max550
Posts: 714
Joined: Tue Nov 13, 2007 9:46 pm

Fuel Tank Inerting Systems

Fri Jul 18, 2008 1:44 pm

I just read yesterday about the FAA requiring airliners to have nitrogen inerting systems installed to prevent explosions like TWA flight 800. I was wondering how it works, but more how much it weighs per plane. Also, are all newly manufactured planes built with them already installed, and if not, which ones are?
I really don't know much at all about the systems, other than that they pump nitrogen into the fuel tank.
Any help is appreciated.
Thanks
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 2:09 pm

So on a 737, the nitrogen generating system is located in the pack bays underneath the wingbox.

They work on the principle that nitrogen and oxygen absorb at different rates and it has a membrane which will be able to mostly separate the oxygen and the nitrogen. The mixture is about 9% oxygen and 91% nitrogen. This won't support combustion in the fuel tank. It is then ducted into the fuel tanks. It's a common misperception that the tanks are 100% nitrogen.

One problem with fuel is that there is a lot of dissolved oxygen in it. When an airplane climbs and the pressure on the fuel decreases, some of the oxygen within the fuel will come out of the liquid and can result in a 40% oxygen environment, which is ripe for combustion.

This link has more than you ever want to know about the subject:
http://www.airweb.faa.gov/Regulatory...CE3F4862570220063BC3B?OpenDocument

[Edited 2008-07-18 07:13:44]
If you have never designed an airplane part before, let the real designers do the work!
 
User avatar
jetmech
Posts: 2316
Joined: Wed Mar 29, 2006 2:14 am

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 2:33 pm

Quoting Max550 (Thread starter):
I was wondering how it works,

Basically, nitrogen is an inert gas. When fuel tanks become empty, the atmosphere inside them is an air / fuel vapour mixture. Pumping nitrogen into the fuel tanks displaces these vapours, and hence, reduces the chances of a fuel tank explosion.

Quoting Max550 (Thread starter):
Also, are all newly manufactured planes built with them already installed, and if not, which ones are?

The 787 will come with a nitrogen generating system as standard. This will be supplied by Hamilton Sundstrand. I'm not sure about the A380 or A350.

http://www.hamiltonsundstrand.com/vg...298f148110VgnVCM100000c45a529fRCRD
http://www.hamiltonsundstrand.com/St...87%20systems%20placement%20map.pdf

Apparently, the key to the system is the air separation modules, which are supplied to Hamilton Sundstrand by Carleton Life Support Systems. These modules may be similar to the following.

http://www.carletonls.com/productsbu/nc1069.htm
http://www.carletonls.com/productsbu/aircraftsurv/pdfs/adv10469.pdf

Apparently, C-17 fuel tanks are rendered inert with nitrogen generated by two of these devices. Each one weighs 52.2 kg (115 lbs). One might assume that the distribution system along with the compressors required to drive it all may add another 104.4 kg (230 lbs) of weight to the complete system.

Regards, JetMech

[Edited 2008-07-18 07:39:43]
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
max550
Posts: 714
Joined: Tue Nov 13, 2007 9:46 pm

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 2:59 pm



Quoting JetMech (Reply 2):
Apparently, C-17 fuel tanks are rendered inert with nitrogen generated by two of these devices. Each one weighs 52.2 kg (115 lbs). One might assume that the distribution system along with the compressors required to drive it all may add another 104.4 kg (230 lbs) of weight to the complete system.

What I found online is that a C-17 MTOW is 585,000lbs, and a 747 is 910,000lbs. Would the system be the same for both or would the system for a heavier plane weigh more? By the same token, would the system for a smaller plane, like a 737 or A320, weigh less or the same?
Will the system be required for smaller planes, like RJ's or props, like the Q400? If so, will their systems weigh less?
 
GST
Posts: 808
Joined: Wed Jun 11, 2008 10:27 am

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 5:09 pm



Quoting Max550 (Reply 3):

First off, I know next to nothing on this subject, so this is more an educated guess.

The larger the tank volume the more nitrogen is required, and the mass of the associated system will be greater. As a rule of thumb, aircraft with greater tank volume tend to be the ones either designed for greater range, or as you say greater MTOW and thus need more powerful engines.
 
roseflyer
Posts: 9606
Joined: Fri Feb 13, 2004 9:34 am

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 6:08 pm



Quoting Max550 (Reply 3):
Would the system be the same for both or would the system for a heavier plane weigh more? By the same token, would the system for a smaller plane, like a 737 or A320, weigh less or the same?

Once you have the system, the difference in weight by doubling capacity is not that much. Components will get bigger, but not linearly. I would guess a few hundred pounds, but that's my best guess.

A few hundred pounds is a lot though. If you really want to know the exact weight increase, I can find out.
If you have never designed an airplane part before, let the real designers do the work!
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 6:32 pm



Quoting Max550 (Reply 3):
Will the system be required for smaller planes, like RJ's or props, like the Q400? If so, will their systems weigh less?

My understanding is that the system is only required for centre tanks, not wing tanks, and therefore many small aircraft won't need it. Having said that, the CRJ has centre tanks.
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 7:16 pm



Quoting GST (Reply 4):
The larger the tank volume the more nitrogen is required, and the mass of the associated system will be greater.

It's not the tank volume that decides the needs of the system. It's the fuel usage rate. You're just pumping gas in at the same rate you're burning fuel.
Anon
 
max550
Posts: 714
Joined: Tue Nov 13, 2007 9:46 pm

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 9:54 pm



Quoting Nomadd22 (Reply 7):
It's not the tank volume that decides the needs of the system. It's the fuel usage rate. You're just pumping gas in at the same rate you're burning fuel.

I hadn't thought about that at all, that makes a lot of sense. But it would still be heavier in a larger aircraft since fuel burn is higher, so you would need more capacity to pump gas in, correct?

Quoting Avt007 (Reply 6):
My understanding is that the system is only required for centre tanks, not wing tanks, and therefore many small aircraft won't need it. Having said that, the CRJ has centre tanks.

I had read that, why don't they require it for wing tanks? Is it only due to cost and weight, or is there less of a risk in the wing tanks?
Thanks for all your replies, my understanding of these systems has increased exponentially since this morning.
 
rwessel
Posts: 2448
Joined: Tue Jan 16, 2007 3:47 pm

RE: Fuel Tank Inerting Systems

Fri Jul 18, 2008 11:26 pm



Quoting Nomadd22 (Reply 7):
It's not the tank volume that decides the needs of the system. It's the fuel usage rate. You're just pumping gas in at the same rate you're burning fuel.

Actually you need a much higher rate - consider an airplane with a mostly empty tank descending from 30,000ft, where the ambient pressure is about 3.5psi, to sea level (14.7psi). During the descent, you'll need enough inert gas to fill the tank with the additional 11.2psi worth of gas.

Put another way, during the descent you need to generate enough gas to fill three-quarters of the tank at sea-level pressures.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 12:46 am



Quoting Max550 (Thread starter):
more how much it weighs per plane

The C-17 estimate is about right...a few hundred pounds.

Quoting Max550 (Thread starter):
Also, are all newly manufactured planes built with them already installed, and if not, which ones are?

737 has it now. 787 will have it from day one. Last time I checked Airbus believed they could comply with the rule without the inerting system for their current production, but A350 will probably have it.

Quoting Max550 (Thread starter):
I really don't know much at all about the systems, other than that they pump nitrogen into the fuel tank.

Actually, they pump low-oxygen air (mostly nitrogen, plus a few % oxygen, plus all the trace gases).

Quoting Max550 (Reply 3):
What I found online is that a C-17 MTOW is 585,000lbs, and a 747 is 910,000lbs. Would the system be the same for both or would the system for a heavier plane weigh more?

More, in general, but the C-17 inerts the whole wing while the 747 just does the center.

Quoting Max550 (Reply 3):
By the same token, would the system for a smaller plane, like a 737 or A320, weigh less or the same?

Less, in general.

Quoting Max550 (Reply 3):
Will the system be required for smaller planes, like RJ's or props, like the Q400? If so, will their systems weigh less?

I believe the rule exempts aircraft under 30 passengers. For anything larger, they have to prove that their flammability is equivalent to an unheated aluminum wing tank. If they can do that, then they don't need it.

Quoting Avt007 (Reply 6):
My understanding is that the system is only required for centre tanks, not wing tanks, and therefore many small aircraft won't need it.

It's required for any tank that's got higher risk than an unheated aluminum wing tank. That is usually the center tank, but that's not an explicit part of the rule.

Quoting Nomadd22 (Reply 7):
It's not the tank volume that decides the needs of the system. It's the fuel usage rate. You're just pumping gas in at the same rate you're burning fuel.

That wouldn't work...you wouldn't be displacing any of the oxygen that was already in there (no fuel tank is ever 100% full).

Quoting Max550 (Reply 8):
I had read that, why don't they require it for wing tanks? Is it only due to cost and weight, or is there less of a risk in the wing tanks?

Less risk on the wing tanks. There is less heating from the aircraft, hence lower flammability.

Tom.
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 6:11 pm

So what are they going to do with the vent system in the center tanks????? cap it off, and how will you evacuate the air as you fuel the tank, obviously check valves, but how well will they actually work at keeping the nitrogen from escaping.

Sounds like a hair brained, "Rube Goldberg" solution for a problem that has occurred once.

Yes I know that the SR-71 used nitrogen in it's fuel tanks as the fuel was burned off, but that was more to keep the aerodynamic forces from crushing the skin into the tanks.

I still think that Flt. 800 took a missile in the belly. So scoff if you must, but that is how I see it.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 7:05 pm



Quoting Ex52tech (Reply 11):
So what are they going to do with the vent system in the center tanks?????

Leave it alone. The air needs somewhere to go. As long as you pump in nitrogen enriched air faster than the tank is breathing in, no problem.

Quoting Ex52tech (Reply 11):
Sounds like a hair brained, "Rube Goldberg" solution for a problem that has occurred once.

The system is actually very nice and simple. The cost/benefit analysis is a disaster though.

Tom.
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 7:21 pm

Quoting Tdscanuck (Reply 10):
That wouldn't work...you wouldn't be displacing any of the oxygen that was already in there (no fuel tank is ever 100% full).


Why would you ever have a high level of free O2 in the tank, unless you manually drained it for maintenance or something? If the incoming vent fed off the Nitrogen system the tank should always be low on Oxygen. The free O2 level in the tank only has to be kept below 8 or 9% if it's the same standard as the system they use on the AH-64s and the DC-9 they've been testing, so you wouldn't have to worry about minor inflows.

I'm not sure if airliners have separate inlet and outlet vents on their tanks, but I'd assume so.

[Edited 2008-07-19 12:24:50]
Anon
 
474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 7:37 pm



Quoting Ex52tech (Reply 11):
Yes I know that the SR-71 used nitrogen in it's fuel tanks as the fuel was burned off, but that was more to keep the aerodynamic forces from crushing the skin into the tanks.

I can't seem to find any reference to this system on the SR-71 in any books or web sites. And I don't remember it being there when I worked on them, so could please provide more details.
 
User avatar
lightsaber
Crew
Posts: 11747
Joined: Wed Jan 19, 2005 10:55 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 7:49 pm



Quoting Tdscanuck (Reply 10):
Quoting Max550 (Reply 3):
What I found online is that a C-17 MTOW is 585,000lbs, and a 747 is 910,000lbs. Would the system be the same for both or would the system for a heavier plane weigh more?

More, in general, but the C-17 inerts the whole wing while the 747 just does the center.

Please remember that the C-17 has an insane decent rate requirement for combat landings into a 'recently acquired' airfield in otherwise hostile territory. Any commercial aircraft would probably be fine with a scaled inerting system of half the capacity.

Quoting Nomadd22 (Reply 13):
The free O2 level in the tank only has to be kept below 8 or 9% if it's the same standard as the system they use on the AH-64s and the DC-9 they've been testing, so you wouldn't have to worry about minor inflows.

That is a reasonable level. No need to go to 0% O2. The flammability curves are very O2 dependent. Anything below 15% provides a dramatic improvement in safety.

Starting on page 24 is a reference appropriate to this discussion. Not that below 9% O2, JetA is self quenching (it will not propagate a spark ignited flame).
www.fire.tc.faa.gov/pdf/ar98-26.pdf

Page 28 goes into oxygen degassing during climb and why that's the more dangerous state of flight. Note that during decent the aircraft is full of COLD fuel. Hence, excluding military aircraft, that stage of flight should not be the primary design requirement. Yes, its good to have the O2 levels in the tank low during decent, but to me 15% seems like a reasonable bound. Its during hot weather climb the risk is greatest.

I'm a combustion engineer by trade; fuel tank ulage explosion risk is something I consider reasonable to reduce the risk. Now, since I do not design the airframe side of the fuel systems, its interesting to see the weight/complexity penalties. Now to go dig through the wings of a few available aircraft to go learn more.  Wink

Lightsaber.
"They did not know it was impossible, so they did it!" - Mark Twain
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 9:29 pm



Quoting 474218 (Reply 14):
I can't seem to find any reference to this system on the SR-71 in any books or web sites. And I don't remember it being there when I worked on them, so could please provide more details.

Well, if you worked on them, then you would know that the airplanes endurance depended on how long the twin 105-litre, and the single 50-litre liquid nitrogen reseviors could supply nitrogen to the fuel tanks.

Endurance time was different on each airplane because of tank sealant deterioration due to exposure to high temperature and "heat soak". The airplanes fresh out of depot maintenance and with new sealant generally had a longer endurance than airplanes that had been out in service for a long time.

This system kept a positive head pressure of gaseous nitrogen on the tanks and prevented "ambient" airloads from crushing empty fuel tanks as the aircraft decended into the denser atmosphere for refueling. Additionally this system "inerted" the the space in the tanks above the heated fuel to prevent fuming and autogenous-ignition.

This information was taken from Paul F Crickmore's book......Lockheed SR-71 The secret missions exposed. Pg.47 paragraph labled "Nitrogen system"
"Saddest thing I ever witnessed....an airplane being scrapped"
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 9:52 pm



Quoting Nomadd22 (Reply 13):
Why would you ever have a high level of free O2 in the tank, unless you manually drained it for maintenance or something?

The tank has to always be vented. So if you just leave the aircraft sitting for a while, eventually you'll get back up to atmospheric (21%) O2 thanks to diffusion alone. Wind will speed that up a lot (a NACA scoop, which is what the vent usually is, generates a pressure differential in a moving airstream). Since you can't control the idle time, you have to design assuming that the ullage starts as normal air.

Quoting Nomadd22 (Reply 13):
If the incoming vent fed off the Nitrogen system the tank should always be low on Oxygen.

But the incoming vent doesn't feed off the nitrogen system...it can't, because you need the vent to function even when the nitrogen system isn't.

Quoting Nomadd22 (Reply 13):
I'm not sure if airliners have separate inlet and outlet vents on their tanks, but I'd assume so.

They usually don't, since there's no real need. There may be climb and dive ports inside the tank (depends on which model you look at) but the actual vent to the outside is usually a single scoop in each wingtip. The vent channel is often just a hat-section stringer on the upper wingskin and is used for venting in and out.

Tom.
 
474218
Posts: 4510
Joined: Mon Oct 10, 2005 12:27 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 10:31 pm



Quoting Ex52tech (Reply 16):
Well, if you worked on them, then you would know that the airplanes endurance depended on how long the twin 105-litre, and the single 50-litre liquid nitrogen reseviors could supply nitrogen to the fuel tanks.

Endurance time was different on each airplane because of tank sealant deterioration due to exposure to high temperature and "heat soak". The airplanes fresh out of depot maintenance and with new sealant generally had a longer endurance than airplanes that had been out in service for a long time.

This system kept a positive head pressure of gaseous nitrogen on the tanks and prevented "ambient" airloads from crushing empty fuel tanks as the aircraft decended into the denser atmosphere for refueling. Additionally this system "inerted" the the space in the tanks above the heated fuel to prevent fuming and autogenous-ignition.

This information was taken from Paul F Crickmore's book......Lockheed SR-71 The secret missions exposed. Pg.47 paragraph labled "Nitrogen system"

I think Mr. Clickmore embellished the function of the nitrogen pressurization a little: The following from the flight manual notes that more nitrogen is used when descending to land or refuel to equalize pressure. But provides no caution of structural failure.

http://www.sr-71.org/blackbird/manual/1/1-58.php
 
ex52tech
Posts: 553
Joined: Wed Dec 06, 2006 2:28 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 10:50 pm



Quoting Tdscanuck (Reply 12):
Leave it alone. The air needs somewhere to go. As long as you pump in nitrogen enriched air faster than the tank is breathing in, no problem.

Is this system only going to be used while on the ground, or shortly after take off? Now if it has been previously discussed as to when the nitrogen will be used, and I just missed that, then I apologize.

But as I see it, as the speed increases so will the venting of the tanks until the system reaches it's volume limit, so if you ARE introducing nitrogen during the entire flight, then I would be inclined to think that you are going to need a lot of nitrogen. Especially as the ambient pressure drops at altitude, and the fuel levels drop. I need a better understanding of how much volume the average airliner vent system displaces, especially at altitude.

I can see the military installing a system on their cargo airplanes, since they are afraid of their own shadows, you open a fuel line on an engine, and you have to have power off the entire airplane........it got a little silly at times.

But to go to the trouble and expense of installing this on a fleet of airliners that have been flying for years without incident (other than Flt. 800) seams a little too paranoid to me.
"Saddest thing I ever witnessed....an airplane being scrapped"
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Fuel Tank Inerting Systems

Sat Jul 19, 2008 11:09 pm

Reading a little history. It seems the break came when the FAA decided that 12% oxygen was good enough, letting the planes use light weight (150 - 250 lbs) polymer membrane separators, instead of the half ton nitrogen generators some military craft use.
The system Boeing tested on their 747 fed the wing tanks as well as the center tank.
I still remember the NTSB not being real happy the A380 didn't get the system.
Anon
 
WingedMigrator
Posts: 1767
Joined: Wed Oct 26, 2005 9:45 am

RE: Fuel Tank Inerting Systems

Sun Jul 20, 2008 4:07 am



Quoting Nomadd22 (Reply 20):
I still remember the NTSB not being real happy the A380 didn't get the system.

It doesn't have a center tank, so inerting wasn't required.
 
tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

RE: Fuel Tank Inerting Systems

Sun Jul 20, 2008 4:30 am



Quoting Ex52tech (Reply 19):
Is this system only going to be used while on the ground, or shortly after take off?

It's on most of the time. The exact operating sequence is proprietary.

Quoting Ex52tech (Reply 19):
But as I see it, as the speed increases so will the venting of the tanks

The vents pressurize the tanks with increasing speed...as speed increases air gets stuffed in, not sucked out. During cruise you only need to keep pace with the rate of fuel loss (very low in air volume terms) plus a little bit for cross flow and diffusion.

Quoting Ex52tech (Reply 19):
But to go to the trouble and expense of installing this on a fleet of airliners that have been flying for years without incident (other than Flt. 800) seams a little too paranoid to me.

There were more than just TWA800, but that's the biggie by far. This argument is exactly what basically every single airline and IATA made to the FAA when the proposed rule first came out. The cost/benefit calculation fails miserably.

Tom.
 
nomadd22
Posts: 1566
Joined: Fri Feb 22, 2008 7:42 pm

RE: Fuel Tank Inerting Systems

Sun Jul 20, 2008 6:09 pm

I tried to work out a quick cost to benefit ratio for these systems. When the EPA considers new regulations they use roughly a $7,000 per life saved standard to decide if they should force industry to comply. At an average of $200,000 per plane for the N2 systems, and taking a wild guess with no basis in fact that it would keep one plane every 20 years from blowing up with loss of passengers, these regs come out to a cost of something like $7,000,000 per life saved.
And, with a real effort to deal with possible ignition sources and knowing not to run hot AC units under mostly empty fuel tanks, one plane every 20 years is probably pessimistic.
I wonder what number the military uses when they decide on new gear for troops.
Anon

Who is online

Users browsing this forum: Bing [Bot] and 15 guests