Digitally Controlled Hydraulics Aid Work on Steel Mill Vessels
Location:Sault Ste. Marie, Canada
At Algoma Steel’s Sault Ste. Marie, Canada plant two huge, 260-ton capacity basic oxygen furnaces feed a pair of thousand-ton vessels. Referred to as “metallurgy stations,” they are used in measurement and adjustment of the content of each batch of steel before it goes to a slab caster or strip mill. The vessels rotate on double spherical ball bearings and tilt for pours.
The trunnion systems supporting the vessels undergo enormous mechanical and thermal stresses. Over time those stresses take a toll, and Algoma found that the eight strap-like supports (called lamella) located around the trunnions were gradually deteriorating.
The vessels would only have to be lifted a fraction of an inch in order to carry out the suspension repairs, but like so many things in life, doing that is not as simple as it sounds.
“The complete vessel assemblies weigh 1012 tons each,” says Algoma Steel mechanical engineer Steve Dew, so they aren’t the kind of thing you just grab with a sling and pick up. Large hydraulic jacks could do the job, but there were no suitable points against which to jack.
Preparations are crucial
The decision was made to lift each vessel assembly with a set of four, 400-ton jacks, so Algoma personnel set about to design, fabricate, and install the necessary jacking brackets. In addition to being plenty beefy, the brackets had to be attached to the vessel securely enough that there would be no danger of their peeling off during the lift.
Welding the brackets in place was complicated by the chrome-molly composition of the massive vessel wall. “We used stick welding with special rods and procedures, as well as inter-pass pre- and post-heats for stress relief. The welds were visually inspected and ultrasonic inspection-checked,” says Steve Dew.
Digital controls play a key role
The actual lifting took only a short time, in part because the four, 400-ton jacks were computer-controlled (synchronous lifting). Close control of the relatively small lift distance was important for both safety and mechanical reasons.
The digital hydraulic control system maintained vertical position accuracy to within 1 mm at each lifting point. Each jack was accompanied by an electric control valve and a high-accuracy position sensor. The position sensors, with mechanical construction analogous to that of a tape measure, unwind a fine wire from a spool to a fixed reference point. Rotation of the spool is monitored with high angular resolution, and the resulting signal goes to the digital controller, which operates solenoid valves at each jack as needed.
One might think of computerized equipment as adding complexity to a task. However, for multi-point lifting, the opposite is often the case. Manually operating the four jacks used for each lift would have required intense coordination, and the possibility of human error is always a concern. For additional safety, each jack was equipped with a manually-set lock ring.
Hydraulic power was provided by a compact, 7.5 hp, 10,000 psi electric pump fitted with an oversize (100 liter) reservoir. The pump unit included a venturi “vacu-valve” in its hydraulic circuit. This arrangement allowed single-acting jacks to be used, yet provided complete “powered” draw-down of the jacks.
“While you’re at it…”
Service on one of the huge vessels required removal of the 10.8-ft diameter bull gear used to tilt the vessel for pours. However, without the bull gear the vessel and trunnion would be free to rotate—certainly a very undesirable situation. Therefore, Algoma staff had to fabricate and weld in place a pair of large stabilizer arms to maintain the vessel.
“We decided to knock out the brick and reline the vessels, so the total lift weight was lighter than originally anticipated,” says Steve Dew, commenting that nonetheless it remained critical “to lift evenly.” Definitely a job for a digitally controlled synchronous lifting system .