Latin American applied research.docx

1、Latin American applied research窗体顶端窗体底端窗体顶端 Citado por SciELO Citado por Google Similares en SciELO Similares en Google Latin American applied researchversinISSN 0327-0793Lat. Am. appl. res.v.34n.4Baha Blancaoct./dic.2004 Control of pusher furnaces for steel slab reheating using a numerical model P.

2、 Marino1, A. Pignotti2 and D. Solis3 Centro de Investigacin Industrial, FUDETEC, 2804 Campana, Argentina 1 sidmrp 2 api 3 sidpsol Abstract Steel slabs are reheated in pusher-type furnaces up to a temperature of 1200oC in the steel sheet manufacturing process. In this article we describe a control sy

3、stem that uses an on-line numerical model to calculate the furnace setpoints in order to improve the heating quality. Examples of actual furnace operation with and without the system are presented to show the improvements that are obtained handling typical non-stationary situations. Keywords Steel I

4、ndustry. Reheating Furnaces. Furnace Control. I. INTRODUCTION In the steel strip manufacturing process (Figure 1), steel slabs obtained from continuous casting are reheated up to temperatures of approximately 1200oC prior to the rolling process. The required temperature at the end of such process ha

5、s to be comprehended within a narrow range determined by the subsequent on-line heat treatment process. Slab reheating in pusher furnaces is one of the sources of variability that produce departures from that narrow range. In the case of SIDERARs hot rolling facility in San Nicols, Argentina, four p

6、usher-type furnaces are used to reheat slabs that are approximately 6m long, between 0.65 and 1.53m wide, and from 0.18 to 0.20m thick. These furnaces are named after the way the slabs are pushed forward inside the furnace. Every time a hot slab has to be discharged to be rolled a new slab is introd

7、uced into the furnace and the intermediate slabs are pushed sideways towards the furnace outlet. In the first part of the furnace the slabs are supported by four refrigerated skids, while near the outlet they lie on a refractory hearth that is intended to diminish the temperature inhomogeneity gener

8、ated by the skids The heating power is supplied by gas burners that use either natural gas or a mixture of natural and coke gases and are arranged in several zones. Typically one preheating zone, two heating zones (an upper and a lower one) and one soaking zone are present (Figure 2). The burners of

9、 each zone are controlled through thermocouple setpoints: a control loop regulates the air and gas flowrates to match the set value with the temperature measured by a properly placed zone thermocouple. Therefore, the problem of furnace temperature control is that of specifying the setpoints that pro

10、duce an adequate slab outlet temperature distribution. To monitor the slab outlet temperature there is an infrared pyrometer at the rougher exit (R4 in Figure 1), which measures the slab longitudinal temperature profile on the upper side of the slab. The mean temperature and the maximum temperature

11、difference of this profile are the target variables of the furnace control and define the heating quality. Although it would be desirable to have a measurement point closer to the furnace outlet, the oxide layer that is formed during the heating process and that is removed by a descaler at the rough

12、er inlet, prevents a reliable measurement prior to the rougher exit. Figure 1. Schematic illustration of the steel strip manufacturing process Traditionally the furnaces are operated manually, based on setpoint tables that correspond to steady state operation. There are also some automatic actions t

13、hat are implemented in the Programmable Logic Controller (PLC) that is used to handle the signals from the process sensors and to regulate the process actuators. Although manual operation gives a reasonable heating quality when the furnace is in steady state, there usually are changes in the slab ge

14、ometry, in the cycle time, in the inlet temperature, and there are downstream events that produce halts in the line. All these situations produce departures from stationary operation and generate variations in the slab mean temperature at the rougher outlet. For instance, to avoid slab overheating i

15、n manual operation, whenever a halt occurs, the gas and air flowrates are automatically decreased by the PLC. When operation is resumed, compensating for this effect is a delicate task that only experienced operators are able to carry out with relative success. In this article we describe a control

16、system based on a numerical model of the process that is used to automatically calculate zone temperature setpoints that are intended to minimize the departures of the slab mean temperature at the rougher exit from the corresponding process objectives. II. NUMERICAL MODELS The use of numerical model

17、s to improve the design of this process has been the objective of several analyses. A very complete steady state model was presented in (Barr, 1995). That model was used to study the influence of the skid configuration on the slab homogeneity. Due to the fact that the models are used in a factory en

18、vironment, the use of standard PC hardware is mandatory, thus restricting the model complexity. However, the continuous increase in the computing power has recently allowed the development of detailed numerical models capable of perform on-line. (Correia et al., 2002) investigate parametrically the

19、use of 2-D zone models to predict the thermal behavior of a continuously operated metal reheating furnace. In (Boineau et al., 2002) a CFD code with a module to calculate the radiative exchanges using a zone formulation was adapted to simulate transients eliminating the fluid dynamic calculation. Al

20、though a series of off-line analyses was presented, on-line results were not given. Also in (Honner et al., 2002), a CFD model was used to calibrate a simpler one which uses adjustable coefficients to evaluate the radiative and convective heat fluxes. On-line results showed a good agreement between

21、calculated and measured values. In a previous paper (Marino et al., 2002) we presented a detailed numerical model of the slab reheating in pusher-type furnaces and showed that the temperatures calculated by the model are in agreement with validation measurements made with instrumented slabs. Model r

22、esults were also successfully compared with the pyrometer measurements at R4. The main model features are: 3-dimensional and spectral calculation of the radiative exchanges in the combustion chamber using the zone method (Hottel and Sarofim, 1967) Detailed description of the radiative properties of

23、the combustion products from RADCAL (Grosshandler, 1993) The combustion product temperatures are calculated from thermocouple measurements 2-dimensional calculation of the slab temperature distribution (neglecting inhomogeneities in the slab width) Similar features are present in models developed fo

24、r different kind of furnaces (Marino and Pignotti, 1997; Altschuler et al., 2000; Marino, 2000). III. FURNACE CONTROL In this section we describe the algorithm that is used along with the model to automatically calculate the zone temperature setpoints that are intended to minimize the departures of

25、the mean slab temperature from the target value. This algorithm includes the evaluation of the effect of changes in the zone temperatures on the slab future thermal evolution. Due to the fact that both the model and the control algorithm have to perform in real time, and that this calculation has to

26、 be updated frequently in order to take into account possible changes in the load geometry, cycle time, thermocouple measurements, etc., there are bounds on the complexity of the algorithm used to calculate the temperature setpoints. The current practice is to perform this calculation every 30 secon

27、ds. A. Control Algorithm In the control algorithm a target average slab temperature at the rougher exit is defined for every slab in the furnace, according to the product and process requirements. It depends on the slab geometry and the strip final thickness. Intermediate target temperatures at the

28、end of the preheating and heating zones are also defined. Figure 2. Longitudinal section of SIDERAR #3 pusher furnace For each zone the updated setpoint temperature is determined by comparing the target temperature at the end of the zone, and the model predicted average slab temperature when it reac

29、hes the end of the zone. In practice, the following equation is solved:=0 where Wi : is a weighing factor that depends on the slab distance to the end of the zone and on the departure of the current slab temperature from its desired stationary value Tci : is the calculated mean temperature that the

30、slab i will have at the end of the zone under the assumption that the slab velocity remains constant. This value is a function of the updated zone setpoint temperature Tobji : is the target temperature of slab i at the end of the zone The summation comprises all the slabs of the zone and also part o

31、f those that are in the preceding zone. Both the obtained zone temperatures and their rates of change are limited in order to preserve the integrity of the refractories. B. Target Offset As the model calculates the mean slab temperature at the furnace outlet, but the heating process reference temper

32、ature is measured with a pyrometer on the slab upper surface after the rougher stand, there is a drop in the mean slab temperature between the furnace and the rougher exit. This drop may vary depending on the transference time, the performance of the descaler, and the refrigeration of the rolling cylinders. Thus, a correction term (offset) is introduced for the target temperature at the furnace outlet. This term is intended not only to compensate for departures from a constant temperature drop from the furnace out