Time to Vent

Fol­low this log­i­cal or­der of hose and vent pro­gres­sion to un­der­stand how diesel fuel is trans­ferred from the pump to your tank to the en­gine.

Cruising World - - Hands-On Sailor - BY STEVE D’AN­TO­NIO

It’s all too easy to take for granted that fuel will make its way from the deck fill to the tank and from the tank to the en­gine. The means by which fuel trav­els and the plumb­ing it uses to make this trip of­ten go over­looked un­til a leak or other fail­ure de­vel­ops.

While fiber­glass, metal­lic pipe or plas­tic are ac­cept­able (with lim­i­ta­tions), the con­nec­tion be­tween the deck fill and the tank is nearly al­ways made of a rub­ber or rub­ber­like hose. Hose is con­ve­nient in that it can be eas­ily routed and it ab­sorbs move­ment be­tween the deck, hull and tank. Hose used in this ap­pli­ca­tion is far from or­di­nary. It must carry a U.S. Coast Guard A2 rat­ing, which means it’s de­signed to, among other things, re­sist ex­po­sure to flame for a min­i­mum of 2½ min­utes. Typ­i­cally, this hose is also re­in­forced with wire to pre­vent kink­ing and crush­ing. Where Amer­i­can Boat & Yacht Coun­cil compliance is sought, fuel-fill hoses must also be in­stalled with dou­ble clamps (this is one of only two lo­ca­tions where ABYC Stan­dards man­date dou­ble clamp­ing; the other is ex­haust sys­tems), which are all stain­less steel, in­clud­ing the screw, and whose band width must be a min­i­mum of a half inch.

Fuel-fill plumb­ing must be self­drain­ing; it should avoid hor­i­zon­tal runs and dips, be­cause they tend to make for slow fu­el­ing, while pro­mot­ing air “blow­back.” All metal­lic com­po­nents in the fuel-fill sys­tem that are in con­tact with fuel, in­clud­ing the deck fill and tank, must be grounded, with re­sis­tance be­tween these com­po­nents and the ves­sel’s ground not ex­ceed­ing 1 ohm (in other words, very low re­sis­tance). The ground­ing wire used for the deck fill must uti­lize a ma­chine screw, i.e., a nut and bolt, rather than a tap­ping screw; the wire can­not be clamped be­tween the hose and deck-fill pipe.

Fuel tanks must be vented to al­low air to es­cape while fill­ing and en­ter as fuel is used. Vent plumb­ing typ­i­cally con­sists of hose that should, if it passes through the en­gine com­part­ment, carry a type-a1 rat­ing. It must be run in such a way as to en­sure it does not trap fuel (no dips or low spots), us­ing a con­tin­u­ous run up­ward from the tank to the vent. The vent plumb­ing must con­nect to the high­est point on the tank, which may call for dual vent con­nec­tions to en­sure it re­mains so while mo­tor­sail­ing (these can be T’d so only one ex­te­rior vent is nec­es­sary). The vent it­self must be lo­cated above the fill or the hose run must in­cor­po­rate a riser, an in­verted U, as close as pos­si­ble to the vent. This ar­range­ment will (or should) pre­vent fuel from be­ing pushed out the vent be­fore it reaches the filler noz­zle at the deck fill dur­ing fill­ing, while min­i­miz­ing the pos­si­bil­ity of wa­ter be­ing pushed into the vent. All fit­tings and con­nec­tions as­so­ci­ated with the fill and vent plumb­ing must be read­ily ac­ces­si­ble (joiner work should not re­quire sig­nif­i­cant dis­as­sem­bly).

Vent place­ment it­self is crit­i­cally im­por­tant; more than a few ves­sels have been plagued by wa­ter be­ing in­gested into fuel tanks via a poorly placed vent. Vents should not be in­stalled on hull sides; in ad­di­tion to risk of sub­mer­sion, this would fre­quently place them be­low the fill un­less they were equipped with a riser.

The O-ring on the deck fill should be in­spected reg­u­larly, and re­placed if it shows any signs of de­te­ri­o­ra­tion. Pe­ri­od­i­cally lu­bri­cat­ing it with Te­flon grease will ex­tend its life and im­prove the seal it makes, keep­ing both rain and sea­wa­ter out of your tank. Fi­nally, the deck-fill cap should be con­nected to the filler neck with a chain or wire.

Next month, we’ll re­view fuel-sup­ply plumb­ing.

Steve D’an­to­nio of­fers ser­vices for boat own­ers and buyers through Steve D’an­to­nio Marine Con­sult­ing (steved­marinecon­sult­ing.com).

It’s im­por­tant that the fuel vent it­self, or its plumb­ing, be routed above the deck fill (top left). If the vent is lower than the fill, spills of­ten re­sult dur­ing fill­ing. Metal­lic deck fills must be elec­tri­cally bonded to the ves­sel’s ground­ing...

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