Thread Content
This post was last edited by Yiding on 2009-11-10 01:04. 1 Process flow and requirements: The blast furnace is an important device used in steel plants for smelting pig iron, while the compressor is the equipment that provides compressed air to the blast furnace. Depending on the capacity of the blast furnace, the specifications of the associated blowers (air volume, air pressure) also vary. Therefore, typical blast furnaces are equipped with dedicated axial flow compressors, and to ensure the safe and normal operation of the furnace, two blowers are usually installed (one as a backup). Due to its advantages such as high efficiency, large flow rate, and wide operating range, axial flow compressors have been widely used in large, medium, and small blast furnaces in recent years. Due to space constraints in Wuhan Iron and Steel Company’s No. 1 blower station, a new AV80 (Unit 5) fully static-vane adjustable axial flow compressor is required. This compressor should serve as the main supply fan for the 3,200 m3 blast furnace (Blast Furnace No. 4), while also functioning as a backup unit for Blast Furnaces No. 1, No. 2, and No. 3, whose capacities are 2,215 m3, 1,560 m3, and 1,460 m3 respectively. To achieve this purpose, the compressor’s air volume range must be 3,000–6,000 m3/min; the air pressure adjustment range must be 0.3058–0.5018 MPa. If the technology of domestic equipment fails to meet these requirements, users clearly state that only imported equipment can be used. Technically, this requirement is special: since it must also serve as a backup unit for Blast Furnaces #1, #2, and #3 with capacities of 2,215 m3, 1,560 m3, and 1,460 m3 respectively, its adjustment range does not meet the requirements. In light of this situation, Shaanxi Blower Company proposed a technical solution that involves using both variable static vane adjustment and variable speed adjustment (the normal operating speeds are: 3,800 r/min, 4,100 r/min, 4,250 r/min) in order to expand the adjustment range of the axial flow compressor. However, this introduces new technical challenges in unit control, as the axial flow compressor needs to adjust its speed while changing the opening of the static vanes. In addition to expanding the components related to unit control, the key issue for the control system is, first and foremost, how to determine appropriately the optimal ranges for adjusting both the speed and the static vanes ； Secondly, during unit operation, how to achieve a seamless transition between speed control and static vane adjustment ； Thirdly, the surge line on which the anti-surge protection control of the axial flow compressor in the control system is based is no longer a conventional curve, but rather a surface formed by a collection of numerous curves; as a result, conventional control strategies alone are insufficient to achieve the anti-surge control and protection functions for the compressor. It is precisely due to these technical challenges that the practice of using both static vane adjustment and speed control in axial flow compressors to expand the range of adjustment remains unexplored in China at present, and is also rare abroad. To address these technical challenges, we conducted thorough research and evaluated various solution options, and carried out numerous experiments and adjustments on-site. Eventually, we succeeded in solving these problems, which earned us positive feedback from Wugang’s customers. The main equipment of this unit – the axial flow compressor – is an AV80-14 type fully static-vane adjustable axial flow compressor manufactured by Shaanxi Blower (Group) Company, while the power equipment consists of NK50/71/32 type variable-speed industrial steam turbines produced by Hangzhou Steam Turbine Factory. The unit control system was designed by Shaanxi Blower (Group) Company, while Xi’an Lanxi Control System Engineering Company was responsible for system configuration and commissioning; relevant departments of Wuhan Iron and Steel Group provided strong support and cooperation. The process flow is shown in Figure 1: Air passes through an air filter and enters the axial flow compressor, which rotates driven by a turbine to generate compressed air of high pressure and large flow rate. This compressed air is then sent to the blast furnace via a hot air stove. 2 System Composition The control system configuration is shown in Figure 2; the control equipment uses the Micro-TDC3000 medium-sized DCS from the American company HONEYWELL. The system consists of a high-speed processor (HPM), an extended I/O rack, a network interface (NIM), a history module (HM), an operation station (US), a local control network (LCN), and a universal control network (UCN). To ensure the safe operation of the blast furnace as well as the stable functioning of the axial flow compressor, the system adopts a fully redundant configuration. 3 System Functions If the axial flow compressor unit is compared to the heart of a blast furnace, then the control system serves as its central nervous system. The system features constant air volume/constant air pressure control, anti-surge control, unit safety interlock protection control, operation control for auxiliary pumps and valves, as well as functions for monitoring the operating status of the entire unit, generating alarms, and recording data. 3.1 Constant air volume/constant air pressure control system: During normal operation of the blast furnace, it is required that the air supply volume delivered by the compressor remain as constant as possible, that is, the compressor operates at a constant air volume. However, due to the many factors that interfere with the air volume in actual production, this inevitably leads to fluctuations in air volume. Therefore, the control system is equipped with a dedicated constant-air-volume regulation system, which uses a combination of controlling the opening of the static vanes and adjusting the speed to achieve regulation. Furthermore, to meet the specific requirement of blast furnaces for a constant compressor outlet pressure under special operating conditions, the constant air volume control system can also be switched automatically or manually to constant air pressure mode, thereby maintaining a constant compressor outlet pressure. This article discusses only variable air volume control as an example. Unit 5 of Wuhan Iron and Steel Group adopts the following 4 optimal operating modes: Mode 1 – the unit’s rotational speed is 3,800 r/min; the static vane adjustment range is 0∽100% (corresponding to a static vane opening of 22∽79°); the maximum air volume is 6,200 m3/min, and the maximum air pressure is 0.3724 MPa. In Condition 2, the unit’s rotational speed is 4,250 r/min; the static vane adjustment range is 0∽100% (corresponding to a static vane opening of 22∽79°). The maximum air flow rate is 6,750 m3/min, and the maximum air pressure limit is less than 0.452 MPa. Condition 3: The unit speed ranges from 3,800 to 4,100 rpm; the static vane adjustment range is 0% to 100% (corresponding to a static vane opening of 22° to 79°). The maximum air flow rate is 6,250 m3/min, and the maximum allowable air pressure is less than 0.45 MPa. In operating condition 4, the speed of the unit is 4,100∽4,250 r/min; the static vane adjustment range is 0∽100% (corresponding to a static vane opening of 22∽79°). The maximum air flow rate is 7,000 m3/min, and the maximum allowable wind pressure is less than 0.45 MPa. Below, taking Condition 3 as an example, the disturbance-free transition from static vane control to speed control is described. As shown in Figure 3, one of the outputs from the constant air volume regulator (PID) passes through the GEL11∽GEL15 piecewise linear function generators and the switches SW101 and SW102, before being sent to the static vane servo controller to control the opening degree of the static vanes ； The other path passes through the GEL16∽GEL17 piecewise function generator and the switching switches SW103, SW104, before being sent to the speed controller to control the turbine speed. C1-- Stator vane opening 32.5° ； C2—Stator blade opening 70.0° ； C3— Rotational speed 3,800 r/min ； C4-- Speed 4,100 r/min ； C4-- The required value for the blast furnace air volume at a rotational speed of 4,250 r/min is used as the setpoint (SP) for PID control, while the actual air supply volume serves as the measured value (PV) for PID. Based on the output signal of the PID (0∽100%), after processing by a piecewise linear function generator, logical judgment is used to determine the setting of the selection modules for SW101, SW102, SW103, and SW104; this determines whether, when adjusting the air volume, the rotation speed should be changed while keeping the static vane opening constant, or the static vane opening should be changed while keeping the rotation speed constant. Below, using the need to increase air volume in a blast furnace as an example, we will explain how this system works. When the unit is operating normally, and the blast furnace requires an increase in air volume, assume that at this time the static vane opening is less than 32.5°. In Figure 3, SW101 is set to switch position S1, while SW102 is set to position S3 ； The static vane controller adjusts the opening of the static vanes based on the output signal from the flow regulator’s PID to regulate the air volume, while SW104 selects mode S1, keeping the unit’s rotation speed at 3,800 r/min. When the blade opening increases to 32.5°, if the air volume still does not meet the requirements, logical judgment is used to take into account the increasing trend of the PID output, and the selection module of SW102 is switched to stage S1, thereby keeping the blade opening at 32.5°. At the same time, SW103 is set to stage S1 and SW104 to stage S2, causing the rotational speed to increase from 3,800 r/min; the air volume continues to rise until it reaches the desired set value. If more air flow is required for production, the rotation speed is increased further. Once the speed reaches 4,100 r/min, the SW104 selection module switches to gear S3, keeping the speed at 4,100 r/min. At the same time, the SW101 selection module switches to gear S2, while SW102 switches to gear S3; the air flow is increased further by adjusting the opening degree of the static vanes until the needs of the blast furnace are met. Similarly, when the blast furnace requires a reduction in air volume, the switching levels of SW101, SW102, SW103, SW104, etc. are automatically selected based on the decreasing trend of the PID output value, the static vane angle, and the actual rotational speed; this allows the set values for rotational speed or static vane opening to be changed or fixed, thereby achieving control of the air volume and satisfying the requirements of the blast furnace’s production process for air supply. 3.2 Surge Control Surge is an extremely unstable and dangerous operating condition for compressors; once it occurs, it can likely cause damage to the compressor. Therefore, the function of the surge control system is to prevent the compressor from operating in the surge zone. However, for compressors that adjust both the stator blades and rotational speed simultaneously, their anti-surge control is more complex and has distinct characteristics. In short, the set of its surge points is no longer a single curve, but a complex surface. To facilitate control, through repeated experiments on-site, its surge point was ultimately simplified to several specific curves. During the unit startup process, the anti-surge valve remains in the fully open position under the control of the control system logic. Once the unit has started up properly and automatic operation is enabled through the control system logic, the control logic automatically identifies the air supply conditions required by the blast furnace (i.e., the operating conditions of the unit), and accordingly selects a specific anti-surge curve for those conditions. At the same time, the control system makes real-time temperature adjustments to this anti-surge curve based on the atmospheric temperature at the compressor inlet. Thereafter, the control system performs PID control on this anti-surge curve function and the measured compressor discharge pressure, with its outputs respectively controlling the two anti-surge valves installed in parallel. The principle of anti-surge control is shown in Figure 4. In Figure 4, the throat pressure difference generated by the compressor during operation is subjected to special functional calculations to produce the anti-surge curve functions for the following 4 operating conditions. GEL01 is the anti-surge curve function at n=3800 r/min (Condition 1) ； GEL02 is the anti-stall curve function at n=4250 r/min (Condition 2) ； GEL03 is the anti-surge curve function at n=3800∽4100 r/min (Condition 3) ； GEL04 is the anti-stall curve function at n=4100∽4250 r/min (condition 4). The curve functions of GEL01, GEL02, GEL03, and GEL04 are subjected to real-time temperature correction using four calculation formulas: CAL_L1, CAL_L2, CAL_L3, and CAL_L4. The control logic selects one of the temperature correction values obtained from these four formulas, based on the air supply requirements of the blast furnace (i.e., the operating conditions), and uses this value along with the actually measured compressor exhaust pressure as the parameters for real-time anti-surge PID control. The corrected value serves as the setpoint for the PID, while the compressor exhaust pressure acts as the measurement value for the PID. The control output of the PID is sent to the two parallel-mounted anti-surge valves BV201 and BV202 through split-range operation (HCC201, HCC202). Under normal operating conditions of the unit, SV should always be greater than PV; however, due to changes in process conditions, it is possible for SV to become less than PV. This situation can lead the unit into a surge condition, and in such cases, the anti-surge PID control will immediately adjust the settings of the two anti-surge valves BV201 and BV202 to open them to a certain degree, so that SV becomes greater than or equal to PV again, thereby bringing the compressor back into a safe operating range and ensuring its safe operation. In addition, this anti-stall control system also features special functions such as anti-stall movement and reset, rapid response of dynamic gain, and anti-saturation. 4 Conclusion The AV-type axial flow compressor, which utilizes both static vanes and speed control mechanisms, was put into use in Wuhan Iron and Steel Company’s No. 5 plant in January 2000. After more than a year of actual industrial operation, it has been proven that this entire unit can meet the production requirements of blast furnaces with different capacities. Moreover, it has been shown that its control system is well-designed, feasible, safe and reliable, operates smoothly, and is easy to operate flexibly. The AV-type axial flow compressor incorporates the new technologies of static vane adjustment and speed control, which not only fills a gap in China but also elevates the overall technology level of axial flow compressors in the country to a new height. To ensure smoother and safer operation and control of the unit, it is recommended that, when selecting equipment for the unit’s control system, priority be given to advanced new control systems with higher operating speeds.