mechanical thermal stresses and creep analysis of boiler …
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MECHANICAL THERMAL STRESSES AND CREEP ANALYSIS
OF BOILER TUBES
1*Tawanda Mushiri
1Lecturer; University of Zimbabwe, Department of Mechanical Engineering, P.O
Box MP167, Mt Pleasant, Harare,
Zimbabwe. *D.Eng. Student; University of Johannesburg, Department of Mechanical Engineering, P. O. Box
524, Auckland Park 2006, South Africa.
Abstract
The boiler tubes are operated continuously at high temperature and pressure. Hence at high
pressure for forces acting on the boiler tubes will be high. This research paper focuses on the
analysis of one of the long term effect of continued application of high pressure on boiler tubes
which causes creeping. By utilizing finite element modelling software, AUTODESK
INVENTOR the effect of pressure with the increase in temperature distribution across the steam
generator tube was evaluated. The increase of heat transfer rate across the wall caused the oxide
scale thickness to grow more rapidly than normal condition. The thermal conductivity in the
boiler tubes, life of boiler tubes and creep damage is also analyzed in this research paper. The
AUTODESK INVENTOR result is analyzed to determine the main and interactive effects of
operating conditions. The effect of steam on boilers and creep damage in comparison with
temperature were researched. Optimum condition identification in order to maximize the
remnant life of the tubes while minimizing the creep damage was done. Creep is the time
dependent deformations that occur when a material is subjected to high level of stresses at
elevated temperature for prolonged period. Matlab was also used to analyse how the failure
occurs.
Key words
Corrosion, fouling, oxide scale formation, embrittlement and graphitization
1.0: INTRODUCTION
According to (Sathyanathan, 2010), a single boiler tube failure in a 500 MW boiler requiring
four days of repair work can result in a loss of more than USD$1,000,000 apart from the
generation loss which is a lot of money which can be saved when critical assessment of the
operation of the boiler tubes is done. A boiler is a closed vessel in which the water is heated up
to convert the water from the liquid phase to superheat steam at specified pressure by addition of
heat in some instances (CleaverBrooks, 2013), the water in the boiler is sometimes used not in
the super heat phase but it will be hot still. Apart from creep other boiler failures occurs due to
corrosion, graphitization, oxide scale formation, slagging and foaming of tubes and caustic
embrittlement. The normal failure type for water tube boiler is the creep (General Electric
Company, 2012), (Karim, Zamani and Shafii, 2013)
Creep is more severe in materials that are subjected to heat for long periods since creep is the
time dependent deformations that occur when a material is subjected to high level of stresses at
elevated temperature for prolonged period so when the time the boiler is exposed to much heat
and pressure increases then the chances of creep are then automatically high and near melting
point (ASME, 1918). Creep always increases with temperature. Unlike brittle fracture, creep
deformation does not occur suddenly upon the application of stress. Instead, strain accumulates
as a result of long-term stress. Creep deformation is “time-dependent” deformation. In order to
encounter this problem simulation programs are widely used to simulate and analyze the
performance of the boiler/steam generator (Karim, Zamani and Shafii, 2013).
A. METHODOLOGY
Simulation software AUTODESK INVENTOR is used to help in simulating the creep rupture
behavior within the steam generator tubes and this data is helpful for preventive maintenance. In
power plant industry basically there are three types of steam cycle that used sub critical steam
cycle, supercritical steam cycle and ultra-supercritical steam cycle. Apart from creeping
graphitization is also caused by operating at high temperatures where by a weak graphite
structure which can break easily so when addressing the effects of creeping we will also be
addressing the effects of graphitization. For sub critical steam cycle power plant, the boiler/steam
generator is operated below critical point of water that is, at a pressure of 22.12 bar and
temperature of 374.15OC (Paakkonen and Sauvula, 2014) . For super critical steam cycle power
plant, the steam generator can be operated above the critical point of water (Sathyanathan, 2010)
and (LARSEN & TOUBRO LIMITED, 2015). Ultra super critical steam cycle power plant
operates at high pressure and temperature which is above 593OC. In sub critical and super critical
power plants the boiler tube materials start to deform wall thinning and creep damage analysis in
boiler tube and optimization of operating conditions at a temperature around 593OC. It is due to
metallurgy of the tube materials moreover, prolong exposure to high temperature and high
pressure, accelerate creep damage due to the presence of preexisting crack, scaling, and
hydrogen (ASME, 1918).
The Larson–Miller relation, also widely known as the Larson-Miller Parameter and often
abbreviated LMP, is a parametric relation used to extrapolate experimental data on creep and
rupture life of engineering materials ( (Bell, 1997). By applying Larson Miller Parameters (LMP)
equation:
Equation 1: Larson-Miller Parameter
𝐿𝑀𝑃 = 𝑇[𝑙𝑜𝑔𝑡𝑟 + 𝐶] Where;
LMP is the Larsen-Miller Parameter;
T is the absolute temperature in degrees Rankine (0F+460);
t is the rupture time in hours;
C is a material specific Constant (.
The constant C is typically found in range of 20 to 22 for metals. The LMP is derived from the
Arrhenius type equation which states that:
Equation 2: Arrhenius Equation
𝑟 = 𝐴. 𝑒−𝛥𝐻/(𝑅.𝑇)
The remaining life of the tube decreases as the oxide scale increases. The life time of the tube
also decreases with longer heating time under high temperature
Fig 1.0: Short term overheating, Long term over heating & Thermal Fatigue failure
B. AIM
Analysis of thermal stresses and creep of boiler tubes to minimize the effect of creeping of the
boiler tubes.
C. OBJECTIVES
Analysis of creep failure time of tubes in reference to operating pressure and temperature
Modeling of the tubes as an end constrained thick wall cylinder subjected to internal and
external pressure at elevated temperature.
2.0: LITERATURE SURVEY
Bailey is the pioneer in the study of design aspect of creep; in 1935 he proposed general
expression for creep in terms of principal stresses based on simple tension test ( (Bailey, 1935)
(Riordan, 2006). Bailey did many several to verify the validity of those general creep expressions
and got good agreement. Satisfactory estimate of a correlation of tension creep test with
relaxation tests was made by Popov in his PhD dissertation work (Popov, 1947). The validity of
methods for simple relaxation is demonstrated on the bases of experimental agreement between
calculated and test results. The formulations for tension creep curves were established. Then the
analytical schemes that appear satisfactory are extended for relaxation with the elastic follow-up.
The consideration of primary creep in the design of internal-pressure vessels was proposed by
Coffin et al (Coffin, 1949). In their paper the evaluation of permanent strains and stresses at a
particular time resulting from loading a thick-walled cylinder under constant internal pressure
and elevated temperature when account is taken to the primary creep characteristics of a given
material. The results are compared with permanent strains obtained by considering secondary
creep as the general bases for pressure vessel design. Till then the commonly accepted bases for
design of pressure vessel at elevated temperatures has been by the use of tensile secondary-creep
data applied to combine steady stress such as the method used by Bailey (Bailey, 1935). They
show how the tensile primary creep characteristics may be utilized in the design of thick-walled
pressure vessels. In other words, they showed the stress-deformation history of the tube from the
time when the pressure is applied initially until its life expectancy, or to the time when steady-
state
Conditions corresponding to secondary creep are reached. Yoh-han Pao and Marin formulate on
analytical theory of creep deformation of materials (Popov, 1947). The theory was proposed for
idealized materials and may be applied to those materials whose behavior conforms to that
idealized material. In the theory, the initial elastic strain, the transient creep strain, and the
minimum rate creep strain are taken to account. Creep analysis of axisymmetric bodies using
finite elements for calculating the creep strains was developed by Greenbaum and Rubinstein
(Popov, 1947). The method involves starting with the elastic solution of the problem and
calculating the creep strains for a small time increment. Those creep strains for a small time
increment. Those creep strains are treated as initial strains to determine the new stress
distribution at the end of the time increment. Next an outline of some of the available literatures
from 1975 to date was done. Lower bounds on rupture times of thick-walled tube in pure torsion
and a hollow sphere under constant internal pressure were obtained by Goel and numerical
values of rapture time for the tube case with different forms of damage rate lows were presented
(ASME, 1918).
The type of damage law assumed makes a significant effect on predicting the time to rapture but
failure in all cases occurs almost instantaneously after the appearance of first crack. A structural
element in creep may rupture in any of the two modes of failure, namely, ductile and brittle.
When a structural component is subjected to high stress levels failure may occur due to the
geometric instability caused by necking; such a failure is called ductile failure. On the other
hand, structures at low stresses and high temperatures may exhibit brittle failure. It happens due
to the degradation of the microstructure of the material. Fissures and voids are usually found
where such a failure occurs and these voids and fissures grow on planes which are perpendicular
to the direction of the maximum principal stress.
When metals are subjected to stress at temperatures in excess of 0.33mT where m T is the
absolute melting temperate, the metal suffers time-dependent creep deformations (Gateway,
2014). In addition, internal damage increases with time and ultimately the metal ruptures.
Therefore, when designing shell structures operating at such evaluated temperatures,
consideration must be made to ensure that creep deformations do not exceed operational
requirements during the life of the component (Popov, 1947). There are six major groups into
which all tube failures can be classified. These six groups can be further divided in to a total of
twenty-two primary types. All high pressure boilers commissioned and put into operation go
through a stabilization period, during which some teething problems occur, including a few tube
failures.
A. CLASSIFICATION OF TUBE FAILURES (Sathyanathan, 2010)
Tube failures are classified as in-service failures in boilers. The failures can be grouped under six
major categories
Stress rupture
Fatigue
Water side corrosion
Erosion
Fire side corrosion
Lack of quality control.
B. TUBE FAILURE DURING STABILIZATION PERIOD
The tube failures in a boiler during initial phase of operation are different from the types that
occur after prolonged operation (ASME, 1918). During the initial period of operation of boiler
the type of tube failures seen are short term overheating, weld failures, material defects, chemical
excursion failure, and sometimes fatigue failures. The short term overheating failure is mainly
due to blockage in the fluid path by some foreign material which gets into the tube surface
during fabrication or during erection of the unit (Bell, 1997). The blockage can also happen
when debris after acid cleaning of the boiler is not removed completely. This failure can be
visually identified by its characteristic appearance of a fish-mouth-like opening and so is also
called as fish mouth failure.
C. RESEARCH GAP
In most analysis, the effect of the stresses weren’t addressed well and therefore there is a gap in
the analysis of forces in the weak metallic structures after the heating and after the metals is
subjected to very high temperatures in which the research will focus on.
(Coffin, 1949) Coffin concluded that in the analysis of a boiler tube and optimization of
operating conditions tubes which are subjected to high temperature, there was more formation of
oxide scale in inner part of the tube surface. Eventually it reduced the tube wall region as the
oxide scale increased. The long term overheating exposure, under high temperature will increase
the formation of oxide scale thickness and will reduce the remaining life of the tube. Hence this
will increase the creep rupture damage (ASME, 1918). Fluctuation of load in boiler operation
will also bring a significant damage to the boiler due to creep damage. He stated that, steam
temperature affected more compared to flue gas temperature as it introduced more oxide scale
inside the boiler tube and the scale growth limiting heat transfer from the outside and causing the
wall tube temperature beyond its normal limit and finally the tubes failed. However, if the steam
generator is operated under optimized conditions, the remnant life can be increased and creep
damage can be reduced, thereby reducing the losses in boiler tubes operation.
Fig 2.0: Schematic Diagram of a Boiler (CleaverBrooks, 2013)
As shown above reheater and superheater tubes in the boilers are exposed to a series of problems
that easily lead to tube failure at high temperature. Generally the problems can be divided into
two categories, which are corrosion related problems and mechanical related problems. The
typical mechanical related problems are creep fracture and overheating while corrosion related
problems encountered in superheater and reheater tube is fireside corrosion. There are many else
of failures occur in different components of boiler tube. However, merely few failures that have
highlighted are of interest in this research.
Fig 3.0: Microstructure of Creep Fracture Mechanisms (Koh, 2002)
Fig 4.0: High temperature on the interior of the boilers (ASME, 1918)
The illustration above shows the effects of the high temperature on the interior of the boilers and
with higher temperatures there is a higher possibility of creep since the internal structures of the
boiler tubes are weakened by the oxidation and the formation of a weak iron oxide layer (rust)
due to the presence of heat and the oxygen is found in the super-heated steam. With the increase
in the time the tube is exposed to such conditions, the oxide layer grows and eventually the tube
fails (Goel, 1975).
3.0: METHODOLOGY
Life expectancy of superheater and reheater stainless steel boiler tubes was predicted by using
the Auto Desk Inventor 2014. Autodesk inventor was used to study the integrity of the boiler
tubes since the superheater and reheater tubes are operated at very high temperature over a long
period of time, the life prediction of the tube can be made as a function of tube temperature,
operating pressure and time.
In the iterative analysis, the performance and the effect of certain parameters of the boiler tube is
evaluated by varying several key parameters, which includes the tube geometry, wall thinning
effect and several other operation parameters. After going through the literature review to
understand on how the development of oxide-scale, wall thinning and other concepts that affect
performance of boiler tubes, the iterative analytical method is implemented into the MATLAB
code. In this research, the boiler tubes are considered as thin walled vessels.
4.0: RESULTS
After the simulation by using Autodesk inventor it was observed that:
Since; 𝑃 = 𝐹/𝐴
Where;
P is the pressure,
F is the force,
A is the area of the boiler tube and hence the area was considered to be constant since the tubes
always have the same area unless they are breaking then pressure X area represent the force
But since the area is constant, then the force due to the pressure is directly proportional to the
force. In the analysis therefore the effect of prolonged effect of force was done to predict the
maximum pressures on the boiler tubes and the results of the analysis are shown below. The
following results were obtained from Autodesk inventor.
And also,
Equation 3: Hoop stress with respect to pressure
𝜎ℎ = 𝑝(𝑟𝑖 +
𝑡2)⁄
𝑡
N/B: In the analysis most emphasis was on the effect of stress where by
𝜎ℎ= hoop stress, MPa
p = operational internal pressure, MPa
ri = inner radius of the tube, m
t = wall thickness of the tube.
A. AUTODESK INVENTOR RESULTS
Fig 5.0: The boilers on the application of pressure
Fig 6.0: The mesh view of the boiler tubes
Fig 7.0: The Von Mises diagram
Fig 8.0: The displacement factor diagram
Fig 9.0: Third principle stress diagram
From the finite element analysis, the maximum force due to the pressure on the boiler tubes is
24Mpa.
B. MATLAB RESULTS
Matlab was used therefore to predict the time for the failure of the boiler tubes and hence the
effect of creep with time was then analyzed
Fig 10.0: Matlab based results of the boiler
The graph above therefore shows the effect of temperature with time and the analysis was done
using Matlab. And the graph was therefore plotted using the plot command using matlab
5.0: RECOMMENDATIONS AND CONCLUSIONS
The boiler tube analysis can be improved by involving other parameters like the effect of time
hence predicting failure on a more realistic approach or using the iterative analytical method
which is numerical estimation using a time step and can only perform analysis for one
dimensional geometry. The Autodesk inventor basically shows the finite element approach of
failure of materials if a certain stress is reached. Richard Edler von Mises as shown in Fig 7.0
shows the maximum stress that can be reached before failure and what it means is not to allow
the von Mises Stress material to surpass the yield stress.
6.0: FURTHER RESEARCH
The scope of the research can be extended by considering the thermal strain experienced in the
tube. The development of thermal stress and thermal strain as a function of temperature, pressure
and time can also lead to boiler tube failure. The thermal strain generated can be studied in
relation to the thermal expansion and temperature and pressure loading of the boiler tube.
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