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Construction & Test of Pressure Vessels, J. V. Tyrrell
CASE S5 -
Problem:
Safety of Pressure Test of Vessels with Defects.
Background:  The Naval Air Propulsion Center, W. Trenton, N.J. has a number of
test cells for turbo prop and jet engines.  Three of these for jet engines
have the ability to simulate operating conditions at altitude. While these
facilities are not the largest of their kind, they are still (in 1985) some of
the most sophisticated.  Intake air is fed into the system severing all cells
by a bank of very large blowers in a parallel configuration.
The air passes through heaters or refrigeration units into one of the test
chambers containing the engine to be tested.  Each test chamber is served by
an exhaust gas cooler.  The engine drives the exhaust and augmentation air
thru the system but it is also drawn out of the gas coolers by a bank of
exhaust blowers similar to but smaller than the intake blowers.  The system is
not only subject to high and low temperatures but operating pressures may vary
from 60 psig to minus 15 psig.
Problem: S5a - Welding repair/stress relief.
Symptoms:
Excessive deformation, jammed doors.
Collection of Facts:  The newest of the altitude test chambers is a cylinder
with its longitudinal axis horizontal and with truncated cones at each end as
transitions to the intake and exhaust (see Fig. 1). The chamber may be
subjected to temperatures ranging from 400 degrees F to minus 70 degrees F,
and pressures from 60 to minus 15 psig.  Jet engines are moved in and out
through a large clam shell type door.  The basic cylinder and the door are
both reinforced by series of circumferential stiffeners.
Radiographic inspection of the welds joining the stiffeners to the shell
revealed that over 90% were defective.  Consequently a large amount of the
welding was repaired.  Because of the temperature and pressure conditions and
the triaxial stress at the junction of the shell and stiffeners, stress relief
of repaired welds was recommended.  The contractor then attempted to
accomplish this by heating the whole chamber.  The chamber was supported on
two concrete piers.  As the temperature rose, the chamber sagged under its own
weight.
Solution:  In the case in question, it was feasible to repair or replace the
damaged chamber. Fortunately, the distortion was not significant for the air
flow.  The door was modified to permit proper closure, and supports for the
internal working platform were adjusted.
Stress relief (particularly in the field) must be carefully considered and
planned or it may do more harm than good.
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