JP2001082702A - Overheat damage diagnosing method for boiler water- wall tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to efficiently and high-precisely detect the overheat damage portion of a bailer water-wall tube and high-precisely diagnose the degree of damage and a remaining life. SOLUTION: (1) Screening of a damage portion and ranking of an evaluation portion are carried out from the oxidation scale thickness of an outer surface on the water-wall tube furnace side. (2) Regarding a portion where an outer surface oxidation scale thickness on the water-wall tube furnace side exceeds a given value, density of a tube inner surface oxidation scale is measured, a portion having a density of 4 g/cm3 or more forms an overheat damage portion. A destructive method (such as a method for measuring a scale amount and a scale thickness) and a non-decretive method (such as, an X-ray CT method and an ultrasonic density method), are given as a method for measuring density of a tube inner surface oxidation scale. (3) An overheat damage portion having density of 4 g/cm3 or more of a tube inner scale is calculated by a method by a method for predicting it from a tube inner surface scale thickness and an overheat supposing time, and an overheat temperature by a method for predicting it from the aforesaid steam oxidation scale thickness. (4) A creep damage rate is calcite from a load stress, calculated from the size (a diameter and a thickness) and the pressure of the water-wall tube, a superheat temperature, an overheat time, and the strength of the material, and a remaining life is diagnosed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for diagnosing damage to a thermal power generation system, and more particularly to a method for efficiently and accurately diagnosing local overheating of a boiler furnace evaporating water wall tube.

[0002]

2. Description of the Related Art Furnace water wall pipes of power boilers are for heating and evaporating high-temperature and high-pressure water by radiant heat of a high-temperature combustion gas of 1000 ° C. or more, and are under severe temperature conditions. It is designed so as not to overheat (overheat) above the temperature. However, if the boiler is operated for a long time, the fluid temperature and heat load at the initial stage of the design will be affected by the flow rate imbalance due to local residue of the inner surface scale of the water wall pipe, and the change in heat absorption due to oxidation thinning of non-pressure parts such as membranes. Can cause overheating damage to the water wall tubing in areas that cannot be predicted from the water.

[0003] In recent boilers for thermal power generation, the load change operation is frequently performed or the low load operation is continued, and the operation time during which the heat load locally increases tends to increase.

[0004]

Conventionally, the material damage over time of the furnace water wall tube has been inspected by measuring the thinning amount at a regular inspection conducted every 1.5 to 3 years or by a nondestructive flaw detection method. ing. At that time, if any damage equal to or more than the specified value is found, it will be replaced with a new material on the spot. Material damage is diagnosed by destructive inspection by extubation at high heat load and high thermal stress.However, the number of inspections is limited and it is extremely difficult to select the most severely damaged area. is there.

On the other hand, in a superheater and a reheater, a method of predicting the superheat temperature from the thickness of the steam oxidation scale on the inner surface of the tube is generally adopted. In a superheater or a reheater, the scale of iron oxide is generated on the inner surface of the tube due to the steam whose inner surface is overheated. This is called steam oxidation. Since the thickness of the scale due to the steam oxidation has a correlation with the superheat temperature, the superheat temperature is predicted from the steam oxidation scale thickness on the inner surface of the tube as described above. That is being done.

This is because the steam oxidation scale thickness y of a carbon steel or a low Cr steel obeys the parabolic law y = (Kp · t) ^0.5 (1) expressed by the following equation (1). It is a method of estimating. Here, Kp is a reaction rate constant, which is obtained by the following equations (2) and (3). Kp = A × Exp (−Q / RT) (2) Equation (2) is an empirical equation based on the parabolic law of steam oxidation, and Q (material constant) and A (material constant) are values obtained by experiments. Kp can be obtained by the following equation: log (Kp) = a + b (1 / T) (3) Here, t: time, T:
Temperature (273 + ° C), a, b: constants of the data regression equation.

As a method of estimating the life of a superheater or a reheater, Japanese Patent Application Laid-Open No. Hei 8-110006 discloses a method of estimating the thickness of the steam oxidation scale generated on the inner surface of many heat transfer tubes under the same design conditions. There is disclosed a method for diagnosing the life of a boiler heat transfer tube, which measures by a non-destructive test, extracts a tube having the largest scale thickness as a representative tube, and diagnoses the life from the representative tube.

[0008] However, if the thickness y of the generated steam oxidation scale can be identified even with a water wall tube, the superheat temperature can be estimated.
In the case of a water wall tube, the same component and the same structure as the iron oxide eluted from the tube oxidized by water vapor (Fe ₃ O ₄ , Fe ₂
O ₃ ) precipitates and adheres from water (that is, from water rather than from a tube), and cannot be distinguished from each other.
Descaled (chemical cleaning,
Therefore, overheating damage cannot be predicted simply from the thickness y of the oxide scale on the inner surface of the tube. In fact, there are some members that do not damage the material even if the oxide scale near 1 mm adheres to the inner surface of the tube.

FIG. 6 is an explanatory view showing the case where steam oxidation scale adhering to the superheater and reheater tubes and scale which adheres to the water wall tube due to precipitation of iron oxide from water.

[0010] Therefore, it can be said that there is no means for finding the overheating damage suddenly occurring in the boiler water wall tube in the current technology.
More recently, there has been a movement to extend the regular inspection interval,
In the meantime, in order to ensure stable operation, it is essential to develop a technology for detecting the overheating damage part of the water wall pipe and a diagnostic technology that can predict the damage several years later with high accuracy.

An object of the present invention is to provide a technique for efficiently and accurately detecting an overheated damaged portion of a boiler water wall pipe and diagnosing the degree of damage and the remaining life with high accuracy.

[0012]

Means for Solving the Problems As a result of intensive research and investigation, the inventors of the present invention focused on the thickness of the outer oxidation scale and the density of the oxidation scale on the inner surface of the tube to reduce the degree of overheating damage of the boiler water wall tube. We found that we could diagnose.

That is, the above object of the present invention is solved by the following constitutions (1) to (3).

(1) A method of diagnosing overheating damage of a boiler water wall tube in a method for diagnosing overheating damage of a water wall tube of a furnace boiler, wherein a detailed diagnosis site is screened and ranked based on the thickness of the oxidation scale on the outer surface of the furnace tube.

(2) In the method of diagnosing overheating damage to the boiler furnace water wall tube, the boiler water wall is used to evaluate or determine the presence or absence of overheating damage from the apparent specific gravity (density) value of the oxide scale on the inner surface of the water wall tube. Diagnosis method of overheating damage of pipe.

In an embodiment of the present invention. The portion where the density of the oxidation scale on the inner surface of the tube is 4 g / cm ³ or more is defined as the overheat damaged portion. As a method for measuring the density of the oxidized scale on the inner surface of the tube, a destructive method (such as a method for measuring the amount of scale and scale thickness) and a non-destructive method (such as an X-ray CT method and an ultrasonic density method) are exemplified.

(3) Based on the thickness of the oxide scale on the outer surface of the water-wall tube furnace, screening of a damaged portion and ranking of evaluation sites are performed. the density of the oxide scale was determined, density and overheating damage site 4g / cm ³ or more sites for density 4g / cm ³ or more overheating damage site within a vessel surface scale, tube inner surface oxide scale thickness and overheating assumed Boiler water wall that predicts from the time, calculates the load stress calculated from the dimensions (diameter and wall thickness) of the water wall pipe and the internal pressure and the creep damage rate from the superheat temperature, the superheat time and the strength of the material, and diagnoses the remaining life Diagnosis method of overheating damage of pipe.

[0018]

FIG. 2 shows a part of the cross section of the boiler water wall tube. Oxidation scales 3 and 4 are formed or adhere to the outer and inner surfaces of the tube, respectively, but the thickness is increased on the furnace side where evaporation occurs at higher temperatures. In addition, as shown in FIG.
In this case, the thickness of the scales 3 and 4 increases on both the outer and inner surfaces of the tube.

The generation rate of the oxidation scale 3 on the outer surface of the tube also increases as the temperature increases, but since the superheat temperature of the heat transfer tube varies depending on the fluid condition on the inner surface side, it is not possible to evaluate and diagnose the degree of damage from the thickness. FIG. 4 is a plot of the relationship between the thickness of the oxide scale 3 on the outer surface of the water wall tube and the degree of overheating damage. Even when the thickness of the outer tube oxidation scale 3 is 200 μm or more, there may be no overheating damage. As described above, it is not possible to diagnose overheat damage only by the thickness of the outer tube oxidation scale 3. However, from FIG. 4, the thickness of the oxide scale on the outer surface of the tube is increased in most cases in the portion where the overheat damage is large. It is possible to rank the evaluation sites.

Next, the relationship between the thickness of the oxidized scale 4 on the inner surface of the water wall pipe and the amount of the scale in the portion having the overheat damage and the healthy portion was plotted. The result is shown in FIG. Pipe inner surface scale (solid mark) 4 from which overheating damage has been observed
Found that even with the same scale thickness, the weight (mg / cm ² ) was larger than that of a healthy pipe (open mark), and that a healthy pipe and an overheated damaged pipe could be clearly distinguished. The solid line in the figure is a line having a specific gravity of 4 g / cm ³ , which means that the tube having an apparent specific gravity (density) of 4 g / cm ³ or more in the pipe inner surface oxidation scale 4 is overheated. This is because the oxide scale 4, which is deposited and adhered from the water on the inner surface of the normal water wall pipe, is generally soft and porous, and thus has a small apparent specific gravity. Can be estimated to have increased.

As described above, the main components of the scale deposited from water and the scale of steam oxidation are iron oxide (Fe).
₃ O ₄ , Fe ₂ O ₃ ), the theoretical specific gravity of these oxides is about 5.5, and the scale generated by steam oxidation is more dense than the scale formed by iron oxide precipitated in water. The density approaches the theoretical value of the density (specific gravity) of iron oxide. In addition, the apparent specific gravity (density) of the deposited scale from water in the sound part may be converted to thickness from the scale amount as about 3 g / cm ³ .

Although it was difficult to identify whether the oxide scale 4 on the inner surface of the tube was an adhered precipitate scale or a steam-oxidized scale based on the scale components, structure, and the results of observation with a cross-sectional optical microscope, it was easily determined by determining the apparent specific gravity. Will be able to

A steam oxidation scale having a significant thickness (usually 50 μm or more) depends on time conditions, but is generated at 450 ° C. or more. become.

If the scale formed on the inner furnace side of the tube is a steam oxidation scale, the superheat temperature can be estimated by using the above-described parabolic law, and the load stress value calculated from the internal pressure and the size can be calculated. The current creep damage rate and remaining life can be calculated from the material strength of the portion.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described with reference to flow sheet drawings. FIG. 1 is a flow sheet of a method for diagnosing overheating damage to a boiler water wall tube according to the present invention. The contents are described in the following order.

An overheat-possible region is predicted from heat load, vapor density, assumed temperature (fluid, gas, metal) and the like. Although this step is not an essential step of the present invention, it is performed to narrow down the overheated site to some extent.

Step of Screening Damaged Sites and Ranking Detailed Damage Diagnosis Sites Based on the Thickness of the Oxide Scale 3 on the Outside Surface of the Tube As shown in FIG. Since there is a clear tendency for the oxide scale 3 on the outer surface to be thicker, a portion where the thickness of the outer oxide scale 3 is 100 to 150 μm or more is selected as the detailed evaluation portion.

The order or ranking of the parts to be evaluated in detail may be in the order of the thickness of the outer oxide scale. As a method for measuring the thickness of the oxide scale on the outer surface of the tube, an ultrasonic method is suitable, but other methods may be employed. If the external oxidation scale 3 is peeled off from the appearance, it is highly likely that the temperature has become higher, and the portion should be evaluated by adding the peeled amount.

A step of measuring the apparent specific gravity (density) of the oxidation scale 4 on the inner surface of the pipe Destructive inspection method (scale amount (mg / c
m ² ) and scale thickness (mm), and calculate the apparent specific gravity) or the non-destructive inspection method (X-ray CT method, ultrasonic density method, etc.) to determine the apparent specific gravity of the oxide scale 4 on the inner surface of the tube ( density)
Is measured.

Step of Determining the Presence or Absence of Overheating A portion where the apparent specific gravity (density) value of the oxidation scale 4 on the inner surface of the water wall tube is 4 g / cm ³ or more is determined as the overheating damaged portion (see FIG. 5). ). The gist of the present invention is to determine the presence or absence of overheating damage from the apparent specific gravity (density) of the oxide scale 4 on the inner surface of the water wall pipe.

Step of Calculating Superheated Temperature from Water Wall Pipe Inner Scale Thickness and Superheating Time If the oxide scale 4 on the water wall pipe inner surface is a steam oxidation scale having a high apparent specific gravity (density), the steam The superheating temperature can be calculated from the parabolic law of oxide scale growth. As described above, the thickness (y: mm) of the steam oxidation scale can be represented by the above equation (1).

STBA2 frequently used for boiler water wall pipes
Taking 0 (0.5Cr-0.5Mo steel) as an example, the following Table 1 shows the calculation results (° C.) of the superheat temperature. The range is given in consideration of data variations.

[0033]

[Table 1]

As described above, the superheat temperature can be uniquely obtained from the thickness of the steam oxidation scale having a large apparent specific gravity and the assumed superheat time.

A process of predicting and diagnosing a creep damage rate and a remaining life from a metal temperature, an applied stress due to an internal pressure, and a material strength.

The following is a calculation result of a calculation example in which a steam oxidation scale of 300 μm is generated at 3,000 h (metal temperature is 590 ° C.) in the calculation example.

The superheat temperature condition (590 ° C.), the dimensions of the water wall tube (outer diameter φ25.4 × 4.2 mm thickness) and the inner pressure (25
From the applied stress (65 MPa) calculated from the pressure stress (MPa) and the creep strength of the material (STBA20), the creep damage rate at 3,000 h is 18%. Further, the remaining life (time until the creep damage rate reaches 100%) under the same conditions can be estimated to be 9,000 h.

The creep damage rate can be calculated from the following equation (4), which is a regression equation between the load stress of the member and the Larson-Miller parameter of the creep life. P = T + (c + log (t)) (4) Here, T is an absolute temperature (K: 273 + ° C.), t is time (h), and c is a constant. P is a value called Larson-Miller parameter (P) determined by temperature and time, and there is a correlation between load stress and P value at which creep rupture can be approximated by a quadratic curve. Use to calculate the damage rate.

For example, from the relational expression between the applied stress and ST of STB410 steel, when the applied stress is 70 MPa, the creep life P is P = 15,500. When the temperature of the part is 500 ° C., the creep rupture time is about 20,0
00 hours.

In the above equation (4), P = 15,500 and c =
The value obtained by substituting 15.753 and T = 273 + 500 to obtain t (about 20,000 h) is the creep life and the current operation (heating) time of 12,000 h, and the remaining life is (20,000). -12,000) and the creep damage rate is (12,000 / 20000).
0) is 60%.

[0041]

The present invention is effective for preventive maintenance because it has the above-mentioned structure and can accurately and efficiently diagnose overheat damage occurring at an unpredictable portion of a water wall pipe of a boiler. Stable power supply in a thermal power plant is possible.

[Brief description of the drawings]

FIG. 1 is a flowchart of a method for diagnosing overheating damage of a boiler water wall tube according to the present invention.

FIG. 2 is a sectional view of a part of a boiler water wall tube.

FIG. 3 is a cross-sectional view of a part of a boiler water wall tube.

FIG. 4 is a diagram showing the relationship between the thickness of the oxide scale on the outer surface of the water wall tube and the degree of overheating damage.

FIG. 5 is a diagram showing the relationship between the thickness and the scale amount of the oxidized scale on the inner surface of the water wall pipe at the part where overheating was damaged and the healthy part.

FIG. 6 is an explanatory diagram showing a case where steam oxidation scale adhering to a superheater / reheater tube and iron oxide from water precipitate and adhere to a water wall tube.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Boiler furnace water wall pipe 2 Membrane 3 Pipe oxidation scale 4 Pipe oxidation scale 5 Overheat damage area

Claims (4)

[Claims] 1. A method for diagnosing overheating damage of a boiler furnace water wall tube, comprising:
A method for diagnosing overheating damage to a boiler water wall pipe, which comprises screening and ranking detailed diagnosis sites. 2. A method of diagnosing overheating of a water wall tube of a boiler furnace, wherein the presence or absence of the overheating damage is evaluated or determined from a density value of an oxide scale on an inner surface of the water wall tube. Overheating damage diagnosis method. 3. The density of the oxide scale on the inner surface of the tube is 4 g / c.
^{3. The} method for diagnosing overheating damage of a boiler water wall tube according to claim 2, wherein a member or a portion of m3 or more is determined as an overheating damage area. 4. Screening of damaged portions and ranking of evaluation sites based on the thickness of the oxidation scale on the outer surface of the water wall tube furnace side, and oxidizing the inner surface of the tube for the portion where the thickness of the outer wall oxidation scale on the water wall tube furnace side is equal to or more than a predetermined value. the density of the scale was measured, density and overheating damage site 4g / cm ³ or more sites for density 4g / cm ³ or more overheating damage site within a vessel surface scale, tube inner surface oxide scale thickness and overheating expected time Boiler water characterized by predicting the superheat temperature from the temperature, calculating the creep damage rate from the load stress calculated from the dimensions and the internal pressure of the water wall pipe, the superheat temperature, the superheat time and the strength of the material, and diagnosing the remaining life. Diagnosis method of overheating damage of wall pipe.