Jan de Jong wrote:
a whole bunch of stuff about series and parallel pumps.
But everyone is getting confused here. The parallel/series
discussion is not about flow or pressure, it is about
redundancy.
Thus the MOST important thing is that in the event of any
one problem, fuel can still flow to the engine. Both pumps
are only used together for take-off and landing, so for
most of the flight it is the single pump that does all the
work.
The problems can be a pump failure, a pump blockage, a
non-return valve blockage, a pipe blockage, a t-piece
blockage, a filter blockage (and others !). The set-up
should be such that any one "problem' as defined above can
be avoided (bypassed) by the use of the different options.
There is no guarantee that you will get to your
destination if (say) a in-line filter blocks up, but you
will have power.
Ideally any by-product of the initial problem (debris ...)
will not affect the alternative fuel supply.
Now the 914 seems to use two pumps with impellers of a
type that cannot have fuel sucked or blown through them
unless the pump is running. This is not true of the
mechanical pump or the electric pump used on a 912/S.
So (for the 914) a parallel system does not need
non-return valves except as a belt, braces and piece of
string option; as one pump running cannot recirculate
through the other.
On the other hand a series (for the pumps) must have a
pipe bypassing each pump in case it fails, and these two
pipes need a non-return valve in them.
So parallel is :
Tank--ILF--- -- pump --
selector---tpc tpc --- engine
Tank--ILF--- -- pump --
So series is :
T---ILF--- -- NRV -- -- NRV --
sel -- tpc tpc ---tpc tpc
-- eng
T---ILF--- -- pump -- -- pump -
But that diagram may not come out how I want ! In all
cases Y-pieces are better than T-pieces.
Parallel has less components, and I am a great believer in
the KISS principle. Even with the NRVs the parallel has
less components.
These components are very reliable (they are approved for
aviation) so it is really best not to go too far into the
what-if solutions as most of the compound problems will
never occur. Or might even occur because of additional
complexity.
KISS :-)
Richard ducking head beneath parapet !
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