A maintenance shutdown at any plant is likely to be both carefully orchestrated and highly stressful, involving multiple trades. At the technologically advanced Gallatin Steel Compact Strip Production Facility in Ghent, KY, in operation since 1995, an 8-day maintenance shutdown in October 2006 included a particularly critical operation: overhaul of the thin slab caster ladle turret slewing bearing. Fortunately, a novel approach using synchronized hydraulic jacks freed up the mill's shop cranes for other necessary tasks. (The turret is located between two overhead crane bays and, although it could have been lifted using the two cranes with a cross-beam, that would have tied up both.)

Plant Overview

The plant converts scrap steel to new hot-rolled steel coils. Scrap is melted in a twin shell electric arc furnace, then passes through a ladle metallurgy facility where adjustments are made to the batch composition as needed. The ladle then moves to the caster and is positioned on a rotating turret. The caster produces slabs, which travel through a 205 m long tunnel furnace to a 6-stand rolling mill, then to a coiler.

Role of Hydraulics

R.G. Kelly Inc designed equipment and wrote a procedure for disassembly of the ladle turret. The approach used synchronized hydraulic jacks to lift the upper turret section off its bearing for placement onto a custom-fabricated transfer car, which then moved it 7 m on a specially fabricated runway. Three D Metal Works did the fabrication and provided project management and turnkey services for the project. (Shown here, the transfer assembly.)

The transfer car incorporated eight 100 ton Enerpac jacks, four on each side, mounted upside down. Hydraulic plumbing and load position sensors were integrated into the car at each jack position. The jacks chosen were double-acting, 100 ton cylinders with 300 mm stroke. The 800 ton lifting capacity provided ample margin for the lift weight of approximately 300 tons.

The Digital Advantage

While the lifting could have been accomplished in a traditional manner, using manually controlled jacks, that's more easily said than done. Manually raising each of the 8 lift points in precisely equal increments is tricky and extremely time-consuming. Too great a variance in the lifting at various points would tilt the turret and cause it to bind on the 432 mm mounting bolts. And unequal lifting can induce internal stresses in the object being lifted.

Enerpac's award-winning Synchronous Lifting System is elegant in its simplicity for the user. The Synchronous Lifting operating system receives electronic signals from position sensors that are attached to the load close to each lifting cylinder. These signals are processed, and based on the results, the computer switches hydraulic control valves on and off. (Pictured, Enerpac's ZE5-Series 700 bar hydraulic pump and digital controller.)

Accurate load position measurement at each lift point is crucial. A nonuniform load exerts different forces at different lifting points. Furthermore, as jacking commences and the load rises, the base of a cylinder may drop slightly as its underpinning compresses. The Sync-Lift position sensors use a thin cable that unwinds from a spool. The end of the cable is attached to the load and the sensor is attached to a stable reference surface (or vice versa). The result is simultaneous load position monitoring accurate to 1 mm at each jack.

Moving the Ladle Turret

The upper section of the turret had to be moved in order to gain access for two milling machines to simultaneously machine the bearing. The first step in doing the move was to assemble the transfer car under the turret. Under each of the eight 100 ton jacks was a 100 ton roller set.

After nuts had been removed from the 90 bolts (M39 x 4, 432 mm long) ringing the approximately 4.5 m diameter bearing, the lift began with 6.35 mm increments. The increments were then increased to 12.7 mm, and next to 19 mm. At each increment, shims were inserted at the jack locations and tack-welded in place. (The 300 ton load, shown here, moves easily on roller sets back over the pedestal/bearing with a couple of chain-alongs.)

After about 40 minutes, the desired lift height of 254 mm was attained, and the turret was moved from above its bearing. The triple roller bearing was replaced, and inspection of the bearing mounting flanges revealed that they still met spec and did not need milling.

The bearing work was completed and the turret set back in place within 24 hours. A critical aspect of the lowering operation was to avoid damage to the new bolts. Even though dowel pins provided side-to-side positioning, avoiding canting the turret as it was lowered was critical.

The Bottom Line

The Enerpac Multiple Points Synchronous Lifting System enabled precise lowering of the assembly to avoid damaging the bolts. The job was completed in 6 days -- one day ahead of schedule. So aside from providing accuracy and control, the Enerpac system helped get the job finished ahead of schedule.