Fuel Tank Inerting - Vent System

[bizdev]

Can anyone help with providing a schematic of a Fuel Tank Venting system on an Aircraft which has been fitted with gas inerting? I understand that the vent system has been modified/re-designed to ensure that Nitrogen is not lost through the vent sysyem?

bizdev

[cessnapete]

The RAF have just fitted a tank inerting system on their TriStar tanker/transports at Brize.

[bizdev]

Yep - and I am aware that BA's new 777's are fitted with the system. I have got schematics of the supply side (Nitrogen generation) but not of the new venting arrangement.

[Tinwacker]

I do not have much data on the B777 venting layout but it is only the centre tank that has the system fitted.

An extract from the AMM suggests that this tank is closed off to venting except during refueling for fuel expansion.

The cross vent check valve opens during overfuel conditions to relieve fuel tank pressure. The cross vent check valve is normally closed to prevent ambient air from going into the center tank.
The cross vent check valve opens during overfuel conditions to relieve fuel tank pressure. The cross vent check valve is normally closed to prevent ambient air from going into the center tank.

The Nitrogen generation is in fact an oxygen reduction system that increases the percentage of nitrogen in a given volume.

The Nitrogen Generation System (NGS) reduces the oxygen content of the air mixture in the center tank to a level which will not support combustion.

TW

[Jet II]

removed by admin -- copyrighted image

[Tinwacker]

Thanks Jet for the schematic,
I just couldn't find it in the AMM or is this from a training aid??

TW

[Mansfield]

I don't have access to any specific information per type or design. However, the AMM statements regarding the cross vent check valve and ambient air are a bit baffling, and perhaps even mis-stated by the Boeing tech writers. More information on the center tank venting design may be necessary.

While climbing, the tank must be able to vent overboard to maintain ambient pressure. If you seal it up, there will be a serious problem with structure in fairly short order. Likewise, during descent, it must be able to draw air in so that ambient pressure can be maintained. Finally, as the fuel volume decreases, air must be introduced to fill the volume.

Unless the design concepts have changed, (and I haven't been involved with this for a number of years), the nitrogen is introduced in order to reduce the oxygen content to around 10 to 15%, which pretty well negates combustion. The problem with the nitrogen supply is that one needs to have enough to replenish the ullage during the climb/descent dynamics. This was one reason the military nitrogen systems that we first looked at were considered inadequate, as they provided nitrogen only during the short periods of combat exposure, not during a long haul flight.

I can see a system which is closed while the pressure differential between tank ullage and outside is static, such as on the ramp. But in the end, there is no way to really trap the nitrogen throughout the flight regime without introducing the enormous problems associated with a pressure vessel as a fuel tank...

[Denti]

Current systems, well, at least the one fitted on 737s since around 2006, constantly generates nitrogen, or rather reduces O2 out of normal bleed air and vents that into the center tank. So you do not need to carry nitrogen, there is more than enough in normal air.

Sadly i don't have any access to the AMM as i'm just a pilot and we don't get that usually. So i cannot post any schematics about that system here. Just know there is a panel in the wheel which shows a green, amber and red light, green means operational, amber degraded or temporarily serviceable, red u/s. Oh, and a placard warning that with the system operational there might be a low oxygen content in there and one should be cautious.

What we get in documentation is only:

Nitrogen Generation System (NGS)

The NGS converts bleed air to nitrogen-enriched air (NEA) during all phases of
flight. The NEA is delivered to the center fuel tank to reduce flammability of the tank. The operation of the NGS is transparent to the flight crew; it does not require any flight crew action to operate the system, nor are there any flight deck indications. The NGS automatically starts operating after take-off and runs continuously through climb, cruise, descent, landing and during taxi for a short period of time. The NGS shuts down after a specified period of time or when bleed pressure is no longer available. The NGS also automatically shuts down during the following non-normal flight conditions:

• Aircraft on the ground and not in test mode
• Either engine is not running in flight
• Fire or smoke detection in the cargo or main deck areas
• Left air conditioning pack overheat
• Center tank refueling valve is open

The fuel tanks are primarily protected by precluding ignition sources; hence
dispatch with the NGS inoperative is acceptable under MEL procedures. The NGS has an operability indicator located in the main wheel well adjacent to
the APU fire control panel.

[Jet II]

Thanks Jet for the schematic,
I just couldn't find it in the AMM or is this from a training aid??


AMM part 1

[Prof. Engineer]

I do not know the changes that have been made to the vent syste. My understanding is that it impedes the airflow from one wing tip to the other. Vent is necessary for the vented fuel tank to allow one cfm of ullage mixture to veent out when one cfm of NEA is supplied to the fuel tank. If there wasw not one the fuel tank will get pressurized and structural damage may occur.

I think a better system is a inerting system that does not use an inert gas. This is described in SAE aper 2009-01-3134. It cn be downloadedfrom SAE website. It requires no modification of the vent system and does not discharge hydo carbon laden ullage gas. There are other advantagess too. I strongly suggest review of the paper "Method and System for fuel tank inerting without an inert gas."

[Prof. Engineer]

Strongly suggest you read SAE paper 2009-01-3134. It may sow seeds in your very fertile mind.

[Prof. Engineer]

In a vented fuel tank the vent has to be open at all times.

1. To discharge hydrocarbon laden ullage gas when the inerting system i operating. A cubic feet of ullage mixture has to vent out for each cubic feet of NEA supplied to the tank.
2. To discharge ullage moxture during climb to depressurize the tank so the tank remains at the same pressure as outside ambient
3. To repressurizethe tank during descent when the outside pressure increases.

I can not think any instant or reason when the vent will not be allowed to do its intended job; vent ullage mixture out or to allow outside air in.

I think the vent modification is to minimize flow through the tanks from one wing tip vent to the other.

Hope the above helps.

[Graybeard]

Some Automobiles of ancient age used exhaust to pressurize the fuel tank, as they didn't have reliable fuel pumps. It had the side benefit of rendering the ullage non flammable. I have such a system in my Model T Ford, which is my daily driver.

Would routing exhaust into the main cabin be practical for starving a fire in a freighter, such as the UPS 747 out of Dubai?

GB

[keel beam]

Would routing exhaust into the main cabin be practical for starving a fire in a freighter, such as the UPS 747 out of Dubai?I would suggest not.

I cannot give specific figures but in the case of Concorde, there was sufficient oxygen in the exhaust gas to enable the reheat to be used.



The Concorde fuel tanks were pressurised at altitude. I am wondering if the Concorde had still been flying today whether the FAA would have wanted/insisted it to have an NGS fitted.

[Prof. Engineer]

You are right the compartments of a freighter can be inerted by using a gas that has low concentration of oxygen. It can be any gas or a mixture of gases. Engine exhaust would qualify. If engine exhaust is used then the exhaust will require some sort of treatment to ensure that it does not react with the materials of construction of the freighter and also with the cargo. You will not need much of this gas as there is no requirement of any minimum ventilation. However note that you will not be able to carry animal cargo (pigs, chicken, etc.) in an inerted compartment. Also, precautions will be necessary during cargo loading and unloading. Low oxygen content atmosphere can create asphyxiation hazard.

[Graybeard]

Thanks for the reply, PE. Is there a level of O2 content that would not asphyxiate, but yet not support combustion? Yeh, I know the processes are the same, but at different rates. Seems worthwhile for the package carriers to study.

GB

[Prof. Engineer]

It is not the oxygen concentration that determines when hypoxic conditions may occur. It is oxygen partial pressure. At 10,000 feet altitude some people brcome hypoxic at oxygen concentration of 21%. I do not think you can carry animals in conditions (oxygen concentrations) that are necessary for fire suppression. Operators loose a lot of animals when the halon fire suppression system is discharged. Halon concentration is very low (3-4%) for fire suppresion.

For a true package transporter it may be worthwhile to have the cargo compartment unpressurized. This may not be good for some electrical or electronic equipment or for cargo that is temperature or altitude sensitive.

[Prof. Engineer]

Flammability Reduction System is far superior than a Fuel Tank inerting system. It is based on the principle that lean fuel air mixtures do not ignite. The fuel air mixture can be rendered lean by several methods. Cooling the ullage mixture outside the fuel tank to condense out fuel vapors is a very simple method. The condensed fuel is returned back to the fuel tank and the chilled ullage mixture is returned back to the fuel tank. The cooling can be done by an inexpensive air cooler. Cold ambient air in flight helps maintain the fuel tank non-flammable. The fuel burn penalty of the system is low, the weight is low and it is safer than a nitrogen system. No vent system modification is necessary and the system does not exhaust out hydrocarbon vapors that may be carciogenic. Suggest you read about it on SAE web site, "Method and system for making a fuel tank inert without an inert gas."

[Prof. Engineer]

Very interesting.
Sitting in the cockpit you have no idea what the system is doing and what is the state of the fuel tank.
What good is this system?
It pollutes outside air with fuel vapors as it exhausts ullage gases.
Please tell me why one would select such a system for one's airplane or one's fleet. I have failed to find one good reason. It is NOT required by law. Other methods, better and cheaper, are available.

If you know of a good reason, PLEASE POST

[Fargoo]

It makes Honeywell a whole load of money with extortionately priced consumable items in the system.

Want to replace those clogged Air Seperation Modules each D check , just bend over here and insert wallet.

Maybe a cynical view but the system is in the MEL as 120 days allowable INOP at the moment which really says it's not that important to the safety of the aircraft.

Someone is making a whole lot of money out of forcing this system through.

[Fargoo]

p.s. I believe it is soon to be FAA mandated on aircraft in US airspace, can't remember the date for this though - anyone?

[Prof. Engineer]

Thanks.

However, Honeywell is the seller and not the buyer.

My interest is to learn from the buyers why would they spend their good money to buy and install a nitrogen Fuel tank inerting system on their plane or their fleet. These systems are expensive. There must be something good about them. I have failed to find anything to write home about. They are not required by law. Also, I will love to learn from the sellers why are they peddling such a system.

Please post your reasons.

Thanks.

[Prof. Engineer]

The regulation requires a Flammability Reduction System. It is performance based. No particular System has been mandated. A few articles stated that inerting is mandated. It is not true. Read the Final rule on FAA website or in the Federal Register.

[Fargoo]

Seems like you already know all the answers.

[mono]

EASA are also expected to mandate installation of a Flammability reduction system.

Whether it's an inert gas or some other system the regulation is expected by the end of the year.

[HAWK21M]

Its Incorporated in AMM ATA chapter 47.

[albertofdz]

Hey guys,

Sorry for bringing up such a dated thread, but I find the whole NGS quite intriguing, given that Boeing does not supply us with much information in our manuals!

The case is that some time back I remember watching a 20 minute or so video which explains the NGS on the 737.

Can anyone provide me with a link for this??? I´ve been searching for it but I just can´t seem to find it!

Many thanks.

[JammedStab]

I am curious as to why a nitrogen generating system would be shut down for such situations as a forward compartment fire or an engine shutdown.

[rigpiggy]

The russian planes of ww2 ran a portion of exhaust into fuel tanks to reduce chances of fire

[CaptainMongo]

For Airbus guys, FAST 44, July 2009 issue, describes the Airbus FTIS:

Publications | Airbus, a leading aircraft manufacturer (http://www.airbus.com/support/publications/)

[FE Hoppy]

Cseries tank inerting system is separate from the vent system. The vent system is no different to any other. The inerting system is always on(when bleed is available) the flow rate varies with phase of flight but it's always pumping and the tanks are always venting.