dxzh, in re your post #1969 of yesterday, on page 99:
Is it a red herring even to contemplate the boost pumps or centre tank pumps allowing the passage of fuel/water when they are selected OFF? I guess the premise of the suction feed test on the ground (as a pure test of the suction feed) would be flawed if it was a possibility that an engine-driven LP could draw fuel through any of the unpowered pumps from centre tank - and thus I would like to discount the idea completely. Yet as a Mad Hatter is drawn to a tea party, I am curious about the possibility given the suggestion in the interim report that air at least might possibly pass through the OJ/jettison check valve (and presumably therefore through the OJ pump when selected OFF too) ...
Centrifugal pumps of conventional design can be operated in series; I have done design work with such arrangements. It does not matter if the upstream pump (in this case) loses its electrical power. There are no in-built internal obstructions to continued flow through a centrifugal pump whether or not its impeller is still free to turn, or siezed in its bearings.
It is conceivable that the housing of such a pump could be fitted with a stop valve held open by a solenoid energized by current from the pump power circuit. I haven't seen any indication that the 777 pumps of which I have details (Eaton Aerospace (UK) wing tank LP boost) have any such feature. This seems undesirable in an aircraft, just my opinion. I've seen systems (non-aircraft) that had such a feature, but only as a separate valve in the piping.
As to air entering the engine LP pump suction, I can only address the fuel path from the main wing tanks. Here the wing tank suction points are above the engine fuel feed in such a way that the engine LP pump would normally operate with a gravity-flooded suction if fed entirely from the wing on that side, with the LP boost pumps of that side off. I can't say I see this as even remotely likely in the air, and use it only by way of example (lacking as yet sufficient info regarding elevations in the center tank plumbing).
Anyway, with a fuel velocity of at most 7 feet per second (8 possibly), I cannot see it likely that significant air would be entrained should the suction of one of the wing LP boost pumps become unflooded itself due to fuel slosh. It isn't like the engine pump was above the fuel source. BTW, the velocity given is for a fuel rate of 35,000 lbs/hr. This is the conventional maximum velocity (for avoiding static charge on the fuel), and may also be seen from the near 2" discharge diameter of the wing LP boost pump casting.
The same principles would apply to the center tanks, but as noted I don't know the elevations of the various components clearly enough to comment. The dimensional detail of the wing LP boost pumps in on the Eaton Aerospace website; earlier I forgot to mention that it is not in the brochures-- instead look under "downloads" for the data sheets, including B777-300 for a comparison to the -200.
Following what I consider to be good example, I stand to be corrected in this matter if necessary. Well, it is a somewhat complex fuel system, although I think it is robust, simple in components and backup/bypass paths, and extremely reliable, at least until now.
OE