After a hesitant start to the replacement of steam ejectors in steel degassing with mechanical vacuum pumps, the latter have become state of the art in vacuum degassing and increasingly in vacuum oxygen de-carburization installations.
While there may have been early concerns about the mechanical vacuum pump’s suitability for production, it has proven reliable in the global steel industry, many with high production cycles. The advantages are clear: the initial higher investment is returned rapidly by significantly lower cost of ownership and reduced downtime, and the lower carbon footprint serves the requirements of modern steel plants in meeting environmental concerns and legislative requirements.
Are steam ejectors the right choice for RH process?
For Ruhrstahl-Heraeus (RH), the situation is slightly different. The required pumping capacity of the system is usually very large, and production runs continually 24/7, often serving twin degassers. RH-treated steel typically comes from an oxygen furnace, which produces steam as a side product while cooling the gases from the furnace. With the availability of steam and increased performance requirements at low process pressure, a steam ejector may be a good fit. Nevertheless, the market for dry mechanical pumps for the RH process is growing.
Mechanical vacuum pumps are a valid option for steam ejectors and have proven reliable in production since 2009. The main drivers for change are lower cost of ownership and a lower carbon footprint.
What is the vacuum task in steel degassing?
Schematic of VD (VOD) and Ruhrstahl-Heraeus (RH-OB) refining station
Vacuum treatments in secondary metallurgy have in common that low pressure is used to remove impurities like nitrogen and hydrogen from molten steel, by lowering the partial pressure. This reaction takes place at the surface of the bath, sometimes with the help of oxygen to remove carbon. The performance requirement of the vacuum pump is determined primarily by the process gas flow, but also by supporting gases such as argon for stirring, camera protection gas, oxygen blowing, if applied, and the leak rate of the refining station. The second parameter is the pump downtime to process pressure, to keep the tap-totap times short, maintaining the liquid steel temperature in the correct range for casting and to maintain production flow. The vacuum tasks of a vacuum tank degasser, VD (VOD) and RH (RH-OB) are in some ways similar (Figure 1). The process time is typically 20 minutes, the process pressure is 0.5 Torr (0.67mbar) and the expected pump downtime would be 5 to 6 minutes with an empty vessel or tank. The main differences in performance requirements are:
- At process pressure: With the circulation of hot steel through the snorkels in an RH treatment, a large surface area of molten metal is exposed to the vacuum during the process. This leads to higher suction speed requirements at the process pressure as compared with the VD.
- For pump down: The volume to be evacuated is smaller on an RH-degasser per degassed ton of steel. In a VD a ladle with hot steel is placed in a tank, which is closed with a lid. The ladle itself is oversized, as it requires a freeboard area for reactions like slag foaming. The tank volume around the ladle must also be evacuated. In the RH, the hot steel is lifted into the cylindric vessel which reduces the empty volume to be evacuated. Both treatments use the volume of the filter for pre-evacuation, which leads to shorter pump down times by equalization of pressures.