Creep Life Estimation of T-22 Reheater Tubes Abhishek Chittoria

Department of Mechanical and Industrial Engineering,

Indian Institute of Technology, Roorkee, India- 247667.

Email Id: abhi.iitr08@gmail.com

Abstract

Reheater tubes of a boiler are exposed to elevated temperature which causes the loss of creep properties of the tube material. Most of the boilers operate at a temperature above 480 degree Celsius, at elevated temperatures an oxide layer starts to form in the tube which increases the temperature of the tube. After certain cycles of exposure the metal starts to deteriorate under stresses and unwanted dimensional changes starts to take place. Under these conditions failure is inevitable. This paper focuses on analyzing various methodologies for damage life assessment and estimation of creep life of the reheater tubes of 2.25cr- 1mo steel.

Introduction

Boiler tubes are generally exposed to higher temperature the superheated steam has a temperature of around 540 degree Celsius. Exposure of tube material at such high temperature causes deformation due to thermally induced stresses. For a SAE-213 grade T-22 steel tube the formation of oxide layer causes more heat to be taken up by tube to maintain heat transfer between flue gas and the steam as a result the temperature of the tube rises and it reaches a temperature where it starts to lose its creep properties known as the excursion temperature. Every time the temperature exceeds excursion temperature the properties of metal is affected. Prolonged exposure to high temperature causes deformation of tube and eventually the tube fails. The strength of boiler tube depends on two factors that are temperature and stress. If any one of these factors exceeds it leads to reduction in rupture time so for a careful analysis of rupture mechanism requires the close attention to these factors.

The stress failure rupture mechanism is classified into: short term overheating and High-temperature creep. The short term overheating refers to exposure of tube material to a temperature higher than excursion temperature for a period of time which is sufficient enough to cause failure of the tube. Tube failure can also occur by high temperature creep. As mentioned above increase in temperature or stress accelerates the creep rate and results in early failure than expected.

Tube Steel Type

ASME specification no.

Babcock and Wilcox◦F(◦C)

Combustion Engineering ◦F(◦C) ASME◦F(◦C)

Riley Stoker◦F(◦C)

Carbon steel SA-178 C 950(510) 850(454) 1000(538) 850(454)Carbon steel SA-192 950(510) 850(454) 1000(538) 850(454)Carbon steel SA-210 Al 950(510) 850(454) 1000(538) 850(454)C-M0 SA-209 Tl … 900(482) 1000(538) 900(482)C-M0 SA-209 Tl a 975(524) … 1000(538) …Cr-Mo SA-213 T11 1050(566) 1025(552) 1200(649) 1025(552)

SA-213 T22 1115(602) 1075(580) 1200(649) 1075(580)Stainless SA-213 321H 1400(760) … 1500(816) 1500(816)Stainless SA-213 347H … 1300(704) 1500(816) …Stainless SA-213 304H 1400(760) 1300(704) 1500(816) …

Table1. Maximum tube metal temperature permitted by ASME code and other boiler manufacturers (REF 1).

Design Considerations

The ASME boiler and Pressure Vessel Code, Paragraph A-150 of section I states the criteria for determining allowable stresses. The allowable stresses are not to be higher than the lowest of the following:• One-fourth of the specified minimum tensile strength at room temperature.• One-fourth of the tensile strength at elevated temperature.• Two-third of the specified minimum yield strength at room temperature.• Two-third of the yield strength at elevated temperature.• Stress to produce 1% creep in 100,000 h.• Two-third of the average stress or four-fifth of the minimum stress to produce creep rupture in 100,000 h, whichever is minimum.

As temperature and stress are the two important factors determining failure, the maximum permitted operating temperature of the boiler tubes of various materials has been shown in table 1. The maximum allowable stresses have been shown as function of temperature in figure 1 for various types of tube materials. For 2.25Cr–1Mo steel, it is the creep or rupture strength that determines the allowable stressat a temperature of beyond 482 ◦C (900 ◦F). Therefore, in the evaluation of creep behavior of Cr–Mo steels, estimation of long term rupture strength has received considerable importance.

Life Cycle Assesment:

One of the approaches in determining the creep life is the parametric evaluation technique.Various parameters have been proposed for the parametric evaluation technique. Some of the parameters have been mentioned as follows:

Fig1. Maximum allowable stresses for various types of tube materials with respect to temperature

Larson-miller Parameter:

The paramter introduced time temperature grouping in the form of T(K + logt) . For a given material , a plot of LMP versus stress resulted in a single plot within scatter as shown in figure 2.

For 2.25Cr-1Mo steel the LMP is defined as:

LMP = T (20 + log t)

Where T is the temperature in degrees and t is the

time to the material is under temperature T.

With the above expression given the hoop stressof the tube we can determine LMP from the plot and through LMP we can predict the time t to failure if the average exposure temperature T is known. Fig2. Plot of stress vs LMP

Manson Hafered Parameter:

this parameter is defined as (REF 1):

f(σ) = log tr – log ta / T - Ta

Where f(σ) is the masson haefered parameter and is plotted against stress to get a linear relationship. The constants ta and Ta are coordinates of point of intercept.

Manson Brown parameter:

The generalized form of manson brown parameter (REF 1) is :

f(σ) = log tr – log ta / (T - Ta )q

Several other parameters can be derived from this parameter .The manson hafered parameter is a special case of manson brown parameter with q=1. It q = -1 and Ta = 0 then it is equivalent to Larson miller parameter.

Though there are many parameters proposed for calculation of creep life but Larson miller parameter is widely accepted and mostly used. In this paper focus is built on calculations using LMP.

Minimum commitment Method:

The method uses a time-temperature-stress relationship general enough to satisfy all commonly used parameters. The specific relationship is established based on the material properties and experimental data.

The general parameter chosen has a form:

f(σ) = M log t + M’ X log t + X

Where M and M’ are temperature independent constants and X is dependent on temperature. It can be shown that when M=0 and M’ = 0.05, the expression reduces to 0.05x (20 + log t), a form which is compatible with the Larson miller parameter. The value of M’ is generally set equal to zero so the expression reduces to

f(σ) = M log t + X

Where X = R1 [ T- Tmid] + R2 [1/T – 1/Tmid]

Where Tmid is the mid value of the temperature range for which data is analyzed and R1 and R2 are constants. MCM is fast emerging promising technique but it hasn’t gained much acceptance.

Life Fraction rule:

As the damage due to creep is cumulative there are procedures designed to estimate the creep life based on the cumulative effect and changing conditions. One of the methods is the life-fraction rule. It is rule for calculating the life expended using time as a measure of damage. When the fractional damages add up to unity failure would occur.

∑ ti/tri = 1

ti/tri represents the ratio of time spent by tube at a particular temperature to the time it would take to cause failure at that temperature. When the reheater tube is exposed to many cycles then for each cycle the above ratio is calculated and when the summation adds up to 1 then failure is postulated to occur.

so the creep rupture failure time can be determined by dividing the projected life of tube into equal intervals and determining life fraction for each interval. From the hoop stress the LMP of the failure can be found out and through LMP tr can be found out for an interval. From temperature records we can easily get t (time for which the tube has been exposed above the excursion temperature) and then we can get the ratio t/tr for that interval. Now for each interval we repeat the same procedure. The probable time to failure is when the summation of the ratio reaches unity. The remaining life of the tube is then predicted by subtracting the service life of tube from the theoretical life.

Although there are several rules proposed for calculation of creep rupture failure time but the life fraction rule is most commonly used and is widely accepted. But there are certain limitations of this rule these are:

(i) This rule is not valid for stress change experiments. Under service conditions where stresses change due to other factors such as corrosion the life fraction rule doesn’t provide the exact life of the tube. The tube life may be less than predicted.

(ii) The rule is valid for variable temperature conditions. In materials that undergo structural changes at high temperature without changes in stress, the kinetics of such changes are governed by the time and temperature of the exposure. Hence service life under fluctuating temperature can be predicted by life fraction rule with greater accuracy.

Estimation based on oxide scale thickness:

With exposure of tube material to high temperature oxidation of material takes place. With time a layer of oxides starts to develop on the inner surface of tube material which hinders the heat transfer. Oxide scale is constituted by a layered structure with compositional and microstructural variations from substrate to the outer interface. Many expressions have been proposed to measure the kinetics of the growth oxide scales and estimation of remaining life (REF 2, 3).

Creep life estimation of reheater tube:

A reheater has around 40 assemblies of coils. Each assembly has many coils stacked up in form of u shapes. The data shown in table 2 is of a tube in a particular assembly number and has been exceeding the prescribed limit of 575◦C consistently over a period of time. 575◦C is considered as an alarming situation in a plant, if the temperature exceeds the limit then damage is inevitable.

SR NO. 1 2 3 4 5 6 7 8Temperature(◦C) 577.02 581.3 579 580 579 578 576 576.57Time (min.) 2.4 3.6 4.8 1.8 3 3 7.8 38.4

Table2. Metal Temperature Excursion report

The temperatures were recorded on monthly basis and average temperature is taken and the time shown is the total excursion time in a month. The tube has already completed 1727 days of its total life. Now we have to calculate the remaining life of the tube. The approach which is demonstrated is based on the application of LMP and life fraction rule. According to the life fraction rule the failure would occur when ∑ t/tr = 1. So we calculate t/tr for each interval which is equal to a month. Time t is directly available in table2 which is the time for which the tube has exceeded the temperature limit. Time tr which is the time required for failure at the average exposure temperature of each interval is to be calculated using LMP. Knowing the maximum allowable stress in the tube we can use figure2 to get the value of LMP. At 575◦C the maximum allowable stress in a SA213 grade T-22 steel is approximately 5.1 ksi according to section I of ASME boiler and pressure vessel code for T22 steel (table 3).

Table 3. Maximum allowable stresses in SA213 grade T22 steel as per ASME code for boilers and Pressure vessel.

From figure 2 the value for LMP at 575◦C is 39000. Using this value time to failure is now calculated for each interval. Though the LMP is computed using 575◦C but the stress values for temperatures few degrees above or below 575◦C doesn’t vary much so the value of LMP is approximately taken as 39000.

SR NO. 1 2 3 4 5 6 7 8

Temperature(◦C) 577.02 581.3 579 580 579 578 576 576.57

Time (min.) 2.4 3.6 4.8 1.8 3 3 7.8 38.4

tr (hrs.) 301720 224906 263278 245824 263278 282016 323745 311240

t/tr 1.32x10-7 2.66x10-7 3.04x10-7 1.22x10-7 2x10-7 1.77x10-7 4.07x10-7 2.06x10-6

Table 4. Life Fractions for the for the given intervals

Form table 4 it can be deduced that ∑ t/tr = 3.67 x 10-6. So, from total life fraction of 1 the tube has consumed a fraction of 3.67 x 10-6. As the tube has already passed 1727 days which corresponds to 41448 hrs of life the remaining life fraction is equal to 0.99999633.

Case Study:

Now to understand creep rupture failure more clearly we have assumed three temperature .i.e. 538,566 and 593◦C. LMP has been calculated for each temperature using the stress vs. LMP plot of figure2.

Table5. LMP and tr at assumed temperatures

Using the LMP relation the time to failure corresponding to each temperature has been calculated. A plot of temperature vs tr has been shown below.

temperature(◦C)0

100000

200000

300000

400000

500000

600000

700000

tr

tr

temperature(◦C) 538 566 593LMP 37500 39000 40000tr 476317.4 651835.8 447601.3

Now we take each temperature and observe life fractions corresponding to different time of exposure to that temperature.

temperature(◦C) 538 538 538 538 538 538time(hrs.) 6 12 18 24 30 36t/tr 1.26x10-5 2.52x10-5 3.78x10-5 5.04x10-5 6.3x10-5 7.6x10-5

Table6. Life fractions corresponding to 538◦C.

temperature(◦C) 566 566 566 566 566 566time(hrs.) 6 12 18 24 30 36t/tr 9.20477E-06 1.84E-05 2.76E-05 3.68E-05 4.6E-05 5.52E-05

Table7. Life fractions corresponding to 566◦C.

temperature(◦C) 593 593 593 593 593 593time(hrs.) 6 12 18 24 30 36t/tr 1.34048E-05 2.68E-05 4.02E-05 5.36E-05 6.7E-05 8.04E-05

Table8. Life fractions corresponding to 593◦C.

1 2 3 4 5 60.000E+00

1.000E-05

2.000E-05

3.000E-05

4.000E-05

5.000E-05

6.000E-05

7.000E-05

8.000E-05

9.000E-05

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