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Description / Abstract:
General
The strain gaging of pressure vessels (also known as pressure
hulls) subjected to the external hydrostatic test pressure loading
serves two purposes. First and foremost, the gaging is to monitor
the structural behavior and response of the pressure vessel under
external pressure load conditions. The resulting strains and
stresses can then be compared to those obtained from the design
analyses performed.
Secondly, proper strain gaging can indicate the onset of
collapse of the pressure hull under the external hydrostatic
pressure test. Theoretically, using the design rules of the latest
ASME PVHO-1 "Safety Standard for Pressure Vessels for Human
Occupancy," standard, the pressure hull will not collapse during
the external hydrostatic pressure test and serves as the proof
test. However, given an unknown circumstance such as an undetected
out-of-tolerance fabrication issue, onset of the collapse of the
pressure hull can be detected by monitoring the strain gages.
Deviation from the predicted strains (stresses) vs. external
pressure is an indicator that the hull is behaving unexpectedly,
deforming more than expected, and possibly be near collapse.
Monitoring the gages during the hydrostatic test can allow the test
to be halted prior to causing significant damage and/or collapse of
the hull.
Two examples of strain gaging a pressure vessel subjected to
external pressure loading are presented herein. The first example,
presented in Section 5.0, shows a basic strain gaging plan useful
for validating strain and stress analyses. This level of gaging
requires a minimal number of strain gages located at general
positions on the hull. The second example, presented in Section
6.0, shows a strain gage layout plan which is useful for not only
validating strain and stress analyses, but also for monitoring the
behavior of the hull during the hydrostatic test. This level of
strain gaging requires the most number of strain gages since gages
are placed at both general locations and regions of concern due to
hull as-built geometries that might initiate collapse.
These two strain gaging examples are provided as illustrative
examples only. These examples in no way establish actual strain
gaging requirements per any code, design rules, or jurisdictional
body. They do not establish required placement gage locations, gage
types to be used, or number of gages. For each hull, the actual
strain gaging plan implemented is a function of many factors, such
as the chamber's configuration, number and size of openings,
attachments, actual as-built geometry, weld details, and whether
just validating an analysis and/or monitoring hull behavior to
preclude collapse. Other factors not mentioned here might also
dictate the placement of strain gages.