**3.5 Transient analysis**

To illustrate the behavior and response of the steam boiler during this transient, the main parameters are presented in curves showing their evolution with time. The curves from **Figures 10**–**15** show the behavior of each selected parameter even during protected or unprotected scenarios. The transient simulation is preceded by a steady-state period equal to 3000 seconds (this time corresponding to the stability of the entire installation at nominal steam boiler load) with a time step size equal to 10<sup>−</sup><sup>3</sup> seconds.

 The temporal variation of feedwater and superheated steam mass flow rate for both scenarios are illustrated by **Figure 12**. Before the accident, the two flow rates are at the same initial value of 374 t/h. After the accident occurrence, the feedwater flow rate decreases instantly, and the superheated steam flow rate vanishes gradually for 90 seconds after the burner shuts down, due to the stopping of steam generation inside the vaporizer tubes. In the second scenario (unprotected), an increase in superheated steam flow rate is observed up to 513.78 t/h which is due to the continuous water vaporization in the vaporizing tubes. At the instant t equals 318 seconds, the flow rate begins to decrease until there is more water in the tubes to vaporize.

**Figure 13** shows the time variation of the pressure in the drum. In the first scenario, following the accident and burners' shutdown, the pressure drops rapidly until 72.44 bars at 58 seconds. From this instant, it continues to decrease but more slowly until the end of the transient. This is due to the cooling of the boiler by the ventilation air. In the second case (unprotected), after the accident, the pressure increases to a value of 81.58 bars resulting from the vaporization. Then at time 313 seconds, the pressure starts to drop, and when it reaches 72.69 bars, it stabilizes at that value until the end of transient.

 The water level in the steam boiler is a key parameter since it indicates the mass of water in the boiler. So, for safety reasons it must be kept in a limited range [28]. The behavior and response of the water level in the drum are shown in **Figure 14**. It is maintained before the accident at the value of 860 mm (setpoint). For the case of the protected scenario, and after stopping the feedwater pump, the level drops suddenly to the value of 359 mm due to the decrease in pressure (**Figure 13**) which generates an intense vaporization of water in the drum. Then it continues to decrease but more slowly until reaching the value of 206 mm at the end of the transient. In the unprotected case, the level decreases slowly and almost linearly contrary to the first case, due to the presence of vapor bubbles in the drum.

**Figure 12.**  *Time variation of feedwater and steam flow rates.* 

**Figure 15.**  *Temporal variation of the superheated steam temperature inlet/outlet of the superheaters.* 

**Figure 15** shows the time variation of the inlet and outlet temperatures of the superheated steam in low-temperature superheater (LTS) and high-temperature superheater (HTS). At steady state, the temperature values at the inlet and outlet of the two superheaters are, respectively, 292.6 and 370.8°C (LTS) and 321.2 and 487.3°C (HTS). After stopping the pump, temperatures decrease after stopping the burners. The protected scenario shows that the inlet temperature of SHT increases to the value of 370.4°C due to the lack of the desuperheating flow following the feed pump stop. Then it decreases to the value of 359°C. The temperatures increase again due to the heat inertia of the flue gases and then begin to decrease linearly. This decrease is caused by the cooling of the superheater by the ventilation airflow.

 For the unprotected case, after stopping the pump, temperatures remain stable and then increase rapidly, reaching very high values of the order of 1720°C. This rise is due to the nonstop of the burners and the decrease of the steam flow rate.

 In the steam boiler, wall tubes are designed to operate under highest heat transfer condition (1), where heat is supplied to the outer tubes' surfaces by the fumes. Therefore, and from the safety point of view, it is very important to know the evolution of the wall temperature of the vaporizer tube of the combustion chamber under accidental conditions. It is a key parameter in the safety analysis of the thermal installation. In natural circulation steam boilers, the vaporization regime is in every way in the form of nucleate boiling in order to ensure the continuous cooling of the wall heated by water [29]. As long as this vaporization regime is maintained, the inner wall temperature remains higher than that of the saturation.

The temporal evolution of the evaporator tube inner wall temperature and the heat transfer coefficient during transient for protected and unprotected scenarios is shown in **Figures 16** and **17**, respectively. Before the accident occurrence, the heat transfer inside the tubes is ensured by the nucleate boiling regime, which is characterized by a moderate internal wall temperature, of the order of 303°C, and a good

**Figure 16.**  *Temporal variation of the inner wall temperature of the vaporizing tubes.* 

**Figure 17.**  *Temporal variation of the heat transfer coefficient in vaporizing tubes.* 

*Numerical Simulation of the Accidental Transient of an Industrial Steam Boiler DOI: http://dx.doi.org/10.5772/intechopen.86129* 

**Figure 18.** 

*Temporal variation of the void fraction in the vaporizer tubes.* 

heat transfer coefficient equal to 20.25 kW/m2 K. Just after stopping the pump, the wall temperature drops from its initial value to 289.38°C, and the heat transfer coefficient drops from 20 to 5 kW/m2 K in a time interval equal to 66 seconds. This drop is caused by stopping the burners; nucleate boiling is therefore stopped. Thereafter, the wall temperature decreases linearly until the end of the transient, and heat transfer is achieved by simple convection.

 In the second scenario, instabilities in the heat transfer coefficient are observed, which implies that there is a poor heat transfer inside the vaporizing tubes, and the inner wall temperature is quasi-constant. From 410 seconds, the boiling crisis appears, leading to the dryout phenomenon. In fact, the liquid film becomes unstable and is depleted under the effect of intense vaporization. Hence, the wall surface dries out, the heat transfer coefficient drops sharply to 45.5 W/m2 K, and the temperature of the inner wall increases rapidly to very high values (3900°C) due to the appearance of the boiling crisis. This temperature is higher than the allowed maximum operating value of the plant (500°C) [2, 22], which leads the melting of the vaporizer tube in the combustion chamber.

It is very important to study the void fraction variation during the transient to understand the flow behavior in both phases. **Figure 18** shows the temporal variation of the void fraction in the vaporizer tubes. For the protected scenario, we can see that before the accident (at steady state), the void fraction is maintained at the value 0.4837. After the accident, an instantaneous increase in the void fraction up to 0.4988 resulting from loss of feedwater is observed. After burner's shutdown, the void fraction becomes almost null, and the flow regime is characterized by liquidphase convection.

During the unprotected case, the void fraction increases to reach unit, between the moment of the accident and the moment of the boiling crisis appearance; as it is shown, there are instabilities in the void fraction during its increase. These are probably caused by poor circulation inside the vaporizer tubes.

#### **4. Conclusions**

Modeling and thermal-hydraulic behavior simulation of an industrial watertube steam boiler during the accidental transient using the RELAP5/Mod3.2 code are presented in this chapter. The transient investigated in this study is the loss of feedwater following the cost down of the feedwater pump. The transient was performed in two steps: the first one concerns the simulation of the protected scenario where the protection systems are operational, and the second one is the simulation of the unprotected scenario.

The results obtained make it possible to analyze and better understand the behavior and response of the installation to the accidental transients by the evolution of the steam boiler thermal-hydraulic parameters. Furthermore, the study clearly demonstrates the protective systems' role in preserving the structural integrity of the steam boiler.

This study has shown that the basic models of RELAP5 code give the possibility of reproducing the main thermal-hydraulic phenomena that may occur in the installation. Thus, it was possible to develop a basic model that can simulate steam boiler operation during normal and accidental transients. In addition, the capacity and reliability of the RELAP5/Mod3.2 code for thermal-hydraulic analysis of conventional thermal installations such as industrial steam boilers have been demonstrated.

Finally it was possible to demonstrate, using RELAP5 modeling capabilities, that in the case of safety and protection system failure, the critical phenomenon of the boiling crisis is established in the combustion chamber which is undoubtedly the cause of the frequent explosion of the steam boilers.
