DIERS 2

[Zebedee1]

The simplified Leung method for evaluating the quantity and quality of two phase flow from vessels exposed to external fire gives extremely high values for liquid flow with very small vessels (25-200 litres in our case) and consequently disproprtionately large vent headers. Flowrates calculated indicate that vessels would be emptied in seconds. Our alternative approach of calculating Swelled Volumes, using the equations of M.A.Grolmes, yields results that seem more sensible. Anybody come across a similar situation?

Our approach has been to compare swelled volume with safety factor of 5% added, then to compare this with actual vessel total volume. If the former is less than the latter then it is assumed that there is full liquid droplet disengagement within the vessel. However not withstanding this, the conservative approach is taken of still assuming two phase flow into the vent with 1% v/v liquid. This gives much lower results than Leung.

 

[StoneCold]

I have brushed up against this topic recently but you are way out of my league. We ended up going with a consulting firm that does a lot of work in this area. Their name is Iomosiac, they can talk to you all day long about DIER's if you want. The owner is the guy to talk to. I guess as a standard disclaimer I should say I am not associated with them in any way except as a former client. I can't answer your question but knowing who to talk to is often half the battle.

Regards
StoneCold

 

[niallmacdubhghaill]

I have found that with small vessels you will have two factors that will result in what seems like an unrealisticly large vent size.
First, with a small vessel you do not get the liquid turnover which results in an amount of bulk cooling, thus reducing the required venting rates. Secondly, the amount of level swell can seem to be disproportionately large, I have found that this again is due to the vessel size. What I think happens is because of the small size of the vessel, you very quickly move into a nucleate boiling regeime causing the liquid in the vessel to flash off quickly. Also, are the tanks operating at a low pressure-around atmospheric pressure? The low pressure differential is often a requirement for seemingly very large relief sizing. If you are tempted to reduce the size of your vent, be very, very sure about this. You need a very solid reasoning behind this move.