PAPER_1A_BANKER_Study of Two Metal Heat Exchanger

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P A P E R
1 A
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Study of Two Metal Heat
Exchanger Failures:
Design, Fabrication, and
Environment Factors
JOHN G. BANKER
ABSTRACT
VICE PRESIDENT CUSTOMERS & TECHNOLOGY
Dynamic Materials Corporation
505 Spine Road
Boulder, Colorado 80501
USA
T: 303-604-3902
F: 303-604-1893
E: [email protected]
The authors’ support was requested
in the post-mortem analysis of two
very dramatic heat exchanger
failures. The failure studies
emphasize the importance of
expertise in design and fabrication
of bi-metal equipment. Both units
were shell-and-tube designs with
explosion clad tubesheets and
reactive metal tubes. One was a
nitric acid process condenser. It was
a zirconium tube unit with zirconiumstainless steel clad tubesheets, and
a stainless steel shell. The 14 year
old unit was out of service when the
failure occurred. The tubesheet clad
face disbonded dramatically while
plugs were being hammered into
leaking tubes. The post-failure study
indicated that a combination of
design and fabrication factors
contributed to ultimate failure. The
other was a titanium tube unit with
an inverted titanium-steel clad
tubesheet. The unit was used in an
offshore natural gas application.
Failure occurred while the unit was
in operating service. During the
event the titanium tubesheet clad
face fully separated from the steel
RICHARD SUTHERLIN, PE
MANAGER, TECHNICAL SERVICES
ATI Wah Chang
1600 Old Salem Road NE
Albany, Oregon 97321
USA
T: 541-967-6924
F: 541-924-6892
E: [email protected]
NEIL HENRY
PRINCIPAL CONSULTANT
ABB Global Consultancy
Daresbury Park
Daresbury, Warrington WA4 4BT
United Kingdom
T: 44 (0) 1925 741020
F: 44 (0) 1925 741212
E: [email protected]
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and all tubes were severed near the
tube-to-tubesheet welds. The
design of the unit was quite unique,
with the titanium clad on the shell
side of the tubesheet and the tubes
welded to the cladding on the back
face. The titanium-steel clad
interface was exposed to the
process gas on the inside of the
tubesheet bore holes. Although
there was minimal moisture present
in the gas, there was sufficient
moisture for corrosion of the steel in
the bore hole. Failure was attributed
to bondzone degradation resulting
from galvanic corrosion in the tube
holes and subsequent titanium
hydride accumulation at the
adjacent clad interface. In this
particular case, it would appear that
the potential for corrosion in the tube
bore was not adequately addressed
at the design stage or that the
operating conditions changed from
those originally anticipated. A broad
poll of clad manufacturers and heat
exchanger fabricators indicates that
this unit and others in the same
facility are likely the only units of this
design to have been manufactured
using titanium-steel construction.
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KEYWORDS
•
•
•
•
•
•
•
•
•
heat exchanger
titanium
zirconium
explosion clad
tubesheet
failure
nitric acid
UNS S32900
HP hydrogen gas
TITANIUM HEAT EXCHANGER
FAILURE IN OFFSHORE
SERVICE
Introduction
In May 2006 the UK Health and
Safety Executive, Hazardous
Installations Directorate, Offshore
Division issued Safety Alert
#1/2006 [1]. This document advised
as follows:
comprised of 550 titanium tubes,
Grade 2, 0.06" (1.6 mm) wall
thickness x 0.75" (19 mm OD). There
was one unusual aspect regarding
the design of the units; to increase
the tube density and unit efficiency,
the tubesheets were turned
backward with the titanium cladding
towards the shell side and the tubes
were welded to the titanium backside as depicted in Figure 1. This also
was the most cost efficient design to
present the titanium surfaces only to
the seawater cooling side. The
ligament width (shortest distance
between tube holes in the tubesheet)
was approximately 0.1181" (3 mm).
The heat exchanger was used in a
horizontal position with the tubesheet
mounted in the vertical at one end as
depicted in the sketch. The top half
of the U-bundle was the hot side, the
bottom half the cold side.
The unit was designed to cool
natural gas drawn from a sub-sea
field. When the field was depleted,
around five years before the failure,
the field was changed to a storage
duty, where gas was injected to the
cavity during the summer months
and withdrawn at periods of high
demand. Cooling was in four stages,
to condense water out of the gas
stream. This cooler was the final
stage of the stream, with cooling
seawater at ambient temperature on
the shell side and the gas cooling to
circa 140°F (60°C) tube-side.
Seawater was being used to
cool high-pressure (HP)
hydrocarbon gas. The shell,
tubes, and titanium cladding
sheet were torn from the steel
tubesheet and propelled
across the deck with sufficient
force to rupture an adjacent
exchanger. The cooling water
pipework and vent pipework
were torn off the shell and the
tube sheet and channel end
were ripped off the supports.
There was a significant and
immediate gas release
followed by ignition and an
explosion. Fortuitously there
were only two relatively minor
injuries, but under slightly
different circumstances there
could have been significantly
more serious casualties.”
Design & Fabrication
“A recent serious incident
occurred that involved the
catastrophic failure of a shell
and tube heat exchanger, and
there is a potential risk of
failure to heat exchangers of
the same, or similar, design.
This notice describes the
incident and outlines the
action that should be taken by
duty holders. The incident on
an offshore gas production
platform occurred when a shell
and tube production cooler
suffered a catastrophic failure.
The subject heat exchangers had
been designed and fabricated around
1982 by a reputable EPC and well
qualified fabricator in the UK and had
been in near continuous service
since. No prior issues with the unit
had been reported. The 66.929"
(1700 mm) OD tubesheets were
explosion clad, consisting of 0.500"
(13 mm) thick titanium, B265 Grade
1, clad to 8" (200 mm) thick steel
forgings,
SA-350
LF2.
The
tubesheets had been explosion clad
by Nobel Explosives in Scotland. The
units were U-bundles with each
Failure Analysis
Subsequent to the failure event, the
expert staff at the UK Health and
Safety Laboratory, Buxton, UK was
tasked with leading both analysis of
the cause of the event and the
related metallurgical failure analysis.
Initial study indicated that there were
no documented records of similar
events worldwide. The authors were
brought in as technical specialists
to assist with the latter. The simple
fact that the clad interface of the
tubesheet
had
apparently
instantaneously disbonded over the
Figure 1. Tube-to-tubesheet configuration.
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full area strongly suggested that the
explosion clad quality may have
been a contributing factor.
Tubesheet Manufacturing
Records
The clad manufacturer’s test records
and the component original material
test reports for both titanium and
the steel were obtained from the
heat exchanger manufacturer’s
Pressure Vessel Certification Data
Book. The ultrasonic inspection
records showed no nonbond
indications in the tubesheet product
area. The bond shear strength
report showed 33,600 psi (224
MPa), well above the specification
minimum of 20,000 psi (138 MPa).
The titanium yield strength was near
the bottom of the specification
range, a condition that is considered
preferable for explosion welding. In
summary, nothing suggested inferior
explosion bond quality in the original
product test records.
Failed Tubesheet Examination
The disbonded titanium cladding
plate had been recovered and was
presented for examination. To
persons skilled in the examination
of explosion clad interfaces, the
bond wave morphology can provide
significant information about the
quality of the original clad product[2].
The disbonded face of the titanium
cladder plate exhibited a very
uniform bond wave pattern. There
were no non-uniform areas which
would suggest anomalies in the
explosive energy or detonation rate
during the cladding process. There
were no indications of internal
stand-off devices or other residual
materials interrupting the collision
front during the cladding event.
However, several conditions were
visually obvious on the previously
bonded titanium surface:
• On the upper half of the
tubesheet, there were
considerable areas of steel
remaining on the surface, as
C O R R O S I O N
Figure 2. Disbonded clad surface, upper side of U-bundle unit.
Figure 3. Disbonded clad surface, lower side of U-bundle unit.
indicated by extensive rusting
of the formerly bonded
titanium face (Figure 2). This
indicated that the separation
partially occurred in the steel
adjacent to the bond. Further,
the presence of a ductiledimple separation nature
could be confirmed by the
nappy feel (Velcro-like) of the
surface. This visual condition
suggested an exceptionally
high quality interface. The
upper half of the unit was the
hot side of the U-tube bundle.
• The prior bond appearance on
the lower half of the tubesheet
was totally different. The wave
uniformity was still obvious.
However, there was no residual
iron and rusting. Also there
was no nappy texture to the
waves. They were well formed
but smooth. These factors
indicated that the failure in the
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lower half of the tubesheet had
occurred at the interface itself,
not partially in the steel. This
was the cold side of the tube
bundle. There was visual
discoloration around the tube
holes on the lower side of the
unit. This discoloration was
accentuated on the lower side
of each hole (Figure 3).
The examination of the steel side of
the disbonded tubesheet provided
additional insight. When examining
the tube holes in the steel from the
formerly bonded side, the following
things were noted:
• In the tube holes on the lower
(cold) side of the unit there
was visible corrosion on the
inside of the steel holes. The
corrosion was predominantly
on the bottom sides of the
horizontal holes and
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predominantly at the end of
the hole adjacent to the
titanium-steel interface. This
data suggested that water
had condensed from the gas
in the cold side of the unit.
This water had naturally set on
the bottom side of the holes
and caused localized
corrosion of the steel.
• No significant corrosion was
visible on the tube holes in the
upper (hot) side of the unit.
The surface of the failed bondzone
was examined metallographically and
spectrographically. On the lower half
of the formerly bonded plate, there
was a very high hydrogen
concentration at the disbonded
surface, approximately 1%. Whereas,
the hydrogen level of the bulk
titanium plate was approximately
0.0010%.
Surface
hydrogen
concentrations on the upper half of
the unit were similar to the bulk
hydrogen levels. The data indicated
that some event had resulted in
hydriding of the titanium bond
surface and that it was limited to the
lower half, or cold side of the unit.
The forensic evidence lead the
investigation team to the following
hypotheses (Figure 4):
• Aqueous corrosion of the steel
in the tube holes resulted in
the generation of atomic
hydrogen which diffused into
the steel.
• Where the steel was very
close to the titanium, this
hydrogen diffused to the
titanium face and was
available to form hydride.
• This part of the unit was operating at a temperature where
titanium hydride was stable.
• Over time the hydrogen
concentration at the interface
gradually increased to the
point where the titanium-steel
interface was covered by a
near continuous layer of brittle
titanium hydride. In this region
C O R R O S I O N
Figure 4. Probable cause of bond deterioration.
significant improvement in heat
exchange surface relative to the
overall size of the unit. This was
clearly a positive benefit to the
owner. In the absence of aqueous
corrosion the unit would likely have
performed admirably for many more
years. The fact that the unit
operated successfully for around 25
years suggests that the deterioration
was extremely slow.
This failure clearly emphasizes
the risk of exposing a thin-walled
titanium-steel interface to a
potentially aqueous corrosion
environment. One key word here is
“thin-walled”. The tubesheet
ligaments were only 0.12" (3 mm)
across. Clearly the exposed OD of
a titanium-steel heat exchanger
tubesheets are frequently exposed
to aqueous corrosion conditions,
but the bond width extending
inward from the OD is huge and the
OD is generally painted, reducing
the natural galvanic corrosion of an
unprotected couple. Although it is
generally a good idea to avoid
exposing a bi-metal interface to a
potentially galvanic corrosive
the bond was no longer tough
like in the top half of the unit,
but was significantly lower in
strength and brittleness.
• Over time the bond strength
gradually degraded to the
point that it was no greater
than operating stress and
residual stress loads on the
unit. At this point some event
initiated failure and the bond
began separating. Once it
began tearing itself apart, the
failure quickly travelled across
the face of the exchanger and
proceeded to tear the bond
apart in the upper section of
the unit where the fracture
proceeded along the steel
side of the interface. (Note:
There can be significant
residual compressive stresses
in bi-metal tubesheets. See
the explanation in the second
part of this paper.)
Retrospection
The design of the unit with a
reversed tubesheet allowed for a
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media, with proper design
considerations it can be done
without concern of catastrophic
failure. The classic example is use
of
aluminium-steel
bi-metal
transition joints to facilitate welding
between aluminium and steel
shipboard components. This highly
successful technology has been
used reliably for over 40 years [3,4].
Corrective & Preventive
Actions
Most heat exchangers with
corrosion-resistant alloy tubes and
clad tubesheets are designed with
the tube extending fully though the
thickness of the tubesheet and
welded to the clad face of the
tubesheet. The bi-metal interface is
locked between the tube on the
inside, the tube-to-tubesheet weld
on the face and one or more rolling
rings machined into the steel tube
hole on the back side. In this case,
the bi-metal interface is not exposed
to corrosive media if it exists.
Today there are other ways of
increasing heat exchange surface
density
that
are
generally
considered better and lower risk
than increased tube density. Similar
or better performance can be
obtained with finned tubes and
conventional
clad
tubesheet
designs at a lower risk.
A
broad
poll
of
clad
manufacturers and heat exchanger
fabricators indicates that this unit
and others in the same facility are
likely the only units of this design to
have been manufactured using
titanium-steel construction.
Plant. Stainless steel plugs were
being driven into the tube holes by
maintenance technicians using a
sledge hammer. With no warning,
the clad face of the tubesheet
separated from the unit in a highly
dramatic, explosion-like event.
Pieces of the zirconium clad face
were thrown nearly 120' (36 m).
Luckily, the three men working on
the unit received only minor injuries.
ATI Wah Chang and DMC were
asked to assist in the Cause and
Corrective Action studies.
The Nitric Acid Plant was a
Weatherly design unit with the
original 304-CE heat exchangers
constructed of 7Mo stainless steel
(UNS S32900). The two units
operated at 140–160°F (60–70°C)
on the tube side and 100°F (38°C)
on the shell side. The stainless units
had been replaced frequently. In
early 1994 a decision was made to
replace the deteriorating stainless
steel units with zirconium. The
design was modified by replacing
the stainless steel tubes with
zirconium 702 and the stainless steel
tubesheets with zirconium-stainless
steel explosion clad. The shell
remained as stainless steel.
The two new units were
installed in December of 1995.
About six months after start-up the
new exchangers were exposed to
hydrofluoric (HF) following an
incident where freon leaked from an
upstream chiller unit. Upon
examination, significant corrosion
of the zirconium was observed in
both units, with one showing
significantly greater damage. The
more severely damaged unit was
removed from service and replaced
with a stainless steel spare. A
replacement zirconium unit was
ordered. The other zirconium unit
was turned around (swapped end
to end) and returned to service. This
unit continued to operate without
significant issues until September
2006. At that time, leaking was
observed in one tube. During the
shutdown, the leaking tube was
plugged and other tube ends were
inspected. Cracks were observed
in 30 other tube-to-tubesheet
welds. These were repaired by
overlay welding. Three months later,
12 more tubes were plugged for
leaks and more weld repairs were
made. In late April 2007, an
additional seven leaking tubes were
repaired. At this time a decision was
made to purchase a replacement
unit. On May 30, 2007 the unit was
again shut down for repair of leaking
tubes. The unit blew apart while the
maintenance team was plugging the
10th hole of the 30 tubes scheduled
to be plugged.
Figure 5. Damaged tubesheet face.
ZIRCONIUM HEAT
EXCHANGER FAILURE IN
NITRIC ACID SERVICE
Introduction
On May 30, 2007 a maintenance
crew at Terra’s Port Neal Plant was
repairing leaking tubes in a heat
exchanger at the #1 Nitric Acid
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Failure Analysis
Figure 5 shows the damaged tubesheet face. Large areas of the
zirconium cladding had been ripped
away from the tubesheet face. Closer
examination revealed several things:
• The zirconium was disbonded
from the stainless steel base
plate over essentially the
complete tubefield.
• The zirconium remained
bonded to the stainless steel
plate over more than 75% of
the tubesheet perimeter.
• The thickness of the zirconium
cladding metal was typically in
the range of 0.080–0.120"
(2–3 mm).
Figure 6 shows the top surface of a
disbonded web section between
four tubes. The width of the cap of
the tube-to-tubesheet welds was
typically 0.200–0.250" (5–6.5 mm).
Figure 6. Top surface of a disbonded
web section .
Figure 7. Bottom Surface of a disbonded
web section .
The machining marks / grooves are
still visibly obvious in the central area
that is not covered by the welds.
Figure 7 shows the opposite
face of this sample (formerly bonded
surface of the zirconium cladding).
The characteristic wavy interface of
the explosion weld is clearly obvious
in the center of the piece. However,
around the tube holes there is a
region typically 0.150" (4 mm) wide
where the explosion weld bond face
is completely disrupted.
Figure 8 presents a cross-section
of the same specimen. Several
significant things are shown here:
• The solidified zirconium weld
Figure 8. cross-section of Webs and Welds from specimen evaluated.
3
1
2
2
1
1. Zr3 Fe eutectic.
2. Light element contamination.
3. Cracks filled with eutectic.
C O R R O S I O N
Continuous Zr-Fe (intermetallic layer at
interface).
1. Typical weld structure.
2. Abnormal due to O, N, and Iron.
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metal contains significant
areas of ZrFe 3 eutectic
indicating that the molten
zirconium weld metal had
penetrated to the stainless
backing metal and mixed with
molten stainless steel.
• Microcracking had occurred
during solidification and the
cracks had back-filled with the
ZrFe 3 eutectic. This indicates
that the cracking had
occurred in the early stages of
solidification.
• Significant light element
contamination (primarily
oxygen and nitrogen) was
observed indicating
inadequate shielding during
welding. These conditions are
well-known to result in brittle
zirconium welds. (Hardness
measurements, not included in
the pictures of this paper,
confirm significant hard areas
particularly in the lower weld
passes.)
• A near-continuous layer of ZrFe intermetallic was observed
at the interface. This is
typically characteristic of
interfaces which have been
heated significantly above
1500°F (800°C).
resulting in the unit’s deterioration
were primarily design and fabrication
related. There was less consensus
regarding the cause of the ultimate
explosive-like failure. The two lead
hypotheses are:
expansion joints in the shell.
The 47.625" OD (1210 mm)
tubesheets were manufactured by
Explosive Fabricators Inc. (now
DMC) in August 1994. The cladding
metal was 0.188" (4.8 mm) nominal
thickness zirconium, SB-551 Alloy
702, manufactured by ATI Wah
Chang. The oxygen level was 650
ppm, well below the 1000 ppm level
considered the upper limit for
optimum explosion cladding [5]. The
purchase order and the production
documents specified that the
zirconium clad face be flat within
0.125" (3.2 mm). Inspection and test
reports indicated that the bond
shear strength test results were
39,000 psi (260 MPa) and that
flatness was within the 0.120" (3
mm) specified.
The fabricator was a highly
regarded shell and tube exchanger
manufacturer who had produced
several of the earlier stainless steel
units for Terra. They had significant
prior experience with clad tubesheet
design and fabrication, but no prior
experience
with
zirconium.
Production records indicate that the
units were fabricated under
considerable delivery pressure.
• An unknown explosive material
accumulated between the
zirconium and stainless steel.
The impacts of tube plugging
initiated an explosion.
Considering that Nitric Acid
and Ammonium Nitrate are
both existing in the facility, this
is a plausible conclusion.
• The design of the unit
inherently resulted in very high
residual stress levels. The
release of these stresses
during the failure event
resulted in a very intense
energy release. Much like a
catapult, the energy release
caused some pieces to be
thrown a long distance.
The authors are in the latter camp.
A reconstruction of what likely
happened follows.
Fabrication
In shell and tube exchanger
manufacture, it is common to
machine the tubesheet faces flat
and parallel prior to beginning
drilling and fabrication. The
machining grooves on the zirconium
clad face, Figure 6, indicated that
these tubesheet blanks were
Hypotheses Regarding the
Cause of the Failure
Design, Cladding &
Fabrication Records
The fabrication drawings were
modifications of the earlier stainless
steel unit drawings. The tube
specification was changed to: Zr
702, 1" OD x 0.065" wall (25 mm x
1.6
mm).
The
tubesheet
specification was changed to
Zirconium explosion clad, consisting
of 3" (75 mm) minimum thickness
stainless steel, SA-240 Type 304L,
clad with 0.188" (4.8 mm) thickness
Zirconium, B-551 Alloy 702. The
tube-to-tubesheet weld design
shows a J-groove weld prep
machined in the cladding face. The
shell of the unit was solid stainless
steel, Type 304L. There were no
C O R R O S I O N
Several teams investigated the
failure and hypothesized regarding
the cause of the unit’s deterioration
and the eventual dramatic failure.
The consensus is that the events
Figure 9a. Schematic showing a flat machined surface resulting in varying cladding
thickness.
Zirconium
S O L U T I O N S
~0.09" to 0.06"tk min.
(2.5 to 1.5 mm)
C O N F E R E N C E
Machined Flat
Bondzone ~0.125Ó
out-of-flat
Stainless Steel
®
0.188"tk max.
(48 mm)
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machined flat. This would have
resulted in a significant reduction of
the zirconium cladding thickness,
see the schematic in Figure 9a. This
would also explain why the
thickness of the disbonded
zirconium cladding pieces was well
below the original cladding
thickness.
In accordance with the heat
exchanger (HX) drawing, the
thickness of the zirconium was
further reduced in the area
immediately adjacent to the tubes
during the machining of the Jgroove weld preps, Figure 9b. It is
easy to speculate that the thickness
was below 0.060" (1.5 mm)
following the two machining
operations, and that the weld preps
might actually have penetrated
through the bondzone into the
stainless steel in some areas.
It is clear from the examination
of the weld metal that the tube
welds penetrated significantly into
the stainless base metal (schematic
Figure 9c), resulting in significant
Following the two hour stress relief
cycle, the parts were slowly cooled
to ambient. The contraction of the
stainless steel base plate was
essentially three times as great as
what the zirconium would have
contracted, if it had been free
standing. However, since the Zr was
fully welded to the stainless, the
zirconium was forced to contract
nearly as much as the much thicker
and stronger stainless steel base
plate. Consequently, the zirconium
cladding layer was left in a significant
compressive residual stress state.
Again, since the zirconium was fully
welded to the stainless steel, the
significant metal removal of the tube
hole drilling did little to affect this.
During
fabrication,
the
tubesheets were welded to the
stainless steel shell prior to insertion
of the tubes. The zirconium tubes
were then inserted into this stainless
steel structure and locked into place
with massive welds at each end.
Welding solidification likely added
some residual tensile stress to the
tubes. But more significantly, the
welding reduced the residual bond
area by roughly 30% by melting it
away, and then greatly reduced the
strength of any remaining bond via
intermetallic formation. On the other
hand, at the perimeter of the
tubesheet in the gasket area, there
was no welding and the bond
remained strong and intact. As a
result of all of this the zirconium
cladding layer became much like a
very large Bellville Spring held back
by the tubes.
During operation, the differential
thermal expansion of the shell and
the tubes further increased the
tensile stresses on the tubes and
the tube welds.
degradation of the explosion clad
interface and in cracked, hard, and
very poor quality zirconium welds,
Figure
8.
The
continuous
intermetallic shown in the lower right
frame of this figure further indicates
that heat of the additional cover
passes significantly reduced the
strength and toughness of the
explosion weld over the full width of
the web between the tubes.
Residual Stresses
Significant residual stresses were
developed during fabrication and
service as the result of differential
thermal expansion. The coefficients
of thermal expansion of zirconium
and stainless steel differ by a ratio of
1:3 (3.2 x 10(-6) in/in/°F for zirconium,
vs. 9.6 x 10 (-6) in/in/°F) for stainless
steel (5.8 and 17.3 mm/mm/°C
respectively). Following the explosion
cladding event, it is common
practice to stress relieve the clad
plate at 600°C (1100°F) to improve
bond ductility and toughness.
Figure 9b. Schematic showing location of deep J- groove weld preparation.
Figure 9c. Schematic showing penetration of zirconium weld into stainless steel base
metal.
Failure Hypothesis
Over time, the cracks in the tubeto-tubesheet welds gradually grew
under the thermally induced
stresses. After 10+ years of
operation these cracks grew into
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PAPER 1A — STUDY OF TWO METAL HEAT EXCHANGER FAILURES: DESIGN, FABRICATION, AND ENVIRONMENT FACTORS
leak paths and the more stressed
tubes began to leak. During tube
plugging, any remaining residual
strength that the tube weld had was
further destroyed. As the number of
plugged tubes increased in the
bottom part of the unit, the stresses
on the adjacent tubes and tube
welds naturally increased. During
the plugging of the 10th hole on
May 30, stresses finally reached the
point that the tubesheet face
unzipped dramatically and the highly
stressed zirconium face released
itself in an explosive-like fashion. In
this instantaneous event, a large
area of zirconium fracture surface
was exposed, releasing more
energy and likely some visible light.
Cause and Corrective Action
The primary causes of this failure
were design and fabrication aspects
which were not aligned with
generally accepted good practice.
Several changes could have
prevented the failure and are
extensively used to produce
successfully performing similar units
worldwide:
• Simply increasing the cladding
metal thickness from 0.188"
(4.8 mm) to 0.375" (9.5 mm)
would have eliminated most of
the problems. Interestingly,
the replacement unit ordered
following the 1996 HF
problem (Freon release) and
produced by the same
fabricator was constructed
with 0.375" (9.5 mm)
zirconium cladding. In a close
review of zirconium clad
tubesheets supplied by EFI /
DMC over the past 20 years,
the four tubesheets for the
two Terra Port Neal units were
the only ones that were not
C O R R O S I O N
unmentored fabricators. Today,
there are many ways to gain
experience other than by trial and
error. The primary reason for the
authors to document these two
unpleasant events in significant
detail is to help others avoid similar
pitfalls in the future. It is very
important to note that although we
have addressed two failures, there
are
thousands
of
titanium
exchangers and hundreds of
zirconium exchangers that are
performing reliably and many that
have performed far in expectation
of that of their owners. a
0.375" (9.5 mm) or heavier
nominal thickness.
• The 0.188" (4.8 mm) cladding
layer would have been
adequate if the surface had
not been machined flat and the
j-groove weld preps had not
been made. There is very
extensive use of 0.188" (4.8
mm) thick titanium cladding in
power plant condenser
construction. If the clad faces
are not machined flat, welds
are made very successfully. In
fact, there is probably more
total clad tubesheet area used
in this single application than
in all other titanium and
zirconium heat exchanger
applications combined.
• The tensile stresses on the
tubes and the tubesheet
welds would have been
significantly reduced if an
adequately designed
expansion joint had been used
in the stainless steel heat
exchanger shell.
REFERENCES
1.
2.
Retrospection
As noted above, the 1996
replacement unit was manufactured
by the same company as the failed
unit and the proposed corrective
actions were taken at that time.
Subsequently this fabricator has
reliably supplied both titanium and
zirconium tubed exchangers using
clad tubesheets. The 1994 Terra
units were a learning event.
Regretfully Terra eventually paid a
significant price for this experience.
3.
4.
5.
CONCLUSION
It is the authors’ observation that
most “problem” reactive metal
vessels and heat exchangers are
the work of inexperienced and
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Issue Brief, Ref
#:HSSE/HSE/IB/2006-2,
“Catastrophic Failure of Shell
and Tube Production Cooler,”
UKOOA Oil and Gas of Britian,
p 145, May 2006.
J. Banker and E. Reineke,
“Explosion Welding,” ASM
Handbook, Vol 6, Welding
Brazing and Soldering, pp
303-305, ASM International,
1993.
C. McKenney and J. Banker,
“Explosion-Bonded Metals for
Marine Structural
Applications,” Marine
Technology, Society of Naval
Architects and Marine
Engineers, pp 285-292, July
1971.
J.G. Banker and J. Visser,
“Reliable Welding of Aluminum
to Steel,” The Naval Architect,
pp 19–20, July / August 2005.
A. Nobili, J. Banker, and C.
Prothe, “Continuing Innovation
in Zirconium Explosion Clad
Manufacturing,” Corrosion
Solutions Conference
Proceedings, ATI Wah Chang,
September 2001.
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