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Date: Wed, 21 Dec 2005 09:38:19 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fuel filter selection
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Michael LaFleur wrote:

> I find it very, very unlikely that both pumps will fail at exactly the 
> same time from debris. Use two filters. The filter before the pump 
> protects the pump and the filter after protects the injectors.
>
I've picked up contaminated gas at stations, and I know others who 
have.  It's rare, but it happens.  I've also known of tanks being 
vandalized with a handful of sand.  Granted, that was high school, but 
do vandals ever grow up?  I think the question hinges on how long you 
think a pump can handle a slug of trash or water.  From the drawings 
I've seen of how the pumps work, there is very little clearance for 
anything but clean fuel.  I don't see a problem with micron sized 
particles killing the pump, but there isn't much margin once you break 
into the visible range.  The problem is compounded by pickup schemes 
that  draw from a blister in the bottom of the tank, which effectively 
insures that all the contaminants are driven directly to the pump.  So, 
the running pump draws in some trash and quickly gives up the ghost.  
The engine hiccups, I get nervous and kick on the second pump, draws in 
some of the same trash, and dies just as quickly.

Al W. doesn't think the pumps are at risk, but he does think there is a 
serious issue with vapor lock which argues  against the filter before 
the pump.  My logic is that I don't trust the input.  I don't trust the 
gas from every station and every can that I will ever draw from to be 
clean and dry, even if it looks that way before I pour it in.

Al is right that we're all just pushing wind until we have some actual 
measurements.  I'll have a system installed in just a few weeks.  I'll 
rig up a test system to measure head loss, vapor pressures, and 
determine what happens when a slug of water hits the pump.

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