 |
| Purchase Information |
| Use this form to request purchase information on API online subscriptions. |
|
|
Document API TR 6AF2 is offered by IHS as part of an online subscription. This subscription contains many documents on the same topic.
You may also purchase this document alone from the IHS Standards Store.
API TR 6AF2 Document Information:
Title
Technical Report on Capabilities of API Integral Flanges Under Combination of Loading—Phase II
American Petroleum Institute
Publication Date:
Sep 1, 2008
Scope:
The evaluation of the load carrying capacity of all API 6A
integral flanges is the objective of this work. The applied loading
includes the end tension and bending moment in addition to the
conventional rated pressure and makeup forces. The effect of a
temperature difference corresponding to 250°F on the inside and
30°F on the outside was also evaluated.
Three-dimensional finite element meshes were generated for each
of the 30, Type 6B, and Type 6BX flanges. The bending moment load
case required a model of one quarter of the flange which was built
up from the smaller segments and the half-bolt superelements. The
computer program SESAM was used to obtain the stresses at selected
critical flange and hub sections and to determine the gasket
reaction due to each of the four unit load cases and the
temperature difference load case. Leakage criterion was defined as
the load combination which reduces the initial makeup compressive
forces in the gasket to zero. The stresses in each defined section
were linearized in accordance with the ASME Section VIII,
Division 2, procedure to determine the membrane and
membrane-plus-bending stress intensities. These stress intensities
were checked against the allowables specified in API 6A, and the
limiting loads were determined. A computer program LCCP was written
to carry out this code check and a LOTUS 1-2-3 Release 3 worksheet
was used to plot the load combination charts.
The results of the analysis carried out indicate that the
leakage criterion governs the capacity of the smaller flanges in
the Type 6B flanges. Leakage was governing for up to 9 in. size
flanges in both the 52.5 ksi and 40 ksi makeups for the 2000 psi
pressure. Leakage was governing the 5 1/8 in.
for the higher pressures. Leakage was also found to be governing
all Type 6BX flanges for working pressures of up to 5,000 psi. For
the 10,000 psi and 15,000 psi flanges, leakage governed only in the
larger size range greater than 2 9/16 in.
Leakage was governing in all the 20,000 psi API 6BX flanges. The
leakage model adopted in this study employs several approximations
that have not yet been evaluated. Therefore, the actual leakage
forces, i.e. load combinations leading to leakage, may be
considerably higher than assumed herein. In reality, the gasket
only leaks when its energized capacity is exceeded.
The state of stress at the stress governing hub section under
the combined loading of makeup, pressure, tension and bending
moment is considered to be "secondary." However, when pressure,
tension, and bending moments are applied together with the
necessary makeup to resist these actions without leakage, the state
of stress is rendered "primary" and, therefore, the allowable
stress intensities are halved. This does not seem to be consistent,
and it may by far exceed the intention of the code. However, the
oversight subcommittee preferred to adopt the conservative route,
which may be overly conservative pending further evaluation.
Therefore, it may be concluded that when the hub stresses are
treated as primary, most flanges do not possess significant reserve
strength beyond the leakage condition. In fact, if the leakage
condition was somewhat conservative, the stress condition may
become governing for most flanges.
The temperature difference of 250°F internal and 30°F external
leads to increases in the load-carrying capacity of the flanges.
This condition is caused by the compressive forces generated in the
gasket due to this temperature difference, and the increase in the
allowable stresses when the self-limiting temperature load
condition is included. It is recommended that a 3-D finite element,
nonlinear material and geometric models of approximately eight
flanges be carried out to determine the actual failure mechanism
that governs the behavior of these flanges. This includes the
prediction of the response of the gasket under increasing load and
a more accurate definition of the leakage mechanism. The
elimination of the raised face does not significantly reduce the
stresses in the hub which caused six Type 6B flanges to fail to
meet the ASME criterion for makeup load only (52.5 ksi for 105 ksi
bolting). The stress intensities were reduced only by about 5% when
the raised face was eliminated, increasing the thickness of the
flange by about 10%. The hub thickness for these flanges had to be
increased by up to about 27% of their existing thicknesses together
with the elimination of the raised face.
The bolt stresses did not govern for any of the flanges
analyzed. Bolt stresses are typically within approximately 67% of
their yield strength due to makeup, pressure, tension, and bending
moment loads. The bolts are expected to be made up to half their
yield. The stresses in the bolts due to temperature differences
increase by about 5 ksi to 7 ksi, which is about 6% to 8% of the
bolt yield stress. The other load conditions (pressure, tension,
and bending moments) increase bolt stress by twice the increase due
to the temperature difference. Therefore, it is concluded that the
bolts will not approach their limiting criterion under the
investigated load conditions.
About IHS
IHS (NYSE: IHS) is a leading global provider of critical technical information, decision-support tools and related services in a number of industries including aerospace and defense, automotive, construction, electronics, and energy. IHS serves customers ranging from large governments and multinational corporations to smaller companies and technical professionals in more than 100 countries. IHS been in business for more than 45 years and employ more than 2,300 people around the world.
