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 Problem: S5b - Pressure testing.
Collection of Facts: The altitude chamber described above is about 40' long.
It was fabricated of a nickel steel (ASTM 203 Gr D with a minimum tensile yield
strength of 37,000 psi) which has good ductility. However, there were many
penetrations and interfaces with other materials. The inlet duct adjoining the
chamber was ASTM A 201 steel and the exhaust duct ASTM A 242. Because of the
welding history and general configuration, there was concern about the safety
of the specified pneumatic test, which was to be conducted at 125% of the
maximum positive pressure, in accord with Section VIII of the ASME Pressure
Vessel Code. The test included the inlet and test sections, with the adjacent
sections segregated by temporary bulkheads and separately tested.
There was even greater concern for the safety of the pneumatic teat for the
exhaust gas cooler which was also a horizontal cylinder, 23' I.D. by about 50'
in length, with conical ends. The cooler had six large cylindrical water tanks
and two smaller headers that were set into the main shell (see Fig. 2). The
configuration introduced points of triaxial stress which limits ductility.
Operating temperatures in the cooler ranged from 3500 degrees F to minus 70
degrees F, and pressures from 60 to minus 15 psig.
The gas cooler was fabricated from a steel frequently used for boilers,
ASTM A 212. The structure also included parts of T1 steel and a variety of
other steels. The basic material was selected for resistance to high
temperatures and had poor characteristics with respect to ductility. The nil
ductility temperature was found by test to be about 42 degrees F. Construction
proceeded into winter and a large brittle type crack appeared. The crack was
repaired by welding. The cooler was scheduled for a pneumatic test similar to
that required for the altitude chamber. Some welding repair had also been
required on the cooler particularly at the junction of T1 steel and the shell
plate.
A hydrostatic test was considered instead of the specified pneumatic test,
however, investigation showed that the weight of water would significantly
affect the stress distribution in the vessels. Nevertheless, a pressure test
was felt to be essential in view of the design and construction history.
Solution: NAVFAC devised a test strategy of using lightweight filler material
to reduce the weight of water and a A&E expert (Jackson & Moreland) was
obtained for testing and inspection. The A&E developed the details and
prepared a test plan. Empty spaces in the vessels were filled with glass foam
insulation material which was checked for resistance to hydrostatic pressure.
As a further precaution, the glass foam was sealed in polyethylene bags and
lashed in place in the vessels. Water was removed from the cooler tanks and
tubes for the test.
The A&E recommended that the ambient temperatures be no less than 70
degrees during the test. This was no problem for the altitude chamber which
was inside a building but the gas cooler was supported by a bare steel frame.
To conduct the test, the frame was draped with reinforced polyethylene plastic
sheeting and portable heaters used. The water used for the test was heated.
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