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Fuel-Water Separators Demystified

Photo by Eddie Berman

Fuel-Water Separator

Belt and Suspenders

A duplex fuel-water separator like the twin Racor 751000 MAX unit shown here makes sense for one giant reason—it lets a boat owner employ one canister to actually process fuel while the other waits in reserve. Should a fuel problem obtrude, all the owner has to do is throw the yellow valve handle and, presto, he’s back in business. This capability constitutes a serious safety feature should an engine begin to cough and choke in a fairway, channel, or under some other restrictive condition. Duplex fuel-water separators can be retrofitted to simplex systems but they aren’t cheap—list price for a 751000 MAX unit is approximately $2,000. The black modular priming pump under the foremost canister is included in that figure, by the way. Racor Division of ParkerHannifin, 800-344-286;

fuel-water separator gauge

The best way to know when to change the filters in a fuel-water separator is to check the vacuum gauge that indicates the level of restriction an engine’s injector pump is having to deal with, typically measured in inches of mercury. Most injector pumps pull about 25 inches of vacuum, according to Racor’s Mark Dickman. “So a restriction of 3 inches is probably fine,” he explains, “but 8 to 10 inches means your fuel is starting to be restricted, and 12 to 15 inches means the injector pump is beginning to cavitate and your engine is probably sucking air.”  

fuel-water separator filter

Dealing with an air-locked diesel isn’t fun. And to obviate the problem, Racor sells a modular electric priming pump that tops off an engine’s fuel system after a filter change, manifolding blunder, or other issue that introduces air into the system. The Racor Turbine Priming Pump, as it’s called, can be retrofitted to either a Racor 900MA or a 1000MA fuel-water separator. It comes in kit form and costs about $500 for a 12-volt version or $850 for a 24-volt version that features a sophisticated brushless electric motor. All kits include controls for helm and engine room, a canister-lid bleed-screw that lets air escape during pump-up, and four bolts that secure the pump module within upper and lower halves of the Racor unit.

fuel-water separator basin

Some folks think the stainless-steel basin at the bottom of a fuel-water separator’s clear-plastic settling bowl is designed to hold spilled or leaking fuel. In reality, the basin is fitted to serve as a UL-approved flame arrestor, a fire-safety device mandated for use on documented vessels and vessels for hire. Manufacturers often fit them on recreational boats these days to satisfy the mandate should said boats wind up in the charter trade.

This article originally appeared in the March 2015 issue of Power & Motoryacht magazine.