API TR 6AF1
Technical Report on Temperature Derating on API Flanges under Combination of Loading
|Publication Date:||1 November 1998|
This report is a continuation to the report on the capabilities of flanges under combined loadings (PRAC 86-21) which resulted in the publication of API Bulletin 6AF. Included herein is an in-depth look into the effect of elevated temperatures on API flanges.
The results in this report are analytical and assume a temperature gradient across the flange as stated in this report. When the flange is insulated on the outside surface, the allowable loads will be higher.
Additional finite element models of five new flanges in API6A, Sixteenth Edition, 1989, which were not in the 1986 Fifteenth Edition, used as a basis for the earlier work, were developed for the combined loading of bolt makeup, internal pressure, tension and bending moment. The API materials were then grouped into four material categories. A thermal analysis was performed (using all 63 axisymmetric finite element models of PRAC 86-21 and the 5 new models) to determine the temperature gradient and resulting thermal stresses at steady state for all four material types at design bore temperatures of350°F and 650°F.
The post-processor program of PRAC 86-21 (calculating the maximum moment capacity for various levels of pressure and tension based on superposition) was partly modified to include thermal effects and produce separate rating curves on the same chart, based on the leak or loss of preload (on the ring joint) criterion and the stress criteria. The stress criteria used were of two types: a) ASME Section VIII, Division 2, allowable stress categories for the flange with the basic membrane stress allowable established by API, and b) allowable bolt stresses as established by API. The results of this post-processing are presented in plots of pressure vs. allowable moment for various tension levels. These new rating charts were developed at two elevated temperatures for all four material categories in Appendices A, B, C, and D respectively.
See Section 4 for details of the axisymmetric analysis and Section 5 for details of the load capacity calculations.
As in the previous report PRAC 86-21, this report does not address the actual gasket contact loads required to make a seal. This report utilizes the leak or loss of preload (on the ring joint) criterion as in Bull 6AF (PRAC 86-21) and Bull 6AF2 (PRAC 88-21) and not the leakage criteria in report PN 90-21.