Page 487 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 453

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.!! 3 .s: 1 / � /,, � _r+++-+----,'-- / 1---l Noles: Based on API RP-520, 1988
.. V �/ On Insulated Vessels Insulation sho!I resist
a,
(/) 2 Q: 21,000 FA·82
.. I <>) �r · .s- .f:!"1./ �.,.," I Dislodgement by Fire Hose Streams .
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3C � " � "- ·1 Surface Area shell include only Totally
Wetted Surface within a Height of 2!) ft.
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100 ,__ __ _.__.... _ _.. i .. � ·,$,·1 ,"
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<( � .}: . ,./_/--+--<-<- <o., .,.._....._ __ ,___. Above Grade or in the Case of Spheres or
e 1-----+-·-+--+-" �, Spheroids to the Elevation of :he
"-1 I ..;:.1 / Maximum Horizontal Diameter.
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5 1----+---' ,.!$'�/ ,'-' v / / i/ The term Grade usually Refers lo Ground
.,W
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4 1-----+--- / V / Grade but may be any Level at which a
Sizable Area of Exposed Flammable
I 7 / Liquid may be Present.
/ / I 1 / / For Flom ma ble Insulation use F = 1.0 or
2 1 ,, 1 Bare Vessel Equivalent.
tQ �� � '-- -3 � l/---"4<.-'. / � -6L/..L.J8-LI- �� '-- �� '----1.3-4.,__5'--'6-'-·8L..L...L----'-2--'- 3 --1.4--L5-L6.J-J8L....W---2'----1.3_L 4 -L 5 � 6-L � 8..W I
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10,00C 100,000 1,000,000 10,000,000 100,000,000
O : Heat Absorbed, BTU I Hr.
Figure 7-30. API fonnula for heat absorbed from fire on wetted surface of pressure vessel. Prepared by pennission, Sizing, Selection and Instal-
lation of Pressure Relieving Devices in Refineries, Part I "Sizing and Seiection," API RP-520, 5th Ed. July 1990, American Petroleum Institute.
the walls and be evaporated only as long as there is liquid Each situation must be evaluated on its own merits or
to drain off the trays, but for a conservative approach, conditions and operating situation, and even its environ-
assume that there are always wetted walls in the column. ment with respect to the plant flammable processing
equipment.
For packed columns, the wetted walls are at least to the
top of the packing, with some entrainment height above Relief Capacity for Fire Exposure
that. Therefore, a vertical packed column of 60 feet of
packing above the liquid level in the sump, which has a
sump of 10 feet and a skirt of 10 feet, would be considered In calculating the relief capacity to take care of exter-
70 feet of vertical height of wetted perimeter, not count- nal fire the following equation is used:
ing the skirt. If the wetted area reached a total height of
80 feet from grade, minus the unwetted area of the skirt
height, because no liquid is in the skirt space, the 80 feet W= Q/L (7-31)
would not be used to establish the vertical height for fire
exposure, but use 60 ft + 10 ft, or 70 feet wetted height.
This is more conservative than code [33a and c], but is where W = weight rate of flow of vapors, pounds per hour
based on experience in investigating the damage levels L = latent heat at allowable pressure, BTU/lb
from fires. Q = total heat absorption from external fire, BTU/hr