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Is it better to plumb these two pumps in series (does the "off" pump flow
through enough) or parallel (does the "off" pump check the reverse flow)?
Are there any redundancy issues that makes one installation safer over the
other?<<
I think the answer depends on what type of fuel pumps they are. If you are
building a carbureted engine and using constant-pressure pumps I believe in
the parallel arrangement. I once suffered an unscheduled landing because of
the series arrangement in my Cherokee. But if you put them in parallel I
would recommend two separate strainers to make the fuel system totally
redundant (except for the selector valve). My problem was that both pumps
flowed through a single strainer, which plugged up with ice crystals. With
the pumps in parallel the fuel pressure will stay roughly constant and the
flow will be limited by the float valve in the carburetor. If the second in
line is a mechanical pump its diaphragm will cease to stroke, effectively
shutting it off. But if the problem is a ruptured diaphragm it will still
leak.
For fixed displacement pumps used in automotive fuel injection systems I
would recommend putting the pumps in series with a check valve paralleling
each. In this way the flow will remain constant with either or both pumps
running and the check valves will allow the fuel to bypass the inoperative
pump.
Gary Casey
Lancair ES project
LML website: http://members.olsusa.com/mkaye/maillist.html
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Please send your photos and drawings to marvkaye@olsusa.com.
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