Page 531 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Applied Process Design 497

Table 7-23 rate of pressure rise is a measure of the speed of flame
Conditions of Failure of Peak Overpressure for Selected propagation and accordingly the violence of the explo-
Structural Components sion. The rate of pressure rise is the slope of the tangent
line (Figure 7-54) through the rising branch of the pres-
Approximate
incident blast sure, which is the time curve of an explosion [53]. The
overpressure greatest rate of pressure rise will occur when an explosive
Structural element Failure (psi) mixture ignites in the center of a vessel. Ignition at any
.����--���--��������--.c.__�� other location will result in a lower rate and somewhat
Glass windows, large Usually shattering, 0.5-1.0
and small. occasional frame reduced explosion pressure. The volume of the vessel
failure. influences the violence of the explosion (Figure 7-55 and
Corrugated asbestos Shattering. 1.0-2.0 Figure 7-57). For example, Bartknecht data [54] shows
siding. that for propane exploding in three separate but different
Corrugated steel or Connection failure, 1.0-2.0 sized vessels, the magnitude of the final maximum pres-
aluminum paneling. followed by buckling. sure is the same for each of about 7 bars, but the time to
Wood siding panels, Usually failure occurs 1.0-2.0 reach this maximum pressure is greater the larger the ves-
standard house at main connections, sel (Figure 7-56).
construction. allowing a whole panel The influence of vessel volume on the maximum rate
to be blown in.
Concrete or cinder- Shattering of the wall. 2.0-3.0 of pressure rise for a specified gas is characterized by the
block wall panels, Cubic Law, i.e., [54] the rate of pressure rise varies for
8 or 12 inches thick each gas.
(not reinforced).
Brick wall panel, 8 Shearing and flexure 7.0-8.0 (dp/dt)ma., (V) 113 =Constant= Kc (7-57)
or 12 inches thick failures.
(not reinforced). V = vessel volume, cubic meters
By permission, Zabetakis, M. G., U.S. Bureau of Mines, Bui. 627 [ 43]. Kc = constant, (bar) (meters/second)

This is valid for the same degree of gas mixture turbu-
miscellaneous conditions, including physiological effects lence and the same ignition source and is illustrated in
of blast pressures on humans and structural damage to Figure 7-58. Influence of the vessel shape is shown in Fig-
facilities. ure 7-56. The behavior of propane is considered repre-
Pressures of deflagration or detonation shock waves sentative of most flammable vapors including many sol-
build upon the existing system pressure at the time of the vents [54]. The maximum explosion pressure does not
initial blast. When a deflagration starts and then builds to follow the cubic law and is almost independent of the vol-
a detonation, the resulting peak pressure can be quite ume of a vessel greater than l liter. For propane, town gas,
high because the final pressure of the detonation builds and hydrogen, the volume relationship can be expressed:
on the peak pressure of the deflagration.
In an enclosure, a peak for initial pressure ratio for a
< - •
1
dp
surface <
deflagration generally can exceed 8:1 of the initial pres- -- .:> • .:>,or,--- =- (7 - 58)
sure. The pressures may build to a ratio of 40: l (reflected V I/3 volume 30 meters dt
pressure) times the initial pressure when a detonation The violence of an explosion is influenced by the ini-
develops. This is the reason detonations can be so disas- tial pressure or pressure of the system in which the explo-
trous. Their final pressure, when built on a deflagration sion takes place. Figure 7-57 illustrates this point for
peak pressure as a base or initial pressure, can be propane and a constant ignition energy source. For low
extremely high. pressure below atmospheric, the explosion reactions are
For example, see Figures 7-53 [ 43) and 7-54. reduced until they will not propagate through the fuel-air
The overpressure levels, that is, final peak less initial mixture [54].
starting pressure, shown in Table 7-23 [ 43] are low refer- For some mixtures, unusual conditions seem to devel-
enced to the magnitude of overpressures attainable from op in the rate of pressure rise at peak explosion pressures
many industrial blasts. due to possible changes in the violence of the reaction
The speed of a combustion reaction will be at a maxi- [54]. Similar results are reported for the level of energy
mum at a certain fuel-air ratio that is generally close to the required for ignition of a mixture related to the concen-
stoichiometric composition. It will be lower, however, for tration range for ignition. Without examining the energy
compositions closer to each of the explosive limits. The level versus concentration at various initial pressures, it