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API TR 6F1 Document Information:
Title
Technical Report on Performance of API and ANSI End Connections in a Fire Test According to API Specification 6FA
American Petroleum Institute
Publication Date:
Apr 1, 1999
Scope:
1 Scope and Applicability
The sizes of interest are shown in Table 1. The ANSI (8) sizes are
adopted by API Spec 6D. The API sizes refer to those in API Spec 6A.
Table 2 shows the materials and gasket types which have been studied.
As can be seen, there are a large number of sizes and combinations of
materials and gaskets.
One very important point is that this Technical Report, and the
research projects which preceded it, are based on the fire test
procedure in API Spec 6FA. Whether or not API Spec 6FA adequately
defines a "true" fire is totally outside the scope of this bulletin.
The test simulates a closed condition with no flow through the
connection. If there were flow through the connector, the heat
transfer response would likely be different. Also, the external loads
from dead weight, piping system thermal expansion, wind, etc., could
significantly affect a joint's performance in a fire. These effects
could be included in the design procedures reported herein, but they
are not in the scope of this report.
1.1 EFFECTS OF FIRE ENVIRONMENT
Exposure to a fire environment around a flanged or clamped joint will
tend to reduce the joint preload required for many seals to function.
In a flanged joint, the flange exterior and bolts will heat up quicker
than the seal and interior flange portion. This thermal gradient
across the joint will cause preload to be lost. If enough preload is
lost, the seal may unseat and leak; lose the contact pressure
necessary to maintain a seal; fail if the seal is not strong enough to
carry pressure load without the restraint from the adjacent contact
surfaces, or fail if the temperature capacity of the seal material is
exceeded.
1.1.1 In addition to preload loss from the thermal gradient, there is
a preload loss due to the reduction of joint stiffness resulting from
increasing temperature. Another factor which can cause preload loss is
yielding of the gaskets, studs, or flanges. Bolt yielding results in
permanent stretching which means permanent preload loss and likely
leakage, either hot or cold.
1.2 SIGNIFICANT VARIABLES
Variables which are significant to this problem are as follows:
1.2.1 Size and Geometry of the Joint
The size will greatly affect the thermal gradient through the joint,
with larger joints having much larger thermal gradient than smaller
joints for a thermal event such as the standard API Fire Test.
However, the larger joints do not get nearly as hot as the smaller
joints for the fire test conditions of this Technical Report. Figures
1 and 2 illustrate this.
1.2.2 Type of Seal
Of particular importance for the seals is the seating load, the
retaining load; amount of pressure energization; whether the seal is
used in joints that make up face-to-face, or with standoff; and the
gasket material.
1.2.3 Material of Construction
Carbon steel has a thermal conductivity approximately three times as
large as that of austenitic stainless steel, and a lower thermal
expansion coefficient. Thus, the thermal response and preload change
of the joints are a very strong function of the material.
In addition, a critical variable is the yield strength of the bolts
and flange at the high temperature of the test. If a large amount of
yielding occurs, the remaining preload will be very small.
1.2.4 Joint Preloads and Internal Pressures
Flanged and clamped joints are normally designed with preloads based
on two considerations: (a) the amount of preload required to seat the
seal, and (b) the amount of preload required for the gasket retaining
load, pressure end loads, and any external forces or moments. Joints
in which the seating load is controlling (highest) will tend to have
more preload margin for the pressure and retaining load case and,
thus, may tend to be more leak resistant in the fire test. Further,
the API Fire Test is at about 75% of the rated pressure. For high
pressure connections, this 75% of rated pressure is a very large
number compared to the retaining load. Thus, it might be expected that
higher pressure joints are more resistant to the fire.
Another consideration is the variability of preload in actual field
joints.
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