Patent Application
20050153647
1. Field of the Invention
The subject invention is directed to a novel design and implementation of chimney flue sections comprised of titanium and its alloys to provide a combination of lower weight, corrosion resistance and flexibility in varied operating environments.
2. Background of the Invention
Power station and other industrial boiler chimneys typically consist of an approximately cylindrical concrete windshield 15 to 20 m diameter and 50-300 m tall, within which are one or more approximately cylindrical flues. When the gas is fed directly from the boiler to the flue it is warm (100° C. to 200° C.) and dry, and a conventional mild steel flue shows insignificant corrosion during operation under these conditions.
If the boiler is burning medium to high (1% or more) sulfur coal, or high sulfur oil, flue gas desulfurization (FGD) is required. This is to minimize the release of sulfur-containing and nitrogen-containing oxides into the environment, and hence minimize the formation of acid rain. In the most common form of FGD, water is sprayed into the flue gas stream, and the resulting sulfur-containing acid-rich solution is neutralized with lime or limestone. The resulting gas stream is cooler (0° C. to 100° C.) and carries moisture and residual acid, which can rapidly corrode mild steel flues, particularly if the dew point is reached under various adverse circumstances.
In order to protect the steel structure, a variety of linings have been employed. These include borosilicate brick, fibre reinforced polymer (FRP) coatings, nickel alloy linings, or titanium linings. Systems with low (50° C.) flue gas temperatures have alternatively been fitted with replacement flues constructed of factory built FRP cylinders, which are stacked in place to form the new flue.
The metallic linings have a useful life of about 15 years, and a reputation for robust and trouble-free operation and low maintenance, given appropriate alloy selection and installation. The nickel alloys used for lining flues have an advantage of simplicity of installation, since they can be welded directly onto an existing steel flue structure. Titanium alloys should not be welded to steel structures and thus another installation method is required.
From 1984 to 1998, resistance brazing titanium claddings to steel backing sheets or batten bars was used for flue installations. These batten bars or sheets can be welded directly to the steel flue structure. Titanium strips are then welded in place to cover gaps between the titanium liner sheets where the steel is exposed. This is considered to be a disadvantage with respect to this practice of the method, since the extensive on-site requires specialist trained personnel, and slows down the installation.
An alternative method was used in the 1990's, where existing brick lined flues were lined with titanium directly, being held in place by a combination of bolts into the brick and mastic between the titanium and the brick. This method requires that the pre-existing brick lining to the flue to be in good condition, and is comparatively slow and laborious.
The invention relates to a flue for use in industrial chimneys to provide corrosion resistance to the passage of corrosive gas through the chimneys. The flue comprises a plurality of formed sections hung from the top of a chimney. These sections may comprise commercially pure titanium or a titanium-base alloy. These sections are connected to each other to form an integral flue.
The plurality of formed and connected sections may include cylindrical sections.
Each cylindrical section may include butt welded edges for completion thereof.
The cylindrical sections may be joined end-to-end to form the flue. This may be achieved by welding.
Alternately, each section may include flanges connected by bolts to adjacent sections using a gasket between the sections for sealing off corrosive gas.
The titanium-base alloy may be a high-strength titanium alloy.
When a new or replacement flue is being installed irrespective of material used, flue foundations are prepared and then a hoist is installed at the top of the concrete windshield. The topmost section of the flue, with as much lining as possible in place, is then placed on the flue foundation, then hoisted up to sufficient height to allow the next highest section to be placed underneath it. The top section is then lowered onto the second section and secured to it, e.g., by bolting flanges or welding together. This process is repeated until the flue reaches the required height. The hoist is then disconnected, leaving the flue substantially freestanding, with some braces to minimize lateral movement.
In order to obtain the advantages of the corrosion resistance and light weight of titanium alloy flue linings, while avoiding the installation and quality assurance issues associated with titanium linings, one must design a flue constructed solely from titanium alloys. The flue is to be designed to be substantially suspended from a device resting on, and transferring load to, the top of the concrete windshield. This could be described as a ‘Chinese Lantern’ design.
If appropriate, the design could utilize cables or tie rods running between the flanges to bear the weight of the flue, rather than carrying the load through the sheet metal walls. Design of the flue is to be made up of cylindrical sections that can be prefabricated in a workshop. The fabrication method is envisaged to uncoil titanium strip from a coil, and form it into cylinders of the flue diameter, with a butt weld to complete each cylinder. Seam welds are to be made between these cylinders in order to build up master cylinders of a height approximately equal to the flue diameter. A ring rolled titanium section is installed by welding it to the top and bottom of each master cylinder to act as a flange and facilitate assembly of the master cylinders to form the flue.
As part of the workshop manufacture, low density insulation is installed onto the outside of the master cylinders to minimize heat loss of the flue gas during operation and facilitate inspection of the chimney. All metallic parts retaining insulation are to be made of titanium to eliminate galvanic (metal-to-metal)
The flue is installed by either bolting the flanges of the spool piece “cylinders” together or welding the spool piece cylinders together under an argon atmosphere using known TiG welding techniques for titanium. This approach leaves the flue under top tension. Side supports can be installed from the inside wall of the windshield to the outside wall of the flue if needed.
Other design variations are possible to increase the load bearing capacity of the flue such as using corrugated titanium sheets or titanium bars or rods as stiffeners in either the axial or circumferential directions. By these methods it should be possible for the flue to withstand sub-atmospheric pressure within the flue.
It may be found desirable to use standard steel cables and fittings to bear the load of the flue. These would run from flange to flange down the outside of the flue, and be tensioned to bear the load. In this case a plastic sleeve or coating may be required to protect the steel cables, and a non-metallic fitting would be used to prevent direct contact between the steel fittings and the titanium flanges. For new chimneys, additional fixtures in the height of the chimney could be utilized to transfer the weight of the flue to the concrete windshield.
Additionally, bellows sections could be incorporated as appropriate to accommodate expansion and contraction as the flue heats and cools. This might include roll forming a shallow angle down the centreline of some titanium strips, making each master cylinder a ‘fold in the Chinese lantern’.
Various titanium alloys could be utilized in different locations in the flue to minimize the cost and maximize the durability. For example, higher strength alloys would be used near the top of the flue where the load is greatest and Pd or Ru bearing grades would be used where the corrosion conditions are predicted to be most severe.
The method of seam welding could be by TIG; Plasma; laser; etc. Alternatively, it may be adequate for some seams to use lock seam joints, as used in the installation of sheet metal roofing. In principle, the above method of flue design and construction could be used for other metallic or non-metallic materials, or a combination of both. Titanium does offer particular advantages in both corrosion resistance and lower density than other metals.
For retrofit chimneys, the flue spool piece section approach also works well. When the segments between vertical brick overlaps inside the chimney is about 9 m spacing, the flue sections should be pre-fabricated to about an 8 m tall ‘can’ in 2-4 cylindrical segments (depending on transportation issues) and constructed with small flanges directed in towards the inside of the cylindrical flue sections. A Teflon gasket can be inserted between the flanges of two adjacent sections to provide a seal. All the holes could be pre-drilled so that assembly of the flue would go quickly when attaching and fastening nut and bolt fasteners. Since there would be the brick for support, the flange could be formed integrally or welded onto the cylindrical ‘can’. The flanges at the top and bottom also are directed inward towards the inside of the cylinder. Pre-made expansion joints are installed between the bottom flange of one can and the top flange of the other can that bridge the offset in the brick. This is similar to the above design for new construction, except the flange would be on the inside of the cylindrical sections instead of on the outside. The expansion joint could be fabricated from titanium or another suitable inert material such as Teflon on-site.
Each 8 m can fabricated from 1.5 mm strip would weigh just over 2,300 lbs (for a 6 m diameter flue). The flanges can be welded onto the parts of the cylindrical sections of the flue in the workshop and a final titanium weldment can be made on-site to complete the fabrication of a single cylindrical spool piece section. The flanges can be bolted together on-site as the upper section is lowered onto the lower section. It might also be possible to rest each individual can or spool piece cylinder on the concrete support coming out from the chimney, thereby eliminating almost any hanging stresses and would make each brick/titanium segment a single unit.
It is estimated that the total installed cost of the titanium flue should be lower than that of a steel flue lined to resist wet FGD conditions using borosilicate bricks or nickel alloys. The high corrosion resistance of titanium under these conditions, and the low maintenance required will ensure advantageous life cycle costs. In the event of a small leak of flue gas through a minor fabrication defect there will be no corrosion damage to the structure since the flue is made entirely of titanium.
With reference to the drawings, and for the present to
With reference to
The term “higher strength titanium alloys” as used herein is defined as a titanium alloy containing additive elements to increase tensile strength to that above the tensile strength of ASTM B265 Grade 2. Examples include Ti Grade 9 and Ti Grade 12.
