Does API 2000 state that the wetted area is some percentage of a liquid full tank?
The wetted area of a tank or storage vessel shall be calculated as follows.
-For spheres and spheroids, the wetted area is equal to 55 % of the total surface area or the surface area to a height of 30 ft above grade, whichever is greater.
-For horizontal tanks, the wetted area is equal to 75 % of the total surface area or the surface area to a height of 30 ft above grade, whichever is greater.
-For vertical tanks, the wetted area is equal to the total surface area of the vertical shell to a height of 30 ft above grade. For a vertical tank setting on the ground, the area of the ground plates is not included as wetted area. For a vertical tank supported above grade, it is necessary to include a portion of the area of the bottom as additional wetted surface. The portion of the bottom area exposed to a fire depends on the diameter and elevation of the tank above grade. It is necessary to use engineering judgment in evaluating the portion of the area exposed to fire.
If so, why do they insist the storage tank liquid is full (API 521 does not follow this convention)? Why do they use a different basis than API 521; rather, why does API 521 not have to use the basis of 30 ft set forth by NFPA?