Auto-Ignition considerations? 1

[Bourbon103]

I have a relief valve on a reactor that we are considering a fire scenario, 85% filled with heptane. My concern is that the normal auto-ignition temperature of heptane is ~399F-433F and at the set pressure of 50 psig, the corresponding vapor temperature is 320F. I have read that a species' auto-ignition temp. lowers with increasing temperature so I am wondering if at 50 psig the auto-ignition temp. would be at or near the corresponding temperature. Thoughts?

 

[Latexman]

AIT is a standard test for relative hazard comparisons, not a fundamental property. There is an induction time associated with a specific size sample at a specific temperature to ignite. For example, a 0.5 liter sample may ignite at 600 C in 10 seconds and an identical 0.5 liter sample may ignite at 800 C in 0.1 seconds. Also, a 0.5 liter sample may ignite at 500 C in 100 seconds and a 100,000 liter sample (vessel) may ignite at 300 C in 100 seconds.

Good luck,
Latexman

 

[Bourbon103]

Ahhhh, good point. Analogous to evaporation rates, I assume. Thanks for the help.

 

[GL431]

Bourbon, I cannot say whether the AIT comes down with increasing temperature (I thought it stays the same). But what interests me, is why AIT is a criterion in your case. I understand that the boiling temperature of heptane at 50 psig is 320F as you say. But as soon as the vapor exits from the PSV the pressure will be atmospheric and the actual vapor temperature will be lower (because of the drop of pressure from 50 psig to atmospheric). This would take you further away from the AIT. Also if your PSV discharges into a flare system - and not into atmosphere - AIT does not become a criterion, no?

 

[MikeInSWLa]

I would like to add that, even if the discharge is below the AIT, that dooes not mean that you are out of the woods as far as the dischagrge being safe (assuming it is going to atmosphere). It is still possible that a vapor cloud could be formed and it could find an ignition source. Heptane, if I am not mistaken, is heavier than air, so it has the potenmtial to migrate to the ground and build up, ans possibly form a cloud within the flammable range.

To determine if you can safely discharge this to atmosphere, you would need to determine how the vapors would become dispersed upon release. If the velocity is hight enough there could be jet-mixing, which would better disperse the vapor and reduce (but not eliminate) the possibility for ignition. API 521 gives good guidance on this and also tells you how to calculate whether jet mixing would occur.

 

[BenThayer]

since this is for "fire scenario" sizing, i would assume that an ignition source is present.

 

[dcasto]

BenT, I agree there, then ask, why worry about auto ignition too, as a matter of fact I'd rather see an auto ignition instead of building up a cloud that WILL ignite.

HOWEVER, the auto ignition would be of a concern with other scenerios, non fire thermal, valve failure, tube rupture ect...

Back to 521 or the need for a vent line/header.