REFLECTOR: Fuel Flow Rate

[Pat Shea]

- Scott Derrick <scott at tnstaafl.net> wrote:

> Hmmm..  I wonder?

-- alventures at cox.net wrote:

>> Not to worry.  Vapor lock would occur long before
>> the sump would be damaged.

--- Chuck Harbert <c.harbert at comcast.net> wrote: 
 
>>> I now also think that it would be a bad idea not
>>> to vent the sump because you could collapse it >>>
from the suction of the pump.


If you cap off the sump vent line the sump can still
vent via both fuel strake vents. In this
configuration, there is a "suction" scenario where
nearly empty strakes can't keep up with sump's gravity
flow rate - but the resulting slight vacuum in the
sump is certainly nothing that's going to compromise
the structural integrity of the sump.

I performed the following extreme test ('cause I
wanted to know): I capped off ALL three vent lines and
let the fuel flow out downstream of the sump until it
stopped. I then turned on the electric fuel pump and
sucked 4 gallons of fuel out of the system.  I had
tapped a vacuum gauge into the system which read about
3.5 in/hg. I'd estimate this is 5-10 times more vacuum
a sump would see just from being non-vented. Brett is
cringing right now... 

This scenario is comparable an airplane (say a spam
can) where a vent line becomes fully obstructed and
the mechanical or electric fuel pumps suck fuel from a
tank until they just can't suck no more. Of course,
Velocity's are a step ahead in this regard since we
have an alternate fuel vent line w/ a check valve.    

However, vapor lock is a whole other issue whenever
your sucking fuel. Oh, and then there's the sump air
pocket from un-ported strake pickups to deal with
too...  

Pat








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