IE77644B1 - An expansion joint - Google Patents

Abstract

An expansion joint has side walls (4) which have a large exposed surface area for radiation of heat (7). Heat transfer to the side walls (4) is reduced by a configuration of base insulation (5). Heat transfer through clamps is minimised by use of insulating washers (15). Heat which reaches the clamps is radiated with the help of channel-shaped clamping members (11). A fabric belt (10) extending between the clamps has a graphite glass cloth protective cuff 20 and a layer 27 which provide insulation and strength. A band (25) of insulation helps to reduce heat transfer but does not extend across the free part of the fabric belt (10) so that temperature on the inside of a chemical PTFE layer (23) is not reduced to the extent whereby there is condensation when gas passing through the joint is at the operating temperature. However, additional protection is provided for an outer silicone cloth layer (21) by an insulating layer (22) which does not extend into the clamped area.

Description

An Expansion Joint The invention relates to an expansion joint for use in high-temperature gas ducts. Such joints are used to absorb thermal expansion and vibration and have application particularly in the electrical generation, cement production and pharmaceutical industries. However, the range of applications is being continuously widened.

Originally, expansion joints comprised a bellows-type arrangement which allowed expansion in one direction. However, such joints had limited suitability in applications where there was a large degree of vibration and the fact that expansion was only allowed in one direction caused maintenance difficulties. For these reasons, fabric expansion joints came into general use some years ago and these allow movement in all directions by use of a clamped fabric belt. Such a joint is described in PCT Patent Specification No. WO 94/16257 (Pathway). In this joint, there is a fabric belt which is mounted between clamps supported on duct side walls. There is additional insulation within the joint.

Whilst this joint appears to be quite effective, it has been found that problems can arise in some industries which can result in a requirement for a large degree of maintenance for the expansion joint. This is particularly true where there are very high temperatures of up to 1000°C from time to time and high-frequency gas expansion and contraction. A reason for these problems is the fact that there are different heat transfer paths within the joint, each with its own heat transfer rate resulting in different heat levels within the joint. For example, one problem which this can lead to is the fact that while there may be sufficient insulation at the clamps of the fabric belt, there is excessive insulation in the free - 2 part of the belt (which of course is not compressed) .

This can cause condensation of acids which damage the » fabric belt. Alternatively, there may be insufficient insulation in the free-part of the belt, which can cause ♦ damage to the non-permeable outer layer.

The invention is directed towards providing an expansion joint which incorporates features to balance the heat flows to the desired levels throughout the joint to minimise long-term maintenance problems.

According to the invention, there is provided an expansion joint for a hot gas duct, the expansion joint comprising joint side walls defining a joint enclosure therebetween and being connectable to a gas duct via a gas baffle, said side walls having free outer surfaces for radiation of heat away from the joint to the environment around the gas duct; a layer of porous base insulation extending around a lower part of the joint enclosure and adjacent part of the side walls, said configuration thereby limiting heat transfer to the joint side walls and acting as a dust barrier for the joint enclosure; a pair of flanges on the joint side walls, each forming part of a clamp for a fabric belt, each clamp being completed by an upper channel-shaped clamping member connected to a flange by a fastener; a fabric belt extending between, and clamped in position by, said two clamps, the fabric belt comprising the following layers in order from within the joint outwardly: a bulk insulation layer of porous insulating material; * a pair of insulation bands mounted in the clamped parts of the fabric belt only and not extending between the clamps, thereby providing differential insulation between the clamped parts of the belt and the free part between the clamps; a cloth insulating layer; a chemical barrier layer; and a gas impermeable outer layer.

In one embodiment, the joint further comprises an additional cloth insulating layer above the chemical barrier at the free part of the fabric belt, but not extending into the clamped parts.

Preferably, the bulk insulation layer has a density in the range of 100 - 150 kg/m3, and preferably a density of approximately 128 kg/m3.

In another embodiment, the fabric belt further comprises protective cuffs of glass graphite cloth extending around the clamped part of the belt.

In a further embodiment, the fabric belt further comprises a layer of silica cloth to provide high initial insulation for heat transfer and mechanical support for the bulk insulation layer.

Preferably, the fabric belt further comprises a wire mesh layer beneath the silica cloth layer to provide mechanical support.

Ideally, the clamp fasteners are bolts extending between the clamp members and the flanges, and insulating washers mounted between the bolts and the flanges.

The invention will be more clearly understood from the following description of some embodiments thereof, given · by way of example only, with reference to the accompanying drawings, in which : - * Fig. 1 is a diagrammatic cross-sectional view of an expansion joint of the invention; Fig. 2 is a more detailed cross-sectional view showing part of a fabric belt of the expansion joint of Fig. 1; and Fig. 3 is a perspective view showing part of the expansion joint.

Referring to the drawings, there is shown an expansion joint of the invention, indicated generally by the reference numeral 1. The joint 1 is mounted on a hot gas duct in a system such as an electrical power plant. The duct has a duct wall 2 to which the joint 1 is connected.

Flow of gas in the duct is indicated by the arrow 3 from which it is clear that ingress of dust into the joint 1 is minimised by part of the duct wall, 2(a), which acts as a gas baffle. The joint 1 has a pair of side walls 4 which extend vertically from the duct wall 2 to provide a large free surface area for heat transfer to the environment by radiation. This heat transfer is indicated by the arrows 7. This helps to reduce heat transfer by conduction through the side walls 4 to the upper part of the expansion joint 1, described below. Further, transfer of heat by radiation to the side walls 4 is minimised by use of a base insulation layer 5 which is placed in position by pins 6 to the side walls 4. The base insulation layer ’ is of U-shaped configuration and extends around the base of the joint 1 within an enclosure defined by the side walls 4. Its purposes therefore are twofold :5 1. It limits the flow of dust into the joint, which dust may be harmful to the fabric belt of the joint, described below. 2. It limits heat transfer by radiation to the side walls 4 and therefore ultimately limits heat transfer by conduction through the side walls 4 to clamps for the fabric belt.

If the circumstances require it, the layer 5 may have an O-shaped configuration rather than a U-shaped configuration.

While achieving the above two objectives, the base insulation layer 5 allows heat transfer by convection through the enclosure as indicated by the arrows 8. The important point is that heat is allowed to transfer up through the joint enclosure by convection so that outer layers of the fabric belt are not attacked by condensed acid.

Each of the side walls 4 terminates in a perpendicular flange 9 which supports a fabric belt 10 together with a channel-shaped clamping member 11. Each member 11 is held in place by a fastener bolt 17 having a nut 16 which engages the flange 9 via an insulating washer 15 to form a clamp. The insulating washer is of fibre glass material. In addition, there may be a thin sleeve of fibre glass material surrounding each of the bolts 17.

It will be appreciated that conduction of heat from the side walls 4 to the clamps is further reduced by use of the insulating washers 15, the action of which tends to maintain heat in the walls from where it escapes as radiant heat, indicated by the arrows 7. If insulating sleeves are mounted around the bolts 17, they would be of benefit in providing a direct route to the clamp members 11 for conducted heat. The configuration of the clamping members 11 is also important as their upstanding webs allow rapid radiation of heat to prevent build-up in the clamped parts of the fabric belt 10. Another important aspect of the clamping members 11 is that they provide rigidity for the clamp and therefore reduce the number of bolt hole penetrations which are required as each bolt tends to bend the clamping member. By reducing the number of bolts, heat conduction through the clamped parts of the fabric belt is minimised. Finally, the arrows 14 indicate the physical movement of the fabric belt 11 to accommodate thermal expansion in the duct and also vibrations etc..

Referring specifically to Fig. 2, construction of the fabric belt 10 is now described in detail. The left clamped part of the fabric belt 10 and portion of the free part of it are shown. A protective cuff 20 of glass graphite cloth surrounds the end of the belt 20 as illustrated in Fig. 2. Describing the layers in sequence from the outside of the joint inwards, the first layer is a silicone coated glass cloth 21 which is gas impermeable and protects integrity of the joint for containment of the gases. The next layer 22 is of glass fibre cloth which extends between the clamped parts of the fabric belt, but not into the clamped parts. It therefore only affects heat transfer through the free part of the belt and acts to protect the outer layer 21 from excessive heat. An important point is that the outer layer 21 is extremely effective at containing gases as it is impermeable, however, it has little resistance to heat. However, because the layer 22 does not extend into the clamped part of the fabric belt 10, it allows heat escape through the clamp members and therefore does not cause build-up of heat at the chemical barrier, described below.

Fig. 3 shows a perspective view of part of the joint. In this view, the feature of the corners of the flanges of the clamping members 11 is illustrated. This helps to avoid stress build-up in the members 11 under hightemperature conditions .

The next layer is a PTFE chemical barrier 23 which extends all of the way across the fabric belt. The type of PTFE material used is pure PTFE which has a high temperature resistance of up to 250°C. The next layer is a 1-ply graphite glass cloth layer which is of similar material to the cuff 20. This provides a large degree of insulation and also helps to protect the PTFE layer 23.

The next layer is a knitted glass material band which is present only in the clamped part of the fabric belt 10 as it does not extend into the free part. It therefore provides additional insulation in the clamped part to help prevent heat transfer to the outer layer 21 and the chemical barrier 23. However, because it does not extend across the free part of the fabric belt 10, it does not cause the additional heat drop on the inside of the PTFE layer 23 which could cause condensation of the gas and therefore acid attack of the various layers. It is therefore very important in achieving a balance whereby heat transfer through the clamped parts of the fabric belt is minimised, while that through the free part is controlled to prevent damage to the outer layer 21, while ensuring that the dew point is not reached whereby condensation can occur.

The next layer is a bulk insulation layer 26 of ceramic fibre which has a density of 128 kg/m3. The thickness of this layer depends on the application and the operating temperatures, however, it would be in the range of 12 mm to 25 mm for operating temperatures of up to 1000°C. ♦ The next layer is a silica cloth layer 27 which has a t. density of 1 kg/m2. This layer has a high insulative property and therefore provides a dual function of providing insulation and also mechanical support for the other layers. This support is backed up by a wire mesh 28 which also extends between and into the clamped ends of the fabric belt 10.

It has been found that this arrangement of fabric belt helps to ensure the correct distribution of heat transfer as described above. For example, heat transfer through the centre of the joint and through the free part of the fabric belt is controlled to ensure that the outer layer is not damaged while at the same time condensation is prevented. Of course, during start-up of the plant and before the operating temperatures are reached in the gas duct, there will be a certain degree of condensation, however, the damage caused for such short time periods will be minimal. The important point is to ensure that there is no continuous condensation while the gas ducts are at the operating temperature. At the same time as controlling temperatures through the centre of the expansion joint, transfer through the clamped parts is also controlled to minimise heat transfer to the outer layers and therefore damage which can be caused to them.

It is important to note that the outer layers at the clamped parts of the fabric belt are compressed and therefore have lower insulating properties. Further, heat ? transfer away from them by radiation is very low. The transfer must be by conduction and therefore operation of · the band 25 is important in minimising heat transfer to the outer layers.

It will be appreciated that the invention provides an overall construction of expansion joint which addresses the various problems which have arisen during long-term use of expansion joints. The features employed are relatively simple and inexpensive to implement.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.

Claims (5)

1. An expansion joint for a hot gas duct, the expansion joint comprising :joint side walls defining a joint enclosure therebetween and being connectable to a gas duct via a gas baffle, said side walls having free outer surfaces for radiation of heat away from the joint to the environment around the gas duct; a layer of porous base insulation extending around a lower part of the joint enclosure and adjacent part of the side walls, said configuration thereby limiting heat transfer to the joint side walls and acting as a dust barrier for the joint enclosure; a pair of flanges on the joint side walls, each forming part of a clamp for a fabric belt, each clamp being completed by an upper channel-shaped clamping member connected to a flange by a fastener; a fabric belt extending between, and clamped in position by, said two clamps, the fabric belt comprising the following layers in order from within the joint outwardly :a bulk insulation layer of porous insulating material; a pair of insulation bands mounted in the clamped parts of the fabric belt only and not extending between the clamps, thereby providing differential insulation between the clamped parts of the belt and the free part between the clamps; a cloth insulating layer; a chemical barrier layer; and a gas impermeable outer layer.

2. An expansion joint as claimed in claim 1 further comprising an additional cloth insulating layer above the chemical barrier at the free part of the fabric belt, but not extending into the clamped parts.

3. An expansion joint as claimed in claims 1 or 2 wherein the bulk insulation layer has a density in the range of 100 - 150 kg/m 3 .

4. An expansion joint as claimed in claim 3 wherein the bulk insulation layer has a density of approximately 128 kg/m 3 . 5. An expansion joint as claimed in any preceding claim, wherein the fabric belt further comprises protective cuffs of glass graphite cloth extending around the clamped part of the belt. 6. An expansion joint as claimed in any preceding claim, wherein the fabric belt further comprises a layer of silica cloth to provide high initial insulation for heat transfer and mechanical support for the bulk insulation layer. 7. An expansion joint as claimed in any preceding claim, wherein the fabric belt further comprises a wire mesh layer beneath the silica cloth layer to provide mechanical support. 8. An expansion joint as claimed in any preceding claim, wherein the clamp fasteners are bolts extending between the clamp members and the flanges, and insulating washers mounted between the bolts and the flanges.

5. 9. An expansion joint substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings .