Page 486 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

452 Applied Process Design for Chemical and Petrochemical Plants

Table 7-8 1. Liquid-full vessels (such as treaters) operate liquid
Environment Factor, F full. Therefore, the wetted surface would be the total ves-
sel surface within the height limitation.
Type of Installation Factor*
Bare vessel 1.0 2. Surge drums (vessels) usually operate about half full.
Insulated vessels+ (These arbitrary insulation condance values Therefore, the wetted surface would be calculated at 50%
are shown as examples and are in British Thermal Units per of the total vessel surface, but higher if design is based on
hour per square foot per °F) greater figure.
(a) 4.0 Btu/hr/sq ft/°F (I in thick) 0.3
(b) 2.0 Btu/hr/sq ft/°F (2 in thick) 0.15
(c) 1.0 Btu/hr/sq ft/°F (4 in thick) 0.075 3. Knockout drums (vessels) usually operate with only a
(d) 0.67 Btu/hr/sq ft/°F 0.05 small amount of liquid. Therefore, the wetted surface
(e) 0.5 Btu/hr/sq ft/°F 0.0376 would be in proportion, but to maximum design liquid
(f) 0.4 Btu/hr/sq ft/°F 0.03 level.
(g) 0.32 Btu/hr I sq ft/ °F 0.026
'Nater-application facilities, on bare vessel** 1.0 4. Fractionating columns usually operate with a normal
Depressurizing and emptying facilitiestt 1.0 liquid level in the bottom of the column and a level of
Underground storage 0.0 liquid on each tray. It is reasonable to assume that the wet-
Earth-covered storage above grade 0.03
ted surface be based on the total liquid within the height
*These are suggested values for the conditions assumed in code [33] limitation-both on the trays and in the bottom.
Par D 5.21. When these conditions do not exist, engineeringjudgment
should be exercised either in selecting a higher factor or in providing
means of protecting vessels from fire exposure as suggested in [33], 5. Working storage tanks' wetted surface is usually calcu-
par. D. 8. lated on the average inventory, but at least 25 ft height,
+Insulation shall resist dislodgement by fire hose streams. For the exam- unless liquid level can reasonably be established as high-
ples, a temperature difference of 1600°:F was used. These conductance er, then use higher value. This should be satisfactory not
values are based on insulation having thermal conductivity of
48TS/hr-ft-°F per inch at 1600°F and correspond to various thickness- only because it conforms with a probability, but also
es of insulation between l and 12 inches. because it provides a factor of safety in the time needed to
**See code for recommendations regarding water application and insu- raise the usually large volume of the liquid's sensible heat
lation. to its boiling point.
+rDcpressurtztng will provide a lower factor if done promptly, but no It is recommended that the wetted area be at least to
credit is to be taken when safety valves are being sized for fire expo-
sure. See [33], Pan i. par. D. 8.2. the height as defined in the definition of area, A.v-
By permission, API-RP-520, American Petroleum Institute, Div. of Note: This author's suggested determination of Aw val-
Refining (1967) and adapted for this current edition by this author ues may be more conservative and not conform exactly to
from later editions of the code (1976) and (1990). Items d, e, and f Code [33a.33c] recommendations. The Code [33a, Part
above from API-RP-520, 5th Ed. (1990). For complete reference, see
the latest code cited in its entirety. 1, Sect D, Par. D.4] reads, "to determine vapor genera-
tion, only that portion of the vessel that is wetted by its
internal liquid and is equal to or less than 25 feet above
a cylindrical vessel. If a formula is not available to accu- the source of flame needs to be recognized."
rately estimate the wetted surface, or the blank diameters
used for fabrication (see Appendix), which would give a 6. Based upon this author's experience in investigating
close approximation of the inside surface of the head, use many industrial fires and explosions, it is suggested that
an estimated area for the dished or elliptical heads as 1.2 X the height limit of 25 feet above "grade" or fire source
cross-section area of the vessel based on its diameter. level is too low for many process plants, and therefore, the
effect of a large external fire around equipment can
reach to I 00 feet with 75 feet perhaps being acceptably
conservative. This author would never use the 25-foot
Surface Area Exposed to Fire
limit, for example, for a horizontal butane storage "bul-
let" tank, 15 feet diameter and raised 15 feet off grade to
its bottom. The fact that any fire will engulf the entire ves-
The surface area of a vessel exposed to fire which is sel, should be considered and the wetted surface should
effective in generating vapor is that area wetted by its be the entire vessel. The same concern applies to a verti-
internal liquid contents. The liquid contents under vari- cal distillation column over 25 feet high. It is this author's
able level conditions should ordinarily be taken at the opinion that the wetted surface should be at least 80% of
average inventory, for example: See note below. the vessel height, recognizing that the tray liquid will wet