TITANIUM SHELL & TUBE HEAT EXCHANGERS ARE FREQUENTLY USED THROUGHOUT THE WORLDS PROCESS, ENERGY, POWER AND PETROCHEMICAL INDUSTRIES.
The equipment is often required for highly corrosive, sometimes toxic and high-pressure applications, so operational integrity is everything.
Such integrity has to be designed in from the embryonic stages of conceptual design. As such, a designer / manufacturer of these products needs a strong, holistic knowledge of codes, standards, materials and manufacturing techniques. Typical requirements include a combination of solid titanium and titanium-steel clad components, including tubesheets, shells, and dished heads. The clad components can present unique issues for, forming, and fabrication. And the high operating temperatures and pressures can present unique design considerations to accommodate the thermal expansion mismatch between titanium and steel.
Our team is adept at meeting these design and manufacturing challenges and has a number of recent references. In recent months, the business was challenged with reverse engineering replacement bundles for a pair of AES type (floating head) debutaniser condensers. Previous inspection records informed the operator that both shell and tube side media should be considered highly corrosive at the service temperatures and flow rates. As a semi emergent requirement in a low oil price climate, there was additional pressure on schedule and cost too. The most economical solution was a titanium clad carbon steel tubesheet (clad on both shell side and tube sides) with titanium tubes, titanium baffles and a floating head with a combination of cladding and loose lining of the carbon steel parent material. The inter-relation between design, manufacture, and inspection issues and the related equipment performance considerations are presented in this report.
TUBE & TUBE SHEET SELECTION
Tube sheets represent the most common and simplest utilisation of titanium clad components with the titanium on the tube-side of the heat exchanger; however, in this application, both sides were clad.
There are several reasons for cladding the tube-sheets, not least of which is it avoids a solid, costly titanium tube sheet which would create dimensional discrepancies in a replacement bundle, due to the differing material stresses.
A choice of titanium for the tubes to handle corrosive conditions leads to a clad tubesheet in order to facilitate the tube-to-tubesheet joining and sealing. Rolled-only joints are generally not acceptable for titanium tubes due to the material’s relatively high yield stress and its ‘elastic memory.’ Due to these properties, the tubes tend to shrink or contract after rolling, creating leaks. Welding of the tubes to the tubesheet is, therefore, required and this requires a titanium face to the tubesheet. In this application, a strength tube to tubesheet weld was specified.
Another reason for the use of titanium-clad tubesheets is to bridge the difference between the material requirements of the shell-side and the tube-side of the heat exchanger. In the chemical process industry, the process fluid is often the corrosive fluid and the heat exchanger is configured for this fluid to be on the tube-side for cleaning and maintenance. This fits well with heat exchanger thermal design as cooling water, condensing steam, boiling water, and hot oil are all heating or cooling media that work well as shell-side fluids. These also have minor or manageable corrosion issues and are usually handled with steel.
The backer material for clad tubesheets is not always carbon steel. The alloy used may be varied to match the material requirements of the shell side. Stainless steel and other alloy backers are frequently used. Forged tubesheet discs are required by the ASME Code for this kind of construction. Cladding of a non-hub side of a forging is not significantly different than cladding a plate material. Explosion cladding of the hub side is considerably more complex and is rarely used. When cladding of the hub side is mandatory, unique design concerns at the clad perimeter must be addressed.
CLADDING METAL THICKNESS CONSIDERATIONS
The titanium that is explosively clad to the tubesheet disc is generally thicker than what is used for cladding on cylinders and heads. The thickness is selected to accommodate the tube to tubesheet welding and any gasket surface machining that is required on the titanium face. The Standards of the Tubular Exchanger Manufacturers Association sets minimums for cladding thickness. TEMA paragraph RCB-7.8 sets the minimum nominal cladding thickness at 3.2 mm when the tubes are welded and 8 mm when they are not. This is a starting point for the consideration and the actual joint configuration and the tube to tubesheet welding technique must be considered. The depth of this chamfer for seal or strength welding plus a machining tolerance and some allowance for weld penetration sets the real minimum titanium cladding thickness in the tube field. Titanium clad tubesheets are manufactured by explosion welding. Explosion welding is a unique technology which uses the energy of a chemical explosive to create fusion and welding conditions. The extreme rapidity of the process does not permit sufficient time for formation of deleterious intermetallic compounds which occur during other, slower technologies. Explosion cladding is a well-developed and industrialized manufacturing technology. Explosion clad materials are manufactured by a limited, but worldwide, group of specialists with the combined explosive and metallurgy expertise required to assure product reliability.
THE REQUIRED MACHINING ON THE TUBESHEET FACE ALSO AFFECTS THE TITANIUM CLADDING THICKNESS AND IS OFTEN A MAJOR DETERMINING CRITERION.
Several factors are at work: the flatness of the disc that can be achieved by the producer; the requirements for gasket surfaces and pass partitions, whether confined gasket joints are required; and whether the full face is required to be machined flat.
When working with low cost metals, the cladding face is commonly machined flat; it simplifies assembly. When working with expensive metals, such as titanium, zirconium, and the nickel alloys, the assembly advantages rarely out-weigh the added metal cost. Generally, tubesheets with titanium clad on the tube-side have a gasket surface for joining with the floating head or channel – otherwise little or no machining is required. Pass partitions when present, require gasket surfaces to be machined across the centre of the tubesheet; whereas without them, machining is required only for the pressure gasket surface is around the periphery of the disc. The requirement for a confined gasket joint is set by service level, usually categorized by the TEMA classes R, C or B. For severe services, corresponding to TEMA R, paragraph R-6.5 requires confined joints. For moderate and chemical process services, corresponding to TEMA C and B, respectively, paragraph CB-6.5 leaves the decision to the user. The effect of confined joints on the thickness is to add 4.8 mm of cladding material for producing the groove.
A clad tubesheet disc is manufactured to be flat within a specified tolerance which is determined by press capacity and technique limits. This tolerance can range from as little as 3 mm on tubesheets under 1500 mm diameter up to as much as 13 mm on tubesheets over 3000 mm diameter. Without pass partitions, an out of flatness condition can be tolerated, thus the centre of the tubesheet doesn’t require machining. For example, cladding thickness determination for a 2000 mm tubesheet with TEMA R pass partitions and confined gasket joints would be as follows and as shown in Figure I. (The specified out-of-flatness of the product permitted in the cladding specification, ASTM B898, Section 10.3.1.2, is 5.5mm)
• Minimum cladding (TEMA): 3.2mm
• Allowance for gasket and pass partition confinement (TEMA): 4.8mm
• Allowance for face out-of-flatness (B898): 5.5mm
• Under gage tolerance from nominal (B898): 1.5mm.
The sum, 15 mm, is the nominal titanium thickness that is required prior to clad manufacture. Assuming standard inch-dimension sizes, the thickness would be adjusted upward to the next standard which is 16mm. The only possibility to reduce this is to improve flatness of the clad disc, which can sometimes be negotiated with the clad manufacturer.
A titanium clad tubesheet introduces minimal additional inspection points into either the fabrication or maintenance of the heat exchanger.
During the manufacturing of the clad tubesheet disc, the bond between the titanium and the backer is tested and proved for its soundness. ASTM Specification B898 requires that all clad plate be ultrasonically tested for bond quality. Two acceptance standards, A and B, are applicable for tubesheets. B898 also specifies that the manufacturer shall assure that the clad shear strength be no less than 140 MPa [20,000 psi]. When supplementary requirement S1 is specified the clad manufacturer is to measure and report the shear strength of the product. Once these inspections have been performed, the disc may be treated as essentially solid during both manufacturing and operation.
The size of the tubesheet is limited by the availability of the parent material. Because of welding and weld quality concerns, the base metal should generally be of one piece. Although cladding alloys are not normally available in the large sizes typical for steel, this is rarely a size limitation. Since the cladding layer is not part of the design strength requirement, it may be pieced together by welding to achieve any size of tubesheet disc. The use of titanium clad tubesheets is appropriate in heat exchanger sizes ranging from 150 mm to 5000 mm [6” to 16 ft.] in diameter and for fixed, floating, or u-tube configurations. Tubesheets with titanium clad on the tube-side are a good solution for tube attachment issues and for joining sides of differing metallurgies. Tubesheets with titanium clad on the shell-side may also be used as in this particular project where the tube sheet features both.
The configuration of the bonnets of a heat exchanger is guided, but not restricted, by the presence of titanium clad tubesheets. A bolted joint between the tubesheet and the remainder of the tube-side allows full freedom for material choice. The floating head may be coated or bare steel, stainless or nickel alloy, or titanium as required by the process material evaluation. Titanium clad floating heads occur often where titanium is specified, as even moderate pressures shift the material choice toward titanium clad for large titanium floating heads.
Conventional floating heads with either clad flat covers or clad dished heads are used frequently. Fabrication of these items involves the standard weld seam batten strip, nozzle liner, and flange face details for covering exposed backer surfaces. Batten strap, or liner, design issues can get quite complex, particularly when pressures and/or temperatures are high. The coefficients of expansion and the elastic moduli of titanium (and the other reactive alloys) are appreciably different from steel. Therco implemented a combination of a clad dish with a semi loose liner around the girth flange of the floating head. This was chosen to assure long, maintenance-free equipment performance.
These loose lined parts are subject to special care and attention during fabrication as well as causing additional inspections for proving the integrity of the fabrication and for ensuring the continuing integrity during operation. Typically, holes are drilled though the steel into the region where the titanium will be welded. This provides a means of providing purge gas to the backside of the titanium weld. The titanium welds that attach and seal the liners are 100% liquid penetrant tested and helium mass spectrometer leak tested.
The purge holes can be used as tell-tale holes are drilled into every space behind the liners, both for the helium leak detection testing and for leak detection during operation. In operation, visual inspections for signs of process fluids leaking out through these holes should be performed regularly. Timely identification of liner failures will minimize backer repairs as the process fluids are highly corrosive to the backer and may quickly degrade the area under the liners. Hot gas cycle testing during fabrication is used to minimize the occurrence of such operational failures. In this test, the titanium clad component is heated and pressurized to operational conditions and cycled, typically, twice, which stresses the titanium liner attachment welds. Afterwards, the integrity of these welds is checked again by the helium leak testing method.
Dished heads from titanium clad plates are generally produced by hot or warm pressing. The cold forming or spinning processes are less used as they induce excessive shear between the backer and the titanium, which can cause disbonding. The pressing equipment of the head former limits the size and shape of the heads available, but standard sized flanged and dished, ellipsoidal, and hemispherical heads are available. For heads larger and thicker than are available from one-piece pressings, segmental heads may be used. In this way a large head may be assembled from a centre dish piece and multiple petal pieces. While larger titanium clad heads and heat exchangers are made possible by this method, the increased number of parts that make up the head also increases the length of weld seams. As each seam requires custom batten strips, this greatly increases the amount of inspections required during fabrication and during operation.
Cones and elbows may also be formed by cold rolling or hot pressing and lined in a similar way. For all formed parts, the layout of the weld seams should be designed with the lining process in mind. Internal structures, such as pass partitions, separation baffles, demister supports, and vortex breakers fabricated from titanium may be directly attached to the internal clad surfaces. These items should be anchored substantially on the integrally clad plate rather than being attached primarily to the loose liners. These internal structures are subject to large thermal stresses that are due to the difference between the thermal expansions of the backing steel shell relative to the titanium internals. Carbon steel expands about 40% more than titanium and stainless steel expands about 80% more than titanium. Special attention must be paid to design of structures that are installed across the whole diameter of the vessel. Flexible sections or bolted joints are helpful for alleviating these stresses. Properly designed internal structures will avoid distortion of the structure, detachment of liners, or disbonding of clad during service.
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