A large bellows is successfully containing nitrogen wiping gas released from gas knives to ensure that the wiping gas remains nearly oxygen-free and thus controls impurities during a process of applying zinc coating to steel strip (hot dip galvanizing). The nitrogen shroud was designed to provide significantly longer service life and better sealing than a previous design.
The shroud was designed by A&A Manufacturing Co, Inc., New Berlin, WI, and Fontaine Engineering, Inc., Bridgeport, WV, for a gas wiping system that is part of a hot dip coating line made for a steel producer. Fontaine is a world leader in designing, manufacturing and supplying technology and equipment used for hot dip coating of strip with zinc and other metals. The company's technology makes it possible to produce coated strip of the highest quality for use in the automotive, appliance and building industries.
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Gas wiping system that removes excess zinc uses a sealed nitrogen atmosphere instead of air. It uses a special shroud bellows that seals the gas knife area to contain the nitrogen gas.
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Maintaining Purity in the Process. The nitrogen shroud is used to enclose the gas wiping knives that deliver nitrogen at pressures from 0-29 psig and flow rates greater than 5,000 scfm toward a coil of steel that has just emerged from a bath of 860F molten zinc or zinc alloy. Pressure and knife proximity are used to remove zinc, thus controlling thickness. Jason Bell, an engineer with Fontaine, explained, "Oxygen-free finishing ensures a cleaner, smoother, more profitable coil of zinc-coated steel, which is used later to produce automobile hoods, roofs, side panels and similar components for manufacturers with stringent quality requirements. A coating produced on a galvanizing line with a nitrogen supply and nitrogen shroud contains fewer impurities and oxides than one from a traditional line that uses air."
In operation, nitrogen is supplied to the gas knives for the wiping process and directly into the bellows through a series of butterfly valves that are controlled by PID loops in the PLC.
The bellows itself surrounds the entire gas knife nozzles and mounts to the knife frames.
Originally, a two-piece bellows was used to simplify installation and removal. The joint in the bellows made it difficult to obtain an effective seal and sealing was further complicated because the previous design had unforeseen interferences after warping. Mr. Bell explained that when the knife height above the bath was changed, the bellows rubbed on the knife nozzles during compression or expansion, due to heat-related nitrogen shroud distortions. He added that the material of the original unit slowly disintegrated because of the heat, zinc splashing and the abrasive zinc dust environment.
The bellows are raised and lowered when the knife height function is activated, approximately 20 times per day. Operation is around the clock, except for occasional plant shutdowns that average less than 30 min per day. The shroud is also moved from an operating position over the pot to an offline position over the mill floor about once every three weeks. This is accomplished by lowering the gas knives to engage the bottom of the shroud and lifting it for travel along a set of linear rails.
Building Longer Life into the Bellows. In order to meet the demanding operating conditions, A&A employed many of the same construction techniques it used to produce large, heavy-duty bellows that connect the front and rear sections of articulated buses. The most important of these approaches was the use of crimped metal bindings to fasten and protect the interior seams that joined the bellows sections. Because Fontaine's customer determined that the shroud would rarely be removed, the bellows was made as a one-piece design instead of being split. This enabled A&A to make each bellows section as a continuous loop and then join it to the other sections by mechanically fastening and crimping the inside edges. Corners were stitched with Kevlar thread to maintain their shape.
Mr. Bell speculated that the bellows shroud is exposed to temperatures between 180F when the wiping process is stopped and approximately 350F when in full operation. The temperature of the molten zinc bath is about 860F, with the bellows located about 12 inches above. Thus, it is also subject to being splashed with molten zinc. In addition, he noted that zinc dust, which looks and feels much like sand, is a major by-product of the wiping process that can contaminate the bellows.
To withstand this high temperature and abrasion while providing the necessary flexibility, A&A used a special silicone-coated fiberglass material and sealed all joints with a silicone compound. To simplify assembly to the gas knife unit, the bellows was furnished with heavy-gauge mounting brackets at both ends, which were finished with a high-temperature paint.
The inside of the bellows measures slightly more than 37 x 210 inches, with a range of motion between 5 1/2 and 46 3/8 inches high. To maintain even expansion of each bellows section and prevent excess strain on the seam areas, the sections are tied together internally with stainless steel cables that limit the amount each one can open.
According to Mr. Bell, much of the success of the project was due to a material that passed Fontaine's rigorous molten zinc splashing and temperature tests combined with A&A's willingness to spend the extra time and effort needed to ensure an effective design and quality product. AF
