Method for rating relief requirements for a CO2 vaporizer?
Method for rating relief requirements for a CO2 vaporizer?
(OP)
I would like advice for the blocked in case for the CO2 side of a vaporizer. The control logic will allow the steam side to reach the header steam pressure. As I understand it the blocked-in venting starts as a liquid expansion case, but as soon as the valve opens vaporization begins and heat transfer increases. Since the heat transfer coefficient on the steam side is the same order of magnitude as on the CO2 side, I'm guessing the overall U value from the exchanger rating form is not necessarily at it’s peak. It underestimates if it includes fouling factors and it may incorporate the subcooling zone and a superheat zone.
I'm thinking the rigorous approach is to do a sensitivity study with heat exchanger rating software, omitting fouling factors, to find the peak local vaporization heat transfer coefficient from the incremental analysis, with the outlet at 110 % of set point and the steam side at full steam pressure. The peak local value from both sides of the tube would be used to compute the maximum overall heat transfer coefficient.
Is there a simplification I can make that I have overlooked?
I'm thinking the rigorous approach is to do a sensitivity study with heat exchanger rating software, omitting fouling factors, to find the peak local vaporization heat transfer coefficient from the incremental analysis, with the outlet at 110 % of set point and the steam side at full steam pressure. The peak local value from both sides of the tube would be used to compute the maximum overall heat transfer coefficient.
Is there a simplification I can make that I have overlooked?





RE: Method for rating relief requirements for a CO2 vaporizer?
Tube rupture case with steam bursting into liquid CO2 not valid, and why ?
RE: Method for rating relief requirements for a CO2 vaporizer?
RE: Method for rating relief requirements for a CO2 vaporizer?
RE: Method for rating relief requirements for a CO2 vaporizer?
Is there any literature you could point me to about using the design U as an acceptable short cut? Maybe I overlooked it, but I didn't find API 521 discussing that approach. They explicitly do remark on ensuring the fouling factors are removed from the calculation.
RE: Method for rating relief requirements for a CO2 vaporizer?
RE: Method for rating relief requirements for a CO2 vaporizer?
Steam flow is driven by the amount of energy that can be consumed on the process side (similarly to distillation column reboilers). Once the CO2 flow reduces, the flow of steam will reduce as well. Assuming equal Q (heat exchange) in the blocked-in case would probably result in a greatly oversized PSV.
Dejan IVANOVIC
Process Engineer, MSChE
RE: Method for rating relief requirements for a CO2 vaporizer?
I agree that there are some cases that justify a more rigorous analysis. Specifically, those are cases in which a rigorous analysis could be justified based on cost (e.g. much smaller relief device) and/or risk reduction (e.g. less hazardous material released to atm). However, in most applications I find that rigorous analyses are not justified. In every case the user is responsible for assessing whether that applies to the specific case at hand.
RE: Method for rating relief requirements for a CO2 vaporizer?
RE: Method for rating relief requirements for a CO2 vaporizer?
don1980, I fully agree with you - he can't go wrong with the PSV designed for the full/design heat transfer rate (reduced for DeltaT ratio at Relief/Design conditions) - if he wants to make sure the equipment is protected against overpressure. I just wanted to make a point that the PSV will likely end up greatly oversized, because heat transfer normally diminishes substantially if the flow of cold fluid is reduced or stopped completely.
We all know this from operating distillation columns, when e.g. reboiler outlet gets flooded and the circulation stops. The amount of heat pumped into the system at these conditions is well below the design rates. Now, this is a different systems and capable of generating very high pressures upon absorption of heat, but some general heat transfer concepts still apply, in my opinion.
Dejan IVANOVIC
Process Engineer, MSChE