Page 538 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

504 Applied Process Design for Chemical and Petrochemical Plants

The distance in which a 500 lb charge will develop a 40 psi flashes to vapor and is ignited by the external heat source
overpressure is approximately 40 ft. (see [81]).

NFPA [34] contains extensive descriptions of Bleves
Example 7-16: Estimating Explosion Damage (also see [82]) and describes them in summary as para-
phrased here with permission: liquefied gases stored in
An overpressure after an explosion is noted as 0.5 psi.
The calculated scaled distance Z is 75 ft/ (lb) 113. Thus for containers at temperatures above their boiling points at
a one pound charge, windows are broken at a distance of NTP will remain under pressure only as long as the con-
75 feet. How far will windows be broken for a 500 lb. tainer remains closed to the atmosphere. If the pressure
charge? is suddenly released to atmosphere due to failure from
metal overstress by external fire or heat, corrosion pene-
tration, or external impact (for examples), the heat
3
Z = 75 = 75/(1)11 = x/(500)1 /3
stored in the liquid generates very rapid vaporization of a
x = 7.937 (75) = 595 fl, windows broken for a 500 lb portion of the liquid proportional to the temperature dif-
change ference between that of the liquid at the instant the con-
tainer fails and the normal boiling point of the liquid.
In general, a reflected shock wave of 55 psi on a human Often this can generate vapor from about one-third to
for 400 milliseconds would be just about the tolerance one-half of the liquid in the container. The liquid vapor-
limit [ 41] (see Table 7-25B). For a more detailed discus- ization is accompanied by a large liquid to vapor expan-
sion of blast scaling and overpressure, see Ref [ 40]. sion, which provides the energy for propagation of vessel
cracks, propulsion of pieces of the container, rapid mix-
Explosion Venting for Gases/Vapors (Not Dusts) ing of the air and vapor resulting in a characteristic fire-
ball upon ignition by the external fire or other source that
Unless there is sufficient explosion data for a specific caused the Bleve to develop in the first place, with atom-
chemical system, including air-mixing or run-away reac- ization of the remaining cold liquid. Often the cold liquid
tions, very few devices can be effective in relieving a con- from the vessel is broken into droplets that can burn as
fined vessel explosion other than a carefully designed they fly out of the vessel. Often this cold liquid can escape
rupture disk, with a good factor of overcapacity. Because ignition and may be propelled one-half mile or more
detonation explosions initiate and travel so fast (see pre- from the initial site. In most Bleves, the failure originates
vious tables/charts), there is a limited chance to relieve in the vapor space above the liquid, and it is this space
the overpressure. Some fast microsecond electronic that is most subject to external overheating of the metal.
detectors based on rate of pressure rise or rate of temper- The transportation industry is subject to more impact
ature rise can and have been used to anticipate runaway failures of vessels that lead to Bleves, The Bleve occurs
reactions and thus create an "early" anticipated release or simultaneously with the impact in most recorded cases,
opening of the relieving device. When such explosive con- but not all. Sometimes there is a delay due to lack of total
ditions are anticipated, special studies such as the DIERS penetration of the vessel with a hole or crack, and time is
technology [51, 67] can be justified to provide informa- needed for the temperature in the container to rise. Fail-
tion for a safe response and control of an explosion. ures of the vessel can lead to fireballs of more than sever-
Other methods such as injection of inerting or reaction al hundred feet in diameter.
suppressants have proven to be beneficial (see Figures 7-
61 and 7-62). The application of water externally to the vapor space
of a vessel or application of insulation can often protect
Bleves (Boiling Liquid Expanding Vapor Explosions). against Bleves.

A relief valve will not usually handle the vapor generat-
This particular type of explosion is less known and
understood, but nevertheless is an important type for ed because its set pressure is usually higher than the boil-
ing point pressure created by the hole or crack in the ves-
damage consideration. This is a type of pressure release
explosion and there are several descriptions. sel; therefore, it will not relieve at the lower pressure.
Kirkwood [30] describes Bleves referenced to flamma- Ref. [ 40] points out that the effects of a Bleve depends
ble liquids as occurring when a confined liquid is heated on whether the liquid in the vessel is flammable. The ini-
above its atmospheric boiling point by an external source tial explosion may generate a blast wave and fragments
of heat or fire and is suddenly released by the rupture of from the vessel. For a flammable material, the conditions
the container due to overpressurization by the expanding described in Ref. [34] above may result, and even a vapor
liquid. A portion of the superheated liquid immediately cloud explosion may result.