Page 516 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

482 Applied Process Design for Chemical and Petrochemical Plants

Table 7-18 straight pipe Ilush with the pipe wall. Reflected side-on
Environmental Factors for Refrigerated Tanks pressure is measured perpendicular, i.e., facing the
oncoming flow.
Datum F Factor
Bare metal vessel IO Pilot Operated Vent Values
Insulation thickness"
6 inches ( 152 millimeters) 0.05''
8 inches (203 millimeters) 0.037" The Code [26] allows pilot operated vent control valves
10 inches (254 millimeters) 0.03'' provided the vent valve can operate automatically in case
12 inches (305 millimeters) or more' 0.025 1' the pilot valves/system failed.
Concrete thickness
Water-application facilities' 10
Depressuring and emptying facilities' IO Explosions
Underground storage 0
Earth-covered storage above grade 0 03
The three basic types of explosions to be concerned
"To rake credit for reduced heat input. the insulation shall resist dislodg-
ment by a fire-hose stream. shall be noncombustible. and shall not decom- about in the chemical and petrochemical environment
pose at temperatures up to 1000 F. If the insulation does not meet these are combustion explosions ( deflagrations), detonation
criteria. the F factor for a bare vessel shall be used. explosions, and BLEVEs or boiling-liquid expanding
"These F factors are based on an arbitrary thermal conductivity of 4
British thermal units per hour per square foot per !degree F per inch of vapor explosions [38].
thickness) and a temperature differential of 1600 F when using a heat input Other than reactive metals explosions, which do not
value of 21.000 British thermal units per hour per square foot in accor-
dance with the conditions assumed in API Recommended Practice 520. truly fall in the types noted above, the two main categories
When these conditions do not exist, engineering judgment should be of explosions are flammable gases, liquids/vapors, and
exercised either in selecting a higher F [actor or in providing other means dusts. Because their sources are different, they cannot be
of protecting the tank from fire exposure
"The insulation credit is arbitrarily limited to the F factor shown for 12 treated in the same manner for discussion.
inches of insulation. even though greater thicknesses may be used. More
credit. if taken. would result in a relieving device that would be impracti-
cally small bur that might be used if warranted by design considerations. Confined Explosions
"Twice the F factor for an equivalent thickness of insulation.
"Under ideal conditions. waler films covering the metal surfaces can
absorb substantially all incident radiation. However. the reliability of A confined explosion occurs in a contained vessel,
effective water application depends on many factors. Freezing tempera- building piping network, or other confined situation. A
tures. high winds. system clogging. unreliability of the water supply. and confined explosion has different characteristics than an
adverse rank surface conditions are a few factors that tna)· prevent
adequate or uniform waler coverage. Because of these uncertainties. the unconfined explosion [ 40]. These explosions may be
use of an F factor other than 1.0 for waler application is generally deflagrations or detonations with the detonation being
discouraged much more destructive due to the higher and more rapid-
'Depressuring devices may be used. but no credit for their use shall be
allowed in sizing safety valves for fire exposure. ly moving pressure wave generated. Schwab [34] states
unequivocally that a vessel containing flammable vapor as
F = environmental factor from Table 4. a mixture when ignited with the resultant pressure
A = wetted surface area, in square feet ( see Table 3. buildup, will explode. If the vessel does not rupture, but
Foot note a). contains the deflagration or detonation, there is no explo-
sion because the requirement for mechanical work has
not been met.
NoTE: The formula above is based on
Combustion explosions are explosions resulting from the
Q = 2 I . 000 A O 82
uncontrolled rapid mixing and reaction of a flammable
as given in AP! Recommended Practice 520. The total heat absorbed, Q. vapor from a flammable liquid with air (or oxygen) ignit-
is in British thermal units per hour. The constant 1107 is derived by ed from an ignition source such as flame, heal, electric
convening the heat input value of2 l .OOO British thermal units per hour per
square foot 10 standard cubic feet of free air by using the latent heat of spark, or static discharge. The combustion is extremely
vaporization at 60 F and the molecular weight of hexane. When the rapid with a flame propagation rate of about 7 feet per
molecular weight, latent heat of vaporization, and temperature of relief second, with the evolution of heat, light, and an increase
conditions for refrigerated hydrocarbons are substituted in the formula
based on hexane. the venting requirements are about equal to the values in pressure [38]. The violence of the explosion depends
for hexane. Hexane has therefore been used as a basis for simplification on the rate at which the energy is released.
and standardization ( see Appendix B for additional information about the
derivation of the formula). A deflagration is a slow burning exothermic reaction sim-
ilar to the combustion explosion, but which propagates
from the burning gases into the unreacted material at a
velocity that is less than the speed of sound in the unre-
By permission: API Std-2000, 3rd Ed. (1982), reaffirmed Dec. 1987[26]. acted material. Most (not all) explosions are deflagrauons,