After nearly 40 years of development in vacuum degassing, the refining function has been continuously expanded, and it has changed from a simple degassing device to a multi-functional processing device. Its fast processing feature enables it to cooperate with the fast pace of the converter. The vacuum circulation treatment is especially suitable for the rapid processing of large quantities of molten steel, and can be satisfactorily matched with large-capacity electric arc furnaces or converters. Due to the advantages of the vacuum cycle treatment method, it has become one of the fastest-growing refining equipment outside the furnace. Multifunctional Refining Equipment .
At present, whether it is new equipment or upgrading of existing equipment, the investment in vacuum degassing treatment projects continues to increase. The reason is that vacuum degassing can significantly reduce costs, improve product quality, and can launch unique Special steel can make steel appreciate.
For economical reasons, manufacturers usually use large equipment to degas a large amount of molten steel, so equipment with great vacuum suction capacity is generally required. These devices can be dirty to operate due to the generation of large amounts of metal fines and oxide dust.
Oil-sealed vacuum pumps, as the energy consumption expenses and equipment maintenance burdens of steelmaking manufacturers are getting heavier and heavier, they hope to adopt dry mechanical vacuum suction systems, relying on this new equipment to better handle dust and save a lot of money .
Compared with previously used devices, the dry mechanical vacuum suction system can significantly save production and maintenance costs and installation sites, while also improving suction speed, operational mobility and total output. The structure of the large Roots-type pre-vacuum pump has a very high dust tolerance, and it is the main equipment of the mechanical vacuum suction system.
Vacuum degassing of molten steel
Vacuum degassing of molten steel is the key operation of secondary steelmaking. It can quickly and effectively remove impurity gases (mainly hydrogen and carbon monoxide) dissolved in molten steel to reduce the dissolved carbon content in molten steel, thereby making the steel quality better, increasing its value and expanding its value. adaptability. Vacuum degassing (VD) and vacuum oxygen decarburization (VOD) are two basic degassing methods.
Vacuum degassing (VD) method The basic operation of VD generally takes 15 to 20 minutes, and is carried out under the pressure of 0.67mbar. Under these conditions, the dissolved hydrogen from the molten steel is sucked into the atmosphere above the molten steel to remove the carbon dioxide gas. This treatment also helps to clean up volatile metal elements (Pb, Sn, As, Sb, Bi, etc.).
After treatment, the residual gas content in molten steel is generally extremely low, such as hydrogen reduced to parts per million?. A soft blow with argon at the end of the treatment also keeps the oxygen content below 15 parts per million.
When performing VD treatment, it is necessary to discharge tens of kg/h equivalent air under the pressure of 0.67mbar. In this way, there must be at least tens of thousands of m3/h of suction speed and a huge multi-stage pump group mainly using Roots type pre-vacuum pump.
Vacuum Oxygen Decarburization (VOD) This treatment is generally used to reduce the carbon content in high chromium stainless steels while avoiding additional loss of chromium due to oxidation. The VOD treatment method is to inject pure oxygen into the molten steel, so that the carbon dissolved in the molten steel becomes carbon monoxide (CO) and carbon dioxide (CO2), and “burns” these carbons. In order to avoid excessive chromium loss, it is generally processed under a pressure of 80-200mbar. During the processing, a large amount of smoke and metal fine powder will be generated, which needs to be collected by large-scale dust removal equipment. The suction capacity (speed) required at such operating pressure is much lower than that of VD, but large Roots-type vacuum equipment is still required.
Other processing methods Other vacuum processing methods that require very high suction capacity are vacuum arc degassing (VAD) and vacuum induction degassing (VID). These treatments use other heating methods to achieve the same purpose.
Suction Performance Requirements
When high-speed vacuum suction conditions are met, a considerable number of large Roots high-vacuum suction pumps should be used in the system, and these pumps must be correctly arranged in stages to maintain an appropriate pressure ratio at both ends of each stage and achieve sufficient suction speed. Both of these targets can only be achieved with a primary pump of adequate capacity.
The selection of the primary pump used in the pre-pumping stage depends on a series of factors, such as degassing method and scale, available facilities on site and user habits, etc., and can be selected as follows. Selection scheme of pre-pumping pump The general requirement of VD for pre-pumping is to ensure a stable suction speed in the range of 10-50mbar.
VOD’s requirement for pre-extraction is to ensure a stable high-speed suction speed in the range of 200-400mbar and to be able to withstand heavy dust and inclusion loads.
Dry vacuum pump unit: Roots vacuum pump + dry screw pump, which can guarantee a satisfactory top pumping speed for VD high vacuum pumping systems at all levels (that is, the best pumping speed in the range of l0-50mbar). Even when large amounts of abrasive dust are generated during VOD processing, these pumps are excellent at solving dust abrasive problems. However, under the condition of severe dust generated by molten steel degassing equipment, a lot of effective operating experience has been accumulated in the use of dry screw pumps. The pump has proven to be easy to operate and extremely reliable.
If the backing pump uses a large liquid ring pump (or water ring pump) for pre-pumping, it is a method that can create a large initial pumping capacity for the high vacuum pumping system, and it is extremely economical and reliable. Make the high-vacuum suction system get fast pumping and strong pre-pumping. The simplicity and reliability of these pumps for fast-moving, high-pressure processes such as VOD have led to their widespread adoption in the steelmaking industry, where they have been a common replacement for steam ejectors for many years. This type of pump has considerable tolerance to dust and other impurities that occur during processing, because most of the dust and impurities can be absorbed by the sealing water and completely removed.
The use of water-sealed liquid ring pumps in molten steel degassing has two major disadvantages:
①Large amount of sealing water
When using the standard operation mode (50% water circulation), the water consumption of a typical 4200m3/h liquid ring pump can be as high as 10m3/h, while the discontinuous operation mode (that is, no water circulation) can reach 20m3/h. The water is to be discharged directly to a waste water treatment plant. The water above the seal water must be clean, while the drain is heavily polluted due to degassing, which can lead to very bad environmental consequences. In order to minimize the abrasion and loss in the pump, it is usually recommended to use a discontinuous operation liquid ring pump for VOD.
②The temperature of sealing water limits the degree of vacuum that can be achieved
Seal water temperature plays a key role in ring pump performance. The technical performance listed for liquid ring pumps is based on a seal water temperature of 15°C. As the temperature of the sealing water increases, so does the temperature of the water vapor. This reduces the vacuum pumping speed and causes cavitation (bubble “explosion”) in the liquid ring pump as the inlet pressure drops to the limit.
Although many manufacturers are equipped with anti-cavitation devices, in the end, the suction speed is greatly lost, and cavitation vibration and noise occur, especially making the ultimate pressure worse. Any such situation may have an unacceptable fatal effect on VD. One or more of the following remedial measures should be taken:
1) Use primary water (probably required anyway);
2) Supercooling the sealing water (the cost of equipment and energy consumption is very high, which may be extremely uneconomical);
3) Add an air injector stage in front of the liquid ring pump (supply air from the exhaust, but some operators are reluctant to do so);
4) A small Roots pre-pumping pump stage is added in front of the liquid ring pump (the cost increases and the equipment becomes complicated).
Example of a typical molten steel degassing treatment system
For a modern molten steel vacuum degassing system with a nominal capacity of 225t of molten steel, its vacuum degassing pressure is designed to be 0.67mbar, while its vacuum oxygen degassing pressure is designed to not exceed 200mbar. The VD processing performance of this vacuum system is 90000L/S under 0.67mbar. In the upstream of the vacuum system, a large cyclone bag dust removal system is specially installed for VOD operation.
Specific information is as follows:
– gas density (standard) : 1.174 kg/Nm3
– air leakage at 30°C : 58 kg/h
– calculated mass flow : 182.8 kg/h
– pressure at pump inlet : 0.67 mbar
– total suction capacity at pump inlet : 240.000 m3/h
– gas density at pump inlet : 0.000762 kg/m3
In order to achieve the suction capacity required by VD, the total pumping speed calculated by EVP VACUUM is close to 90000L/s, and 9 sets Dry Type Vacuum units are required: Roots screw unit (Roots pump of ZJP-10000, ZJP-2500+ ZJP-600 Roots pumps, backing pump needs to use LG-300 dry screw vacuum pump).
For more details, please contact EVP VACUUM.