Hello,

I am trying to calculate a relief rate of a very cold, unwetted vessel (-196°C). The problem is that at these cold temperatures, i.e. negative Rankine temperatures, the usually applied API-521 equation: W = 0,1406 (sqrt(MP1))*(A*((Tw-T1)^1,25))/(T1^1,1506) breaks down, because of the negative number^1,1506. Even at a temperature of -100°C, the formula outputs huge numbers because of the small value in the denominator.
Does anyone know other standards, with which I could calculate the relied load?

Kind regards

Now that is interesting, negative Rankine temperatures...

Rankine scale is an absolute temperature scale (similar as Kelvin scale), therefore there are no negative Rankine temperatures (see Wikipedia page on Rankine scale ).

[°R] = ([°C] + 273.15) × 9⁄5

Therefore, -196°C equals 138.9 °R

Tw and T1 in equation 12 of API Std 521 are both expressed in °R. Note that on pages 42 and 43 of API Std 521 a few times a reference to SI units is made, but this was later corrected in the Errata published June 2007. API Std 521 Errata

Dear gvdlans,

thanks for the reply. Of course you're right and it all makes sense now.

Kind regards,

FrankSchleck

gvdlans,
You always have sharp eye . Thumb up for you.