JP5601812B2 - Gasket for nuclear power plant - Google Patents

Description

  The present invention relates to a gasket for a nuclear power plant used for piping of a nuclear power plant (reactor etc.), in particular, expanded graphite, aramid fiber, rubber binder, vulcanization accelerator and vulcanizing agent. The present invention relates to a sheet gasket formed from a material containing a main rubber chemical.

  Traditionally, asbestos-type gaskets have been used for this type of seat gasket, but asbestos has been prohibited from use and restricted due to health and environmental health issues such as causing emphysema. Recently, non-asbestos-based gaskets tend to be used frequently.

  This tendency is the same in nuclear power plants, and non-asbestos-based gaskets are used instead of traditional asbestos-based gaskets. For example, Patent Document 1 discloses such a non-asbestos-based gasket. As described above, a sheet gasket formed from a material containing expanded graphite, aramid fiber, rubber binder, rubber chemicals, and the like has been proposed.

  By the way, since nuclear plants require extremely strict safety and durability, austenite such as SUS304 having excellent corrosion resistance is required for metal plant parts (for example, reactor structural materials and piping). Stainless steel is used. However, even in such a pipe made of stainless steel, stress corrosion cracking (SCC) may occur in the flange depending on the gasket loaded between the flanges, which is mainly due to chloride ions eluted from the gasket. It is said that. Therefore, as a nuclear power plant gasket, it is preferable to use a gasket having a chloride ion concentration of less than 100 ppm as a temporary guide. Incidentally, asbestos-based gaskets, which are traditional gaskets for nuclear power plants, remove chloride ions by subjecting asbestos minerals used as raw materials to chloride ion removal treatment (boiling treatment or washing treatment with running water) in advance. Therefore, it did not contain chloride ions above the reference value (100 ppm).

Japanese Patent Laid-Open No. 2008-201988

  On the other hand, some of the conventional non-asbestos gaskets have characteristics similar to those of asbestos gaskets as disclosed in Patent Document 1, and are substitutes for asbestos gaskets as a gasket for nuclear power plants. There is something you can expect. However, the conventional non-asbestos gaskets including the gasket disclosed in Patent Document 1, which can be used as an alternative to asbestos gaskets, have a chloride ion concentration of 100 ppm or more, and conversely chloride ions. Those with a reduced concentration are significantly inferior in thermal creep properties to asbestos-based gaskets, and in any case, no material that can be suitably used as a gasket for nuclear power plants has been proposed. It is.

  The present invention has been made in view of such circumstances, and restricts the chloride ion concentration to be lower than the above-mentioned general standard (less than 100 ppm), and suppresses thermal creep and corrosion of metal flanges as much as possible. An object of the present invention is to provide a nuclear power plant gasket that can be suitably used as an asbestos-based gasket substitute in plant piping and the like.

A gasket for a nuclear power plant of the present invention that has solved this problem includes expanded graphite, an aramid fiber (aromatic polyamide fiber), a rubber binder, and a rubber chemical mainly composed of a vulcanization accelerator and a vulcanizing agent. In the sheet gasket formed from the contained material (hereinafter referred to as “gasket raw material”), each content of the rubber binder, rubber chemicals and vulcanization accelerator in the gasket raw material (mass ratio with respect to the total mass of the gasket raw material) Rubber binder: 10 to 18 mass%, rubber chemical: 3 to 5 mass%, and vulcanization accelerator: 0.8 to 2.0 mass%, and zinc oxide not containing chloride ions is used as the vulcanization accelerator. It is proposed that the chloride ion concentration be limited to 10 ppm or less. The rubber binder is contained for the purpose of improving the sealing property, but if the content is less than 10 mass%, the sealing property cannot be sufficiently improved, and conversely if the content exceeds 18 mass%. There is a possibility of adversely affecting the tensile strength. Rubber binding materials include nitrile rubber (NBR), styrene butadiene rubber (SBR), natural rubber (NR), acrylic rubber (ACM), isoprene rubber (IR), chloroprene rubber (CR), butadiene rubber (BR), and butyl rubber. (IIR), ethylene propylene rubber (EPDM), fluoro rubber (FKM), or a combination of two or more selected from these can be used, and NBR is generally excellent in oil resistance, compression recovery, and mechanical strength in a well-balanced manner However, the content is preferably set to 10 to 18 mass% in consideration of restoration properties and the like. The rubber chemicals are blended for the purpose of accelerating the vulcanization of rubber and improving the thermal creep property, and are mainly composed of a vulcanizing agent (crosslinking agent) and a vulcanizing accelerator. In addition to vulcanizing agents and vulcanization accelerators, rubber chemicals can be used in addition to anti-aging agents, scorching agents, sulfur, plastics, depending on the purpose of heat resistance, oil resistance, acid resistance, light resistance, color tone, etc. Agents, pigments, etc. are included, but the content of rubber chemicals should be 3 to 5 mass% in consideration of thermal creep properties and vulcanization acceleration effects. Especially, the chloride ion concentration for vulcanization accelerators In consideration of the above, the content needs to be set to 0.8 to 2.0 mass%.

In a preferred embodiment of such a gasket for a nuclear power plant, zinc oxide not containing chloride ions is used as a vulcanization accelerator as described above . Specifically, as such zinc oxide, specifically, JIS K5102 ( It is optimal to use one kind of zinc white, two kinds of zinc white, or three kinds of zinc white as defined in (Retired on Feb. 20, 2009).

  Moreover, in the gasket for nuclear power plants of this invention, it is preferable that the total content (mass ratio with respect to the total mass of a gasket raw material) of the expanded graphite, an aramid fiber, and a rubber binder in the said gasket raw material shall be 40-50 mass%. If the total content is less than 40 mass%, it is difficult to ensure a sealing property equivalent to an asbestos-based gasket. Conversely, if it exceeds 50 mass%, the thermal creep characteristics may be adversely affected.

  Further, each content of expanded graphite and aramid fibers in the gasket raw material (mass ratio with respect to the total mass of the gasket raw material) needs to be set so as to ensure sealing performance, strength, etc. equivalent to an asbestos-based gasket, The expanded graphite is preferably set to 15 to 35 mass%, and the aramid fiber is preferably set to 4 to 15 mass%. That is, expanded graphite is to ensure the resilience of the gasket during heating, but if the content is less than 15 mass%, it is not possible to ensure the predetermined resilience, conversely, if the content exceeds 35 mass%, The sealability and strength (tensile strength) may be adversely affected, and thermal creep properties may be reduced. Further, the aramid fiber contributes to the improvement of the tensile strength and the thermal creep property, but when the content is less than 4 mass%, such a function is not sufficiently exhibited, and conversely, when the content exceeds 15 mass%, There is a possibility that the sealing property and the like are lowered.

  In addition to the above, the gasket raw material contains inorganic fibers and inorganic fillers as necessary. Inorganic fibers are included to improve heating strength, especially to prevent thermal deformation (thermal creep) during initial heating and to improve resilience, and include rock / slag fibers, alumina-silicate fibers, and alumina fibers. Ceramic fibers such as zirconia fibers and rock wool, glass fibers such as quartz glass fibers and high silicate glass fibers, carbon fibers, phosphate fibers, and metal fibers are used. Further, the inorganic filler mainly functions as a plugging material and contributes to improvement in sealing properties, compressive strength, and heat resistance. Generally, the content (mass ratio with respect to the total mass of the gasket raw material) is 20 to 40 mass. %, Preferably one or more selected from kaolin, sepiolite, clay, talc, heavy calcium carbonate, light calcium carbonate, silica, mica, barium sulfate, carbon black, alumina, vermiculite, etc. Furthermore, inorganic powders such as powders of hydrous silicate minerals mainly containing silicon and aluminum, magnesium, iron, alkaline earth metals and alkali metals, and natural mineral powders such as wollastonite can be used.

  The gasket for a nuclear power plant of the present invention has a chloride ion concentration further reduced (less than 10 ppm) from the general standard (less than 100 ppm) described at the beginning, and it effectively prevents the occurrence of corrosion on thermal creep and metal flanges. It can be prevented and can be suitably used as an asbestos-based gasket substitute in a nuclear power plant, and its practical value is extremely large.

  As an example, a disk-shaped gasket (joint sheet) No. 1 was prepared by a conventional method (hot roll method or the like) using each gasket raw material shown in Table 1. 1-No. 8 was produced. This example gasket No. 1-No. In No. 8, NBR was used as a rubber binder, and zinc oxide containing no chloride ions (corresponding to three types of zinc oxide defined in JIS K5102 described at the beginning) was used as a vulcanization accelerator. Rubber chemicals include vulcanizing agents, vulcanization accelerators, anti-aging agents, and the like. These contents are summed with the contents of vulcanizing agents in Table 1 It is described in the column of “agents”.

  In addition, as a comparative example, each gasket raw material shown in Table 1 was used, and an example gasket No. 1-No. 8 with the same shape (disk shape) by the same process. 9-No. 12 was produced. This comparative gasket No. 9, no. 10, the constituent materials of the gasket raw materials (aramid fibers, rubber binders, expanded graphite, inorganic fibers, inorganic fillers, rubber chemicals) are the same as in Example gasket No. 10. 1-No. The same one used for 8 was used. Comparative gasket No. 11, no. 12, except that zinc stearate (“Zinc stearate N” manufactured by Tamnan Chemical Co., Ltd.) was used as a vulcanization accelerator, the components of the gasket raw material (aramid fiber, rubber binder, expanded graphite, Inorganic gaskets, inorganic fillers, rubber chemicals (excluding vulcanization accelerators) 1-No. The same one used for 8 was used.

Thus, each gasket No. 1-No. No. 12 was subjected to a heat creep property confirmation test. This confirmation test was conducted for each gasket No. 1-No. 12 is sandwiched between a stainless steel (SUS304) cradle heated to approximately 200 ° C. and a pressing cradle for 1 minute at an appropriate surface pressure (98 N / mm ² ), and the gasket diameter before and after the test is measured. Thus, the thermal creep rate is obtained. That is, the thermal creep rate R is determined by R = ((((d2) ^2- (d1) ² ) / (d1) ² ) × 100 (%) from the average diameter d1 before the test and the average diameter d2 after the test. ). The quality of the thermal creep property is determined by the thermal creep rate R, and the lower the thermal creep rate R, the better the thermal creep property. In general, in order to be excellent in thermal creep property, it is preferable that R ≦ 40%.

  The results are as shown in Table 2. The example gasket is No. Except for 8, the thermal creep rate R was in the above-mentioned preferable range, and it was confirmed that the thermal creep property was excellent. Example Gasket No. No. 8, since the total content of the aramid fiber, the rubber binder and the expanded graphite slightly exceeds the above preferred range (40 to 50 mass%), the thermal creep rate R exceeds the above preferred range (R = 42%) slightly exceeds the range, and is not particularly inferior in thermal creep. On the other hand, comparative gasket No. 9, no. No. 10 has an extremely high thermal creep rate and is inferior in thermal creep properties because the contents of the rubber binder, the vulcanization accelerator and the like deviate from the preferable ranges described above.

  In addition, each gasket No. 1-No. For No. 12, the chloride ion concentration (ppm) was measured by ion chromatography. The results were as shown in Table 2.

  As can be seen from Table 2, Example Gasket No. 1-No. No. 8 has a chloride ion concentration of 10 ppm or less, can effectively prevent metal corrosion such as SCC, and is suitable as a gasket for a nuclear power plant. On the other hand, comparative gasket No. 11, no. For No. 12, the chloride ion concentration is extremely high despite the fact that the content of each component of the gasket raw material does not deviate from the above preferred range. Comparative gasket No. 9, no. As for No. 10, although the content of each component of the gasket raw material deviates from the above-described preferred range, the chloride ion concentration is equal to or close to that of the example gasket. From these points, it is understood that the level of chloride ion concentration depends largely on the type of vulcanization accelerator. To reduce the chloride ion concentration, zinc oxide (oxidation) defined in JIS K5102 as a vulcanization accelerator is used. It is understood that it is optimal to use zinc).

Further, each gasket No. 1-No. About 12, the corrosiveness confirmation test with respect to the metal flange was done. That is, under the same conditions as gaskets loaded between piping flanges (made of austenitic stainless steel such as SUS304) in a nuclear power plant, each gasket No. 1-No. No. 12 flange corrosion was confirmed. Specifically, each gasket No. 5 is placed between metal plates made of SUS304. 1-No. No. 12 was held and the corrosion state of the metal plate surface (contact surface with the gasket) after a predetermined time was visually confirmed. The temperature of the metal plate was normal temperature, the surface pressure due to the clamping pressure of the gasket was 2.9 N / mm ² , and the clamping time was 72 hours.

  The results were as shown in Table 2. In Table 2, “◯” indicates that the metal plate surface is not corroded at all, “△” indicates that the metal plate surface is slightly corroded, and the metal plate surface is significantly corroded. When it is, it is indicated by “x”.

  As is apparent from the results shown in Table 2, Example Gasket No. 1 having a chloride ion concentration of 10 ppm or less. 1-No. 8 and comparative gasket No. For No. 9, no flange corrosion occurred. In contrast, Comparative Gasket No. 1 with a chloride ion concentration slightly exceeding 10 ppm. In Comparative Example Gasket No. 10, comparatively no flange corrosion occurred but the chloride ion concentration greatly exceeded 10 ppm. 11, no. No. 12 has significant flange corrosion. In particular, comparative gasket No. 1 having a chloride ion concentration exceeding the general reference value (100 ppm) described at the beginning. As for No. 12, the degree of flange corrosion was so great that it was confirmed to be clearly unsuitable as a gasket for a nuclear power plant.

  Therefore, flange corrosion is mainly caused by the chloride ion concentration of the gasket, and it is understood that limiting the chloride ion concentration to 10 ppm or less makes it extremely effective as a gasket for a nuclear power plant. . In order to be suitable as a gasket for a nuclear power plant, it is necessary to reduce the fluorine ion concentration (generally less than 5 ppm is preferable). 1-No. Regarding No. 8, the fluorine ion concentration is 1 ppm or less, and it is understood that the nuclear power plant gasket of the present invention is greatly reduced in fluorine ion concentration (1 ppm or less).

Claims (4)

A sheet gasket formed from a material containing expanded graphite, aramid fiber, a rubber binder, and a rubber chemical mainly composed of a vulcanization accelerator and a vulcanizing agent,
The contents of the rubber binder, rubber chemicals and vulcanization accelerator in the material are as follows: rubber binder: 10-18 mass%, rubber chemical: 3-5 mass%, and vulcanization accelerator: 0.8-2.0 mass%. A nuclear power plant gasket characterized in that zinc oxide containing no chloride ions is used as the vulcanization accelerator and the chloride ion concentration is limited to 10 ppm or less. The nuclear plant gasket according to claim 1, wherein the total content of expanded graphite, aramid fiber, and rubber binder in the material is 40 to 50 mass% . Non-asbestos joint sheet in which the content of expanding the black lead in said material characterized in that it is a 15 to 35 mass%, according to claim 1 or claim 2. The nuclear plant gasket according to any one of claims 1 to 3, wherein the content of the aramid fiber in the material is 4 to 15 mass%.