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Hydrate Formation During Depressurisation Activity

hydrate formation depressurisation

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#1 TruptiS


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Posted 06 August 2019 - 12:48 AM

Dear All,


I have performed depressurisation calculation in TGnet software for a 32", 33 km buried gas pipeline operating at 90 bar (g) and depressurisation initial temperature is 20°C. The depressurisation time calculated in TGnet is 60 hours.


It is manual stage wise depressurisation performed by sequential opening of the valves/RO's after certain time intervals in such a way that the peak flow doesn't go above design flowrate of the existing vent stack.


I wanted to check whether there was hydrate formation during the depressurisation. So I plotted the graphs of TGnet depressurisation and Unisim hydrate formation together in an excel sheet.


The hydrate formation curve is pressure v/s temperature whilst the depressurisation curve is pressure/temperature v/s time. Hence, I have plotted the temperature/pressure points (at respective time) of depressurisation in the hydrate formation graph in an excel sheet and I see both the graphs intersecting each other. It means that there will be hydrate formation during the intial phase of depressurisation.

Please find the graph attached.


I wanted to know whether this is a correct method of checking hydrate formation during the depressurisation activity. 


If yes, what are the other prevention measures against hydrate formation other than methanol injection.


Anyone has done similar activity and has any experience regarding this matter. Please share.




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#2 PaoloPemi


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Posted 07 August 2019 - 11:45 AM

it is difficult to comment as you do not indicate how you calculated hydrate formation temperature (or pressure), also I cannot guess if values are correct since you do not provide the composition of mixture (required for hydrate phase equilibria),

anyway, to give an aswer I assume that inlet is ever in gas phase with constant composition, if that is the case you can simply calculate hydrate formation temperature at the different pressures calculated during depressurization (I guess by TGnet), 

to verify formation of hydrates you can calculate hydrate formation pressure (or temperature) or solve a multiphase flash (at specified P and T)  including hydrate phase, solving a multiphase flash allows to estimate hydrate fractions,

as general rule with hydrates : better to be conservative as the errors (i.e. calculated hydrate formation pressures  vs experimental)  depending from formers, structures and contions can be high (see https://www.prode.com/en/hydrate.htm  for some estimates)

to answer the other question, yes there many methods to reduce the risk, inhibitors, heating  my (old) copy of GPSA gives some indication, also you may find useful books as Clathrate Hydrates of Natural Gases, Natural Has Hydrates etc. both books provide methods to estimate hydrate formation conditions,

the selection of most convenient method to adopt to reduce the risk of hydrate formation  should consider the resoirces available at site (for example the cost fo energy etc.)

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