POWER July 2011 - 59

MATERIALS
Titanium tubing remains arguably one of the
most tested of the competing ASTM condenser
tube materials requiring full eddy current, ultrasonic,
and a final pneumatic proof testing. Supplementing
this rigorous testing, improvements
in the manufacturing and installation process at
both the condenser fabrication facility and in the
field have led to improved overall quality and
increased product throughput.
It is particularly noteworthy that the latest
computer-controlled technology is now being
applied within the process stream to dramatically
impact automatic tube end welding
and mechanical expansion of the tube-totubesheet
joint, resulting in improved and
consistent joint strength and seal. Both new
unit designs and modular changeouts have
benefited from these innovations.
Current in-field retubing projects can now
employ these sophisticated computer modeling
techniques and associated process tooling
advancements, allowing highly successful
mechanical tube joint expansion of thin-wall
titanium tubing into softer tubesheet material
such as Muntz Metal and Naval Rolled Brass.
Table 1. Chemical requirements of
Grade 2 titanium. Source: Valtimet Inc.
Component
Carbon (max.)
Oxygen (max.)
Nitrogen (max.)
Hydrogen (max.)
Iron (max.)
Others (max. each)
Others (max. total)
Titanium
Weight (%)
0.08
0.25
0.03
0.015
0.3
0.1
0.4
remainder
Mechanical expanded
2. Even more groovy. The ID groove
enhancement process is now commonplace
where thin-wall tubing can be successfully expanded.
Source: Valtimet Inc.
18
16
14
12
10
8
6
4
2
July 2011 | POWER
Expanded + ID groove
Expanded + welded
This groove enhancement process to the inside
diameter (ID), as it is known (Figure 2),
is now commonplace where thin-wall tubing
(as thin as 24 BWG/0.022 in. wall thickness)
can be successfully expanded, resulting in
pull-out loads well within the bandwidth of
best practices (Figure 3). This process was
not possible even five years ago.
Following the success of these early retube
projects, today's in-field retubing projects can
now use sophisticated computer modeling techniques
and associated process tooling advancements
that allow highly successful mechanical
tube joint expansion of thin-wall titanium tubing
into softer tubesheet material such as Muntz
Metal and Naval Rolled Brass. These tooling advancements,
coupled with ID groove enhancement,
are now commonplace where thin-wall
tubing (such as 24 BWG/0.022 in.) can be successfully
expanded, resulting in pull-out loads
well within the bandwidth of ASME Code Section
VIII, Division I Nonmandatory Appendices
requirements and best practices acceptability.
It is also noteworthy that should titanium clad
materials be used for the tubesheet, as opposed to
a solid B-265 plate, seal welding of the tube-totubesheet
joint is strongly suggested, if not mandatory.
A mechanical expanded-only joint will
not provide the proper sealing and will potentially
expose the parent carbon steel to corrosion
products that would invade the annulus between
the tube and drilled tubesheet (Table 2).
Generating companies in Florida, Arizona,
South Carolina, and Alabama have recently selected
titanium for their retubing projects. They
selected Gr. 2 titanium tubing not only because
of its acknowledged 40-year corrosion immunity
but also for recent innovations in thin-wall tubing
expansion into a softer tubesheet material.
When titanium condenser tubing is used in
both new and in-field retubed units, a more generous
cleanliness factor (Uc) can be considered
when performing the heat transfer calculations.
Values such as 90% Uc are commonplace in
today's designs. A higher cleanliness factor can
reduce the amount of heat transfer surface or,
alternatively, increase performance as a result of
reducing the overall fouling factor. Additional
benefits could include reduced circulating water
losses when thin-wall tubing is used and, because
titanium is immune to underdeposit pitting,
a tubeside freshwater flush is not needed.
Stainless alloys and other candidate materials,
on the other hand, can be irreparably damaged
should corrosive water be left standing in the
tubes during an off-line condition.
Corrosive Activity by Any Name Is
Still Corrosion
During the early years of the titanium retub3.
Holding power. A series of tests were performed using 1-inch-diameter titanium Gr. 2 B-338
tubes with a Gr. 2 B-265 tubesheet to determine the load necessary to pull the tubes from the
tubesheet after proper installation. The three pull-out tests include standard mechanical expansion,
mechanical expansion with an ID groove, and mechanical expansion plus tube-to-tube sheet welding.
Test data on the first two installation options demonstrate that an acceptable tube joint is possible
within a shop manufacturing environment. The tests also found that mechanical expansion alone
produced marginal pull-out loads for thin-wall tubes and that thin-wall tubes (30 BWG and 27 BWG)
installed with only mechanical expansion were problematic. Source: Valtimet Inc.
Danger area
500
1,000
1,500
2,000
Pull-out load (lb)
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59
2,500
3,000
3,500
4,000
Wall reduction (%)

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POWER July 2011

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