JPS61265311A - Method and device for superintending life expectancy of high temperature structure - Google Patents

Abstract

PURPOSE:To prevent a structure from breaking down beforehand by calculating the life span of a machine member according to the changing amount of minute damage on the surface of the member so as to control a steam control valve and steam temperature. CONSTITUTION:A signal from a metal temperature detector 11 mounted on a steam turbine casing is input to a thermal strain arithmetic and logica unit 12 in order to calculate thermal strain on the surface of the casing. Comparing this thermal strain with the prescribed value by a judging device 13, when it is above the prescribed value, a signal is transmitted to a steam temperature controller 14 so as to operate a boiler 16 toward the direction of dropping steam temperature while valve opening is reduced by a steam control valve controller 15. Also, crack progress speed is calculated by an arithmetic and logic unit 19 according to a signal from a crack progress detector 18 mounted on the inner surface of the casing, damage level is assessed by the judging device 13 and the life span up to limited crack size is figured out in order to control opening of the control valve and stream temperature.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a method for evaluating the lifespan of mechanical structures, and particularly a method and method for evaluating the remaining life of mechanical structures that are subjected to repeated loads or load fluctuations at high temperatures. Relating to a lifespan monitoring device.

[Background of the invention]

For example, a steam turbine is a mechanical structure that is subjected to repeated loads in a high-temperature environment, and its members are subject to repeated thermal stress due to startup and stoppages, or load fluctuations during operation, resulting in fatigue damage to the members due to repeated loads. and creep damage due to constant load under high temperature. As a result, the component has a tendency to accumulate damage due to fatigue and creep loading, causing macroscopic cracks in the component and compromising its structural integrity. In such cases, if the life of the structure is not evaluated and monitored, there is a risk of damage to the equipment and a major accident at the plant. In particular, it is important to monitor the lifespan of rotors, casings, and various valves in terms of their strength.

In conventional structural design, a sufficient safety factor was set for the rotor and casing based on the creep strength to ensure high strength reliability.

Therefore, in the past, plants were operated without evaluating the lifespan of equipment. However, at present, there are many thermal power plants that have been operating for long periods of time, and there is also the problem of material deterioration of these components, so there is a strong demand for highly accurate remaining life evaluation technology for rotors, casings, etc.

On the other hand, in recent years, as the capacity of steam turbines has increased and operating conditions have become more severe due to changes in the form of electricity demand, the accumulation of damage to rotors and casings is accelerating, causing cracks to propagate and penetrate through the plate thickness, resulting in serious accidents. It is now possible to consider the possibility of inviting
The situation is such that equipment life evaluation technology is desired.

However, there are currently very few effective components for casings and rotors, life evaluation techniques, and life monitoring systems. If the strength lifespan and degree of damage to components of blinds and casings can be calculated, monitored and evaluated with high precision, plant reliability will be greatly improved. However, a satisfactory configuration that makes this possible has not yet been proposed. Regarding the rotor, a system has been proposed that estimates the thermal stress generated in the rotor by measuring the steam temperature around the rotor and the inner surface temperature of the casing, and controls the operation of the turbine from the rate of change and absolute value of the thermal stress. There is.

For example, Japanese Patent Laid-Open No. 50-149804.

[Purpose of the invention]

The purpose of the present invention is to calculate the life of a component based on the amount of change in microscopic damage on the component surface in a mechanical structure that is subject to fatigue and creep damage due to repeated loads or variable loads at high temperatures, and thereby prevent the occurrence of damage. The object of the present invention is to provide a method and device for evaluating the remaining life of mechanical structures that can prevent damage to the structures and ensure the safety and reliability of the plant.

[Summary of the invention]

In the present invention, we focus on the microscopic damage that occurs on the surface of a component at the beginning of its life under repeated loads in the high-temperature creep region, and calculate the life consumption rate by quantitatively clarifying the growth process of this damage. It is characterized by evaluating the remaining life of high-temperature equipment.

The principle of the remaining life evaluation method of the present invention will be explained below.

Figure 3 shows the low cycle fatigue life at 650 CVc of austenitic stainless steel, which is a structural material for main steam pipes of turbines, etc., and clarifies the time point at which cracks occur and the life at rupture of a smooth round bar test piece. In this way, when focusing on microscopic cracks of 0.05 m, which are about the size of crystal grains, the rupture life occurs at an early stage. As a result, it was found that most of the lifespan leading to fracture is the growth process of minute cracks, and if this process can be quantitatively understood, the accuracy of lifespan evaluation will be greatly improved. FIG. 4 shows the growth behavior of this minute I-crack. Since crack growth is exponential, the vertical axis VC is the logarithm of the crack length, and the horizontal axis is the number of repetitions. In any load strain range, the growth process can be well approximated by a straight line in the figure.

Incidentally, it is known that fatigue life depends on the load strain rate and load waveform at high temperatures. Figure 5 shows the results of the investigation. Even in tensile-holding waveforms and low-speed, high-speed sawtooth triangular waveforms that cause significant limp damage, most of the life until failure is spent in the growth process of minute cracks. In this way, even under conditions in which creep, which is damage to components under constant loads of centrifugal force and internal pressure under high temperatures, which is the actual machine condition, and fatigue damage caused by repeated starting and stopping are combined, the length of minute cracks is small. A linear relationship is established between the logarithm log2a of VC and the number of repetitions N. That is, the following equation is obtained.

log 2a = C, N = (1
) Therefore, the growth rate of a small crack is da/dN = C' a ・−・(
2) In equation (2), if the initial crack length is taken as the grain size,
2a(, = 0.05+ms Final crack length is the experimental value, 2at:' 10txxz, and if both sides are integrated, the constant C'ゝ\ is C' = Z3N/Nf-N.

By the way, the crack initiation life N0 is N because cracks occur early in the entire life. =O. Therefore, as a new relationship between crack length and life ratio, 1ag2a = Z3N/Nf + log2ao ”(
3) This equation means that if the crack length is determined, the life ratio N/N f , that is, the life consumption rate (damage amount) can be determined. Figure 6 shows the relationship between minute cracks and life ratio in a high-temperature (#h?cycle) fatigue test under unreasonable conditions based on equation (3).

Despite various load conditions, Factorof2
It is confirmed that the experimental data falls within the range of variation, and the relationship of equation (3) holds true. Therefore, the present invention was devised based on this new fact.

When actually using the relationship of vc (3), it can be rearranged as shown in the following equation. That is, log2a/2ao= 2.3N/Nf ”・(
4) If the initial crack size 2ao is known in advance, then by calculating the amount of growth from then on, (4)
) The amount of damage (N/Nf) can be found from the relationship of the equation.

By the way, since the propagation rate of minute cracks cannot be evaluated using conventionally used fracture mechanics parameters, the relationship between the total strain width Δεt and the crack propagation rate is calculated as shown in Figure 7. A unique relationship is obtained. The crack growth rate is expressed in a dimensionless manner so that it can be applied to various crack shapes. From this relationship, if the amount of applied strain is known, the crack propagation rate can be determined, and the growth rate of the damage amount can be determined.

The feature of the present invention is to evaluate the life of a mechanical structure based on the relationship between the growth rate of minute cracks and the amount of damage to the member, and thereby to control operating conditions and prevent damage to the structure. It is in.

[Embodiments of the invention]

Examples of the present invention will be described in detail below.

FIG. 1 shows a high-pressure stage steam turbine, and shows one embodiment of the present invention applied to a steam turbine remaining life management system. As shown, the steam turbine consists of an upper casing and a lower casing 2. The high-temperature, high-pressure main steam 5 passes through the control valve chamber 3 and enters the first stage blade section 4 . Thereafter, the steam passes through the high pressure stage and enters a reheater (not shown) as high pressure exhaust 6, where it is again heated to high temperature and pressure, and then enters the intermediate pressure stage as reheated steam 7. After passing through the intermediate pressure stage, most of the steam goes to the low pressure stage as intermediate pressure exhaust, and the rest exits the casing as bleed air 9.

Such a casing is a mechanical structure that is exposed to high-temperature, high-pressure steam, and excessive thermal stress is applied to the members when the casing is started or stopped. In addition, it simultaneously suffers fatigue damage due to this repetition and creep damage due to voltage load under high displacement conditions. Therefore, a life management system as shown in FIG. 2 is applied to casings that are subjected to severe thermal stress. Reference numeral 11 denotes a casing meta/I/filling degree detector, and the output of this detector is sent to a thermal strain calculating section 12 to calculate the thermal strain of the casing, for example, using the following equation.

etc = a (Tm TS) where 6 tons, thermal strain on the casing surface, α ke,
Linear expansion coefficient % of the casing material Tm is the average salinity in the thickness direction of the casing, and Ts is the casing surface temperature. T s r Tm can also be calculated by measuring the steam temperature, etc.

The casing surface thermal strain calculated in this way is
The determination is made by the determination unit 13 based on the relationship between the strain and the crack propagation speed shown in FIG. 7, and if it is less than the specified value, no controller m is activated. This specified value is a thermal strain that is within the so-called fatigue limit of the material when the amount of crack growth in the member is sufficiently small within the design life and the growth rate is high enough not to cause any problems. By the way, the calculated strain is
If the temperature exceeds the specified value, the determiner 13 operates and controls the steam temperature control device to control the 4Vc suppression signal in a direction to lower the temperature of the steam being sent. At the same time, the opening degree is suppressed by the control device 15 that controls the opening degree of the steam control valve. Whether the controlled variable has been faithfully executed is constantly monitored by calculating thermal strain by detecting metal temperature, and setting appropriate operating conditions. However, even if the thermal strain is appropriately controlled in this manner, due to load fluctuations and repeated starting and stopping, minute cracks develop and grow on the surface of the member as described above. Reference numeral 18 denotes a crack growth detection device such as a crack gauge using the electric potential/I/method, which is installed on the inner surface of the casing. The crack growth rate is calculated by the crack growth rate calculator 19 based on the ratio of the number of load fluctuations ΔN found by measuring the strain fluctuations.

The crack growth rate Δa/ΔN calculated in this way is
The determining device 13 compares the growth rate of the damage amount with a preset critical crack size, and evaluates the degree of damage according to equation (4)vc. These values are monitored by an alarm display device shown at 20. Furthermore, the determiner 13 performs a life evaluation from the current crack growth rate to the critical crack size, and if the critical crack size is not reached within a predetermined period of use, no control signal will be issued, but if excessive If the damage occurs and there is little margin for the critical crack size, a suppression signal is sent to the steam temperature control device 14 and the steam control valve opening control device in order to suppress thermal strain in order to reduce the crack growth rate. shall be issued. As a basis for this, the relationship between double force strain and crack propagation rate shown in Figure 7 is used. By performing such control from time to time by calculating thermal strain and measuring the growth rate of minute cracks, highly accurate life management can be performed even when operating conditions change.

〔Effect of the invention〕

According to the present invention, damage to structures can be prevented and safety and reliability of the plant can be ensured.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a steam turbine according to an embodiment of the present invention, and FIG. 2 is a diagram showing a life monitoring device for a steam turbine according to the present invention.
From Figure 3! FIG. 7 is a diagram for explaining the present invention in detail. DESCRIPTION OF SYMBOLS 11... Metal temperature detector, 12... Thermal strain calculation part, 18... Crack growth detection device, 19... Crack growth rate calculator, 13... Judgment device, 14... - Steam temperature control device, 15... Steam control valve control device, 16... Boiler, 17... Turbine.

Claims (1)

[Claims] 1. In a method for monitoring the remaining life of a high-temperature structure subjected to repeated loads or variable loads, the thermal strain occurring on the surface of the member is calculated based on the surface temperature of the member and the average temperature, and the thermal strain is determined by a specified value. If it is larger than the above value, the steam control valve and steam concentration are adjusted to control the thermal strain to an appropriate magnitude, and the growth rate is measured using a microcrack growth detector provided on the surface of the member. Calculating the degree of damage and remaining life of the member from the relationship between the thermal strain and the growth rate of minute cracks, and controlling the steam control valve and steam temperature so as to suppress the degree of damage within a set value within a predetermined period. A method for monitoring the remaining life of high-temperature structures. 2. In a remaining life monitoring device for high-temperature structures subjected to repeated loads or variable loads, a detector that detects the temperature of the surface of a member and the amount of crack growth;
a computing unit that calculates the thermal strain and the crack growth rate per number of load changes based on the detected values from the detector; and a computing unit that calculates the thermal strain and the crack growth rate per number of load changes based on the detected values from the detector, and the values from the computing unit, the allowable limit thermal strain value, and the critical crack value that are determined in advance. A determination device that compares the dimensions and determines the degree of damage progression to be suppressed within a set life based on the crack growth rate, and based on the determination, issues a signal for controlling the operation of the turbine and boiler; 1. A remaining life monitoring device for a high-temperature structure, comprising: a steam temperature control device for controlling the operation of the turbine; and a steam control valve control device for controlling the operation of the turbine.