Hello,
There is sour emulsion storage tank system that will be trucked out every day. The idea is to tie the truck vent to low low pressure flare system so as not to vent the sour gas from the truck vapour space (due to out breathing) to the atmosphere.
It is reported by the client that vapor space in the truck has 20% of the LEL. Once connected to LLP flare system, the truck vapour stream is mixed with the flashed gas from the storage tank as well as flare purge gas. I would like to confirm if this mixture can create a hazardous condition by 1) making an explosive mixture 2) causing flash back from the flare flame. Following are the criteria that i believe applies:
1) In order to prevent the formation of explosive mixture, I believe the hydrocarbon concentration should be above UEL (rich in hydrocarbon), which is about 15% for methane and less for other hydrocarbons (i.e. air concentration should be below 85% ).
2) However, there is criteria in API-521 that the oxygen level should be less than 1% below the buoyancy seal (translating to less than 5% air) and 4 to 8% for velocity seal (less than 20 to 40% air)
These two criteria has a significant difference in terms of air concentration to be achieved. If I stick with the second criteria, I should inject a huge volume of fuel gas to reduce the air concentration.
I would like to know where the second criteria come from and which criteria I should use in my calculations.
Thanks for your help in advance,
Amin