There was a mounded propylene storage tank put into use in China in 2020, with a capacity of 3,300m3, working at room temperature under the working pressure of 1.56MPa. Other parameters are as follows: inner diameter 7600mm, length 68700mm, filling factor 0.9, and wall thickness 48mm (cylinder)/seal head (semi-spherical head). The medium is propylene, and material is A516 Gr.70. The tank has not undergone an overall inspection yet so far, and the wall thickness monitoring system has not identified any obvious corrosion yet.

### 3.1 Failure Possibility Calculation

*F* G, the failure possibility of similar equipment, is selected for the storage vessels specified in Table 2 of GB/T 26610.4[13]. Since there are 4 possible leakage sizes, the failure possibility of similar equipment for this mounded propylene storage tank should be the sum of the 4 leakage size probabilities, i.e., *F*G=0.000156.

*F* E: The correction coefficient of equipment. As mentioned earlier, what has the greatest influence on the correction factors of equipment is the technical module factor, whose value is 10 ~ 100 times the sum of the general condition factor, mechanical factor and process factor. Therefore, the technical module factor should be taken into consideration when the correction factors of equipment are calculated.

According to the basic information about mounded propylene storage tank evaluation, the tank may suffer from corrosion thinning on its inner and outer walls and wet hydrogen sulfide stress corrosion cracking inside. Fatigue damage is excluded because the tank was just put into service recently and has not been filled with a medium for too many times. Besides, the possibility of brittle fracture is extremely low, so it is also excluded this time. To sum up, for the technical module factor, thinning sub-factor and stress corrosion cracking sub-factor need to be calculated.

Thinning sub-factor calculation:

The medium stored in the tank is propylene, which may contain traces of H2S, which causes acid water corrosion in the presence of water. Moreover, before the storage tank was put into service, it had been considered that it might fail to go through a periodic inspection. So, its internal and external surfaces are covered with an anticorrosive coating, and equipped with corrosion probes. The monitoring data show that there is no obvious corrosion thinning. In summary, it is assumed that this tank suffers from general corrosion, and the corrosion rate is set to 0.05mm/year. Although the tank does not suffer from atmospheric corrosion because its internal and external surfaces are covered by soil, it may suffer from soil corrosion. The corrosion rate is also set to 0.05mm/year. The total wall corrosion rate is 0.1mm/year.

Service length a = 2 years, total corrosion rate r = 0.1mm/year, and wall thickness t = 30mm (head thickness), so ar/t = 0.0067.

Inspection is considered invalid because no test has been conducted. The thinning sub-factor is set to 1 in accordance with Table C.7 in GB/T 26610.4[13].

Safety factor correction: the operating pressure is 1.56 MPa, and the safety factor is set to 0.5 in accordance with Table C.8 in GB/T 26610.4 [13].

Online monitoring correction: the tank is equipped with corrosion probes there is hardly any liquid flowing in the tank; according to Table C.9 in GB/T 26610.4 [13], the online monitoring correction factor is set to 10, based on acid water corrosion (not greater than 6.10mm/s).

So, the final thinning sub-factor is equal to initial thinning sub-factor× safety factor/online monitoring factor = 0.05.

Stress corrosion cracking sub-factor calculation:

There is H2S in the medium in the mounded propylene storage tank, indicating that there may be a sulfide stress corrosion cracking mechanism. The sulfide stress corrosion cracking factor is co-determined by the maximal severity index of stress corrosion cracking and the frequency of inspections with the highest validity.

The mounded propylene storage tank made of A516 Gr.70, which is not highly sensitive to sulfide stress corrosion cracking, so it is set to a low value. According to Table D3 in GB/T 26610.4[13], the severity index is 1. The tank has not been inspected yet since it was put into use. According to D.10 in GB/T 26610.4[13], the stress corrosion cracking sub-factor is 1. The factor is adjusted to be 2.14 with time going by.

The total technical module factor is equal to the sum of thinning sub-factor and stress corrosion cracking sub-factor, 2.19.

*F* M: The assessment coefficient of management system: According to the results of assessment, the management factor *F*M=1.0.

*F* L: The impact factor of defect beyond tolerance: There is no defect beyond tolerance in this device, *F*L=1.0.

Therefore, the failure probability *F* is 0.00034164, and the failure possibility is 3, according to Table 1, GB/T 26610.4[13].

### 3.2 Failure Consequence Calculation

The medium in the mounded propylene storage tank is propylene, which is flammable and explosive but slightly toxic. Therefore, for consequence calculation, personal injury caused by combustion explosion is primarily taken into consideration and expressed in area (m2).

The calculation can be conducted by referring to qualitative consequence calculation method offered in GB/T 26610.3[12], because the area of impact from the combustion explosion of a mounded storage tank is smaller than that of an aboveground device[9–11]. Therefore, the calculated consequence area is multiplied by an overburden mitigation factor to determine the final consequence level.

By reference to Table 3 ~ 5 in GB/T 26610.3[12], the chemical factor CF = 12 according to the characteristic parameters of propylene.

By reference to Table 6 in GB/T 26610.3[12], the damage quantity factor DQF = 45 according to the density of liquid propylene and the filling capacity of the propylene tank.

According to the boiling point and autoignition temperature of propylene at normal atmospheric pressure, as well as Table 7 in GB/T 26610.3[12]and Table 8 in GB/T 26610.3[12], the state factor SF = 6 and AF = 13.

The propylene in the tank is liquid, and the pressure factor PRF=-10.

According to the score of 10 items in Section 6.3.2.6, GB/T 26610.3[12], the safety protection factor CRF=-8.

According to the sum of the above 6 items, the area of impact from initial combustion explosion is 58.

Considering the characteristics of mounded equipment, the area of impact from its explosion is smaller than that of aboveground equipment. So, the damage distance to buildings is taken into consideration only. According to the research data from Zhou Jiahong[8], the damage to buildings caused by equipment explosion in soil is only 35.1% of that in the air. Therefore, the consequence of mounded equipment assessment can be multiplied by a consequence mitigation factor, 0.4, to reflect the role of overburden soil in reducing consequence.

The final consequence area is 23.2, and the consequence grade is B according to Table 2.

According to the risk matrix diagram(See Fig. 1), the risk level of mounded propylene storage tank is low.

### 3.3 Risk Level Comparison

At present, propylene storage spheres are generally used for propylene storage in China’s petrochemical industry. This time, a 3000m3 propylene storage tank was selected for risk assessment to compare the risk level of aboveground propylene storage spheres with that of mounded propylene storage tanks (3300m3).

The basic parameters of a 3000-m3 propylene storage sphere owned by a petrochemical plant are as follows: Put into use in 2019, operating pressure 1.77 MPa, operating at normal temperature, diameter 18,000 mm, filling factor 0.9, wall thickness 54 mm, material Q345R, medium propylene, insulated. A comprehensive inspection was carried out for the first time in 2022. The measured minimum wall thickness was 53.7mm, and the inner surface was covered with 50% magnetic powder; defects were not identified in 50% ultrasound check, and obvious corrosion was not identified in macroscopic inspection.

Quantitative failure probability was calculated according to GB/T 26610.4[13]. For the technical module factor, the thinning sub-factor, stress corrosion cracking sub-factor and external damage sub-factor were taken into consideration. Calculations were made by the above steps. The thinning sub-factor was 0.5; the stress corrosion cracking sub-factor was 10; the external damage sub-factor was 0.5.

The failure probability of similar equipment, correction coefficient of equipment, evaluation coefficient of management system and impact factor of defect beyond tolerance were calculated, respectively, results as follows: *F*G=0.000156, *F*E=2.0, *F*M=1.0 and *F*L=1.0. Finally, the failure probability of this propylene storage sphere was found, 0.000312, and the failure probability was 3 according to the table.

Qualitative failure consequence was calculated according to GB/T 26610.3[12]. The chemical factor CF = 12, damage quantity factor DQF = 45, state factor SF = 6, autoignition factor AF = 13, stress factor PRF=-10, and safety protection factor CRF=-5. The sum of the factors was 61, and the consequence was D according to the table. The risk level was medium to high.

Table 1

Comparison of risk level between mounded storage tank and aboveground storage sphere

Equipment | Capacity | Failure Possibility | Failure Consequence | Risk Level |

Mounded propylene storage tank | 3300m3 | 3 | B | Low risk |

Propylene storage sphere | 3000m3 | 3 | D | Medium-high risk |