Phoenix's plans pay off on Zr project
by Richard W. Jenkins, Phoenix, A Division of Kodiak Industries, Inc.
Phoenix fabricated this zirconium heat
exchanger, which has been in service for more than a year now without any
problems The fabrication
of zirconium equipment occasionally requires heat treatment of all welds to
eliminate corrosion problems associated with the as-welded microstructure. This
heat treatment can present challenges unless one plans ahead.
In early 1995, Phoenix received an order for a large heat
exchanger requiring post weld heat treatment of zirconium welds. The Houston,
Texas-based fabricator immediately began planning.
Phoenix's Engineering Department, which has over 100 years'
combined experience in designing and building various heat exchangers,
recognized that the zirconium components would create some unique challenges
during the heat treatment process. It would also affect the methods used to
fabricate this particular heat exchanger and require some additional controls
on quality to ensure that the heat treatment did not adversely affect the
integrity of the unit.
The zirconium heat exchanger presented some difficult
challenges. The tubes were sixteen-ft-long seamless zirconium 702. The tube
sheets were 3/8-in.-thick zirconium 702 plate, explosively bonded to 2
1/2-in.-thick carbon steel plate. The shell of the heat exchanger was carbon
steel. The tubes were welded to the zirconium portion of the clad tubesheet.
Heat treatment of these welds was an area of concern due to limitations on
rates of both the heating and cooling cycles and the differences in thermal
expansion between zirconium tubes and the carbon steel shell.
The determination of an accurate thermal model based on
conductive, convective, and radiant heat flow during the heat treatment cycle
is an extremely complex problem. A worst case
simplification appeared to be the best approach.
Phoenix decided to use strip heaters to heat the face of the
tubesheets where the welds are located. This created the following conditions:
1.The most severe thermal expansion of
the tubes was expected to occur near the center of the bundle with the outer
perimeter of tubes acting as a heat shield.
2.The average metal temperature of the tubes was expected to
approach the temperature of the tubesheet (1300°F) due to the relatively long
heating times.
3.The outer perimeter tubes were expected to be at a
significantly lower temperature (900°F).
4.The average shell temperature was estimated to be 300°F
due to radiant and convective heat from the tubes.
These conditions give rise to the following potential
challenges:
1.Differential thermal expansion of the
zirconium tubes versus the carbon steel shell.
2.Differential thermal expansion of the core tubes versus
the perimeter tubes:
a. Core
tubes at 1300°F [4.7 x 10-6 x (1300-70) x 192 in.= 1.1 in.]
b. Perimeter tubes at 900°F [4.3 x 10-6 x
(900-70) x
192 in. = .68 in.]
c. Shell
at 300°F [6.37 in. x 10-6 x (300-70) x 192 in. = .28 in.]
d. Differential core tube versus
perimeter tube = .42 in.
e. Differential core tube versus shell
= .82 in.
While the expansion joint of this exchanger was designed to
accommodate a movement of approximately 1/2 in., it would be of no benefit to
the movement of the core versus perimeter tubes. This movement could have
caused significant bending in the tubesheet with possible permanent damage to
some of the tube-to-tubesheet welds. Therefore, the project team decided on a
solution that didn't rely on the expansion joint movement.
After reviewing all considerations, Phoenix determined the
following sequence was the best approach:
1. Heat-treat the unit in the vertical
position with only the top tube-to-tubesheet welds performed. The bottom
tube-to-tubesheet welds would not be closed until after the top tubesheet was
heat-treated. Tubes would be allowed to expand through the bottom tubesheet
during the initial heat treatment. Each tubesheet would be heat treated
separately.
2. Heat-treat the top tubesheet and, at temperature, measure
the thermal expansion of tubes in relation to the bottom tubesheet.
3. Perform a dye check of the top seal welds to assure that
the welds were not affected by the heat treatment cycle.
4. Seal weld the tubes to the bottom tubesheet and invert
the heat exchanger to place the bottom tubesheet at the top position.
5. Heat treat the bottom tubesheet at
the same time, applying enough separate heating to the shell to expand the
shell to match the measured tube expansion.
6. Perform a dye check of the bottom seal welds to ensure
the welds were not affected by the heat treatment cycle.
Phoenix used unique procedures to heat treat the Zr welds in
this unit
Phoenix chose Cooperheat to perform the actual
heat-treating. The La Porte, Texas-based company used flexible ceramic pad
(FCP) heating elements to accomplish post weld heat treatment. Cooperheat used
1-in.-thick 300 series stainless steel mesh to support the electrical
resistance heaters above the tubesheet. This prevented heating elements from contacting
the Zr tubesheet. It used 16 heating elements to cover the entire
54-in.-diameter tubesheet. Cooperheat technicians individually controlled and
monitored each element throughout the heating cycle.
An additional band of heating elements extended 8 in. below
the tubesheet on the carbon steel shell to achieve an even expansion of the
heat exchanger. The entire heating apparatus was then covered by insulation.
According to Phoenix's Engineering Manager, Rocky
Kuykendall, "Cooperheat did an outstanding job with the heat treatment
process." The customer asked for a temperature of 1425°F + 25°F
across the entire tubesheet for each heat treatment. Cooperheat held a maximum
range of 20°F across the entire tubesheet for each heat treatment.
The end result of this operation was a heat exchanger that
has now been in service for over one year with no problems.
In the fabrication of zirconium equipment, Phoenix has found
that advance planning can eliminate potential problems. When heat treatment of
zirconium is required, the special qualities of zirconium must be taken into
account.
For more information, contact Phoenix at 281-821-5297 and
ask for Rick Jenkins or Herschel Lain.
Q&A: MIC
by Te-Lin Yau, Corrosion Engineer
Question:
Why is Zr resistant to MIC?
Answer:
In corrosion, MIC stands for microbiologically induced or
influenced corrosion. Apparently, there isn't a consensus as to whether the
presence of living organisms is the primary reason or just a contributor to
corrosion of common metals and alloys. There is no
confusion, however, about the immunity of zirconium (Zr) to MIC. Results of
long-term tests in natural waters confirm Zr's resistance to MIC. Still, we can
suggest a few reasons.
Most organisms are sulfate-reducing bacteria. Metabolic
processes may produce corrosives, such as sulfuric acid, inorganic or organic
sulfides and organic acids. Common metals and alloys have a high affinity for
sulfur and its compounds. Conceivably, it is possible for bugs to eat certain
metals. Metabolic products simply make it much worse for common metals and
alloys.
Although zirconium is a reactive metal, it has little
affinity for sulfur and its compounds. Because of its reactivity, Zr is
typically protected by a layer of hard, adherent oxide film. Taking a bite of
Zr would be tough on the heartiest bug. Furthermore, it resists attack by most
inorganic and organic acids.
In addition, changes in oxygen potential, salt
concentration, pH, etc. from organisms won't bother Zr. Cathodic depolarization
associated with anaerobic growth is unfavorable to certain metals/alloys but
not Zr. Consequently, materials specifiers should not overlook Zr as the
structural material for any process equipment when natural water is used as the
cooling medium.
Developments in the CPI
Zirconium Meeting
The Organizing Committee of the first "International
Conference on the Use of Zirconium in Organics" is seeking papers. The
conference will be held September 8-10, 1997 in Gleneden Beach, Oregon at the
Salishan resort.
This event will bring together chemical producers, equipment
designers, and fabricators to address all issues concerning the use of
zirconium in organic environments. Although the conference focuses on organics,
inorganic chemicals will be covered as well since they are present in many organic
environments.
Conference topics include the performance of zirconium in
organic environments; comparisons between zirconium and other materials;
equipment design and fabrication; ideas on enhancing process efficiency,
improving product quality, and reducing costs; and safety. Other topic
suggestions are welcome.
Please send a paper title and a short abstract by January
31, 1997 to Te-Lin Yau, Wah Chang, P.O. Box 460, Albany OR 97321 or fax the
information to 541-967-6987.
The Conference will also feature a small exhibit hall and
two evening events. Companies interested in exhibiting or hosting a meeting
event can contact Kirk Richardson at 541-967-6955.
Scenic Salishan, located on the rugged Oregon Coast, offers
two golf courses, putting greens, tennis courts, and many other amenities. The
conference should prove to be an event to remember.
For further details about the conference, call 541-967-6977.
Ellet celebrates 75th
British Columbia-based Ellet Industries is celebrating 75
years of metals fabrication.
Founded in 1921 by Coppersmith Sidney J. Ellet, the
company's early emphasis was on fabrications using copper and brass for
breweries, distilleries, food and beverage facilities, and local shipyards.
As metals developed, Ellet moved from copper and brass metal
smithing into the world of stainless steel fabrication and into new markets,
such as the Petrochemical and Pulp and Paper Industries. Ellet also kept pace
with the demands for corrosion resistant fabrications in the many faceted
Chemical Processing Industry, working with new materials as they were
developed.
Today, Ellet continues on the cutting edge of the metals
fabrication industry, specializing in highly corrosion resistant metals, such
as zirconium, titanium, and niobium; marketing its skills globally; and
shipping equipment to the USA, Indonesia, China, Japan, Thailand, Australia,
New Zealand, Taiwan, South America, Pakistan, and Europe.
For more information about Ellet Industries, call
604-941-8211.
Ellet technician welding zirconium tubes to zirconium/clad
tubesheets
CPI, Ti groups add resources
Wah Chang's Chemical Processing Industries (CPI) and Ti/Nb
Products Sales Departments recently added valuable manpower resources.
Parry Walborn is now in charge of all aspects of Wah Chang's
CPI-related business -from sales and marketing through production to QA and
shipping. Walborn recently helped build the company's chemicals and high purity
titanium businesses and is considered one of its top managers.
Rob Henson changed positions from titanium salesman to
Process Industries Sales Manager. Henson now helps with sales to the CPI in
addition to maintaining his customers in the Mineral Processing Industries. His
background includes 12 years in corrosion research.
Steve Williamson also joins the CPI sales staff.
Williamson's background includes over 15 years in Wah Chang's X-ray and
Physical Testing Lab as well as work in quality assurance and production.
Tony Nelson recently moved from Wah Chang's QA Department to
Sales' Ti/Nb Products Department. Nelson's customers will undoubtedly benefit
from his 22 years' experience in QA and inspection.
Parry Walborn
Rob Henson
Steve Williamson
Tony Nelson
