JPS5817327B2 - Thermal stress accident protection method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for protecting against accidents due to thermal stress, and particularly to a method for protecting against turbine thermal stress accidents suitable for use in turbines that require rapid startup/shutdown and large load changes.

In high-temperature, high-pressure steam turbines, there are many problems caused by thermal stress and uneven heat distribution that occur during startup and shutdown or load changes.

Examples include strength failure of the casing or rotor due to thermal stress, low cycle fatigue failure due to repeated stress, creep love due to residual stress, and contact accidents due to elongation of the casing and rotor due to uneven heat distribution.

Conventionally, measures have been taken to prevent these accidents, such as (1) reducing the thickness of the casing, (2) tightening control conditions and operating with a margin.

However, due to power supply conditions, rapid startup/stopping and large load changes are now required, and operation with a narrower margin from the limit value has become necessary.

Therefore, in order to meet the demands as much as possible and ensure safe operation, it has become necessary to monitor restrictions such as thermal stress more strictly than before.

Furthermore, as mentioned above, thermal problems can lead to various accidents, so it is necessary to comprehensively deal with the problems caused by them.

The purpose of the present invention is to solve the above-mentioned drawbacks of the prior art, to prevent accidents mainly caused by thermal stress, and to provide a turbine thermal stress accident system that is highly safe, and also enables rapid start-up and shutdown and large load changes. It is to provide a protection method.

The features of the present invention are as follows.

(1) Continuously monitor various limiting conditions within the turbine caused by thermal stress and uneven heat distribution, make comprehensive judgments, and determine operational targets (allowable load change range, load change rate set value, etc.).

(2) Monitor each limiting condition according to the type of accident and set limits on driving targets separately.

(3) Introducing the concepts of priority and urgency among restrictive conditions to coordinate requests for driving freedom and accident prevention;
It is possible to realize a system that is safe and can respond to changes in load.

The present invention will be explained below with reference to the drawings.

In realizing the present invention, it is of course convenient to use a control digital computer.

However, since it is thought that the invention will be easier to understand if explained in a bird's manner, the following explanation will be given assuming that the apparatus is constructed using analog dedicated cards.

Therefore, when the present invention is realized by a computer,
In the following description, it is sufficient to consider "devices" that perform independent functions as units or steps of each digital calculation.

FIG. 1 is a block diagram showing the basic concept of the present invention.

In the figure, 1 is a thermal stress calculation device, 2 is a life consumption rate calculation device, and 3
4 is a creep amount calculation device, 4 is an elongation difference calculation device, 5 is a thermal stress strength fracture prevention device, 6 is a life consumption rate management device, 7 is a creep rubber prevention device, 8 is a contact prevention device between the turbine stationary part and the rotating part. , 9 is an output management device, 10 is a conventional plant control device, 100 is an actual load signal, 101 is a load request signal, 102 is a main steam pressure signal, 103 is a main steam temperature signal, 104 is a casing inner and outer surface temperature signal, 105 106 is a thermal stress predicted value signal, 106 is a thermal stress current value signal, 107 is a life consumption rate signal, 108 is a creep amount signal, 109 is an elongation difference predicted value signal, 110 is a load change rate limit value signal, 111
112 is a load limit value signal, 112 is an allowable thermal stress signal, 113 is a load change rate setting value signal, 114 is a load setting value signal, 11
5 is a differential expansion signal, 116 is a reheat steam temperature signal, 117 is a high pressure exhaust chamber temperature signal, and 118 is a low pressure exhaust chamber temperature signal.

Below, the word "signal" will be omitted for each signal.

Next, the operation and effects of the accident protection device configured as described above will be explained.

FIG. 2 is a block diagram showing a thermal stress calculation method.

First, in the steam flow rate predictor 11, load 100, load request 1
01, the main steam flow rate change is predicted in time series using the load change rate set value 113.

Next, using the steam table 12, the main steam enthalpy 201 is estimated from the main steam pressure 102 and the main steam temperature 103, and the predicted main steam flow rate 2
00 and the enthalpy 201, the turbine first stage steam temperature (θst) 202 is estimated.

Next, the thermal stress σ is estimated using the following equation.

Here, E: Young's modulus β: linear expansion coefficient ν: Poisson's ratio' (9mS: surface metal temperature θm: metal average temperature, also surface meta), temperature θ age and metal average temperature θ□
is found as a solution to the following differential equation.

Here, M: Mass of the turbine C: Specific heat a: Fuel transfer coefficient A: Heat transfer area λ: Thermal conductivity d: Turbine metal wall thickness θml: Metal intermediate temperature OmB: Metal bore temperature θ81: Turbine first stage steam temperature n: Number of divisions Next, the thermal stress prediction maximum value determiner 15 compares the thermal stress calculated using equation (1) with the maximum value calculated previously, and if the current value is smaller, the previous value is used. Maximum predicted thermal stress value σp
Output as max 105.

In addition, if the previous value is larger than this time, the maximum value storage device 1 is used to calculate the next load change or thermal stress at startup.
Clear 6.

Conversely, if the current value is larger, the contents of the maximum value storage device are replaced and a recalculation command 206 is output.

Next, a method for calculating the life consumption rate Lf is shown in the following equation.

Here, al: constant determined from the material and structure of the turbine m: constant i: i sample number Next, a method for calculating the creep amount e will be shown.

FIG. 3 is a block diagram showing a creep amount calculation method.

First, the strain calculation device 17 calculates the strain amount (ε) 207.

The calculation formula is shown below. Next, in block 18, this strain amount ε(i) 207, thermal stress σ(i), metal average temperature θm(i), function g: a function determined from the material of the turbine, and further, in block 19, this Assuming that the creep rate e (-→ continues for ΔL time), determine the creep increment (Δe) 209.

Therefore, the creep amount 108, ie, e(i), is determined by the following equation.

Next, we will show how to predict the difference in elongation.

FIG. 4 is a block diagram showing a method for predicting the difference in elongation.

First, in block 21, the steam temperature distribution in the turbine (θaj
)210.

Assuming that the inside of the turbine is divided into n parts and decreases linearly from the inlet to the outlet, the steam temperature distribution is expressed by the following equation.

Here, θaj ' Steam temperature of the j-th divided region θsn: Turbine outlet steam temperature Next, the metal temperature distribution of the turbine casing (Om,
j) 211 and rotor metal temperature distribution (θmcj)2
12 is estimated in block 22.

Estimation is performed by solving the following equation.

Here, k: Constant for correcting mass M: Mass C: Specific heat a: Heat transfer coefficient A: Heat transfer area suffix r: Rotor C: Casing m: Metal S: Steam j: Division position number Next, (8 ) formula, calculate the elongation difference (△1) from the temperature distribution of the metal.
109 is estimated in block 23.

The estimation follows the following formula.

Next, methods for preventing accidents under the various limiting conditions described above will be described.

First, the thermal stress strength fracture prevention device 5 operates under σ>σ1 (event condition) and takes the following measures.

(See Figure 5) (1) If σpmax > σ2 (target thermal stress), then σp
maX possible.

Limit the load change rate so that

(2) If σpmax>σ8 (elastic limit value), limit the load setting value.

(3) If σpmax>σ4 (−〇, 9 a5 ), trip the turbine.

Next, the life consumption rate management device 6 takes the following steps.

(1) If Lr(i)<Lfr(i) (standard life consumption rate curve: see Figure 6), output the thermal stress determined by the 1-cycle standard life consumption rate △L f r as the target thermal stress σ2 (the standard life consumption rate curve: see Figure 6). (See Figure 7).

(2) If Lf(season>Lfr(i), then △Ltr
A target thermal stress σ2 determined by K(I, f(i)−Llr(i)) is determined and output.

(3) Lf(i)>Lfr(i)+△Lfrt(1
)......(If △L'frL (1" allowable width),
Limit the load change rate setting value.

In addition, in the creep-rubbure prevention device γ, the amount of creep is 1
Monitor 08 and determine the margin time until creep love (8th
Depending on the magnitude of (see figure), (1) alarm, (2) load change rate restriction, and (3) reverse operation instruction are taken.

In addition, the contact prevention device 8 monitors the predicted expansion difference value 109 and compares it with a predetermined expansion difference limit value to take measures such as (1) limiting the load change rate and (2) instructing a reverse operation.

Finally, the output management device 9 integrates the outputs of each unit accident prevention system described above, and determines the priority of the output request (an evaluation factor determined by the degree of damage caused by the occurrence of the accident) and the degree of urgency. (an evaluation factor determined by the amount of time left until the accident occurs).

For example, the priority is determined as follows.

In other words, in the thermal stress accident protection method, reverse operation〉load limit〉
Load change rate limit> alarm.

If the priority values are the same but the request values are different, the request value with the highest degree of urgency is selected and output.

Therefore, the effects of the present invention are listed as follows.

(1) By installing an output control device that summarizes the request signals from each prevention device against turbine accidents caused by thermal stress and uneven heat distribution, and selects those with high priority and urgency, the Mutual interference that occurs can be eliminated, and flexible and appropriate measures can be taken depending on the plant status.

(2) Since the limit conditions are monitored on the accident side, the margin from the limit value can be reduced, enabling highly efficient plant operation.

(3) It is easy to configure an automated system including accident protection.

(4) Since a prevention device is provided for each accident, the construction of the accident protection system is easy.

In the above explanation, everything has been explained using a dedicated analog device, but as mentioned earlier, it is more convenient to realize the present invention by using a control digital computer than by using a dedicated analog device.

In this case, there is no need to specifically show a flowchart, and the calculation process corresponding to each accident unit may be performed as in this embodiment.

(1) The method of installing a prevention device for each accident, integrating the request signals from each prevention device, and determining the output based on priority and urgency is a method for preventing accidents not only for turbine thermal stress but also for various plants. Can be used.

(2) In this system, the evaluation level for output determination is 2.
Levels (priority, urgency) are set.

This means that accidents caused by turbine thermal stress are long-term (lifetime consumption rate, creep) and short-term (thermal stress,
This is because it is difficult to unify them into one evaluation level because they are divided into two types (difference in growth).

However, for general target plants, either evaluation method is often sufficient.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a block diagram showing a method for calculating thermal stress, Fig. 3 is a block diagram showing a method for calculating creep amount, and Fig. 4 is a block diagram showing a method for calculating the amount of creep. A block diagram showing how to calculate the difference, Figure 5 is a characteristic diagram of stress and strain, Figure 6 is a standard life consumption rate curve, Figure 7 is a relationship diagram between 1 cycle life consumption rate and thermal stress, Figure 8 is a creep characteristic diagram. Explanation of symbols, 1: Thermal stress calculation device, 2: Life consumption rate calculation device, 3: Creep amount calculation device, 4: Elongation difference calculation device, 5: Thermal stress strength fracture prevention device, 6: Life consumption rate management device, 7: Creep love prevention device, 8: Contact prevention device, 9: Output control device.

Claims (1)

[Claims] 1. Calculating the current value of thermal stress generated in the plant and the predicted value after a predetermined time from the actual load signal of the plant, the load request signal to the plant, and the signal indicating the status of thermal energy of the plant, the current value of thermal stress a step of calculating the lifetime consumption rate of the plant from the current value of thermal stress and the factors that cause thermal stress in the plant; a step of calculating the amount of creep occurring in the plant from the current value of thermal stress and the factors that cause thermal stress in the plant; A step of calculating the elongation difference, and outputting an operation signal determined by the magnitude relationship between the thermal stress, life consumption rate, creep amount, and elongation difference calculated in each step and the limit value set in advance for each of the above. and selecting one of the operation signals according to a predetermined priority and outputting it as an operation signal for the plant.