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Clearing Propylene With Nitrogen

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#1 Rick2018@@@


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Posted 09 December 2018 - 07:45 AM

What risks or dangers are there to consider when purging propylene from a vessel or pump with N2. I.e.... after venting off pressure from a bullet vessel or pump if there is liquid lying in the bottom what might any risks be when needing to use N2 to push the small amount of liquid propylene out the bottom. To put it another way, will the added pressure from the incoming N2 cause the propylene to change in any way as to harm the pump or metal. Thanks.

#2 Technical Bard

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Posted 09 December 2018 - 12:00 PM

None, so long as the nitrogen isn't higher pressure than the vessel MAWP...

#3 Art Montemayor

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Posted 09 December 2018 - 01:51 PM



I would inert your Propene tank as I would all the liquefied petroleum liquids I’ve done in the past; with consideration and caution about any residual, saturated liquid left in the tank at normal storage pressure.

You don’t tell us the whole story of the scenario.  For example, what are the vessel conditions (as well as any connected equipment, such as pump(s), etc.) with respect to contents, pressure and temperature?  You must identify the phases existing inside the vessel and connected equipment.  This is important in order to know just what the dickens it is that you are initially going to purge out of the tank when you subject it to nitrogen pressure.


You are very correct in being primarily concerned with any potential dangers or hazards involved in such an operation and taking the time to discuss and analyze the situation.  The only hazards involved with nitrogen in this operation are, as Technical Bard states, the nitrogen pressure and - I would add - the possibility of asphyxiation (but that should be remote if handled properly).  The main concern I would have is the identification of the propene phase(s) existing in the system.  If your tank is at normal operating pressure and temperature, there is a possibility of liquid propene still existing in the system.  You can either prove or disprove this by level readings or slight drainage at the lowest system point.  If you are not sure, I would depressure the system through the lowest point through a manual expansion into a vessel connected to your flare system where you normally will burn the purged gases.


You have to be careful in depressurizing your propene vessel depending on the amount of any liquid inside.  Refer to the attached thermo-physical data on propene and you can easily see the resulting low temperature you can generate if you vent the propene gases first instead of the liquid.  Depending on the situation, you could generate a vessel temperature below the normal -20 °F the assumed steel material can take.  This is one of the problems you want to avoid and why disposing of any propene liquid is the main problem.  Once all liquid is disposed of safely, you can proceed to sweep the vessel with nitrogen to your heart’s content without any risk - as long as you don’t exceed the vessel’s MAWP (which I assume you know perfectly well).  Planned and strategic points of injecting the inerting nitrogen are always the smart way to inert the vessel quickly and safely - with continuous analysis of the propene contents in the exit purge.


I hope this experience helps out.

Attached File  Propylene Thermo-Physical Properties.xlsx   367.28KB   14 downloads


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