7.
ENVIRONMENTAL COMPATIBILITY. Although minor corrosion problems can
occur in an industrial environment, marine environments present the most
serious source of environmental corrosion for hyperbaric chamber piping
systems. Corrosion is usually caused by mist-laden sea air. The Corrosion
Handbook by Herbert H. Uhlig (Reference 4) contains data on such corrosion.
MIL-STD-777 also contains guidance. For specific problems, such as stress
corrosion or crevice corrosion, it may be necessary to seek out references on
the particular materials of interest. Marine research laboratories can often
provide such data.
8.
ENGINEERING PROPERTIES. The most important of the engineering
properties which are considered during material selection are thermal
conductivity, coefficient of thermal expansion, strength, and nil-ductility
transition temperature.
a.
Thermal Conductivity and Expansion. Differential expansion in a
piping system can cause severe stress and distortion, resulting in component
failure or malfunctioning. Calculations to determine the effects of
temperature changes in a piping system require values for the thermal
conductivity and coefficient of thermal expansion for the piping system
materials. These values can be obtained from the ASME Code or from
authoritative source data such as publications of the National Bureau of
Standards.
b.
Strength. Calculations to determine the structural adequacy of
piping systems require values for the yield strength, ultimate strength, and
fatigue strength of the piping system materials.
CAUTION: Values from materials handbooks cannot be used because
there are many different approaches or formulas to
calculating structural adequacy and each formula
represents an attempt to estimate (1) calculation
accuracy, (2) material strength, (3) fabrication
variations, (4) material sensitivity to defects,
and (5) loading variations. Each calculation
approach is an attempt to provide an adequate,
but not excessive, safety factor for the probable
variations in manufacturing conditions and in service
loading conditions. Extensive industrial
experience has gone into the selection of calculation
approaches and appropriate stress values
for the ASME Code.
While materials handbook values may be used to compare materials roughly, the
actual values used for calculations must either be taken from the tables in
the ASME Code or they must be selected using the same philosophy that was
used to select the ASME Code values.
c.
Nil-Ductility Transition Temperature. When some carbon steels are
reduced in temperature, their notch toughness is reduced to the point that
they have nil-ductility. The temperature at which this occurs can be as high
as 35 to 45 deg. F. Failures from this effect can be sudden and
catastrophic, therefore the piping system designer must avoid such failures
by selecting a material which has a nil-ductility temperature well below the
minimum operating temperature. (A design approach to this problem is
discussed in Chapter 2.)
Previous Page
