Qantas 744 Depressurisation
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Atmospheric pressure at 30 000' : ~ 4psi.
Assume adiabatic expansion:
Pressure x (Volume ^ 1.4) = constant. [1.4 is a typical value for a diatomic gas, like oxygen]
Therefore:
Pressure1 x (Volume1 ^ 1.4) = Pressure2 x (Volume2 ^ 1.4)
(Volume2 / Volume1) ^ 1.4 = (Pressure1 / Pressure2)
(Volume2 / Volume1) = (Pressure1 / Pressure2) ^ (1/1.4)
P1 = 1800 psi
P2 = 4 psi
Therefore:
(Volume2 / Volume1) = (1800 / 4) ^ (1/1.4)
(Volume2 / Volume1) = 78.6
That is, the gas in the cylinder will expand to almost 80 times its compressed volume if the cylinder ruptures.
That assumes that the cylinder didn't lose any heat while it climbs to altitude - fairly reasonable, I think.
If I've messed up the maths, someone please let me know.
Assume adiabatic expansion:
Pressure x (Volume ^ 1.4) = constant. [1.4 is a typical value for a diatomic gas, like oxygen]
Therefore:
Pressure1 x (Volume1 ^ 1.4) = Pressure2 x (Volume2 ^ 1.4)
(Volume2 / Volume1) ^ 1.4 = (Pressure1 / Pressure2)
(Volume2 / Volume1) = (Pressure1 / Pressure2) ^ (1/1.4)
P1 = 1800 psi
P2 = 4 psi
Therefore:
(Volume2 / Volume1) = (1800 / 4) ^ (1/1.4)
(Volume2 / Volume1) = 78.6
That is, the gas in the cylinder will expand to almost 80 times its compressed volume if the cylinder ruptures.
That assumes that the cylinder didn't lose any heat while it climbs to altitude - fairly reasonable, I think.
If I've messed up the maths, someone please let me know.
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In contrast, the forward cargo doors on a 747 are not of this type. They are held closed by mechanical locks/latches (well, most of the time...).
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We are discussing the "doors" on the positive pressure relief valves -- 2 small butterfly doors on the forward left side of the fuselage, both of which are open in the pix shown (link in post #906, page 10).
I would be surprised if friction from a gas flow would raise the orifice temperature significantly. The hot connector that some of us may remember from pumping up bicycle tyres is a result of the adiabatic compression of the air in the pump body. A more likely cause of a high degree of heating is that when the pressure differential across the orifice is so high that the exit speed would be supersonic, a shock wave appears and limits the speed of the outflow. It would be the flow across the shock wave that produced the metal-melting temperature apparently observed in some cases. It's almost 50 years since I last looked at the mathematics of the situation in detail, so I'm not going to bore most of you with even further off-thread computational minutiae.
All of the above seems to assume flow. How does one define flow in an explosive rupture removing all definition of an orifice
I'm still waiting for confirmation that the body of the O2 bottle actually exploded and then I can limit my own speculation somewhat
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Everyone (incl. Cabin Crew) still wears masks at 10,000ft.
rogerd,
thanks for the info on the IFE operation during depressurization events.
Xeque,
The clip I had in mind, this one, shows people still wearing their masks, although it also shows the IFE screen displaying the altitude as 10,000ft, so it must be after the emergency descent, and the masks would no longer be required.
Probably cabin crew haven't told them yet that the ambient air is fine to breathe; they may not even have realised it, wearing portable masks themselves.
The comment on this one, on the other hand, is BS. ("Watch the altitude drop", showing the descent for landing, coming down from 10,000ft, and not the emergency descent. I've seen no video of that.)
Bernd
thanks for the info on the IFE operation during depressurization events.
Xeque,
The clip I had in mind, this one, shows people still wearing their masks, although it also shows the IFE screen displaying the altitude as 10,000ft, so it must be after the emergency descent, and the masks would no longer be required.
Probably cabin crew haven't told them yet that the ambient air is fine to breathe; they may not even have realised it, wearing portable masks themselves.
The comment on this one, on the other hand, is BS. ("Watch the altitude drop", showing the descent for landing, coming down from 10,000ft, and not the emergency descent. I've seen no video of that.)
Bernd
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I'm stumped
All of the above seems to assume flow. How does one define flow in an explosive rupture removing all definition of an orifice
All of the above seems to assume flow. How does one define flow in an explosive rupture removing all definition of an orifice
Sorry, this is off topic,but associated with the explosive force of the cylinder in the frame of the main thread theory.
Mech Prentice. Thanks for the maths (way over my head) but I DO understand the 80-fold expansion and the force with which it would slam against the fuselage....not forgetting that it could well have slammed a large chunk of flattened bottle against said skin.
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Mech-prentice -- Assuming the cylinder ruptured prior to the hull breach then Volume 2 would be the pressurized fuselage, which was closer to 12 psi.
What about the potential energy?
The cylinder should have contained 3,260,000 cc (115 cubic feet) Oxygen @ 128 barg (1850 psig), 21 deg C (70 deg F).
Generally, for a diatomic gas whose volume is V, the internal energy is (1.4)PV
What about the potential energy?
The cylinder should have contained 3,260,000 cc (115 cubic feet) Oxygen @ 128 barg (1850 psig), 21 deg C (70 deg F).
Generally, for a diatomic gas whose volume is V, the internal energy is (1.4)PV
1.4 * 128 * 3,260,000 = 584,192,000 cubic cm-atm
...............................= 59,193,254 joules
...............................= 8 kg dynamite
.
Corrections are invited!
...............................= 59,193,254 joules
...............................= 8 kg dynamite
.
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Un-screwed up
but I don't understand how the body exploded in the bench-vice merely as a result of the valve being unscrewed.
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Static possible cause of exolosion in oxygen cylinder
I have observed that a high pressure gas cylinder when leaking through a small constriction,(in this case a partly opened oxygen supply cock) can generate a static charge sufficient to discharge a 3 inch spark.
I saw this happen on 2 different flights , fortunately there was no subsequent ignition.
Transferring fuel without an earth connection comes to mind.
I saw this happen on 2 different flights , fortunately there was no subsequent ignition.
Transferring fuel without an earth connection comes to mind.
Second Law
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Macha 948
The cylinder should have contained 3,260,000 cc (115 cubic feet) Oxygen @ 128 barg (1850 psig), 21 deg C (70 deg F).
Generally, for a diatomic gas whose volume is V, the internal energy is (1.4)PV
Generally, for a diatomic gas whose volume is V, the internal energy is (1.4)PV
1.4 * 128 * 3,260,000 = 584,192,000 cubic cm-atm
...............................= 59,193,254 joules
...............................= 8 kg dynamite
pV = nRT based sums agreed but ........being properly pedantic, ......no allowance is being made here for rate of expansion, so 8 kg of an NG based explosive is an absolute i.e. instantaneous discharge limit case.
Time dependence is critical methinks.
Yes, yes we have inadequate data.
CW
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Perhaps it would be advantageous to consider two extremes - one being leakage of the gas through an orifice (varying in size from a minor leak, through a 'venturi' that restricted flow, to a 'porthole' large enough to empty the contents (almost) instantaneously) and the other being instantaneous rupture of the container (as happens with a child's balloon).
It's obvious (even to the layperson) that different conditions (as far as dissipation of energy) apply to the extremes.
It is a fact that the expansion of the pressurised gas requires heat from the surroundings which will be chilled by the escaping gas - this in itself could alter the ductility of the metal of the struture of the aircraft making it brittle and subject to 'snapping' rather than deforming, so a jet of escaping oxygen could 'blast' a hole in the skin of the hull.
Of course, the cylinder itself would also chill and (conceivably) become fragile (regardless if it was metal or polycarbonate or whatever its material). I believe that the composite cylinders have aluminium liners, and, being highly conductive (as regards heat) the aluminium would reach a lower temperature more rapidly than the outer skin of 'plastic'. Differential contraction (despite the residual pressure) could cause delamination in extreme circumstances (IMO). Thus an initial 'controlled' leak could develop into explosive expansion because of failure of the cylinder structure.
Once 'uncontrolled' expansion of the previously contained pressurised oxygen occurs generating shock-waves, probably supersonic in propagation (which is why we hear a bang from a ruptured child's balloon . . . ).
It's not beyond reasonable speculation to imagine a combination of a leak causing localised supercooling of the hull wall ('weakening' it) followed by rapid acceleration of the leak into a rupture of the cylinder.
The rest (part of the cylinder exiting through the hull wall with another piece 'blowing' up through the cabin floor followed by debris being sucked-out of the breach in hull after the shock-wave has triggered the pressure doors in the cabin wall) is 'inevitable' - when the holes in the cheese line up . . .
It's obvious (even to the layperson) that different conditions (as far as dissipation of energy) apply to the extremes.
It is a fact that the expansion of the pressurised gas requires heat from the surroundings which will be chilled by the escaping gas - this in itself could alter the ductility of the metal of the struture of the aircraft making it brittle and subject to 'snapping' rather than deforming, so a jet of escaping oxygen could 'blast' a hole in the skin of the hull.
Of course, the cylinder itself would also chill and (conceivably) become fragile (regardless if it was metal or polycarbonate or whatever its material). I believe that the composite cylinders have aluminium liners, and, being highly conductive (as regards heat) the aluminium would reach a lower temperature more rapidly than the outer skin of 'plastic'. Differential contraction (despite the residual pressure) could cause delamination in extreme circumstances (IMO). Thus an initial 'controlled' leak could develop into explosive expansion because of failure of the cylinder structure.
Once 'uncontrolled' expansion of the previously contained pressurised oxygen occurs generating shock-waves, probably supersonic in propagation (which is why we hear a bang from a ruptured child's balloon . . . ).
It's not beyond reasonable speculation to imagine a combination of a leak causing localised supercooling of the hull wall ('weakening' it) followed by rapid acceleration of the leak into a rupture of the cylinder.
The rest (part of the cylinder exiting through the hull wall with another piece 'blowing' up through the cabin floor followed by debris being sucked-out of the breach in hull after the shock-wave has triggered the pressure doors in the cabin wall) is 'inevitable' - when the holes in the cheese line up . . .
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bsieker
I think you're right. All the clips I've seen seem to be from the same source but your first one is longer - the others have been edited with 'journospeak' added. Watch the altitude drop from 10,000 FEET not Metres. Maybe the reporter made that error.
I think you're right. All the clips I've seen seem to be from the same source but your first one is longer - the others have been edited with 'journospeak' added. Watch the altitude drop from 10,000 FEET not Metres. Maybe the reporter made that error.
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News at 10 ...
Seems someone at the Cairns Post reads PPrune too ...
Cairns.com.au - Qantas blast
I'm sure I saw these photos here first.
Cairns.com.au - Qantas blast
I'm sure I saw these photos here first.
The Reverend
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Intruder,
The butterfly doors are held in place by a spring mechanism and forced open by escaping air pressure from the valves. The valves close after pressure relief but the doors have to be manually reset.
OK... What is the mechanism that holds the doors closed? What trips that mechanism? How is it made fail-safe?
The butterfly doors are held in place by a spring mechanism and forced open by escaping air pressure from the valves. The valves close after pressure relief but the doors have to be manually reset.
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That is, the gas in the cylinder will expand to almost 80 times its compressed volume if the cylinder ruptures.
That assumes that the cylinder didn't lose any heat while it climbs to altitude - fairly reasonable, I think.
If I've messed up the maths, someone please let me know.
That assumes that the cylinder didn't lose any heat while it climbs to altitude - fairly reasonable, I think.
If I've messed up the maths, someone please let me know.
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The butterfly doors are held in place by a spring mechanism and forced open by escaping air pressure from the valves. The valves close after pressure relief but the doors have to be manually reset.
Other possibility is that the valves rebounded off their seats when the valve impacted the door frame not too far away -- the "jostling" I mentioned earlier.
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Other possibility is that the valves rebounded off their seats when the valve impacted the door frame not too far away.
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I do apologise if this has been discussed before, but I do find the door handle damage quite strange. It has been bent inwards toward the door, but not upwards, as you would imagine, if it had been hit from directly below. Their is no other visible damage to the bottom edge of the handle. It had been hit with enough force to deform it inwards and move it towards the open position, breaking the internal mechanism and then embedding itself into the door panel, but you imagine that you would see chips or dents on the lower edge.
Discuss!
Discuss!