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Part 1: Long Term Pressurized Graphite Gasketed Joint Tests: Long Term Pressurized Graphite Gasketed Joint Tests

PVRC project 96-09G is the first of a series of pressurized media tests and it investigates the effect of long term pressurized hot air exposure on the performance of two types of reinforced flexible graphite based gaskets, gasket A with a thin flat metal reinforcement, and gasket B with a thick corrugated metal. The gaskets are tested in NPS 3, Class 150 bolted flanged joints at 700F under a constant internal air pressure of 110 psi. Gaskes were aged until about 90% relaxation of their initial compressive stress was achieved. Very different rates of stress relaxation and leak rate increase over time are observed for the two gasket types. In the case of gasket A, the degradation of the properties was relatively slow and the test lasted 342 days. For gasket B, a much faster rate of degradation was recorded during two tests that lasted respectively 34 and 46 days. This behavior was not expected and cannot be explained for the time being.

A comparison made between predicted stress relaxation and experimentally measured long term stress relaxations of gasket A demonstrates that the PVRC quantification tools Ag and Qpg developed for RFG sheet gasket qualification, are making accurate service life predictions in the case, of flexible graphite sheet gaskets with a thin flat metal reinforcement.

Part 2: Long Term Pressurized Graphite Gasketed Joint Tests: Long Term Performance of Flexible Graphite Based Gaskets Under Nitrogen Exposure

PVRC project 99-04G is the second of a series of pressurized media tests and it investigates the effect of long term pressurized hot nitrogen exposure on the performance of two types of reinforced flexible graphite based gaskets, gasket A with a thin flat metal reinforcement, and gasket B with a thick corrugated metal. The gaskets are tested in NPS 3, Class 150 bolted flanged joints at 700F under a constant internal nitrogen pressure of 110 psi. Very different rates of stress relaxation and leak rate increase over time are observed for the two gasket types. In the case of gasket A, the degradation of these properties is relatively slow and the test lasted 18 months, the maximum planned test duration. For gasket B, a much faster rate of degradation is recorded and the test had to be stopped after 11 months when stress relaxation was above 95% of the initial compressive stress. This behavior was not expected and cannot be explained for the time being.

A comparison made with the long term pressurized hot air test results of PVRC project 96-09G shows that for both gaskets, the compressive stress decrease over time is about 50 % less for hot nitrogen exposure. Also, gasket leak rates during the nitrogen exposure tests are significantly lower than those recorded during the air exposure tests. Depending on the gasket type and the exposure time, the leak rates measured in the nitrogen tests are found to be between 10% and 60% of the air test leak rates. Finally, a method that can adjust the PVRC qualification tools for the calculation of long term gasket temperature in the case of a joint pressurized with a non-oxidizing fluid is presented. The method is based on the establishment of a recommended Ag value that is specific for an internal fluid exposure condition.

Part 3: Long Term Pressurized Graphite Gasketed Joint Tests: Additional Long Term Pressurized Joint Tests on Corrugated Flexible Graphite Gaskets

PVRC Project 00-BFC-12G is an experimental study that investigates the possible causes for the fast rate of stress relaxation and leak rate increase of a flexible graphite gasket reinforced with a thick corrugated metal core, gasket B, that was recorded during long term pressurized joint tests performed for PVRC Project 96-09G [1]. The influence of three parameters is selected for this investigation: the initial compressive stress, the gasket width and the gasket brand. A new brand of gasket, gasket C, similar in construction to gasket B, is selected to perform three of the four tests of the study. The fourth test is performed with gasket B initially compressed at 7500 psi. The results of the new series of tests are combined with those of Project 96-09G to perform the analysis.

Results of the tests carried-out on gaskets B and C with two levels of initial compressive stresses, 5000 and 7500 psi, indicate that the stress relaxation and the leak rate increase of both types of gaskets are not significantly influenced by the initial stress applied. It is also found that the gasket width parameter tested on gasket C, does not have an important effect on the degradation rate of the measured gasket properties. The fast degradation rate of gasket B can only be explained by the third parameter, the gasket brand. The comparison between the test results of gaskets B and C reveals a tremendous difference between the degradation rates, the degradation rate of gasket C being much slower. Overall, it is found that stress relaxation and leak rate change of gasket C during the long term pressurized tests are more in accordance with the perceived field performance for this type of gasket.

The visual inspection of the gasket specimens after the tests shows that the flexible graphite layers of gasket C are cut into narrow strips by its corrugated metallic core, while for gasket B, layers were found to be in one piece. One possible explanation for the better performance of gasket C could be an encapsulation of the graphite strips that is achieved by a metal to metal contact between the corrugations of the metallic core and the flange faces. The high degradation rate of gasket B could be caused in part by the absence of this protective mechanism.

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Published: 2005 Number of Pages: 84 File Size: 1 file , 3.3 MB