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Steelmaking: The one-crane solution Nucor’s melt shop in Jewett, Texas, USA, has shaved US$10.2 million from its capital expense and operating costs and is approaching zero unplanned, crane-related downtime in eight years.

*Author

WHEN Nucor began designing a new melt shop for its Jewett, Texas, mini-mill in 2002, the centrepiece of the design was what came to be known as the “one-crane solution.” Nucor conceptualised the $100 million mini-mill expansion with a single crane for each critical function: one charge crane, one ladle crane, one caster-maintenance crane, and one billet-handling crane. Industry-watchers were sceptical, both of the single-crane approach and the unconventional cranes selected, as it was the first time a US steelmaker had purchased hot-metal cranes of this capacity featuring AC variable-frequency controls.

Nucor’s primary motivation for the new melt shop was to improve the cost-competitiveness of its operation. Since the mill’s products are merchant-grade bars, angles and light structurals, cost-per-ton is a primary factor in improving market share.

“The team’s mission was to build a melting and casting facility that would provide a competitive advantage in safety, quality and operational performance for years to come,” said John Farris, Jewett’s

maintenance manager at the time and now mill manager. “To accomplish this, our team challenged tradition and searched for different ideas from around the world and in other industries. We could not get comfortable with the traditional notion of a back-up crane and instead decided to find a crane manufacturer capable of building a single crane with built-in redundancy. Konecranes was selected as the partner to innovate our severe duty crane application,” he said.

The concept of purchasing a single crane for each function was revolutionary, particularly in an operation whose goal was zero unplanned downtime for its cranes. In practice, the one-crane solution saved Nucor significantly in initial equipment, building, long-term maintenance costs and improved safety conditions, and required nearly no unplanned downtime in its first eight years of operation.

An AC-powered approachAs Nucor’s material handling partner in this venture, Konecranes worked with the steel producer to design four AC-powered cranes whose extensive redundant

features gave Nucor the reliability of two cranes in one. The AC variable-frequency controls also eliminated some of the worst maintenance headaches of DC–powered cranes.

Tommy Massey, current maintenance manager at the Jewett mill, implemented parameters for tracking all of the costs and downtime attributed to the cranes since their 2004 installation. According to Massey’s analysis of the data and to Damon Burrow, Nucor’s melt shop electrical lead, the AC motors are more economical than their DC counterparts. “Many long-timers in the industry believe DC is the only way to go because of the amount of torque and life you get out of them,” Burrow said. “We have far fewer problems from AC motors, and they are much less expensive to replace than a DC motor.”

The Jewett melt shop, with a capacity of 1.2Mt/y, replaced an outdated facility at the same site with only 850kt capacity. Another major factor in Nucor’s cost-reforming strategies for the new melt shop was overall building cost. Nucor has estimated that the one-crane solution

reduced its building cost by 30%, or more than US$4 million.

Replacing three 45t electric arc furnaces (EAFs) with a single 90-ton EAF cut melt time from 70 minutes to 35 minutes at 3,000 degrees. Even so, the new EAF bay with its 225-foot runway is very compact: it would have required at least another 100 feet of runway and two additional bays in both the charge and ladle areas to accommodate a redundant crane, and the added square footage would have required more capacity for the emission control system.

In addition, a traditional two-crane solution would have required purchasing back-ups for the charge, ladle and billet cranes. Instead, Nucor and lifting partner Konecranes met the challenge of reliability in single cranes by essentially combining the mechanical and electrical resources of two cranes into one structural package.

On each of the four cranes, the bridge and trolley are equipped with four to eight drive motors. Even with half the drives out of operation the trolley or bridge can still operate, although at reduced speeds and with longer acceleration times. Similarly, the main hoists on the ladle, charge and billet cranes are equipped with two hoist motors and differential gearboxes that permit them to remain fully functional at reduced speeds in the event of failure of one of the main hoist motors or inverters, as well as a spare AC inverter. A simple switching process is all that is needed to bring the spare inverter on line, further adding reliability to the one-crane set-up.

“Looking at the initial cost of two cranes versus one – you pay more by having redundancy on the crane but not the same cost as buying two individual cranes,” Burrow said. “Having only one crane for each job gives us more time to work on other equipment to keep reliability of the entire facility higher,” he added.

Massey said the downtime cost savings realised through the redundant features

is a major plus: “To replace one of those drives might take us 30 hours, and a repair could take 15-24 hours. With a spare installed, in a matter of 30 minutes we can switch the drive and then schedule a repair of the problem drive. It’s given us a significant savings over repair or replace time.” In its first eight years, the mill attributed no significant downtime to the cranes.

Safety considerationsAccording to Massey, the control e-house has provided a safe working environment for maintenance crews engaged in service or diagnosis on the cranes. Work gets done faster with fewer trips up on the crane, and most work can be done inside the 80°F e-house, rather than the 140-145°F ambient temperatures in the melt shop.

He noted that the redundant features, notably gearboxes and drives, allow workers to keep cranes operating without having to address issues while the shop is under pressure for production. Also, the

design of the control systems with the interlocked safety features that are built in to the operating system have had a significant impact on reducing the amount of operational errors that can damage the crane. “The greatest contribution to safety is that the cranes are reliable,” Massey said, so that crews spend less time on the cranes.

The cranes also benefit from a ground-based diagnostics feature; however, given their reliability, the feature is seldom used. “A maintenance worker can check many conditions without actually getting on the crane, which has rotating equipment and pinch points to think about,” Burrow said. “The job becomes safer and more effective – fewer hazards need to be mitigated to get the same sort of information you’d normally have to go on the crane to retrieve.”

With an eye to managing costs, the Nucor and Konecranes team also worked to design the cranes with a high commonality of spares, since waiting for parts is not a cost-effective option. “If we have one hoist motor spare in the warehouse it’s good in one of five spots,” Massey said. “The cost savings is in the inventory value we have to keep to maintain reliability.”

Maintenance planning becomes more important in a single crane scenario, as maintenance must be preventative rather than relying on back-ups. “In a traditional dual-crane system you will have a crane that is always available as your back-up,” Burrow said. “With a single crane system you have to plan out when you will do work and repairs much more in- depth.”

Burrow said that a good example of this philosophy is Nucor’s billet crane, which services both rolling mills and the melt shop and handles 2.4Mt of material per year. This makes the billet the hardest-working crane in the facility, and, therefore, the most difficult to schedule for maintenance.

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“One of the very best aspects of this whole project has been the co-operation between Konecranes and Nucor in

designing the reliability into these cranes.” Tommy Massey, maintenance manager

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In the years since Nucor purchased their four melt shop cranes, lifting technology has continued to advance. Nucor is now upgrading its charge crane to incorporate DynAReg, one of Konecranes’ newest technologies that recaptures the braking energy of the hoist and recycles it into the electrical grid. “On the current drive system there are resistors outside the e-house, exposed to dust, dirt and heat. With the new DynAReg system there is no need for those high-maintenance resistors,” Massey said.

Cost savings and unexpected benefitsMassey and Burrow calculated that Nucor’s Jewett melt shop saved over

US$10.2 million in building, equipment and maintenance costs with its crane program in its first eight years of operation while successfully moving 8Mt of steel. A side-by-side comparison of the cranes they purchased alongside a more traditional two-crane solution shows where the savings can be found (See box p30).

In addition to cost savings, Burrow said that components like the redundant drives and motors led to unexpected benefits. “We can also use those as troubleshooting tools if we are having problems or want to do maintenance. We can use all these options to get the crane back in operation faster. Before having redundant systems, we had to completely change a piece of equipment to find out if what we thought

was the issue was really it,” he said. “I would definitely say that the one-

crane solution at our facility has changed the way we do things,” Burrow said. “A lot of concerns voiced when we first went with the one-crane solution have fallen by the wayside. We’ve proven over the last eight years that it can be done and done reliably.”

“We’re very happy with the reliability we’re seeing after eight years, as the cranes have not caused any significant downtime. We attribute that to two factors: the design and quality of the cranes themselves, and the preventative maintenance practices we’ve established around a single-crane solution,” Massey said. “One of the very best aspects of this whole project has been the co-operation between Konecranes and Nucor in designing the reliability into these cranes.”

“All of the things we wanted to build in, Konecranes was successful in giving us,” Nucor’s mill manager John Farris said. “Our operating cost reflects that. With the technology that is available now, and with what’s been proven here, I can’t imagine anyone designing a new meltshop with double cranes today [unless] … specific material handling demands dictated that having two cranes was the only way to do the work.” t

All four of Nucor’s melt shop cranes are top-running, double-girder designs featuring AC inverter controls.

Nucor’s radio-controlled charge crane is rated CMAA Class F at 200t, with two auxiliary hoists rated at 75t and 25t. The crane is also rated at CMAA Class B at 300t for handling major equipment during maintenance outages.

Its process begins when the charge crane picks up an 80t bucket loaded with around 100t of scrap steel, and conveys it to the EAF, ferrying 35 – 40 loads each day. The 5,000 cubic foot capacity bucket required for one charge operation weighs 198t when full. The crane’s 75t auxiliary hoist is used to open the charge bucket, and the 25t auxiliary allows the operator to change furnace electrodes and access areas that cannot be reached by the 75t hoist. The 25t hoist is equipped for magnet use, further enhancing its maintenance value. Total travel distance for the charge crane is only 100 feet, with a vertical lift of 90 feet.

The 200t ladle crane is a virtual twin to the charge crane, but without the 25t auxiliary hoist, and with the addition of an operator’s cab. Both cranes are designed

for continuous duty in extremely harsh environments. Heat shields protect key areas of the cranes, which are engulfed in flame at each furnace charge and exposed to searing heat during ladle handling in the caster aisle. This crane picks up the ladle of molten steel from a ladle car and moves it to an alloying station and then to the caster, which converts the steel into billets. After depositing the ladle, the crane returns for another load. The ladle crane travels 130 feet from the alloying station to caster, with a vertical lift of 90 feet.

The caster maintenance crane, CMAA service class D, is rated at 100t with a 35-ton auxiliary hoist. It is used to clean the slag out of empty ladles and prepare them for their next use. This crane is also equipped for magnet use on its auxiliary hoist. The caster maintenance crane has horizontal travel of more than 400 feet, and vertical lift of 90 feet.

Nucor’s billet crane, rated CMAA class E, is a two-magnet crane with a 75t main hoist and a 12.5t auxiliary. It has a unique trolley that allows turning the hoisting machinery through 270 degrees of rotation to position the magnets in any orientation required for billet handling.

Crane-by-crane: Examining the details

This enables it to stack billets side-by-side until they have a square stack, then rotate 90 degrees to build the next stack, preventing domino-like falls in the 1,200-foot-long billet bay. It handles up to 19 6.25-inch square billets at a time. The billet crane travels 1,200 feet with a vertical lift of only 50 feet.

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Nucor has been able to verify the cost savings of the one-crane solution at least partially because Konecranes initially offered pricing on a traditional, two-crane solution that was not selected. Looking back at those equipment cost numbers, circa 2002, and extrapolating what costs would have been to operate and maintain seven traditional cranes compared to four redundant cranes, Nucor’s Tommy Massey and Damon Burrow developed a comparison of the two solutions.

Maintenance cost comparisonWhen Nucor’s Jewett melt shop became operational in 2004, crane maintenance costs were divided into three categories for tracking and analysis. (See box.) This allows managers to see what part of the crane incurs cost, making it easier to make decisions about where maintenance dollars should go. Nucor’s system monitors 40 different data capture areas on the charge crane.

Overview of cost savings totaling US$10.2 million

1. Labour that team spends on the cranes

2. Material checked out of MRO inventory

3. Purchases of non-inventory parts and services required for cranes

Comparison of equipment costs: Single redundant design versus. 2-crane, non-redundant

For (2) Cranes per Runway, without Redundancy,

As-Purchased assuming that would eliminate the 100 Ton Crane.

Items:

Ladle Crane 2,093,174 3,843,000 for (2)

Charge Crane 2,148,112 3,925,000 for (2)

100 Ton Crane 731,889 - (100 T not needed if (2) Ladle Cranes purchased)

Billet Crane 1,493,000 2,822,000 for (2) Source:

M. Laughlin, Konecranes, 20-June-2013

Spares 655,888 655,888 Lot

Install 792,069 1,092,069 Lot

Building Length n/a 2,000,000 Additional

Ladle Bay

Building Length n/a 2,000,000 Additional Source:

Ladle Bay D. Burrow, Nucor-Jewett, 26-June-2013

Maint. Costs 5,349,015 7,134,72 Using 1.4 as 2nd Crane Maintenance Factor

(See Tab 2)

Total $13,263,147 $23,472,682

10,209,535

Savings