Innovative Vacuum-Tank Degassing Technologies: Well-Established Metallurgical Performance Figures Achieved by Using Dry Mechanical Pumps

摘要

With increasing demands for highly performing and at the same time efficient and running-costs saving vacuum-degassing equipment, dry-mechanical pumps have been confirmed a proper choice to completely fulfill the expectations of steel-making companies worldwide. Regarding the production of both special carbon steel and stainless steel grades, the usage of mentioned pumps allowed reaching simultaneous remarkable results from different viewpoints in various Danieli worldwide installations (treating up to 200 t of liquid steel). As a matter of fact, in front of the replacement of a widely spread and renowned steam-ejector pumps technology with the recently developed technology embodied by the dry-mechanical vacuum pumps (MVP), well-established metallurgical performances in terms of carbon-hydrogen-nitrogen-sulphur removal have been repeatedly attained while meeting at the same time the client's expectations concerning, for example, running-costs and "push-button" availability. Regarding running-costs it is already well known that by replacing a Steam Ejector Vacuum Pump equipped with Water Ring Pumps (WRP-SVP) with a MVP degassing system allows a running-cost reduction of around 70-90%. It must be pointed out that one of the main differences between the degassing systems equipped with MVP and SVP is their designed suction capacity: for a VD-VOD process the usual provided specific suction capacity of the MVP is ranging between 1.0-1.5kg/h/t of liquid steel, while for SVP it is ranging between 2.5-3.0kg/h/t of liquid steel. This difference originates from the higher MVP specific investment-costs (euro/kg/h) if compared to SVP, especially for heat sizes higher than 100-120 t even though year after year the capital costs of the MVP become more and more competitive. The practically identical results obtained from both pump systems from productivity and metallurgy point of view, despite the difference in the designed suction capacity for the same equipment, are justified by the following reasons: the designing of the overall VD-equipment and process when a MVP is included in the plant has been optimized in order to diminish as much as possible additional gas loads as, for example, the equipment leakages and the additional nitrogen/argon load to the pumps. The lower suction capacity is directly linked with an increased importance of proper maintenance activities. SVP (if not provided with an upstream textile filter) has to compensate a gradual performance loss caused by the dust deposit on the steam-ejector body internal wallsv For MVP-based equipment all the volume dbwnstream the Main Shut-Off Valve (MSOV) is partially or deeply pre-evacuated before starting a VD-VOD treatment. Thanks to the very flexible, modular, redundant and maintenance-friendly equipment design, random problems occurring to the singular mechanical pumps can be easily solved without affecting the VD-VOD plant availability and metallurgical performance. Whenever a pump malfunction starts to appear, depending on the MVP layout configuration, either the singular pump is isolated from the remaining pumps by closing both pump upstream and downstream on/off valves or the pump pertinent skid is isolated from the system by closing its skid valve. It must be highlighted that even when working with one or more pumps isolated due to temporary malfunctions, the final quality of steel can be still guaranteed. With a particular regard to the VOD process, a very short overall process-time has been repeatedly performed (averagely less than 58min under-vacuum time at ChMP~1) while guaranteeing the expected required quality of the final refined stainless steel. Moreover, in several VOD installations (ranging from 6t up to 1 l0t of liquid steel) an efficient automatic process control such as dynamic pressure-regulation during oxygen-blowing has been attained thanks to the controllable variable-speed drives with which each mechanical pump is provided.