At 3% LHE loss per day that would have been some time back offshore.
Reclaim went HPG a long time ago with minimal gas loss. Well as long as it was tested with the diver still sitting on the bell stage that is. (Oil Endeavour DSV for Sub Sea International Forties Bravo 1979 Krasburg system)…
I was talking about the liquid Nitrogen used as a refrigerant to reclaim pure helium from chambers, not closed-circuit surface-based diver recirculating systems like Divex. It's been a while since I installed my first units, but the vacuum on Dewars has not improved significantly. Today, most offshore operations I know of are either using chemical filters for reactive gasses only or membrane helium purifiers for mid-range purity — mostly because they are “good enough” and supplying LN
2 offshore is a huge PITA.
Cryomech - Helium Recovery System
Here is an image of Dewars used offshore:
For others reading this, gas from saturation diving chambers is vented to atmosphere and captured in big flexible bags. It is primarily Helium and mostly “contaminated” with O
2, N
2, CO
2, water vapor, miscellaneous “bio-generated” gasses, and maybe oil vapor. A HP compressor sucks the gas out of the collection bag and runs it through the reclaimer. High-purity reclaimers use chemical filters to capture most of the gasses that solidify at LN
2 temperatures — below −196 °C (77 K; −321 °F) and would clog up the system with “ice”.
What’s left goes into a heat exchanger cooled by LN
2 and all the gasses except Helium and trace amounts of Hydrogen condense out as liquids. The helium then heads to HP storage cylinders.
I have seen some Helium Cryogenic units (liquefying the Helium). They produce ultra-high purity, but are higher maintenance and even less efficient (energy and LN
2 usage). The LHe then has to be boiled-off (allowed to warm) and then pumped into HP storage.
...But for on board oxygen generation you need IMHO to decide first your purity. Most industrial medical units use 90% and argue that in medical terms patents only need 90% not so a diver.
The better blocks run to 93% to 95% and even then the gaseous concentration of CO CO2 need to be agreed with the supplier first as 10% CO and over 400 ppm CO2 are not uncommon, so beware OK for medical use maybe on the surface but for filing rebreather cylinders?
Also be careful as to the percentage tolerance in generators is +/- 3% of purity and they are temperature sensitive. Typical discharge pressure is 9 to 10 psig
For <98% plus purity you will just about double the plant size and increase cost. It adds an additional generator stack and needs more air and produces at higher pressure. Alternative is use a bigger generator than HP compressor and increase the produce discharge pressure to lower the flowrate and increase the purity.
90+% purity is manageable for open-circuit Nitrox continuous blending and the numbers for open-circuit decompression on 10/90 N
2/O
2 aren’t bad. Nobody has told me anything about rebreathers or treatment chambers… yet.
The question remains, which units have proven reliable in this application? Reading manufacturer’s literature and listening to salesman isn’t the same as hearing the good, bad, and ugly from people using it at sea out in the middle of nowhere (industrially speaking).
