WO2004003423A1

WO2004003423A1 - Pre-insulated pipe

Info

Publication number: WO2004003423A1
Application number: PCT/DK2002/000432
Authority: WO
Inventors: Jan Rasmussen
Original assignee: A/S Star Pipe
Priority date: 2002-06-27
Filing date: 2002-06-27
Publication date: 2004-01-08
Also published as: AU2002315245A1

Abstract

A pre-insulated pipe such as a district heating service pipe comprising one or several inner pipes carrying a gas or a fluid, and an insulating foam preferably a polyurethane (PUR) foam surrounding the inner pipe and optionally a durable outer casing, e.g. of a polyethylene material surrounding the foam and the inner pipe. Surrounding the foam core and preferably applied inside of the optional PE-casing is provided a diffusion- regulating layer of a barrier material such as ethyl-vinyl-alcohol (EVOH) to control the individual passage of gases in and out of the PUR foam so that the thermal conductivity is maintained or preferably reduced over the lifetime.

Description

PRE-INSULATED PIPE

TECHNICAL FIELD

The present invention relates to pre-insulated pipes of the type comprising one or several inner pipes carrying a gas or a fluid, and an insulating foam, preferably polyurethane foam (PUR), surrounding said inner pipe or pipes.

Pipes of this kind are being used in many different applications, e.g. district heating piping, hot or cold water distribution, gas distribution, where the medium should keep its temperature high or low within the insulated pipe. Moreover, it is a prerequisite to have a pipe insulation, which maintains its insulation efficiency essentially unchanged over many years, preferably 50 years or more.

BACKGROUND OF THE INVENTION

For district heating service pipes it is essential for economic reasons to waste as little energy as possible between the heating plant and the consumers, so it is important to provide as much insulation as possible in the limited space, where the pipes are to be placed.

In addition pre-insulated pipes are a valuable means to maintain a certain temperature of gases or fluids in many industrial processes.

Moreover, the official European standard EN 253 dealing with district heating pipes is stating fixed rules for the construction and dimensions of the pipes, the materials and the pertinent parameters, among which is the insulation property, which must not degrade very much over time.

According to EN 253 the heating pipe is comprised of at least three members: An inner service pipe of steel, an outer Polyethylene-casing and between said pipes or tubes is arranged an insulating foam. Polyurethane (PUR) foam has proved to be a very expedient insulating material, and said standard specifically defines the insulation material as being PUR foam.

However, PUR has shown the disadvantage that over the years the insulation efficiency degrades, apparently due to gas diffusion between the foam cells and the environment. It is believed that the degradation comes from the fact that the blowing agents, mainly carbon dioxide and cyclopentane in the foam cells are disposed to let themselves replace by nitrogen and oxygen, said nitrogen and said oxygen showing higher thermal conductivities, thereby reducing the overall insulation efficiency of the foam. In older foam blowing techniques, also CFC gases are used as blowing agents.

In order to overcome such gas diffusion it is known to provide an aluminum foil on the inside of the PE-casing of current pipes and outside of the PUR layer, which is a very efficient way of setting up a two-way barrier to gases. Unfortunately, this also means that the desired dissipation of CO₂ without being replaced by less favorable gases is not going to happen, and the process step of applying an aluminum foil to the inside of the PE-casing is labor intensive and thereby cost sensitive.

DISCLOSURE OF THE INVENTION

Accordingly, it is the object of this invention to avoid or at least reduce the extent to which the inappropriate gas diffusions occur and to allow for some CO₂ dissipation out of the PUR foam.

Moreover, it is a further object of the invention to present a solution according to which it is easy to apply the gas diffusion regulating barrier on the outside of the PUR foam. With such barrier material it is also essential to obtain a reliable adhesion to the PUR foam and to the PE-casing, if used.

According to the invention, a layer of a diffusion-regulating material or barrier material is applied around the insulating PUR-foam in order to protect the foam from penetrating gases and preserve its low thermal conductivity over the life time.

A surprising result of applying said barrier material is that not only will the destructive gases be stopped or substantially hindered in advancing the aging of the PUR foam, but the foam might initially experience a reduction in thermal conductivity due to dissipation of CO₂ from inside the cell cavities. Furthermore, as the ingress of deteriorating gases is to a large extent avoided, the well-known aging of PUR foam is not experienced to any significant degree with the barrier material in place. By this way great energy savings could be foreseen.

Another advantageous effect of the barrier material is that the material is easy to apply not only in manufacturing the pre-insulated pipes, but also in situ molding of pipe bends and T-pieces takes advantage over the handy application of the barrier material.

In one embodiment of the invention, the diffusion-regulating layer is comprised of a barrier material such as an ethyl-vinyl-alcohol (EVOH).

In another embodiment of the invention, the district heating pipe is further comprising a durable outer polyethylene casing surrounding the PUR foam and having said barrier layer applied to the inside surface thereof.

In still another embodiment of the invention, the diffusion-regulating layer is co- extruded with the outer PE-casing.

A further embodiment of the invention is the application of the barrier layer during in situ molding of a piping.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed part of the present description, the invention will be explained in more detail with reference to the exemplary embodiments of a pre-insulated pipe according to the invention shown in the drawings, in which

Figure 1 shows one end of a usual service pipe according to EN 253, Figure 2 shows one end of one embodiment of a service pipe according to the invention, Figure 3 shows one end of another embodiment of a service pipe according to the invention, Figure 4 is a diagram illustrating the impact of the aging process in the PUR foam on the thermal conductivity over time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

European standard EN 253 "District heating pipes - Preinsulated bonded pipe systems for directly buried hot water networks - Pipe assembly of steel service pipes, polyurethane thermal insulation and outer casing of polyethylene" defines the required quality properties of district heating pipes. Among others, the standard defines the PUR foam by its minimum specific gravity (80 kg/m³), maximum cell size (0,5 mm), maximum thermal conductivity (0,33 W/mK) and lifetime (> 30 years). Moreover, the PE-casing is defined by a minimum thickness.

The standard does not take into consideration that the inner service pipe might be produced of other materials than steel, but other materials capable of transporting hot gas or fluid could equally well be used as seen from a technical point of view only. Such materials could be e.g. copper, aluminum, PEX, etc. Furthermore, said service pipes could be flexible pipes as well.

The PUR foam stated in the standard is used because it presents a very good insulation means, and so far the pipe manufacturers have not introduced an alternative material.

For the outer casing, standardization has stated that the accepted material is high- density polyethylene (HDPE), while also here other materials might be equally useful. PE is durable but in terms of gas permeability it definitely shows less favorable properties.

The Danish Technological Institute has tested pre-insulated service pipes produced in accordance with EN 253 and found the insulation efficiency reduced by more than 10% over the expected lifetime, and according to a recent technical dissertation "Long- Term Thermal Performance of Polyurethane-insulated District Heating Pipes", 2001 , Maria E Olsson of Chalmers Technical University, the degradation is said to be increased by about 30%. Such degradation or aging is referred back to inexpedient diffusion of gases in and out of the PUR foam, such that the cell structure initially containing approximately 50-50 mixtures of cyclopentane and carbon dioxide ends up containing a mixture of oxygen and nitrogen, which results in a less favorable insulating foam.

In an attempt to solve this problem, pre-insulated pipes have been manufactured with an aluminum foil attached to the inside surface of the PE-casing, whereby is obtained a substantially 100% active diffusion barrier. However, such aluminum foils have to be glued to the PE-casing, which is a complex and labor-consuming process. A further solution would be the use of a PE-coated aluminum foil, but this means a complex and expensive product compared to what is suggested by the invention.

The invention is based, at least in part, on the surprising discovery that it will be possible to improve the long-term insulating properties by protecting the polyurethane foam in insulated service pipes through the application of a diffusion-regulating layer, where advantage is taken from the fact that the barrier is not equally efficient to stop every prevailing type of gas, and that especially CO₂ passes easier through the chosen barrier than other gases do.

In Figure 1 is shown a usual pre-insulated district heating pipe 1 which complies with the EN 253 standard and comprises an inner pipe 2 carrying a gas or a fluid, and an insulating foam 3, preferably a polyurethane foam (PUR), surrounding the inner pipe 2, and further surrounded by a durable PE-casing 4.

Figure 2 shows a first embodiment of the invention, where the essential parts of a pre- insulated pipe including the service steel pipe 2 and the insulating foam 3 are surrounded by a gas-diffusion-regulating layer 5 with barrier properties allowing more CO₂ to diffuse than other prevailing gases such as O₂ and N₂.

Figure 3 shows the same pipe as in Figure 2 but further comprising a durable outer PE-casing 5 to keep the pipe and insulation layer free of damages from the handling and presence in the ground or other installation site.

The EVOH demonstrates a very good adhesion to as well the PUR foam as the PE casing, which is pertinent for the pipe assembly to be diffusion tight. In a preferred embodiment of the invention, a pre-insulated pipe 1 according to Figure 3 is comprised of an inner steel pipe 2 transporting the gas or fluid at a temperature of about 130°C, a layer of PUR foam 3 surrounding the inner pipe 2, a gas-diffusion- regulating layer or barrier layer 5 of an EVOH material available from EVAL Company of the U.S.A. sheathing the PUR foam 3, and a PE-casing enclosing and adhering tightly to the EVOH layer. One of the EVAL^® grades of barrier material, e.g. F101 B offers a permeability (cm³.20 μm/m².dag.atm at 25°C) of 0.81 for carbon dioxide 0.27 for oxygen and 0.017 for nitrogen. Consequently, the barrier material presents about 50 times stronger hindrance to nitrogen than to carbon dioxide while the hindrance against oxygen is at least 3 times stronger than it is to carbon dioxide.

The diagram in Figure 4 illustrates the thermal conductivity of PUR foam in different types of insulated pipes at different stages of the service life cycle. Traditional pipes show an increasing conductivity, while the aluminum-foil-encased pipe is essentially not changing, while the pipe according to the invention shows an improving insulation performance in the initial stage followed by a slowly developing increase in conductivity but not exceeding the aluminum-foil-encased pipe within the service lifetime, if proper dimensions are chosen.

Initially, the blow molding of PUR foam generates a high number of cells filled with gases of the blowing agents and reaction gases such as cyclopentane and carbon dioxide, whereof the cyclopentane shows a 30% lower thermal conductivity than do the carbon dioxide. The carbon dioxide tends to diffuse out of the cells leaving behind the less thermal-conductive cyclopentane and accordingly, the PUR foam increases the insulation efficiency, if not oxygen or nitrogen takes up the space. The barrier material according to the invention will police the gas diffusion to the benefit of an advantageous insulating foam, while later in the service lifetime some of the undesired gases may eventually penetrate the foam and to some extent raise the conductivity.

From a manufacturing point of view, the EVAL^® product is very useful because it can easily be applied by spraying means, which extends the area of use to also cover in situ insulating piping with odd geometry, being a very tough and expensive problem to solve with aluminum foils.

The chemical reaction between the EVAL^® product and the isocyanate of the foam will produce a strong mutual adhesion easily complying with the requirements of EN 253. As mentioned earlier, there may be diffusion-regulating-barrier materials, e.g. EVOH materials, which can serve same purpose as the EVAL^® products, and it is obvious that no matter the choice of barrier material it will be covered by the scope of the invention.

The previously mentioned use of CFC gases, which in most developed countries is abandoned, is of course no exception to the coverage of the present invention.

It is obvious to those skilled in the art that modifications and variations of the embodiments described are possible without falling outside the scope of the invention and as defined by the following claims.

Claims

1. Pre-insulated pipe (1) of the type comprising one or several inner pipes (2) carrying a gas or a fluid, and an insulating foam (3), preferably a polyurethane foam surrounding the inner pipe (2), characterized by a diffusion-regulating layer (5) being applied on the outside of the foam.

2. Pre-insulated pipe according to claim 1, ch aracte rized by the diffusion- regulating layer (5) having barrier properties showing a stronger barrier to O₂ an N₂ than to CO₂.

3. Pre-insulated pipe according to claim 1 or 2, charact erized by the diffusion- regulating layer (5) being comprised of an ethyl-vinyl-alcohol (EVOH).

4. Pre-insulated pipe according to claim 2 or 3 and further comprising an outer casing (4) of a durable material, preferably a polyethylene casing, characterized by the diffusion-regulating layer (5) being provided between the polyethylene casing (4) and the polyurethane foam (3).

5. Pre-insulated pipe according to claim 2 or 3 and further comprising an outer casing (4) of a durable material, preferably a polyethylene casing, characterized by the diffusion-regulating layer (5) being provided on the outside of the polyethylene casing (4).

6. Pre-insulated pipe according to claim 4 or 5, charact erized by the diffusion- regulating layer (5) being coextruded with said outer casing (4).

7. Pre-insulated pipe according to claim 4 or 5, charact erized by the diffusion- regulating layer (5) being applied by spraying onto the polyethylene casing (4).

8. Pre-insulated pipe according to claim 4 or 5, charact erized by the diffusion- regulating layer (5) being applied by smearing onto the polyethylene casing (4).

9. Pre-insulated pipe according to any of the preceding claims, characterized by the inner pipe (2) being a flexible pipe.

10. Pre-insulated pipe according to any of the preceding claims, characterized by the inner pipe (2) being made of steel, copper, aluminum, PEX or other suitable material.

11. Pre-insulated pipe according to any of the preceding claims, characterized by the diffusion-regulating material (5) and the outer casing material (4) being intermixed before application onto the insulating foam (3).