Considering the emergence of potential ignition sources after the diffusion of leaking gas in the tank area, which may lead to combustible gas clouds meeting ignition caused by deflagration accidents. Assume that within the scope of the leaking gas hazard area, an ignition source is set to trigger a combustible gas cloud deflagration accident, this process occurs in a three-dimensional open unconstrained space, the explosion of the combustible gas cloud in this case is usually a deflagration phenomenon (Li et al., 2020) that is an explosive field generated by a non-ideal explosive source. Due to the nature of the leaking medium and the influence of local weather, the diffusion process of the leaking gas is complex, and the final formation of the gas cloud concentration distribution is irregular and varied in shape, thus its detonation intensity is concerned. Therefore, there is a need to study the consequences of the detonation of gas clouds formed after gas leakage in large tanks. However, it is difficult to directly evaluate the detonation strength of the actual irregularly shaped and non-uniformed gas cloud, so the model needs to be simplified to reduce the computational difficulty.
Firstly, the gas leak dispersion simulation can be derived from the lower explosion limit of the combustible gas dispersion range, to calculate the volume of leaking combustible gas. Secondly, the equal volume sphere method is used to transform the actual gas cloud into a spherical region of the same volume, and it has been verified that the explosion overpressure values calculated by this method are closer to those of the actual diffusion area method. The actual gas cloud is equivalent to a spherical gas cloud with a radius of 5.35 m. This spherical gas cloud is filled with methane and air-premixed gas, and the concentration is within the explosive limit, the region involves a high temperature of 1800 K for ignition and detonation, and its deflagration results are analyzed as follows.
5.1 Deflagration overpressure assessment
The ignition of the combustible gas clouds in a very short period through the rapid expansion of external work, creating a rapid increase in surrounding air pressure thus causing injuries and property damage. The explosion is usually in the form of shock wave to the surrounding people and buildings. When the shock wave pressure reaches a certain range, it will cause different degrees of injuries to people and buildings.
Figure 10 is the maximum overpressure field for 1ms at x = 42 m and z = 1.5 m concerning the cross-sectional deflagration overpressure cloud, it can be seen that the explosion overpressure occurs at the center of the explosion with a small circle, surrounded by concentric circles radiating outward, and away from the center of the explosion the overpressure value gradually weakened. The overpressure value of explosion center is of more than 130 KPa. During the whole process of deflagration, the overpressure region is changing dynamically.
With the rise in height, the maximum overpressure value increases firstly then decreases. When the distance is 0.5 m away from the ground, the maximum explosion overpressure is 31 KPa. When the distance is 2.5 m away from the ground, the maximum explosion overpressure is 115 KPa. When the distance is 5.35 m away from the ground, the maximum explosion overpressure reaches a peak of 132 KPa. When the distance is 11m away from the ground, it is clear that the deflagration overpressure cloud radiation range decreases, at this time the maximum explosion overpressure is 4.65 KPa.
The destructive effect of the blast wave on the target is usually measured by the peak overpressure, i.e. only when the overpressure ΔP of the blast wave reaches a certain critical value, it will cause a certain degree of damage or injury to the target. Table 2 gives the degree of damage to the building structure caused by the explosion overpressure, and Table 3 gives the corresponding degree of damage to personnel.
Table 2
The extent of damage to the building structure by explosion overpressure
Overpressure reaching the of the building structure(KPa) | Degree of damage to a reinforced concrete structure |
3–10 | Minor damage |
10–30 | Minor damage |
30–50 | Moderate damage |
50–80 | Serious damage |
80–100 | Total damage |
Table 3
The extent of damage to personnel by explosion overpressure
Overpressure reaching the surface of the body (KPa) | Degree of damage to personnel |
20–30 | Mild injury |
30–50 | Moderate injury |
50–100 | seriously injury |
100 | Fatal injury |
400–600 | Death |
In order to evaluate the area affected by the overpressure injury, the spatial distribution of the critical overpressure contour after the tank leak explosion was established. Figure 11, Fig. 12, and Fig. 13 depict the spatial distribution of the overpressure at 1ms for the "field-shape ", "one-shape" and "L-shape" tank zone distributions, respectively. Burst overpressure spatial distribution map shows that the tank leakage hazard area generated a hemispherical overpressure wave centered on the ignition source, the further away from the ignition source, the lower the shock wave overpressure value, and the wider the impact area.
For P = 2 KPa, the shock wave has the largest area of influence, covering an area of 1241.43 m2, the shock wave radius reaches 11.5 m, according to the explosion overpressure guidelines, this overpressure will not cause damage to buildings and people, so the radius of 11.5 m is defined as the minimum safe radius of the explosion. P = 20 KPa shock wave covers an area of 1369.6 m2, the radius of hazard is 9.52 m, this overpressure will cause mild damage to the building structure, causing minor injuries to personnel. P = 50 KPa shock wave covering an area of 373.023 m2, the radius of harm is 5.74 m, this overpressure will cause serious damage to the building structure, causing serious injuries to personnel. P = 110 KPa has a very small impact, its shock wave covering an area of 119.09 m2, the radius of harm is 3.14 m, this overpressure will cause complete damage to the building structure and cause fatal injury to personnel.
For P = 2 KPa, the shock wave has the largest area of influence, covering an area of 1288.42 m2, the shock wave radius reaches 11.96 m, according to the explosion overpressure guidelines, this overpressure will not cause damage to buildings and people, so the radius of 11.96 m is defined as the minimum safe radius of the explosion. P = 20 KPa shock wave covers an area of 1365.43 m2, the radius of hazard is 9.98 m, this overpressure will cause mild damage to the building structure, causing minor injuries to personnel. P = 50 KPa shock wave covering an area of 380.597 m2, the radius of harm is 5.84 m, this overpressure will cause serious damage to the building structure, causing serious injuries to personnel. P = 140 KPa has a very small impact, its shock wave covering an area of 291.766 m2, the radius of harm is 4.99 m, this overpressure will cause complete damage to the building structure and cause fatal injury to personnel.
For P = 2 KPa, the shock wave has the largest area of influence, covering an area of 1288.81 m2, the shock wave radius reaches 11.97 m, according to the explosion overpressure guidelines, this overpressure will not cause damage to buildings and people, so the radius of 11.97 m is defined as the minimum safe radius of the explosion. P = 20 KPa shock wave covers an area of 1361.6 m2, the radius of hazard is 9.95 m, this overpressure will cause mild damage to the building structure, causing minor injuries to personnel. P = 50 KPa shock wave covering an area of 379.615 m2, the radius of harm is 5.84 m, this overpressure will cause serious damage to the building structure, causing serious injuries to personnel. P = 140 KPa has a very small impact, its shock wave covering an area of 287.661 m2, the radius of harm is 4.96 m, this overpressure will cause complete damage to the building structure and cause fatal injury to personnel.
To summarize the analysis of the three distribution arrangements of the tank area, it can be seen that the radius of influence of the three types of tank area distribution is not very different, but the one-shaped and L-shaped arrangement of the distribution of the storage tank coverage is more extensive.
5.2 Deflagration High-Temperature Assessment
Gas cloud explosion in the generation of shock wave overpressure at the same time will also release high temperature, and the high temperature on the staff in the tank will cause some injury. Different temperatures on the extent of the human injury as shown in Table 4.
Table 4
Degree of injury to personnel by temperature
Temperature (K) | 361 | 391 | 453 |
Degree of injury to personnel | After 10-minutes exposure, the human body will be in a dangerous condition | Whole body burns | Serious injury |
According to the fire temperature damage criteria, the critical temperatures for personnel damage and severe damage are 391 K and 453 K. The critical temperatures for partial damage and complete damage to the steel structure are 673 K and 873 K. Figure 14, Fig. 15, and Fig. 16 show the influence areas of the critical damage temperature for the three types of tank area distribution, i.e. "field-shape ", "one-shape" and "L-shape", respectively.
When the tank area is distributed in the shape of a field, a character and an L-shape, the area covered by the temperature of 391 K is 633.176 m2、659.838 m2 and 659.372 m2 respectively, and the area covered by the temperature of 873 K is 516.625 m2、547.349 m2 and 542.429 m2 respectively. 873 K has a small area of influence and has spread to the two storage tanks, and the continuous fire will cause the covered The area of influence for 391 K is slightly larger, and personnel working in this area will suffer severe burns and even life-threatening injuries. Comparing the area covered by the hazardous area of the three tanks, it can be seen that the one type covers the widest area, followed by the L type, and the field type covers the smallest area.