ASME STP-PT-036-2010 pdf download.BOLTED FLANGED CONNECTIONS IN ELEVATED TEMPERATURE SERVICE.
2 LITERATURE RESEARCH 2.1 High Temperature Joint Behavior The 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 bolts 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 mechanical failure. The figure demonstrates the significant effect of the relative flexibilities on the joint life.
One of the interesting points to come from this analysis was that there 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 defined as leakage, rather than mechanical rupture. However, this relationship is somewhat misleading due to the limitations regarding the assumptions 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 flange stress corresponding to the reduction in material Young’s modulus and yield strength with increasing temperature. 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 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 100,000 hrs without maintenance and 10,000 hrs with maintenance (re-tightening).
Bailey continued his work in the field of high temperature 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 BS10 for standard piping flanges in order to accommodate the increasing pressures and temperatures associated with steam power generation. The first report presented by the committee [5] detailed tests on a standard BS10: Table T, 8 inch flange at 900°F to 1000°F and 1450 psig steam pressure. The flanges were assembled, heated, steam applied internally and held steady at the operating conditions until noticeable leakage from the joint occurred. The gaskets, when used in these tests, were either metal or a very thin asbestos fiber (1/64 inch thick) and therefore the influence of the gasket on the joint behavior was neglected. The paper includes a diagram 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.ASME STP-PT-036 pdf download.