JPH0665923B2 - Prevention method of pipe end erosion of feed water heater - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a pipe end erosion prevention method for a feedwater heater of a thermal power plant.

[Background of the Invention]

In feed water heaters used for thermal power plants, it is known that erosion occurs on the inlet side of heat transfer tubes. To prevent this erosion, for example, as shown in Japanese Utility Model Publication No. 43-25843, a method of preventing turbulence by providing a rectifying means on the inlet side of the heat transfer tube, or a heat transfer tube as disclosed in Japanese Patent Laid-Open No. 50-83849 Although a method of providing a rubber lining on the inner surface of the method has been proposed, the method of providing the rectifying means has a drawback that the flow resistance of the water supply increases, and the method of providing the rubber lining lacks durability when the water supply temperature rises. There was a problem.

[Object of the Invention]

The present invention provides a method for preventing the occurrence of tube end erosion without requiring any special means such as a rectifying means.

[Outline of Invention]

Erosion at the inlet of the heat transfer pipe of the feed water heater does not occur in all of the feed water heater, but is concentrated in those where the feed water temperature is within a specific temperature range. Based on the new knowledge that the less scale adheres and the more scale adheres, the less erosion is, it is easier to artificially attach the scale by magnetizing the heat transfer tube where scale does not adhere easily. I tried to prevent erosion.

Example of Invention

Figure 1 shows a system diagram of a thermal power plant, which uses a boiler 10
Steam generated in 1 goes through the main steam pipe 141 and the high-pressure turbine 102.
Is supplied to the high-pressure turbine, and the exhaust gas is reheated.
It is reheated in 3 and passes through the reheat steam pipe 142, works in the reheat turbine 103, and drives the generator 104 directly connected to the turbines 102 and 103.

The reheat turbine exhaust gas is condensed in the condenser 106 to be boiler feed water, passes through the condensate pump 107, the condensate demineralizer 108, and the condensate booster pump 109, passes through the low pressure feed water heaters 111, 112, 113, and the deaerator 114. Leading to. The feed water deaerated by the deaerator 114 is gradually heated while passing through the high-pressure feed water heaters 117 to 110 after being increased in pressure by the water feed pump 116 and returned to the boiler 101. The low pressure feed water heaters 111 to 113 and the high pressure feed water heaters 116 to 110 are connected to the extraction lines 121 to 121 of the turbines 103 and 102, respectively.
Heated by bleed steam via 123, and 131-134. The thick line in FIG. 1 indicates the main system of steam or feed water.

FIG. 2 is a cross-sectional view of the feed water heater. The boiler feed water enters the water chamber 1 through the feed water inlet nozzle 2 and the U-shaped heat transfer tube 5
Through the water supply outlet nozzle 3. A plurality of heat transfer tubes 5 are provided to form a tube group and are opened to the water chamber 1 by the tube plate 4 and supported by the tube support plate 6 and the tie rod 7 with respect to the body 9. Has been done. On the other hand, the heated steam extracted from the turbine enters the fuselage 9 through the steam inlet nozzle 10, and the condensed water that has finished the heating action by the water heater located on the high pressure side of this water heater is also the drain inlet nozzle. It flows from 11 into the body 9.

These heating fluids come into contact with the surfaces of the heat transfer tubes 5 of the tube group, exchange heat with the feed water flowing in the tubes, condense, form drains 12, and are discharged from the drain outlet 14 at the bottom of the body. It

FIG. 3 is a detailed cross-sectional view of the end of the heat transfer tube. Significant erosion was observed near the boundary of the welded portion 20 at the inlet, and almost no erosion was observed at the outlet side of the feed water. It is considered that the cause of the erosion at the end of the heat transfer tube is that when the feed water in the water chamber 1 flows into the heat transfer tube, it becomes a turbulent flow near the inlet to generate a vortex and a kind of cavitation.

Therefore, FIG. 4 shows the results of inspecting the erosion state of the pipe end of each feed water heater in a thermal power plant that has been operating for several years. As a result of the inspection, no erosion of the pipe end was observed in the feed water heaters 111 to 113 on the low pressure side, and no scale adhered. For the high-pressure feed water heaters 116 to 120,
As shown, the two on the relatively low pressure side, namely 117 and 118
However, erosion was hardly observed in the two high pressure side, 119 and 120. As for the two on the high-pressure side, the scale was attached almost all over the inner surface of the heat transfer tube and the surface of the tube sheet that contacted the feed water.

The reason why erosion does not occur in the low-pressure feedwater heater is that the feedwater pressure is low, so that the flow of feedwater at the heat transfer tube inlet is stable and that it is near the condensate demineralizer, and the scale in the feedwater is small. Since the concentration is low, it is presumed that the erosion is extremely light even if turbulence occurs at the heat transfer tube inlet.

On the other hand, in the high-pressure feed water heater, the one on the high-pressure side is not eroded. It is presumed that the scale attached to the inner surface of the heat transfer tube functions as a protective film against erosion.

Therefore, using the same model as the inlet of the heat transfer tube, when the feed water with a constant impurity (iron oxide) (100 ppm) was flowed, the results of an experiment on the feed water temperature, the amount of scale adhesion, and the amount of erosion are shown in Fig. 5. . The pH of the feed water is 9.3, the oxygen concentration in the feed water is 10 ppb or less, and the flow rate of the feed water is 7.3 m / sec. As a result of continuous operation for 100 hours, the erosion amount becomes maximum when the feed water temperature is 150 ° C and decreases rapidly with increasing temperature. I'm running. on the other hand,
The scale deposit tends to increase rapidly when the temperature exceeds 200 ° C., which is similar to the example of the actual plant shown in FIG.

From these results, it can be seen that the higher the feed water temperature, the easier the scale adheres, and the more the scale adheres, the less the erosion.

Therefore, the present invention artificially makes it easy to attach a scale to a feedwater heater in a region where the feedwater temperature is relatively low and where the scale is difficult to attach.

Most of the scale attached to the heat transfer tube of the feed water heater is iron oxide. Therefore, if the heat transfer tube is magnetized, the scale easily attaches even if the feed water temperature is low. So 10
A scale collected from an actual thermal power plant for the water supply of
An artificially made scale (composition Fe ₃ O ₄ : d−Fe ₂ O ₃ = 3: 1)
To produce a solution with a scale concentration of 10 ppm, and apply this solution to 1 / mi of the surface of a permanent magnet with a magnetic strength of 3K gauss.
The amount of adhesion to the magnet was measured when it was circulated at a flow rate of n. The result is shown in FIG. The scale collected from the actual plant, that is, the actual scale, was adsorbed to the magnet in about 15 minutes, whereas the artificial scale was saturated after 50 minutes and the adsorption amount was about 80%. It was

As a result, the actual scale has the property of being easily attracted by magnetism, and considering the time margin until erosion occurs in an actual thermal power plant, it seems that the magnetic strength does not have to be so strong. From other experiments, it was confirmed that the scale adhered within a few days if it was about 600 gauss.

Magnetizing a steel tube heat transfer tube does not require so high skill. Since the steel pipe is a ferromagnetic material, for example, when performing an eddy current flaw detection test, a degaussing operation after the test (generally, it is always necessary to perform demagnetization operation to prevent adhesion of cutting chips and the like). If omitted, residual magnetism can be left.

In this case, since degaussing operation is not performed after the eddy current flaw detection inspection, it is necessary to consider that chips and the like do not adhere. Therefore, this inspection is preferably performed after the assembly of the feed water heater is completed and the foreign matter is removed by water flushing.

Fig. 7 shows the situation of the eddy current flaw detection inspection.
Enters the water chamber 1 and inserts the test coil 75 provided at the tip of the cord 73 wound up by the cord winch 72 into the heat transfer tube 5 and inspects the change in the magnetism of the coil 75 with the oscilloscope 76. And the current of the coil 75 is the flaw detector main body.
Controlled by 77, the display of oscilloscope 76 is recorded on pen recorder 78.

In this inspection, the heat transfer tube 5 is magnetized by the magnetism of the coil 75, and if a special degaussing operation is not performed after the inspection is completed,
A remanence of about 600 Gauss can be obtained.

When the eddy current flaw detection test is not performed, after the feed water heater is assembled, a coil for magnetizing in the heat transfer tube is inserted and a current can be applied to magnetize.

The feed water temperature of the feed water heater is about 150 ° C to 300 ° C, which is lower than the magnetic transformation point (780 ° C) of steel in this temperature range, so that demagnetization during operation is extremely small.

〔The invention's effect〕

As described above, according to the present invention, it is possible to prevent corrosion of the end portion of the heat transfer tube without increasing the flow resistance of the feed water.

[Brief description of drawings]

1 is a system diagram of a thermal power plant, FIG. 2 is a cross-sectional view of a feed water heater, FIG. 3 is a detailed cross-sectional view of a heat transfer tube end,
FIG. 4 is a comparison diagram showing scale adhesion and erosion conditions in an actual plant, FIG. 5 is a characteristic diagram showing erosion amount and scale attachment amount, FIG. 6 is a scale adhesion characteristic diagram, and FIG.
The figure is an explanatory view showing the situation of the eddy current flaw detection inspection. 1 ... Water chamber, 4 ... Tube plate, 5 ... Heat transfer tube.

Claims (3)

[Claims] 1. A feed water heater having a heat transfer tube made of a magnetic material, wherein the heat transfer tube is magnetized to allow water to pass through while leaving a residual magnetism, and a scale in the water is attached to the heat transfer tube. A method for preventing erosion of a pipe end of a feed water heater, which is characterized by preventing erosion of a pipe end by the scale. 2. The method for preventing erosion of a pipe end of a feed water heater according to claim 1, wherein the heat transfer tube is magnetized by excitation during eddy current flaw testing of the heat transfer tube. 3. The pipe end erosion prevention of a feed water heater according to claim 1, wherein the magnetization of the heat transfer tube is applied only to a feed water heater having a feed water temperature of 150 ° C. to 230 ° C. Method.