JPH08121991A - High anti-corrosion type corrugated pipe - Google Patents

Abstract

PURPOSE: To provide a pipe having a high anti-corrosion against sea water and having a superior heat transfer characteristic. CONSTITUTION: High Cr ferrite stainless steel containing Cr of 25% or more and C+N limited to a value of 150ppm or less is applied as a raw material for pipe. The pipe is worked to have a corrugation. High anti-corrosion can be attained by high Cr, a cold machining work is improved under a limitation of C+N, resulting in that a corrugate machining can be attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stainless steel highly corrosive-resistant corrugated pipe used as a heat transfer pipe of a heat exchanger, and more particularly to a pipe and / or pipe such as a condenser of a power plant. The present invention relates to a highly corrosion-resistant corrugated pipe suitable for a heat transfer pipe of a seawater heat exchanger through which seawater flows. In this specification,% means% by weight unless otherwise specified.

[0002]

2. Description of the Related Art In a power plant, steam sent from a boiler to a turbine is cooled by a condenser and returned to the boiler. A condenser, which is a type of heat exchanger, has a large number of heat transfer tubes that are arranged at predetermined intervals.By passing seawater inside each heat transfer tube and letting steam pass outside, steam is converted into seawater. To cool.

As a heat transfer tube for a condenser, a copper alloy tube made of aluminum brass or the like is widely used because of its excellent thermal conductivity. However, it cannot be said that the copper alloy tube has sufficient corrosion resistance to seawater. For this reason, conversion to titanium pipes and stainless steel pipes, which have excellent corrosion resistance to seawater, is being pursued, and among them, stainless steel pipes with relatively low material costs are considered promising.

[0004] As a stainless steel used for a heat transfer tube of a heat exchanger in which seawater flows in a condenser such as a condenser, Cr is used.
High Cr ferritic stainless steel containing 25% or more of is suitable. This stainless steel is called super stainless steel, and 29% Cr-4% Mo-2% Ni is a typical component system.

Super stainless steel represented by 29% Cr-4% Mo-2% Ni can exhibit high corrosion resistance even in a seawater environment by containing 25% or more of Cr. However, the heat transfer coefficient is inferior to that of a copper alloy such as aluminum brass. The thermal conductivity of each is shown in Table 1. The thermal conductivity of super stainless steel is only about 1/6 that of aluminum brass. On the other hand, the strength of super stainless steel is considerably higher than that of copper alloy. Therefore, it is attempted to compensate for the poor heat transfer coefficient by reducing the wall thickness of the tube.

[0006]

[Table 1]

For example, as a heat transfer tube made of super-stainless steel corresponding to a heat transfer tube made of aluminum brass having a wall thickness of 1.0 mm, a heat transfer tube having a wall thickness of 0.5 mm, which has almost the same strength, is used. There is.

[0008]

The thin-walled heat transfer tube made of such super-stainless steel is superior in corrosion resistance to seawater as compared with a copper alloy-based thick-walled heat transfer tube having the same strength. However, the heat transfer rate is still poor. The reason is as follows.

Since the thermal conductivity of super stainless steel is about 1/6 of the thermal conductivity of copper alloys, the difference in thermal conductivity should be sufficiently compensated for by reducing the thickness to about 1/2 as described above. I can't. When thinning more than 1/2,
Even super stainless steel lacks strength.

When steam is passed through the heat transfer tube and seawater is passed through the heat transfer tube, heat is generally transferred from the steam side to the seawater side as shown in FIG. As can be seen from the figure, heat transfer is greatly hindered on the seawater side. In a heat transfer tube made of stainless steel, marine organisms are more likely to adhere to the inner surface of the tube than in a heat transfer tube made of a copper alloy, and the heat transfer coefficient on the seawater side is thereby greatly reduced.

In order to increase the heat transfer coefficient of the heat transfer tube, corrugating is effective, for example, in Japanese Patent Laid-Open No. 62-114.
It is well known from Japanese Laid-Open Patent Application No. 731. The corrugating process is a grooving process for spirally squeezing the pipe from the outer surface side with a roller, and the spiral grooved pipe subjected to the process is called a corrugated pipe (FIG. 2). As described in Japanese Utility Model Application Laid-Open No. 57-127327, this corrugating process is usually applied to thin-walled pipes of iron, copper, aluminum, etc., which have good workability. It may also be applied to stainless steel pipes, as described in JP-A-51-14867. However, the super stainless steel having the Cr content increased to 25% or more has extremely low ductility and low toughness as compared with ordinary stainless steel. Therefore, cracking occurs when corrugating is performed, and the cracking becomes more remarkable in thin-walled pipes. Therefore, in the case of a thin wall tube made of super stainless steel,
In reality, it is impossible to improve the heat transfer coefficient by corrugating.

For these reasons, the heat transfer tube made of superstainless steel is superior in corrosion resistance to seawater, but still inferior in heat transfer coefficient, as compared with the copper alloy heat transfer tube.

An object of the present invention is to provide a highly corrosion-resistant corrugated pipe having excellent corrosion resistance against seawater and excellent heat transfer coefficient.

[0014]

The present inventor has been researching stainless steel cleaning for a long time. In the process, this time, the amount of (C + N) in super stainless steel was 150 pp.
It has been found that when the thickness is reduced to m or less, corrugation of the thin-walled pipe becomes possible, and thereby the low heat transfer coefficient which is a problem in the thin-walled pipe of super stainless steel is solved.

The present invention has been made on the basis of the above findings, and contains 25% by weight or more of Cr and 15% of (C + N).
A gist of a high corrosion-resistant corrugated pipe characterized in that a spiral groove is formed by corrugating in a pipe made of high Cr ferritic stainless steel limited to 0 ppm or less.

In the highly corrosion-resistant corrugated pipe according to the second aspect, the pipe inner diameter of the mountain portion which is not subjected to corrugation is Di.
In this case, the groove depth H is 0.01 × Di or more and the groove pitch P is 0.6 × Di or less.

[0017]

The highly corrosive corrugated pipe of the present invention is used, for example, as a heat transfer pipe of a seawater heat exchanger such as a condenser in which seawater flows, and shows excellent corrosion resistance. A turbulent flow is generated in the pipe, which suppresses the adhesion of marine organisms to the inner surface of the pipe and reduces the water film resistance on the inner surface of the pipe, resulting in a high heat transfer coefficient.

The material of the corrugated pipe of the present invention is Cr 2
High Cr ferritic stainless steel (super stainless steel) containing 5% or more and limiting (C + N) to 150 ppm or less. Other components are not particularly limited, but usually M
o: 1 to 8%, Ni: 3% or less, Si, M
There is no problem even if n, P, S and other alloy elements are contained in the same strength as general ferritic stainless steel.

If Cr is less than 25%, the corrosion resistance is insufficient. The upper limit is preferably 35% or less in order to suppress embrittlement. Mo is added for the purpose of improving pitting corrosion resistance,
If it is less than 1%, there is no effect, and if it exceeds 8%, the steel becomes brittle and the cost increases, which is not practical. Ni is added for the purpose of improving workability, but if it exceeds 3%, grain boundary precipitation of intermetallic compounds is caused and corrosion resistance deteriorates.

The workability in corrugating, that is, the cold workability is greatly affected by the toughness and ductility of the material. C and N accelerate the precipitation of carbonitrides, reduce the toughness and ductility of the material, and deteriorate the cold workability. Therefore, C +
N is limited to 150 ppm or less, and 120 ppm or less is particularly desirable. If C + N exceeds 150 ppm, cracking will occur during corrugation processing, making the processing difficult. Regarding the lower limit of the (C + N) amount, the lower the (C + N) amount, the better the workability. Therefore, the lower limit is not particularly specified, but from the viewpoint of the cost of refined steel, 30 ppm or more is preferable.

In high Cr steel such as super stainless steel, it is generally difficult to remove impurities such as C and N. For example, powder of iron ore or the like is produced in the steel refining process by VOD (vacuum refining). C + by blowing up
It is possible to limit N to 150 ppm or less.

The flat tube, which is the base tube of the corrugated tube, is
Usually, it is manufactured by an electric resistance welded pipe made of a stainless steel strip having the above composition. Then, this flat tube is subjected to a well-known corrugating process to obtain the corrugated tube of the present invention.

The wall thickness of the flat tube is preferably 0.7 mm or less in order to reduce the material cost and improve the heat transfer coefficient. The lower limit of the wall thickness is preferably 0.3 mm or more in order to secure the mechanical strength.

In the corrugated pipe of the present invention, the groove depth H and the pitch P have a great influence on the heat transfer coefficient. According to the investigation by the present inventor, when H ≧ 0.01 Di P ≦ 0.6 Di Di: the pipe inner diameter of the unprocessed mountain portion (the inner diameter of the flat pipe), the heat is equal to or higher than that of the copper alloy pipe of the same strength It was found that the transfer rate was obtained. Therefore, the groove depth H is preferably 0.01 Di or more, and the groove pitch P is preferably 0.6 Di or less.
Regarding the upper limit of the groove depth H, when the groove depth is increased, the pressure loss inside the tube is rather increased, wasteful energy is required, and the amount of vapor condensation outside the tube is increased.
Due to the surface tension of the condensate, the residence time remaining in the groove becomes longer and the heat transfer coefficient on the outer surface also deteriorates.
It is desirable that the lower limit of the groove pitch P be 0.3 Di or more, since the heat transfer coefficient becomes saturated and the pressure loss increases as the pitch becomes smaller.

The high corrosion resistant corrugated pipe of the present invention is particularly suitable for a heat transfer pipe of a heat exchanger in which seawater flows inside or outside the pipe such as a condenser of a power plant, or both, and is applied to the heat transfer pipe. It is possible to obtain the corrosion resistance and the heat transfer coefficient which are superior to those of the conventional copper alloy tube.

[0026]

EXAMPLES Examples of the present invention will be shown below, and the effects of the present invention will be clarified by comparison with Comparative Examples.

Ten kinds of high Cr ferritic stainless steels A to J whose chemical compositions are shown in Table 2 were melted by an electric furnace to a VOD powder top blowing method. The VOD powder top-blowing method promotes decarburization reaction and denitrification reaction by spraying powder of iron ore etc. into molten steel by VOD, as described above, to reduce (C + N)
Is a method of achieving

The molten steel ingots of each steel were agglomerated and hot rolled to cold rolled into a thin plate having a thickness of 0.5 mm. Furthermore, using each thin plate as a material, an outer diameter of 25.4 by a continuous welding pipe production line
mm, an inner diameter of 24.4 mm (Di), and a wall thickness of 0.5 mm, a thin electric resistance welded steel pipe (flat pipe) was manufactured. Then, each flat tube was corrugated with a groove depth H of 1.44 mm (= 0.06 Di) and a pitch P of 14.4 mm (= 0.6 Di).

A Charpy test was carried out on a test piece taken from a hot-rolled sheet, and a test piece taken from a flat tube was immersed in seawater for 6 months to investigate whether crevice corrosion occurred or not. Further, Table 3 shows the results of an examination as to whether or not cracks would occur during corrugation.

Since all of the steels A to J are high Cr ferritic stainless steels containing 25% or more of Cr (super stainless steel), crevice corrosion did not occur in the corrosion resistance test. Among them, Steels A to E had a C + N of 150 ppm or less, and thus had good toughness, and cracks and flaws did not occur even during corrugation. However, steel F with C + N exceeding 150ppm
Samples J to J had low toughness and cracked due to corrugation, making it impossible to manufacture corrugated pipes.

By the way, the outer diameter having the same strength is 25.4 m.
An aluminum brass tube having a thickness of 1.0 mm and a wall thickness of 1.0 mm caused crevice corrosion in the corrosion resistance test.

[0032]

[Table 2] * Outside the scope of the present invention

[0033]

[Table 3] * ○ No corrosion × Corrosion ** ○ ○ No cracks × Cracks

Next, the flat tube of the steel A was corrugated under various conditions, and the heat transfer coefficient of the obtained corrugated tube was measured. In the measurement of the heat transfer coefficient, the apparatus of FIG. 3 was used, seawater was passed through the pipe, steam was passed outside the pipe, and the heat transmission coefficient was determined by the following formula. The flow rate of seawater was 1.0 m / s, and the steam temperature was 35 ° C. (vacuum). K = Q / S · (T _o −T _i ) / (T _s −T) K: Heat transmission coefficient (W / m ² K) Q: Cooling water amount (m ³ / s) S: Pipe surface area (m ² ) T _O : Seawater outlet temperature (K) T _i : Seawater inlet temperature (K) T _s : Steam temperature (K) T: Cooling water temperature (K)

The heat transmission coefficient of the flat tube without corrugation was 3200 W / m ² K. Table 4 shows the ratio of the heat transmission coefficient of each corrugated pipe when this is set to 1, together with the corrugating conditions. FIG. 4 shows the influence of the groove depth H and the pitch P on this ratio. An aluminum brass tube of the same strength (a flat tube having an outer diameter of 25.4 mm and a wall thickness of 1.0 mm) has a heat transmission coefficient of 3630 W / m ² K (ratio 1.13).

The corrugated pipes a to i of steel A have a higher heat transfer coefficient than the flat pipe without corrugation. The heat transfer coefficient of the corrugated pipe increases as the groove depth H increases and the groove pitch P decreases, and the groove depth H is 0.01 Di.
As mentioned above, it exceeds the aluminum brass tube (flat tube) of the same strength with the groove pitch P of 0.6D or less.

[0037]

[Table 4] * Desirable conditions are deviated ** No corrugated (flat tube)

[0038]

As described above, since the high corrosion-resistant corrugated pipe of the present invention is made of high Cr ferritic stainless steel, it has very good corrosion resistance against seawater. Moreover, since the cold workability of the high Cr steel, which has been considered difficult to corrugate, is improved and corrugated, the heat transfer coefficient is high. Therefore, it can be applied to a heat transfer tube of a seawater heat exchanger in which seawater flows, such as a condenser of a power plant, and its life can be remarkably extended without lowering the heat transfer coefficient.

In particular, the high corrosion-resistant corrugated pipe according to claim 2 has a higher heat transfer coefficient than the copper alloy pipe which is considered to have a good heat transfer coefficient.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a state of heat transfer in a heat transfer tube of a condenser.

FIG. 2 is a schematic diagram showing the shape of a corrugated tube.

FIG. 3 is a schematic diagram showing a schematic configuration of a heat transfer coefficient measuring apparatus.

FIG. 4 is a chart showing the influence of the groove depth and groove pitch of the corrugated tube on the heat transfer coefficient.

Claims (2)

[Claims] 1. A weight ratio of Cr is 25% or more, and (C +
A highly corrosion-resistant corrugated pipe, characterized in that a spiral groove is formed by corrugating in a pipe made of high Cr ferritic stainless steel with N) limited to 150 ppm or less. 2. The groove depth H is 0.01 × Di or more, and the groove pitch P is 0.6 × Di or less, where Di is the inner diameter of the corrugated pipe. The highly corrosion-resistant corrugated pipe according to claim 1.