CONCERNS ABOUT LCO2 PIPELINE 1

[gladkenn]

Can pumping LCO2, 350 meters away be ok without fear of vaporization and freezing P/L?

 

[Montemayor]

gladkenn:

There are several things missing from your query:

1) What are the conditions of the liquid CO2? Is it High Pressure liquid (above its critical pressue, -1,071 psia, & below its critical temperature, 88 oF)? Or is it Low Pressure liquid (usually around 225 -250 psig & -10 oF)? It can be anything in-between these conditions, as long as it is above the triple point, where dry ice starts to form.

2) Is your 350 m P/L buried? or is it above ground?

3) What do you mean by "freezing" your P/L? Do you mean you expect to get below the triple point (where solid dry ice starts to form)? Or are you referring to the freezing of the humid air or dirt on the exterior of the P/L? If the CO2 is "neat" (no other components), the only way you can freeze the interior of the P/L (in a true, engineering sense) is to get below the triple point (75 psia & -70 oF).

Pumping LP LCO2 a distance of 350 m is no sweat in a properly insulated and maintained line. This can be done with a minimum of pressure drop and vaporization. If you can't tolerate vaporization, stay away from pumping the saturated liquid (which is what I've mentioned above). To eliminate vaporization you must furnish subcooled liquid to the degree that it can absorb any heat pickup as sensible heat without reaching the boiling point at the pumping pressure. Of course, the less the pressure drop (the bigger the diameter) of the P/L, the less work done on the liquid and the less the heat pickup.

So, to answer you basic question: Yes, you can pump LCO2, 350 meters away without fear of vaporization and freezing the P/L --- as long as you stay within the parameters I spell out above. Without any basic data to base myself on, I can't furnish any other comment or recommendation.

Art Montemayor
Spring, TX

 

[gladkenn]

Thank you for the answer Art Montemayor!
I apologized for the lack of details. I am actually trying to transport LCO2 liquid(250psig, -8 oF) from one tank to another above ground.One thing that concerns me though is that the tank, 350 m away, has no condenser unit. Only a vaporizing unit for CO2 usage. The volume of LCO2 that can be transported may be lesser due to the evaporation inside the destination tank. Can you enlighten me about this?

 

[Montemayor]

Glad:

Saturated liquid CO2 at -8 oF has a pressure of 266.58 psia (251.88 psig at average sea level elevation) so it might appear that you do not have saturated liquid. Although you didn’t state it, I’ll assume you have saturated liquid anyway – that’s the normal method of storing and transporting LCO2 in the industry.

If you can’t subcool your LCO2 and you have no means of condensing your generated vapor at the target tank, you have reason for being concerned about the results from the proposed pumping method. Since I don’t know the nature of the target tank, its MAWP, its vaporizing load, and whether the vapors from that same tank are constantly and steadily being withdrawn, I would advise you against pumping saturated liquid through the 350 m distance. I also don’t know the size of the transport pipe, the type of pump, the piping configuration, the flow rate, etc., so I don’t even have an idea of the quantity of heat that can be potentially generated during the operation.

You state that “The volume of LCO2 that can be transported may be lesser due to the evaporation inside the destination tank”. Less than what? The hazard event that can develop (as I think you already know) is that you will generate sufficient saturated vapor in your operation so as to overpressure the target tank and all connected piping and equipment. In my opinion – and more importantly, due to my experience in the LCO2 business – you must not depend on any safety relief valves to “save the day” by relieving the excess, generated vapor. This LCO2 application is a very unique and precarious design situation and it will take me at least another paragraph to explain to you and others why it is so and what can occur to generate even more serious hazards.

When a LCO2 system is relieved of over-pressure by a conventional safety relief valve, a rapid release of the 2-phase system occurs that often entrains sufficient quantities of the saturated liquid into the PSV orifice itself. This event is more pronounced if it occurs in a pipe. As soon as the LCO2 (not vapor CO2) is allowed to expand across the PSV orifice, dry ice is formed – i.e., you have expanded below the triple point and are now in the zone of SOLID CO2 (-109 oF). The dry ice immediately starts to dam up and jams the PSV in the OPEN position, not allowing the spring to re-close it and, as a result, the vented system is reduced down to essentially atmospheric pressure. There is nothing you can do to avoid this from happening, once it starts. All you can do is stand there and see your inventory in the system depleted and the involved equipment reduced in temperature down to -109 oF. Here is where the situation turns hairy and dangerous. If you, like everyone else I know, have your tank and piping constructed of normal steel – such as A516 and A53 & 106 – then you have developed some serious carbon steel brittle failure possibilities. Once carbon steel is subjected to such abnormal, low temperatures, its crystalline structure is affected and the Charpy V-notch test verifies that it is unsuited for pressure applications. I have seen this happen more than once and it’s happened to me as well. The situation is more common than most people will admit because of the expensive results: the affected carbon steel is deemed useless and dangerous for the application and has to be scrapped – this includes the tank. You don’t want to go there.

You should either:

1) size the pumping operation appropriately to reduce the heat input
2) reduce the amount of pumped LCO2
3) ensure that the formed saturated vapor is safely vented elsewhere.

I hope these observations and comments are of some help.


Art Montemayor
Spring, TX