FIELD

The present invention relates to a bellows arrangement. BACKGROUND

Bellows parts are used in pipe extensions, and extendible bellows compensators, for instance. Bellows compensators are devices, which compensate for the volume variations of a fluid in a main vessel. The vessel may house an electric, heat generating device such as a transformer, for instance. The operation of a bellows compensator is based on contraction and expansion of a bellows part of the compensator, whereby the volume of the compensator changes.

Bellows parts of the bellows compensators as well as pipe extensions are subject to several requirements. On the first hand, in some application environments it is utmost important that the bellows compensator is leakage free, and will stay such for a long period of time. That suggests for increasing the thickness of the walls of the bellows part. However, this is contradictory to a requirement that the bellows part should be flexible such that it would not cause excess pressure to vessel, which would suggest making the walls of the bellows as thin as possible thus compromising the safety of the device.

Current bellows parts fail to adequately combat these contradictory ob- jects.

SUMMARY

An object of the present invention is to provide a bellows arrangement so as to alleviate the above disadvantages. The object of the invention is achieved with the invention, which is defined in the independent claims. Some embodiments are disclosed in the dependent claims.

The present invention provides the important advantage in that the structure of the bellows part is as robust as possible, however not compromising the operational flexibility of the system. Furthermore, the structure improves the testing possibilities of the bellows part. DRAWINGS

Figure 1 shows one embodiment of a bellows compensator arrangement; and

Figure 2 shows one embodiment of a test port arrangement; and 5 Figure 3 shows one embodiment of a method.

DETAILED DESCRIPTION

Figure 1 shows an embodiment of a bellows compensation arrangement 100. Such a bellows compensation arrangement is typically used in pressure compensation of a vessel having variable volume fluid therein. In an embodiment, i o the system to be compensated is a subsea electric device, such as a transformer, frequency converter or a switchgear, for instance, or it may be a dry land device, such as a transformer. The electric device is placed within the vessel, which is filled with insulating fluid, such as transform.

The bellows compensator may be substantially cylindrical but also other

15 forms are possible. Its basic parts are a bellows part 120 and end plates 102, 104 fixed substantially to the opposite ends of the bellows part. Practically, the bellows compensator may comprise end rings 106, 108 at both ends of the compensator fixed to their respective end plates 1 02, 104, whereby the bellows part 120 is fixed to the end rings 106, 108.

20 There is a fluid connection between the vessel housing the subsea device, and the pressure compensation arrangement 1 00, which fluid connection is not shown in Figure 1 . The purpose of the fluid connection is to allow insulation fluid to enter and exit the pressure compensator 100 when the volume of the fluid in the vessel increases or decreases due to heating or cooling of the fluid. The

25 volume variations of the bellows compensator take place by extension and contraction of the bellows part.

The fluid connection may comprise a pipe, which has one end in the vessel, and one end at the pressure/volume compensation arrangement 100. In an embodiment the pipe may enter the arrangement through a stationary end plate,

30 which may be either the first end plate 102 or the second end plate 104 of the bellows compensator. The other end plate may be movable such that the corrugation wall of the bellows part 1 20 may extend and contract in longitudinal direction, which is perpendicular to the level of the horizontal end plates 102, 104 in Figure 1 .

The end plates may be relatively thick metal or composite plates. The end rings may be made of thinner metal plate but anyway thicker than the thin bel- lows part, which may comprise two or even more overlapping thin metal bellows parts. By way of example, the end plate may have a thickness of about 10 mm, the end ring about 3 mm, and the bellows part about 0,5 mm.

Although Figure 1 shows only the metal bellows part 120, there may be provided an additional outer rubber bellows part that encompasses the metal bel- lows part. The rubber bellows may be directly attached to the end plates 102, 104. Between the metal bellows part 120 and the rubber bellows there is an intermediate space, which may be filled with an intermediate fluid. The intermediate fluid may be the same fluid as the insulation fluid that is within the vessel and the bellows part 120. Alternatively the intermediate fluid can be a different fluid than the insulation fluid, but may however be mixable with the insulation fluid. Thus, in a breakage situation of one of the bellows part 120 or the rubber bellows, the operation of the system is not endangered.

Figure 1 shows also an embodiment of a test port arrangement 1 30 for testing that the separate bellows sheets of the bellows part 1 20 are tight such that there is no leakage in the sheets itself or their attachment to the end rings.

Although Figure 1 shows the bellows part 120 in the context of a bellows compensator, the bellows part may alternatively be used as a flexible pipe extension. In such a case, the end rings are not welded to an end plate but are connected to a pipe by welding.

Figure 2 shows a closer view of the test port arrangement. Referring to

Figure 1 , Figure 2 shows only the upper part of the arrangement such that only the top end plate 202 of the compensation arrangement is visible. The structure at the lower end of the compensator may be similar as also shown in Figure. That is, the lower end of the bellows compensator is attached to a lower end ring, which is at- tached to a lower end plate. There may be provided another test port arranged to the other end ring if desired.

The end ring 206 may be fixed to the end plate 202 by welding 207. The end ring(s), and the bellows part may be substantially cylindrical such that the welding 207 forms a circle on the end plate 202. The end ring 206 may be substantially perpendicular to the end plate 202. When the end rings have been welded to their respective end plates, a closed interior space is formed by the end plates, the end rings and the bellows part. Additionally, if a rubber bellows is in- volved around the bellows part 220, another closed space is formed between the rubber bellows and the metal bellows 220 and the end rings.

As Figure 2 shows, the bellows part 220 comprises two metal bellows layers, 222, 224. The bellows layers are made of the metal plates, which have been bent and welded by a longitudinal (vertical in Figure 2) weld to a cylinder. This longitudinal weld is one possible leakage risky point in the manufacture of the of the bellows arrangement. As Figure 2 shows, the bellows part 220 may comprise at least two bulges alternately bulging to opposite sides of the vertical middle of the bellows part.

The inner bellows 222 and the outer bellows encompassing the inner bellows may overlap each other thus being in very close proximity to each other. Overlapping refers here to that the contours of bellows follow each other, that is, if there is a bulge in the first bellows layer 222, there is a similar bulge also in the second bellows layer. Practically, the bellows layers may be so close to each other that they touch each other.

In an embodiment, there may be provided a separating member between the inner and outer bellows for keeping the bellows at least partly separated from each other. This is especially important at high depths, because the ambient pressure tends to press the bellows against each other thus decreasing the flexibility of the metal bellows. The separating member may comprise a wire mesh and/or grease, for instance. The grease may be fed to the space between the bellows via the test port.

The using of two bellows layers as shown in Figure 2 provides the advantage that individual layers can be made of relatively thin material. In this way the structure stays flexible and the bellows extends and contracts fairly easily. In this way the pressure compensator causes as little pressure to the insulation fluid, which is advantageous especially in high-pressure environments.

However, the use of the two or more layers causes the challenge that it is difficult to tell if individual layers 222, 224 and their attachment points are leak- age free. That is, even if the bellows part 220 would be found as leakage free, it is still not known if both layers 222, 224 are leakage free. That is, it may be that one of the layers is leakage free but the other layer is faulty. The structure of Figure 2 attempts to solve this problem by verifying that both layers are non-defective as explained further in the following.

The inner bellows 222 and the outer bellows 224 may be both fixed to the end ring 206 such that the bellows sheets are arranged to opposite sides of the end ring, that is the inner bellows is attached to the inner surface of the end ring, and the outer bellows is attached to the outer surface of the end ring.

In this way the attachment of each individual layer can be carried out separately, which is substantially easier than if the two bellows sheets would be arranged on the same side of the end ring. Separate attachment also reduces the risk of leakage in the horizontal welding seams 223, 225.

In an embodiment, the welds 223 and 225 are through-welds going through the respective bellows sheets 222 and 224. The through weld may be arranged at a distance from the end of the bellows sheet. That is, there is a portion of the sheet 222 between the welding point 223 and the end wall 202. In this way the risk of leakage at the welding seam is reduced because the welding is not at the end of the sheet. Alternatively, the welding may be at the end of the bellows.

The welds 223, 225 of the bellows sheets 222, 224 may be arranged at different heights in the end ring 206. The weld 223 of the inner bellow 222 may be arranged closer to the end plate 202 than the weld 225 of the outer bellow 224. This allows mounting of a test port 230 in the area between the welds 223, 225. Alternatively, a cutting is made to the bellows which is on the same side of the end ring as the test port. The cutting may arranged such that it is capable of surrounding the test port from other sides except from the end of the bellows. In such embodiment, the inner bellows and outer bellows can be welded on the same or substantially the same height if desired.

In one embodiment, the test port 230 is fixed to the outside of the end ring 206 by welding 231 . The test port may comprise a flow channel 234, which extends through the end ring 206 from the outer side of the end ring to the inner side of the end ring. There may additionally be provided a groove 236 in the end ring 206, which groove extends to the lower end of the end ring from the attach- ment position of the test port 230. The groove 236 thus forms a flow channel from the test port to the space 228 between the inner bellows 222 and the outer bellows 224. In another embodiment, the test port channel does not lead through the end ring but there is provided a channel interior of the end ring, which channel has an opening in the free end of the end being the end opposite to the end which is fastened to the end plate. The test port may have a channel which leads to the channel inside of the end ring. To both embodiments, the common factor is that there is provided a channel which leads from the test port to the space between the bellows layers.

When the test port is arranged as shown in Figure 2, the risk of leakage in test arrangement is avoided as much as possible. In the first place, the bellows layers can be attached separately and individually significantly reducing the leakage risk. Furthermore, the test port may be attached to the relatively thick end ring, whereby any leakage risk when attaching the test port with a thin bellows layer is avoided. The channel formed into the end ring may further improve flow of fluid between the test port and the space between the bellows layers. The test assembly of the figures provides the important advantage that both layers of the metal bellows part 120 can be individually tested against leakage.

Although Figure 2 shows that the test port leads from the space be- tween the inner layer and the end ring, the arrangement may be inverse to that. That is, the bellows sheets may be attached such that the inner sheet is lower than the outer sheet in vertical direction. The test port assembly may then be arranged on the side of the inner bellows such that the test channel leads from the interior of the bellows system to the space between the outer bellows and the end ring. However, the optimal solution is to position the test port as shown in Figure 2, because this arrangement provides the possibility to test the tightness of the metal walls also after the end plates have been welded to the end rings. That is not possible if the test port is in the inside, unless there is a test connection arranged to the end plate and leading to the test port. Testing the condition of the metal bel- lows is preferable also after the welding of the end plates, because the welding process of the end plates and/or eventual transportation of the bellows arrangement may cause cracks to the thin bellows walls. Although Figure 2 shows the test port placed under the end plate 202, the test port arrangement may alternatively comprise a channel which extends through the end plate 202 so that the intermediate space 228 can be tested from the outside of the bellows arrangement. The channel may comprise a pipe, for in- stance, which leads to the exterior of the compensator through an opening arranged in the end plate.

Furthermore, although Figure 2 shows a two bellows sheet arrangement, there may be more sheets than that. For instance, the bellows part 220 may comprise three overlapping layers in which case the two-layer construction of Fig- ure 2 may be modified as follows. The third sheet may be arranged to the same side of the end ring 206 as the inner sheet 222. The welding of third sheet is higher than the welding 223 of the inner sheet 222, that is, between the welding point 223 and the end plate 202. The test port for testing the space between the inner layer and the third sheet would be then arranged between the welding point 223 and the welding point of the third sheet. The test port would be fixed on the same side of the as the test port 230 and would have a channel leading through the end ring 206.

Although Figure 2 shows that the outer sheet is arranged closer to the free end of the end ring being the end which is not attached to any further part, the situation may also be inverse. That is, alternatively the inner bellows may be attached closer to the free end of the end ring than the outer bellows, in which case the test port is arranged to the inner surface of the end ring and extends through the end to the outer side of the end ring.

The testing procedure is now explained with reference to Figure 3.

In 300, the two metal bellows layers are welded from their both ends to the end rings as shown in Figure 2. In 302, the test port is attached by welding to the end ring as also shown in Figure 2. The tightness of the attachments of the bellows parts to the ends rings is tested by the method steps 304 to 308.

In 304, a helium detector is coupled to the test port, and vacuum is as- pirated via the test port to the space between the bellows layers. Although reference is made to helium in the following, other gases are applicable as well.

In 306, helium is then sprayed to the inner surface of the inner bellows, and/or to the outer surface of the outer bellows. In 308, it is monitored if the helium detector detects any leakage of helium to the space between the bellows layers. An eventual leakage may be at the welds of the bellows to the end ring, or in a longitudinal weld when a rectangular sheet has been welded to a cylindrical bellows part. Alternatively to spraying helium on the surface(s) of the bellows, the whole bellows arrangement may be applied to a space filled with monitoring gas, and eventual leakage of the gas to the space between the layers is being monitored.

The above mentioned testing may be carried out in two phases such that first only the inner bellows is sprayed whereby an eventual leakage therein can be detected. When the inner bellows has been verified to be leakage-free, the testing can be continued by spraying the outer bellows with helium and detecting presence of helium in the space between the bellows layers.

The above mentioned test procedure may be executed when the bellows parts have been welded to the end ring but before the end ring has been welded to the end plates, where after the inner surface of the inner bellows is no longer accessible.

To verify the tightness of the bellows arrangement in different manufacturing stages after the test disclosed in Figure 3, a subsequent leakage test may be carried out when the end plates have been welded to the end rings. Further- more, yet another leakage test may be carried out when the rubber bellows have been fastened to the end plates.

In an aspect there is provided a subsea pressure compensator, comprising an inner bellows, and an outer bellows encompassing the inner bellows, which inner bellows and outer bellows are arranged at least partly overlapping each other. The subsea pressure compensator comprises a test assembly for testing the tightness of the space between the inner bellows and the outer bellows. The test assembly may comprise one or more of a test port, a groove formed to the end ring, and a hole formed to the end ring. The channel may be formed by a test channel of the test port alone, the channel of the test port in combination with the groove or the hole of the end ring.

In an embodiment, the inner bellows is attached to a first side of the end ring, and the outer bellows is attached to a second side of the end ring. If the end ring is in a vertical position as shown in the drawings, one of the sides is the inner side on the right and the other one of the sides is the outer side on the left.

In an embodiment, he inner bellows and outer bellows are attached to the end ring at different heights, and the test port is arranged on the same side of the end ring as the bellows whose attachment level is closer to the space between the inner bellows and the outer bellows. The test port can thus be arranged either on the same of the end ring as the inner bellows or the outer bellows. In such cases the test port is arranged higher, that is being further away from the free end of the end ring than the bellows being on the same as the test port. The free end refers here to the end of the end ring that is opposite to the end of the end plate.

In an embodiment, the test channel of the test port passes through the end ring, and has an opening that faces the bellows arranged on the opposite side of the end ring than the test port.

In an embodiment, the end ring has a groove on the opposite side of the end ring than the test port, which groove coincides with the test channel of the test port thus together forming the channel of the test assembly.

In an embodiment, the end ring has a hole formed between the sides of the end ring, which hole has an opening facing the space between the inner and outer bellows, and which hole connects to the test channel of the test port thus together forming the channel of the test assembly.

In an embodiment, the inner bellows and the outer bellows are fixed from their first end to a first end ring, and from the second end of the bellows to a second end ring such that a closed space, except the channel of the test assembly, is formed between the inner bellows, the outer bellows and the first and second end rings.

In an embodiment, the welding for attaching the bellows to the end ring goes through the bellows part and is arranged at a distance from the end of the bellows part.

In an embodiment, the inner and outer bellows have substantially a form of a cylinder, which is open from both ends.

In an embodiment, the end rings are cylindrical.

In an embodiment, the subsea pressure compensator comprises a first end plate for closing the first end ring and a second end plate for closing the sec- ond end ring such that a closed inner space is formed between the end plates and interior of the inner bellows.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven- tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.