B737 Scavenge Jet Pump

[POOKEY]

BOEING STATE ON PAGE 12.20.2 OF VOLUME 2 THAT FUEL IS TRANSFERED FROM THE CENTER TANK
TO MAIN TANK No 1 UNDER CERTAIN CONDITIONS.

MY ACTUAL UNDERSTANDING IS THAT FUEL IS " NOT TRANSFERED " AT ALL.
WHEN THE " SCAVENGE JET PUMP " ( APROX 80 KG/HR ) OPERATES, IT SYPHONS FUEL FROM THE CENTRE TANK AND
COMBINES IT WITH FUEL SUCTION FEED FROM MAIN TANK No 1 AND SENDS IT TO THE ENGINE VIA THE
No 1 FUEL MANIFOLD.
THE " ILLUSION " OF THE TRANSFER OF FUEL COMES ABOUT BECAUSE THE DECREASE IN FUEL TANK QUANTITY
IN MAIN TANK No 1 IS LESS THAN THAT IN MAIN TANK No 2 ( PROVIDING FF ARE APPROX SAME ) GIVING A FUEL IMBALANCE.

DOES ANYONE ELSE SEE IT THIS WAY ( IE PAGE 12.20.4 ) ?

[avioniker]

From the SDS section 28-10-00-005:

FUEL STORAGE - CENTER TANK SCAVENGE SYSTEM

General
The center tank fuel scavenge system increases the quantity of fuel you can use.

Functional Description
The left forward boost pump supplies motive flow to a jet pump. The jet pump removes fuel from the center tank and transfers it to main tank
1. The jet pump has no parts that move. It is on the left front spar. A fuel scavenge shutoff valve controls fuel sent to main tank 1. When the fuel level in main tank 1 decreases to 4,487 lbs (1,990 kgs) the float valve opens.
The minimum fuel transfer rate for the fuel scavenge system is 220 lbs/hour (100 kgs/hour). The fuel transfer rate for the fuel scavenge system is generally between 220 lbs/hour (100 kgs/hour) and 450 lbs/hour (200 kgs/hour).

I'd say you have a pretty good understanding of the system..

[LEM]

No, mates, fuel is actually transferred from the center to the main tank.
The forward pump creates an additional flow to the tank itself, and as Bernoulli demonstrated, this creates a low pressure which "pulls" the center tank fuel into the main one.
Even if the main tank is full initially, it is this circulation of fuel which creates the suction to the main tank anyways. In this particular case, the center tank fuel will "be ready" to replace the main tank fuel used by the engine until the operation stops.

More complicated to explain than actually is.
I'm talking about classic, don't know if NG is different, but a scavenge jet pump is a scavenge jet pump. :ok:

[POOKEY]

THANKS AVIONKER .

LEM...I AM NOT SURE ABOUT THE CLASSIC'S J.T.P.
BUT IF THEY ARE BOTH THE SAME ARE YOU SAYING THAT THE J.T.P IS OPERATING AS SOON AS No1 FWD FUEL PUMP IS OPERATING ?

IN THIS CASE THAT WOULD MEAN THAT FUEL FROM THE CENTRE TANK WOULD COMMENCE TRANSFERING TO THE MAIN TANK No 1 AS SOON AS THERE IS SUFFICENT VOLUME AVAILABLE IN THE MAIN TANK No 1. IF THIS IS THE CASE THAN THE CLASSICS IS DIFFERENT TO THE NG.

THANKS FOR YOUR REPLY AND YES I AM SURE IT IS "More complicated to explain than actually is. "

CHEERS MATE

[B73567AMT]

Both the Classic and Next Gen Center Tank Scavenge have only one thing in common:
The fact that they transfer the fuel from the center tank to the number 1 tank.
Thats it!!!

They are activated differntly. The scavange circuit for the classic is activated manually. When the flight crew gets the low pressure lights in the center system, the switches are placed off and then on. All this logic is sent to a box (EAU) which activates the scavenge circuit for 20 minutes. This normally takes place when the center tank is pretty much empty.

On the B737NG however, everything is automatic. It activates when the No.1 Main tank is about half full something like 4,300 lbs. At this capacity a float switch is activated which opens the scavenge shut off valve. This allows the flow of fuel from ctr to no.1.

Yes there will be a bit of an imbalance since the no.1 main tank is being supplied with fuel from the center while contunuing to burn its current supply. So the no.1 tank should have more fuel.

[Captain Stable]

Pookey, as a general request not only to you but to others, can you please post in future in upper and lower case please?

Two reasons - firstly it makes it much easier to read than a post all in capitals, and secondly because by long internet tradition, use of all caps is considered shouting.

Thanks.

[tykon]

To all,

In the NG, fuel is always being transferred. from centre to wings.

As pilots, you would know that you aren't generally allowed to transfer (with your boost pumps) anything under roughly 4 -500kg.

In the Ng, your scavenge is always on, so on a long flight, you will notice the centre decrease, but maybe not on a shorter flight.

In the older 737, there was a 20 min scavenge once centre pumps turned off, and as long as your wing tank pump(namely left forward I think) was on, but now it is always on.

hope this is totally correct, but feel free to correct me if you want, as this is from memory.

[LEM]

BUT IF THEY ARE BOTH THE SAME ARE YOU SAYING THAT THE J.T.P IS OPERATING AS SOON AS No1 FWD FUEL PUMP IS OPERATING ?

No, the fuel scavenge shutoff valve has to open for the main fwd pump to be able to do this job. ;)

[4Screwaircrew]

I have pulled the Boeing book from the shelf, B 737-300.

Center Tank Scavange Pump

"When both center tank fuel pump switches are turned OFF, the fuel scavange shutoff valve opens. this allows fuel pressure from the main tank No. 1 forward pump to operate the center tank scavange jet pump, which transrefrs the remaining center tank fuel to main tank No. 1. Ater 20 minutes, the fuel scavange shutoff valve automatically closes."

Which is how I have alwaysbelieved it to be, I have no knowledge of the NG so perhaps someone can come up with a Boeing quote for that?;)

[avioniker]

Just in case nobody wants to scroll to the top, here's the Boeing quote again:

From the SDS section 28-10-00-005:

"FUEL STORAGE - CENTER TANK SCAVENGE SYSTEM

General
The center tank fuel scavenge system increases the quantity of fuel you can use.

Functional Description
The left forward boost pump supplies motive flow to a jet pump. The jet pump removes fuel from the center tank and transfers it to main tank
1. The jet pump has no parts that move. It is on the left front spar. A fuel scavenge shutoff valve controls fuel sent to main tank 1. When the fuel level in main tank 1 decreases to 4,487 lbs (1,990 kgs) the float valve opens.
The minimum fuel transfer rate for the fuel scavenge system is 220 lbs/hour (100 kgs/hour). The fuel transfer rate for the fuel scavenge system is generally between 220 lbs/hour (100 kgs/hour) and 450 lbs/hour (200 kgs/hour)."

[LEM]

OK , avioniker, that's NG. Works automatically.

Classic: works when the center pumps are manually turned off. :ok:

[B73567AMT]

Classic: Works when Center pumps are turned off and then back on.

[fruitloop]

Classic :Works when centre pumps are turned "on"then "off"