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Products and solutions, 2006-11-03, 11:00 AM

High-tech laser resists heat and fire

At ThyssenKrupp Steel AG´s hot-strip mill in Duisburg-Bruckhausen, one of the regular chores performed by maintenance specialists ThyssenKrupp Anlagenservice is the repair of the doors on the slab-pusher furnace. Although the work itself is relatively simple for the repairers, the inevitable and recurring downtime does cause headaches for the mill operators.

The Duisburg mill rolls annually some 3.1 million tonnes of steel into strip - a process more or less automated except for a few manual interventions such as loading the slabs into the furnace. On pusher-type furnaces, the slabs are entered into the furnace using a manually controlled pusher device. As a consequence, all the slabs already inside the furnace are pushed forward and - in a continuous cycle - the first of the slabs (FIFO) then exits the furnace to enter the rolling mill. In the process, there is the danger that the first slab collides with the exit door of the furnace while the latter is still shut. Even though these doors comprise two halves measuring 1.4 by 6.4 meters and weigh around 3.5 tonnes, they may still be damaged through the collision, leading to a severe disruption of production. And even if the damaged furnace door might still operate under emergency conditions, the fact that the pusher is unable to retract the slab package does lead to prolonged downtime.

For a long time the plant operators were faced with the problem of accurately detecting the position of the slab edge with respect to the furnace door and then switching off the pusher whenever a predefined clearance to the door was crossed. Temperatures of up to 1,350 °C and leaping flames in the measuring zone not only mean an uncomfortable environment but other physical effects also made it difficult to select a suitable identification technique.

From proposal to prototype

Norbert Remer, electrical engineer at the hot-strip mill, came up with the idea of using a laser to detect the position of the first slab with respect to the furnace door. The idea was then enacted by ThyssenKrupp Anlagenservice that, together with Remer, drafted in no time at all, a drawing broad version to be developed into the prototype of a laser distance-measuring device.

This is the way it works: The laser's measuring point quickly identifies a change in distance whenever the slab edge encounters its beam; a signal is then emitted that triggers the safety shutoff. The slab's light and infrared emissions are reliably blocked by the sensor with the result that the only signal actually processed is that relayed by the distance laser. Due to optical in-homogeneities not visible to the human eye, the distance measured does have a tolerance of +/-5 millimeters yet this has virtually no impact on edge detection. What is measured is not the absolute distance but, for the purpose of edge location, simply a significant jump in the measured distance and hence a measuring accuracy of less than one millimeter is easily sufficient for the job in hand.

Uppermost objective: operational reliability

So this was the theory on which the model was based. Even though the measuring and processing principles did possess all the physical properties to perform the required functions, the ultimate criterion was nonetheless the reliability or ruggedness of the device under actual operating conditions," explains Dr. Peter Maresch, Head of Engineering at ThyssenKrupp Anlagenservice. "Automation enhances operational reliability only to the extent in which the necessary components themselves do not become prone to failure. And this is especially true of the tough conditions prevailing in a hot-strip mill where, within an environment of heat, humidity, and dust, instrumentation systems are often at the limits of their capability." This is exactly where the experience acquired by ThyssenKrupp Anlagenservice's maintenance experts came into play since they succeeded in persuading the mill operators to give project go-ahead on one of the three furnaces.

The entire project was implemented from drawing board to reality by TAR Automation, already experienced in laser measuring systems, working to the engineering specifications of Project Manager Murat Celik and his crew. "It's hard to wish for a better customer than ThyssenKrupp Anlagenservice," comments TAR General Manager Alfred Rachner. The engineers' specifications were so detailed that we were more or less able to start implementation right away."

Protection against heat and power outage

To safeguard the system against the heat, the device is housed in an air-cooled structure with electric Peltier cooling which, combined, ensure an operating temperature in the range of 35 to 45 °C. A fire-protection door with temperature sensor and electric opening (spring-operated in the event of power outage) safeguards against surging flames. "This is an area in which water cooling - certainly more effective - cannot be used since a failure in the water lines could lead to explosive vapors," is how Celik explains his selection of system.

A second backup safety system to ensure the success of the cooling equipment is given by an ongoing measurement of temperature within the device and in its direct environment. Additionally measured is the flow of cooling air in order to protect the device even before any possible rise in temperature. As soon as the permissible ambient temperature of 85 °C is exceeded or the cooling air systems fails, the device immediately retracts from the danger zone into a safe position. An ET200 triggering device mounted in an air-conditioned cubicle allows the digital signals plus temperature and pressure values to be interlinked for plant control. The distance laser is integrated into the automatic control system via a Profibus (Process Field Bus).

Included among the services provided by ThyssenKrupp Anlagenservice are not only the development of the system but also most of its installation. This extended from the construction of the installation platform via the furnace and the development of the sliding frame mechanism to the electrical engineering system including voltage supply, the data lines, and the connecting points for the coolant air: all that was left for the customer to do, ThyssenKrupp Steel, was to connect the unit to the main systems and carry out the reprogramming work. Additionally, all this installation work was carried out parallel to production, in other words, no downtime at the hot-strip mill. "We can say that the work was done as accurately as the laser now functions," says a satisfied Central Department Manager Klaus-Ludwig Messner. He is now considering whether to install a laser measuring system on the other two furnaces.

The facts speak for themselves: the furnace currently featuring the laser device has an annual output of around 70,000 tonnes. During the more than two years in which the slab position detector has been used, there has not been a single instance in which a slab has caused damage by colliding with the furnace door. Previously, there had been two such instances annually. "The elimination of a single collision outweighs the capital outlay and so, in addition to the engineering success, the cost savings are also remarkable," sums up Maresch.

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