Ring joint cover

Abstract

The present invention provides a ring joint cover that prevents damage to the ring joint gasket from direct contact between the flanges when the ring joint gasket is installed between the flanges of a pipe and the flanges are tightened, thereby improving the sealing performance between the ring joint gasket and the flanges. [Solution] A metal ring joint cover 1 having an annular shape that is fitted onto a metal ring joint gasket 6, the ring joint cover 1 having an inner circumferential wall 2 on the inner circumference side, an outer circumferential wall 3 on the outer circumference side, and a bottom surface portion 4 for connecting the inner circumferential wall and the outer circumferential wall, wherein at least one through hole 5 is formed in the bottom surface portion 4.

Description

Technical Field

[0001] The present invention relates to a ring joint cover. More specifically, the present invention relates to a ring joint cover attached to a ring joint gasket used to seal fluid in a pipe by attaching it between flanges of a pipe and then tightening the flanges, and a ring joint gasket including the ring joint cover.

Background Art

[0002] Ring joint gaskets are used, for example, in petrochemical plants, ships, etc. in pipe flanges, pressure vessels, valve bonnets, etc. that handle high-temperature and high-pressure steam and gas, oil, solvents, etc. As a ring joint gasket, a ring joint gasket made of a shape memory alloy has been proposed (for example, see Patent Document 1).

[0003] When the ring joint gasket is attached between the flanges of a pipe and the flanges are tightened, the ring joint gasket rubs against the flanges, damaging the ring joint gasket and the flanges, and reducing the sealing performance between the ring joint gasket and the flanges.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention has been made in view of the prior art, and aims to provide a ring joint cover and a ring joint gasket equipped with the ring joint cover that can prevent damage to the ring joint gasket and the flange by direct contact when the ring joint gasket is installed between the flanges of a pipe and the flange is tightened, and can also improve the sealing performance between the ring joint gasket and the flange. [Means for solving the problem]

[0006] The present invention (1) A ring joint cover having an annular shape that is fitted onto a metal ring joint gasket, the ring joint cover having an inner circumferential wall on the inner circumference side of the ring joint cover, an outer circumferential wall on the outer circumference side of the ring joint cover, and a bottom surface for connecting the inner circumferential wall and the outer circumferential wall, wherein at least one through hole is formed in the bottom surface. (2) The ring joint cover described in (1) above, wherein the hardness of the metal used for the ring joint cover is the same as or lower than the hardness of the metal used for the ring joint gasket. (3) The ring joint cover according to (1), wherein the metal used for the ring joint cover is at least one metal selected from the group consisting of aluminum, copper and nickel, and (4) A ring joint gasket comprising a ring joint cover having a ring joint cover as described in any of (1) to (3) above attached to at least one of the upper and lower surfaces of a ring joint gasket, wherein the hardness of the metal used for the ring joint cover is the same as or lower than the hardness of the metal used for the ring joint gasket. Regarding.

[0007] In this invention, both the upper and lower limits of the numerical range are included within that numerical range. [Effects of the Invention]

[0008] According to the present invention, when a ring joint gasket is installed between the flanges of a pipe and the flanges are tightened, it is possible to prevent damage caused by direct contact between the ring joint gasket and the flanges, and to improve the sealing performance between the ring joint gasket and the flanges. The present invention also provides a ring joint cover and a ring joint gasket equipped with the ring joint cover. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic plan view showing one embodiment of the ring joint cover of the present invention. [Figure 2] Figure 1 is a schematic longitudinal cross-sectional view of the XX portion of the ring joint cover of the present invention. [Figure 3] Figure 1 is a schematic longitudinal cross-sectional view showing another embodiment of the XX portion of the ring joint cover of the present invention. [Figure 4] Figure 2 is a schematic longitudinal cross-sectional view showing the ring joint cover with a ring joint gasket installed inside, and the ring joint cover fitted into the gasket groove of the pipe flange. [Figure 5] This is a schematic longitudinal cross-sectional view of a ring joint gasket equipped with a ring joint cover, in which ring joint covers having the cross-sectional shape shown in Figure 2 are attached to both the upper and lower surfaces of the ring joint gasket. [Figure 6] Figure 2 is a schematic diagram illustrating the details of the ring joint cover. [Modes for carrying out the invention]

[0010] The ring joint cover of the present invention is a metal ring joint cover having an annular shape that is mounted on a metal ring joint gasket.

[0011] The ring joint cover of the present invention has an inner peripheral wall that exists on the inner peripheral side of the ring joint gasket, an outer peripheral wall that exists on the outer peripheral side of the ring joint gasket, and a bottom surface portion for connecting the inner peripheral wall and the outer peripheral wall, and at least one through hole is formed in the bottom surface portion of the ring joint cover.

[0012] Since the ring joint cover of the present invention has the above-described configuration, when the ring joint gasket is mounted between the flanges of the pipe and the flanges are tightened, not only can it prevent the ring joint gasket and the flanges from being damaged by direct contact, but also the sealing performance between the ring joint gasket and the flanges can be improved.

[0013] The ring joint cover of the present invention and the ring joint gasket provided with the ring joint cover will be described in detail below with reference to the drawings. However, the present invention is not limited only to the embodiments described in the drawings.

[0014] FIG. 1 is a schematic plan view showing an embodiment of the ring joint cover of the present invention.

[0015] The ring joint cover 1 shown in FIG. 1 has an annular shape in its planar shape. However, the present invention is not limited by the planar shape of the ring joint cover 1, and preferably has a planar shape corresponding to a ring joint gasket (not shown) that is mounted on the ring joint cover 1.

[0016] Examples of the planar shape of the ring joint cover 1 include a circle shown in FIG. 1, polygons such as a square and a rectangle, but the present invention is not limited only to such examples. When the ring joint cover 1 has a polygon, it is preferable that an arc is formed at a corner of the polygon. The circle herein includes not only a perfect circle but also shapes such as an ellipse and a track ellipse in the concept.

[0017] The size in the planar shape of the ring joint cover 1 varies depending on the type of the ring joint gasket to be mounted on the ring joint cover 1 and its use, etc., and thus cannot be determined unconditionally. Therefore, it is preferable to appropriately determine according to the type of the ring joint gasket and its use, etc. When the planar shape of the ring joint cover 1 is, for example, a circle, the diameter in the planar shape of the ring joint cover 1 is usually about 2 mm to 3 m.

[0018] The ring joint cover 1 is made of metal. Examples of the metal include aluminum, copper, nickel, brass, tin, lead, pure iron, very soft iron, tantalum, gold, silver, alloys of these metals, etc., but the present invention is not limited only to such examples. Among these metals, from the viewpoint of improving the sealing performance between the ring joint gasket and the pipe flange when the ring joint gasket provided with the ring joint cover 1 is mounted on the pipe flange, at least one metal selected from the group consisting of aluminum, copper, nickel, brass, pure iron and very soft iron is preferable, and at least one metal selected from the group consisting of aluminum, copper and nickel is more preferable. When two or more kinds of metals are used, the two or more kinds of metals can be used as an alloy.

[0019] Incidentally, the Vickers hardness (also called Vickers hardness) of metals is as follows: for example, aluminum is 15 Hv, copper is 50 Hv, nickel is 200 Hv, brass is 60-190 Hv, iron is 45-80 Hv, gold is 25 Hv, silver is 25 Hv, and lead is 5 Hv. Note that the Vickers hardness of metals is the value measured under the following conditions.

[0020] [Measurement conditions for Vickers hardness] • Measuring device: Micro-Vickers hardness tester [Mitutoyo Corporation, model number: HM-210] • Load: 50-500 gf (adjustable depending on the material type and thickness of the sample) • Pressing time: 15 seconds • Number of measurement points: 5 • Vickers hardness: The average value of Vickers hardness measured at 5 locations is used.

[0021] Figure 2 is a schematic longitudinal cross-sectional view of the XX portion of the ring joint cover 1 shown in Figure 1.

[0022] As shown in Figure 2, the ring joint cover 1 has an inner circumferential wall 2 located on the inner circumference side of the ring joint cover 1, an outer circumferential wall 3 located on the outer circumference side of the ring joint cover 1, and a bottom surface portion 4 for connecting the inner circumferential wall 2 and the outer circumferential wall 3. The inner circumferential wall 2 is located on the inner circumference side of the ring joint cover 1 and is the wall of the surface that contacts the perpendicular to the ring joint cover 1, the outer circumferential wall 3 is located on the outer circumference side of the ring joint cover 1 and is the wall of the surface that contacts the perpendicular to the ring joint cover 1, and the bottom surface portion 4 is a member located on the bottom surface of the ring joint cover 1.

[0023] The inner surface 2a of the inner circumferential wall 2, the inner surface 3a of the outer circumferential wall 3, and the bottom surface 4a of the bottom surface 4 preferably have a shape corresponding to the ring joint gasket attached to the ring joint cover 1, from the viewpoint of improving sealing performance by ensuring close contact between the ring joint cover 1 and the ring joint gasket attached to the ring joint cover 1. The inner surface 2a of the inner circumferential wall 2, the inner surface 3a of the outer circumferential wall 3, and the bottom surface 4a of the bottom surface 4 refer to the inner surfaces of the ring joint cover 1 where a space exists enclosed by the inner surface 2a of the inner circumferential wall 2, the inner surface 3a of the outer circumferential wall 3, and the bottom surface 4a of the bottom surface 4 in the schematic vertical cross-sectional view shown in Figure 2.

[0024] In the embodiment shown in the schematic longitudinal cross-sectional view of the ring joint cover 1 in Figure 2, corners are formed at the boundary between the inner circumferential wall 2 and the bottom surface 4, and at the boundary between the outer circumferential wall 3 and the bottom surface 4. However, corners are not necessarily required at each boundary, and curved surfaces may be formed. For example, if the outer circumferential surface of the ring joint gasket fitted to the ring joint cover 1 is curved, then, as shown in Figure 3, curved surfaces may be formed at the boundary between the inner circumferential wall 2 and the bottom surface 4, and at the boundary between the outer circumferential wall 3 and the bottom surface 4. Therefore, in the present invention, the boundaries between the inner circumferential wall 2 and the bottom surface 4, and the boundaries between the outer circumferential wall 3 and the bottom surface 4 do not necessarily have to be clearly defined.

[0025] Figure 3 is a schematic longitudinal cross-sectional view showing another embodiment of the XX portion of the ring joint cover 1 shown in Figure 1.

[0026] The thickness of the ring joint cover 1, in other words, the thickness of the inner circumferential wall 2, outer circumferential wall 3, and bottom surface 4, cannot be determined in general terms as they vary depending on the type of metal used for the ring joint cover 1, the type of ring joint gasket fitted to the ring joint cover 1, and its application. However, it is usually preferably 0.2 to 0.8 mm, and more preferably 0.3 to 0.7 mm.

[0027] In Figure 2, the height h from the upper end to the lower end of the ring joint cover 1 cannot be determined in general, as it varies depending on the type of ring joint gasket to which the ring joint cover 1 is applied and its application. Therefore, it is preferable to determine it appropriately according to the type of ring joint gasket and its application. The height h of the ring joint cover 1 is usually about 3 to 10 mm.

[0028] At least one through hole 5 is formed in the bottom surface 4 of the ring joint cover 1.

[0029] One of the features of the present invention is that the ring joint cover 1 of the present invention has at least one through hole 5 formed in the bottom surface portion 4 of the ring joint cover 1. In the ring joint cover 1 of the present invention, since at least one through hole 5 is formed in the bottom surface portion 4, compared to a ring joint cover in which no through hole 5 is formed, it is possible to not only further improve the sealing performance between the ring joint gasket and the pipe flange, but also to suppress the occurrence of damage such as cracks in the ring joint cover 1.

[0030] The number of through holes 5 formed in the bottom surface 4 of the ring joint cover 1 is at least one. Therefore, the number of through holes 5 formed in the bottom surface 4 of the ring joint cover 1 may be just one, or it may be multiple, such as 2 to 6, and the present invention is not limited by the number of through holes 5. From the viewpoint of production efficiency of the ring joint cover 1, the number of through holes 5 is preferably 1 to 6. When multiple through holes 5 are formed in the bottom surface 4 of the ring joint cover 1, it is preferable that each through hole 5 is provided at equal or nearly equal intervals.

[0031] The diameter of the through-hole 5 is not particularly limited, but it is preferably about 0.5 to 3 mm in order to improve the sealing performance between the ring joint gasket and the pipe flange and to suppress damage such as cracks in the ring joint cover 1.

[0032] As shown in Figure 4, a ring joint gasket 6 is fitted to the ring joint cover 1, and the ring joint gasket 6, which includes the ring joint cover 1, is fitted into the gasket groove 8 of the pipe flange 7.

[0033] Figure 4 is a schematic longitudinal cross-sectional view showing the ring joint gasket 6 installed inside the ring joint cover 1 shown in Figure 2, and the ring joint cover 1 installed in the gasket groove 8 of the pipe flange 7.

[0034] The inner surface (not shown) of the ring joint cover 1 preferably has a shape corresponding to the ring joint gasket 6 attached to the ring joint cover 1, from the viewpoint of ensuring a tight seal between the ring joint cover 1 and the ring joint gasket 6 attached to the ring joint cover 1, as described above.

[0035] When the ring joint gasket 6, which includes the ring joint cover 1, is inserted into the gasket groove 8, the bottom surface 4 of the ring joint cover 1 and the inner wall (not shown) of the gasket groove 8 come into contact at point P on the inner circumferential wall 2 of the ring joint cover 1 and point Q on the outer circumferential wall 3 of the ring joint cover 1. Therefore, the through hole 5 is located between point P and point Q on the bottom surface 4 of the ring joint cover 1.

[0036] Examples of ring joint gaskets 6 include ring joint gaskets with a circular longitudinal cross-sectional shape, ring joint gaskets with an elliptical longitudinal cross-sectional shape, ring joint gaskets with a hexagonal longitudinal cross-sectional shape, ring joint gaskets with an octagonal longitudinal cross-sectional shape, ring joint gaskets with a wedge-shaped longitudinal cross-sectional shape, and ring joint gaskets with a triangular longitudinal cross-sectional shape, but the present invention is not limited to these examples. The longitudinal cross-sectional shape of the ring joint gasket 6 is usually selected according to the application of the ring joint gasket 6.

[0037] The hardness of the metal used in the ring joint cover 1 is preferably the same as or lower than the hardness of the metal used in the ring joint gasket 6, from the viewpoint of suppressing damage caused by direct contact between the ring joint gasket 6 and the pipe flange 7 via the ring joint cover 1. Similarly, the hardness of the metal used in the pipe flange 7 is preferably the same as or lower than the hardness of the metal used in the pipe flange 7, from the viewpoint of suppressing damage to the pipe flange 7.

[0038] Examples of metals that can be used in the ring joint gasket 6 include pure iron, mild steel, copper, brass, stainless steel (SUS304, SUS304L, SUS316, SUS316L, etc.), chromium molybdenum steel, Monel, titanium, nickel, aluminum, and Inconel 600, but the present invention is not limited to these examples. The type of metal used in the ring joint gasket 6 is selected appropriately depending on the application of the ring joint gasket 6.

[0039] The ring joint cover 1 is attached to at least one of the upper and lower surfaces of the ring joint gasket 6. Whether the ring joint cover 1 is attached to one of the upper and lower surfaces of the ring joint gasket 6, or to both, is optional and can be appropriately selected depending on the application of the ring joint gasket 6.

[0040] Examples of product forms for the ring joint cover 1 include a single ring joint cover, a pair (2) of ring joint covers, and a ring joint gasket with a ring joint cover attached to the upper and / or lower surface of the ring joint gasket. However, the present invention is not limited to these examples.

[0041] Figure 5 shows a ring joint gasket 9 equipped with a ring joint cover 1, which has the cross-sectional shape shown in Figure 2, attached to both the upper and lower surfaces of the ring joint gasket 6. Figure 5 is a schematic longitudinal cross-sectional view of the ring joint gasket 9 equipped with the ring joint cover 1.

[0042] In the ring joint gasket 9 equipped with the ring joint cover 1 shown in Figure 5, the ring joint cover 1 is attached to both the upper and lower surfaces of the ring joint gasket 6. However, depending on the application of the ring joint gasket 6, the ring joint cover 1 may be attached to only one of the upper or lower surfaces of the ring joint gasket 6.

[0043] When a ring joint gasket 9 equipped with the ring joint cover of the present invention is installed between the flanges (not shown) of a pipe and the flanges are tightened, the sealing performance between the ring joint gasket 6 and the flange can be further improved compared to a ring joint gasket 9 equipped with a ring joint cover that does not have a through hole 5, and damage such as cracks in the ring joint cover 1 can be suppressed.

[0044] Therefore, the ring joint gasket 9 equipped with the ring joint cover of the present invention can be suitably used as a sealing member for pipes in, for example, thermal power plants, nuclear power plants, steam engines of steam turbine ships, oil refining lines, petrochemical process lines, semiconductor manufacturing lines, and other machinery and equipment. [Examples]

[0045] Next, the ring joint cover and the ring joint gasket equipped with the ring joint cover of the present invention will be described in more detail based on examples, but the present invention is not limited to the embodiments described in such examples.

[0046] Examples 1-3 Multiple ring joint covers 1, each having the planar shape shown in Figure 1 and the longitudinal cross-sectional shape shown in Figure 2, were fabricated as ring joint covers.

[0047] More specifically, a ring joint cover 1 was used, which had a circular planar shape as shown in Figure 1, a longitudinal cross-section as shown in Figure 2, and an upward-facing opening. As the metal constituting the ring joint cover 1, aluminum, copper, or nickel was used, as shown in Table 1.

[0048] As the ring joint cover 1, the ring joint cover shown in Figure 6 was used. Figure 6 is a schematic diagram illustrating the details of the ring joint cover 1 shown in Figure 2.

[0049] A ring joint cover 1 was manufactured, which has an inner circumferential wall 2 located on the inner circumference side of the ring joint cover 1, an outer circumferential wall 3 located on the outer circumference side of the ring joint cover 1, and a bottom surface portion 4 for connecting the inner circumferential wall 2 and the outer circumferential wall 3.

[0050] The height h of the ring joint cover 1 was 5 mm, the outer diameter R1 of the upper surface of the ring joint cover 1 was 107 mm, and the inner diameter R2 was 83 mm, while the outer diameter r1 of the lower surface of the ring joint cover 1 was 104 mm, and the inner diameter r2 was 87 mm. The thickness of the ring joint cover 1 was 0.4 mm.

[0051] The inner surface of the ring joint cover 1 has a shape corresponding to the ring joint gasket 6 that is fitted to the ring joint cover 1. As shown in Figure 4, when the ring joint gasket 6, which includes the ring joint cover 1, is inserted into the gasket groove 8 of the pipe flange 7, a through hole (diameter: 1 mm) 5 is formed in the bottom surface 4 of the ring joint cover 1, as shown in Figure 6, between the contact point P that contacts the inner diameter side wall of the gasket groove 8 and the point Q that contacts the outer diameter side wall of the gasket groove 8.

[0052] An octagonal ring joint gasket made of R24 (Vickers hardness: Hv45) extremely soft steel was used as the ring joint gasket 6, and a pipe flange 7 made of SFVC2A (forged carbon steel for pressure vessels, Vickers hardness: Hv140) as specified in JIS G 3202 was used as the pipe flange 7. In addition, SNB material fully threaded stud bolts (SNB7, 7 / 8-9UNC) were used as bolts to fasten the pipe flange 7. A lubricant (Bostik, product name: Never-Seez) was applied to the bolts.

[0053] A ring joint gasket 9 equipped with a ring joint cover 1 was manufactured by attaching the ring joint cover 1 to the upper and lower surfaces of the ring joint gasket 6, respectively.

[0054] Next, as shown in Figure 4, the ring joint gasket 9, which includes the ring joint cover 1 obtained above, was sandwiched between the pair of pipe flanges 7, 7, and the pipe flanges 7, 7 were tightened with bolts, adjusting the tightening torque to 250 N·m.

[0055] The bolts were tightened in accordance with JIS B2251, with eight bolts tightened diagonally in the following order: 10% (25 N·m), 20% (50 N·m), 60% (150 N·m), and 100% (250 N·m) of the tightening torque of 250 N·m. After that, the bolts were tightened four times in the circumferential direction with a tightening torque of 250 N·m.

[0056] Next, a thermal history was applied to the piping to which the pair of pipe flanges 7,7 obtained above were joined. More specifically, the piping was heated in an electric furnace from room temperature (25°C) to a temperature corresponding to the metal constituting the ring joint cover 1 (aluminum: 350°C, copper: 500°C, nickel: 760°C), and after maintaining that temperature for 15 hours, it was allowed to cool to room temperature. This cycle constituted one thermal history, and after applying the thermal history to the piping 10 times, the sealing performance, durability, and scratch resistance of the piping were investigated based on the following method. The results are shown in Table 1.

[0057] [Sealing properties] The sealing performance of the piping was evaluated by introducing nitrogen gas at the pressures shown in Table 1 into the piping, subjecting it to a 10-cycle thermal history, and then recovering the nitrogen gas that leaked between pipe flanges 7,7 using the water displacement method, based on the following evaluation criteria.

[0058] [Evaluation Criteria for Sealing Performance] ◎: The amount of nitrogen gas recovered is 3 × 10 -4 Pa·m 3 Less than / s (Leakage: Less than 1 mL / 360 seconds) (Pass) ×: The amount of nitrogen gas recovered is 3 × 10 -4 Pa·m 3 / s or more (fail)

[0059] [Durability] After subjecting the piping to a 10-cycle thermal history, the piping was disassembled, the appearance of the ring joint cover was observed, and its durability was evaluated based on the following evaluation criteria.

[0060] [Durability evaluation criteria] ◎: No abnormalities were found in the ring joint cover. ×: Cracks or other abnormalities are observed in the ring joint cover.

[0061] [Scratch resistance] After subjecting the piping to a 10-cycle thermal history, the piping was disassembled and the appearance of the ring joint gasket was observed, and its abrasion resistance was evaluated based on the following evaluation criteria.

[0062] [Criteria for evaluating scratch resistance] ◎: No abnormalities were found in the ring joint gasket. ×: The ring joint gasket shows signs of wear, such as scratches.

[0063] Comparative Examples 1-3 In Examples 1 to 3, a piping system was fabricated in the same manner as in Examples 1 to 3, except that a ring joint cover without a through hole was used as the ring joint cover 1. The sealing performance, durability, and scratch resistance were then evaluated. The results are shown in Table 1.

[0064] Comparative Examples 4-6 In Examples 1-3, a piping system was fabricated in the same manner as in Examples 1-3, except that a ring joint cover was not used, and the sealing performance, durability, and scratch resistance were evaluated. The results are shown in Table 1.

[0065] [Table 1]

[0066] From the results shown in Table 1, it can be seen that the ring joint gaskets 9 equipped with the ring joint cover 1 obtained in each example and Comparative Examples 1 to 3 have excellent sealing properties, as the ring joint cover is attached to the ring joint gasket, and therefore, compared to Comparative Examples 4 to 6, scratches on the ring joint gasket can be suppressed as described in the scratch resistance column.

[0067] Furthermore, the ring joint gasket 9 equipped with the ring joint cover 1 obtained in each embodiment has a through hole formed in the bottom surface of the ring joint cover, and therefore exhibits superior sealing performance even at high temperatures compared to Comparative Examples 1 to 3, which do not have a through hole formed in the bottom surface of the ring joint cover. This also indicates that the ring joint cover does not suffer damage and therefore has superior scratch resistance. [Explanation of Symbols]

[0068] 1 Ring joint cover 2 Inner wall 2a Inner surface of the inner circumferential wall 3 Outer wall 3a Inner surface of the outer wall 4 Bottom part 4a Bottom surface of the bottom part 5 Through hole 6. Ring joint gasket 7 Pipe flange 8 gasket grooves 9. Ring joint gasket with ring joint cover

Claims

1. A ring joint cover having an annular shape, which is fitted onto a metal ring joint gasket, and having an inner circumferential wall on the inner circumference side of the ring joint cover, an outer circumferential wall on the outer circumference side of the ring joint cover, and a bottom surface for connecting the inner circumferential wall and the outer circumferential wall, wherein at least one through hole is formed in the bottom surface.

2. The ring joint cover according to claim 1, wherein the hardness of the metal used in the ring joint cover is the same as or lower than the hardness of the metal used in the ring joint gasket.

3. The ring joint cover according to claim 1, wherein the metal used for the ring joint cover is at least one metal selected from the group consisting of aluminum, copper, and nickel.

4. A ring joint gasket comprising a ring joint cover, wherein the ring joint cover according to any one of claims 1 to 3 is attached to at least one of the upper and lower surfaces of a ring joint gasket, characterized in that the hardness of the metal used for the ring joint cover is the same as or lower than the hardness of the metal used for the ring joint gasket.

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