Fuel Tank Inerting - Vent System
 

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

bizdev
 

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

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.

Tank Venting
 

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

TW

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

TW

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...

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:

AMM part 1

Vent System
 

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."

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

Fuel tank Inerting- Vent System
 

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.

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

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.

Inerting a Freighter or Cargo airplane
 

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.

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

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.

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."

NGS Operation
 

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

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.