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Nace Applicability Criterion For Liquid Streams

nace h2s partial pressure

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#1 Dieguito2


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Posted 10 October 2018 - 08:30 AM

I have a doubt regarding on how to apply Annex C.2 of NACE ISO 15156-2 2009 Standard. For a gas free liquid system, the H2S partial pressure for assess the applicability of the Nace Standard, i.e. pH2S>0.05 psia, is calculated at a pressure corresponding to the bubble pressure at the operating temperature (Pb) and for the H2S molar fraction of the gas phase in equilibrium with the liquid at the bubble point (xH2S). Thus: pH2S=pB*xH2S. In my application I have a bi-phase liquid stream HC+H2O. What bubble pressure I have to consider the one related to the HC phase or the one of H2O phase? Since SSC corrosion can happens only in presence of an aqueous phase, to me it seems more correctly to consider the equilibrium bubble pressure of the H2O phase alone, but Annex C of Nace standard states: “For a liquid-full pipeline downstream of gas separation units, a good approximation for bubble-point pressure is the total pressure of the last gas separator” That is it seems to consider the bubble pressure of the HC phase. Regards

#2 Technical Bard

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Posted 12 October 2018 - 11:04 PM

The NACE method means you need the composition of H2S in the bubble point vapour.  So instead of your equation, it should be pH2S = pB * yH2S, where yH2S is the mole fraction of H2S in the vapour phase that forms at bubble point.  Using a simulator, setting the vapour fraction to 0.001 will generate a small vapour fraction with an accurate composition.  The simulator will handle both the H2S evolution from the oil and water phases.  If you don't have a simulation, the vapour composition in the last separator could be used.  This method uses the partial pressure in the vapour phase as a proxy for the sulfide ion concentration in the liquid (which is what really matters to the corrosion and cracking mechanisms).


If you look in the other NACE standard (MR0103 - ISO17945), you will also find some other criteria for liquid systems, ppmw of H2S in the aqueous phase, ppmw cyanide and pH that could lead to sulfide stress cracking.  

Edited by Technical Bard, 12 October 2018 - 11:06 PM.

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