737 Max version can transfer fuel using cabin pressure

[JammedStab]

On the BBJ 8 that is...

"The BBJ MAX 8 may be fitted with up to eight auxiliary belly tanks, but seven or fewer tanks afford the most practical tradeoff between range and payload. Three forward and four aft belly tanks hold 24,455 lb. of fuel, for a total capacity of 70,370 lb. The aux tanks transfer fuel to the center tank using cabin pressure. While the wing and center tanks can be filled in 15 min., Boeing estimates that it will take another half hour to refuel the seven aux tanks."

BBJ MAX 8: It?s Bigger, Flies Farther And Is More Efficient | BCA content from Aviation Week (http://aviationweek.com/bca/bbj-max-8-it-s-bigger-flies-farther-and-more-efficient)

[A and C]

May be I’m getting confused but it was my understanding that the BBJ Aircraft all used cabin pressure to move the fuel into the centre tank.

[inbalance]

I flew a Bonanza 30 Years ago which was able to do the same.

[wiedehopf]

May be I’m getting confused but it was my understanding that the BBJ Aircraft all used cabin pressure to move the fuel into the centre tank.

that would mean that the wing tanks need to withstand the same pressure the cabin needs to withstand.

this is unlikely as it would incur additional weight.

the other direction is the one needed in flight and therefore more interesting.
if the center tank can only be filled on the ground that's perfectly allright.

and getting the fuel from the center tank that is at cabin pressure into the wings is trivial you just open a valve. (making sure the tank can get air from the cabin so it doesn't collapse)

[CL300]

Instead of using electric boosts pumps they are using ejector pump driven by regulated air pressure, ( same air as the cabin is pressurized with). I think this is the way to read it.
The writer must have experience in other airframes that are not using bleed air for fuel transfer.

[A and C]

In a word NO using cabin air pressure does not mean pressurising the whole fuel system to cabin pressure. Quite clearly the system would apply a regulated pressure to the fuel and stop the flow when the fuel tank is empty.

The vent/ surge system does vent the whole fuel system and maintain the whole fuel system above ambient pressure to help gravity feeding of fuel should the boost pumps fail, so in the very worst situation of unregulated cabin pressure getting into the centre tank this air would be vented overboard via the vent/ surge system.

[wiedehopf]

In a word NO using cabin air pressure does not mean pressurising the whole fuel system to cabin pressure. Quite clearly the system would apply a regulated pressure to the fuel and stop the flow when the fuel tank is empty.

The vent/ surge system does vent the whole fuel system and maintain the whole fuel system above ambient pressure to help gravity feeding of fuel should the boost pumps fail, so in the very worst situation of unregulated cabin pressure getting into the centre tank this air would be vented overboard via the vent/ surge system.

Isn't the center tank inside the pressure vessel?
If so you indeed need full cabin pressure to get fuel into the center tank or the center tank needs to withstand the cabin pressure and it is directly vented to the outside.

anyway i see the problem now.

you said in your post moving fuel INTO the center tank.
but that's not what you meant i guess.

[A and C]

The B737NG centre tank is not inside the pressure vessel, it is infact just like the wing tanks in that it is in the area between the front and rear wing spars and reaches across the Aircraft from just inboard each engine pylon.

The extra fuel carried in the BBJ is in tanks contained in the cargo holds that are pressured. The fuel from these tanks is transferred into the centre tank by regulated cabin pressure so that it can be used in the same way as it is in the Airliner variants.

The fact that the tanks in the cargo holds are at ( regulated ) cabin pressure and the Center tank is at ( slightly ) above ambient pressure allows the fuel to be transferred into the centre tank without having to resort to fitting transfer pumps or boost pumps and modifications that would vastly complicate the fuel system and take it away from the airliner standard.

[PDR1]

Aren't all the tanks pressurised anyway, to stop the fuel boiling off at altitude?

PDR

[wiedehopf]

The B737NG centre tank is not inside the pressure vessel, it is infact just like the wing tanks in that it is in the area between the front and rear wing spars and reaches across the Aircraft from just inboard each engine pylon.

Thanks a lot, makes perfect sense.
Oh well that took much more explaining than should have been necessary, sorry about that :)

[A and C]

The answer is “ partly “ the fuel tanks are kept at slightly above ambient pressure, a NACA duct is installed under each wing ( outboard ) to apply air pressure into the vent /surge system.

This has a number of functions, it prevents cavitation in the boost pumps and in the event of a double boost pump failure helps the main tanks to gravity feed.

Some how with temperatures in the range of -40c I can’t see anything boiling off, in fact the most likely fuel issue at altitude is waxing of the fuel if the temperature goes below about -45c, this can cause blockage of the fuel filters.

The later B737NG Aircraft also have a system to increase the nitrogen levels in the fuel tanks, this is intended to reduce the ( very unlikely ) risk of explosion. The risk of this is highest on a hot day when the centre tank is almost empty.