Hartstoc
Well Known Member
I’m in the process of installing this fully redundant dual fuel pump assembly with independent external check and relief valves that I’m assembling using exquisite parts obtained from Don Rivera at Airflow Performance.
Return lines not shown here will carry bypass fuel from the relief valves back to the tank of origin through a two-stage selector valve, and power will be provided by a redundant electrical system including two powerful EarthX batteries. The engine-driven fuel pump will be eliminated completely. I’ll describe this design and the reasoning behind it in a future thread, but here I want to share my exploration of the electric pump’s innards.
I will be depending upon these sealed, non-serviceable pumps full-time. Even though they are rated for a 100% duty-cycle, I thought It would be a good idea to look inside one. There are many variant automotive applications for these pumps with assorted inlet/outlet configurations and applications, and I was able to obtain a nearly identical new one on eBay at a good price for the purpose of an “autopsy”. This photo shows the innards of that pump spread out , and a detailed photo album with captions appended to each photo can be viewed using the link below it:
(Edit note:the dismantled pump is an airtex E8228. It is externally very similar to those supplied by AP. Don Rivera indicated to me that they no longer use Airtex pumps, but that the Airtex equivalent to the ones they do use would be P/N E-2315. The pumps in the top photo are Delphi FD0011’s, rated at 106PSI. I wanted to keep the thread generic because, from what I’ve seen in various parts diagrams, most are pretty similar and the same concerns would apply to all roller-vane pumps.)
https://public.fotki.com/Hartstoc/anatomy-of-a-fuel-pump/?view=roll#1
The experience of dismantling this pump was both reassuring and a bit frightening. Quality, materials and construction of all components is quite impressive and supports the 100% duty-cycle rating, but the tortured pathway taken by fuel as it passes through this pump really got my attention. The pump is completely flooded internally with, and lubricated by, the fuel passing through it. The motor’s brushes, contacts and rotating armature(which beats the living bejesus out of the passing fuel) are all fully submerged. Most of these pumps were initially designed to also be entirely submerged full time inside gas tanks, and have been variously adapted to external installations. When submerged inside a tank, much of the pump’s heat can migrate out directly through the sidewalls, but when mounted externally nearly all heat must be carried away by fuel passing through the pump itself. The heat involved here is substantial, as nearly all energy from the 4-8 Amps(at 12V) consumed continuously ends up as heat imparted directly into the passing fuel.
All of these sealed roller-vane type pumps are constant-displacement, meaning that they must be allowed to pump a fixed flow of 30-50GPH or so, depending on their size.
Based upon what I saw, I’ve drawn two personal conclusions. These are my own personal opinions and all are welcome to offer theirs in reply. I know the second one will be controversial, so I leave it to each reader to decide if what I have to say rings true:
1- Running a tank dry while operating on one of these pumps could be a really bad idea for a couple of reasons. They are not very good at self- priming and cannot pump vapor effectively. A fair amount of pressure may be required get fuel flowing through system components like the flow divider and to the injectors. I’m planning on installing a manual bypass of the relief valve circuit to allow free flow back through the return lines in the event that the pump needs to purge air and prime itself should a tank be run dry inadvertently. Another possible concern is internal arcing and detonation. Though impossible with the pump completely full of fuel(no oxygen), it is conceivable that this could happen at least briefly within a pump that has been run dry and becomes filled with an explosive air-fuel mixture.
2- I’ve concluded that these pumps should never be installed in aircraft unless full-sized fuel return lines are provided to route bypass flow, which is always a far greater volume than throughput flow to the engine, back to the tank from which the fuel is drawn so as to allow it to cool and to harmlessly dissipate any vapor that may have formed. Of particular danger, wether used as a backup boost pump or as the sole source of fuel pressure, are pumps of this type equipped with short-loop recirculation of bypass fuel back into the pump inlet, be it internal or external.
Consider that this recirculated fuel is being severely agitated, heated, pressurized, and then passed through a relief valve where the pressure drops to near zero each and every time it makes a circuit around this little loop. If the pump is flowing 35GPH and the fuel burn is 7GPH, then 80% of the total flow is essentially locked into the recirculation loop.
If the fuel supply is cold, the throughput of 7GPH may well be capable of stabilizing full system temperature at a level below the fuel vapor point at the relief valve exit. On the other hand, if the incoming fuel is warm, and perhaps the flow has been reduced for a descent, the temperature of the recirculating fuel will rise, and a tipping point will eventually be reached at which the hot fuel will generate a lot of vapor when exiting the relief valve. This vapor is not easy to re-liquify, and these pumps do not pump it effectively, so in short order this positive feedback loop will impede flow to the engine, and the pump will enter into an ever-hotter, destructive, vapor-locked condition that can only be remedied by shutting it down and waiting a very long time for it to cool down.
Some may wonder why, if I’m right, the combination of high pressure engine driven pumps with short-loop recirculating electric pumps as backup/boost pumps has not caused big problems in the hundreds, maybe thousands, of aircraft so equipped. I think it is partly due to the very low failure rate of the engine driven pumps, and typically very limited use of the boost pumps, mostly at very high throughput flows during takeoff and climb. This cannot be taken as proof that such installations are inherently safe, only that few pilots ever venture into their danger zones.
As an aside, all of this also suggests that if you are flying with an electric backup/boost pump having short-loop recirculation, and you find yourself using it as backup in an emergency scenario, you should not “baby” the plane, but rather fly full-throttle, full rich to the nearest safe landing area to maximize throughput to the engine and insure that the pump stays cool.- Otis
(Edit note: As often happens tapping into VAF “group wisdom”, comments later in this thread have led me to a much better pump than the one pictured here- details can be found below)
Return lines not shown here will carry bypass fuel from the relief valves back to the tank of origin through a two-stage selector valve, and power will be provided by a redundant electrical system including two powerful EarthX batteries. The engine-driven fuel pump will be eliminated completely. I’ll describe this design and the reasoning behind it in a future thread, but here I want to share my exploration of the electric pump’s innards.
I will be depending upon these sealed, non-serviceable pumps full-time. Even though they are rated for a 100% duty-cycle, I thought It would be a good idea to look inside one. There are many variant automotive applications for these pumps with assorted inlet/outlet configurations and applications, and I was able to obtain a nearly identical new one on eBay at a good price for the purpose of an “autopsy”. This photo shows the innards of that pump spread out , and a detailed photo album with captions appended to each photo can be viewed using the link below it:
(Edit note:the dismantled pump is an airtex E8228. It is externally very similar to those supplied by AP. Don Rivera indicated to me that they no longer use Airtex pumps, but that the Airtex equivalent to the ones they do use would be P/N E-2315. The pumps in the top photo are Delphi FD0011’s, rated at 106PSI. I wanted to keep the thread generic because, from what I’ve seen in various parts diagrams, most are pretty similar and the same concerns would apply to all roller-vane pumps.)
https://public.fotki.com/Hartstoc/anatomy-of-a-fuel-pump/?view=roll#1
The experience of dismantling this pump was both reassuring and a bit frightening. Quality, materials and construction of all components is quite impressive and supports the 100% duty-cycle rating, but the tortured pathway taken by fuel as it passes through this pump really got my attention. The pump is completely flooded internally with, and lubricated by, the fuel passing through it. The motor’s brushes, contacts and rotating armature(which beats the living bejesus out of the passing fuel) are all fully submerged. Most of these pumps were initially designed to also be entirely submerged full time inside gas tanks, and have been variously adapted to external installations. When submerged inside a tank, much of the pump’s heat can migrate out directly through the sidewalls, but when mounted externally nearly all heat must be carried away by fuel passing through the pump itself. The heat involved here is substantial, as nearly all energy from the 4-8 Amps(at 12V) consumed continuously ends up as heat imparted directly into the passing fuel.
All of these sealed roller-vane type pumps are constant-displacement, meaning that they must be allowed to pump a fixed flow of 30-50GPH or so, depending on their size.
Based upon what I saw, I’ve drawn two personal conclusions. These are my own personal opinions and all are welcome to offer theirs in reply. I know the second one will be controversial, so I leave it to each reader to decide if what I have to say rings true:
1- Running a tank dry while operating on one of these pumps could be a really bad idea for a couple of reasons. They are not very good at self- priming and cannot pump vapor effectively. A fair amount of pressure may be required get fuel flowing through system components like the flow divider and to the injectors. I’m planning on installing a manual bypass of the relief valve circuit to allow free flow back through the return lines in the event that the pump needs to purge air and prime itself should a tank be run dry inadvertently. Another possible concern is internal arcing and detonation. Though impossible with the pump completely full of fuel(no oxygen), it is conceivable that this could happen at least briefly within a pump that has been run dry and becomes filled with an explosive air-fuel mixture.
2- I’ve concluded that these pumps should never be installed in aircraft unless full-sized fuel return lines are provided to route bypass flow, which is always a far greater volume than throughput flow to the engine, back to the tank from which the fuel is drawn so as to allow it to cool and to harmlessly dissipate any vapor that may have formed. Of particular danger, wether used as a backup boost pump or as the sole source of fuel pressure, are pumps of this type equipped with short-loop recirculation of bypass fuel back into the pump inlet, be it internal or external.
Consider that this recirculated fuel is being severely agitated, heated, pressurized, and then passed through a relief valve where the pressure drops to near zero each and every time it makes a circuit around this little loop. If the pump is flowing 35GPH and the fuel burn is 7GPH, then 80% of the total flow is essentially locked into the recirculation loop.
If the fuel supply is cold, the throughput of 7GPH may well be capable of stabilizing full system temperature at a level below the fuel vapor point at the relief valve exit. On the other hand, if the incoming fuel is warm, and perhaps the flow has been reduced for a descent, the temperature of the recirculating fuel will rise, and a tipping point will eventually be reached at which the hot fuel will generate a lot of vapor when exiting the relief valve. This vapor is not easy to re-liquify, and these pumps do not pump it effectively, so in short order this positive feedback loop will impede flow to the engine, and the pump will enter into an ever-hotter, destructive, vapor-locked condition that can only be remedied by shutting it down and waiting a very long time for it to cool down.
Some may wonder why, if I’m right, the combination of high pressure engine driven pumps with short-loop recirculating electric pumps as backup/boost pumps has not caused big problems in the hundreds, maybe thousands, of aircraft so equipped. I think it is partly due to the very low failure rate of the engine driven pumps, and typically very limited use of the boost pumps, mostly at very high throughput flows during takeoff and climb. This cannot be taken as proof that such installations are inherently safe, only that few pilots ever venture into their danger zones.
As an aside, all of this also suggests that if you are flying with an electric backup/boost pump having short-loop recirculation, and you find yourself using it as backup in an emergency scenario, you should not “baby” the plane, but rather fly full-throttle, full rich to the nearest safe landing area to maximize throughput to the engine and insure that the pump stays cool.- Otis
(Edit note: As often happens tapping into VAF “group wisdom”, comments later in this thread have led me to a much better pump than the one pictured here- details can be found below)
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