JPH0329644Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Purpose of the Invention] (Field of Industrial Application) The present invention relates to a main steam stop valve that is incorporated into a main steam pipe and used as a main element of a safety device for a steam turbine.

(Prior Art) As is well known, in large-capacity steam turbines, the main steam introduced into the steam chamber of the nozzle box is first A so-called all-round injection operation is used, in which the injection is directed towards the stage vanes.

When this operation method is adopted, the main steam stop valve incorporated in the main steam pipe has a main valve that shuts off the main steam flowing to the steam turbine, and a small valve that forms a flow bypassing the main valve inside the main valve. . When the steam turbine is driven, by opening this small valve, a portion of the main steam flows downstream of the main valve, warming up each device in the system.

By the way, a boiler that supplies main steam to a steam turbine uses a large number of heat transfer tubes as components such as an evaporator and a superheater, and when feed water passes through these heat transfer tubes, it is heated and becomes saturated steam and even superheated steam. However, during this process, oxidized scale adhering to the inner surface of the metal heat exchanger tube gets mixed into the steam, and these oxidized scales are carried along with the steam to the main steam stop valve. This oxide scale contains hard iron oxides such as Fe ₃ O ₄ , and if this iron oxide adheres to the small valves or valve stems in the main steam stop valve and has been operated for a long period of time, it will cause damage to these parts. will be eroded.

To deal with this problem, the method shown in FIG. 4 has been commonly used in the past.

That is, in the figure, numeral 1 is the main valve body, 2
indicate small valves, and when the valve stem moves upward, the small valve 2 moves, and during this time, the distance between the small valve 2 and the main valve body 1 increases, and the distance from the steam inlet 4 to the steam chamber increases. 5 and from the steam chamber 5 to the steam outlet 6 is formed.

Here, the method of preventing erosion is to weld a cobalt-based superalloy (trade name: Stellite) to the part that descends from the throat part of the small valve 2, that is, the skirt part 7, and increase the height of that part. (The black areas in the figure indicate the overlay areas).

(Problems to be solved by the invention) As mentioned above, in the conventional technology, the general method is to increase the hardness of the skirt portion 7 of the small valve 2. Mitigation Measures” (Thermal and Nuclear Power Generation Vo1.36
No. 351 (December 1985 issue), the cobalt-based superalloy has almost the same corrosion resistance as the material used for small valve 2 (1Cr1Mo0.25V alloy steel), so it is When used for a short period of time, erosion occurs in the build-up area, and sometimes the erosion progresses and there is a danger that even the valve stem 3 will be eroded. In the unlikely event that the valve stem 3 is severely eroded, the main steam stop valve, which is expected to stop the steam turbine in an emergency and act as a safety device, will not be able to function, and the operation of the steam turbine will be disrupted. .

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a main steam stop valve that can prevent corrosion accidents caused by oxidized scale on the valve stem.

[Structure of the invention] (Means for solving the problem) The main steam stop valve according to the invention has a covered cylinder formed by overlapping cylindrical elements each having a hardened layer in a certain area of the valve stem that connects the small valves. It is characterized by being made to cover.

(Effect) For example, as stated in the above research paper,
The hardened layer produced on the surface of steel by boriding has a Vickers hardness of approximately Hv1500, and has extremely high corrosion resistance. Therefore, even if the oxide scale flying from the boiler collides at high speed, the surface will hardly be eroded. However, this hardened layer has a thickness of about 0.03 to 0.08 mm, and if a large amount of oxidized scale were to fly, the hardened layer might be attacked. When the hardened layer is eroded at this time, the base material begins to be rapidly eroded, and thereafter the thickness of the base material becomes significant. However, if there is another new hardened layer in contact with the base material, the oxide scale will collide with this hardened layer, and corrosion can be stopped there. The coated cylinder with a multilayer structure of the present invention is intended to reduce erosion of the valve stem by stress based on this principle.
Even if the outer cylindrical element is eroded and the next cylindrical element is eroded, a multi-layered covering cylinder with a hardened layer on each cylindrical element is installed in advance at the location where the valve stem is expected to be eroded. This prevents erosion of the valve stem itself at the expense of the covered cylinder during the period between one periodic inspection and the next periodic inspection.

(Example) Hereinafter, an example of the present invention will be described with reference to FIGS. 1 to 3.

In FIG. 1, reference numeral 11 indicates a main valve body, and 12 indicates a small valve. As the valve stem 13 moves, the gap between the main valve body 11 and the small valve 12 widens, and the steam Small valve 12 for a certain period of time when starting the turbine
is configured to work. This invention is valve stem 13
A covering cylinder 14 having a multilayer structure is provided over a certain area continuous with the small valve 12, that is, over a length corresponding to the skirt portion 7 of the conventional small valve 2. This covering cylinder 14 is manufactured by the method described below, and its lower part rests on the shoulder 15 of the valve stem 13, as shown in FIG. The male screw 16 and female screw 17 are screwed together and fixed to each other.

The above-mentioned covered cylinder 14 is made by the following procedure. That is, as shown in FIG. 3, each cylindrical element 14a, 14b, 14c, 14d is, for example
Each piece is made of 1CrMo0.25 low-alloy heat-resistant steel and finished with a different inner diameter so that they can be stacked with a slight gap between them.

Next, these cylindrical elements 14a, 14b, 14c,
14d is subjected to boriding treatment to form a hardened layer on its outer surface. This boriding treatment _is carried out by _{immersing FeB} or
It forms a layer of Fe ₂ B.

Further, the formation range of the hardened layer is the cylindrical elements 14a, 1
4b, 14c, and 14d are defined as a limited section L in the longitudinal direction. When welding the upper part of the covering cylinder 14, if boron is present in the built-up part 14W, welding defects are likely to occur. This is to avoid affecting the raised portion 14W.

The covered cylinder 14 configured as described above is attached to the valve stem 13.
If the outer cylindrical element 14a is mounted on the cylindrical element 14a, even if oxide scale collides with the outer cylindrical element 14a, the surface will be extremely hard with a Vickers hardness of about 1500 Hv due to the boriding treatment.
Because it has a layer of FeB or Fe ₂ B, it is hardly eroded under normal circumstances. Even if a large amount of oxidized scale were to fly and attack the hardened layer of the cylindrical element 14a, and furthermore, the base material would be completely lost, the next layer provided in contact with the outer cylindrical element 14a A hardened layer of the cylindrical element 14b appears, and since this second cylindrical element 14b serves as a wall to block the oxidized scale from flying in, the erosion quickly subsides. If the erosion can be stopped up to the second cylindrical element 14b from one periodic inspection to the next periodic inspection, the next cylindrical element 14c and 14d can be used as they are, and if the erosion has progressed to the cylindrical element 14c, New cylindrical elements 14a, 14b,
Together with the cylindrical element 14d remaining after preparing 14c,
Furthermore, it will be restored as a covered cylinder 14 in preparation for operation until the next year's regular inspection.

[Effects of the invention] As explained above, in the present invention, a certain area of the valve stem connected to the small valve is covered with a covering cylinder composed of overlapping cylindrical elements each having a hardened layer. Even if the outer cylindrical element is eroded due to oxide scale, the second cylindrical element
In addition, in some cases, the third cylindrical element double or triple protects the valve stem, which reduces the risk of valve stem breakage accidents, increasing the reliability of the main steam stop valve that acts as a safety device. It has the excellent effect of significantly increasing the

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing the main parts of the main steam stop valve according to the present invention, Fig. 2 is an enlarged sectional view showing the mounting part of the covering cylinder of the invention, and Fig. 3 shows details of the covering cylinder. The configuration diagram and FIG. 4 are cross-sectional views showing the main parts of a conventional main steam stop valve. 11...Main valve body, 12...Small valve, 13...Valve stem, 1
4...Covered cylinder, 14a, 14b, 14c, 14d
...Cylindrical element, 15...Shoulder, 16...Male thread, 17...
Female thread.

Claims (1)

[Claims for Utility Model Registration] (1) In a main steam stop valve having a small valve driven by a valve stem built in a main valve body, each of a certain region of the valve stem connected to the small valve is 1. A main steam stop valve, characterized in that the main steam stop valve is covered with a covering cylinder formed by overlapping cylindrical elements having hardened layers. (2) The main steam stop valve according to claim 1, wherein the cylindrical element has a hardened surface layer produced by boriding treatment.