Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)24

Development of Boiler Life Estimation System

KOUJI SASAKI*1

YOSHIMICHI KAWAKAMI*2

MASAKAZU KAMIBAYASHI*1 KENJI YAMADA*1SHIGEHIRO SATOU*3NOBUHIRO OSAKI*4

Since many power plant boilers in operation have already reached 150,000 to 350,000
hours of operating time, determining the amount of remaining life is becoming increasingly
important to prevent the problem of boiler tube leakage due to long-term operation. It has so far
been difficult, however, to logically determine the appropriate time of inspection, resulting ingreat cost and effort. Based on a statistical analysis of inspections over more than 30 years,
with data classified by boiler specifications such as operational age, type of combustion, designpressure and design temperature, etc., Mitsubishi Heavy Industries, Ltd. (MHI) has created aboiler life estimation system intended to direct the determination of detailed inspection times bypredicting the tube creep remaining life and the remaining life of tube thickness reduced bycorrosion. This paper introduces an outline of the system.

|1. Introduction
As boiler plants show age deterioration through long-term operations, the remaining life ofeach section must be evaluated and necessary repairs – as well as the periodical replacement ofcomponents – must also be implemented. This allows the prevention of accidental failures andproblems and enables continuous, safe, long-term operation until the next planned shutdown.
Accordingly, as a support tool for determining appropriate inspection times, we have createda remaining life estimation system, i.e., a creep remaining life estimation for high temperatureparts (superheater tubes and headers) that operate at temperatures around 500 ºC in which tube
thickness reduction is taken into consideration and a corrosion remaining life estimation of tubes(water tubes, furnace tubes and economizer tubes) that operate at around 300 ºC, based on boiler
type and operational characteristics. Furthermore, this system has another feature that assists withinspection planning by displaying the key inspection points of a particular boiler using a databaseof past damaged examples. The contents of this system are described below.

|2. Input of boiler specifications
Figure 1 shows the login screen of the system. A user of the system can make evaluations
and change input conditions while accessing the administrative server over the Internet andconsulting with the boiler user.
Figure 2 shows the input screen of the particular boiler type and its operational
specifications: identification name of boiler, boiler type, steam pressure, steam temperature,
combustion system, fuel and completion year, etc. These specifications constitute some of theparameters for the tube creep and corrosion evaluations, which are described below and areregistered as individual boiler data. These data can be utilized as necessary.

*1

Yokohama Research & Development Center, Technical Headquarters*2

Manager, Yokohama Research & Development Center, Technical Headquarters*3

Manager, Yokohama Machinery Works*4Manager, Power Systems Service Management Department, Power Systems Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)
25In addition, on the screen below to the right, the number of past incidents of damage in thefurnace tube, superheater tube and header, etc., of the same type of boiler is displayed based on theoperational age of the particular boiler being evaluated.

Figure 1

Login screen of system

A user of the system can make evaluations and change
input conditions while accessing the administrative
server over the Internet and consulting with the boiler
user.

Figure 2 Input screen of the particularboiler type and its operational
specifications
When selecting the boiler type, a drawing of theselected boiler type is displayed automatically. Input
data such as steam pressure, steam temperature andfuel, etc., are subsequently reflected in the creep andcorrosion lifetime estimation of the boiler.

|3. Extraction function of past damage examplesWhen selecting the “number of past incidents of damage” section on the above screen, theoperational age, the frequency of damage and the replacement results of each section are displayed
in a column chart (Figure 3) and the current operational age of the particular boiler being
evaluated is also indicated by an arrow, so the positioning of the particular boiler is confirmed
compared with the frequency of past damage. By using the tabs at the top of the screen, a list of
past damage examples can be selected (Figure 4). When selecting the targeted point from the list,a situational photograph, the causes and a summary of the countermeasures for the particulardamage at the targeted point are displayed (Figure 5), which helps the user understand thephenomenon that occurred.

Figure 3 Operational age vs. frequency ofdamage and replacement results(example for the superheater tube)
The relationship between the operational age andfrequency of damage by factor and the replacement
results of the superheater tube can be displayed. The
operational age of the particular boiler is also indicatedby an arrow in the figure.

Figure 4 Screen to select damage examples
for each point
Points where incidents of damage are likely to occur
are displayed. When selecting a point, concrete
examples of damage are displayed (Figure 5).

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)
26

Figure 5

Display of damage examples

Past damage examples, their causes andcountermeasures are indicated for each selectedpoint.

|4. Creep remaining life estimation for superheater tube
The creep remaining life estimation for the superheater tube is conducted according to thefollowing procedures.
(1) Input data
Input the nominal thickness, outside diameter and material (selected from a list) for thesuperheater tube, in addition to the boiler type and operational age described in chapter 2.
(2) Evaluation factors
The following evaluation factors are automatically selected by the system, where a
database is created based on an analysis of in-house and external data, taking into consideration
the tube material and boiler type that were entered.
(i) Larson-Miller parameter constant for each material(ii) Corrosion rate equation, where design temperature is a parameter(iii) Scale deposit rate equation, where design temperature is a parameter(iv) Temperature correction coefficient based on past examinations of cutting tubes(3) Life estimation method
In each year from start up, tube-temperature increase values affected by scale thickness
and tube-stress increase values affected by tube thickness reduction by corrosion are calculated
and then the creep damage rate for each year is totaled. The point where the totaled damage
rate exceeds 100% is determined as the remaining life. In addition, the relationship between thetube temperature and scale deposit rate is exemplified in Figure 6 and the relationship between
the tube temperature and tube thickness reduction rate is exemplified in Figure 7. As suchdiscrepancies in the data for each evaluation factor are taken into consideration, the remaining
life evaluation is calculated as the range of the upper limit and lower limit.
Figure 8 shows an example of the remaining life evaluation results.

Figure 6
Example of the relationshipbetween tube temperature and scale
deposit rate for the superheater tube
Boilers that have a high design temperature have atendency toward considerable scale deposits in thesuperheater tube, which contributes to the increase oftube temperature and toward a reduction in the creep
remaining life for the superheater tube over time.

Figure 7
Example of the relationshipbetween tube temperature and tube
thickness reduction rate for thesuperheater tube
Boilers that have a high design temperature have atendency toward considerable tube thickness
reduction in the superheater tube, which contributes
to the increase of tube stress and toward a reductionin the creep remaining life for the superheater tube
over time.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)
27

Figure 8 Example of creep remaining
life estimation for superheater tube
When selecting evaluation conditions such as
tube material, etc., the operational age versus
scale deposit thickness and tube temperature
variation due to heat transfer deterioration aredisplayed, as are the operational age versus tube
thickness reduction by corrosion and tube stressincrease. Furthermore, the creep remaining life
estimation results are displayed withconsideration for the above.

|5. Corrosion remaining life estimation of water tubes, furnace tubes
and economizer tubes
The corrosion remaining life estimation of water tubes, etc., is conducted according to thefollowing procedures.
(1) Input data
Input the nominal thickness, outside diameter and material (selected from a list) of theparticular tube, in addition to the operational age and combustion system, etc., as describedabove.
(2) Evaluation factors
Based on automatic selection by the system, a corrosion rate equation that varies with thecombustion system and fuel (heavy oil, multi-fuel, gas and coal, etc.) is selected for each tube,
i.e., water tubes (generating tube), furnace tubes and economizer tubes. In addition, if the boiler
is situated outdoors, exterior corrosion of the tubes due to rainwater may occur. In this case,when selecting an installation location as outdoors when entering boiler specifications inchapter 2, a rainwater corrosion evaluation is also conducted.
(3) Life estimation method
First, according to JIS B 8201, an allowable stress value (σa) corresponding to the designtemperature for each tube material is extracted and then the required thickness of the tube (tsr1)
is calculated using the allowable stress value (σa), the outside diameter of the tube and thedesign pressure of the tube, pursuant to the same standards. In a comparison between therequired thickness of the tube (tsr1) and the minimum thickness of the tube (tsr2) differently
determined corresponding to the outside diameter of the tube, the greater value is taken as therequired minimum thickness of the tube (tsr).
Furthermore, the tube thickness reduction by corrosion is calculated for each fuel and foreach combustion system and, in the case of outdoor installation, the tube thickness reduction by
rainwater corrosion is also calculated. The point of time where the tube thickness falls below
the required minimum thickness of the tube (tsr) is determined to be the lifetime of the tube.
Figure 9 shows an example of input data and evaluation results described above.

Figure 9 Example of corrosion
evaluation of water tube,
furnace tube and economizer
tube
When selecting conditions such as
combustion system and fuel, etc., the tube
thickness change versus operation age isdisplayed. The point of time where thetube thickness falls below the required
minimum thickness of the tube isdetermined to be the lifetime of the tube.

Mitsubishi Heavy Industries Technical Review Vol. 47 No. 2 (June 2010)28|6. Creep remaining life estimation of header and main-steam pipe
The creep remaining life estimation of the header, etc., is conducted according to thefollowing procedures.
(1) Input data
Input the particular location and tube material (selected from a list), in addition to theoperational age, etc.
(2) Evaluation factors
Based on automatic selection by the system, an equation for the creep remaining life
consumption rate for the header and main-steam pipe is selected, where a database is createdby statistical analysis based on conventional inspection results for each material.
(3) Life estimation method
The range of life is calculated using both the upper and lower limit equations of the creep
remaining life consumption rate.
Here, the inspection results data of the header and main-steam pipe for this evaluation
database are acquired from the “damage classification method,” the “number density of creep
cavity method” and the “A parameter method,” etc. Therefore, since it is considered that thismethod has a greater error of estimation than that of creep remaining life estimation for thesuperheater tube described above in which sample tubes have been drawn and creep tests havebeen conducted, the 50% point of the remaining life obtained by this method is used as theestimated remaining life, according to “the guidelines concerning remaining life evaluation” by
the Ministry of Economy, Trade and Industry.
Figure 10 shows an example of input data and evaluation results described above.

Figure 10 An example of creep remaining
life estimation of header andmain-steam pipe
When selecting conditions such as tube material,etc., the creep remaining life estimation results aredisplayed.

|7. Conclusion
We have created a boiler life estimation system to predict the creep remaining life of
superheater tubes, etc. and the corrosion remaining life of water tubes, etc., by using operating
conditions such as the boiler type, operational age, fuel and combustion system, etc., based on theextensive inspection and replacement data obtained from 1,500 cases of industrial boilers overmore than 30 years. This system allows us to predict the future risk of malfunctions in boilers
currently in operation prior to carrying out detailed inspections, helping in the creation of practical
maintenance plans.
In addition, since the number of new boilers, such as biomass boilers, etc., has increased
recently, we are also accumulating data regarding these new boilers. We are committed tocontinuing to meet the needs of various maintenance services in the future.

References
1.
2.
3.

Japanese Industrial Standard JIS B 8201 (2005)
Nishimura et al., Mitsubishi Power Boiler and Piping Life Assessment System to Extend Legal
Inspection Interval, Mitsubishi Juko Giho, Vol. 37 No.1 (2000) pp. 46-49
The Guidelines Concerning Remaining Life Evaluation, NISA-234c-05-7 (2005), by Nuclear andIndustrial Safety Agency, Ministry of Economy, Trade and Industry