REFLECTOR:RE:check valves

[KeithHallsten]

I'm inclined to agree.  One thing you can't overlook with check valves is
the resistance to flow that's added.  In a high-pressure system this may be
negligible, but when we have only a tiny head pushing the fuel to the sump
it could be critical!  The pressure necessary to overcome the spring and
start flow is called the "cracking pressure", because that's the pressure
that will just crack the valve open.  I suspect that most check valves have
a cracking pressure that's higher than the pressure we have available to
make the fuel flow in the first place.  If that was the case, no fuel would
flow at all, and a sump tank vent would prevent the fuel pump suction from
helping!  At best, they would substantially restrict the flow to the sump as
well as from the sump.

This brings up the other check valve we are advised to add to the fuel
system - the valve in the vent system high on the engine bulkhead that is
supposed to provide an alternate vent inlet to let the fuel flow if the
belly vent ices over or is plugged by mud daubers.  What is the recommended
valve for this application, and what is its cracking pressure?  Here we have
the advantage that the fuel pumps will suck the pressure in the sump tank
down in the case of a blocked vent inlet, but I'd still like to have some
assurance that the cracking pressure is low enough to really let the valve
serve its intended function!

Keith


----- Original Message ----- 
From: "Brett Ferrell" <bferrell@123mail.net>
To: <reflector@tvbf.org>
Sent: Monday, July 28, 2003 7:10 AM
Subject: Re: REFLECTOR:RE:check valves


> I'll bite.  Although it *should* be OK, all thinngs being equal, I think
it's a bad idea because all things are never equal.  There are a lot of ways
the tanks might get out of balance, and with open lines they can rebalance
themselves and avoid a lot of trimming.
>
> I think in the example of the tip tanks, they're meant to be able to be
completely drained, and you wouldn't want them refilling in a steep
(uncoordinated) turn.  You shouldn't routinely have the strakes running dry,
and even if you get a little undesirable cross flow, the moment arm isn't as
big a concern as with a tip tank.
>
> Brett
>
> Quoting Alexander Balic <alex157@direcway.com>:
>
> > I was thinking of this also- about using check valves in the lines from
the
> > mains to the sump to keep the fuel always moving in the correct
direction-
> > anyone else done this? any other comments on why is a good/bad idea?