Degree of destruction of objects (buildings, structures, transport)

Degree of destruction of objects (buildings, structures, transport)

depending on the overpressure

Objects

Degree of destruction, (overpressure, kPa)

Little (10-20)

Medium (20-30)

Intense (30-50)

Shop with a light metal frame

10-20

20-30

30-50

Brick buildings

8-12

12-20

20-30

Railway Tanks

20-40

40-60

60-90

Truck

20-40

40-50

Power lines

20-40

50-70

80-120

Pipeline ground

Pipelines (on the flyover)

20-30

30-40

40-50

Ground fuel storage tanks

10-30

30-50

50-100

Underground fuel storage tanks

30-50

50-100

100-200

Thermal power plant

10-15

15-20

20-25

Water tower

10-20

20-40

40-60

Wooden houses

10-20

20-30

Table 6.2

The value of heat impulses, corresponding to the ignition of materials

Materials

Thermal impulse, kJ/m²

Boards, rubber

230-400

Wood shavings, paper

330-500

Tarpaulin

420-500

Dry wood

500-670

Roofing (roofing material)

580-810

Chipboard

160-200

Table 6.3

The value of heat impulses and excessive pressure, corresponding to the degree of human injuries

Degree of injury

Thermal impulse, kJ/m²

Overpressure, kPa

Secure

<80

<10

Light (bruises, hearing loss)

80-100

10-40

Moderate (bleeding, dislocation, concussion)

100-400

40-60

Hard (concussion)

400-600

60-100

Lethal

>600

4. The intensity of heat radiation from the explosion of FA and GA mixtures at a distance R, kJ/m²·s we can find by the formula:

                                       J = Q₀·F·T,                                   (6.5)

where Q₀ is the specific heat of the fire, kJ/m²·s (Table 6.4); F is the angular coefficient characterizing the relative location of the source of the explosion and the object of the economy:

,                                       (6.6)

T is the thermal transparency of air:

                                                   (6.7)

Table 6.4

Thermal technical characteristics of some substances

Substance

Heat of fire,
кJ/m^{2 ·}s

Heat of combustion,
кJ/kg

Acetone

28,4

Petrol (gasoline)

1780÷2200

Kerosene

Methyl alcohol

20,9

Mixture of methane, propane, butane

40-50

Oil

43,7

Ethanol

8200÷10000

33,8

Furfural

Fuel oil

 

5. Next, we determination of the duration of the fireball exposure burning, s:

t_fb =(0,45 ÷0,85)·                          (6.8)

where М is the mass of GA/FA mixtures, kg.

6. Heat impulse U, kJ/m², we find by the formula:

                                          U = J · t_fb,                                (6.9)

where t_fb is the duration of the existence of the fireball, s, is determined by the formula (6.8).

Note: In formulas (6.5 – 6.7 and 6.9) it is necessary to calculate each parameters 4 times for each subzones (subzones of full, intense, medium and little damage) respectively. You should keep it in your mind.

7. Determination of the irretrievable loss of people from the impact of the shock wave, people:

N = 3 ∙ P ∙ M^0,666                              (6.10)

where P is the population density in a given locality, thousand people per km²; M is the mass of GA/FA mixtures, ton.

8. Determination of the damaging effect of the shock wave and heat impulse is provided by comparing the calculated values of ΔP_f and U_t with the table values (Tables 6.2, 6.3).                                                                                  

Example.We calculate the scales of the zones of ES in case the explosion of tanks with flammable liquids. The calculation will be carried out for the tank farm of the gasoline storage, which is used to supply gasoline to the oil extraction plant. In the tank farm are installed 4 tanks for receiving and storing gasoline, with a volume of 25 m³ each. The density of the population in this area is 3 thousand people per km².

Physicochemical properties of gasoline: volatile flammable liquid,
ρ = 790 kg/m³; t_flash = 33 °С, specific heat of fire Q₀ = 1700 ÷ 2200 kJ/m²·s. We calculate the damage zones in the explosion of tanks with extraction gasoline:

It is necessary to determine the radius and characteristics of the explosion zones during the explosion (overpressure in each zones of ES, fireball burning exposure time, intensity of heat radiation heat impulse) and to assess the damaging impact of shock waves on people and objects if the shop with a light metal frame is located at a distance of 400 m from the fuel storage site.

Solution.We calculate the damage zones in the explosion of tanks with extraction gasoline. First of all, we calculate the mass of FA mixtures:

М = (4·790·25)·0,9=71100 kg.

During the explosion, five zones of damage are formed: blasting (detonation), the action of the products of the explosion (fireball), the action of the shock wave, heat damage and toxic smoke.

1. First zone – zone of blasting (shattering) action, formula (6.1):

 m,

2. Second zone – zone of the fireball, formula (6.2):

                                  R_fb=1,7·R₁=1,7·72,5=123.25 м,                           

3. Third zone – zone of action of the shock wave. Radiuses of subzones full (ΔР_f = 50 kPa), intense (ΔР_f = 30 kPa), medium (ΔР_f = 20 kPa) and little (ΔР_f=10 kPa) damage we will find by the formula (6.4).

Radius of subzones full damage:

 m

Radius of subzones intense damage:

 m

Radius of subzones medium damage:

 m

Radius of subzones little damage:

 m

4. The intensity of heat radiation we determine by the formula (6.5).

Firstly, we calculate the thermal transparency of air by the formula (6.7) for each zone:

Т_full = 1–0,058ln306,7 = 0.66;

Т_intense = 1–0,058ln413,1= 0.65;

Т_medium = 1–0,058ln528,7 = 0.63;

Т_little = 1–0,058ln818,3 = 0.61;

The angular coefficient F we calculate by the formula (6.6) for each zone respectively:

The intensity of heat radiation on the distance R₃:

J_full = 1200·0,128·0,66 = 101,4 kJ/m²·s

J_intense = 1200·0,078·0,65 = 60.84 kJ/m²·s

J_medium = 1200·0,05·0,63 = 37.8 kJ/m²·s

J_little = 1200·0,022·0,61 = 16.1 kJ/m²·s

5. The duration of the fireball burning we determine by the formula (6.8):

t_fb = 0,65· =26.9 s

6. Heat impulse U, U, kJ/m², we find by the formula (6.9):

U_full = 101,4·26,9 = 2727.66

U_intense = 60,84·26,9 = 1636.6

U_medium = 37,8·26,9 = 1016.82

U_little = 16,1·26,9 = 433.09

7. The irretrievable loss of people from the impact of the shock wave according to the formula (6.10) will be:

N = 3 ∙ 3 ∙ 71.1^0,666 = 154 people

Finally, we have to choose the main measures for protection:

– deepening, embankment of tanks with gases, fuel;

– installation of fire protective walls, partitions, etc.;

– location of tank farms outside the zones of emergency situations (ΔРf <10 kPa, U <100 kJ/m²);

– Compliance with fire and explosion safety regulations.

Conclusion.As a result of the explosion, the shop with a light metal frame at the distance from center of explosion 400 m received intense damage (see Table 6.1). By the magnitude of the heat impulse 1636.6 kJ/m², it can be assumed that people in the open area can get fatal burns. The safe distance for people is 818.3 m.