any opening into the hull. Fuel-tank vents
also require a backfire flame mesh with
30 wires per inch (1.18 wires per mm) each
way. This prevents flames from spreading
down into the vent pipe, which seems pru-
dent. CFR Yacht and ABYC require that the
mesh be “cleanable.” In practice, this means
that the mesh or the vent unit with the mesh
has to be removable. Presumably, if you can
remove it, you can clean it.
Best practice is that each tank have a
dedicated vent line. Essentially, all vents
must rise continuously from the tank.
There can be no sags or dips, which hold
fluid or debris, in any vent line. If unavoid-
able, short horizontal runs (no more than
18 inches (45 cm) are acceptable, but such
horizontal runs increase the chance of a
potential clog in the vent line. As long as
the vent run rises continuously, you can
combine vents from different tanks into
one line. You can do this only if the total
cross-sectional area after joining is greater
than the combined cross-sectional area of
the individual lines. Such large-diameter
vent lines won’t fit off-the-shelf vent
fittings.Don’t share vent lines with tanks contain-
ing other liquids, even if the shared cross-
section area is adequate. For instance, if you
shared a vent line from a diesel and a fresh-
water tank, the bad taste in the fresh water
from migrating fuel-oil gases would be rather
unpleasant. Microbes from black and gray
water can exacerbate algae growth and con-
tamination in fuel tanks.Vents Under Pressure
Large vessels may be fueled under pressure
rather than from a simple fuel-pump nozzle.
This places still-higher demands on the vent
lines. Clearly, if the vent lines were too small,
internal tank pressure could build to a burst-
ing point. For tanks filled under pressure, the
vent lines must be the same diameter as the
fill pipe or larger. My practice has been to
install the two standard^3 / 4 - inch (20 mm) vent
lines described previously plusan additional
vent line having the same ID as the fill or
slightly larger.Vent Spill Prevention
Several manufacturers offer inexpensive units
to reduce fuel spills through the vent. Some
devices whistle continuously as the tank is
filled, changing pitch noticeably as it nears
capacity. This warns you to ease back on the
nozzle handle and top off the last few gallons
slowly. Racor’s Lifeguard fuel/air separator
(Figure 5-6) traps small overflows in an
enclosed plastic globe and routes them back
to the tank (Figure 5-7). Larger overflows
cause a ball-type check valve to seal the vent
closed. If you wrap a rag around the fill nozzle,
sealing it in the fill-pipe opening, the pressure
buildup caused by the check valve closing
shuts off the fill nozzle when the tank is full.
Should something go wrong and the fill noz-
zle does not shut off, the check valve will open
again as the pressure further increases, elimi-
nating any chance of rupturing the tank.
Another option is to install a small, cus-
tom, overflow collection tank in the vent line
(Figure 5-8). For space reasons, this may only
be practical on larger vessels. Usually about
2 gallons (7.57 L) in capacity, the vent enters
the bottom of the collection tank to one side.
A horizontal baffle is built into the tank a
quarter of the way above the bottom, butPART TWO: FUEL SYSTEMS
New EPA Gasoline Vent Filter
Requirements ProposedAs of February 2008, the Environmental Protection Agency (EPA)
is planning new regulations for gasoline fuel systems on boats.
The anticipated changes are intended to reduce vapor escaping
from the tank through the vent and also to reduce fuel spills. The
most prominent feature of the new requirements will be a carbon-
filled canister on the fuel vent line between the tank and the vent
opening. A secondary feature is required to keep liquid gasoline
overflow from entering this carbon-filled canister. This additional
feature is a check valve to be installed in the vent line, as close to
the tank as possible. The check valve will shut flow immediately if it
senses liquid fuel, but will remain open for air and vapor. Other
new features may include better pressure-sensor shutoff response
on the fuel fill nozzle to further reduce spills. This may be inte-
grated with the configurations of the check valve and fill pipe.
Current plans are to phase in the carbon canister and its re-
lated check valve on gasoline vent lines for engines of 500 hp
(373 kW) or higher starting in 2009, and for all gasoline engines
as of 2010. Go to http://www.epa.gov for information on the latest devel-
opments. These changes will apply only to gasoline, not to diesel.