Grasshopper
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- Jun 28, 2012
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Reading threads concerned with the risk of implosion of a carboy under vacuum, most recently the threads on vacuum degassing, I am left with the impression that some people may not be thinking about this risk correctly. Specifically, some comments suggest that implosion risk can be significantly reduced by decreasing the vacuum from 25 inches of Hg to 20 inches and that the risk can be further reduced by pulling vacuum only on a carboy full to the shoulder with liquid. With the caveat that I might not be looking at this correctly either, let me offer my thoughts.
It is not the vacuum on the inside of the carboy that can cause failure but the higher pressure on the outside pushing in. The driving force for failure is the differential pressure across the walls (the delta P). For ease of calculation, I am going to approximate the atmospheric pressure at 15 psi and full vacuum at 30 inches of Hg in construction of the following table.
.....Vacuum.............Inside Pressure............Delta P
.... 30 (in. Hg)........... 0 (psi).................. 15(psi)
.... 25 ..................... 2.5 ..................... 12.5
.... 20 ..................... 5 ........................ 10
Thus we see that the difference in force across the walls of the carboy from 25 to 20 inches vacuum is not that large, 12.5 psi vs. 10 psi and that there is not all that much difference between full vacuum and 20 inches (15 vs. 10). Thus, if there is a significant risk of implosion at full vacuum, we shouldn’t feel all that comfortable at 20 inches. Unless we know from experimental data or reliable calculations what the failure point is likely to be, we really have no idea how comfortable/uncomfortable we should be. If the failure point turns out to be 100 psi then I would likely be comfortable even at full vacuum but if it turns out to be 20 psi, then I am going to be uncomfortable even at 20 inches knowing that flaws, scratches etc. will reduce the fail point. Personally, I think that the failure point is higher rather than lower given the lack of reports of carboy implosions on this forum (failures of thin wall demijohns not withstanding) but that is just a guess.
Filling the carboy with liquid may reduce the consequences of implosion (see below) but doesn’t change the probability of failure all that much. The increase in pressure at the bottom of a full carboy due to the liquid is about 0.7 psi (about 18 inches of water) so at 20 inches of vacuum, the delta P at the bottom is only reduced from 10 to 9.3 psi.
Implosion implies a failure where the glass shards from the crushing failure caused by excessive external pressure are accelerated to a velocity sufficient to create a flying glass hazard. The liquid in a full carboy will dissipate the energy of the flying glass and eliminate this hazard (if not the hazards associated with spilling 6 gallons of red wine on a white rug).
So how else can we mitigate the hazards of flying glass? In chemistry lab we used to tape the outside of vacuum receivers so that the tape would keep the glass from flying in the event of an implosion. Likewise one could tape the outside of a carboy with duct tape or clear packaging tape. Or one could put the carboy inside a bag or pillow case to stop the flying glass. I use a plastic garbage bag for this which I think is probably sufficient to minimize the hazard. I also keep the carboy in a secondary container to minimize the mess from spilled liquid.
Others may have better methods.
It is not the vacuum on the inside of the carboy that can cause failure but the higher pressure on the outside pushing in. The driving force for failure is the differential pressure across the walls (the delta P). For ease of calculation, I am going to approximate the atmospheric pressure at 15 psi and full vacuum at 30 inches of Hg in construction of the following table.
.....Vacuum.............Inside Pressure............Delta P
.... 30 (in. Hg)........... 0 (psi).................. 15(psi)
.... 25 ..................... 2.5 ..................... 12.5
.... 20 ..................... 5 ........................ 10
Thus we see that the difference in force across the walls of the carboy from 25 to 20 inches vacuum is not that large, 12.5 psi vs. 10 psi and that there is not all that much difference between full vacuum and 20 inches (15 vs. 10). Thus, if there is a significant risk of implosion at full vacuum, we shouldn’t feel all that comfortable at 20 inches. Unless we know from experimental data or reliable calculations what the failure point is likely to be, we really have no idea how comfortable/uncomfortable we should be. If the failure point turns out to be 100 psi then I would likely be comfortable even at full vacuum but if it turns out to be 20 psi, then I am going to be uncomfortable even at 20 inches knowing that flaws, scratches etc. will reduce the fail point. Personally, I think that the failure point is higher rather than lower given the lack of reports of carboy implosions on this forum (failures of thin wall demijohns not withstanding) but that is just a guess.
Filling the carboy with liquid may reduce the consequences of implosion (see below) but doesn’t change the probability of failure all that much. The increase in pressure at the bottom of a full carboy due to the liquid is about 0.7 psi (about 18 inches of water) so at 20 inches of vacuum, the delta P at the bottom is only reduced from 10 to 9.3 psi.
Implosion implies a failure where the glass shards from the crushing failure caused by excessive external pressure are accelerated to a velocity sufficient to create a flying glass hazard. The liquid in a full carboy will dissipate the energy of the flying glass and eliminate this hazard (if not the hazards associated with spilling 6 gallons of red wine on a white rug).
So how else can we mitigate the hazards of flying glass? In chemistry lab we used to tape the outside of vacuum receivers so that the tape would keep the glass from flying in the event of an implosion. Likewise one could tape the outside of a carboy with duct tape or clear packaging tape. Or one could put the carboy inside a bag or pillow case to stop the flying glass. I use a plastic garbage bag for this which I think is probably sufficient to minimize the hazard. I also keep the carboy in a secondary container to minimize the mess from spilled liquid.
Others may have better methods.
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