122 PROFESSIONAL BOATBUILDER T o clean diesel fuel before it reaches a vessel’s main engine or generator, many yards take a conven- tional approach: pass it through vari- ous filters made of pleated cellulose or synthetic elements, and a gravity-type water separator. The element-filter concept is an old one, and over time science has nearly perfected this type of filtration, with most manufacturers claiming 99% or greater efficiency on solids, water, and other contaminants. Given this record, it’s hard to imagine there’s much room or need for improvement. Well, today’s dominant filter tech- nology may be effective, but it’s far from maintenance free. Element filters must be inspected and periodically disassembled and thoroughly cleaned, especially if they’re exposed to gross or overwhelming contamination. If they become clogged with asphaltine, bio- logical detritus, or water, they’ll do their job by preventing passage of this contaminating matter, along with stopping the flow of fuel, which will cause the engine to surge, behave erratically, or shut down entirely. This isn’t a fault. When filters clog, they’re performing exactly as they are designed to. But a qualified improvement over the traditional filter, especially when it comes to polishing or preliminary fuel filtering, is a centrifuge. S wedish engineer and chemist Carl Gustaf de Laval, though not its inventor, is acknowledged as the father of the modern separating centrifuge. An accomplished inventor in his own right, de Laval worked toward the latter part of the 19th century building high-speed steam turbines with special nozzles of his design (de Laval nozzles), involving reduction gears with lots of lubrication. One challenge he faced: contamination of turbine oil with water in the form of steam. While searching for a reliable, inexpensive method to quickly remove A variation on century-old dairy separation-technology finds its niche in the engineroom of today’s long-range vessel. Centrifugal Filtration Left—Though costly compar ed to a conventional fuel filter , the moder n centrifugal separator is compact and ef ficient, capable of r emoving tiny par ticles of debris and water . And, there’s no filter element to change. Text and photographs by Steve D’Antonio [E d i t o r s N o t e : Please refer to “So You Think You Know Diesel,” page 112, also by Steve D’Antonio.]
Transcript
122 PROFESSIONAL BOATBUILDER
T o clean diesel fuel before itreaches a vessel’s main engine or
generator, many yards take a conven-tional approach: pass it through vari-ous filters made of pleated cellulose orsynthetic elements, and a gravity-typewater separator.
The element-filter concept is an oldone, and over time science has nearlyperfected this type of filtration, withmost manufacturers claiming 99% orgreater efficiency on solids, water, andother contaminants. Given this record,it’s hard to imagine there’s muchroom or need for improvement.
Well, today’s dominant filter tech-nology may be effective, but it’s farfrom maintenance free. Element filtersmust be inspected and periodicallydisassembled and thoroughly cleaned,especially if they’re exposed to gross oroverwhelming contamination. If theybecome clogged with asphaltine, bio-logical detritus, or water, they’ll dotheir job by preventing passage of thiscontaminating matter, along withstopping the flow of fuel, which willcause the engine to surge, behaveerratically, or shut down entirely. Thisisn’t a fault. When filters clog, they’reper forming exactly as they aredesigned to.
But a qualified improvement overthe traditional filter, especially when itcomes to polishing or preliminary fuelfiltering, is a centrifuge.
Swedish engineer and chemist CarlGustaf de Laval, though not its
i n ven to r, i s a cknow ledged a sthe father of the modern separatingcentrifuge. An accomplished inventorin his own right, de Laval workedtoward the latter part of the 19thcentury building high-speed steamturbines with special nozzles of hisdesign (de Laval nozzles), involvingreduction gears with lots of lubrication.One challenge he faced: contaminationof turbine oil with water in the form ofsteam. While searching for a reliable,inexpensive method to quickly remove
A variation on century-old dairyseparation-technology finds itsniche in the engineroom of today’slong-range vessel.
CentrifugalFiltration
Left—Though costly compar ed to a conventional fuel filter , the moder n centrifugal separator is compact andefficient, capable of r emoving tiny particles of debris and water . And,there’s no filter element to change.
Text and photographs bySteve D’Antonio
[Editors Note: Please refer to “So You Think You Know Diesel,”page 112, also by Steve D’Antonio.]
OCTOBER/NOVEMBER 2008 123
water and other contaminants fromthe oil, he opted for the centrifugeseparator principle, a product he’dalready improved and developedsuccessfully for other applications.
Interestingly, while de Laval’s workwith turbines and gear reductionhad an impact on the engineering andmaritime world—the first steam-turbine yacht, Turbinia, sea-trialed in1894, was for a time the fastest vesselin the world (see ProfessionalBoatBuilder No. 113, page 32)—hisimprovements to the centrifuge wenttoward less exotic pursuits. In 1878de Laval patented the first continu-ous centrifugal milk/cream separator.The Laval dairy separator went on tobecome an immensely successfulproduct as was his vacuum milkingmachine, both of which were manu-factured under license all over theworld. The first commercial applica-tion of his centrifuge separator fornon-dairy purposes was for processingfish oil, in 1882.
A salad spinner is the most prosaicand clear example of centrifugal
force in action. It spins washedgreens at high speeds, flinging waterout through the holes in the spinningbasket, so the salad makings emergerelatively dry. The centrifuge workslike an amplifier of gravity. Forinstance, if you thoroughly mix oiland water, or fine sand and oil, in ablender, the emulsions would remainmixed for quite some time, days oreven weeks perhaps. Because of dif-fering densities, they would eventu-ally separate, though not completely.In many cases, a permanent emulsionlayer will remain; this is particularly
true for liquids such as fuel or lubri-cating oil and water or coolant. Byrapidly spinning this emulsion in acentrifuge, however, the compara-tively weak force of gravity thatwould normally act upon these sub-stances is multiplied several thousandtimes over. It’s not unusual for cen-trifuges to impart 6,000 times theforce of gravity (6,000 g’s) on the liq-uid within while spinning at between7,000 and 9,000 rpm. By comparison,you might pull 2 or 3 g’s when brakingor cornering rapidly in an automobileor falling off a large wave while underway in a boat.
Through this enhanced gravitationalforce, even the smallest particles areslung out of a liquid. Centrifuge man-ufacturers routinely cite and guaranteeparticle removal from diesel fuel andlight lube oils down to 5 microns,while practical applications suggestparticles as small as 0.5 micron areeasi ly removed through a cen-trifuge’s high-speed rotation.Additionally, a centrifuge is capableof separating liquids of differentdensities such as fuel and water. Inmany designs, the water is bled offcontinuously and automaticallywithout requiring any action on thepart of the operator.
The rotational energy for nearly
Traditional pleated-paper fuel-filter elements do a fine job of r emoving dir t,debris, and water , but ther e’s a limit totheir capacity and the volume of fuelthey can filter .
Once a pleated element is packed withcontaminants, fuel flow is r educed orstopped; the polishing system is thenoff line until a new element isinstalled. Centrifuges can captur e andhold a lot of debris without r estrictingfuel flow. Some r equire manualcleanout; others ar e self-clean ing.
124 PROFESSIONAL BOATBUILDER
all centrifugal fuel “separators”(because they lack filter elements, itdoesn’t seem right to refer to them asfilters) is derived from an AC or DCelectric motor. Through a gearbox thismotor may also operate a feed/dis-charge pump moving fuel into andout of the centrifuge. In the case ofindustry-leading Alfa Laval separators,a “water seal” is established inside thecentrifuge by adding a small quantityof water prior to operation. Because ithas a greater density than the fuel, thewater creates a seal or dam of sorts
that prevents fuel from drain-ing from the separator whileit’s rotating, but allows waterseparated from the fuel todrain from the unit.
Most centrifugal separatorsrely on buffer plates or disc stacks,which further enhance the efficiencyof particle and water removal fromthe fuel (and other fluids). The sink-ing or slung particles and water mol-ecules strike a cone-shaped disc, whicharrests their travel and removes themfrom the fuel more quickly. Centri-fugal force causes these particlesand water to migrate to the peripheryof the centrifuge’s bowl, where thesolids adhere to the bowl’s surfaceand the water is pushed out throughthe water seal. In some models, the
solids are also automatically dischargedthrough a “sludge port.”
The average fuel centrifuge aboarda moderately sized vessel is roughlythe size of a small air-conditioningcompressor pallet and weighsbetween 80 and 150 lbs (36 and 60kg). It’s capable of pumping between150 and 200 GPH (568 and 757 l). Apopular model, the Alfa Laval MIB303, is at the larger end of theseranges and requires between 450 and700 watts at 24VDC, 120VAC, or230VAC. Like most centrifugal separa-tors, it’s a hands-off unit: after addingwater, you start it and walk away.Water drains automatically, and thebowl needs to be cleaned only whenit’s full: about a measured cup (0.2 l)of compressed solids.
There’s an important distinctionthat must be made at this stage of ourdiscussion. Quite a few conventionalprimary-fuel-filter designs advertise,variously, a “cyclone,” “turbine,”and/or “centrifuge” component that’sdesigned to “spin off large particu-lates and water droplets” (the empha-sis is mine). Note that the centrifuge
Gross contamination of mostconventional paper elementsand their filter bodies will bringfuel flow to a halt. In sever ecases, even a liberal supply of fuel filters may be quicklydepleted.
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OCTOBER/NOVEMBER 2008 125
in many primary filters, includingthose from Racor, Separ, Baldwin, andothers, is not a gimmick. It is indeedeffective at spinning out large particu-late debris and water droplets. Thespeed at which the fuel moves andspins through these passive centri-fuges, however, imparts a tiny fraction
of the g-force of a dedicated, activecentrifugal separator. While the fil-ters cited above are effective anddesirable components in a primary-and polishing-filter arrangement,as passive centrifuges they are notcapable of, nor are they designed to,remove dirt, debris, and water in their
single-digit micron or molecular form.Thus, despite the word centrifuge todescribe one of the steps such filtersemploy for water and debris removal,there’s little similarity between the
Above—A gauge that monitors pr essur eat the clean-fuel outlet of a centrifugalseparator will sound an alar m; the feedpump will be tur ned of f if pressur edrops beyond a pr eset limit. Right—Many conventional primar y fuel-filter designs include passive centrifugeelements capable of spinning out lar geparticulate debris—but lacking the g-force of a tr ue centrifuge.
Centrifugal separators like this one (anAlfa Laval MB303) have few movingparts and often operate for thousandsof hours with little or no maintenance.
126 PROFESSIONAL BOATBUILDER
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the scientific community, centrifugalseparators are relied on for thepurification and separation of all sortsof liquids, from cutting fluids formanufacturing, to wine, beer, andfruit juice processing, to steam-turbinelubricating oil, blood handling, and ofcourse dairy production. Biodieselrefining has also proven to be idealfor centrifugal separation, primarilybecause it requires the separation ofsolids from a liquid as well as separa-tion of liquids of differing densities.
Diesel is the fuel so many vesseloperators are concerned about (see“So You Think You Know Diesel,”page 112). A centrifugal separator/polisher is ideally suited to meet theirneeds; but centrifugal separators,because of their “active” design, arenot good primary fuel filters. (For fail-safe primary filtration it’s better to rely
An Alfa Laval separator—installedaboard a 65'/19.8m motor yacht—isplumbed via a manifold to each of thevessel’s tanks, and polishes fuelbefore any is pumped to a day tank.
two processes. They should not beconfused.
The last two oceangoing ships Iwas aboard—the M/V Endeavour,
a 295' /90m ice-capable expeditioncruise ship designed for high-latitudetravel (see PBB No. 89, page 16); andthe M/V Ever Renown, a nearly1,000' /305m containership—wereequipped with centrifugal separators,
and with good reason. When you’reat sea, cruising far from the nearestport, self-sufficiency is a priority.Centrifugal separators for fuel andlube oil purification, among otherthings, contribute to that indepen-dence, because they require no filterelements, have few moving parts, andare designed to operate continuouslyfor months without service.
Throughout general industry and
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OCTOBER/NOVEMBER 2008 127
on something with no moving parts.)Centrifuges work particularly well aspolishing system “purifiers” that scrubfuel before it gets to the element fil-ters. They serve this very purpose onthe ships I mentioned above, as wellas aboard many recreational vessels.The only regular service that cen-trifuges require is periodic replace-ment of O-rings, and removal ofsludge from some models.
So why aren’t centrifuges univer-sally adopted in the marine indus-
try? Their price tags. These unitsrequire precise machining and robustconstruction, and, predictably, thatcomes at a cost. The smallest AlfaLaval separator that would be suitedfor diesel fuel polishing, the MAB102B, lists for about $8,500, while theMIB 303 lists for $10,000–$12,750,depending on configuration.
Is it that much better, more effec-tive, and less trouble than a conven-tional filter to warrant the additionalexpense? I believe the answer may beyes, depending on the vessel’srequirements. Where industrial filtra-tion/separation technology is con-cerned, centrifuges are the last wordin getting dirt and water out with thegreatest reliability and least fuss andmaintenance. If a vessel is equippedwith especially large tanks, or if it’s
traveling to locations where fuel maybe of more questionable quality thanusual, or if an owner desires an ultra-reliable and effective polishing sys-tem, then the centrifugal separatormay be the best approach.
It’s worth noting, because thesesystems are installed primarily in com-mercial applications like oceangoingships, and power plants, where theyare exceptionally long-lived and reli-able. Many units are still in service
after 30 and, in some cases, 50 yearsof operation.
About the Author: A former full-service yard manager and longtimetechnical writer, the author now workswith boat builders, owners, and othersin the marine industry as “SteveD’Antonio Marine Consulting LLC.”His book on marine systems will bepublished by McGraw-Hill/Inter-national Marine in 2009.
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A 40-year-old centrifugal separator . It’sstill going str ong aboar d an expeditioncruise ship that operates fr om Arctic toAntarctic waters, wher e fuel filtration iscritical but the near est spar e filter-element may be thousands of miles away.
