Reconstruction of ladle electronic scale with high temperature Load Cell

 [Summary] The article focuses on the environmental conditions and analysis of the installation and use of steel ladle electronic scales; the structural design of the scale body with high temperature sensors; the effective measures that should be taken to overcome the impact of harsh environmental conditions on the electronic scale; and the various impact resistance of electronic scales , Measures to prevent high-temperature radiation, prevent molten steel from burning, and prevent tipping; it has a certain guiding role and reference value for the design and transformation of electronic scales used in high-temperature environments.

 [Key Word] Ladle electronic scale, High temperature load cell, High temperature radiation, Weighing Scale body structure

1:Overview

In order to improve steel quality and reasonably control manufacturing costs, metallurgical enterprises generally need to install two sets of electronic weighing systems in the process of molten steel refining and continuous casting to measure online.

1.1. The vehicle-mounted ladle electronic scale is used to detect the liquid level when the molten steel is injected into the ladle, and to control the amount of alloy material added during refining to improve the quality of the smelting, and to weigh the molten steel in the ladle.

1.2. The turntable ladle electronic scale is used to monitor the continuously falling molten steel level in the ladle during continuous casting, and to control the flow in a timely manner. When the molten steel level is close to the slag, the tap hole is closed in time.

It can be seen from the above that the operating quality of the ladle electronic scale in the continuous casting process plays a very important role in the quality of steel and the improvement of work efficiency.

2:Environmental conditions and analysis of ladle electronic scales on site 

At present, the ladle electronic scales used in China, whether they are made domestically or imported from abroad with continuous casting equipment, have different structures, but their installation positions are basically the same. Generally, one is designed under the ladle support base. The electronic scale with a special structure enables the left and right ear seats of the ladle to be supported on the electronic scale frame; generally, it needs to have the ability to withstand high temperature, anti-sputtering, anti-shock and resist the external horizontal impact to ensure the normal operation of the equipment and meet the The need for continuous casting process. This article focuses on the structural design characteristics of using domestic high-temperature sensors to design and transform ladle electronic scales.

2.1On-site environmental characteristics of the on-board ladle electronic scale

In the process of carrying molten steel, a special carrier must be used to carry it, which is generally called a ladle car. The car body size varies according to the size of the ladle. The body of a general carrier vehicle is about 7 meters long, about 5 meters wide, and about 2.6 meters high. The rail track is used as the running route. Before tapping, the baked empty ladle is hoisted by a crane and placed at a specific position on the ladle car from a high altitude. The ladle has been baked for several hours before being hoisted, and the surface temperature reaches 60℃～70℃. After the ladle is placed, the truck is driven to the furnace mouth of the tapping furnace, and molten steel or molten iron with a tapping temperature as high as 1600°C flows in Inside the ladle, the site environment is relatively harsh.
The main manifestations are: a. In order to prevent molten steel from splashing and heat preservation, although there are heat insulation boards or firewalls around the furnace mouth, the ambient temperature around the truck can still rise to several hundred degrees. b. During the tapping process, the inclination angle of the furnace changes with the tapping volume. In order to ensure that the ladle mouth is consistent with the furnace mouth, the position of the tapping car is changed accordingly, but in the actual operation process, there will be asynchronization phenomenon, and molten steel overflow often occurs. There is thick steel slag on the surface of the ladle car. c. When the tapping car runs from the tapping furnace to the refining zone, use high-power electrodes to heat and smelt for 20-30 minutes. At this time, the temperature of the ladle wall has reached several hundred degrees, and the temperature under the ladle lug can be as high as 250℃. . d. The ladle car will produce lateral impact when it is running.   

2.2: On-site environmental characteristics of rotary table ladle electronic scale

The threat to the rotary table ladle electronic scale mainly comes from the high temperature radiation of the ladle and the vertical impact force on the scale body during lifting. The main manifestations are: a. The distance between the rotary table wall of the continuous casting equipment and the ladle is generally about 1500mm. In order to prevent high temperature radiation, in the design, radiation protection measures with a cover are generally taken. However, in actual use on site, the ladle is generally not covered, and the temperature at the installation position of the electronic scale is very high, reaching about 200°C. b. During the hoisting process, because the operating room of the crane is higher from the ground, the operation based on experience during landing will sometimes cause a high drop and great impact on the scale body. c. Crane hoisting ladle is a high-altitude operation, and it is more difficult for the ladle support ears to fall into the support seat of the electronic scale on the wall of the turntable.

3:Scale body structure of ladle electronic scale

Renovating the installation site of the ladle electronic scale has problems such as the threat of molten steel splashing, the threat of high temperature radiation, the vertical impact during lifting and the horizontal impact of the operation of the carrier, and the difficulty of positioning at high altitude. At present, domestic ladle electronic scales are generally designed and installed on the supporting frame of the ladle supporting ear seat with a special scale body, one on the left and one on the left, and a guide frame for accurate alignment. The following describes the structural characteristics of the two scales.

3.1 Structure of ladle electronic scale designed with double shear beam type high temperature load cell

The ladle electronic scale designed and reformed with a double shear beam sensor as the weighing fulcrum is composed of a high temperature sensor, a load-bearing beam, a protective plate, an approach guide frame, and a final guide frame. (See Figure 1)

The load cell used in ladle electronic scales include large-tonnage domestically produced high temperature sensors, sensors from Schenck in Germany and UTL from Spain (see Figure 2 for the appearance of domestic LC708 load cell).

Figure  2

The above-mentioned high-temperature sensors have a common feature: large tonnage range, wide external dimensions, and plane load-bearing. There are 4-6 M24 mounting screw holes on the load-bearing surface, which can ensure that the load-bearing beam and the sensor are firmly connected, safe and reliable.
The following problems must be solved in the transformation design of ladle electronic scale:

3.1.1 High-altitude hoisting ladle ears into the scale body
When the crane gantry hook hung a huge ladle and placed it on the position of the scale, the crane operator operated visually from dozens of meters away. If there is no guiding device around the scale, it is difficult to make the ladle enter the designated position normally. In order to ensure safe operation and make the ladle smoothly enter the scale body, guide devices are designed and installed on the left and right sides of the scale body (see Figure 1 for the structure of the guide device). The guide frame is welded into an integral structure with high-quality alloy steel. One high and one low are fixed on both sides of the scale body. The top of the guide frame is made into a certain slope to give the lifting operator an obvious reference mark. When the wrapping ears are close to the guide frame, the left and right inclined planes will drive the wrapping ears into the scale body smoothly, which improves operation safety and protects the scale body at the same time.

3.1.2. Ensure that the ladle is fully stressed

To ensure that the ladle is not affected by external forces during measurement, the lugs are fully supported on the load-bearing beam of the scale body to improve the measurement accuracy. When designing the scale body, an approach guide frame and a limit frame must be added to the load-bearing beam.

When the ladle supporting ear enters the weighing seat, it is difficult to ensure that the ladle sits on the load-bearing beam of the weighing box without friction and no force, because when the ladle is close to the side of the guide device, the friction is divided. Force is bound to affect weighing accuracy. In order to overcome this influence, a final guiding device should be installed on the load-bearing beam. Under the action of the inclined surface of the device, the ladle is forced to approach the center and finally sit on the load-bearing beam. Since the final guiding device and the load-bearing beam are integrated, no external force will be generated, and the weighing accuracy will not be affected. It can be seen from Figure 1 that if the top of the inclined surface of the final guide is not close to the inner side of the vertical surface of the guide, the ladle will be stuck on its top, causing the ear seat to tilt; in addition, the inclined surface of the final guide must be large enough Angle, so that the horizontal force generated after the ladle contacts this inclined surface is sufficient to push the ladle to the center, therefore, the final guide device must be designed with sufficient height.

3.1.3: Overcome horizontal impact

When the ladle lug is guided into the scale body and the car body through the final guiding inclined plane for lateral movement, a large horizontal impact force will be generated, and this huge impact force will cause a certain horizontal displacement of the load-bearing beam. It is understood from the on-site usage that it is for this reason that many ladle electronic scales fail in use. In the transformation design, the special design of the sensor should be used to bear the huge horizontal impact force, so that it has sufficient resistance. For example, if a high-strength planar bridge sensor is used, the maximum allowable lateral load is between 20-150t. Will not be damaged. During installation, the load-bearing beam of the sensor is used to form an integral structure with the load-bearing beam. When a horizontal impact force is generated, the load-bearing surface bears the shear force in the horizontal direction. The body can be used normally.   

3.1.4 Overcome vertical impact
When a ladle of hundreds of tons falls, the peak value of the vertical impact force generated is huge, and its vertical impact force has a fatal effect on the sensor. The best way is to install a hydraulic device in the lower part of the scale body, the hydraulic device will bear the vertical impact force, and then slowly fall onto the load-bearing beam. However, this structure is difficult to realize on the actual use site, especially the equipment transformation is difficult. Therefore, the electronic scale of this kind of structure can be used to make use of the structural characteristics of the sensor itself to enlarge the range of the sensor under the premise of meeting the requirements of the instrument sampling signal to resist vertical shocks. Since this type of electronic scale is generally a process scale, it does not require high accuracy. When selecting a sensor, a large range sensor can be used as much as possible. For example: a 150t ladle scale can use 4 100t sensors with a sensitivity of 2.0mV/V. Suppose the bridge voltage is 10V and the division value is 100kg. Through calculation, each division value still has 2.5: μV signal output, and the meter is enough to sample. This can greatly improve the overall impact resistance of the electronic scale and prolong the service life of the load cell.

3.1.5 Protective measures against high temperature and molten steel splash

The high temperature threats of vehicle-mounted ladle electronic scales and rotary table ladle electronic scales come from the molten steel splashing from the tapping furnace mouth and the high temperature radiation of about 1600℃ in the ladle, as well as the conduction of the temperature of the ladle wall to the scale body. The on-site test results show that the temperature of the scale body can reach about 200℃-250℃. In view of this situation, the high temperature resistance 200℃-250℃ load cell should be used in the transformation design, and the scale body should be added to prevent molten steel sputtering. Heat insulation board, the sensor adopts high temperature resistant bridge sensor, and its lead wire is introduced into the line pipe from the middle of the scale body, and the wire is not routed from the outside. The periphery consists of a downflow plate and an inner lining plate to form a labyrinth for double protection, so that the sensor will not be burnt out even if molten steel overflows.

3.2: Electronic scale designed with cylindrical high temperature load cell
The steel-clad electronic scale designed and reformed with a cylindrical sensor as the weighing fulcrum is basically the same in appearance as the electronic scale designed with a bridge sensor. Both have load-bearing beams, close to the guide frame, and the final guide frame internal design structure different. (See figure 3 for appearance)

It is composed of approach guide frame, final guide frame, load-bearing beam guide column, guide sleeve, anti-lifting plate, high temperature sensor, protection plate, etc.

When the ladle enters the scale body, take the approach guide frame as a reference target, and slide the bag ears along the slope of the guide frame into the inside of the approach guide frame. When entering the ear of the scale body and sitting on the load-bearing beam, it is limited by the final guide frame and divided into two directions of XY axis to ensure that the ear and the wall are not affected by any external force. When a horizontal lateral force is generated, it is overcome by three guide posts, and the sensor is not affected. Since a certain point of the ear of the ladle first falls on the load-bearing beam when the ladle is falling, the load-bearing beam will lift up and tilt the sensor. When the scale body is designed, an anti-lifting device should be installed inside to make the sensor receive normal force. In the structural transformation design, in addition to the previous horizontal impact resistance and peripheral protection, the following characteristics are mainly considered:

3.2.1 Positioning and horizontal impact on load-bearing beams

When the ladle moves on the scale body, huge horizontal force will be generated due to inertia. If the scale body does not have a limit device, it will inevitably damage the sensor and even bring safety hazards to production. It can be seen from the figure that three guide posts and guide sleeves are added inside the scale body to bear the horizontal impact force while maintaining the horizontal state of the load-bearing beam. For the convenience of installation, there is a gap of about 1mm between the guide posts and the guide sleeves; in order not to affect the weighing accuracy, high temperature resistant buffer rubber is pressed into the bushing. This structural design, in fact, only allows the load-bearing beam to move up and down linearly, limiting the horizontal displacement. In this scale body structure, the sensor cannot use the plane bridge sensor used in the above scale body, but a double-spherical pendulum sensor. This type of sensor can provide a certain amount of horizontal displacement, and the weight sensor is not affected. Commonly used high temperature sensors include domestic high temperature sensors such as LC402. (See Figure 4 for appearance)

3.2.2 Anti-warping design of load-bearing beam
The ladle descends from a high altitude and enters the scale body. It is impossible for the lug support seat and the load-bearing beam to contact the entire plane at the same time, and because there is a certain gap between the guide column and the guide sleeve, this will inevitably cause the load-bearing beam to instantly rise and jump; if not Adopting the anti-lifting limit device will inevitably cause the sensor to tip over, and in severe cases, it will cause the load-bearing beam to tilt, which will bring hidden dangers to safe production. Therefore, it is very necessary to add anti-warping devices on the left and right sides of the sensor when designing the transformation. In the design, two sets of anti-saw plates can be welded on the load-bearing beam and the bottom plate of the scale body, keeping them on the same centerline, and limit holes are added to the riser plates, and limit pins are used to limit the position. When designing, keep a certain gap between the limit hole and the limit pin. Generally, the maximum limit gap should not cause the sensor to tip over.

4:The Conclusion:

The use of high temperature sensors to design and transform ladle electronic scales can overcome the impact of high temperature environments on electronic scales. It has been widely used in