In recent decades, the demand for higher-quality, refined steels has increased steadily. At the same time, steel plants operate under the increasing pressure to design their processes more energy-efficient and reduce CO2 emissions drastically. Higher-grade steels which are demanded e.g. by the automotive or aerospace industry require further treatment in the so-called secondary metallurgical processes, which are often carried out under vacuum.
Degassing, especially those with oxygen insufflation, as in VOD and RH-OB methods, produce potentially explosive gases. Vacuum components and equipment with ATEX approval enable safe and cost-efficient solutions for mechanical vacuum solutions.
“Today’s international standards for mechanical vacuum systems are based on the latest generation Roots-type vacuum pumps and dry screw-type vacuum pumps. Selecting such modern mechanical pump solutions also offers outstanding process control possibilities, and employs a very reliable design, enabling the pumps to survive inside the harsh steel plant environment. By installing standard pumps in multiple arrangements, even highest suction requirements can be fulfilled with competitive pricing, while focusing on high safety standards” explains Uwe Zoellig, Senior Manager Market Segment Process Industry.
In all processes for steel degassing under vacuum gases and vapors are formed that might be dangerous. Essentially CO and H2 as well as the vapors of volatile metals are released from the steel surface. Metal and metal-oxide vapors partially condensate on cold plants elements and generate a fine dust. Such dust must be efficiently separated in special bag filters, cyclones and containers and therefore does not reach the vacuum pumps.
Explosion protection in vacuum plants especially those that generate a large quantity of CO by the use of oxygen for decarburization so far has been assured by overpressure flaps at the metallurgical reaction vessels, alarm devices for water leakage, sensors for pressure and temperature as well as emergency venting with nitrogen. With the introduction of mechanical vacuum pumps efficient installations for gas cooling and dust separation became necessary.
During a typical steel degassing process potentially flammable fuels as CO or H2 are produced, but the appearance of such gases alone does not create a danger. Only if the gas is mixed in the right concentration with oxygen, the gas mixture becomes dangerous. Therefore, oxygen represents a quite substantial threat for the assessment of explosion risks. Yet, oxygen cannot be eliminated, as leakages cannot be ruled out completely, and the use of oxygen is mandatory for certain manufacturing processes. However, the user should reduce the introduction of oxygen to the minimum. For this reason the usage of primary vacuum pumps which require to be cooled by the inlet of big amounts of ambient air (air-blast cooling) is therefore not recommendable, as this additional air will increase the chances to build up an explosive gas-mixture.
Today, the standard mechanical vacuum pumps already fulfill high requirement for safety. Nevertheless, in case there are uncertainties regarding the flammability of gas mixtures which need to be handled by the pump sets, the user will have to conduct a risk analysis of the various plant parts to define the relevant explosion protection zones. The result will most probably be the definition of an explosion Zone 1 for the inner part of the vacuum system. For this assessment, components with an ATEX-certificate Category 2 (inside) for gases can be the easy solution. Thus way the user can reach the highest safety standard for his employees with relatively low investment expenditure. The same considerations are also valid for all vacuum process with forced decarburization like VD-OB and RH-OB, or for processes with natural decarburization like VCD as well as for the VD-degassing of fully killed steel grades in presence of a reactive slag, especially in connection with higher leak rates. The off-gas composition and the moment of appearance of flammable mixture are very different in these processes.
ATEX vacuum solutions from Oerlikon Leybold Vacuum consist of pumps and components that meet the specification of ATEX Cat 2 (i) G IIC T2. This equipment can be combined into ATEX certified vacuum systems. To meet the requirements, an additional gas cooling and temperature control between the pumps is provided to prevent gas temperatures from exceeding the defined limits. Furthermore, all pumps are controlled and monitored by a specially programmed frequency converter.
Regarding the vacuum pump set, potential ignition sources such as overheating or electrostatic charging must be considered, which will be achieved with the usual attention during design and manufacturing of ATEX-certified pumps. In particular, the pumps must be protected efficiently against overload by too high pressure differences in order to avoid excessive temperatures.
This is valid for all possible operating point starting with high suction pressures, passing to medium operating pressures, that are to be hold over a longer time as for example during delayed pump down in the VD-process or during the oxygen blow phase in the VOD-process, down to lowest end pressures with its high compression ratios.
Capable products alone do not guarantee trouble-free operation of the degasser. User errors can be made with an insufficient system design. The vacuum pump supplier must be able to combine the single products into a smart turnkey solution to ensure a satisfied user. The modern and highly performant vacuum solutions of Oerlikon Leybold Vacuum consist of two different standard pump models only, combined into three-stage vacuum systems. Such three stage designs allow highest suction speed combined with lowest power consumption.