JP7607876B2 - Bellows type expansion joint - Google Patents

Description

The present invention relates to an expansion joint used in piping paths that guide fluids such as liquids and gases, and in particular to a bellows-type expansion joint that has a bellows shape.

Countless pipelines run throughout a wide variety of production facilities, from everyday lifelines to cutting-edge energy fields and space development. In these pipelines, expansion joints with a bellows shape absorb the repeated displacement and bending deformation acting on the pipes with the bellows section, reducing the load and preventing damage to the pipes.

Bellows-shaped expansion joints are disclosed, for example, in Japanese Patent Laid-Open Publication No. 2-195096 (Patent Document 1) and Japanese Utility Model Publication No. 2-39103 (Patent Document 2).

Austenitic stainless steel (e.g., SUS304, SUS316L) generally has good corrosion resistance and workability, and is reasonably cost-effective, so it is used as a material for bellows-type expansion pipe joints from the standpoint of economy and workability. The use of stainless steel bellows-type pipe joints allows them to flexibly deform even in the event of an earthquake, land subsidence, etc., and ensures water permeability, etc.

The expansion joint disclosed in Patent Document 1 is made of a single layer, whereas the expansion joint disclosed in Patent Document 2 is made of multiple layers.

Patent Document 2 describes an expansion joint that is installed in a piping system in which a fluid exists within the piping, and is characterized by a configuration in which a boundary maintaining layer made of a highly ductile metal is used as the inner layer, and a shape maintaining layer made of a high-strength metal is used as the outer layer.

Japanese Unexamined Patent Publication No. 2-195096 Publication No. 2-39103

Various fluids flow through pipe joints, including highly corrosive liquids, gases, and powders. Expansion joints are also used in applications where liquids containing microorganisms flow.

Austenitic stainless steel is known as a material that exhibits good corrosion resistance, so it is also effectively used for pipes that guide highly corrosive fluids.

The inventors of the present application have found that, for a straight pipe without irregularities, general-purpose austenitic stainless steels such as SUS304 and SUS316L provide good corrosion resistance to highly corrosive liquids, but that in the case of a bellows-shaped expansion joint, corrosion is likely to occur on the inner surface of the bellows-shaped portion. In other words, in an expansion joint with a bellows shape, even when no highly corrosive liquid is flowing, liquid remains in the bellows portion, and this portion repeatedly dries and wets, and they have recognized the problem that the corrosion resistance of the general-purpose austenitic stainless steels (SUS304, SUS316L, etc.) that have been used conventionally is insufficient.

The first step of this invention is to recognize the problem specific to bellows-type expansion joints, namely, the problem of corrosion progressing due to highly corrosive liquid remaining in the bellows section.

In recent years, major steel manufacturers have begun to sell highly corrosion-resistant steels such as duplex stainless steels (SUS329J4L, etc.) and super stainless steels (SUS312L, etc.). However, their prices are several times higher than those of general-purpose stainless steels (SUS304, SUS316L, etc.), so their use in thick-walled bellows-type expansion joints with increased pressure resistance results in high costs.

The invention described in Patent Document 2 proposes an expansion joint with an inner boundary maintaining layer made of a highly ductile metal and an outer shape maintaining layer made of a high strength metal, in order to improve both the boundary maintaining performance and shape maintaining performance of piping bellows and achieve a high amount of expansion and contraction and a high number of repetitions (long life). This publication does not recognize the issue of corrosion resistance that is unique to bellows-type expansion joints.

The object of the present invention is to provide a bellows type expansion joint that exhibits good corrosion resistance against highly corrosive fluids while preventing excessive increases in costs.

The present invention is a bellows-type expansion joint having a bellows shape, and is equipped with an inner layer that contacts the fluid flowing inside the pipe, and an outer layer that overlaps the inner layer. A characteristic of this joint is that, in terms of the pitting corrosion index represented by "Cr + 3.3Mo + 16N", the material that constitutes the inner layer has a pitting corrosion index of 35 or more, and the material that constitutes the outer layer has a pitting corrosion index of 26 or less.

In one embodiment, the material constituting the inner layer is stainless steel containing, by weight, 19-26% Cr, 2.50-7% Mo, and 0.08-0.30% N, and the material constituting the outer layer is stainless steel containing, by weight, 16-20% Cr, 3% or less Mo, and 0.07% or less N. Preferably, the stainless steel constituting the inner layer and the stainless steel constituting the outer layer are austenitic stainless steel.

In another embodiment, the stainless steel constituting the inner layer is a duplex stainless steel having ferritic and austenitic phases, and the stainless steel constituting the outer layer is an austenitic stainless steel.

For example, the stainless steel constituting the inner layer is JIS standard SUS312L or SUS329J4L, and the stainless steel constituting the outer layer is JIS standard SUS304, SUS304L, SUS316 or SUS316L.

The diameter of the expansion joint is, for example, 50 to 3600 mm, and the thickness of the inner layer (T1) is, for example, 0.3 to 3.0 mm. In this case, the ratio (T2/T1) of the thickness of the outer layer (T2) to the thickness of the inner layer (T1) is preferably 1 to 5.

A bellows-type expansion joint according to another embodiment has a second outer layer that overlaps the outer layer. The material that constitutes this second outer layer has a pitting corrosion index of 35 or more.

In yet another embodiment of the bellows-type expansion joint, the material constituting the inner layer is a nickel alloy containing Ni as the main component. The nickel alloy contains, for example, by weight, 14 to 17% Cr, 14.5 to 17.5% Mo, and 0.07% or less N. An example of such a nickel alloy is JIS standard NW0276.

If the material constituting the inner layer is a nickel alloy whose main component is Ni, the material constituting the outer layer is, for example, stainless steel.

The bellows-type expansion joint according to the present invention, configured as described above, has an inner layer that is in direct contact with the passing fluid and is made of a highly corrosion-resistant material with a pitting corrosion index of 35 or more, and an outer layer that overlaps this inner layer and is made of a general-purpose material with a pitting corrosion index of 26 or less. This prevents excessive increases in costs and provides good corrosion resistance against highly corrosive fluids.

Photographs showing the procedure for an experiment evaluating the corrosion resistance of a straight pipe and a bellows pipe. 1 is a photograph showing the occurrence of rust on a straight pipe. 1 is a photograph showing the state of rust formation on a bellows tube. 6 is a photograph showing the procedure of an experiment to evaluate the corrosion resistance of SUS304 and SUS312L bellows pipes. 1 is a photograph showing the state of rust formation on a SUS312L bellows pipe. 1 is a photograph showing the state of rust formation on a SUS304 bellows pipe. 1 is a photograph showing the occurrence of stains on the bellows tube of NW0276.

Conventional bellows-type expansion joints with a bellows shape typically have a single-layer structure made of general-purpose austenitic stainless steel such as SUS304 or SUS316L. This general-purpose austenitic stainless steel is known as a material that exhibits good corrosion resistance, and also has high ductility and good formability, so it is commonly used as a material for pipe joints that guide highly corrosive fluids.

The inventors of the present application came to suspect that, although a straight pipe without any irregularities would exhibit good corrosion resistance against highly corrosive liquids if made of general-purpose austenitic stainless steel such as SUS304 or SUS316L, in the case of an expansion joint having a bellows shape, liquid would remain in the bellows section even when no highly corrosive liquid was flowing, and this section would repeatedly dry and wet, so that even higher corrosion resistance would be required.

Therefore, we first conducted an experiment to evaluate the corrosion resistance of a highly corrosive liquid using a straight pipe made of general-purpose austenitic stainless steel and a bellows pipe made of general-purpose austenitic stainless steel and having a bellows shape.

[Corrosion resistance evaluation experiment for straight pipes and bellows pipes]

1) Test Sample The material of both the straight pipe and the bellows pipe was SUS304, a general-purpose austenitic stainless steel.

The dimensions of the straight pipe and bellows pipe are as follows:
Straight pipe: nominal diameter 100A, length 220mm, plate thickness 0.6mm, number of layers 1
Bellows pipe: nominal diameter 100A, length 220mm, plate thickness 0.6mm, number of layers 1
Bellows part: Height 20 mm, pitch 30 mm, number of ridges 5

2) Equipment and instruments used Hot plate: Economy hot plate manufactured by AS ONE Co., Ltd., product number 1-9385-22
Enamel container: Noda Horo Co., Ltd., product number TK-58

3) Fluid used: Marine Art Super Formula (artificial seawater for testing and research) manufactured by Tomita Pharmaceutical Co., Ltd.

4) Ingredients of artificial seawater used and amount per 1 L when dissolved Sodium chloride: 22.1 g
Magnesium chloride: 9.9 g
Calcium chloride: 1.5g
Anhydrous sodium sulfate: 3.9 g
Potassium chloride: 0.61 g
Sodium bicarbonate: 0.19 g
Potassium bromide: 96 mg
Borax: 78mg
Anhydrous strontium chloride: 13 mg
Sodium fluoride: 3 mg
Lithium chloride: 1 mg
Potassium iodide: 81 μg
Manganese chloride: 0.6 μg
Cobalt chloride: 2 μg
Aluminum chloride: 8 μg
Ferric chloride: 5 μg
Sodium tungstate: 2 μg
Ammonium molybdate: 18 μg

5) Immersion conditions Room temperature: 20℃
Water temperature: 50℃
Amount of water: 2000ml
Amount of artificial seawater: 76g
Concentration of artificial seawater: (76/2076) x 100 = 3.66%
Immersion time: 15 minutes Exposure time: 30 minutes

6) Test procedure A photograph showing the test procedure is shown in Figure 1. The specific procedure is as follows.

A) Place an enamel container on each hot plate and pour the solution of artificial seawater into the container.

a) When the water temperature reaches 50°C, place the test sample in the enamel container with the solution and leave it for 15 minutes.

c) Remove the test sample and leave it for 30 minutes.

D) The above B) and C) constituted one cycle, and the rusting condition was observed. Before the next cycle, evaporated water was added back in, and the concentration in the enamel container was adjusted to the initial state before moving on to the next cycle.

7) Test results The state of rust on the straight pipe is shown in Figure 2, and the state of rust on the bellows pipe is shown in Figure 3.

To check for rust formation, the surface of the material was observed from a distance of 30 cm, and the number of spots that could be determined to be rust was compared.

As shown in Figure 2, in the case of a straight pipe, there was no rust after one cycle, but after two cycles visible rust of about 1 mm or less appeared, and after 10 cycles rust of about 1 mm or less appeared in several places. After 25 cycles, stains were observed, and after 50 cycles the stains became darker. After 75 cycles, some of the stains began to discolor.

As shown in Figure 3, in the case of the bellows tube, after one cycle, several visible spots of rust measuring less than 1 mm appeared, and after 10 cycles, several spots of rust measuring more than 1 mm appeared. After 25 cycles, a large amount of rust appeared in the bellows-shaped part (boundary surface), and after 50 cycles, the number of rusted spots increased, and after 75 cycles, the number of rusted spots further increased.

8) It was found that when the evaluation material was the general-purpose austenitic stainless steel SUS304, rust was more prominent in the bellows pipe than in the straight pipe. From this result, it was recognized that it is necessary to use a material with higher corrosion resistance than general-purpose austenitic stainless steel such as SUS304 as the material for the inner surface of the bellows pipe that may come into contact with highly corrosive fluids.

The components of SUS304, SUS304L, SUS316 and SUS316L, which are representative general-purpose austenitic stainless steels, are shown in the table below.

The pitting corrosion index, expressed by the following formula, is an index that indicates the pitting corrosion resistance of stainless steel.
Pitting corrosion index=Cr+3.3Mo+16N

For SUS304 and SUS304L, the pitting corrosion index is approximately 20, calculated with a Cr content of 19%, Mo content of 0.2% (as an unavoidable impurity), and N content of 0.02% (as an unavoidable impurity).

For SUS316 and SUS316L, the pitting index is approximately 25.6, calculated based on Cr content of 17%, Mo content of 2.5%, and N content of 0.02% (as unavoidable impurities).

Considering the results of this study, it can be concluded that general-purpose austenitic stainless steels with a pitting corrosion index of 26 or less are not the best material to form the inner surface of a bellows tube that carries highly corrosive fluids.

[Corrosion resistance evaluation experiment for SUS304 and SUS312L bellows pipes]

1) Test sample: A bellows pipe made of SUS312L, an austenitic stainless steel, was used as a test sample of the present invention. A bellows pipe made of SUS304, a general-purpose austenitic stainless steel, was used as a test sample of a comparative example.

The dimensions of the bellows tube are as follows:
Bellows pipe: nominal diameter 100A, length 220mm, plate thickness 0.6mm, number of layers 1
Bellows part: Height 20 mm, pitch 30 mm, number of ridges 5

2) Equipment and instruments used Hot plate: Economy hot plate manufactured by AS ONE Co., Ltd., product number 1-9385-22
Enamel container: Noda Horo Co., Ltd., product number TK-58

3) Fluid used: Marine Art Super Formula (artificial seawater for testing and research) manufactured by Tomita Pharmaceutical Co., Ltd.
The ingredients and amounts are the same as previously described.

4) Immersion conditions Room temperature: 20℃
Water temperature: 50℃
Amount of water: 2000ml
Amount of artificial seawater: 76g
Concentration of artificial seawater: (76/2076) x 100 = 3.66%
Immersion time: 15 minutes Exposure time: 30 minutes

5) Test procedure The photograph showing the test procedure is shown in Figure 4. The specific procedure is as follows.

A) Place an enamel container on each hot plate and pour the solution of artificial seawater into the container.

a) When the water temperature reaches 50°C, place the test sample in the enamel container with the solution and leave it for 15 minutes.

c) Remove the test sample and leave it for 30 minutes.

D) The above B) and C) constituted one cycle, and the rusting condition was observed. Before the next cycle, evaporated water was added back in, and the concentration in the enamel container was adjusted to the initial state before moving on to the next cycle.

6) Test results The state of rust occurrence on the bellows pipe made of SUS312L is shown in Figure 5, and the state of rust occurrence on the bellows pipe made of SUS304 is shown in Figure 6. The state of rust occurrence was evaluated by observing the surface of the material from a distance of 30 cm, and comparing the number of spots that could be determined to be rust.

As shown in Figure 5, in the case of a bellows tube made of SUS312L, there was no rust after one cycle, visible rust of about 1 mm or less appeared after two cycles, and rust of about 1 mm or less appeared in several places after 10 cycles. Rust of about 1 mm or less appeared in several places after 25 cycles, and stains began to appear on the boundary surface of the bellows-shaped part after 50 cycles. Then, after 75 cycles, some of the stains discolored (rusted).

As shown in Figure 6, in the case of a bellows tube made of SUS304, after one cycle, visible rust of about 1 mm or less appeared in several places, and after 10 cycles, rust of about 1 mm or more appeared in several places. After 25 cycles, a large amount of rust appeared in the bellows-shaped part (boundary surface), and blue rust was also observed. After 50 cycles, the area of rust increased, and after 75 cycles, the area of rust increased further.

7) It was clearly observed that when the material evaluated was a bellows pipe made of SUS312L, rust was less likely to occur than when the bellows pipe was made of the general-purpose austenitic stainless steel SUS304. From this result, it was recognized that SUS312L, which has higher corrosion resistance, is more suitable than general-purpose austenitic stainless steels such as SUS304 as a material for forming the inner surface of a bellows pipe that may come into contact with highly corrosive fluids.

The components of SUS312L are shown in the table below.

For SUS312L, the pitting corrosion index is approximately 43, assuming Cr content of 20%, Mo content of 6%, and N content of 0.2%.

The pitting corrosion index of SUS304 is approximately 20.

Considering the results of this study, it can be seen that a material with a higher pitting corrosion index (e.g., 35 or higher) is more suitable as a material for forming the inner surface of a bellows tube that carries highly corrosive fluids than general-purpose austenitic stainless steels with a pitting corrosion index of 26 or less.

[Materials with high pitting index]

The pitting corrosion index of the austenitic stainless steel SUS312L is approximately 43, making it suitable as a material for forming the inner surface of bellows pipes that carry highly corrosive liquids.

The composition of SUS329J4L, a duplex stainless steel with ferritic and austenitic phases, is shown in the table below.

In the case of SUS329J4L, if the Cr content is 25%, the Mo content is 3%, and the N content is 0.19%, the pitting corrosion index is approximately 38. This pitting corrosion index value is more than 10 higher than the pitting corrosion index of 26 for general-purpose austenitic stainless steel, making it an appropriate material for forming the inner surface of a bellows tube that passes highly corrosive liquids.

In the case of stainless steel, if the pitting corrosion index is 35 or more, which is about 10 higher than the 26 of general-purpose austenitic stainless steel, it is recognized as a suitable material for forming the inner surface of a bellows tube that passes highly corrosive liquids.

Materials other than stainless steel, as long as they are alloys that exhibit high corrosion resistance, are recognized as suitable materials for forming the inner surface of a bellows tube through which highly corrosive liquids pass. For example, nickel alloys exhibit high corrosion resistance and can be used as materials for forming the inner surface of a bellows tube. This point will be explained in more detail later.

[Multi-layered bellows tube]

Highly corrosion-resistant materials with a pitting corrosion index of 35 or more, typically highly corrosion-resistant stainless steel, generally have a high market price, so manufacturing a bellows-type expansion joint with this highly corrosion-resistant material would be very expensive. Therefore, in order to suppress the increase in costs, it is possible to form a coating made of a highly corrosion-resistant material on the inner wall surface of the bellows-type expansion joint, which comes into direct contact with highly corrosive fluids. However, if a corrosion-resistant coating is formed on the bellows-shaped part by plating or the like, there is a possibility that the thickness of the coating will vary, and parts with poor corrosion resistance will appear in some places. In addition, there is a possibility that the corrosion-resistant coating will peel off as the bellows deforms.

Instead of using a corrosion-resistant coating, it is possible to use a multi-layered bellows pipe with an inner tube (inner layer) of a constant thickness made of a highly corrosion-resistant material, and an outer tube (outer layer) of a constant thickness overlapping the inner tube. In this case, to prevent costs from increasing, the outer layer is made of a material with a low market cost.

However, since bellows tubes are repeatedly subjected to loads such as bending stress, it must be avoided that multi-layer bellows tubes have inferior strength compared to single-layer bellows tubes. The inventors of the present application investigated the life span of single-layer bellows tubes and multi-layer bellows tubes when repeatedly loaded under the same conditions.

Specifically, the calculated lifespan was calculated using bellows strength calculation formula II described in JIS B2352.

The conditions for calculation are as follows:

When the above conditions were applied and calculations were performed, the following results were obtained.

Multilayer structure (0.8 mm x 3 layers): 15,968 times Single layer structure (2.4 mm x 1 layer): 706 times The above lifespan numbers were calculated by the following formula.

First, the symbols required for strength calculations are shown in the table below.

[In the case of a three-layer structure]
First, the total mountain displacement e is calculated.

Next, calculate the stress Sp.

Next, calculate the stress Sd.

Next, calculate the lifespan Nc.

[Single-layer structure]
First, the total mountain displacement e is calculated.

Next, calculate the stress Sp.

Next, calculate the stress Sd.

Next, calculate the lifespan Nc.

The above results show that, with all conditions other than the number of layers kept the same, a three-layer bellows tube has a lifespan 22.6 times longer than a single-layer bellows tube. Therefore, in terms of resistance to repeated loads, a multi-layer bellows tube is deemed superior to a single-layer bellows tube.

[Inner and outer layer materials]

The materials for bellows tubes with inner and outer layers are discussed below.

(1) Both the inner and outer layers are austenitic stainless steel Austenitic stainless steel has high ductility and good formability. It also has better weldability than ferritic stainless steel and martensitic stainless steel. Considering these points, it is desirable to construct the inner and outer layers from austenitic stainless steel. For the inner layer that comes into direct contact with highly corrosive fluids, it is desirable to use a highly corrosion-resistant stainless steel with a pitting corrosion index of 35 or more, and for the outer layer that overlaps the inner layer, it is desirable to use a general-purpose stainless steel with a pitting corrosion index of 26 or less.

For example, the following combination of materials is desirable:

Inner layer: SUS312L
Outer layer: SUS304, SUS304L, SUS316 or SUS316L

The components of the above materials are as shown in Tables 1 and 2. The pitting corrosion index values are as follows:
SUS312L:43
SUS304, SUS304L: 20
SUS316, SUS316L: 25.6

If we look at the amounts of Cr, Mo, and N, which have a significant effect on corrosion resistance, the amounts fall roughly into the following ranges for the above materials.

In the case of the inner layer (SUS312L), the procedure is as follows.
Cr: 19-21% by mass
Mo: 6 to 7% by mass
N: 0.16 to 0.25% by mass

In the case of the outer layer (SUS304, SUS304L, SUS316 or SUS316L), the following applies.
Cr: 16-20% by mass
Mo: 3 mass% or less N: 0.07 mass% or less (as an unavoidable impurity)

(2) Inner layer is duplex stainless steel and outer layer is austenitic stainless steel. SUS329J4L, which is a duplex stainless steel having ferritic and austenitic systems, also exhibits good corrosion resistance. The composition of SUS329J4L is as shown in Table 3, and its pitting corrosion index is 38.

SUS329J4L contains 24-26% Cr, 2.5-3.5% Mo, and 0.08-0.30% N, and has a pitting corrosion index of 35 or more, so it exhibits good corrosion resistance. Therefore, it is suitable as a material for forming the inner layer of bellows pipes.

The preferred combination of materials is as follows:

Inner layer: SUS329J4L
Outer layer: SUS304, SUS304L, SUS316, SUS316L

(3) Cr, Mo and N Contents of Stainless Steel Considering the contents of (1) and (2) above, the Cr, Mo and N contents of the highly corrosion-resistant stainless steel constituting the inner layer are as follows:

Cr: 19 to 26%
Mo: 2.5-7.0%
N: 0.08-0.3%

The Cr, Mo and N contents of the general-purpose stainless steel that constitutes the outer layer are as follows:

Cr: 16 to 20%
Mo: 3% or less N: 0.07% or less

The amount of N in general-purpose stainless steel is an unavoidable impurity, but it is considered to be less than the amount in highly corrosion-resistant stainless steel, so it is set at 0.07% or less.

The preferred combination of materials is as follows:

[Thickness of inner and outer layers]

The diameter of the bellows type expansion joint that the present inventors intend to use is in the range of 50 to 3600 mm. When a multi-layered bellows pipe having an inner layer and an outer layer is subjected to a force greater than the design limit and cracks occur, it is believed that the layer with the greater plate thickness will crack first. Since it is the inner layer that comes into contact with highly corrosive liquids, it is desirable to avoid the inner layer cracking before the outer layer.

From the above viewpoints, it is desirable to set the ratio (T2/T1) of the thickness of the outer layer (T2) to the thickness of the inner layer (T1) in the range of 1 to 5. From the viewpoint of cost, it is also preferable to set the thickness of the expensive inner layer to 1/2 or less of the total thickness of the inner and outer layers.

In order for the inner layer to maintain high corrosion resistance over a long period of time, it is desirable for the inner layer to have a thickness of 0.3 mm or more and 3 mm or less. In terms of the seam welding ability of thin plates, it is also desirable for the thickness to be 0.3 mm or more.

From the standpoint of formability, it is preferable that the upper limit of the total thickness of the inner and outer layers is 6.0 mm.

[Bellows tube with 3 or more layers]

If the external atmosphere of the bellows-type expansion joint is, for example, an environment involving salt damage, in addition to constructing the inner layer from a highly corrosion-resistant material, a second outer layer may be placed on the outer layer and this second outer layer may be constructed from a highly corrosion-resistant material.

The material combinations in the above embodiment are, for example, as follows:

Inner layer: SUS312L or SUS329J4L
Outer layer: SUS304, SUS304L, SUS316 or SUS316L
Second outer layer: SUS312L or SUS329J4L

[Inner layer made of nickel alloy]

Nickel alloys that exhibit high corrosion resistance can be used as the material for the inner layer of a multi-layered bellows tube. The inventors of the present application compared the corrosion resistance of a bellows tube made of a nickel alloy that meets JIS standard NW0276 with that of a bellows tube made of stainless steel (SUS312L, SUS304).

The corrosion resistance evaluation experiment for SUS312L is shown in Figure 5, and the corrosion resistance evaluation experiment for SUS304 is shown in Figure 6. As mentioned above, a comparison of the two shows that SUS312L, which has high corrosion resistance, is more suitable than general-purpose austenitic stainless steels such as SUS304 as the material for constructing the inner surface of a bellows pipe that may come into contact with highly corrosion-resistant fluids.

Bellows-type expansion joints may be installed in locations with more severe temperature and corrosive environments. For example, in the case of bellows-type expansion joints used in salt production equipment, even highly corrosion-resistant stainless steels such as SUS312L may not be able to withstand the severe corrosive conditions. For bellows-type expansion joints intended for installation in such severe corrosive environments, it is preferable to use an inner layer made of a nickel alloy.

The inventors of this application conducted an experiment to evaluate the corrosion resistance of a nickel alloy bellows tube that meets JIS standard NW0276.

The components of NW0276 are shown in Table 6 below.

When calculating the pitting corrosion index of the above nickel alloy, the Cr content was set to 15.5%, the Mo content to 16%, and the N content to 0.02%, which is the amount contained as an unavoidable impurity. As a result, the pitting corrosion index is Cr + 3.3Mo + 16N = 68.62.

The conditions for the corrosion resistance evaluation experiment of the above nickel alloys are the same as those for the experiments on SUS312L and SUS304 in Figures 5 and 6, so a repeated explanation will not be given here.

Figure 7 shows the occurrence of stains on a bellows tube made of nickel alloy (NW0276).

As shown in Figure 7, in the case of a bellows tube made of NW0276, no stains were observed after one cycle, stains of about 1 mm or less appeared in several places after two cycles, and stains were observed on the boundary surface of the bellows-shaped part after 10 cycles. After that, no increase in stains was observed after 25 cycles, 50 cycles, or 75 cycles, and there was no discoloration of the stains even after 75 cycles.

Compared to the rust occurrence on the SUS304 bellows tube shown in Figure 6 and the rust occurrence on the SUS312L bellows tube shown in Figure 5, it can be seen that the NW0276 bellows tube shown in Figure 7 exhibits higher corrosion resistance.

In this experiment, we used a nickel alloy material that meets the JIS standard NW0276, but any nickel alloy that contains Ni as the main component and contains 14 to 17 mass% Cr, 14.5 to 17.5 mass% Mo, and 0.07 mass% or less N is likely to exhibit similar corrosion resistance.

In multi-layer expansion joints used in more severe corrosive environments, it is desirable to make the inner layer out of the nickel alloy described above and the outer layer out of general-purpose stainless steel. If a second outer layer is provided to cover the outer layer, the second outer layer may be made out of the nickel alloy described above.

In the above explanation, specific embodiments have been used as examples, but the scope of the present invention is not limited to the embodiments, and various modifications or changes are possible within the scope of the present invention or within an equivalent range.

The present invention can be advantageously used in bellows-type expansion joints that guide highly corrosive fluids.

Claims (8)

An expansion joint having a bellows shape , which is used in a piping path for guiding a liquid ,
an inner layer in contact with the liquid flowing through the tube;
an outer layer overlying the inner layer;
The material constituting the inner layer is a stainless steel containing, by weight, 19 to 21% Cr, 6 to 7% Mo, and 0.16 to 0.25% N,
A bellows type expansion joint, wherein the material constituting the outer layer is a stainless steel containing, by weight, 16 to 20% Cr, 3% or less Mo, and 0.07% or less N. An expansion joint having a bellows shape, which is used in a piping path for guiding a liquid,
an inner layer in contact with the liquid flowing through the tube;
an outer layer overlying the inner layer;
The material constituting the inner layer is a duplex stainless steel containing, by weight, 24 to 26% Cr, 2.5 to 3.5% Mo, and 0.08 to 0.30% N;
A bellows type expansion joint, wherein the material constituting the outer layer is a stainless steel containing, by weight, 16 to 20% Cr, 3% or less Mo, and 0.07% or less N. The stainless steel constituting the inner layer is SUS312L according to the JIS standard,
2. The bellows type expansion joint according to claim 1 , wherein the stainless steel constituting the outer layer is SUS304, SUS304L, SUS316 or SUS316L according to JIS. The stainless steel constituting the inner layer is SUS329J4L according to the JIS standard,
3. The bellows type expansion joint according to claim 2, wherein the stainless steel constituting the outer layer is SUS304, SUS304L, SUS316 or SUS316L according to JIS standard . The diameter of the expansion joint is 50 to 3600 mm,
The plate thickness (T1) of the inner layer is 0.3 to 3.0 mm,
5. The bellows type expansion joint according to claim 1 , wherein a ratio (T2/T1) of a plate thickness (T2) of the outer layer to a plate thickness (T1) of the inner layer is 1 to 5. a second outer layer overlying the outer layer;
5. The bellows type expansion joint according to claim 3, wherein a material constituting the second outer layer is SUS312L or SUS329J4L according to JIS standard . An expansion joint having a bellows shape, which is used in a piping path for guiding a liquid,
an inner layer in contact with the liquid flowing through the tube;
an outer layer overlying the inner layer;
The material constituting the inner layer is a nickel alloy containing, by weight, 14 to 17% Cr, 14.5 to 17.5% Mo, and 0.07% or less N;
A bellows type expansion joint, wherein the material constituting the outer layer is a stainless steel containing, by weight, 16 to 20% Cr, 3% or less Mo, and 0.07% or less N. 8. The bellows type expansion joint according to claim 7 , wherein the nickel alloy is JIS NW0276.