Volumetric flow rate for jet fuel

[Rob130]

We are a group of C-130 flight engineers trying to determine the amount of fuel that can be transferred in a manner not covered in the operations manual.

We need to determine a GPM or GPH for fuel flow into a tank.

The pipe length is 3 feet, the tube is .5 ID 6061 AL, Fluid density is .03 lbs/gal (JP5), Kinematic viscosity is 8.5mm/2/s, pressure is 40 psi from pump and line is open to tank with no back pressure.

I have tried some online calculators but lack the true engineering back ground to be confident in the result. Any help would be greatly appreciated.

TIA, Rob130

 

[katmar]

Rob,

Your density data is wrong. JP5 would have a density of around 6.7 lb/USgal (SG=0.8).

If the ID is 0.5 inches then you will get a flow of around 28 USGPM. If the ID is 0.5 feet then the flow would be 6000 USGPM.

Both of these calculations assume that the piping on the suction side of the pipe is not a restriction, and the pump would continue to supply 40 psi at the calculated flows.

regards
Katmar

 

[blackwed]

The mechanical guys here are sharp but they will need some more info. I'm a mere electrical but will try to predict some of the Q's. What's your pump's rated volumetric output? What kind of pump is it? What's the piping configuration on the upstream side of the pump and will it provide sufficient suction pressure to support the pump's rated output? With my limited knowledge, I'd expect 40 psi on one end of a 3' tube and zip on the other could really impart some velocity to a liquid. Maybe more than you really want!? Maybe enough to erode the aluminum tubing, maybe enough that the feed to the pump can't keep up? Like I said, you came to the right place.

BTW, density for jet 5 is .788-.845 kg/L or nominally 6.75 #/gal.

DB

 

[blackwed]

Katmar beat me to the density. Guess I was composing while he was submitting.

DB

 

[katmar]

Hi Blackwed - yes, we must have overlapped a bit there, but I was pleased to see that you voiced exactly the same concerns that I did.

BTW, I see I made a typo in the last paragraph. It should of course read
"Both of these calculations assume that the piping on the suction side of the pump is not a restriction, and the pump would continue to supply 40 psi at the calculated flows."

katmar

 

[Rob130]

Thanks for the resonse!

I made another mistake, the tube is .5 in but the fitting the fuel passes thru into the tank is .375 in.

The pump is centrifugal pump with a rated output of 40 psi no flow and 27,400 gph at 12 psi.

Assume the only demand on the pump is the .5 tube and fuel passing trhu the .375 in fitting, fuel being pumped into the airspace above the fuel in an unpressurized tank.

 

[blackwed]

To move liquid thru a 0.5" tube at 27400 gph (456 gpm) the velocity will be around 735 fps (over 500 mph) and thru the 0.375 orifice the numbers are like 1300 fps or something in excess of 900 mph!! A) it won't happen and B) you're glad of it. What's going to happen to whatever it hits!? Plus, whatever the pump's discharge rate is, the suction side has to be fed at the same rate. I don't think gravity will give you 450 gpm unless you have like a 4" pipe.

Anyway, I'm guessing the two points you gave us are the extremes on the pump's curve, i.e., shut off and runout? For a pump capable of that performance to discharge into this small a opening, my guess is that you'll be operating way way way closer to the 40 psi and no flow end than to the other, in which case you probably won't move enough product thru the pump to keep it from overheating. It sounds like you're working on a real worst case scenario kind of effort here. My gut reaction is that it'll break something before it can do you any good.

Like I said though, I'm electrical and the real pipe and pump guys may have some tricks of the trade I don't know about.

DB

 

[Rob130]

Those are the pumps specs and what it is capable of under high demand. The pumps are fuel boost pumps that are submerged in fuel to keep them cool. They are designed to provide a higher volume of flow than maximum engine demand.

We are trying to determine a volumetric flow thru a .375 in. open line into a fuel tank.

It is one of those trivia questions/C-130 urban lengends dealing with the question of how much fuel you can transfer, inflight, using the 40psi out put of a fuel pump, thru an open valve designed to bleed residual pressure from the crossfeed fuel manifold and into the #2 fuel tank thru a .375 in. ID bulkhead fitting.

The vavle is designed to be open only while the primer valve switch is depressed, and closes when the valve switch is released. The valve orifice is 1 inch and is not the restriction. If, while the switch is depressed you open the circuit to the valve, it will remain in the open position and any fuel in the manifold will flow into the #2 tank.

I would like to be able to inform our instructors and students as to the rate, in pph/ppm, they could expect to see in this scenario.

Thanks again, Rob130

 

[blackwed]

So the small line is not the only place the pump is sending fuel. If I understand correctly, you're just slip-streaming a little product off what your pump is sending to the turbines?

To simplify it a little, if you had 40 psi behind a 0.5" hole in a bulkhead, you'd move about 31 gpm. Change the hole to 0.375 and flow drops to the 16-17 gpm ballpark. I don't think we're too far off for the purpose of proving or busting an "urban legend." I wouldn't try to fly my numbers over any mountains though.

So, what's the story that's going around? What kind of rates have supposedly been seen?

DB

 

[Rob130]

Nobody ever ventured a guess or tied to find the answer. They seemed content to say it could be done but not what could be expected!

 

[katmar]

I re-ran my numbers with the 0.375" fitting and my predicted flowrate dropped to around 22 gpm (=1,320 gph) at 40 psi. This flowrate is much closer to the zero flow point than the 27,400 gph point, so the pressure should not drop much. You would probably get something like 20 gpm or 8,000 pounds per hour.

I agree with blackwed - if I was flying in that plane I would much rather know that someone had measured the actual flow than rely on my own calcs!

 

[aviat]

Those pumps are designed to slip when at pressure. As you know the way they are tested is to dead end in a manifold and read the pressure. As long as it meets the min pressure, it will provide the required flow for one engine. As pumps wear they are able to provide less flow.

How much flow you get though the cross feed primer valve would depend on how much excess flow you have in the pump(s). But using that valve could cause some problems. Take a situation where number one pump was weak, but still meets tests, and could barely feed its engine, then you open number one cross feed valve and primer valve. This would cause number one engine to be starved of fuel and can do damage to engine.

The primer valve was designed to bleed the cross feed manifold after draining and pressure tests. What you are planning to do is not approved; otherwise it would be in the manuals. If something should go wrong there would be a lot of questions asked, especially if the instructors are teaching unapproved practices to students.

 

[Rob130]

The pump is only used to feed the cross feed manifold, all engines would be on tank to engine feed. 40 psi out of aux or external tanks.

We do not teach, endorse or recommend this procedure. As you say, if it is not in the Operators Manual we do not teach it as a procedure or "technique".

This information will be used to discourage engineers from performing a common though unapproved procedure. After extened ground operation with the APU running (fuel supplied by #2 tank) you can get an unbalanced fuel load.

Engineers will hit the prime and pull CB to put fuel back in #2 to balance fuel. So what happens when they forget about it during ground ops? The open prime valve will continue to fill the tank at a rate faster than burn off, the topping valves are not in play and the excess flow into the tank can fill the tanks 3% airspace and cause the tank to vent on the ground.

AVIAT "But using that valve could cause some problems. Take a situation where number one pump was weak, but still meets tests, and could barely feed its engine, then you open number one cross feed valve and primer valve. This would cause number one engine to be starved of fuel and can do damage to engine."

Very true, but the engine fuel low pressure warning light would illuminate and the pumps are designed to gravity feed so the engineer would see this (hopefully). The real problem would be if the pump failed, air would be drawn into the crossfeed manifold with the prime valve open, the engine pumps would suck air and not fuel! Good/Bad Scale - Bad.

I want to try and keep people from outsmarting themselves when they think they know how something works but really don't!

 

[aviat]

To get your flow rate just set ups the pumps and valves the way you want to measure. After it pumps for a few seconds record fuel in #2 tank and start a stopwatch. Let it run for 5 to 10 minutes then record the quantity in #2 again. Find the difference between the two readings and divide by the time in minutes and that will give pounds per minute. If you require flow in liters or gallons per minute; divide by the density.

You could do it on more that one airplane and average them. Density changes with temperature. If you have a wide range of temperatures you may want to compensate for that.

 

[Rob130]

Good suggestion, unfortunatly we don't have access to an aircraft, simulators only.