ASME STP-PT-036-2010 BOLTED FLANGED CONNECTIONS IN ELEVATED TEMPERATURE SERVICE《高温设备中螺栓法兰连接》.pdf

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1、 STP-PT-036 BOLTED FLANGED CONNECTIONS IN ELEVATED TEMPERATURE SERVICE Mt: STANDARDS TECl-INOlOGY, llC Date oflssuance: October 17, 2010 This report was prepared as an account of work sponsored by ASME Pressure Teclmologies Codes and Standards and the ASME Standards Technology, LLC (ASME ST-LLC). Ne

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9、d Flanged Connections in Elevated Temperature Service STP-PT-036 TABLE OF CONTENTS Foreword . . . vi Abstract vii INTRODUCTION . 1 2 LITERATURE RESEARCH . 2 2.1 High Temperature Joint Behavior . 2 2.2 Mechanical Effects ofTemperature on Joint Behavior . 16 2.3 Code Status I 6 2.4 Gasket Creep Behavi

10、or. . 16 2.5 Material Relaxation Behavior. . 17 3 CREEP BEHAVIOR . 18 3.1 Definition of Creep Law and Material Properties . I 8 3.2 Finite Element Modeling . 27 3.3 Approximation of Creep/Relaxation Behavior Using Code Stresses 32 4 EXPERIMENTAL METHODS AND RESULTS . 36 4.1 Experimental Methods 36 4

11、. 1.1 Bolt Relaxation . 36 4. 1.2 Joint Relaxation 37 4.2 Experimental Results . . . 40 4.2.1 Bolt Relaxation . 40 4.2.2 Joint Relaxation 41 5 CONCLUSIONS . 48 6 RECOMMENDATIONS . 49 References . . 50 Acknowledgments 54 LIST OF FIGURES Figure 1 -From Baumann 3, page 1336 . 2 Figure 2- Early Stress R

12、elaxation Relationship from Bailey 4, page 149 3 Figure 3-Relaxation Diagram from Gough 5, Fig. 24, page 263 4 Figure 4-Joint Life Diagram from Tapsell 9, Fig. 11 , page 448 . 6 Figure 5-Bolt Creep Comparison from Tapsell 9, Fig. 13, page 450 6 Figure 6-Bolt Load Factor Graph from Kerkhof10, Fig. 1,

13、 page 152 . 7 Figure 7- Relaxation Relationships from Johnson 11, page 431 7 Figure 8- Flange Ring Relaxation Graphs from Johnson I I, Fig. I 1, page 432 9 Figure 9- Carbon Steel Pipe Flange Strength vs. Temperature from Johnson 11, Fig. 28, page 448 9 Figure 10-Tensile Test Relaxation Graphs from J

14、ohnson I I, Fig. 33, page 456 10 111 STP-PT-036 Bolted Flanged Connections in Elevated Temperature Service Figure 11 - Comparison of Flange Test Results vs. Creep Tests from Bernhard 13, Fig. 12, page 122 11 Figure 12- Illustration ofRelaxation Rules from Cooper, et. al. 14, Fig. 3 this work led to

15、flanged joint design rules, such as the ASME Section Vill, Division I , Appendix 2 method that was introduced in the 1940s and has remained largely unchanged since that time. Other international methods of design have been introduced recently, most notably the CEN EN-13555 method. However, none of t

16、he current methods address design of a bolted joint in the creep range. The requirement for the design of high temperature joints was identified during the initial development of the design methods, but unfortunately a concise design method was never documented in a code or standard. This is somewha

17、t understandable, given the myriad of complexities involved with analyzing the significance of creep on a bolted joint. In fact, the present design methods are also inadequate even when addressing low temperature operation that involves creep and relaxation of the components 1. Unfortunately, even w

18、ith the more powerful analysis methods available today, the researchers of the 1930s actually appeared to be closer to resolving high temperature flange design than more recent research efforts. In part, this lack of advances in the design of high temperature flanges is probably due to the fact that

19、 most flanges do not operate in the creep range for the materials of construction and therefore the vast majority of flanges have given admirable service. ln addition, a creep “failure“ of a flange is most likely to be a relatively small leak which is easily rectified by re-tightening the bolts duri

20、ng operation. Such “failure“ does not often warrant management attention and therefore does not garner industry attention as an issue requiring resolution. It may also be easily demonstrated that industry, as a whole, has learned to accept bolted joint leakage 2 and therefore relatively little effor

21、t has been directed towards reducing the frequency of joint leakage. The need for improvement in the design of high temperature flanged joints was identified to ASME and this project was funded by ASME, starting in August 2007, to examine the requirements for high temperature (in the flange material

22、 creep range) flange design. The intent of the project is to examine the requirements for high temperature flange design and provide guidance for inclusion of design methods into the modern ASME pressure vessel design codes. Throughout the project, it was kept in mind that high temperature flange jo

23、ints are a relatively small portion of the flange population, and that improvements in Finite Element Analysis (FEA) and computing power are now to the point where very large non-linear creep problems can be solved relatively easily. Therefore, while the fundamentals of high temperature flange desig

24、n using code equations were included in the assessment, the initial starting point for the project was to formulate guidelines for FEA of the creep problem, based on comparison with relatively scarce flange creep test data. It is recognized that these guidelines may actually be the most appropriate

25、implementation of high temperature flange design, due to the inherently critical nature of most high temperature flanges. The following literature search looked at fundamental research in high temperature flange joints, especially with respect to papers including experimental verification of results

26、. In addition, the area of the mechanical effects of temperature on bolted flange was included, as any assessment of flange creep must be made at the initial operating stress conditions, rather than at the ambient conditions. In addition, the subject of gasket creep behavior was examined. This subje

27、ct has had extensive research across a variety of gasket types. but there is very little tie-in with actual high temperature (creep regime) behavior of the bolted joint. STP-PT-036 Bolted Flanged Connections in Elevated Temperature Service 2 LITERATURE RESEARCH 2.1 High Temperature Joint Behavior Th

28、e primary driver for initial efforts in high temperature flange design came from improvements in the steam power generation industry that resulted in higher steam temperatures and pressures being used in the late 1920s and early 1930s. The initial work of Baumann 3 in 1930 looked at the creep of bol

29、ts and flange components of the joint and proposed a joint “life“ relationship that accounted for the relative creep strength and relative flexibility of the bolt and flanges (Figure 1). The concept of joint life was related as a measure of time before flange leakage, rather than flange or bolt mech

30、anical failure. The figure demonstrates the significant effect of the relative flexibilities on the joint life. l.f t l + n q = 1 e parame er = 1 + p _ flexibility of flange n - flexibility of bolts _ creep rate of flange p - creep rate of bolts A. Creep rate-stress relation. B. Rigid flanges (q = 1

31、). C. F lange flexibility equal to bolt flexibility (q = 2). D. FJange flexibility four times bolt flexibility (q = 5 ). 10 I (). -z-Figure 1 - From Baumann 3, page 1336 o, I Baumann also made several conclusions from his examination of joint creep characteristics, including: That a bolt stress that

32、 induced a creep strain rate of less than 1 o8 would give satisfactory life That if the steam load alone induced a creep strain rate of greater than 108, visible leakage would occur in less than one year That flange flexibility is very desirable. A flange with similar flexibility as compared to the

33、bolts will double the creep life of the bolt That with a rigid flange the life is independent of the bolt length or flange thickness. 2 Bolted Flanged Connections in Elevated Temperature Service STP-PT-036 However, as noted by Baumann, his treatment assumed tensile creep in series of the joint compo

34、nents. In the case of the flange, the creep is in bending and therefore the assumptions made by Baumann were conservative by comparison to the real case. The creep of flange joints was further studied by Bailey 4 in 1935 and the concept of stress relaxation of components in series was developed furt

35、her using a creep life relationship under a tensile diminishing stress (Figure 2). By using this relationship and the methods outlined in Appendix I of the paper, it is possible to determine the remaining stress (j) in each joint component from an initial stress (/o) for a given time period (t). How

36、ever, once again this assumed that the bolt and flange creep were identical in nature, which is not the case, as the flange is predominantly in bending and the bolt predominantly in tension. Taking the law C = Afn to apply to the material over a timet in which the stress falls from the initial value

37、 /o to the value f, application of equation (9), Appendix I, p. 234, to the case of simple tension gives since x0 = /0/E 1 and c0 = A/011 and/= 1 (n - l )EAtn-l Figure 2-Early Stress Relaxation Relationship from Bailey 4, page 149 One of the interesting points to come from this analysis was that the

38、re was little advantage to tightening the joint to a value exceeding twice the final stress at “failure.“ Similarly to Baumann, failure of the joint was defmed as leakage, rather than mechanical rupture. However, this relationship is somewhat misleading due to the limitations regarding the assumptio

39、ns of similarity of the bolt and flange creep and also, as detailed! by Bailey, the fact that the actual value is significantly higher than two, as there is a reduction in bolt and fl ange stress corresponding to the reduction in material Young s modulus and yield strength with increasing temperatur

40、e. Therefore, the initial stresses at the start of creep are significantly lower than the initial assembly stress and it is the stress at the start of creep to which the factor must be applied. It was also stated in the paper that the sum of the individual component elastic and creep strains at any

41、given time would be equal to the initial elastic strain imparted by the assembly load. Bailey suggested in the paper that a suitable life for a joint might be I 00,000 hrs without maintenance and 10,000 hrs with maintenance (re-tightening). Bailey continued his work in the field of high temperature

42、bolted joint behavior as chairman of the Institute of Mechanical Engineers Pipe Flanges Research Committee that studied flange creep from 1936 to 1954. The focus of the study was to provide additional flange ratings to the British Standard BS I 0 for standard piping flanges in order to accommodate t

43、he increasing pressures and temperatures associated with steam power generation. The first report presented by the committee 5 detailed tests on a standard BSl O: TableT, 8 inch flange at 900F to l000F and 1450 psig steam pressure. The flanges were assembled, heated, steam applied internally and hel

44、d steady at the operating conditions unt il noticeable leakage from the joint occurred. The gaskets, when used in these tests, were either metal or a very thin asbestos fiber (1164 inch thick) and therefore the influence of the gasket on the joint behavior was neglected. The paper includes a diagram

45、 illustrating the concept of bolt stress relaxation (Figure 3) where the elastic relaxation of the bolt strain during the test (A-G) is shown relative to the creep strain of the bolt and other joint components. Measurements of the bolt lengths and flange deflections both before and after the tests w

46、ere used to determine the proportions of the 3 Bolted Flanged Connections in Elevated Temperature Service STP-PT-036 In a I 938 paper, Waters 7, who was one of the original developers of the methods presently used in the ASME code, developed an assessment of the effect of creep on loose-ring flanges

47、 and predicted that, on an allowable stress basis, the fact that the flange was creeping in bending would mean that the stress relaxation occurring at extreme fibers of the ring would result in an improvement in stress distribution (lower peak stresses) and therefore justification could be made that

48、 high temperature flanges could, in fact, be thinner than their low temperature counterparts. However, he also noted that it was likely that flange deformation and subsequent leakage would limit the life of the joint and it would be necessary to increase the flange thickness to a point where the def

49、ormation during the flange life did not result in leakage. The paper outlines a method of assessing the creep of the joint by finite time increments and accounting for the stress distribution in the flange ring at each time increment. It is suggested that assessment of the flange behavior may be possible by using a combination of tensile and bending creep tests. The paper also makes the same conclusions regarding the sum of the elastic and creep stra