JPS5929762B2 - steam generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to steam generators, and in particular to liquid metal (primarily liquid sodium) heated steam generators used in fast breeder reactor plants.

For example, in a shell-and-tube steam generator used in a fast breeder reactor plant, heat exchange is performed by flowing the heated fluid through the tube side and the heating fluid through the shell side. , heat transfer of the heated fluid in the latter half of the two-phase flow region where the amount of heat exchanged is maximum is unstable, resulting in large temperature changes in the heat exchanger tube in this region and large thermal stress cycles on the tube wall of the heat exchanger tube. This results in the addition of

For this reason, there was a possibility that the heat exchanger tubes would be damaged due to thermal fatigue after long-term operation.

The object of the present invention is to provide a steam generator in which the heat exchanger tubes are not damaged due to thermal fatigue even after long-term operation. It has a thermal resistor that covers a part of the outer surface, and detects the point where the fluid to be heated transitions from the nucleate boiling state to the film boiling state, the so-called dryout point. It is characterized by being configured so that the position of the dryout point can be maintained in the portion of the heat transfer tube covered with the heat resistor by controlling the flow rate ratio with the fluid. By suppressing the amount of heat exchanged in the latter half of the phase flow region, especially near the dryout point, the thermal stress cycle applied to the heat transfer tubes is suppressed, and a steam generator with high integrity even after long-term operation is obtained. be able to.

Below, we will use a liquid sodium heating steam generator as an example.
We will explain the phenomena that occur inside heat exchanger tubes.

Figure 1 shows the temperature distribution on the sodium side and water side in a shell-and-tube steam generator. The horizontal axis is the position (m) measured from the water side in the tube axis direction, and the vertical axis is the temperature. ('C), E and F are respectively,
The flow rate ratio of sodium to water (GN/GW) is 8 and l.
The figure shows the case of O, where the solid line shows the temperature on the water side and the dotted line shows the temperature on the sodium side.

As is clear from this figure, in both cases, the maximum temperature difference between the sodium side and the water side is located in the region where the temperature on the water side is constant, that is, in the latter half of the two-phase flow region. , Therefore, the amount of heat transferred from the sodium side to the water side is greatest in this region.

This region also corresponds to the so-called dryout point where the heat transfer on the water side transitions from the nucleate boiling state to the film boiling state.

H and I in the figure represent GN/GW of 8 and 1, respectively.
It shows the dryout point in case of 0.

Therefore, when the dry-out point is reached and the nucleate boiling state transitions to the film boiling state, the heat transfer coefficient usually rapidly decreases to about 1/1O.

If this dryout point is stable at a fixed position, it will have little effect on the health of the steam generator, but the dryout point constantly fluctuates due to the influence of various fluctuation phenomena inherent in two-phase flow. .

Figure 2 schematically shows the mechanism by which temperature fluctuations occur in heat exchanger tubes due to fluctuations in the dryout point.The horizontal axis represents steam dryness, and the vertical axis represents temperature.J and M are, respectively, Liquid sodium and water/steam, K and L indicate the outer and inner walls of the heat exchanger tube, 0 indicates the dryout point,
Lines drawn up and down with arrows at both ends indicate the range of temperature fluctuations.

That is, when a cycle in which the heat transfer coefficient changes between 1 and 1/work 0 before and after the dryout point O is added, a large temperature fluctuation cycle occurs in the heat exchanger tube, particularly on the inner wall of the heat exchanger tube.

Due to this temperature fluctuation cycle, the amplitude of the thermal stress cycle applied to the tube wall becomes relatively large, and it is difficult to suppress it below the yield stress of the heat exchanger tube material under any operating conditions, and after long-term operation, a certain degree of fatigue may occur. Accumulation is unavoidable.

Based on these study results, the present inventors reduced the fluctuation of the dryout point and at the same time covered the heat transfer tube in the area where the dryout point occurs with a thermal resistor, thereby reducing the heat flux near the dryout point. As a result, the temperature cycle of the heat exchanger tube wall caused by the fluctuation of the dryout point, as well as the fluctuation amplitude of the thermal stress cycle, can be suppressed, thereby improving the long-term health of the steam generator. This made it possible.

Examples will be described below.

The steam generator 1 shown in FIG. 3 consists of a shell 2 and a large number of heat exchanger tubes 5. The shell 2 is composed of an upper shell 3 and a lower shell 4, and the heat exchanger tubes 5 are composed of a feed water downcomer section 6 and a steam riser tube. It consists of part 7.

The water supply downcomer section 6 has one end connected to the water supply header 11 provided in the upper shell 3, and the steam riser pipe section T has one end connected to the steam header 12 provided in the upper shell 3. The steam riser section 7 is wound in a helical coil around a cylindrical inner shroud 13.

Further, a cylindrical heat shield 8 is arranged concentrically with the internal shroud 13 between the water supply downcomer section 6 and the steam riser section 7 .

A sodium inlet nozzle 19 is provided through the upper shell 3 and is connected at one end to a ring header 20 which includes a riser region 25 formed between the heat shield 8 and the inner shroud 13. A sodium distribution pipe 21 is attached to be inserted into the pipe.

A sodium outlet nozzle 22 is provided at the bottom of the lower barrel 4 .

In addition, a nozzle 23 for discharging a reaction product when sodium and water react is provided at the top of the upper body 3.

The outer surface of each heat exchanger tube 5 located in a part of the steam riser pipe portion 7 is covered with a heat resistor 30 and a heat flow suppression section 31
Configure.

4 and 5 show the covering state of the thermal resistor 30. FIG. 4 shows a structure in which a plurality of heat transfer tubes 5 are covered together, and FIG. 5 shows a structure in which a plurality of heat transfer tubes 5 are covered together. The heat flux suppressing section 31 covered with these thermal resistors 30 is provided over 20 sections of the entire length of the steam riser pipe section 7.

In a steam generator having such a structure, high temperature sodium flows from the sodium inlet nozzle 19 through the ring header 20 and the sodium distribution pipe 21 into the riser region 25.

The inflowing sodium descends through this region 25 and flows out from the sodium outlet nozzle 22.

On the other hand, water is supplied from the water supply header 11 to the water supply downcomer section 6 of each heat transfer tube 5, descends within the water supply downcomer section 6, and then ascends through the steam riser section γ.

Then, while passing through the steam riser pipe section 7, it is heated by sodium and becomes steam, which flows out from the ladder 12 into steam.

In the heat flux suppression section 31 of the steam riser pipe section γ, a thermal resistor 30 is attached to the outer surface of the tube, and the heat flux in this section is suppressed to about half that of other sections.

The heat resistor 30 is made of stainless steel with a relatively low thermal conductivity, but if the wall thickness is increased, the same chromium or molybdenum steel as the heat exchanger tube can be used.

Figure 6 shows the analysis results for the amplitude of temperature fluctuation on the inner wall of the heat exchanger tube, where the horizontal axis shows the ratio of the thermal resistance of the thermal resistor to the thermal resistance of the heat exchanger tube, and the vertical axis shows The temperature fluctuation amplitude (°C) of the inner wall of the pipe is taken, and when P and Q t R are 30, 50, and 100 load, respectively, S indicates the allowable range of temperature fluctuation, and the thermal resistance It can be seen that by providing a body, the amplitude of temperature fluctuation tends to be suppressed.

In addition, the amplitude of temperature fluctuation is also affected by the load. For example, assuming that thermal fatigue does not accumulate on the heat exchanger tube wall even if operation is continued at 30% load, the thermal resistance of the heat exchanger tube is 1.
It can be seen that it is sufficient to provide a thermal resistor having a thermal resistance approximately five times higher.

FIG. 7 shows an embodiment of a control device for maintaining the dryout point within the heat flux suppression zone 31.

The temperature of sodium, which is a heating fluid, is increased by exchanging heat with primary sodium heated in a nuclear reactor (not shown) in an intermediate heat exchanger 41.

This heated sodium is supplied to the steam generator 1, flows down the riser section region 25, and exchanges heat with water or steam flowing inside the heat transfer tube of the steam riser section γ.

The temperature of the sodium flowing down the riser section region 25 is determined by at least two regions upstream and downstream of the heat flux suppression section 31.
The temperature is measured by a thermometer 46 provided at each point.

The sodium that has undergone heat exchange in the steam generator 1 is pressurized by a sodium pump 42 driven by a pump drive device 43 and returns to the intermediate heat exchanger 41 again.

On the other hand, the feed water whose pressure has been increased by the feed water pump 44 driven by the pump drive device 45 enters the steam generator 1 from the ladder 11 to feed water.

While the feed water rises in the heat exchanger tube of the steam riser section γ, it exchanges heat with sodium, is boiled and superheated, and is discharged from the steam header 12 to the outside of the steam generator.

This superheated steam is used to drive a steam turbine (not shown) to generate power.

At this time, the pump drive device 4 that drives the water supply pump 44
The 50 rotational speed is regulated by a signal regarding the sodium temperature in the riser region 25 detected by the thermometer 46 and is controlled in such a way that the dryout point is always within the heat flux suppression zone 31 .

Figure 8 is an explanatory diagram showing that the position of the dryout point can be detected by the signal related to the sodium temperature.As seen in Figure 1, in a steam generator that performs countercurrent heat exchange, boiling progresses. , so-called, as the dryness of the steam increases, the temperature difference between the sodium and the steam increases.

On the other hand, it is well known that when the dryness of steam increases and a dryout phenomenon occurs, the heat transfer coefficient decreases rapidly.
Therefore, the heat flux, which is determined as the product of the temperature difference between lithium and vapor (referred to as heat transfer coefficient), exhibits an extremely large value at the dryout point.

Furthermore, considering the heat balance, the heat flux is proportional to the temperature gradient in the flow direction of sodium, and therefore the position of the dryout point is detected as the point where the temperature gradient in the flow direction of sodium shows the maximum value. be able to.

The horizontal axis in FIG. 8 indicates the position of the dryout point in the riser pipe region 25, and the vertical axis is centered on the heat flux suppression section 31 (this portion is indicated by U in the figure) within the riser pipe region 25. Temperature measurement points (A, B and C) set at two points each on the top and bottom of the
, D) shows the temperature difference in sodium temperature.

However, A, B, C, and D indicate the positions of temperature measurement points in order from upstream to the flow of sodium, and the distance between A and B and the distance between C and D are set in advance to be equal. has been done.

In addition, V and W in the same figure. X is (TA TB
)t (TCTD)t(TA-TB) -(To-TD
) is shown.

As mentioned above, the temperature difference (TA-TB) or (T.

-TD) is a value proportional to the average heat flux between A and B or between C and D, and becomes maximum when the dryout point exists between A and B or between C and D, respectively.

Therefore, the difference between both temperatures is (TA-TB)-(
To-TD) is the eighth point corresponding to the position of the dry-out point.
The value will be as shown by X in the figure.

That is, if a dryout point exists upstream of the heat flux suppression section 31, (TA-TB)-(TC-TD
) indicates a positive value, and if a dryout point exists downstream of the heat flux suppression section 31, (TA-TB)-(T
C-TD) indicates a negative value.

Further, (TA TB) (TCTD) becomes 0 when the dryout point is located at the center of the heat flux suppression section 31.

Therefore, the signal (TA-TB)-(Tc-TD) can be a signal that is approximately proportional to the position of the dryout point from the center of the heat flux suppression section 31 at least within the section A-D. Recognize.

Therefore, in the steam generator shown in Fig. 7, if each detection signal of the thermometer 46 is combined to become -('rA-TB) + ('ro-TD), if this value is positive, the dryout point is reached. This means that the position is on the downstream side of the heat flux suppression section 31.

Therefore, in such a case, the rotation speed of the pump drive device 45 of the water supply pump 44 is increased to increase the water supply flow rate.

That is, by reducing GN/GW, the dryout point can be moved into the heat flux suppression section 31.

Conversely, if this value is negative, it means that the position of the dryout point is on the upstream side of the heat flux suppression section 31, and the number of rotations of the water supply pump drive device 450 is reduced to reduce the water supply flow rate. In other words, by increasing 0870w, the dryout point can be moved into the heat flux suppression section 31.

By controlling the operating state of the steam generator 1 in such a way that the position of the dryout point is always within the heat flux suppression zone 31, excessive thermal stress is not applied to the heat exchanger tubes 5, so that It becomes possible to provide a steam generator that is still highly sound even after being operated for a long period of time.

FIG. 9 shows another embodiment, which differs from the embodiment shown in FIG. 7 in that the measurement result of the thermometer 46 is given to the pump drive device 43 of the sodium pump 42.

That is, the measured value of the sodium temperature in the riser section region 25 is synthesized to become (TA-TB) cT'o-TD), and this signal is used to control the rotation speed of the pump drive device 43 that drives the sodium pump 42. used.

In this case, if the value of (TA-TB)-('rc-'rD) is positive, the position of the dryout point is on the upstream side of the heat flux suppression section 31, so the sodium pump 420 By increasing the rotation speed and increasing GN/GW, the position of the dryout point can be moved into the heat flux suppression zone 31.

Conversely, if the value of (TA-TB) (TCTD) is negative, the position of the dryout point is within the heat flux suppression section 31.
Sodium pump 420
Reduce rotation speed.

As a result, by reducing GN/GW, the position of the dryout point can be pulled back into the heat flux suppression section 31.

Even when such a control method is adopted, effects similar to those of the above-mentioned embodiment can be obtained.

As described above, the steam generator of the present invention can suppress fatigue of the heat exchanger tubes due to thermal stress caused by temperature fluctuations in the heat exchanger tube walls, so even after long-term operation, the steam generator has high soundness. It provides a steam generator with excellent reliability and is a key to industrial effectiveness.

[Brief explanation of drawings]

1 and 2 are explanatory diagrams of the dry-out phenomenon of a steam generator, FIG. 3 is a sectional view of the main part of the steam generator of the present invention, and FIG.
5 and 5 are cross-sectional views of different embodiments of the heat exchanger tubes of the steam generator of the present invention, FIG. 6 is a diagram showing the target temperature fluctuation of the steam generator of the present invention, and FIG. 7 is a diagram showing the temperature fluctuation target of the steam generator of the present invention. FIG. 8 is a diagram showing the operating principle of the steam generator of the present invention, and FIG. 9 is a cross-sectional diagram of the essential parts of another embodiment of the steam generator of the present invention. be. Explanation of symbols, 1...Steam generator, 2...
- Shell, 5... Heat exchanger tube, 6... Water supply descending pipe section, I... Steam rising pipe section 530...
Thermal resistor, 31...Heat flux suppression section, 42...
... Sodium pump, 44 ... Water supply pump, 43, 45 ... Pump drive device, 46 ...
····thermometer.

Claims (1)

[Scope of Claims] 1. In a shell-and-tube type steam generator in which a fluid to be heated is passed through the tube side and a heating fluid is flowed through the shell side for heat exchange, the steam generator is located in a part of the tube in the longitudinal direction and is located on the outer surface of the tube. a thermal resistor covering the heating fluid, a means for detecting the position of the dryout point of the heated fluid, and a flow rate of the heated fluid and the heated fluid based on the detection result of the dryout point position obtained by the means. and means for controlling the ratio of the heat resistor to maintain the position of the dryout point at a portion of the tube covered with a heat resistor. 2. The means for detecting the position of the dryout point of the heated fluid measures the temperature gradient in the flow direction of the heated fluid on the upstream and downstream sides of the portion of the pipe covered with the thermal resistor, and A steam generator according to claim 1, further comprising means for determining a temperature gradient difference signal. 3. The steam generator according to claim 1, wherein the means for controlling the ratio of the flow rates of the heating fluid and the heated fluid is configured to be controlled only by a change in the flow rate of the heated fluid. 4. The steam generator 5 according to claim 1, wherein the means for controlling the ratio of the flow rates of the heating fluid and the fluid to be heated is configured to be controlled only by a change in the flow rate of the heating fluid. A steam generator according to any one of claims 1 to 3, wherein the heating fluid is water and the heating fluid is a liquid metal. 6. The steam generator according to claim 5, wherein the liquid metal is liquid sodium.