There are basic hazardous conditions and misunderstandings within this topic and I am forced to repeat some facts that I’ve made known in past threads regarding the storage, use, and application of liquid and gaseous CO2.
Gsgopi makes some misleading (?) statements in his/her original post. This is understood as a lack of necessary process training and understanding of phase equilibria and thermodynamics. Both subjects have a lot to do with the application (which is not accurately described). Gsgopi states: “I am using liquid co2 in my setup for cooling inside the oven”. This doesn’t say what are the conditions of the liquid CO2. It makes a big difference in understanding what he is doing without a flow diagram or P&ID resource.
It is further stated: “The 300psi regulated Co2 gas is passed into VT-300 satellite reservoirs which gives liquid co2 as output.” This then implies that the liquid CO2 fed to the oven must be low pressure, liquid CO2 at approximately 250 – 300 psig and -8 to +1 oF. This cold, liquid CO2 pick up latent heat of vaporization and is recovered as a saturated gas by the “VT-300” apparatus described in the referenced website.
The VT-300 is apparently nothing more than a recovery unit comprised of a refrigerating unit and a liquid reservoir and it recovers the generated CO2 vapors for temporary storage and subsequent use later. It apparently lacks the refrigeration capacity to maintain the captured CO2 in the liquid state when subjected to heat gain from the surrounding ambient conditions. The OP fails to state this and this has to be assumed. This badly described data has caused misconception and wrong advice from the subsequent posts. This just causes confusion, but the most important point here is that the VT-300 unit apparently has been either badly specified prior to purchase or has been sadly designed.
No low pressure liquid CO2 tank (which is the international norm for storing CO2) should be equipped with a low-cost rupture disc. The reason for this is clearly seen and understood by studying the thermodynamic properties on the valuable information furnished by our old friend, Morten. This basic data should be well-understood because it is the underlying information necessary to avoid a hazardous situation or to mitigate it when dealing with a liquefied, compressed gas such as CO2. Morten is almost correct in his comment referring to the result of venting liquid CO2. Whenever liquid CO2 is vented, the result will be DRY ICE (solid carbon dioxide at -107 oF). This is the same effect that is seen when a CO2 fire extinguisher is actuated during a fire. However, the fire extinguisher is at ambient temperature (80 oF) and consequently at approx. 1,000 psig. In this case, the liquid CO2 is at 300 psig (and +1 oF) and when it expands due to the sudden release of a rupture disc, the liquid in the reservoir is converted to dry ice at -107 oF. This is OK, as long as the parent metal of the reservoir is made with properties to resist such a low temperature without undergoing subsequent brittle fracture (as is the case with normal carbon steels). This is a “stealth” type of hazard that can occur if the reservoir metal cannot withstand such a temperature “shock”. Normally, design engineers take this into consideration when confronting this type of possible scenario and fabricate the vessel out of low-temperature steel or stainless steel (more expensive!). Another solution is to install safety pressure relief valves (two) that reseat after relieving the excess pressure. However, these are more expensive than a simple rupture disc. One can quickly conjure up what a supplier would supply if left to sell the most “competitive design” due to a lack of specifications or data sheets from the buyer. The lack of demonstrated process knowledge in this application raises my concern and my writing this post.
In my opinion, both Compositepro and Morten raise a valid and important safety issue regarding this application. I recommend that VA-Tran Systems be advised what is happening with regards to their equipment and an inquiry made as to what is their opinion and what can be the consequences of these over-pressure incidents. Certainly a PSV can replace the rupture disc; but what has occurred with the ability of the reservoir to safely continue to store the liquefied CO at 300 psig? If there is a justifiable reason for continued storage of LP CO2 liquid, then additional storage refrigeration is required – which translates into more capital monies.
The comment by chicopee does not apply because what is clearly being used here is low pressure, saturated liquid CO2 at 300 psig and not high pressure liquid CO2 at ambient temperature. The HP CO2 would not work in this process.
