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Engineering
& Design > Service Life Factors
General
Corrosion • High Velocity/Chemical
Abrasives • High Cycle/Chemical
Media
Stress
Corrosion • Intergranular
Corrosion/Attack • Pitting
Corrosion • Fatigue
High Flow
Velocity • Torsion
• Vibration
• Low Pressure
Applications
The following describes
various service life factors in corrugated metal hose applications. The
information is based on our experience as a manufacturer of metal hose,
braid products and metal hose assemblies. While this information is intended
to be a general guide, each application should be evaluated individually
because of the many variables that affect service life of metal hose assemblies.
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General Corrosion
Uniform attack through the entire corrugated length of the metal
hose assembly is mostly described as general corrosion. Attack on
the alloy is affected by chemical concentration, temperature and
the type of alloy from which the metal hose is manufactured. Some
typical areas of attack include the root or bottom of the corrugation
and in the heat affected weld area.
Most stainless
alloys form a protective film of stable oxides on the surface when
exposed to oxygen gas. The rate of oxidation is dependent on temperature.
At normal temperatures, a thin film of oxide is formed on the alloy
surface. Higher temperatures will cause oxidation to proceed more rapidly.
The oxides that
form on copper or nickel alloys are of a nonporous oxide formation.
A nonporous oxide formation will provide a protective layer on the
surface but if the layer is removed, no protection is provided to
the underlying metal.
Factors when
selecting piping and corrugated metal hose assembly materials should
consider that the piping is a rigid member and the hose assembly
will be subject to flexing. As outlined later in the Service Life
Factors section, several factors associated with flexing affect
the service life of metal hose.
Service life
may be affected by factors external to the metal hose assembly.
Consideration should be given to the chemical composition of the
environment surrounding the hose assembly as well as the media being
transferred when selecting an alloy.
OmegaFlex does
not publish corrosion resistance data because of the many variables
present in metal hose applications. Many reference materials are
available and provide accurate corrosion data. The Corrosion Data
Survey published by the National Association of Corrosion Engineers
(NACE) is one of the many sources of reference for corrosion resistance
information.
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High Velocity/Chemical
Abrasives
Turbulent flow of abrasive chemical media over the alloy surface may
cause accelerated corrosion or erosion-corrosion. Liquids or gases
that have suspended solid particles will wear or remove the oxide
protective film and leave the alloy exposed and more susceptible to
corrosion. Some forms of flow assisted corrosion include terms such
as cavitation or impingement. Reducing the velocity or incorporating
a liner in the metal hose assembly may reduce the effects of this
type of abrasion. |
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High Cycle/Chemical
Media
Applied stresses such as flexing or cyclic motion may reduce the oxide
film surface effectiveness against corrosion. Cracks, resulting from
cycling of the hose assembly, form in the protective oxide layer on
the surface of the alloy thus reducing the effectiveness against corrosion.
The introduction of a corrosive environment often eliminates the fatigue
limit of the alloy creating a finite life regardless of stress level. |
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Stress Corrosion
The detailed mechanism of stress corrosion is complicated and not
well understood. The process of stress corrosion seems to be one
of initial formation of corrosion pits and crevices, and subsequent
fracture due to stress concentrations associated with the crevices.
Stress corrosion cracks often follow crystal boundaries in the grain
structure of the alloy. Visual examination of high cycle/chemical
media and stress corrosion failures appears similar. Application
data specifying media, temperature and movements is very useful
in order to determine the exact cause of failure.
Chlorides and
caustics are the media most frequently found to cause stress corrosion
cracking. Relieving stresses or selection of an alloy known for
resistance to the conveyed media are possible ways to reduce this
type of failure.
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Intergranular
Corrosion/Attack
Corrosion along the grain boundaries of the metal may occur and
the grains of metal separate from the mass causing loss of strength
and ductility. Failure due to loss of ductility is also known as
brittle fracture. Alloys such as 304L or 316L have been developed
to reduce the effects of intergranular corrosion. These low carbon
alloys have a grain structure that is more resistant to corrosion.
Intergranular
attack may occur when certain grades of stainless steel such as
304 or 316 are subjected to temperatures beyond 800°F. Chromium
can precipitate out of solution, bonding with carbon and forming
chromium carbides on grain boundaries at this temperature and above.
Reduced protection from the loss of chromium when combined with
corrosive media leaves the grain structure exposed to possible corrosive
attack.
Using a stabilized
grade of stainless steel, such as T321, is an effective method of
preventing sensitization. Stabilized alloys sacrifice the stabilizing
element to the carbon thus preventing loss of chromium in the grain structure.
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Pitting Corrosion
Highly localized attack with the appearance of a relatively sharp
or well-defined boundary and a surrounding area that appears unattacked
is referred to as pitting corrosion. Pitting may occur in crevices,
inclusions, imbedded iron or other metals, also items such as marine
organisms in sea water adhering to metal surfaces or grease. |
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Fatigue
Failure of a progressive nature due to the flexing of the corrugations
is known as fatigue. Stress generated by flexure, pulsation, torsion,
vibration and flow induced vibration are some causes for fatigue
failure. Continual small cracks form in the metal. Fatigue cracks
often originate at small imperfections, such as non-metallic inclusions,
within the metal. Stress will concentrate at the crack and further
cycling will increase the size of the crack until a complete fracture
occurs. Fatigue failures normally occur as a circumferential crack
at the top or bottom of the corrugation.
Additionally,
high flow velocity may cause the corrugations to vibrate at a high
frequency and resonance vibration may occur. See High Flow Velocity
below. Increasing the bend radius will decrease the stress level
in the individual corrugations. Changes in corrugation count of
the hose or control of the motion may also increase hose life.
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High Flow
Velocity
Applications where the flow of a liquid or gas is above manufacturer
recommended levels and a liner is not incorporated into the hose
assembly design often results in premature fatigue failure. The
high flow velocity causes the corrugations to vibrate at a high
frequency and, if the vibration is near the natural frequency of
the hose, failure will occur very quickly.
Spider web type
cracks and fractured pieces of metal breaking from the corrugations
are typical appearances for this type of failure. Reducing the velocity
by increasing the hose diameter or the use of an interlocked type
liner are possible ways to avoid high flow velocity failures.
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Torsion
Rotation about the longitudinal axis develops a shear stress in the
metal that can cause premature failure. Twisting the metal hose assembly
during installation or as a result of movement in two planes can produce
cracks that start circumferentially on the crown or outside of more
than one corrugation and progress longitudinally. Torsion is one of
the most common causes for premature metal hose failure. Incorporating
a lay line on the metal hose assembly will provide means for determining
if the hose is rotating about the longitidinal axis. |
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Vibration
Vibration failures start as very small or irregular cracks, primarily
close to the vibration source, around the circumference of the corrugation.
The cracks may progress to the corrugation wall in the form of a “Y”.
Extreme braid wear on the crown or top of the corrugation is usually
present. If the vibration is near the natural frequency of the hose,
failure will occur very quickly. Corrugated metal hose may be harmonically
tuned to compensate for damaging frequencies. |
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Low Pressure
Applications
Caution must be used when unbraided metal hose assemblies are used
in low-pressure applications such as engine exhaust. Proper installation
practices, as outlined by the Expansion Joint Manufacturers Association
(EJMA), utilizing piping guides and anchors must be observed to prevent
premature failure of the metal hose assembly. The addition of braid
should be considered for vibration attenuation. |
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