1、Designator: Meta Bold 24/26Revision Note: Meta Black 14/16STP-PT-024DEVELOPMENT OF BASICTIME-DEPENDENT ALLOWABLE STRESSES FOR CREEP REGIME IN SECTION VIII DIVISION ISTP-PT-024 DEVELOPMENT OF BASIC TIME-DEPENDENT ALLOWABLE STRESSES FOR CREEP REGIME IN SECTION VIII DIVISION I Prepared by: Charles Bech
2、t IV Greg Hollinger Charles Becht V Becht Engineering Co. Date of Issuance: November 21, 2008 This report was prepared as an account of work sponsored by ASME Pressure Technologies Codes and Standards and the ASME Standards Technology, LLC (ASME ST-LLC). Neither ASME, ASME ST-LLC, Becht Engineering
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9、onic retrieval system or otherwise, without the prior written permission of the publisher. ASME Standards Technology, LLC Three Park Avenue, New York, NY 10016-5990 ISBN No. 978-0-7918-3189-2 Copyright 2008 by ASME Standards Technology, LLC All Rights Reserved Time-Dependent Allowable Stresses in Se
10、c VIII Div I STP-PT-024 iii TABLE OF CONTENTS Foreword . iv Abstract v 1 INTRODUCTION . 1 2 METHODS 2 2.1 Method for Occasional Loads . 2 2.2 Method for Time Dependent Design Considering Creep 3 3 RESULTS 7 3.1 Results for Occasional Loads 7 3.2 Results for Time Dependent Design Considering Creep. 1
11、0 4 CONCLUSIONS AND RECOMMENDATIONS 12 4.1 Occasional Loads 12 4.2 Time Dependent Design Considering Creep. 12 References 13 Appendix A - Example Calculations for Time Dependent Design Considering Creep . 14 Appendix B - Material Data for Examples 1-10. 52 Acknowledgments 56 LIST OF TABLES Table 1 -
12、 Proposed Allowable Stress Parameters. 7 LIST OF FIGURES Figure 1 - Example Yield Stress and Allowable Stress Versus Temperature 5 Figure 2 - Comparison of Existing Allowable Stresses with Proposed Allowable Stresses 9 Figure 3 - Maximum Time at Load 10 STP-PT-024 Time-Dependent Allowable Stresses i
13、n Sec VIII Div I iv FOREWORD This document was developed under a research and development project which resulted from ASME Pressure Technology Codes the reduction factor for 800H is set at 0.9; and the reduction factor for 2-1/4 Cr 1 Mo, for a 100,000 hour duration at the highest temperature 1100F,
14、is 0.79. As an initial recommendation, we recommend that a material strength reduction factor of 0.8 be applied to all materials other than austenitic stainless steel. Further study could provide more specific yield strength reduction factors that provide more precise distinction between materials.
15、However, the benefit of being able to design based on yield strength should in most all circumstances outweigh the debit of including the material strength reduction factor. A means to define what loading conditions may be included for this allowable stress basis is required. We set an objective of
16、a total duration of ten hours. Thus, in design, the total loading duration of all occasional loads for which these time independent allowables would be used would be ten hours. We then evaluated a variety of materials, as described in Section 3.1, for these proposed short term allowables. The allowa
17、ble duration of the occasional load was calculated for a variety of materials, as described in Section 3.1. The allowable life, using a basis consistent with the code rules (e.g., 80% min. stress rupture for the stress and duration), was determined. Based on this study, it was determined that an add
18、itional factor was required to achieve the desired ten hour allowable load duration. The additional limit is that in no case may the stress exceed four times the allowable stress in Section II, Part D 4. Considering the yield strength reduction factor, and the desired permissible load duration of te
19、n hours, the recommended limits for evaluation of stresses that include earthquake or extreme wind are as follows. 1. All materials other than those cited below: 0.64 Sy2. Austenitic stainless steel: 1.0 SyTime-Dependent Allowable Stresses in Sec VIII Div I STP-PT-024 3 3. Other materials with stres
20、s strain characteristics similar to austenitic stainless steel: 0.8 Sy4. But not greater than 4S (per Section II, Part D, Table 1A). 2.2 Method for Time Dependent Design Considering Creep To perform time dependent design, creep properties must be considered. However, these properties as a function o
21、f time and temperature are only provided with Section III, Subsection NH, within the ASME Boiler and Pressure Vessel Code, and there only for a very limited number of alloys. Time dependent properties for more alloys are provided in ASME FFS-1 2 but design using these properties could arguably be co
22、nsidered to require a greater degree of sophistication than is desirable for Section VIII, Div 1 applications. This report proposes a relatively simple approach to explicitly consider time, without requiring material data beyond what is provided in the existing allowable stress tables. The basis for
23、 the allowable stresses for Section VIII, Div 1 construction is the following (per ASME Section II, Part D, Appendix 1). At temperatures above the temperature for which time dependent properties govern the allowable stress (termed the transition temperature, the highest temperature for which allowab
24、le stresses are provided that is not governed by creep properties), the creep properties govern. ST/3.5 or 1.1 STRT/3.5 2/3 Syor 2/3 SYRYor 0.9 SYRYFavgSR avg, 0.8 SR minScFrom ASME Section II, Part D, these values are defined (quoted) as: Favgmultiplier to average stress for rupture in 100,000 hr.
25、At 1500F and below, Favgis 0.67. Above 1500F, it is determined from the slope of the log time-to-rupture versus log stress plot at 100,000 hr. such that Favg= 1/n, but it may not exceed 0.67. RTratio of the average temperature dependent trend curve value of tensile strength to the room temperature t
26、ensile strength. RYratio of the average temperature dependent trend curve value of yield strength to the room temperature yield strength. Scaverage stress to produce a creep rate of 0.01%/1,000 hr. SRavgaverage stress to cause rupture at the end 100,000 hr. SRminminimum stress to cause rupture at th
27、e end of 100,000 hr. STspecified minimum tensile strength at room temperature, ksi. SYspecified minimum yield strength at room temperature, ksi. n a negative number equal to log time-to rupture divided by log stress at 100,000 hr. The relationship for creep properties between time and temperature fo
28、r a given stress is commonly expressed in terms of the Larson-Miller parameter. The equation is of the form: ( ) ( )460 /1000 logLMP T A B t=+ +(1) A and B material dependent constants T temperature (F) T time (hours) STP-PT-024 Time-Dependent Allowable Stresses in Sec VIII Div I 4 The Larson-Miller
29、 parameter is a constant for a given stress. Given the temperature and duration for a value of stress, the Larson-Miller parameter may be calculated. The proposed approach is to use the Larson-Miller parameter to calculate allowable stresses (on the same creep bases as are used to set the basic Code
30、 allowable stresses) for shorter and longer durations, for a given temperature and stress. While strictly applicable to extrapolating rupture properties, this can be done with the existing allowable stress tables, as follows. 1. Calculate the stress for an operating condition. 2. Determine the tempe
31、rature from the allowable stress table for which that calculated stress would be the allowable stress. 3. Calculate the Larson-Miller parameter based on the temperature determined from step 2 and a duration of 100,000 hours. 4. Given the Larson-Miller parameter that has been determined to be applica
32、ble for the value of stress, from step 3, calculate an allowable duration for the actual temperature of the operating condition. This can be done for a number of operating conditions, and then combined in a typical time fraction summation approach, with the sum of the time fractions limited to 1.0.
33、The approach requires consideration of operating conditions. That is, the expected combinations of actual operating pressures and associated metal temperatures. There would still be a design pressure and temperature used in the design process to set the minimum required wall thickness. But short ter
34、m high temperature events may not govern in establishing the design condition. After establishing the basic required metal thicknesses, time dependent life calculations can be performed, including a specified life, and additional short term operating conditions. By using operating pressures and temp
35、eratures for this life assessment, design lives that are significantly greater than 100,000 hours can be calculated, even when the calculated stress for the design condition is equal to the allowable stress. This provides a uniform prediction of life, which is independent of margins placed on temper
36、ature and pressure when setting the design conditions. There are three additional considerations 1. When the temperature for an operating condition is below the transition temperature. 2. When the calculated stress for an operating condition has an associated allowable stress that is higher than the
37、 allowable stress at the transition temperature. 3. When the calculated stress for an operating condition is less than the stress at the highest temperature for which allowable stresses are provided in the allowable stress table. For the first condition, the temperature is below a temperature for wh
38、ich significant creep effects are anticipated with an additional margin (as the allowable stress at that condition is based on strength properties). For operating conditions at temperatures equal to or below the transition temperature, the condition is not considered in the life fraction calculation
39、. For the second and third condition to be considered, first consider a typical curve for a material showing the allowable stresses based on tensile strength versus creep properties as a function of temperature. See Figure 1. In the temperature range where creep governs the allowable stress, the slo
40、pe of the allowable stress versus temperature curve is continually decreasing. This effects how conditions below the transition temperature and above the maximum temperature can be readily considered. Time-Dependent Allowable Stresses in Sec VIII Div I STP-PT-024 5 Sy versus Creep Rupture Base Allow
41、able Stress for 1.25 Cr0510152025300 200 400 600 800 1000 1200 1400Temperature (F)Stress(Ksi)SyCreep Rupture Based Allowble StressFigure 1 - Example Yield Stress and Allowable Stress Versus Temperature For temperatures below the transition temperature, the values of allowable stress could be used, w
42、hich would be conservative. This is because a lower temperature would be selected for calculating the Larson-Miller parameter, which will result in a lower value of the parameter and associated shorter lives when used to calculate life at the operating temperature. Noting that the slope is continual
43、ly decreasing, a less conservative, but conservative nonetheless, approach can be used. The first two values of allowable stress above the transition temperature can be used to extrapolate the time dependent component of allowable stress to lower temperatures. This will always result in a lower valu
44、e than the actual material properties would yield, with a resulting, conservatively low value of the Larson-Miller parameter. At temperatures above the maximum temperature for which allowable stresses are provided, the curve cannot be conservatively extrapolated. Therefore, when the stress is below
45、the minimum allowable stress value that is provided, the associated maximum temperature should be used to calculate the Larson-Miller parameter. Again, this will provide a lower temperature than the actual material properties would yield, which will provide a conservatively low value of the Larson M
46、iller parameter and further life prediction. The follow steps summarize the proposed method. 1. Given i, tiand Tifor number (i) of short-term and long-term elevated temperature cases. 2. Compute the Von-Mises (effective) stress for each case. Alternatively, max. principal stress for consistency with
47、 VIII-1. 3. Use effective or max. principal stress for each case to find the corresponding temperature “TDi” temperatures from II-D; this presumes that such temperature is associated with the allowable stress controlled by stress rupture (i.e., this is an elevated temperature condition). 4. Compute
48、the LMP(t1000000, TDi)ifor each case with API-579 LMP material constants and 100,000 hr. (the “basis” for VIII-1 creep rupture life). STP-PT-024 Time-Dependent Allowable Stresses in Sec VIII Div I 6 5. Compute the allowable time (ti)ifor each case using the LPM for each case with ti=(10)exp(ai) wher
49、e ai=LMP(t,T)i/(Ti=480)-C. 6. Compute the life consumption, for each case; i=ti/(ti)i. 7. =iDesign acceptable if 1.0. Time-Dependent Allowable Stresses in Sec VIII Div I STP-PT-024 7 3 RESULTS 3.1 Results for Occasional Loads Table 1 summarizes material properties, allowable stress for occasional loads of short duration and allowable duration. The Yield Strength values at elevated temperatures were taken from ASME FFS-1 2. There are differences between the II-Part D Yield Stresses and the ASME FFS-1 yield stresses. These differences are considered to be insignifi
