1 reply to this topic

[Conchubhar]

Hi all,

Apologies if this has been discussed before, i just cant a thread on it.

I am sizing a relief system for a fire-case scenario, based on a reactor full of solvent, and am unsure whether to size based on single or 2 phase relief.

The standard proecure in my company is to firstly employ level swell calculations, based on the HSE workbook/DIERS.
When i do these calcs i get 2-phase flow for my system, and hence a large relief system.

I read a paper by Fauske today, "Properly Size Vents for Nonreactive and Reactive Chemicals", in which he says that for non-foamy/reactive systems, during a fire boiling at the walls of the vessel will only occur, hence actual level swell will be minimal, and 2 phase flow is unlikely to happen.

I'd like to hear people's thoughts on whether single phase relief models are acceptable for relief of non-foamy, non-reactive system, and if it is, how justified.

Thanks!

[DalMc78]

I think this is the only thread on this topic and I am a bit suprised. I have also come across a similar problem for low pressure storage tanks which are currently operated at levels which would give two phase flow according to the standard DIERS method for estimating the onset of 2 phase flow where fully dispersed bubbles are assumed.

In answer to your question - I think the answer may be yes. You need to consider if the act of pressure relief will cause the pressure to drop and so "bump" liquid causing it to froth into the vent. Im sure the reference you talk about states that it is only applicable to low pressure vessel for this reason.

There are however further aspects to this which I hope I could get some guidance on from the more experienced posters.

DIERS (Appendix 1-B6) says:

"For essentially liquid filled atmospheric storage vessels where little or no over pressure can be tolerated, an emergency relief device can be sized for vapour flow for a non reactive scenario (i.e. fire exposure) if the vapour velocity into the vent at the swelled liquid height is less than that critical entrainment velocity and the vessel contains a non-viscous and non foamy fluid."

The problem to me is fully evaluating the level swell which is caused by liquid thermal expansion and volume increase due to bubble voidage in the zone near to the vessel wall. Fauske et al have written a number of papers (see http://www.fauske.com/chemPapers.html) on the subject of two-phase onset. The one referenced in the DIERS section above, "The onset of Two-phase venting via entrainment in liquid filled storage vessels exposed to fire", says the effect of level swell due to bubbles can be essentially be ignored.

Another of the Fauske papers written but not available on the website, ”Vapour-Liquid Disengagement in Atmospheric Liquid Storage Vessels Subjected to External Heat Source”, allows the level swell due to bubbles at the walls to be evaluated but uses data for water only. Chart data would need to be extrapolated to use fluids other than water. I am not entirely confident of the applicability of doing this but to me is is better and more conservative to calulate the swell due to bubbles rather than ignore the effect.

The DIERS method seems to be overly conservative regarding tank inventories to avoid two phase flow in a fire relief case as it is explicitly intended for use where bubbles are present throughout the liquid. It would be good to find a reference that summarises the necessary approach for low pressure vessel fire cases but I have not been able to find any.

Do others think the Fauske "Bubble swell" method described can be generalised to cover other liquids via extapolation? Is it safe to ignore bubble level swell as stated in the DIERS Fauske reference?

Thanks.