Instrumentation configuration of VD furnaces in steel mills

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VD is the abbreviation for vacuum degassing in steel mills. As an important processing technique for producing various types of steel, such as pipeline steel and oil tank steel, vacuum degassing in steel mills is attracting increasing attention from metallurgical enterprises. By 2006, the 120t VD furnace at Nanjing Steel Plant had set a high domestic record of consecutive production over a dozen batches, while the VD furnaces at Anshan Iron and Steel Group Company also enjoyed a high level of adoption in the industry. Jigang once achieved the goal of producing 15 furnaces in a single shift continuously, and there have been few reports of higher numbers since then. As a piece of equipment with a high degree of automation in terms of instrumentation, the VD furnace in a steel mill relies on automated instruments for 95% of its control and monitoring functions; 51% of the faults encountered are related to the instrumentation used in these VD furnaces. In addition to being automated and featuring electronic controls, the VD furnace in steel mills is also related to mechanical and thermal engineering fields. Many steel mills have weak technical capabilities in instrument maintenance; the newly installed VD equipment has not been put to full use. Below, based on the author’s experience in handling numerous shutdown failures since first coming into contact with the equipment in 2001, and in conjunction with specific failure phenomena encountered during training at Ansteel and on-site at Jinan Iron and Steel, an analysis is presented from a technical perspective. Analysis of automation instrument failures in the VD furnace of a steel plant ◆ The HMI displays inaccurate vacuum levels. Figure 1 shows the basic principle of VD degassing; this is the most common configuration of VD equipment. This pump mainly consists of multi-stage main and auxiliary pump bodies, as well as three condensers. Pumps at all levels consist of a Laval nozzle, a suction chamber, and a diffuser. The condenser consists of a shell, water spray plates, and cooling water distribution pipes. The water and steam system includes condensate, steam, steam-water separators, etc. The control section includes a PLC electrical control system and an instrument control system. http://yunrun.com.cn/upload/201910/13/201910131505307405.png Figure 1: Schematic diagram of vacuum degassing in a steelmaking plant. Multiple primary and auxiliary jet pumps are started sequentially; the injection speed of the working steam is increased to over 1.5 Mach using Laval nozzles. The pressure difference generated enables this steam to mix with the high-temperature furnace gas at the inlet, forming a mixed gas. After passing through the diffuser throat, this gas slows down and its pressure increases as it enters the subsequent condenser. After being cooled in the subsequent condenser, the condensed substances flow together with the cooling water from the bottom outlet of the condenser into the hot well water seal tank. Some of the furnace gas is discharged through the outlet above the condenser to the next stage of jet pump or into the atmosphere. The vacuum created in this process creates a pressure difference between the atmosphere and the vacuum chamber, forcing the gases present in the molten steel to emerge. Under the action of argon stirring, the gases and impurities in the molten steel at the bottom rise continuously and are discharged along with the furnace gas, thus completing the degassing process of the molten steel. It can be seen that the formation and maintenance of a vacuum are key to degassing the molten steel. In the initial stage of operation and after a period of stable production, single-crystal silicon pressure transmitters (yunrun.com.cn/product/788.html) are prone to issues where the rate of vacuum decrease becomes abnormal. In such cases, it is necessary to first inspect the two transmitters used for measuring vacuum levels. In particular, the low-pressure single-crystal silicon pressure transmitter model YR-ER102-B1CABNN1NNC1 with a measurement range of 0–20 kPa (0–200 mbar) may experience blockages in its pressure measurement pipelines due to dust or steel slag, thereby preventing accurate and timely pressure measurements. In addition, special attention should be paid to inspecting the steam jet vacuum pump; such problems can also occur if the nozzle of the vacuum pump becomes detached or deviates from its axis. 1. Vacuum system leakage: What is meant by system leakage is what is commonly referred to as external leakage; it usually occurs at the flanges connecting the instrumentation valves and at the upper part of the valve cores. Since the entire system is under negative pressure, it is very difficult to detect such leaks. There are usually two methods used on-site: one is to light a cigarette and bring it close to the suspected area, then observe the direction in which the smoke drifts ； Second, use very thin paper and stick it to the suspicious area; if it sticks, it indicates severe air leakage. 2. Air leakage from the nozzle joint gasket: The vacuum pump consists of three parts, and the nozzle is the part prone to failure; in particular, the flange connecting the nozzle is very likely to become loose under the impact of high-temperature steam. Based on experience, inspections should be carried out after 100–300 cycles; it should be opened by the 500th cycle at the latest, otherwise internal leakage will occur soon. Only by opening and disassembling the nozzle, replacing the gasket, and tightening the flange can air leakage be prevented. 3. Air leakage at the system’s vacuum pipeline flanges, the welds of the pressure tapping pipes for instruments, and the tank lid. Since the VD vacuum degassing system operates by placing the steel ladle inside a vacuum chamber for degassing, the system has a relatively large vacuum volume, of around 250 m3 (including the pipelines). There are over 100 flange sealing points throughout the system, with the total length of the sealing lines extending to approximately 180 meters. For dealing with leaks, one approach is to perform weld repairs, tighten bolts, or replace the gasket with a new one ； Second, check for the presence of the following conditions: ① Damage to the sealing strip of the camera lens viewing window ； ②The silicone rubber seal of the vacuum tank is crushed, damaged, aged, or there is a gap at the joint of the seal ； ③There are cracks at the weld ； ④Improper handling of the thread after feeding it caused air leakage between the vacuum cover and the vacuum tank ； ⑤Damage to the valve port of the vacuum instrument valve ； ⑥The instrument valve for filling the vacuum pipeline or the instrument valve for draining water from the vacuum pipeline is in the open position ； ⑦The flow meter measurement orifice plug has come loose or is blocked ； ⑧The drain pipe of the condenser and the exhaust pipe outlet of the start-up pump do not extend into the water seal tank, etc. Regarding leaks in the VD vacuum tank system, emphasis should be placed on inspecting dynamic seals, such as the condition of the tank lid sealing ring. Internal leakage is mainly caused by: ① Damage to the check valve connected to the main condenser (such as detached or misaligned valve caps) ； ②The gasket at the nozzle joint is damaged. Due to the long pipelines in the system and inadequate maintenance of the equipment, the sealing performance at some of the equipment interfaces has declined, and there are cracks in certain units. In particular, the failure of the check valves has led to increased internal and external leakage, **which raises the exhaust volume of the exhaust system and increases the exhaust load on each pump. ◆The system’s evacuation time is too long. Typically, VD equipment can reduce the vacuum level to below 200 Pa (2 mbar) within 6–8 minutes; as long as this time does not exceed 10 minutes, the production rate of a continuous caster with a 270-mm thick slab—at 0.8–1.0 m/min—will not be disrupted. However, in the event of a fault, the vacuum level cannot be reduced below 200 Pa no matter how long the evacuation process lasts, because the vacuum created by the steam has reached equilibrium with the air that has leaked in. Unless the leaks are sealed or the vacuuming capacity is increased, the vacuum level will never drop below 200 Pa. Based on experience in on-site maintenance, the pressure transmitter used to measure vacuum level is a crucial device. Sometimes, clogging of the filter screen ahead of it can cause an increase in the time required to evacuate the system; this fault is rather subtle, and cleaning out the dust accumulation is necessary to restore normal operation. 1. Clogging and erosion of the nozzle in vacuum pump ejectors: Typically, vacuum pumps are designed with five stages, and all steam ejectors operate in series to compress the flue gas stage by stage. The nozzle is made of stainless steel and features high precision in manufacturing. Blockages or erosion can affect the expansion of steam at the Laval nozzle; therefore, it is necessary to clean it regularly and check the fixing condition of the steam nozzle on its base, as there are very strict requirements regarding the coaxiality between the pump body and the steam nozzle during installation. Use the downtime for maintenance to disassemble the nozzle and clean out foreign objects (as shown in Figure 2); blow out the steam inlet pipe if necessary. http://yunrun.com.cn/upload/201910/13/201910131648350561.png Figure 2: Schematic diagram of foreign objects attached to the nozzle of a jet pump. The nozzle of a certain stage of the ejector is blocked, resulting in reduced performance; or too much dust has accumulated in the diffuser throat of the ejector, blocking the airflow pathway. When the dust accumulation at the throat of the diffuser reaches a certain thickness, it alters the axis of the diffuser. At this point, the high-speed steam emitted by the Laval nozzle creates turbulence inside the diffuser, which results in a significant deterioration of the pump’s operating characteristics and affects its pumping performance. 2. Erosion and blockage of the condenser nozzles: On the basis of confirming that testing instruments such as electromagnetic flowmeters are functioning properly, analyze the changes in the flow rate of water entering the condenser. Since the joints between the nozzle and the steam are usually made of carbon steel, erosion is generally severe in these areas. Failures in these locations are not only difficult to detect but also directly affect the vacuum-pumping capacity, as disrupted steam flow can ultimately hinder the ability to create a vacuum; however, the flow rate indicated by the instruments usually increases at such times. The water nozzles are made of ordinary Q235 material, which is highly prone to rust. Under prolonged exposure to high-volume water flow, they can easily become detached. Based on changes in the flow rate at the instrument, it is advisable to periodically open the manhole to inspect the nozzles as part of the monthly maintenance schedule; this also allows for checking whether any blockages have occurred. For components that are old and worn out due to corrosion, conduct overhaul inspections, replace them with new parts, and replace the pump if necessary. 3. Dust accumulation in the vacuum system – Dust accumulation in the vacuum system is a common cause of failures. A dust separator is installed between the vacuum tank and the main vacuum cut-off instrument valve, and it needs to be opened regularly for cleaning and ash removal (every 100–300 cycles). At the same time, the inner wall of the vacuum pump needs to be cleaned regularly; otherwise, excessive dust accumulation on the inner wall will affect the vacuum level, as shown in Figure 3. http://yunrun.com.cn/upload/201910/13/201910131649517792.png Figure 3: Dust accumulation inside the vacuum pipeline of the VD furnace. 4. The impact of condensate water temperature and water quality. The purpose of installing a condenser in the system is to condense and remove the condensable gases from the earlier stages, thereby reducing the load on the injection pumps in the later stages. The working principle of the condenser relies on the convection between cooling water and steam; ultimately, the cooling water absorbs the steam and converts it into condensed water, which is then discharged through the drain at the bottom of the condenser into a tank. While the steam is condensed, the separated furnace gas is discharged to the atmosphere or into the next stage of pump through an exhaust port. When the cooling effect is poor, the condensable gases in the hot flue gas cannot be completely condensed into a liquid state; as a result, there is an excessive amount of flue gas in the condenser, which leads to a decrease in vacuum level. The amount of cooling water, water temperature, water quality, and fouling of the condenser are the main factors affecting the cooling effect. Given other fixed conditions of the system, increasing the contact area between the cooling water and the high-temperature mixed flue gases, as well as prolonging their contact time, is the fundamental design principle for ensuring adequate heat exchange between them and achieving the desired cooling effect. The design concept of the VD condenser is to use a sieve-type cooling water distribution plate that provides a large contact area between the cooling water and the high-temperature flue gas, allowing the cooling water to flow downward in the form of droplets and streams through the holes in the plate. However, due to the high dust content in the high-temperature flue gas during degassing, scaling on the surface of the distribution plate reached 10–20 mm, and almost all of the φ8 mm holes became blocked. This effectively resulted in a overflow-type distribution plate, reducing the efficiency of the condenser and becoming one of the main reasons for the decrease in vacuum level in the VD system. At this point, it is necessary to regularly clean the condenser and carry out water purification treatment. Whether the spray holes in the condenser are unobstructed is directly related to the cooling efficiency of the system, and thus closely associated with the vacuum pumping capacity. In practice, it has been found that condensers are prone to being clogged and scaled by debris. Measures for dealing with condenser blockages: ① Install protective screens at the inlets of the chilled water and hot water pumps to prevent larger particles and debris from entering ； ②Add water purification equipment to improve water quality and delay the accumulation of scale within the system. Firstly, it is necessary to ensure that the water is clean, free of impurities and scale, as otherwise this can lead to blockages in the condenser’s condensation pores. Secondly, with the total amount of water being relatively fixed, excessive water flow within the condenser can impair the performance of the vacuum pump; therefore, it is crucial to regulate the water distribution across each stage of the condenser. Therefore, the opening degree of each valve is determined during the vacuum test, and it is not easily changed ； If any changes are necessary, it’s best to leave marks so that everything can be restored to its original state. The high temperature of the cooling water severely affects and limits the vacuum pumping capacity, resulting in an extended vacuum pumping time or even an inability to achieve the desired level of vacuum. Therefore, regular maintenance of temperature measuring instruments is an essential step. 5. Excessive air curtain flow at the tank lid camera: To monitor the condition of the molten steel during production, two types of observation methods are generally employed on the VD tank lid: an manual viewing window and an automatic camera. The former does not fill the tank with gas, while the latter, for the purpose of protecting the lens, has an air shield that introduces a certain amount of compressed air into the tank. It is generally opened during the production of the reduced-cap version, as the heat radiation from the tank lid is so strong that the solenoid valve cannot operate for an extended period of time; therefore, the interlock here is usually set to the normally open state. If the pressure and flow rate are not appropriate, the gas blown into the tank will result in a longer pumping time. ◆Abnormal surging noises are heard from the ejector and pipes. Such problems are quite obvious; by categorizing them based on the phenomena observed on site, the following aspects should be taken into consideration when such faults occur: 1. The working steam pressure is too low, resulting in an increase in condensate water and a phenomenon similar to \"water hammer\". In this case, the steam pressure should be increased. 2. Insufficient cooling water volume or excessive temperature, resulting in abnormal operation of the condenser. This is a common fault; in such cases, it is effective to increase the water pressure to above 0.45 MPa and lower the water temperature to below 30°C, while also adjusting the opening degree of the inlet valves for each stage of the condenser appropriately. 3、Too much dust has accumulated in the diffuser throat of the ejector, blocking the airflow passage. There is no other solution to this phenomenon other than inspection and cleaning. 4. For components with a long service life that are subject to wear and corrosion, major inspections should be carried out, replacement parts should be used, and a new pump should be installed if necessary. 5. The moisture content in the working steam is too high; improve the functionality of the steam-water separators and drum traps, optimize boiler operation, and repair or replace the traps. 6. The temperature of the working steam is too high; the degree of superheating exceeds 20–30°C. Adjust or maintain the steam cooling device to keep the degree of superheating within the range of 0–15°C. 7. If a red alarm appears for the instrument valve on the HMI, check the area where the alarm is triggered. At this point, it is necessary to make appropriate adjustments to the limits of the area where the alarm has occurred; generally, this will allow normal operation to be restored ； If the valve does not respond sensitively enough, it is necessary to check the cylinder and the air supply pressure. 8. Communication failures in the automated instrument control network usually occur at the network connections of remote stations; this can result in a group of valves failing to function properly. It is necessary to regularly inspect the network connections to prevent oxidation and loosening. Maintenance of automated instruments in VD furnaces in steel mills 1. Leak detection of instrument equipment in the vacuum degassing system. To minimize leaks in the vacuum degassing system, leak detection is an essential step that is carried out throughout the entire process, from the factory testing of various system components to on-site assembly and commissioning. The entire system is mainly divided into three areas: ① Vacuum pump area instruments. All components of the vacuum pump must undergo pressure testing and leak detection before leaving the factory; the possibility of air leaks in the pipelines is minimal. However, the presence of numerous monitoring instruments complicates things, and after assembly on-site, leak detection is still required using the positive-pressure soap bubble method, involving the use of compressed air at a pressure of 0.15–0.2 MPa for 24 hours. The relevant standard is an average pressure drop of no more than 2% per hour; practice has shown that an average pressure drop of less than 0.4% per hour is fully achievable. ②Instrumentation equipment in the vacuum pipeline area: After assembly in this area, leak testing must also be carried out using the positive pressure soap bubble method; in particular, the components of various instrument valves are key areas for leak detection. ③VD vacuum chamber and vacuum gauge: The VD vacuum chamber must be accepted after undergoing a negative pressure leak test, with a leakage rate not exceeding 1000 Pa·L/s. If the volume of the vacuum chamber is 170 m3, then the pressure rise should be less than 20 Pa/h. It is also possible to check whether the low-pressure transmitter operating in the range of 0–2×104 Pa is functioning properly; this is sufficient to meet the requirements of steelmaking degassing systems. 2. The impact of the cleanliness of the inner wall of vacuum pipes on vacuum pumping. Theoretically, the cleaner the inner wall of the pipe, the better it is for improving vacuum pumping efficiency. However, in practical manufacturing, strict standards have been established: any debris on the inner wall must be removed so that the surface is smooth, without any irregularities, allowing the true color of the steel sheet to be visible. Before entering the vacuum pump, it is necessary to ensure that there is no CO inside, that there is sufficient oxygen (above 18%), and that a dust mask is worn. 3. The influence of cooling water flow rate on vacuum pumping. Theoretically, for vacuuming, it’s not the case that the greater the cooling water volume, the better. Because, with an excessive amount of cooling water, on the one hand, the area occupied by rising steam is reduced, which increases resistance, slows down the flow rate, reduces the exhaust velocity, and thus slows down the increase in the system’s vacuum level ； On the other hand, since the diameter of the condenser is designed based on the maximum amount of cooling water, if the water volume is too large while the drainage speed remains constant, the cooling water may flow back into the jet pump, causing a phenomenon similar to siphoning, which leads to an increase in water level and occupies space within the condenser. However, in practical operations, it is often assumed simply that the cooling water volume is insufficient based on a high outlet water temperature, and the cooling water volume is increased blindly, which ends up having the opposite effect. A proper way to determine whether the cooling water volume for the condenser is appropriate is as follows: ① Determine whether the cooling water volume is too large or too small based on the actually measured flow rates of cold and hot water, the temperature difference (the difference in water temperature and the temperature loss of the molten steel), as well as the correction factor; adjust the water volume accordingly ； ②The amount of cooling water is determined indirectly by referring to the corresponding pump curve tables based on the actual head of the cold and hot water pumps, and the water volume is adjusted accordingly ； ③Adjust the water pressure, lower the water temperature, and adjust the opening degree of the inlet valves for each stage of the condenser. 4. Impact of external leakage points on vacuum level: Due to the long length of the pipelines in the entire system and the numerous connection points, the likelihood of external leakage increases accordingly. Although the external leakage at each point has a relatively small impact on the vacuum level, generally around 60–260 Pa (0.6–2.6 mbar), the cumulative effect of leaks at various points is significant. To quickly and effectively eliminate various leakage points, in addition to checking fasteners such as flange connection screws, it is also highly effective to wrap a layer of plastic wrap around the outer edge of the joint surface. 5. Maintenance cycle for automated instruments in VD furnaces: ① Daily maintenance: Check the operating status of the valves using the HMI display on the control console. ②Weekly maintenance: dust cleaning, inspection of limit switches, hose connections, and seals. ③Monthly maintenance: Inspect the discharge valve, compressed air filter, steam on/off valve, seals of the main isolation instrument valve, and the condenser. If the condenser discharge pipe is cold, it is very likely that there is a problem with the condensation valve. ④Annual maintenance: Disassemble the cooler, inspect and clean it ； Clean the hot well and check whether the nozzles from level 1 to level 3 are loose. Many companies in China are now producing VD steel grades, and in fact there are significant differences among them. The quality of products produced at a vacuum level of 200 Pa differs from that of those produced at 300 Pa, 400 Pa, and 500 Pa; it is generally difficult for ordinary users to determine the quality of the molten steel. The quality of the 100×106 grade molten steel produced by some enterprises differs significantly from that of molten steel with a quality level of 50×10-6 or lower; these issues ultimately need to be resolved by improving the level of automation and control as well as by further reducing the vacuum level. Currently, China’s manufacturing sector has an increasing demand for high-end metallurgical materials, and the VD process will also play an important role in the production processes of various metallurgical enterprises. Enhancing the understanding of the peculiarities of VD equipment, particularly the special role of the automation instruments in VD furnaces in steelmaking processes, is essential for unlocking the potential of VD technology.