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The Facet pump in this position is a two edged sword. It can help
eliminate a vapor lock when on but it is a restriction that can create a vapor
lock when off. A properly laid out fuel system should not require
another pump to prime the main EFI pump. It is a bandaid IMO. Again,
any restriction other than a strainer to stop the 'big chunks' before the EFI
pumps is not a good thing.
Tracy
Without
a Facet pump between the tank and the HP and you have a vapor lock at the
HP pump you will have no flow. With the Facet at the tank it will
prime the HP pump by pushing liquid fuel into the HP pump. Once
the vapor lock is gone, the HP can suck through the Facet even when it is
running. How much flow is really coming out of an HP pump with a normal
FI system with a return to the tank. I doubt is it 30 + gph. I
guess I's saying the HP pump will be operating at much less than its max.
rated capacity and the system with the Facet between the tank and the HP pump
should work fine. I'm considering this system for my RV
7A.
Dennis H.
Jim Sower wrote:
<... HP pumps are
capable of moving LOTS more fuel than a Facet pump ... HP pumps would be
drawing fuel through the Facet pump ... because the Facet can't keep up
...>
You're right. I hadn't thought of that. Facet
pumps deliver 30 gph. I'll check how much my HP pumps are putting
out. Wonder if Facet delivers more at lower head pressure, objective
being to push fuel through filter and plumbing so there's less chance of
pressure drop that could cause the fuel to vaporize.
Back to the
drawing board ... maybe ... Jim S.
Marvin Kaye wrote:
Jim Sower <canarder@frontiernet.net>
wrote:
"""
I have a canard, but I will have a Facet pump
near (and below) the wing tanks
to PUSH the fuel through the filter
and fuel flow transducer. I don't want
the HP pumps to SUCK
through the filters and etc. for fear of vaporizing the
fuel.
"""
This doesn't make any sense to me, but perhaps I'm missing
something. The HP pumps are capable of moving LOTS more fuel than a
Facet pump. Consequently, it seems to me that the HP pumps would
actually be drawing fuel through the Facet pump when they're switched on,
simply because the Facet can't keep up with what's being drawn out of the
sump tank by them. Additionally, a return system needs 2 flow
transducers, one for the feed line and one for the return... then the
display instrument's electronics deduct the return flow from the feed flow
to properly calculate actual through-the-injectors instantaneous flow
data. (The EI fuel flow instrument uses an FFDM-1 (fuel flow
differential module) to do the job, GRT EIS does it itself, as do other
flow instruments with both feed and return inputs.)
As long as the
filters are rated to flow as much fuel as the HP pumps are capable of
pushing I don't see that (vapoization) as an issue. The filter
elements do need to be kept clean, and are a replace-at-annual item.
<Marv>
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