API 2000, clause 4.3.3.2.2, tanks having surface area >= 260 square meter and design pressure <= 0.07 barg, required venting capacity is 19910 Nm3/hr of equivalent air.

Also Emergency venting capacity for low pressure tank is derived from following Equation 1B of API 2000, clause 4.3.3.2.1

Nm3/hr = 881.55 x QF/L x (T/M)^0.5}

Now here,

Q = heat input = 4129700 watts (For tanks having wetted surface >= 260 square meter and design pressure <= 0.07 bar g) as per API 2000, Appendix B.
F = environment factor
L = latent heat of vaporisation
T = Reliving temperature of vapor at reliving pressure (If it is not know than considered bubble point of stored liquid)
M = Molecular weight of vapor

Now when we calculate venting capacity by above said two method using hexane as a stored liquid we are getting different venting rate.

So what approch we should considered for emergency venting rate calculation for other liquids having very high bubble point.?

Nirav, sorry for the late reply.

The standard says that 4.3.3.2.2 has "a lesser degree of accuracy" and also please read Appendix B.

Then keep in mind that under the best of circumstances these are still estimates and that emergency venting equipment is very inexpensive when compared to the value of the stored product, the equipment and not the least, the human life and welfare.

I would always be conservative with emergency venting as it is the last line of defense between an intact and a ruptured tank.

This may not be a scientific analysis of the code, and if you need that you should at least go to this link

http://committees.api.org/standards/tech/index.html

as well as

http://committees.api.org/standards/tech/reti.html

and also click onto the link at the bottom of that page for STD 2000 and look that spreadsheet over for general background.