What is the role of HP (high pressure) and LP (low pressure) fuel pumps in aircraft like the A320? Why are two different ones needed?
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$\begingroup$ There are no HP vs LP pumps in an A320. The various pumps supply the same pressure. Some models have jet pumps that use electric pump pressure to transfer fuel. Where did you see mention of two different kinds of electric pumps on A320? $\endgroup$– Juan JimenezCommented Nov 16, 2022 at 0:42
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$\begingroup$ hursts.org.uk/airbus-technical/html/ar01s19.html It's shown in the design diagram. It talks about how the engine master switches control the LP & HP shut-off valves, can you please elaborate what exactly are they? $\endgroup$– Sachin ChaudharyCommented Nov 16, 2022 at 3:27
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3$\begingroup$ @JuanJimenez Clearly, you can see A320 has HP & LP pumps in edit :) ! $\endgroup$– Sachin ChaudharyCommented Nov 16, 2022 at 5:04
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1$\begingroup$ You need HP pump to push fuel into the combustion chamber against the chamber pressure, and also power the hydraulics of the engine. But the HP pump is at the engine and quite far from the tank, so you really can't rely on it to suck fuel from the tank for itself, unless you pressurize the entire tank. This is just like a car: you have a LP pump at the tank and a HP pump (or two) at the engine. $\endgroup$– user3528438Commented Nov 16, 2022 at 7:14
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1$\begingroup$ I think these pumps are in the engine. So more of a question about why the engine needs it than the airframe. $\endgroup$– BowlOfRedCommented Nov 16, 2022 at 7:30
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2 Answers
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The question asks about all low pressure fuel pumps on commercial aircraft but the diagram depicts only one specific instance (of a two stage high pressure pump) and is confusingly labeled.
The low pressure pumps move fuel around the aircraft where only volume is needed. These pump through comparatively large pipes with low back pressure, so they operate at less than 60psi. Fuel is used to maintain balance, reduce flutter susceptibility and for cooling in addition to providing an energy source for the engines. The a320 has six low pressure pumps serving the center and inside fuel tanks. A series of valves control where fuel goes. The outer tanks gravity feed the inner tanks, so no pumps are needed.
The high pressure pumps are attached to an engine and atomize fuel into a burnable spray inside the combustion chamber, where high pressure is needed to overcome combustion pressures as high as 600psi in modern engines. This nozzle test is at 200psi but in operation the pressure may be 1000psi or higher.
The high pressure pump typically consists of multiple stages as a single stage cannot achieve the desired pressure. Shown is a two stage high pressure pump with a low pressure centrifugal boost stage on the right followed by a high pressure gear stage to the left.
This matches the OP's diagram depicting a single mechanically driven engine fuel pump (in red) containing a centrifugal low pressure and geared high pressure 'pumps' (in yellow) more easily understood as stages.
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$\begingroup$ There's some confusion here. The LP fuel pump is in the engine and is mechanically driven. As you can see from the FCOM schematic, the LP fuel pump is after the LP fuel valve. The pumps you're referring to are the tank pumps, they're electrical and sit in the tanks. $\endgroup$ Commented Nov 18, 2022 at 19:54
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$\begingroup$ @Pilothead That's my point about the LP valve. In your first picture, you can see that the 6 tank pumps are upstream of the LP valve. In OP's schematic, the LP fuel pump is downstream of the LP valve. That shows they are different pumps. If you look at the A320's overhead fuel panel, those pumps are marked as e.g. "L TK PUMPS 1/2". $\endgroup$ Commented Nov 19, 2022 at 0:10
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$\begingroup$ @user71659 So two different points: one - the question was not limited to the diagram and specifically stated a "canonical answer" about low and high pressure pumps was wanted. This requires inclusion of the pumps I described. I am not saying they are the one in the diagram. two - the low pressure 'pump' in the diagram is the low pressure stage of the two stage high pressure pump in the engine, with confusing labeling. The diagram draws a box around both 'pumps' and calls it a pump. If you have any documentation to the contrary, I'd like to see it in an answer. $\endgroup$ Commented Nov 19, 2022 at 2:00
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$\begingroup$ @Pilothead You're trying to say the two pumps marked in one red box are a ONE single unit? I think they are their own distinct and individual pumps and not "Stages" on a one pump. $\endgroup$ Commented Nov 20, 2022 at 5:46
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$\begingroup$ @SachinChaudhary Marked in one red box labeled "Fuel Pump" in your diagram. This whole discussion is immaterial to the question you asked, which is role. $\endgroup$ Commented Nov 20, 2022 at 21:33
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In general LP pumps supply fuel to the engine from the fuel tank. HP pumps supply fuel to the injectors. This principle works not only for aircrafts but even on modern cars. It's sometimes possible for engine to work if other pump is not working but with less reliability usually if LP pump is broken. HP pump at the engine ensures that you don't need long lines to have high pressure. If you don't have LP pumps you can easily run into vapor bubble issues in the fuel line. If LP pump fails engine still can use suction feed from HP pump.
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$\begingroup$ You said LP pump supplies fuel to the engine, and HP pump supplies fuel to injectors. What do you mean? What's the difference? To what part of "engine" LP pump supplies fuel..? Fuel injectors are also a part of engine.... What do you mean? You're saying same thing essentially $\endgroup$ Commented Nov 21, 2022 at 7:11
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$\begingroup$ @SachinChaudhary. Fuel generally moves around the airplane at comparatively low pressure ( < 60 psi). At the very end of its journey, as it's about to go into the nozzles, the pressure gets boosted far higher (in a 2-stage process that Pilothead described, up to hundreds of PSI). That boost comes not outside the engine but by a part of the engine, the HP fuel pump. Read the 1st two sentences of this answer understanding that the LP pumps are in/near the fuel tanks, but the HP pumps are in/part of the engine. Does that make more sense? $\endgroup$– Ralph J ♦Commented Nov 23, 2022 at 0:29