Tank Venting Requirement

A lot of information missing. What fluid you are storing in this tank? Where does nitrogen go out continuously, if you go on in-breathing at the rate of 250 SM3/hr? Which fluid is being pumped out at 6 m3/hr?

N2 line velocity can be taken about 8 m/s, after calculating actual volume flow at 15 mbar pressure and prevailing temp.

fluid is CS2 and water level is maintained above the CS2 fluid. The N2 is vented out to atm through breather valve in case if pressure exceds 15 mbar

Where does the 250 sm³/hr figure come from?

from API std.2000 edn 5th page no. 7 Table 2B requirement of thermal venting capacity. for 1500 m3 tank it is 253 Nm3/hr.

Even with a totally empty tank there has to be a temperature variation of about 50ºC to get a thermal breathing of 247 sm³/hr. If the gas space volume is less than the tank volume, The temperature excursion will be proportionately higher. Something does not seem to be right.

sir, tank is at ambient temp. there is no as such temp. variation.

There might be a need to supply the 250 nm³/hr of N₂ if there somehow is rapid cooling inside the tank, but this should not need to be continuous. Your N₂ valves and piping should be sized for this large value, but the N₂ supply should not need to sustain it continuously; it is more of a peak requirement. The 6 nm²/hr withdrawal from the tank would require continuous N2 replenishment of 6 nm²/nr.

Agreed, there's something wrong here, specially as OP says there is no temperature change. I'm not familiar with API 2000, but maybe it's being misinterpreted. Perhaps it says a store of 250 m³ must be available. Also if the N₂ can be stored when it comes out on tank filling the consumption could be quite small.

Codey

I am a bit unsure of the units of measurement here? What is meant by SM3/hr? Is this supposed to be cubic meter/hr under standard state conditions? Other responders indicated a unit of measurement of something like nm3/hr. What is that? Nanometers cubed per hour? The MKS system of units originally was a pretty good, self-contained system, but if it becomes "bastardized" then we shall have to resort to farthings per fortnight.

nm3/hr means normal m³/h. Strictly speaking it should be written Nm³/h, but in a gas context most people will understand. Normal conditions are 0°C, 1 atmosphere (760 mm Hg).

Codey