Page 510 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

476 Applied Process Design for Chemical and Petrochemical Plants

Table 7-17
Total Rate of Emergency Venting Required for Fire
Exposure Vs. Wetted Surface Area
3
(wetted area vs. ft. of free air/hr, 14.7 psia, 60°F)
Venting Requirement Venting Requirement
Welted Area" ( cubic feet of free Welted Area" (cubic feet of free
(square feet) air " per hour) (square feet) air" per hour)
20 21.100 350 288.000
30 31,600 400 312,000
40 42,100 500 354,000
50 52. 700 600 392,000
60 63.200 700 428,000
70 73, 700 800 462,000
80 84.200 900 493,000
90 94,800 1000 524.000
100 105,000 1200 557 ,000
120 126,000 1400 587,000
140 147,000 1600 614,000
160 168.000 1800 639,000
180 190,000 2000 662.000
200 211,000 2400 704,000
250 239.000 2800 742,000
300 265,000 >2800"
NoTE: Interpolate for intermediate values. The total surface area does not include the area of ground plates but
does include roof areas less than 30 feet above grade.
"The welted area of the tank or storage vessel shall be calculated as follows: For spheres and spheroids. the
wetted area is equal to 55 percent of the total surface area or the surface area to a height of 30 feet 19. 14 meters).
whichever is greater. For horizontal tanks, the welted area is equal to 75 percent of the total surface area. For
vertical tanks, the welted area is equal to the total surface area of the shell within a maximum height of 30 feet
19.14 meters) above grade.
h At 14. 7 pounds per square inch absolute ( 1.014 bar) and 60 F ( 15. 56 C).
"For welted surfaces larger than 2800 square feet (260. J square meters), see 1.3.2.1. 1.3.2.2. and 1.3.2.4.

By permission: API-Std-2000, 3rd Ed., 1982, reaffirmed Dec. 1987, American Petroleum Institute [26].

In accordance with code [26] the inbreathing capacity such conditions the venting requirements may be
is to be determined by the following: increased above the normal design levels previously
reviewed. Emergency venting may be by [26]:
1. Use maximum liquid flow out of tank (considered as
oil by code) as equivalent to 560 cubic feet of free air 1. Weak roof-to-shell attachment for fixed roof tanks, as
per hour for each 100 barrels ( 4,200 gallons) per hour per API-Std-650 for Weuled Steel Tanks for Oil Storage.
of maximum emptying rate. This applies to oils of any The joint fails and excess pressure can be relieved
flash point. Also includes gravity flow conditions. (above the "normal" design provided). Such tanks do
2. For thermal in breathing use at least the cubic feet of not require additional emergency vent equipment;
free air per hour given in Table 7-14, column head- however, it can be provided in order to prevent the
ing number 2. This also applies to oils or fluids of roof seam failure with its attendant replacement/
any flash point. maintenance requirements. This type can only be
used outside of a building, not confined.
Convert the free air rates to the proper product in the 2. For fixed roof tanks without the weak roof design,
tanks using the corrections outlined in a previous para- the required total venting at the time of emergency is
graph. Keep in mind that the manufacturer's rating tables determined as below, since normal and "thermal"
are in free air; however, the actual process calculations venting and in breathing can be ignored. The capac-
provide flows in terms of the actual liquids at actual tem- ity of normal outbreathing equipment can be count-
peratures and pressures. It is important that the manu- ed toward emergency requirements.
facturer be given the actual fluid conditions to ensure (a) Tanks designed for 1 psig or below are to have the
proper capacity rating. emergency venting rate determined from Table 7-
17. For more than 2,800 ft. of exposed wetted
2
Emergency venting of large tanks is usually associated internal surface, no increase in venting is
with external fire conditions around the tanks. Under required. All the code recommendations assume