Propane - Nitrogen
Propane - Nitrogen
(OP)
Hello
With reference to this thread:thread124-181137: Safe operating conditions for Autorefrigeration potential
if there is propane in pipework and one wants to blow this propane out with nitrogen to a vessel, would there be a concern that the temperature in the pipework would drop to below the MDMT? The pipework material is charpy rated for minus 49°C. As I understood the discussion in that thread (which is closed), because one does not have propane liquid in contact with equilibrium propane vapor, that the temperature would not be as normally predicted. Related to this, does anybody have a usable reference to propane-N2 VLE data? Or LPG-N2 VLE data?
Thanks for your feedback in advance.
With reference to this thread:thread124-181137: Safe operating conditions for Autorefrigeration potential
if there is propane in pipework and one wants to blow this propane out with nitrogen to a vessel, would there be a concern that the temperature in the pipework would drop to below the MDMT? The pipework material is charpy rated for minus 49°C. As I understood the discussion in that thread (which is closed), because one does not have propane liquid in contact with equilibrium propane vapor, that the temperature would not be as normally predicted. Related to this, does anybody have a usable reference to propane-N2 VLE data? Or LPG-N2 VLE data?
Thanks for your feedback in advance.





RE: Propane - Nitrogen
RE: Propane - Nitrogen
The first response in the referenced thread was important--auto-refrigeration is a phase-change effect. If your propane is all gas, then blowing it out with nitrogen is a reasonable way to create an inert atmosphere. If it is liquid, then you have to be concerned about flashing and the coinciding refrigeration. You didn't say what phase the propane is in.
David
RE: Propane - Nitrogen
RE: Propane - Nitrogen
NOW, the issues are what kind of stress will the pipe encounter because of contraction due to the cold, heck, if the line was install at 110 F and the tewmp was -20F (safe for ordinary carbon steel) then installing a line when it was 40F out and going down to -50F has less contraction than the first case.
RE: Propane - Nitrogen
Would you say it is safe to blow out liquid propane from a long pipe of SA-333 material (10" pipe, approx 40 t of propane) with nitrogen?
The total pressure (i.e. the partial pressures of nitrogen and propane added) will go down from the starting pressure (approx. 116 psig) to atmospheric. The way I understood it, the liquid propane would chill to the bubble point temperature corresponding to its partial pressure, which in this scenario could be below 14,7 psia. The liquid propane would cool the material it is in contact with, in this case the pipe wall, down as it itself is chilled. I do not know what that temperature would be, but it could be lower than minus 45 °F, I think.
Would the piping be safe for the duration of such an operation, which could last a few hours as I expect?
Do you know of a case (industrial/pilot/lab) in which this has been done either intentionally or unintentionally and whether it was safe or unsafe? Unfortunately I do not have reference cases in which this has been done and that is what I am searching for.
I think I understand what you are saying in your first response above, I just want to make sure that you understand the data for my case.
Thanks in advance
RE: Propane - Nitrogen
Ever hear of cryogenic heat treating?
http://www.metalscience.com/techinfo_ASM.php
If you look into B31.3, there is a section on low temperature zero to low stress piping. There are 10,000 plants that have propane refrigeration systems with -20F rated steel that blowdown that piping and the steel goes to -50F, there is no issues.
RE: Propane - Nitrogen
To see the power of evaporative cooling simply get wet and stand in the wind. You feel cold. Question: How is this possible when water boils at 212F? Answer: evaporative cooling. You are standing in a draft of air (not a draft of steam) so you cool down to the dew point rather than the boiling point.
best wishes,
sshep
RE: Propane - Nitrogen