Edition 01/2017

The current oversaturation of the global steel market is calling for an even stronger focus on the refinement process. In light of this development, VOD (vacuum oxygen decarburization) and AOD (argon oxygen decarburization) installations are gathering importance in terms of technical interest. Because both methods work with vacuum, modern solutions that generate this vacuum reliably and economically are in demand. Electromechanically-driven roots type blowers and screw vacuum pumps are replacing traditional steam jet pumps. The move away from process steam is significantly improving energy efficiency.

More energy-efficient

The benefits are obvious: When a cascading network of electrically-driven roots type blowers and screw vacuum pumps is used, steam jet pumps are no longer necessary. This substitution also ensures that the complex pipework infrastructure between central steam boiler and the pumps can be eliminated. The same applies for steam generating boilers that are no longer required. Another benefit of vacuum generation via electromechanically-driven pumps: The AERZEN units require electrical energy only when they are operating. So the energetically dubious and expensive standby mode, which is needed to keep the steam boiler constantly pressurised, becomes superfluous. The more intelligent generation of a demand-based vacuum therefore notably increases the energy efficiency of VOD and AOD processes– without compromising output or product quality. The efficiency of this method is currently being demonstrated by a new VOD system in a foundry in Linz, Austria, which is equipped with this very technology.
VOD and AOD were the two methods of further reducing the carbon content of stainless steels. The difference is that with VOD, the gases dissolved in liquidsteel are encouraged to bubble out at the surface solely by applying a vacuum at a pressure of around 100 and 200 millibars– not least because the partial pressure of the carbon monoxide is reduced. With the AOD method – argon oxygen decarburization– carbon as well as other impurities are selectively oxidised and expelled from the melt by blowing in an argon/oxygen mixture. With both methods, the liquid steel is transported into the pressure-tight boiler in crucibles. Any vacuum present after the boiler is evacuated must be maintained for a defined time of up to half an hour to ensure reliable decarbonisation with high reproducibility of the steel quality.

Large installations for small footprints

AERZEN accomplishes this task with a combination of positive displacement blowers of series GQ and screw compressors of series VRa. In a current project that went into operation in a steelworks in China n 2016, the four-stage setup delivers a pump output of 635,000 cubic meters per hour at a vacuum of 0.67 mbar absolute (0.5 torr). The benefit of the AERZEN solution lies primarily in the dimensions of the positive displacement blower and screw compressors. The assemblies are among the biggest in the world and are designed for the tough operating conditions in the steel industry. In addition to the requirement for fast pump-out times, this also includes operational reliability within a redundant system, which has few installations. The combination of positive displacement blowers and screw vacuum pumps is ideal because, for most of the conveying flow, the energetically-efficient roots principle is applied and the vacuum drops further only in the low pressure range of screw compressors. To enhance operational reliability, the systems can be interconnected in their respective vacuumstages. If a unit drops out, the network is still able to generate the vacuum.“We must hold this for a good 20 minutes. This completes the process and we increase the pressure again gradually in the opposite”, says Uwe Grosskurth, Key Account Manager for vacuum blowers at AERZEN, describing the process. The finely-tuned processes are crucial to achieving the desired quality. “We don‘t want to transform scrap into just scrap – but into something viable”, says Grosskurth, not forgetting that scrap metal is often the raw material in steel refinement. “We ensure the perfect vacuum – with installations that work in harmony with one other.”Perfectly matched means combining and dimensioning compressor and blower in such a way that no assembly runs constantly at the performance limit or gets too hot, while others never leave their partial load range. A few large assemblies rather than several small ones: For Uwe Grosskurth, the AERZEN strategy creates only benefits for steelworks. “The fewer the machines, the more time, space and money available for the pipework.” And where there are no pipes, there is no risk of leaks either.“Every tiny leak impairs efficiency.” And then the leaner machine fleet is even more easily integrated into the control and command level, since there are fewer frequency converters for controlling the speed of the drive motors. “Our solutions have the best footprint in the world”, emphasises Grosskurth. In terms of operational safety too, less is more. Each assembly saved reduces the outage risk within the mean time between failure (MTBF) calculations.

Outlook

AERZEN estimates that as many as 600 steel refining plants worldwide are still generating their vacuum with steam. “Our customers are very keen convert. High erenergy efficiency and falling maintenance costs simply speak for themselves”,says Grosskurth, mentioning return-oninvestment times of three to four years as further motivation. “We have over 40 years‘ experience with steel and iron and the process knowledge we have amassed enables us to effectively support retrofits together with experienced installation manufacturers.”