## 3 Analyzed Model  
The configuration of the new system for BF gas treatment has been engineered with a CFD analysis to find out the most effective solution for reducing the openings of the emergency bleeder valves on the BF top (Bleeder 1 and Bleeder 2), which release dirty gas to the atmosphere, and to minimize modifications on the plant. The concept was to partially by-pass the gas cleaning circuit in order to reduce the localized pressure losses and consequently the BF pressure. The aim was to clean the gas as much as possible and partly send it to the network and partly release it to the atmosphere through a new bleeder valve located downstream the cleaning equipment. Obviously, this solution is adoptable only for medium gravity events that were anyhow very frequent in the original plant. Around 80% of the top bleeder valves' openings were caused by medium gravity events. In case of very serious events, the bleeder valves on the BF top have to open. In order to open the bleeder valves on the BF top only in case of a very serious event, their opening threshold, related to the BF throat pressure, are modified, as shown in Table 2 (new arrangement). The original scheme of the BF gas-cleaning system is composed by:  
* Bleeder valves at the BF top (B1 and B2 from now);
* One dust catcher;
* Two cyclones in parallel;
* A DN2800 pipe 60 m long feeding three lines in parallel, each one composed by a wet scrubber and an electro filter (one of these lines is in stand-by);
* A gasometer which feeds the network. The new arrangement considers an additional bleeder valve at the reserve scrubber outlet (B3 from now, on named clean relief valve) and some by-pass circuits, which can be opened or closed by suitable tapping valves, to provide different operating scenarios (Figure 4). In particular, a new 60 m long pipeline DN800 has been added in order to reduce also the distributed pressure losses of the circuit, if necessary. Figure 5 shows a detail of the analyzed model, highlighting the tapping point valves to by-pass one or more sections of the cleaning equipment. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Valve** & **DN** &
\begin{tabular}{c} **Opening Threshold** \\ **(mmWC)**2  & **Closure Threshold** \\ **(mmWC)** \\ \hline \hline \multicolumn{4}{c}{Original Arrangement} \\ Bleeder 1 & 500 & 950 & 850 \\ Bleeder 2 & 500 & 1050 & 950 \\ counterweight check valve & 1000 & 1200 & 1050 \\ \multicolumn{4}{c}{New Arrangement} \\ Bleeder 1 & 500 & 1050 & 950 \\ Bleeder 2 & 500 & 1050 & 970 \\ counterweight check valve & 1000 & 1200 & 1050 \\ Bleeder 3 (new) & 800 & 850 & 800 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Opening and closure threshold of the bleeders in the original system and in the new arrangement. The CFD transient analysis of the described system has been evaluated with the software Ansys Fluent, using the RNG k-epsilon turbulence model. The fluid used is an ideal gas with the properties of AFO gas (specific heat, conductivity, and viscosity functions of temperature and molecular weight). The composition of the ideal gas, given in species mole fractions, is as follows: N\({}_{2}\) 0.477, CO 0.225, CO\({}_{2}\) 0.187, H\({}_{2}\) 0.047, H\({}_{2}\)O 0.064. The analyzed model considers:  
* The nominal diameters and the lengths of the pipes to correctly simulate the distributed pressure losses;
* The equipment volumes (not the shape) to simulate the correct expansion volume for the pressure wave;
* Gas behavior in the equipment is simulated with a simplified model, in particular pressure losses of the equipment are simulated with suitable surfaces (Figure 6), acting as porous-jump, calibrated on the basis of pressure field values in normal operating conditions;  
Figure 4: Analyzed model. Figure 5: Detail of the analyzed model. The tapping points are: V0, the blast furnace outlet; V1, the dust catcher inlet; V2, cyclone inlet; V3, cyclone outlet. As long as the operating conditions are normal, all the bleeder and tapping point valves are closed.