Propane - Nitrogen

[bbl585]

Hello

With reference to this thread:thread124-181137

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.

 

[JLSeagull]

If the propane is at an elevated pressure the autorefrigeration could reduce the outlet of a pressure relief valve below -20 degree F.

 

[zdas04]

bbl585,
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

 

[bbl585]

It is liquid propane. The concern is that the temperature might be lower than the atmospheric boiling point of propane due to the forceful blowing through of nitrogen.

 

[dcasto]

you ahave zero need to worry about auto refrigeration and low temperatures. propane at -49 C would have to be at under 12 psia, or equal to over 5000 feet altitude. PLUS, since there is no hoop stress, there is nothing to cause the pipe to even remotely fail. I've blowndown ethylene lines at -120F and then put them in servive for the last 25 years.

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.

 

[bbl585]

Dear dcasto

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

 

[dcasto]

look at the charts for propane, at 14.7 the boiling point is -40F. What do you think they do in Moose Jaw Canada when the outside temp is below -40F, do they just shutdown any pipeing and abondon it?

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.

 

[sshep]

I think there is a chance of getting lower than -40F boiling point at the place where liquid is evaporating if the N2 blowing across it at a good rate. This is evaporative cooling. I think the risk of getting too far below the boiling point is low, but not sure. It depends on diffusion and heat coming from the outside- i.e. evaporation has to get its heat from somewhere.

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

 

[Compositepro]

This problem can be avoided by controlling the tank pressure during blow-down. Why would you use nitrogen until all the liquid had vaporized? The main danger for the piping and vessels is large temperature gradients more so than the brittle point of the steel. There will be large stresses between cold and warm parts which depend on the structural design and where the liquid pockets are.