Limiting Oxygen Concentration for an Isomerate Storage Tank 3

[Bandit1422]

I am looking for a basis for LOC (same as Minimum Oxygen Concentration) when applying nitrogen purge to an Isomerate storage tank. Can anyone explain to me the tactics in developing this? I understand it is based on LEL and have seen variance based on pressure, altitude, etc.... Can someone help me gain a little better understanding of this? As well i understand a common practiced safety factor is 25% of the LEL.

 

[Bandit1422]

I read through the thread that you provided and got a little higher value based on the calculations i was referred to. I have found that toluene's experimental LEL is around 1.26 and you multiple that by the 9 decided from the stoicmetric conversion and you get around 11.4 compared to the 9.5 discussed in the thread. This created some confusion as well as the basis for using these calculations:

NE = S { NEi * I%i }
where I%i = fractional quantity of ith Inert component
NEi = nitrogen equivalent of ith Inert component
CLF = Sigma { F%n } / Sigma { F%n / CLn } = low limit of Flammable portion in Air
CLM = Sigma { I%i + F%n } / Sigma { F%n / CLn } = low limit conc of total Mixture in Air
where F%n = fractional quantity of nth Flammable component
CLn = lower limit of nth Flammable component

How do they differ and which is preferred?

Finally so essentially applying a safety factor of 40% would equate to (LOC-(LOCx.4))

 

[25362]

My intention was [≤]0.4 LOC.

 

[Latexman]

I have found that toluene's experimental LEL is around 1.26 and you multiple that by the 9 decided from the stoicmetric conversion and you get around 11.4 compared to the 9.5 discussed in the thread. This created some confusion . . .

Most of the time, different estimation methods give different results. The BEST value comes from test data in a qualified lab. On safety issues it boils down to, are you willing to bet your life and livelihood on an estimate. If not, have it measured.

Good luck,
Latexman

 

[Bandit1422]

Im going to assume that the reason for the large safety factor would be because the level of variation in comparison with the hazard associated. Is there any background (book references etc...) providing supplemental information in using a 40% safety factor?

 

[25362]

In addition.
The same book by Crowl & Louvar gives an example of how to estimate the LFL and HFL of a mixture of air with flammable gases, based on Le Chatelier's empirically derived equation to determine if a certain mixture is flammable or not.

LFL_mix = 1[÷][Σ] (y_i/LFL_i)​

and

UFL_mix = 1[÷][Σ] (y_i/UFL_i)​

Where

y_i = mol fraction of the flammable gas i in the mixture with air
LFL_i = LFL of the flammable gas i in the mixture
UFL_i = UFL of the flammable gas i in the mixture

The authors advise us that empirical formulas like these have their limitations. Therefore, I suggest following Latexman's advice.

BTW, the lowest (roughly) estimated LOC doesn't necessarily go with the lowest LFL. Take, for example:

Metane: LFL = 5.0. LOC = 5.0[×]2 = 10.0
Hexane: LFL = 1.1. LOC = 1.1[×]9.5 = 10.45
Ethylene: LFL = 2.7. LOC = 2.7 [×]3 = 8.1
Heptane: LFL = 1.1. LOC = 1.1[×] 11 = 12.1
Acetone: LFL = 2.5. LOC = 2.5[×]4 = 10.0

 

[Bandit1422]

If you would be interested in helping i came up with a Limited Oxygen Concentration Calculator for this isomerate material based on Estimated (Stoichiometric) Calculations, Experimental Calculations I was able to find, and calculations using LeChateleirs Mixing Rule being that the mixture is typically constant. I could send it to either one of you for a quick review. If not thanks I greatly appreciate your help in assissting me to understand this information.

 

[25362]

http://d.yimg.com/kq/groups/3862917/197829902/name/N2+Purging+For+PV.pdf

http://www.fire.tc.faa.gov/pdf/TG3.pdf

http://www.airproducts.com/~/media/downloads/article/N/en-nitrogen-blanketing-article-312-11-013.pdf