pipe, such as a steel pipe, and further comprising initially applying a corrosion protective coating to the pipe. 49281 - 13

4. A method as claimed in any one of the preceding claims, wherein the foamable plastics compositions are nolyurethane compositions.

5. A method as claimed in any one of the preceding claims, wherein the innermost layer of foamed plastics insulation is stable at temperatures of up to 35O°F (177°C) end the outer layer or layers are stable only at temperatures of up to 200°F (93.5°C).

6. A method as claimed in any one of the preceding claims, wherein the innermost layer is rigid.

7. A method as claimed in any one of Claims 1 to 5, wherein the innermost layer is semi-rigid.

8. A method of providing pipe with foamed plastics insulation, substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

9. A pipe when insulated by a method as claimed in any one of the preceding claims.