JP2018179244A - Safety valve, nozzle and disc used for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a safety valve that has no variation in set pressure at which a safety valve operates even when being exposed to a high-temperature steam atmosphere for a long term, and to provide a valve seat and a valve element used for the same.SOLUTION: There is provided a nozzle 2 or disc 3 in a safety valve 10. An iron oxide with 20 nm or more is preliminarily formed on a stainless steel with a mirror surface in at least seal surface 8 in the nozzle 2 or disc 3.SELECTED DRAWING: Figure 1

Description

The present invention relates to a safety valve and a nozzle and a disk used therefor, and more particularly to a safety valve provided in a container and piping used for high temperature oxidizing fluid such as steam, and a nozzle and a disk used therefor.

The safety valve is installed at a location where pressure is applied, such as tanks for storing gas, power generation boiler equipment, piping of a chemical plant, etc., and plays a role of preventing explosion and damage of equipment.
The safety valve is one that automatically opens the valve body and discharges the fluid when the pressure on the inlet side of the valve reaches or exceeds a preset pressure. A structural diagram of a representative safety valve is shown in FIG.

Usually, the safety valve is disposed at the top of the pressure vessel, piping, etc. The valve seat for closing the fluid is composed of a metal nozzle and a disc, and the sealing surface is polished to a mirror state by lapping.
The fluid closing force acting on the valve seat is provided by the spring located at the top. The spring force is adjusted so that the safety valve operates in response to the designated pressure (set pressure). When the fluid pressure of the pressure vessel, piping, etc. rises to the set pressure during plant operation, the upward force acting on the disc by the fluid pressure overcomes the spring force, and the safety valve operates. At this time, since the disk moves upward, fluid flows between the nozzle and the disk and flows on the secondary side (lateral direction). It is the original function of the safety valve to prevent the internal pressure of the pressure vessel and piping from rising by this.

The use of this safety valve in steam service is described below.
Nozzles and disks, which are members directly in contact with the fluid in high temperature oxidizing services such as water vapor, which are free of particularly corrosive components (such as halogen) in the fluid, are generally 18Cr. -8 Ni austenitic stainless steel is often used.
It is well known that, in piping members used for high temperature oxidizing fluids such as steam, oxidation scale is generated inside the piping. In the case of stainless steel, which is a main component of piping members, it has been reported in several papers that the oxide film formed is an iron-based oxide mainly composed of Fe ₂ 0 ₃ and Fe ₃ 0 ₄ (Non-patent documents 1 to non-patent document 4)

These papers also report that the state of oxidation varies depending on fluid temperature, fluid composition (air oxidation and water vapor oxidation), chemical composition of constituent materials, surface finish (grain size) and the like.
However, its research scope is to address various problems caused by the peeling of the oxidation scale from the boiler superheater tube of the thermal power plant, and solves the problems occurring inside the safety valve as in the present invention described below. It is not something to try.

Patent No. 2976301

Journal of the Japan Institute of Metals and Materials (1972) Volume 36 Effect of grain size on water vapor oxidation of austenitic stainless steel High Temperature Steam Oxidation Resistance and Hydrogen Penetration of Japanese Iron and Steel Institute of Japan Vol. 71, No. 1 (2007) 68-75 Fe-Cr Alloy High Temperature Properties of Stainless Steel Masao Kikuchi SanyoTechnical Report Vol. 21 (2014) No. 111-27 Study on high temperature oxidation behavior of ferritic stainless steel in water vapor containing atmosphere: Deterioration mechanism of Cr203 film Kei Yamauchi HUSCAP / HokkaidoUnversity COIIection of Scholarly and Academic Papers 2003, 3.25 Influence of oxidation treatment conditions on the thermal oxide film structure of SUS316L and corrosion resistance in chlorine gas Haruo Yuri, et al. Zairyo-to-Kankyo, 44, 481-486 (1995)

By the way, in order to improve the sealability, it is required to improve the sealability of the safety valve, and the seal surface which is the contact portion between the nozzle and the disc is mirror finished. However, it was found that there is a possibility that the pressure at which the safety valve operates changes when mirror finishing.
An object of the present invention is to provide a safety valve and a nozzle and a disc used therefor, which have no fluctuation in the set pressure at which the safety valve operates even when exposed to a high temperature steam atmosphere for a long time.

The invention according to claim 1 is a nozzle or a disc in a safety valve, and at least a sealing surface is a nozzle or a disc characterized in that an iron oxide of 20 nm or more is previously formed on stainless steel having a mirror surface. .

The invention according to claim 2 is a nozzle or a disc in a safety valve, and at least a sealing surface is characterized in that an oxide containing chromium oxide as a main component is formed in advance on stainless steel having a mirror surface. Or a disc.
The invention according to claim 3 is a control valve, a pressure reducing valve, and other devices used under an oxidizing atmosphere such as steam or the like, and a valve seat portion, a guide portion, and other portions having a reduced function due to oxide film growth have mirror surfaces. An apparatus characterized in that an iron oxide of 20 nm or more is formed in advance on stainless steel.
The invention according to claim 4 relates to a control valve, a pressure reducing valve and other devices used under an oxidizing atmosphere such as steam or the like, and a valve seat portion, a guide portion and other portions having a reduced function due to oxide film growth have mirror surfaces. It is an apparatus characterized in that an oxide containing chromium oxide as a main component is formed in advance on stainless steel.
The invention according to claim 5 is the nozzle or disc according to claim 1 or 2, or the apparatus according to claim 3 or 4, wherein the stainless steel is austenitic stainless steel or ferritic stainless steel having a Cr content of less than 20% by weight. It is.
The invention according to claim 6 is a nozzle or disc characterized in that at least the seal surface of the nozzle or disc in the safety valve is made of a material that does not allow iron oxide to grow in an oxidizing atmosphere.
The invention according to claim 7 relates to a control valve, a pressure reducing valve and other devices used under an oxidizing atmosphere such as steam or the like, and a valve seat portion, a guide portion and other portions where the function is deteriorated by oxide film growth are iron oxides. It is an apparatus characterized by being made of a material that does not grow
In the invention according to claim 8, the material which does not grow iron oxide in the oxidizing atmosphere is austenitic stainless steel, nickel based alloy, cobalt based alloy, titanium based alloy or aluminum based alloy having a Cr content of 20% by weight or more. The nozzle or disc according to claim 6, which is another non-ferrous heat-resistant alloy material, or the device according to claim 7.
The invention according to claim 9 is the nozzle or the disk or the device according to claim 8, wherein the nickel base alloy is a precipitation hardening alloy or a solid solution strengthening alloy.
The invention according to claim 10 is the nozzle or the disk or the device according to claim 9, wherein the precipitation hardening alloy is Stellite (registered trademark) and the solid solution hardening alloy is Hastelloy (registered trademark).
The invention according to claim 11 is the nozzle or the disk or the device according to claim 8, wherein the cobalt-based alloy is stellite.
The invention according to claim 12 is a nozzle or a disc in a safety valve, and at least a sealing surface is a coating made of Cr, Co, Ni, Al, Ti or an alloy or an oxide or nitride thereof on stainless steel having a mirror surface in advance. It is a nozzle or disc characterized by being formed.
The invention according to claim 13 is a safety valve using any one or both of the nozzle according to any one of claims 1 to 12 or a disc.
The invention according to claim 14 is the safety valve according to claim 13, wherein the safety valve is superheated steam or other high temperature oxidizing fluid.

The actions and effects of the present invention will be described together with the findings obtained when making the present invention.
The seal surface (valve seat) made of austenitic stainless steel before being exposed to the water vapor environment is polished to a mirror-like state by lapping, and is in a state where there is only an extremely thin natural oxide film on the surface. By exposing this to a high temperature oxidizing atmosphere for a predetermined time, oxygen intrudes into the inside and directly reacts with the alloy component, and the oxide film grows thick.

The oxide film is shared while increasing the contact portion between the oxide film grown on the nozzle side and the oxide film grown on the disk side. By acting like an adhesive, the common part of the oxide film is added to the spring force acting on the valve seat in the form of a fixing force. Then, the setting pressure is increased by the amount of the fixing force.
When this is operated again, the set pressure returns to the original value. This is because once the operation is performed, the sticking state is released, and the setting pressure becomes zero because the sticking force which prevents the operation becomes 0.

As described above, the safety valve needs to have a function to release the fluid to the secondary side at a preset pressure, but if a sticking phenomenon occurs, a security problem such as an increase in the set pressure will occur.
In an actual operation, an operation test is performed with an actual fluid at the time of start-up, or a minute foreign matter bites, and the contact area of the valve seat decreases. Vibration during transportation or operation of the plant causes the valve seat to tilt, and the low regrind level during maintenance causes the valve seat seal surface to be rough and there are many factors that do not stick, which may obscure the problem. There are many.
However, a safety valve that creates an ideal smooth sealing surface and the valve seat is in intimate contact will maximize this sticking problem.

It is a conceptual diagram which shows the structure of a safety valve. It is a graph which shows the relationship between the test temperature and adhesion surface pressure which concern on a reference example. It is a graph which shows the component distribution of the depth direction immediately after grinding | polishing which concerns on the reference example shown in FIG. 2, and after a water vapor exposure test. It is a graph which shows the relationship between the test temperature in Example 1, and adhering power with the case of a prior art example. It is a graph which shows the component distribution of the depth direction immediately after grinding | polishing in the case of the stellite in Example 1, and after a water vapor exposure test. It is a graph which shows the component distribution of the depth direction immediately after grinding | polishing in the case of SUS310S in Example 1, and a water vapor exposure test. It is a graph which shows the relationship between the test temperature which summarized the prior art example and Example 1-6, and adhering power. It is a graph which concerns on Example 7 and shows the relationship between the test temperature at the time of applying this invention to only one of a nozzle and a jig, and adhering power. It is a graph which concerns on Example 8 and shows the relationship between the test temperature at the time of changing the thickness of the iron oxide formed beforehand, and adhering power. It is a graph which concerns on Example 8 and which shows the component distribution of the depth direction after the steam exposure test in each thickness of an oxide.

(First form)
In the aspect of the invention according to claim 1, the safety valve is a nozzle or a disc in the safety valve, and at least the sealing surface is previously formed with an iron oxide of 20 nm or more on stainless steel having a mirror surface.
Here, as stainless steel, stainless steel having a Cr content of less than 20% by weight is preferable. In the case where Cr is 20% or more, as described later, even if iron oxide is not formed in advance, it has the effect of reducing the adhesion.

As stainless steel in the present invention, the following stainless steels can be used.
Austenitic
SUS201; Ni (3.5-5.5%), Cr (16-18%), Mn (5.5-
7%), N (0.25% or less)

SUS202 :: Ni (4-6%), Cr (17-19%), Mn (7.5-10%), N (0.25% or less)
SUS301: Ni (6-8%), Cr (16-18%)
SUS302: Ni (8-10%), Cr (17-19%)
SUS303: Ni (8-10%), Cr (17-19%), Mo (0.60% or less can be added)
SUS304: Ni (8-10.5)
%), Cr (18-20%)
SUS305: Ni (10.5-13%), Cr (17-19%)
SUS316: Ni (10-14%), Cr (16-18%), Mo (2-3%)

SUS 317: Ni (11-15%), Cr (18-20%), Mo (3-4%)

Martensite
SUS403: Cr (11.5-13%)

SUS420: Cr (12-14%)
SUS630: Ni (3-5%), Cr (15-17.5%), Cu (3-5%), Nb (0.15-0.45%)
Ferrite
SUS405: Cr (11.5-14.5%), Al (0.1-0.3%)
SUS430: Cr (16-18%)
SUS430LX: Cr (16-19%), Ti or Nb (0.1-1.0%)

  In the present invention, the thickness of iron oxide is 20 nm or more. By setting the thickness to 20 nm or more, the effect of reducing the sticking strength is generated. The formation of the iron oxide is preferably performed by heating the stainless steel in an air atmosphere at a temperature above the working fluid temperature of the safety valve for a predetermined time. The heating temperature is preferably 200 ° C to 500 ° C. 250 ° C. to 450 ° C. are more preferred. As shown in FIG. 2, the iron oxide adhering surface pressure rapidly rises at 250 ° C. or more, and this rise is considered to be based on the formation of iron oxide as described above, so heating at 250 ° C. or more is preferable. The heating time may be appropriately selected in consideration of the thickness. For example, 5 to 6 hours are preferable.

(Second form)
The invention according to claim 5 uses a material which does not grow iron oxide in the oxidizing atmosphere. As such a material, austenitic stainless steel having a Cr content of 20% by weight or more is used.
It was confirmed that no iron oxide was formed even when the austenitic stainless steel containing 20 wt% or more of Cr was exposed to high temperature steam. That is, when this material was exposed to water vapor at 250 ° C. or higher, it was confirmed that Cr-rich and not Fe-rich oxides were formed, and the growth of Cr-rich oxides did not cause an increase in adhesion. .

As 20 wt% or more of the Cr-containing stainless steel, for example, the following materials are listed.
SUS309S: Ni (12.00 to 15.00), Cr (22.00 to 24.00)
SUS310S: Ni (19.00 to 22.00), Cr (24.00 to 26.00)

SUS317J2: Ni (12.00 to 16.00), Cr (23.00 to 26.00)
SUS329J1: Ni (3-6), Cr (23-28), Mo (1-3)

(Third form)
This embodiment is a mode in which chromium oxide instead of iron oxide is formed at least on the seal surface in austenitic stainless steel having a Cr content of less than 20% by weight.
The formation of the oxide containing chromium oxide as a main component may be performed, for example, by the method described in Patent Document 1 or Non-Patent Document 5.

For example, after electropolishing stainless steel, baking is performed to remove water from the surface of the base material, and then heating is performed at a temperature of 400 ° C. or more in an oxidizing gas atmosphere having an impurity content of 1 ppb or less. An oxide containing chromium oxide as a main component can be formed on the surface of the base material.
In addition, a Cr film is formed on the stainless steel surface by sputtering, thermal spraying, and other methods, and then, after baking, it is heated at a temperature of 400 ° C. or more in an oxidizing gas atmosphere, for example, and chromium oxide is mainly contained as a film. An oxide may be formed.
The thickness of the oxide containing chromium oxide as a main component is not particularly limited, but is preferably 5 nm or more, more preferably 10 nm or more, from the viewpoint of the sealing property and the effect of suppressing the generation of adhesion.

(Fourth form)
A non-ferrous heat-resistant alloy is used as a material that does not form iron oxide. For example, nickel base alloys, cobalt base alloys, titanium alloys or aluminum alloys are mentioned.
As a nickel base alloy, a precipitation hardening alloy (eg, Inconel 718) which is hardened by precipitation of a compound such as Nb or Ta, a solid solution type alloy (eg, Hastelloy C) or the like which is solid solution strengthened by containing Mo is preferable. .
These alloys are not easily oxidized even in a high temperature corrosive environment, and are not oxides that cause an increase in adhesion even if oxides are formed.
The cobalt-based alloy is preferably, for example, stellite. This material is also resistant to oxidation and the oxide is not an oxide that leads to an increase in adhesion.

(Fifth embodiment)
The above-mentioned form is formation of an oxide, and is an example formed by reaction with a base material by heat treatment.
On the other hand, in the present embodiment, at least the seal surface is covered with a film other than the oxide. Moreover, you may form a film on the surface by methods other than the method by reaction with a base material.
For example, at least the sealing surface of stainless steel may be mirror-finished, and Cr, Co, Ni, Al, Ti or an alloy or an oxide or a nitride thereof may be formed by sputtering, spraying, plating or the like. Similar effects can be expected for films such as Al (Al ₂ O ₃ , AIN), Ti (TiO, TiN), and Ti—Al—N.

(Mirror surface)
The sealing surfaces of the valve body and the valve seat are mirror-finished to enhance the sealing performance. The surface roughness is preferably 10 nm or less in Rmax (Rz in JIS B 0601: 2001). The effect of preventing adhesion of the present invention is particularly effective when it is in the range of 3 nm to 10 nm.

(safety valve)
If the valve seat and the valve body of the present invention are used for only one of the nozzle and the disc, the effect of suppressing the sticking force is produced. Therefore, it is also possible to use conventional valve seats that are difficult to replace, and replace only the easily replaceable valve body with the valve body according to the present invention.
(Other equipment)
The invention also applies to equipment used under steam (superheated steam) or other oxidizing atmospheres. As a valve, for example, a control valve and a pressure reducing valve can be applied other than the safety valve.
Moreover, it is applied even if it is apparatuses other than a valve. The present invention is also applicable to an apparatus configured by combining members having portions in which members are kept in contact with each other in an oxidizing atmosphere and the contact surfaces are separated or moved as appropriate. Such members include, for example, a valve seat portion, a guide portion and the like.

(Reference example)
A safety valve consisting of austenitic stainless steel SUS304 valve seat and valve body finished in mirror surface was exposed to saturated water vapor. The exposure temperature was changed in the range of 140 ° C. to 300 ° C. The exposure time is 19 hours.
The result of having investigated water vapor temperature and adhering surface pressure (adhesion load per unit area) is shown in FIG. The test temperature on the horizontal axis in FIG. 2 is the temperature of the water vapor that is the fluid. That is, the temperature at which the valve body and the valve seat of the safety valve are exposed. It can be seen that sticking occurs above a certain temperature.

FIG. 3 shows the distribution in the depth direction immediately after polishing and after the water vapor exposure test. The sputtering time on the horizontal axis is the time when sputtering was performed from the surface at a sputtering speed of 6 nm / min. Therefore, the value obtained by multiplying the time on the horizontal axis by 6 becomes the depth (nm) (the same applies hereinafter). It can be seen that the oxide film, which was 6 nm immediately after polishing, has grown to 60 nm or more.
As shown in FIG. 3, the surface of the austenitic stainless steel valve seat after the water vapor exposure test is an oxide film mainly composed of iron, and adhesion is generated in the valve seat.
Example 1

The results of the water vapor exposure test with various materials different from this are shown in FIG.
In the drawings, 304 is SUS 304, 310 is SUS 310, IN 718 is Inconel 718 (registered trademark), HC 276 is Hastelloy C 276 (registered trademark), ST6 is Stelite 6 (registered), and ST12 is Stelite 12 (registered trademark). In these materials, no adhesion occurs under the conditions of adhesion with austenitic stainless steel.

The distribution in the depth direction immediately after polishing and after the steam exposure test of Stellite, which is a Co-Cr alloy among them, is shown in FIG. It can be seen that the oxide film, which was 3 nm immediately after polishing, has grown to 30 nm or more after the water vapor exposure test. Similar to austenitic stainless steel, oxide film growth is observed, but no sticking has occurred. That is, it should be noted that, in the case of the same oxide but Co oxide or chromium oxide, it is completely different from iron oxide. It can be said that it is effective as a sticking prevention measure not to form and grow an Fe-based oxide film on the outermost surface of the valve seat sealing surface from the result more than the sticking power.
(Example 2)

Cr 18%, which produces large adhesion in a saturated water vapor environment. Heat treatment was performed on Ni 8% austenitic stainless steel SUS 304 at 425 ° C. for 6 hours to grow an oxide film in advance. Oxide growth was performed for both the nozzle and the disk. A high-temperature steam exposure test at 285 ° C. was performed on a safety valve combining a nozzle and a disk on which an oxide film has been grown in advance. The results are shown in FIG.
In FIG. 7, the sample represented by “304_G / 304_G” is a sample according to the present embodiment.
In this example, the result that no sticking occurred was obtained.
(Example 3)

A metal material having an anti-sticking effect is formed as a protective film on the valve seat sealing surface by CVD, sputtering or the like. Useful metal components include Cr, Ti and Al. In this case, a 265 ° C. water vapor exposure test was performed when chromium nitride was formed on the austenitic stainless steel side surface on one side.
However, in the present example, the protective film was formed only on the nozzle. The disc is untreated.
The results are shown in FIG. “304_CrN / 304” in FIG. 7 is a sample according to this example. As compared with the case where the protective film is not formed, the result is obtained that the adhesion is suppressed to 1/3 or less.
Example 4-1

In the present embodiment, austenitic stainless steel of 20% by weight or more of Cr was used.
By using an austenitic stainless steel with 20% or more of Cr as it is, the composition of the oxide film in the state of water vapor exposure changes, and the growth of the outermost surface of the Fe-based oxide film can be suppressed. FIG. 6 shows the distribution in the depth direction of the austenitic stainless steel of Cr 20% or more immediately after polishing and after the water vapor exposure test. Compared to the depth direction distribution of the Crl 8% austenitic stainless steel of FIG. 3, it can be seen that the vicinity of the outermost surface is rich in Cr, and the growth of the oxide film is suppressed.
The evaluation test results of this example are shown in FIG. “310S / 310S” in FIG. 7 is a sample according to this example. In the high temperature steam exposure test at 250 ° C., the result of not sticking is produced.
(Example 4-2)

In this example, SUS310S was used as austenitic stainless steel of 20 wt% or more of Cr. In this example, a process of forming a chromium oxide film on the stainless steel surface was performed prior to the group address.
That is, after electrolytic polishing, baking was performed to remove water from the surface of the base material, and then heating was performed at a temperature of 400 ° C. or more in an oxidizing gas atmosphere having an impurity content of 1 ppb or less. Thereby, an oxide mainly composed of chromium oxide was formed on the surface of the base material. In addition, this oxide is a passivity film | membrane, and is represented by "G + F" in this specification and drawing.
A high temperature steam exposure test was conducted on this sample. The results are shown in FIG. In FIG. 7, “310S_G + F / 304” is a sample according to this example. This sample used the material which processed to the nozzle, and the disc used untreated SUS304 austenitic stainless steel.
(Example 5)

In this example, SUS304, which is less than 20% by weight of Cr, was used. In this example, a passive film was formed in the same manner as in Example 4-2.
That is, a chromium oxide layer not containing iron oxide was formed on the surface.
A high temperature steam exposure test was conducted on this sample. The results are shown in FIG. In FIG. 7, “304_G + F / 304_G + F” is a sample according to this example. In addition, this sample processed the nozzle and the disk.
In this example, even in the exposure test at 280 ° C. to 290 ° C., almost no sticking force was generated.
Even in a normal Cr18% austenitic stainless steel, a Cr203 film can be deposited on the outermost surface by heat treatment. In Al-containing stainless steel, it is also effective to precipitate Al203.
(Example 6)

An example using an oxidation resistant material with small Fe content such as Ni alloy Ni-Mo alloy, Co-Cr alloy, etc. is shown in FIG. Even in this case, the result is that adhesion does not occur. Even with these materials, oxide films are grown, but adhesion between the films is not generated. (It differs from the case of Fe oxide film.)
The adhesion evaluation results in the water vapor exposure test of the above example are summarized in FIG.
In a Crl 8% austenitic stainless steel abrasive product, sticking occurs in a saturated water vapor environment of 200 ° C. or higher.
Even in the case of high-grade oxidation resistant materials and austenitic stainless steels in which iron oxide is formed in advance, the temperature range of valve seat fixation can be expanded.
(Example 7)

The eight samples on the left side of FIG. 8 are the results of the water vapor exposure test in the case where the antisticking treatment of the present invention is applied to only one side of the nozzle, which is a safety valve component, and a jig.
As shown in the left eight results in FIG. 8, even if it is a countermeasure on only one side, a certain effect is obtained.
In this case, when it is intended to prevent sticking of a valve seat to a safety valve used for a certain period of time, it becomes possible to treat by re-using a nozzle which is difficult to replace parts and replacing only the disc side. The result is very useful for maintenance.

Reference Signs List 1 inlet 2 nozzle 3 disc 4 outlet 5 hody 6 spring 7 valve rod 8 sealing surface (seat)
10 Safety valve

Claims (14)

A nozzle or disc in a safety valve, wherein at least a sealing surface is pre-formed with an iron oxide of 20 nm or more on stainless steel having a mirror surface. A nozzle or disc in a safety valve, wherein at least a seal surface is a chromium oxide-based oxide preformed on stainless steel having a mirror surface. Control valves, pressure reducing valves, and other devices used under steam or other oxidizing atmospheres. Valve seat parts that are degraded by oxide film growth, guide parts, and other parts are stainless steel with a mirror surface of 20 nm or more iron A device characterized in that the oxide is pre-formed. Control valves, pressure reducing valves, and other devices used under steam or other oxidizing atmospheres. Valve seat portions that are degraded by oxide film growth, guide portions, and other portions are chrome oxide on stainless steel having a mirror surface. An apparatus characterized in that an oxide which is a main component is formed in advance. The nozzle or disc according to claim 1 or 2, or the device according to claim 3 or 4, wherein the stainless steel is an austenitic stainless steel or a ferritic stainless steel having a Cr content of less than 20% by weight. A nozzle or disc characterized in that at least the sealing surface of the nozzle or disc in the safety valve is made of a material that does not grow iron oxide in an oxidizing atmosphere. Control valves, pressure reducing valves, and other devices used under steam or other oxidizing atmospheres, valve seat parts that are degraded by oxide film growth, guide parts, and other parts are made of materials that do not allow iron oxide to grow Equipment to feature. The material which does not grow iron oxide in the oxidizing atmosphere is austenitic stainless steel, nickel base alloy, cobalt base alloy, titanium base alloy or aluminum base alloy or other non-ferrous heat resistant alloy material having a Cr content of 20% by weight or more. A nozzle or disc according to claim 6 or an apparatus according to claim 7. 9. The nozzle or disc or device according to claim 8, wherein the nickel base alloy is a precipitation hardening alloy or a solid solution strengthening alloy. 10. The nozzle or disc or device according to claim 9, wherein the precipitation hardening alloy is Stellite (registered trademark) and the solid solution hardening alloy is Hastelloy (registered trademark). The nozzle or disc or device according to claim 8, wherein the cobalt-based alloy is stellite. A nozzle or a disc in a safety valve, wherein at least the sealing surface is characterized in that a film consisting of Cr, Co, Ni, Al, Ti or an alloy or an oxide or a nitride thereof is previously formed on stainless steel having a mirror surface. Nozzle or disc. A safety valve using any one or both of the nozzle according to any one of claims 1 to 12 or a disc. The safety valve according to claim 13, wherein the safety valve is superheated steam or other high temperature oxidizing fluid.

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