Tank Inerting
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Guys, you can’t burn a liquid. It won’t happen. What burns is the vapor or “gas” that comes from that liquid. If you were to make a spark within a filled fuel container - not drop it in, since the fumes would definately ignite - you should see no reaction. Again, as long as you didn’t find an air pocket. It’s when you make the fuel mix with the air that you produce that explosion that takes you down. And yes, now is when I expect someone to have multiple examples to prove me wrong.
To the colder argument making less pressure, yes. If you cool a compressed gas it will decrease in pressure. And if you heat it up it will increase in pressure. If you vent that gas you reduce both the pressure and the temperature of the gas that comes out. For those of us who don’t fully understand boiling point vs. pressure on a liquid, the lower the pressure of a closed system the lower the boiling point of that liquid in that system. As an example, water normally boils at 100C/212F at sea level. This would be at the roughly assumed normal pressure at sea level of 101.32 kPa / 14.7 psi. If you were to keep the water at the same temperature but reduced the pressure it would begin to boil at some point. If you want a painful demonstration ask someone about the bends.
Why should anyone care about the pressure in the system or the boiling point? Hiccups in power are not pleasant, and no fuel to the engine - either due to the tanks being empty or not - means the engine stops working. How long can you fly a brick? Keep in mind, you only have that backup generator for ~ 7 mins.
Sorry for going long winded again.
-Babite -
Must of been a problem for SR-71 devs.
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you can boil a liquid simply by reducing the pressure upon it, applying no external heat. the molecular motion within the fluid itself will cause the reaction. fuels and chemicals often more easily than water. i think that point is being missed.
Must of been a problem for SR-71 devs.
it was.
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you can boil a liquid simply by reducing the pressure upon it, applying no external heat. the molecular motion within the fluid itself will cause the reaction. fuels and chemicals often more easily than water. i think that point is being missed.
We use a vacuum chamber to remove air from epoxies in the composite lab. The epoxies boiled but it was air bubbles that increased in volume and floated to the surface. I would say the epoxy was not turning into a gas.
many liquids will vaporize at ambient temps and pressures. Higher temps and less pressure can change the rate.
I’m kind of curious about how much fuel loss is due to venting to maintain correct tank pressure. Tank pressure to high and something breaks.
If you want a painful demonstration ask someone about the bends
The Bends has nothing to do with vaporizing anything or changing liquid to a gas. Bends has to do with nitrogen saturation in the bones or joint. The body can retain more nitrogen under pressure. During a rapid change in pressure the nitrogen bubbles increase in size and cause problems, iirc.
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I’m kind of curious about how much fuel loss is due to venting to maintain correct tank pressure. Tank pressure to high and something breaks.
No fuel should be loss because of tank venting unless on of the valves is not working. You do get a constant flow of air from the fuel vent under the left wing and if one of the valve is bad you will see fuel come out of it during refueling. You may find picture or videos of the F-16 in-flight refueling and you will see some fuel come out from the bottom of the left wing. It will look like a mist or smoke.
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Like this one. This F-16 is carrying 3 tanks, for some reason carrying three tanks tends to cause some fuel venting.
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LOL, 3 pages of tank inerting and is not even implemented in BMS.
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Some of us are just having too much fun
The ECS “fuel forcing” loop runs from the externals-wings-internals-reservoirs. If the internal series of tanks (wing, F/A, res) are devoid of air then the transfer marches along in sequence with the ECS pushing on the externals and the “light at the end of the tunnel” is beyond the res’s. There’s a small opening on the exit of the res’s so any fuel that bleeds through (instead of to FFP) is dumped back into wings. Unless the externals and internals are maintained at separate pressures then externals can never “catch up” to too empty internals but can only replace internal fuel 1:1. For example with half full internals and full externals the externals won’t empty faster than the engine uses fuel even though there is plenty of room inside. Be aware the “vent” isn’t just an open pipe but a pressure relief valve.
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I am considering buying this document http://arc.aiaa.org/doi/abs/10.2514/6.1981-1638
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LOL, 3 pages of tank inerting and is not even implemented in BMS.
Can be useful if one day …
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Some of us are just having too much fun
The ECS “fuel forcing” loop runs from the externals-wings-internals-reservoirs. If the internal series of tanks (wing, F/A, res) are devoid of air then the transfer marches along in sequence with the ECS pushing on the externals and the “light at the end of the tunnel” is beyond the res’s.
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wings-internals-reservoirs have their own fuel pump with there own feed line. For the fuel to be pushed by ECS pressure, pump pressure must be less than the ECS pressure.When pump pressure is greater than ECS the fuel travel backward in the transfers line. The internal tanks keep the reservoir tank full and the wing tank keeps the internal tank full.
EDIT:
There could be a PPH where suction is greater than ECS pressure and transfer pump pressure. The fuel flow toward the rev boot pump using both transfer pump and transfer lines.There’s a small opening on the exit of the res’s so any fuel that bleeds through (instead of to FFP) is dumped back into wings.
That does not dump into the wing tank. It’s hydraulic pressure that powers the wing tank pump. Yes, the electric reservoir tank boost pump hydraulic powers the wing tank pump.
Unless the externals and internals are maintained at separate pressures then externals can never “catch up” to too empty internals but can only replace internal fuel 1:1. For example with half full internals and full externals the externals won’t empty faster than the engine uses fuel even though there is plenty of room inside.
This looks good to me. The external tanks do not have a fuel pump for savaging. I’m not sure it’s possible. Maybe if the pilot didn’t switch wing/centerline feed when the tank/s empty. <shrug>> Be aware the “vent” isn’t just an open pipe but a pressure relief valve.
what is max pressure in the tanks? Is there an altitude where the fuel “boils” and reaches that relief pressure?
jmo, from d1 schematics.</shrug>
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Some of us are just having too much fun
The ECS “fuel forcing” loop runs from the externals-wings-internals-reservoirs. If the internal series of tanks (wing, F/A, res) are devoid of air then the transfer marches along in sequence with the ECS pushing on the externals and the “light at the end of the tunnel” is beyond the res’s. There’s a small opening on the exit of the res’s so any fuel that bleeds through (instead of to FFP) is dumped back into wings. Unless the externals and internals are maintained at separate pressures then externals can never “catch up” to too empty internals but can only replace internal fuel 1:1. For example with half full internals and full externals the externals won’t empty faster than the engine uses fuel even though there is plenty of room inside. Be aware the “vent” isn’t just an open pipe but a pressure relief valve.
Not sure if I catch all of that. The externals tanks can’t supply fuel as fast as the engine can use it at hi power settings. I wish Cali was around, he is a F-16 fuel system technician.
Maximum fuel transfer rate is 18,000 pph from the 300-gallon fuel tank or 30,000 pph from the 370-gallon fuel tanks. Maintaining fuel flow above these values while the external tank(s) is feeding results in a decrease of internal fuel. Prolonged operation under these conditions may result in the rapid depletion of fuselage fuel and render fuel transfer by siphoning action inoperative. Without siphoning action, fuel transfer to the fuselage tanks is provided by the wing turbine pumps at a maximum rate of 6000 pph. A fuel flow rate greater than 6000 pph continues to deplete fuselage fuel. Under these conditions, the external fuel tank(s) may appear slow to feed and a fuel imbalance may result. Prolonged AB operation in a three tank configuration may result in engine flameout prior to depletion of external fuel.
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…Caper, Not sure if I get all of that. F1 and A1 tanks have transfer pumps (electrical), AFT reservoir has one boost pump and FWD reservoir has 2 electrically powered boost pump (on C model, revers on D models). F-2 tanks have no pumps AFAIK. There are transfer pump ( fuel pressure driven) in the wing. Only the FFP is hydraulically operated by Hydraulic system A.
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Caper, Not sure if I get all of that. F1 and A1 tanks have transfer pumps (electrical), AFT reservoir has one boost pump and FWD reservoir has 2 electrically powered boost pump (on C model, revers on D models). F-2 tanks have no pumps AFAIK. There are transfer pump ( fuel pressure driven) in the wing. Only the FFP is hydraulically operated by Hydraulic system A.
I know you don’t get it. lol
Read your own post.
Without siphoning action, fuel transfer to the fuselage tanks is provided by the wing turbine pumps at a maximum rate of 6000 pph.
The wing turbine pump does not operate on Hydraulic oil. It operates on fuel pressure from the reservoir boost pump. It still is and can be called a hydraulic turbine.
There are two transfer system that feed the reservoir tanks. The suction transfer lines and the transfer pump lines.
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Not sure if I catch all of that. The externals tanks can’t supply fuel as fast as the engine can use it at hi power settings. I wish Cali was around, he is a F-16 fuel system technician.
Maximum fuel transfer rate is 18,000 pph from the 300-gallon fuel tank or 30,000 pph from the 370-gallon fuel tanks. Maintaining fuel flow above these values while the external tank(s) is feeding results in a decrease of internal fuel. Prolonged operation under these conditions may result in the rapid depletion of fuselage fuel and render fuel transfer by siphoning action inoperative. Without siphoning action, fuel transfer to the fuselage tanks is provided by the wing turbine pumps at a maximum rate of 6000 pph. A fuel flow rate greater than 6000 pph continues to deplete fuselage fuel. Under these conditions, the external fuel tank(s) may appear slow to feed and a fuel imbalance may result. Prolonged AB operation in a three tank configuration may result in engine flameout prior to depletion of external fuel.
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This is a quote that closes it out for me. Fuel flow under 12000ppm is when the suction transfer line act like a return lines for the transfer pump lines and the transfer pump lines can top off the tanks. During maneuvers, the suction transfer line can suck air because of the fuel sloshing away form the pickup. One function of the transfer pump lines is to fill the tanks and remove the air. Note: that the reservoir tanks have a separate air removal system. It’s very important to keep the reservoir tanks full w/o air to supply air bubble free fuel to the engine during maneuvers. (RC model stuff)
Also, at full AB at sea-level max PPM is what(?) 40,000ppm, iirc. When feeding on the c/l tank, 30kppm comes from the c/l tank and the other 10kppm come from the a/c tanks. If the wing tank empty the suction transfer line will suck air for the ECS pressure. Have you sucked on a drinking straw with a hole in it….same thing. So, ppm from the external tanks is limited to the 6000ppm wing transfer pump (x2 12000ppm). Now you are feeding 12kppm on the external and 28k on the internals.
I still think Fred is right about external not be able to “catch up” to fill the other tanks.
you guys are way to focused on the ECS pressure.
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@casper
Yes there are plenty of elec, fuel press, hyd-driven pumps too. I was ignoring those and focusing on just the ECS path but it is a good point that since it all works on pressure deltas that some parts of the system could be flowing backward if its boost pressure was inferior to a parallel system. It is not surprising that ECS pressurization is rarely or never the “prime motivator” for internal fuel flow.hat does not dump into the wing tank. It’s hydraulic pressure that powers the wing tank pump.
I was looking at the tank pressurization (blue with black dots) that come from the reservoirs and seem to terminate in the wing tanks. I guess that’s air ejectors? Legend shows check valves but it makes sense for those to be the ejectors (or at least directly after them), a sort of air loopback.
normal 4.7–6.4 psi, combat 1.0-3.0 psi is the schedule. Vapor pressure JP-8 at 100°F is 0.029 psi and there’s a relief mechanism for oil/fuel cooling at 200°F (100F sounds like a reasonable operating temp). For reference 40,000’ ISA is ~2.6 psi. But yay Maxwell-Boltzmann distribution you’re going to have some vapor even if ambient is above vapor pressure. To answer the resulting question “what fraction is evaporated at various temperatures and pressures” I don’t know except more or less. The joy is that temperature as a single number is an average. In reality each molecule has an individual temperature with a distribution. What fraction is above the bond energy breaking point is some statistical analysis. In either case, no, fuel on its own shouldn’t cause overpressure (unless it’s on fire ). The cause would be ECS excess or refueling (either AAR or ground).
@mvgas
If you think of A B C as external, internal, and engine there is no way to blow fluid from tank A to fill up tank B unless some fluid goes from B to C. If A is full and B is partial there’s no way to “fill tank B from tank A.” The only way to get fluid out of A is for that same amount to go to C. Reservoir tanks don’t have this problem since they have air ejectors.
F-2 “saddle” Has two pumps driven by bleed fuel pressure off the reservoir outputs.
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I was looking at the tank pressurization (blue with black dots) that come from the reservoirs and seem to terminate in the wing tanks. I guess that’s air ejectors? Legend shows check valves but it makes sense for those to be the ejectors (or at least directly after them), a sort of air loopback.
That’s system removes air from the reservoir tanks. I think there an air/fuel separator and air is expel from the tank and injected into tank pressure system.
There’s a small opening on the exit of the res’s so any fuel that bleeds through (instead of to FFP) is dumped back into wings.
“exit of the res’s” sounds like Red on the legend (Boost Pump Pressure). “small opening” sounds like bleed fuel pressure (red w/black dot) that powers the wing transfer pump.
normal 4.7–6.4 psi, combat 1.0-3.0 psi is the schedule. Vapor pressure JP-8 at 100°F is 0.029 psi and there’s a relief mechanism for oil/fuel cooling at 200°F (100F sounds like a reasonable operating temp). For reference 40,000’ ISA is ~2.6 psi. But yay Maxwell-Boltzmann distribution you’re going to have some vapor even if ambient is above vapor pressure. To answer the resulting question “what fraction is evaporated at various temperatures and pressures” I don’t know except more or less. The joy is that temperature as a single number is an average. In reality each molecule has an individual temperature with a distribution. What fraction is above the bond energy breaking point is some statistical analysis. In either case, no, fuel on its own shouldn’t cause overpressure (unless it’s on fire ). The cause would be ECS excess or refueling (either AAR or ground).
I was wonder if jet fuel @high altitude behaves like propane at sea-level. At 100°F propane is at 200psi!!!
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Guys, you can’t burn a liquid. It won’t happen. What burns is the vapor or “gas” that comes from that liquid. If you were to make a spark within a filled fuel container - not drop it in, since the fumes would definately ignite - you should see no reaction. Again, as long as you didn’t find an air pocket. It’s when you make the fuel mix with the air that you produce that explosion that takes you down. And yes, now is when I expect someone to have multiple examples to prove me wrong.
To the colder argument making less pressure, yes. If you cool a compressed gas it will decrease in pressure. And if you heat it up it will increase in pressure. If you vent that gas you reduce both the pressure and the temperature of the gas that comes out. For those of us who don’t fully understand boiling point vs. pressure on a liquid, the lower the pressure of a closed system the lower the boiling point of that liquid in that system. As an example, water normally boils at 100C/212F at sea level. This would be at the roughly assumed normal pressure at sea level of 101.32 kPa / 14.7 psi. If you were to keep the water at the same temperature but reduced the pressure it would begin to boil at some point. If you want a painful demonstration ask someone about the bends.
Why should anyone care about the pressure in the system or the boiling point? Hiccups in power are not pleasant, and no fuel to the engine - either due to the tanks being empty or not - means the engine stops working. How long can you fly a brick? Keep in mind, you only have that backup generator for ~ 7 mins.
Sorry for going long winded again.
-BabiteGotta love a guy who knows his chemistry.
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Frederf/Caper, I get that, what I meant is that you guys make it sound more complicated that it needs to be. I think about it the same way I did when defueling the jet for maintenance. Everything feed the reservoirs in sequence, external to wing, etc. Air and fuel in each tank will adjust accordingly. The system have way to get air if a vacuum is form, called negative pressure relive valve on the back bone ( it would look like a golf ball size round vent). Overpressure will vent out of fuel vent under left wing. Anything else is just over thinking it in my opinion.
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The system have way to get air if a vacuum is form, called negative pressure relive valve on the back bone ( it would look like a golf ball size round vent).
thats new. One way to get a vacuum in the tank is if the ECS/tank pressure fails.
I was kind of done with this threat. Maybe you should bring it up on the next threat that we derail.