Page 548 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

514 Applied Process Design for Chemical and Petrochemical Plants

Maximum Pressure During Venting
I 11111
Ks,, bar· m/sec P,ed• bar ga
" ir--- 50 0.2
/1,- 75 0.3
,, /v--100 0.4 ' ........
, / -150 0.5 x.r-,
I/ / -200 0.6 r-, r-,
' " r-, r-, r-
"1, I/ v / 250 " <,
�I/ 1/V/ 300 0.8 xr-, r-, r-..."'
/VV,, 1.0---..... "'' r,. r- r-
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r., L/Vi/ v- 500 1.6--.... ":" I' � ['.__" I., �
1'.' .'\. "" "
r,."',.. ., " � "'V ,1 / v-600 2.0- "" . "I'- r-, <, -, I/ I., �
r,1/ , / r,. " r-r- r- I\. -, <, I'\: v I/ I, i.,.
<,
.... " I'\, [',I' r- "'), ,.... Vy �
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.
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I
100 10 0.1 10 100 1000
---- Vent Area, m 2 Vessel Volume, m 3 ----
Figure 7-65A. Venting nomograph for dusts, P stat = 0.1 bar ga. Reprinted with permission, NFPA 68-1988, Deflagration Venting, (1988) Nation-
al Fire Protection Association, Quincy, MA 02269. See note Figure 7-63A.
but sufficiently accurate for industrial use, care must be Example 7-18: Use of the Dust Nomographs
used in selecting the conditions for design. Classification
of a dust in a specific class is not assurance of the proba- A storage silo for lignite particles (not lumps) is 10 ft
bility of a dust explosion [54]. diameter by 12 ft tall. 'What protection is needed to guard
against destruction from a dust explosion (deflagration)?
The dust hazard classes for deflagrations are given in
Table 7-28, with data in Tables 7-28 and 7-30A, B, C, D, From Table 7-30C, lignite is dust hazard class 1, with K.t
and E. = 151 bar-m/ sec.
Using a nomograph requires only the vessel volume in 1. Vessel volume= 1t ( L0) (12)/4 = 942.4 cu ft
2
meters, selecting the dust class. St-1, St-2 or St-3 from = 942.4/35.3 = 26.7 cu meters
Table 7-28. Using Tables 7-29 or 7-30 select the K,t value L/D = 12/ 10 = 1.2, length/diam. is ok.
determined experimentally. The reduced pressure, P red» 2. Pred, bar ga = 2.0 = (2) (14.5) = 29 psig max. allowable
(maximum pressure actually developed during a vented vessel internal pressure. Note: 1 bar= 14.5 psi
deflagration, termed reduced explosion pressure) must 3. Reading vent area (Figure 7-650) = 0.70 sq meters
not exceed strength of vessel (see earlier discussion) and = 0.70 X 10.8 sq ft/sq
the P,tat• i.e., the vent device release pressure. Note that meter
the static activation pressure, P,t, must be determined = 7.56 sq ft
from experimental tests of the manufacture of relief pan- select, P,i.at = 0.1 bar ga, vent device release pressure
els such as rupture disks. = (0.1) (14.5) = 1.45 psig.