Page 509 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Applied Process Design 475

Code requires: 1200 cu ft of free air /hour for each ( 26,000- 21, 000)
4200 gallons/hour maximum filling rate = 500 + ( 1000 - 500 )
( 42,000- 21, 000)

. . (1200)
SCFH free arr required = -- ( 21, 000) = 6, 000 = 619 SCFH free air@ 14.7 psia and 60°F
(4200)
Total vacuum relief required:
2. Thermal outbreathing:
Using Table 7-14, for 26,000 gallon capacity,
required vent = 371 SCFH free air at 14.7 psia = 2000 + 619 = 2619 SCFH free air
and 60°F. Interpolating, vent required for 26,000
gallon capacity. Select vacuum relief valve by referring to man-
ufacturer's tables at vacuum relief setting of 0.5
oz/in. and 1.0 oz/in. maximum allowable vacu-
2
2
3. Total pressure relief requirement um for tank. Note, no correction is required as is
shown for section A above, since air is flowing in,
and not benzene.
= 6000 + 371 = 6371 SCFH free air
C. Emergency venting requirements (bare tank)
4. Total corrected pressure relief requirement Total external surface exposed to fire:

Ve (1337) 6371 (1,337) = nD (height) = 1t(15) (20) = 942.5 ft. 2
Ve =----=
1
LM 112 (169)(78.11) 1 2
Use entire shell area, since height< 30 ft
= 5709 SCFH free air al. 14.7 psia and 60°F when Wetted area: Entire 20 feet above grade. From
benzene in tank. Table 7-17 at 942.5 ft. 2, CFH free air relief:

5. Read manufacturer's capacity tables or charts to 900 ft. = 493,000 SCFH
2
select model and size of pressure relief, reading 1,000 ft. = 524,000 SCFH
2
table to closest capacity, but toward larger size if in
between. Consult manufacturer for final selection. For 942.5 ft. 2:

( 942.5 ·-· 900)
B. Vacuum (lnbreathing) Volume= 493, 000 + (542 ,000 - 493, 000)
( 1000- 900)
1. The draw-out on emptying is given as 250 gpm.
Using the code requirements, free air: Venting Rate = 513,825 SCFH free air at 14.7 psia and 60°F

Corrected venting rate for benzene in tank:
[(560 cu fl air/4,200 gal/hr)] [(250) (60)] = 2000 cu
ft free air/hr at 14.7 psia and 60°F , 513 825 (l 337)
V = ' ' = 459 945 SCFH
1
c (169)(78.11) 12 ,
2. Thermal inbreathing:
From code table (using vacuum), Table 7-14. For relief valve sizing, refer to manufacturer's tables at
2.0 oz/in. pressure setting and 3.5 oz/in. maximum allow-
2
2
able tank pressure.
For tanks of 26,000 gallon capacity: Inbreathing requires vacuum relief for the tank. The
usual conditions arise from liquid flow out of the tank or
from condensation or contraction of the vapors by reduc-
At 21,000 gal., SCFH air= 500 tion in temperature of the tank contents caused by atrnos-
AL 42,000 gal., SCFH air = 1,000 pheric changes (not by system mechanical refrigeration).