Message-ID: <002f01bf5b7a$db8c3580$a1818ed1@pavilion>
From: "J. N. Cameron" <CIC@centurytel.net>
To: "Lancair List" <lancair.list@olsusa.com>
Subject: Fuel line vents; fittings
Date: Mon, 10 Jan 2000 08:55:29 -0600
Mime-Version: 1.0

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    The viscosity of air is around 1/6000th that of fuel, so the flow of air
through a 1/4" diameter vent line would probably keep up with fuel flow
through something like a 6" fuel line.  Sizing the vent line is more a
matter of preventing obstruction than providing adequate air flow.  A small
cone-shaped screen over the opening might be a good safety addition, as it
would provide a larger opening area and keep the vent line from being
plugged by a bug.
    On the subject of fittings, I've long felt that Swagelok fittings are
far superior to AN fittings and have used them in my business for 20+ years.
They are easy to use:  cut a clean tube end, tighten 1-1/4 turns (or to fit
an inexpensive gauge supplied by Crawford), and Voila!  You have a solid,
leak-tight join that will outlast the tube.  Swagelok fittings come in a
variety of materials, including brass, steel, stainless, aluminum and nylon,
are widely available and relatively inexpensive.  The superiority of these
fittings has given Crawford a dominant position in the industry -- the
second- and third-place companies (Cajon, Parker) are selling Swagelok
copies!
Seems to me that the use of AN flare fittings is a legacy from 1930's
milspecs.  Can't see any reason to use 'em.  Flaring a tube creates a
stress-weakened end that's subject to fatigue failure, and any imperfections
in the flare surface can make a leaky joint.  Furthermore, there's no
reliable way to gauge the proper tightening tension from fitting to fitting
and material to material.  The availability of several different sealing
gaskets for flare fittings testifies to difficulties with them.
Jim Cameron  (once & future ES)


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