## 5 Optimal Solution  
As a result of considerations made through CFD analysis of the circuits, the third option, circuit C3, has been adopted. This option is profitable in terms of cost savings and optimization of results, reducing the amount of gas relieved into the atmosphere at lower temperature and lower dust content.  
The new configuration introduces one bypass line with hydraulically operated valve (dirty relief valve) on cyclones and a new relief hydraulically operated valve (clean relief valve) to the atmosphere, installed on the top of the wet scrubber standby unit (Figure 17).  
The wet scrubber is connected to the inlet gas circuit and disconnected from the downstream gas network: Water is continuously injected in order to keep the system ready in case of relief valve intervention. In the event of overpressure, the gas is further cleaned and cooled in the wet scrubber before being discharge into the atmosphere.  
Figure 16: Evolution over time of pressure in circuit C3 compared with the no bleeders case in a low severity event. **(A)** BF top pressure; **(B)** cyclones outlet pressure; **(C)** network pressure.  
CFD analysis highlighted that in case of low and medium overpressure, circuit C3 reduces the network pressure (compared to the operating conditions) while discharging the gas to the atmosphere through the clean relief valve. To optimize the design, the effectiveness of the cyclone by-pass (dirty relief valve), without the clean relief valve, has been studied for the three different severe grade events. The behavior in case of overpressure was compared for:  
* The circuit without any emergency valve;
* Circuit C3 with a by-pass opening at 850 mmWC without a clean relief valve;
* Circuit C3 with the simultaneous opening of the clean relief valve and by-pass at 850 mmWC.  
The simulated pressure curves are given in Figures 18-20.  
It is worth noting that the by-pass alone manages to keep the top pressure below 1050 mmWC in case of medium severity events and the network pressure naturally increases in any case.  
The combined action of the by-pass and clean relief valve in the atmosphere (C3) covers all cases at best.  
In the final arrangement, the intervention of relief valves (dirty and clean) takes place in sequence according to different set point thresholds.  
For low overpressures, the opening of the cyclone bypass alone (dirty relief valve) can counteract transient phenomena; in this case, the emission to the atmosphere is zero.  
If the overpressure phenomena are greater, the clean relief valve will be opened to the atmosphere: In this case, the exhaust gas is in any case cleaned and cooled by the passage through the scrubber (which, for original design, guarantees a dust content lower than 1 g/Nm\({}^{3}\), 20-30 times less than a discharge from the BF top bleeders). In any case, the real phenomena last only a few seconds.  
Both relief valves will be electro-hydraulically actuated to have an adequate response time and the combined action of the two valves will be managed completely automatically by the control system. In the same way, valves are closed sequentially once the pressure comes down.  
This system is expected to drastically reduce the opening of the BF top bleeders, which will only be necessary in the event of extraordinary and major events.  
Figure 17: Scheme of the optimal solution.  
Figure 19: Evolution over time of pressure in the circuit with by-pass opening at 850 mmWC without Clean Relief Valve and C3 compared with the no bleeders case in a medium severity event. (**A**) BF top pressure; (**B**) cyclones outlet pressure; (**C**) network pressure.  
Figure 18: Evolution over time of pressure in the circuit with by-pass opening at 850 mmWC without Clean Relief Valve and C3 compared with the no bleeders case in a low severity event. (**A**) BF top pressure; (**B**) cyclones outlet pressure; (**C**) network pressure.