Submarine cabin altitude
Discussion
Mr Pointy said:
I'm guessing that it's unlikely as the crew would suffer from the bends if they didn't decompress before surfacing & opening the hatches.
My knowledge of diving/pressure issues is pretty basic but you'd pretty much make emergency evac difficult if not impossible wouldnt you?The atmosphere inside a nuclear submarine is kept within a closely monitored band; pressure, CO, CO2, O2, NO, levels (and others) are constantly read in order to maintain the atmosphere within an 'MPC' Maximum Permissible Content level. The atmosphere parameters are constantly changing and If for any reason the MPC levels are breached there are regulations associated with the requirement to vent to outside atmosphere (whilst at Periscope Depth) and to change the air inside.
MBBlat said:
As stated 1 atm - the pressure differences at depth are too great for any increase to make much difference.
A quiz question for submarine designers - does the air inside the pressure hull have a weight?
The air will have mass, but will also have a buoyancy (-weight, sort of) as it displaces a greater mass of water when it is diving. A quiz question for submarine designers - does the air inside the pressure hull have a weight?
Similarly a large hot air balloon may have both a weight and a mass of say a tonne when packed, a weight of 0Kg when hot inflated and buoyant, but with a mass of (say) 8 tonnes due to the volume of air in the balloon...
HTH
Pressurising the hull would definitely reduce the stresses it is subjected too, however, the complications it would create would be far more difficult to overcome than creating a strong enough hull.
The biggest problem is how to avoid bending the crew (giving them all a decompression injury). The first issue is the amount you wish to pressurise it to. Recreational divers are generally limited to 40 metres, which is equivalent to a pressure of 5 atm (1 atm on the surface, plus 1 tm for every 10 metres you descend. Using the Professional Association of Diving instructors (PADI) tables, you have a maximum bottom time of 8 minutes if you do not want to incur mandatory decompression stops. The advantage of sticking to these limits is you can ascend to the surface any time, however, it severely limits the effectiveness of the sub.
Let's say they want to dive to 40m for an hour. We have to chuck the PADI tables away and go for something a little more serious. Many technical divers will use the Buhlmann ZH-L16 decompression model, which is incorporated into dive tables, dive computers, and decompression software. In the latter case, I have an iPhone app called 'Baltic' that will produce a dive plan. I have just run this example through the app; assuming the gas mix is air throughout the dive, for an hour on the bottom, we are going to need to do 3 hours and 42 minutes of decompression stops on the way up.
The above example assumes the sub is pressurised to the ambient pressure. The sub could be pressurised to a lower figure, but it would not achieve much. According to Wikipedia, the Astute class submarines are tested to over 300 metres or 31 atm. With the sub maintaining an ambient pressure of 1 atm, the pressure acting on the hull will be 30 atm. Lets say we pressurise the hull to say, just 3 atm (equivalent to 20 metres deep), in order to reduce the deco penalty, the pressure on the hull is only reduced to 28 atm, so it's pointless exercise.
Let's say we did pressurise the hull to ambient pressure, there are problems with the gas mix we need to overcome. The most important is oxygen toxicity. As the ambient pressure increases, oxygen becomes toxic. On the surface, we breathe an oxygen partial pressure of 0.21 atm. As we descent 10 metres (2 atm) this increases to a ppO2 of 0.42 atm. At 20 metres, it increases to 0.63 atm, and so on. The accepted safe limit is 1.4 atm when working, which is equivalent to 56 metres.
So let's say we want to go deeper, we need to reduce the O2 content in the atmosphere. Let's say we are diving to 100 metres (11 atm), we would need a mix with an O2 content of 12%. On the surface, that gives us a ppO2 of 0.12 atm; the problem with that is we need something like 0.16 atm to sustain life. For a diver, this isn't a problem. For the descent, they will use a richer 'travel mix' from the surface and switch to the weak mix when the increase pressure makes it breathable. In a sub, they would have come up with a system that allows the O2 content in the sub's atmosphere to reduce according to the pressure increase.
The next problem is nitrogen narcosis. As the pressure increases, nitrogen does strange things and begins to impair your judgement. There are divers that claim to dive to 60 metres and beyond on air without getting narked, but most of these are probably fortunate not to have had to deal with an equipment problem.
To reduce the effect of narcosis, we put helium in the mix. I won't go into details of the calculation techniques, but the idea is you put enough helium in to make the gas mix equivalent to diving air at 30 metres - this is known as the Equivalent Narcotic Depth (END). There are no depth related issues with breathing helium, so the helium content can be kept constant. The trouble is it is expensive, and it will make every crew member talk like Joe Pasquale.
A further problem is Central Nervous System (CNS) oxygen toxicity. As well as watching you do not exceed a certain ppO2, we have to also monitor the time we spend breathing elevated O2 levels within a set period. This could be prevented by keeping the ppO2 at 0.21 atm at all times, but this will result in a ridiculous deco penalty.
Now we need to ascend. If we keep the pressure inside the sub equal to the ambient pressure, the sub is going to have to do a sh*t-load of deco on the way up. This can be reduced by upping the O2 content in the atmosphere (a diver will switch to his richer deco mix in this situation), but filling a nuclear sub with a flammable gas is not a wise idea. Also, as others have already said, if you had to escape from the sub, it makes getting bent almost inevitable.
The alternative option is to maintain pressure in the hull, but surface the sub, however, we are now simply applying stresses from the opposite direction.
TL:DR
The biggest problem is how to avoid bending the crew (giving them all a decompression injury). The first issue is the amount you wish to pressurise it to. Recreational divers are generally limited to 40 metres, which is equivalent to a pressure of 5 atm (1 atm on the surface, plus 1 tm for every 10 metres you descend. Using the Professional Association of Diving instructors (PADI) tables, you have a maximum bottom time of 8 minutes if you do not want to incur mandatory decompression stops. The advantage of sticking to these limits is you can ascend to the surface any time, however, it severely limits the effectiveness of the sub.
Let's say they want to dive to 40m for an hour. We have to chuck the PADI tables away and go for something a little more serious. Many technical divers will use the Buhlmann ZH-L16 decompression model, which is incorporated into dive tables, dive computers, and decompression software. In the latter case, I have an iPhone app called 'Baltic' that will produce a dive plan. I have just run this example through the app; assuming the gas mix is air throughout the dive, for an hour on the bottom, we are going to need to do 3 hours and 42 minutes of decompression stops on the way up.
The above example assumes the sub is pressurised to the ambient pressure. The sub could be pressurised to a lower figure, but it would not achieve much. According to Wikipedia, the Astute class submarines are tested to over 300 metres or 31 atm. With the sub maintaining an ambient pressure of 1 atm, the pressure acting on the hull will be 30 atm. Lets say we pressurise the hull to say, just 3 atm (equivalent to 20 metres deep), in order to reduce the deco penalty, the pressure on the hull is only reduced to 28 atm, so it's pointless exercise.
Let's say we did pressurise the hull to ambient pressure, there are problems with the gas mix we need to overcome. The most important is oxygen toxicity. As the ambient pressure increases, oxygen becomes toxic. On the surface, we breathe an oxygen partial pressure of 0.21 atm. As we descent 10 metres (2 atm) this increases to a ppO2 of 0.42 atm. At 20 metres, it increases to 0.63 atm, and so on. The accepted safe limit is 1.4 atm when working, which is equivalent to 56 metres.
So let's say we want to go deeper, we need to reduce the O2 content in the atmosphere. Let's say we are diving to 100 metres (11 atm), we would need a mix with an O2 content of 12%. On the surface, that gives us a ppO2 of 0.12 atm; the problem with that is we need something like 0.16 atm to sustain life. For a diver, this isn't a problem. For the descent, they will use a richer 'travel mix' from the surface and switch to the weak mix when the increase pressure makes it breathable. In a sub, they would have come up with a system that allows the O2 content in the sub's atmosphere to reduce according to the pressure increase.
The next problem is nitrogen narcosis. As the pressure increases, nitrogen does strange things and begins to impair your judgement. There are divers that claim to dive to 60 metres and beyond on air without getting narked, but most of these are probably fortunate not to have had to deal with an equipment problem.
To reduce the effect of narcosis, we put helium in the mix. I won't go into details of the calculation techniques, but the idea is you put enough helium in to make the gas mix equivalent to diving air at 30 metres - this is known as the Equivalent Narcotic Depth (END). There are no depth related issues with breathing helium, so the helium content can be kept constant. The trouble is it is expensive, and it will make every crew member talk like Joe Pasquale.
A further problem is Central Nervous System (CNS) oxygen toxicity. As well as watching you do not exceed a certain ppO2, we have to also monitor the time we spend breathing elevated O2 levels within a set period. This could be prevented by keeping the ppO2 at 0.21 atm at all times, but this will result in a ridiculous deco penalty.
Now we need to ascend. If we keep the pressure inside the sub equal to the ambient pressure, the sub is going to have to do a sh*t-load of deco on the way up. This can be reduced by upping the O2 content in the atmosphere (a diver will switch to his richer deco mix in this situation), but filling a nuclear sub with a flammable gas is not a wise idea. Also, as others have already said, if you had to escape from the sub, it makes getting bent almost inevitable.
The alternative option is to maintain pressure in the hull, but surface the sub, however, we are now simply applying stresses from the opposite direction.
TL:DR
- Pressurising the sub to little more than atmospheric pressure is pointless.
- Pressurising the hull beyond atmospheric pressure creates all sorts of technical and safety issues and restricts the operation of the sub.
Some great insight from the PH massive as always. So for your run of the mill nuclear sub, it's not feasible.
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
dvs_dave said:
Some great insight from the PH massive as always. So for your run of the mill nuclear sub, it's not feasible.
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
dvs_dave said:
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
A human is still a human, and you want your Special Ops people to be on top performance when they get out, not staggering about clutching their heads.dvs_dave said:
Some great insight from the PH massive as always. So for your run of the mill nuclear sub, it's not feasible.
What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
No. Pressurising the crew gives them no benefit whatsoever. What about special ops craft? Military, OGS and exploration craft? Do they run more exotic gas mixes, pressures and decompression profiles for the benefit of the mission over the safety of it?
These days, composite fibre winding tech allows us to form highly tensile filament wound "hoops" to help retain internal pressures, but i'm not sure how easy it would be to reverse the fibre loading to provide a net positive force on the outside skin of the subs pressure hull?
(ie for a compressed gas cylinder (or something we want to spin at very high speed) we tightly wrap it high tensile, press stressed carbon filaments, which acts against the pressure pushing outward on the walls. We need to do the opposite to provide compressive relief)
(ie for a compressed gas cylinder (or something we want to spin at very high speed) we tightly wrap it high tensile, press stressed carbon filaments, which acts against the pressure pushing outward on the walls. We need to do the opposite to provide compressive relief)
Historically, the internal air pressure of diesel submarines used to increase when dived (leakage from high pressure air systems and internal venting). This increase could be sufficient to cause the conning tower hatch to fly open when released during surfacing, occasionally followed by a submariner hanging on to the hatch wheel.
V8 Fettler said:
Historically, the internal air pressure of diesel submarines used to increase when dived (leakage from high pressure air systems and internal venting). This increase could be sufficient to cause the conning tower hatch to fly open when released during surfacing, occasionally followed by a submariner hanging on to the hatch wheel.
Mentioned in this...
https://www.amazon.co.uk/dp/B00KDJM0CO/ref=dp-kind...
The other advantage of not pressurising the hull is that it makes emergency escapes a lot easier. A friend of mine exited a Diesel/electric sub from about 300ft with no ill effects
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