WO2008025529A1

WO2008025529A1 - Pipe section comprising a tubular insulating element of a fire retarding material

Info

Publication number: WO2008025529A1
Application number: PCT/EP2007/007541
Authority: WO
Inventors: Lars Elmekilde Hansen; Kurt Munk Petersen; Herbert Pieper
Original assignee: Rockwool International A/S
Priority date: 2006-08-29
Filing date: 2007-08-29
Publication date: 2008-03-06

Abstract

A pipe section (1) comprising a tubular insulating element (2) having an outer surface (3) and an inner surface (4) and an insulating material comprising mineral wool, such as glass wool, stone wool or slag wool, arranged between the outer surface (3) and the inner (4) surface wherein the insulating material has an annular outer section extending inwardly from the outer surface, an annular inner section extending inwardly from the inner surface and an annular core section between the annular outer section and the annular inner section. A fire retarding material (7) is arranged in the mineral wool in such manner that the concentration of the fire retarding material (7) in the mineral wool is higher in one or two secions selected from the annular outer section, the annular inner section and the annular core section, than in the other sections or section. Methods for producing such a pipe section are also described.

Description

PIPE SECTION COMPRISING A TUBULAR INSULATING ELEMENT OF A FIRE RETARDING MATERIAL

The invention relates to a pipe section in accordance with the introduction of claim 1. The invention also relates to a method for producing such a pipe section.

Pipe sections comprising mineral wool as the insulating layer have proven to have good insulating properties as well as good resistance against fire. The last property is important when pipe sections are used for fire protecting pipes conveying a flammable liquid, sprinkler pipes, circular ventilation ducts, or to protect circular constructional steel members, hangers for ventilation ducts or sprinkler pipes, or when pipes penetrate vertical or horizontal fire rated partition walls.

Generally, the resistance against fire is increased when the thickness of the insulating layer is increased. Thus, when a structure such as a circular constructional steel member or a pipe conveying a flammable liquid is to be fire protected, the thickness of the insulating layer must be adjusted in such manner that it prevents the structure from being exposed to a critical temperature for a desired time, which is often 30, 60, 90 or 120 minutes. Traditionally this is accomplished by simply using thicker pipe sections for higher fire rated structures. However, often it is desirable to use as thinner insulating layer as possible in order to minimise the bulkiness of the structure. There is therefore a desire to reduce the diameter of the pipe section, but maintaining the good fire protecting properties.

WO 97/20780 describes a man-made vitreous fibre product through which is substantially uniformly distributed a particulate endothermic material. The chosen endothermic material works as a fire retarding material since it is stable at up to 200 °C, but decomposes endothermically at temperatures above 200 ⁰C. If the product is shaped as slabs or plates it is suitable for use in e.g. fire doors or for fire protection of steel structures. It is also mentioned that the product can be shaped as a pipe or be used as granulate.

In WO 97/20780 the fire retarding material is substantially uniformly distributed in the product which certainly adds to the resistance against fire. However, when the product is a pipe section the uniform distribution of the fire retarding material has a number of disadvantages. In case of external fire the outermost fire retarding material will quickly be exposed to temperatures well above 200 ⁰C, and the fire retarding properties will quickly disappear. The innermost fire retarding material, i.e. closest to the insulated pipe or structure, will not be exposed to temperatures above 200 ⁰C until very late, probably not until other structures have already failed. Similarly, in case of internal fire, e.g. fire within a ventilation duct, innermost fire retarding material will quickly be exposed to temperatures well above 200 ⁰C, and the fire retarding properties will quickly disappear, while the outermost fire retarding material will not be exposed to temperatures above 200 ⁰C until very late. Therefore, in relation to fire protection the fire retarding material in the sections closest to the outer and inner surfaces of the pipe section can be considered superfluous and adds unnecessary to the weight and costs of the pipe section. Another disadvantage of having the fire retarding material distributed uniformly in the pipe section is that during handling of the pipe section fire retarding material will fall off from the inner and outer surfaces and from the axial opening slit which is most often present in pipe sections, unless all surfaces are covered by e.g. a glass fleece.

Based on these observations it is an object of the invention to provide a pipe section that has improved fire protection properties compared to conventional pipe sections, but does not have the disadvantages of the pipe section mentioned above.

This object is achieved by providing a pipe section as stated in the characterizing part of claim 1. Hence, the invention involves providing a pipe section that has a higher concentration of fire retardant material in one of the annular sections than in another of the annular sections. This means that the concentration of fire retarding material can be higher in one or two sections selected from the annular outer section, the annular inner section or the annular core section, than in the other sections or section.

Thereby is obtained a pipe section that has good fire protection properties without superfluous amounts of fire retarding material. Compared to conventional pipe sections without any fire retarding material the outer diameter of the pipe section can be reduced and still provide the same fire protection properties. Compared to pipe sections provided with a uniformly distributed fire retarding material the pipe section according to the invention has a reduced weight.

When the pipe section is designed for use in an environment where a fire is most likely to occur inside the pipe, i.e. adjacent the inner surface of the pipe section, it can be advantageous to have a higher concentration of fire retarding material in the annular inner section and optionally also the core section than in the outer section. An example of a situation where this embodiment could be used is when the pipe is used to carry a flammable substance such as a fuel like oil or natural gas, or a substance that is used in an industrial process.

In contrast, when the pipe section is designed for use in an environment where a fire is most likely to occur outside the pipe, i.e. adjacent the outer surface of the pipe section, it can be advantageous to have a higher concentration of fire retarding material in the annular outer section and optionally also in the core section than in the inner section. An example of a situation where this embodiment could be used is when the pipe is used to carry a non-flammable substance, such as water, through an environment in which there is an increased likelihood of fires occurring, such as in a chemical factory or industrial works.

In the preferred embodiment, there is a higher level of fire retarding material in the annular core section than in the annular outer section or the annular inner section. In this embodiment, particularly efficient protection is provided by the fire retarding material and the pipe section is suitable for use in an environment where the threat of fire can be from either inside the pipe, or outside the pipe.

Furthermore, in this embodiment, there is a reduced tendency for dusting or falling off of fire retarding material from the inner and/or outer surfaces.

In a first embodiment the fire retarding material is present in the annular core section only. Thereby is ensured that no fire retarding material is exposed to the outer and inner surfaces whereby the risk of fire retarding material falling off from these surfaces is eliminated.

The annular core section may have a radial thickness of between 1% and 90%, preferably between 5% and 50%, of the radial thickness of the insulating element. Correspondingly, the annular inner and annular outer sections each have a radial thickness of between 5% and 49.5%, preferably between 25% and 47.5% of the radial thickness of the insulating element.

According to the preferred embodiment of the invention, the more concentrated the fire retarding material is in the annular core section the better fire protecting properties of the pipe section. However, it is also desirable that the pipe section maintains its integrity, and therefore the annular core section must have a certain radial width.

The fire retarding material is preferably a particulate material that decomposes endothermically at increased temperatures, such as particulate material that liberates water at temperatures above 180 °C. It may be a carbonate or hydrate, and preferably it is magnesium hydroxide. The particulate material should have a mean particle size of between 0.5 mm and 10 mm, preferably between 2 mm and 5 mm.

Such particulate material is able to maintain its water liberating properties even after having been through a curing oven as it is conventional in the manufacturing of mineral wool pipe sections.

It is also an object of the invention to provide a new method for producing a pipe section with the features described above. Generally, pipe sections comprising an insulating layer of mineral wool are made by first preparing a web of mineral wool which is then treated in different manner depending on the pipe section to be produced.

Thus, it is known to produce pipe sections by producing batts of mineral wool with a cured binder and cutting the pipe sections from the batts. Another known method is to wind an uncured web of mineral wool around a mandrel and then cure the binder in the mineral wool and remove the pipe section from the mandrel. A third method is to form pleatings or wavings of a web of mineral wool with an uncured binder, curing the binder, cutting the pleatings into smaller pieces consisting of half pleatings, removing material from each smaller piece by milling to form it into a half-annular shape and finally joining two such half- annular pieces to form the pipe section.

In order to produce the new pipe section according to the invention new methods have been developed, which methods include the following known features: preparing a web of mineral wool, adding a fire retarding material to the mineral wool during or after preparation of the web of mineral wool, and manufacturing the pipe section from the mineral wool. These features are known from WO 97/20780 mentioned above. In order to achieve a pipe section according to the invention the method also comprises that the fire retarding material is added to the web of mineral wool in such manner that after manufacturing of the pipe section from the mineral wool the concentration of the fire retarding material in the mineral wool is higher in one or two sections selected from the annular outer section, the annular inner section or the annular core section, than in the other sections or section, preferably the concentration is higher in the annular core section between the outer surface and the inner surface than in the annular outer and inner sections.

In a first embodiment for producing a pipe section according to the invention the fire retarding material is added to a surface of the web of mineral wool, and subsequently the web of mineral wool is wound around a mandrel. By this method a so-called wound pipe section is made. Such a pipe section is relatively rigid and provides excellent heat insulating properties.

The fire retarding material is preferably added to a surface of the web immediately before winding whereby the risk of loosing fire retarding material during handling is minimised.

In order to achieve a pipe section with good integrity the fire retarding material is added to the surface of the web in a length that corresponds to 1-10 windings, preferably 2-5 windings.

In a second embodiment for producing a pipe section according to the invention the fire retarding material is added to one or two sections of the web of mineral wool, preferably added predominantly to the core section only and subsequently the following steps are performed:

• pleating the web into a plurality of pleatings,

• dividing the pleated web at a centre plan to form smaller pieces each comprising half a pleating, • mechanically working each of the smaller pieces to a semi-annular shaped body,

• joining two semi-annular shaped bodies to form a pipe section.

In either of the two embodiments for producing a pipe section the fire retarding material is preferably a particulate material that decomposes endothermically at increased temperatures, such as particulate material that liberates water at temperatures above 180 ⁰C. It may be a carbonate or hydrate, and preferably it is magnesium hydroxide. The particulate material should have a mean particle size of between 0.5 mm and 10 mm, preferably between 2 mm and 5 mm.

The invention will be described in the following with reference to the drawings in which

Figure 1 shows a pipe section according to a preferred embodiment of the invention provided with a particulate fire retarding material provided substantially only in an annular core section of the insulating layer;

Figure 2 shows schematically a first method for producing a pipe section according to a preferred embodiment of the invention as shown in Figure 1 ; and

Figures 3a and 3b show schematically a second method for producing a pipe section according to a preferred embodiment of the invention as shown in Figure 1.

Figure 4 is a diagrammatic illustration (not to scale) of apparatus for use in a process of making a web suitable for use in making the pipe section of the invention;

Figure 5 is a diagrammatic illustration of two positions between which the baffle and ejection apparatus may be moved; Figure 6 is a perspective view of the underside of a baffle for use in the invention;

Figure 7 is a side view of the same baffle;

Figure 8 is an under view of a different baffle for use in the invention;

Figure 9 is a side view of the same baffle;

Figure 10 is a view from beyond the distal end of the extensions shown on the baffle of Figure 9;

Figure 11 is a cross section of a web suitable for use to make the product of the invention; and

Figure 12 is a diagrammatic illustration (not to scale), of a side view of a baffle.

Figure 1 shows a pipe section 1 according to the invention. The pipe section comprises an insulating material that forms a tubular insulating element 2 with an outer surface 3 and an inner surface 4. The insulating material comprises mineral wool, such as glass wool, stone wool or slag wool, where stone wool is preferred due to its excellent fire properties.

The outer surface 3 of the insulating material is preferably provided with a protective covering 5, such as an aluminium foil or the like, that preferably is glued to the insulating material as it is conventional in the field of pipe sections.

The inner surface 4 is preferably not covered. This is also conventional. The pipe section 1 is provided with an axial opening slit 6 that allows the pipe section 1 to be opened and mounted on a pipe (not shown). The axial opening slit 6 preferably has a radial extent of up to 4/5 of the outer diameter D of the pipe section 1. Thus, the axial opening slit 6 extends radially all the way through one layer of the tubular insulating element 2 and well into to the diametrically opposite layer. This design allows the pipe section 1 to be opened with only a minor risk of breaking the pipe section 1.

As it clearly appears from Figure 1 the pipe section 1 is provided with a fire retarding material 7 that is arranged in an annular core section 8 while the sections being closer to the outer surface 3 and inner surface 4 are free or substantially free of fire retarding material 7. Alternatively, the fire retarding material could be arranged predominantly in an annular inner section 22 or annular outer section 21.

The fire retarding material 7 is preferably a particulate material that decomposes endothermically at increased temperatures. One example of such material is magnesium hydroxide that liberates water at temperatures above 200 ⁰C. The magnesium hydroxide may have mean particle size of between 0.5 mm and 10 mm, preferably between 2 mm and 5 mm. Another example is aluminium hydroxide that liberates water at temperatures above 180 ⁰C. Other materials may also be used.

Figure 2 shows a first embodiment for producing a pipe section 1 according to the invention. In this method the pipe section 1 is wound from a thin web 9 of mineral wool, preferably stone wool. The web 9 is wound around a mandrel 10 as it is known when producing wound pipe sections. Before or during the winding of the web 9 fire retarding material 7 is distributed on the upper surface of the web 9 at a predetermined area A. Preferably the area A spans the whole width W of the web 9 such that fire retarding material 7 is present continuously from one end of the pipe section 1 to the other. The fire retardant material is applied so that it is predominantly in the annular inner section 22, the annular core section 8 or in the annular outer section 21. However, if desired the fire retarding material 7 may also be applied only in a narrower area such that it does not extend to the edges 10 of the web 9. In such case the fire retarding material 7 is not exposed at the end of the pipe section 1 and the risk of falling off material is eliminated.

Preferably the length L of the area A corresponds to at least one full revolution of the winding such that it is ensured that the fire retarding material 7 is present annularly in the pipe section 1. It is possible that the fire retarding material 7 is applied at an area A with a length L that is a little shorter than one winding of the pipe section 1. An advantage of this is that it would then be possible to provide the axial opening slit 6 in an area without fire retarding material 7. Thereby the risk of fire retarding material 7 falling off at the axial opening slit 6 is eliminated. This embodiment would require accurate position control during manufacturing of the pipe section 1 which may add to the costs.

In a preferred embodiment the length L of the area A corresponds to a plurality of windings, such as up to 10 windings. This of course increases the thickness of the annular core section 8 provided with the fire retarding material 7, but it ensures better integrity of the insulating material than if the same amount of fire retarding material 7 is concentrated in only one winding. The preferred length of the area A should depend on the amount of fire retarding material 7 that is to be added to the pipe section 1. Thus, it should be ensured that the pipe section 1 still has good integrity and does not delaminate after winding. Normally, 2-5 windings provided with the fire retarding material 7 would suffice.

The fire retarding material 7 is applied by an applicator 11 positioned above the web 9 to be wound. The applicator positions the materials in the appropriate areas of the web, i.e. so that it becomes concentrated in the annular inner, core or outer section depending upon where the fire retardant material is likely to be needed. The applicator 11 may be of any construction that provides the desired distribution of fire retarding material 7. The actual construction of the applicator 11 depends on the physical properties of the fire retarding material 7, i.e. whether it is applied in dry or wet form, the particle sizes, etc. A preferred applicator 11 is the baffle described below.

Figures 3a and 3b show a second embodiment for producing a pipe section 1 according to the invention. In this method the pipe section 1 is produced from a thick web 12 that undergoes different process steps. The web 12 is produced in such manner that the concentration of a fire retarding material 7 is higher in the regions that will become one or two sections selected from the annular outer section 21 , the annular inner section 22 and the annular core section 13, than in the region that will become the other sections or section, and is preferably higher in the regions that will become the core section 13. This may be accomplished by any of the methods described in WO 99/51536 or in PCT/EP2006/08068. Alternatively, a new method for making mineral wool pipe section may be used, as outlined below. The web 12 with the fire retarding material 7 concentrated in the core section 13 is then pleated as shown in Figure 3a by means of an up and down moving mechanism 16 to form pleatings 17. The pleated web 12 is now led to a curing oven 18 that cures the binder provided in the web 12, such that stable pleatings 17 as shown in Figures 3b are formed. The pleated web 12 is then divided at a centre plan to form separate smaller pieces 19 that each comprises half a pleating 17. These smaller pieces 19 are then worked mechanically, e.g. by milling, to semi-annular shaped bodies 20. Two such semi-annular shaped bodies 20 are then joined to form a pipe section 1. This process without a fire retarding material 7 is described in WO 97/01006.

Since the initial web 12 is provided with a fire retarding material 7 in the core section 13 the fire retarding material 7 will be present in the core section of each of the semi-annular shaped bodies 20 and thus in an annular core section 8 of the pipe section 1 , when two such semi-annular bodies 20 are interconnected.

Webs that have a depthwise distribution of fire retarding material that are used to make pipe sections according to the invention are preferably made by a new method that is specially adapted to make a product having a controlled depthwise distribution of fire retarding material and which allows easier and more accurate control of the distribution of the fire retarding material and which can more easily be applied to conventional apparatus which utilises centrifugal spinners which rotate about a substantially horizontal axis.

The method involves making a man-made vitreous (MMV) fibre product comprising, through the thickness, a core layer between adjacent layers. Once the web is made into a pipe section, for example in accordance with the methods shown in Figures 3a and 3b, the core layer becomes the annular core section, and the adjacent layers become the annular inner sections and annular outer sections depending upon the orientation.

The method of making a man-made vitreous (MMV) fibre product comprising a core layer 132 between adjacent layers 133 comprises; fiberising a mineral melt using a spinner comprising one or more fiberising rotors 101 , 102, 103, 104 which rotate about a substantially horizontal axis, and entraining the fibres in air 105 travelling substantially horizontally as a cloud of fibres, collecting the fibres from the cloud as a web 1 14 on a permeable collector

111 which travels continuously along a path comprising, in sequence, an initial collecting zone A, an intermediate collecting zone B and a final collecting zone C whereby the fibres of the core layer 132 are collected on the collector 111 in the intermediate collecting zone B and the fibres of the adjacent layers 133 are collected on the collector 111 in the initial and final collecting zones A, C, and forming the MMV fibre product from the web 114, wherein one or two layer selected from the core layer 132 and adjacent layers 133 comprises MMV fibres mixed with fire retarding material and the other layer or layers comprise MMV fibres optionally mixed with the fire retarding material in an amount which is substantially less, and wherein the fire retarding material is directed downwardly through the cloud of fibres as a region of downwardly directed particles 134 which extends, at the surface of the web 114, across the width of the web and in the length direction substantially only in one or two of the collecting zones A, B and C.

The fire retardant material may simply be injected through a slot which has substantially the dimensions of the region of downwardly directed particles. However, it is preferable to achieve the downward supply of fire retarding material by ejecting a stream of the fire retarding material on to the underside of a baffle, or a plurality of streams on to the undersides of a plurality of baffles (i.e., each stream is directed on to the underside of a different baffle). The or each baffle is in or above the upper part of the cloud. The baffle is shaped, or the baffles are shaped, to deflect the fire retarding material downwardly as a diverging region of downwardly directed particles which, at the surface of the web, extends across substantially the entire width of the web and extends in the lengthwise direction substantially only one or two of the collecting zones.

The baffle (and in particular its underside) can have any shape appropriate for converting the stream of particulate fire retarding material into the desired size and shape of the region of falling fire retarding material at the surface of the web.

Accordingly it is necessary to select the baffle according both to the configuration of the stream of particles injected on to it and having regard to the dimensions of the region over which the particles are to be distributed on the surface of the web. The baffle must also be shaped so as to deflect the particles laterally in order that the falling particles fall through a region which diverges both in the lengthwise direction and the widthwise direction.

The lateral spreading may be achieved by providing ribs on the underside of the or each baffle wherein the ribs diverge outwardly in the direction of ejection in order to deflect the particles transversely across the web.

Instead of or in addition to relying on ribs of this type, the undersurface can be shaped so that it has the form of a shallow V, in the form of a central base and a wing which extends upwardly from each side of the base. The combination of the smooth downward inclination of the baffle lengthwise and the transverse shaping of the wing is selected so as to obtain the desired amount of outward deflection of the particles. The inclination of the wings, even at their steepest point close to the base, is usually quite low so that the two wings usually make an angle of at least 160° and frequently 165 or 170° up to about 179°. Put another way, the overall slope of the wings from the base to the outermost point of each wing is generally in the range 1 % to 15%, often around 3 to 10%. Each wing usually diverges, so as to promote lateral spreading.

The apparatus of Figure 4 comprises a cascade spinner 101 having a plurality of rotors 102, 103 and 104 arranged as a cascade in conventional manner whereby each rotor rotates about a substantially horizontal axis, and wherein the cascade is provided with air ducts 105 through which air is forced substantially horizontally around the rotors and therefore the entire spinner. Mineral melt is fed down a gutter 106 from a furnace 107 on to the top rotor off which it is thrown centrifugally in sequence on to the other rotors, whereby fibres are thrown off the rotors and are entrained in the air from one or more air ducts 105. The fibres are carried forwards from the rotors as a cloud of fibres into a collecting chamber 108 which has a roof 109 and a base 110 defined by a continuously moving permeable collector. The collector travels substantially horizontally away from the spinner and thus fibres gradually accumulate on the collector as it travels from an initial position 112 (below which substantially no fibres are collected) to a final position 113, at which the web 114 has reached its final maximum depth.

A duct 115 ejects a stream of fire retardant material on to the underside of a baffle 116 which is positioned in an opening 120 in the roof 109 to deflect the particles downwardly on to the collector. The arrangement of the ejector and the baffle is such that the stream 134 of particles is spread out laterally and longitudinally so as to extend across substantially the entire width of the web and so that substantially all the particles are collected on the web between points 117 and 118. Accordingly the initial collecting zone (A) extends between points 112 and 117, the intermediate collecting zone (B) extends between points 117 and 118 and the final collecting zone (C) extends between points 118 and 113.

The baffle 116 can be partly inside and partly outside the chamber during operation but usually it is mainly or wholly inside the chamber. In order that the chamber can be used for normal manufacture when the baffle is not required, preferably the baffle is adjustable between an operating position and a dormant position. These two positions are shown in Figure 5 where the dormant position is in solid line and the operating position is in dashed line. Preferably there is a slide 117 which is slid backwards to provide the opening 120 when required and which is then slid forwards to close the opening when the baffle 1 16 has been raised to the dormant position.

One preferred baffle is shown in Figures 6 and 7 and comprises a baffle plate 121 by which the plate can be mounted on a suitable open framework 122 rigidly connecting the baffle to the outlet 123 of the ejector 115. As shown in Figures 6 and 7, the baffle is curved downwardly and carries on its underside diverging ribs 124 for promoting lateral spreading of this stream of particles. The distal end 125 of the baffle carries spaced-apart extensions or fingers 126 which are similarly curved downwardly. The varying length and angulation of the underside of the baffle which is encountered by the stream of particles results in increased lengthwise spreading (i.e., increased distance between points 1 17 and 118) than is obtainable if the distal end is a uniform straight edge.

In Figures 8 and 9 the extensions 126 at the distal end 125 of the baffle are curved downwardly at a similar angle to the plate. However, as shown in Figure 12, it is preferred for the protrusions 126 to curve downwardly more steeply that the remainder of the baffle.

Instead of or in addition to having the ribs 124, the underside of the plate can be as shown in Figures 8 to 10. The underside of the plate in Figure 8 is shown as having a central base 127 and wings 128 and 129 which extend slightly upwardly from the base 127. The view shown in Figure 10 (of the ends of the fingers 126) indicates the degree of upward inclination. It will be apparent that this can be quite small, since the combination of the curve in the lengthwise direction and the outward spreading of the bafflθ between the front edge which includes the mounting 121 and the distal edge 125 can thereby easily result in the desired lateral and longitudinal spreading of the stream of particles. In a typical example the width of the distal edge 125 across the entire baffle is typically 150 to 200mm and the extent by which each wing 128 and 129 is inclined upwardly from the base 130 is indicated by the small distance "d" between point 130 and point 131. Typically the inclination is such that the distance between points 130 and 131 (i.e., the extent to which each wing extends upwardly) is between 2 and 15mm, usually around 5 to 10mm, when the width of the distal edge 25 is 150 to 200mm.

Figure 11 is a cross-section of a mineral wool web having a core layer 132 between two facing layers 133. The core layer 132 and the facing layers 133 all have substantially the same content of binder and mineral fibre, but the core layer 132 has a very much higher concentration of magnesium hydroxide particles or other particulate fire retarding material than the facing layers, which are preferably substantially free of the fire retarding material. The batt can then be formed into a pipe section as described above with reference to figures 3a and 3b.

Claims

1. A pipe section (1) comprising a tubular insulating element (2) having an outer surface (3) and an inner surface (4) and an insulating material comprising mineral wool, such as glass wool, stone wool or slag wool, arranged between the outer surface (3) and the inner (4) surface, wherein the insulating material has an annular outer section extending inwardly from the outer surface, an annular inner section extending inwardly from the inner surface and an annular core section between the annular outer section and the annular inner section and a fire retarding material (7) is present in the mineral wool, characterized in that the concentration of the fire retarding material (7) in the mineral wool is higher in one or two sections selected from the annular outer section, the annular inner section and the annular core section, than in the other sections or section.

2. A pipe section according to claim 1 , characterized in that the concentration of fire retarding material is higher in the annular core section than in the annular outer section or the annular inner section.

3. A pipe section according to claim 2, characterized in that the fire retarding material (7) is present in the annular core section (8) only.

4. A pipe section according to claim 3, characterized in that the annular core section (8) has a radial thickness of between 1% and 90%, preferably between 5% and 50%, of the radial thickness of the insulating element (2).

5. A pipe section according to any one of claims 1 -4, characterized in that the fire retarding material (7) is a particulate material that decomposes endothermically at increased temperatures.

6. A pipe section according to claim 5, characterized in that the particulate material liberates water at temperatures above 180 ⁰C.

7. A pipe section according to claim 5 or 6, characterized in that the particulate material is a carbonate or hydrate.

8. A pipe section according to claim 7, characterized in that the particulate material is magnesium hydroxide.

9. A pipe section according to any one of claims 5-8, characterized in that the particulate material has a mean particle size of between 0.5 mm and 10 mm, preferably between 2 mm and 5 mm.

10. A method for producing a pipe section (1) comprising a tubular insulating element (2) having an outer surface (3) and an inner surface (4) and an insulating material comprising mineral wool, such as glass wool, stone wool or slag wool, arranged between the outer surface (3) and the inner surface (4), wherein the insulating material has an annular outer section extending inwardly from the outer surface, an annular inner section extending inwardly from the inner surface and an annular core section between the annular outer section and the annular inner section and said method comprising the following steps:

• preparing a web (9; 12) of mineral wool,

• adding a fire retarding material (7) to the mineral wool during or after preparation of the web (9; 12) of mineral wool,

• manufacturing the pipe section (1) from the mineral wool, characterized in that the fire retarding material (7) is added to the web (9; 12) of mineral wool in such manner that after manufacturing of the pipe section (1) from the mineral wool the concentration of the fire retarding material (7) in the mineral wool is higher in one or two sections selected from the annular outer section, the annular inner section and the annular core section, than in the other sections or section.

11. A method according to claim 10, characterized in that the fire retarding material (7) is added to a surface of the web (9) of mineral wool, and subsequently the web (9) of mineral wool is wound around a mandrel (10).

12. A method according to claim 11 wherein the fire retarding material (7) is added to a surface of the web (9) of mineral wool using a baffle (1 16).

13. A method according to claim 11 or 12, characterized in that the fire retarding material (7) is added to a surface of the web (9) immediately before winding.

14. A method according to claims 11 or 13, characterized in that the fire retarding material (7) is added to the surface of the web (9) in a length (L) that corresponds to 1 -10 windings, preferably 2-5 windings.

15. A method according to claim 10, characterized in that the fire retarding material (7) is added to one or two sections (13) of the web (12) of mineral wool, preferably is added predominantly to the core section only and subsequently the following steps are performed:

• pleating the web (12) into a plurality of pleatings (17),

• dividing the pleated web at a centre plan to form smaller pieces (19) each comprising half a pleating (17),

• mechanically working each of the smaller pieces (19) to a semi-annular shaped body (20),

• joining two semi-annular shaped bodies (20) to form a pipe section (1).

16. A method according to any one of claims 10-15, characterized in that the fire retarding material (7) is a particulate material that decomposes endothermically at increased temperatures.

17. A method according to claim 16, characterized in that the particulate material liberates water at temperatures above 180 ⁰C.

18. A method according to claim 16 or 17, characterized in that the particulate material is a carbonate or hydrate.

19. A method according to claim 15, characterized in that the particulate material is magnesium hydroxide.

20. A method according to any one of claims 11 -19, characterized in that the particulate material has a mean particle size of between 0.5 mm and 10 mm, preferably between 2 mm and 5 mm.

21 A method according to any of claims 15 to 20 wherein the steps of preparing a web (109; 112) of mineral wool and adding a fire retarding material (107) to the mineral wool during or after preparation of the web (109; 112) of mineral wool, are carried out by fiberising a mineral melt using a spinner comprising one or more fiberising rotors (101 , 102, 103, 104) which rotate about a substantially horizontal axis, and entraining the fibres in air (105) travelling substantially horizontally as a cloud of fibres, collecting the fibres from the cloud as a web (114) on a permeable collector (111 ) which travels continuously along a path comprising, in sequence, an initial collecting zone A, an intermediate collecting zone B and a final collecting zone C, wherein the web has a core layer between adjacent layers and, following manufacture of the web into a pipe section, the core layer becomes the annular core section, and the adjacent layers become the annular inner sections and annular outer sections, and the fibres of the core layer (132) are collected on the collector (11 1 ) in the intermediate collecting zone B and the fibres of the adjacent layers (133) are collected on the collector (111) in the initial and final collecting zones A₁ C, wherein one or two layers selected from the core layer (132) and adjacent layers (133) comprises MMV fibres mixed with fire retarding material and the other layer or layers comprise MMV fibres optionally mixed with the fire retarding material in an amount which is substantially less, and wherein the fire retarding material is directed downwardly through the cloud of fibres as a region of downwardly directed particles (134) which extends, at the surface of the web (114), across the width of the web and in the length direction substantially only in one or two of the collecting zones selected from the initial, intermediate and final collecting zones (A, B and C).

22. A method according to the claim 21 wherein downward supply of the fire retarding material is provided by ejecting a stream in air of the particulate additive on to the underside of a baffle (116), or by ejecting a plurality of streams in air each on to the underside of a different baffle (116), wherein the or each baffle is in the upper part of the cloud or above the cloud of fibres and wherein the baffle is shaped, or the baffles are shaped, to deflect the fire retarding material downwardly as a diverging region (134) of falling particles which, at the surface of the web, extends substantially across the width of the web and extends lengthwise substantially only in one or two of the collecting zones.

23. A method according to claim 22 in which the underside of the or each baffle has spaced-apart extensions (126) on its distal edge (125) in order to increase the length of the intermediate collecting zone.

24. A method according to any of claims 22 to 23 in which the or each baffle (116) carries ribs (124) on its underside which diverge outwardly in the direction of ejection in order to deflect the particles transversely across the web and/or the underside of the or each baffle comprises a base (127) and a wing (128, 129) extending upwardly from one or both sides of the base and shaped to deflect the particles transversely across the width of the web.