Nucor's new paradigm
In a mini-mill expansion, the steelmaker did away with redundant overhead cranes, slashing costs and space significantly without sacrificing performance.
By Gary Forger, Editorial Director -- Modern Materials Handling, 7/1/2004
The price of steel is very much on the minds of many these days as prices continue to soar. While there are several reasons offered for the recent run up, ultimately it all comes down to the basics of supply and demand.
That makes the startup in May of Nucor Steel's $100 million mini-mill expansion in Jewett, Texas a welcome addition to U.S. steel capacity. When it hits its stride, the expansion's throughput will be 1.2 million tons a year of structural steel for construction.
Even more noteworthy than the timely capacity expansion is the new view that Nucor has taken of the overhead bridge cranes that it installed at a total cost of $7 million.
Quite simply, there are no backup cranes for any of the four major functions in the facility, explains Dick Rappolee, project manager and auxiliary mechanical supervisor at Jewett. In the past, Nucor, along with the rest of the steel industry, always had a redundant backup crane for every crane in use. That not only increased crane and other costs but expanded the overall footprint of the facility.
'We challenged ourselves with the crane issue from a cost standpoint, asking if it was possible to do the work we needed to do with a single crane for each function versus the traditional concept of two,' says John Farris, maintenance manager. 'In short, we did not see the redundant crane adding value to our operation.'
Instead, the new melt shop was de-signed in such a way that only a single crane is in place for each of the four functions. The four functions are: charge, ladle, caster maintenance and billet (see plant layout drawing below).
Each crane (KCI Konecranes) is outfitted with maintenance-efficient AC variable frequency controls and on-board redundant features. Reportedly, this is the first time that an American steelmaker has opted for such large hot metal cranes with these AC controls.
Farris estimates that the new crane paradigm reduced building and crane costs by 30%.
By example, he says this. 'Our melting bay [charge crane] with a 225 foot runway is very compact by most standards. It would have required at least another 100 feet of runway system to accommodate a redundant crane. In addition, the added square footage would adversely affect the size and effectiveness of the emission control system [for the plant].'
'We were challenged to make this expansion happen for a set number of dollars. And we did it by thinking differently than we had in the past,' says Rappolee.
Upgrading the melt shopAs the largest steel producer in North America, Nucor considered this project to be a major opportunity to improve its competitive position globally. Furthermore, the company has long had a strong focus on profitability.
To get there, increased throughput with maximum efficiencies was an underpinning of the Jewett expansion. The new melt shop replaces an outdated facility at the same site with only 850,000 tons of capacity. Capacity is now nearly 50% higher.
Consolidation was the name of the game from the start. Where there had been three, 45-ton electric arc furnaces, there is now only one 90-ton furnace. Even cycle times changed. In the past, it took 65 to 70 minutes to melt 45 tons of scrap steel to begin the process of making construction-grade billets. The new facility melts 90 tons in just 35 minutes at 3,000 degrees.
Equally important as increasing throughput, says Rappolee, was building a world-class operation.
To begin, materials flow needed to be improved with the old facility featuring some significant materials handling bottlenecks. Furnaces were often waiting for the charge crane to deliver steel. Now, Rappolee says, there is no waiting. Furthermore, there was not free movement of the ladle crane and caster maintenance crane, further slowing down the process. Now, those two systems do not interfere with each other although they are on the same runway.
Crane by craneEach of the functional areas is designed to maximize materials flow. The process begins with dump trucks bringing in scrap steel and dumping it into a below-ground scrap bucket. Fully loaded, the bucket, which weighs 80 tons empty, holds 100 tons of steel, and is filled 35 to 40 times a day. The bucket is carried by the charge crane to an electric arc furnace.
Total travel distance for the charge crane is only 100 feet. Rappolee says standard distances are up to 300 feet. Vertical lift is 90 feet.
The charge crane is a top-running double-girder design. It is outfitted with a 200-ton hoist to handle the charge bucket. A 75-ton hoist on the charge crane opens and closes the charge bucket. And a 25-ton hoist is for maintenance. It allows the operator to change furnace electrodes and access areas that cannot be reached by the 75-ton hoist. The operator controls the crane's movements from a remote radio controller.
Next in the process is the ladle crane. It is a twin to the charge crane except it does not have a 25-ton hoist and does have an operator's cab.
Once the steel is melted initially, it is poured into a ladle that is moved by cart to a secondary heating station. When that process is finished, the crane carries the ladle to what is called a caster where molten steel is converted into billets.
The ladle crane bay is a total of 570 feet. Vertical lift is 90 feet and horizontal travel is only 130 feet, roughly half of what a standard ladle crane would travel.
The remainder of that bay is for the caster maintenance crane, which cleans slag out of empty ladles and prepares them for their next use. This, Rappolee says, frees up the ladle crane for its primary purpose.
Vertical lift of the caster maintenance crane is 90 feet. And because it handles much lighter loads, it is rated at only 100 tons and has an auxiliary hoist of 35 tons.
Although it has the same top-running double-girder design of the other cranes, the billet crane is quite different from the others. To begin, it has the longest horizontal run of the four – 1,200 feet. It takes finished billets from the melt shop to inventory and the roll mill, which has been where it stands since the plant originally opened. Vertical lift is only 50 feet.
Rappolee explains that the billet crane operates 36 minutes an hour, which is 'pretty high.' The norm, he says, is about 20 minutes per hour for a billet crane.
The crane features two magnets with a 75-ton main hoist and 12.5-ton auxiliary hoist. A unique trolley allows the main hoist to rotate 270 degrees to position the magnets in any orientation for billet handling. The crane can handle up to 19 billets at a time.
Living without redundancyWith only one crane for each function, crane reliability is essential to the success of the expansion.
'All equipment in this facility is there for maximum reliability,' says Farris. 'We produce steel to make profits. And that means we don't want to be good at working on equipment.'
The first step Nucor took was to opt for AC variable frequency controls instead of traditional DC controls. The switch to AC eliminates brushes and commutators, reducing maintenance costs and extending maintenance intervals. Furthermore, the AC squirrel cage motors used on Nucor's cranes isolate the rotor from the windings, eliminating the carbon dust buildup problem common in steel mills.
While that move was an important step to ensuring high uptime for the cranes, it was not the only one. All four cranes are outfitted with four to eight drive motors. Half the trolley and bridge drives can be out on any crane and it can still operate at a normal speed, explains Farris.
But the backup equipment doesn't stop there. On the ladle, charge and billet cranes, the three most key to continued operation of the plant, the main hoists are equipped with two motors and differential gearboxes. With half its motors out, a hoist can continue to operate at reduced speeds.
As the plant ramps up to full capacity, Rappolee and Farris say that initial performance of the cranes is as expected. And expectations are high here. Nucor expects an 18% annual return on the assets in the plant.
'We were challenged to do this handling system for a pre-set budget that required us to move away from working in traditional ways. And best of all, we did just that,' says Rappolee.
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