3 replies to this topic

[fernandocamano]

Hi friends,

 

I know there is been a lot of questions and answers regarding how to calculate the wetted surface area of a tank. However, none of them mention what is written in ISO 28300. So lets take a look at the Terms, definitions and abbreviated terms:

 

wetted area 

surface area of a tank exposed to liquid on the interior and heat from a fire on the exterior

 

So far so good, is pretty clear to me,  now, let's see the calculus, this is where I'm having trouble to figure out, I've been thinking about this for months, really  , but can't understand the following:

 

it says:

 

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

 

So, let's say I have this Vertical Tank:

 

Raidus: 15.6'

height: 18'

 

I'm using the formula for area of a cylinder: 2πrh+2πr²  = 2π*15.6*18+2π(15.6)²  then I susbtract the area of the top and bottom of the tank since it won't be exposed to the fire.

 

The result is =  2940.54 square feet

 

What I don't understand and what is driving me nuts to understand is this last sentence: 

 

"...the wetted area is equal to the total surface area of the vertical shell to a height of 30 ft above grade"

 

To a height of 30 ft,  what this means? that I have to use 30 as my height instead of 18? or what?

 

Now, let's say I have the same Tank but this time is horizontal.  My surface area is the same, 2940.54 square feet right? 

 

ISO 28300 says it is the 75% of the surface, so no prob, 2940.54 * .75 = 1323.24, so far, so good. 

 

Now, same thing, I can't understand the following

 

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

 

"...Or the surface area to a height of 30 ft above grade whichever is greater..."

 

shall I replace the length of my tank with 30 feet in the formula and if it is greater than my previous calculus, should I take this value ?

 

Thanks for your comments and have a great day

[fallah]

fernando,

 

I think it wants to say if the height of the tank is more than 30 ft, you should get the surface area up to 30 ft elevation from ground; on the other hand, if the height of the tank is below 30 ft, you should get the surface area of the tank at its full elevation which is lower than 30 ft...

[breizh]

Fernanado ,
Consider this resource too .

Breizh

[curious_cat]

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

 

"...Or the surface area to a height of 30 ft above grade whichever is greater..."

 

shall I replace the length of my tank with 30 feet in the formula and if it is greater than my previous calculus, should I take this value ?

 

Since both your dimensions are less than 30 ft I don't think you need to account for the 30 ft clause.