Blog · Sourcing guide · 2026-07-23
Marine separators spare parts — a sourcing guide for Alfa Laval, GEA Westfalia and Mitsubishi
Fuel-oil and lubricating-oil centrifugal separators are recurring engine-room maintenance items. Mismatches often begin when the bowl configuration, gravity-disc size or gasket kit is selected without confirming the exact model, serial number and oil duty. Underneath most of those mismatches sits one distinction that decides what the machine even has. A purifier separates water and solids from oil and sets the oil/water interface with a gravity disc chosen for the density and temperature of that oil. A clarifier removes solids and the small quantities of water that come with them, runs with a sealing disc where the gravity disc would go, and has no interface to set. And a machine built on the ALCAP arrangement has no gravity disc at all: a transducer in the clean-oil outlet measures water content and triggers the discharge, so the unit follows fuel density on its own. A request for a gravity disc against that third kind of machine has no answer, and the round trip that discovers it costs a day. This guide covers three frequently encountered OEM families, the data buyers should send and where OEM and documented alternative routes may apply. It does not state lead times or prices.
The three OEM families
- Alfa Laval — MAB, MAPX, MMB, MOPX, S-separator (S926/946/821), P-separator (S/P/PX-Line), Fuel-Oil (FOPX/FOPX-J) and Lube-Oil (LOPX) families installed across commercial fleets.
- GEA Westfalia — OSC, OSD, OSE and OTC centrifugal separator families, including self-cleaning configurations used on European and Asian builds.
- Mitsubishi Kakoki (SJ / SelfJector) — SJ series (SJ700, SJ2000, SJ3000). Common on Japanese-built vessels.
Older units may bear MAPX, MMPX or FOPX designations. Similar appearance or family history does not prove interchangeability; confirm the exact plate, serial number and parts documentation. Family history is a particularly weak guide on this equipment, because the visible machine changed less across generations than the internals did.
Component families in a separator RFQ
- Bowl components — bowl body, top disc, bottom disc, disc stack, distributor, sliding bowl or operating piston, and separate paring-disc assemblies where fitted. These rotating and precision-matched parts require exact unit identification. The bowl is a balanced rotating assembly, so a single component exchanged inside it is a balancing question and not only a fitment question, and the disc stack is matched to the bowl it came from rather than to the model name. Where a stack is being renewed it is worth saying how many discs are in the machine and whether any are missing, because a stack running short changes the separation the machine achieves before it changes anything a gauge will show.
- Gravity discs — marked by inner diameter in millimetres. Selection depends on the separator model, oil density, temperature and throughput. A request for a "gravity disc" without the current marking and operating data is incomplete. Two answers make the line workable straight away: the marking on the disc now in the machine, and the density and temperature the oil is actually handled at. The disc bore sets where the oil/water interface sits in the bowl. Too large a bore moves that interface outward and oil starts leaving on the water side; too small and water carries over into the clean oil. Neither shows up immediately, which is why the disc is selected from figures rather than from the last one fitted.
- Gasket and seal kits — kit contents differ between minor, intermediate and major maintenance scopes and can vary by model revision. Say which scope is planned rather than naming the kit, because the same word covers different contents between makers and the scope is what the workshop is actually working to. Consider an alternative kit only after the complete material and dimensional specification is confirmed.
- Drive components — worm gear, worm wheel, ball bearings, drive shaft, spindle. Critical alignment tolerances. The worm gear and worm wheel are supplied as a pair and worn as a pair, so replacing one against a worn other shortens the life of the new part; and the spindle bearing arrangement is specified with its own damping and preload, which is why a bearing designation alone is not enough on this position.
- Control and monitoring — sensors, control valves, water-content transmitters, control-board modules. Increasingly integrated with vessel automation. On a machine with automatic interface control the transducer, the discharge valve and the control module are one system, so a fault read as a sensor problem is worth describing by what the machine actually did — discharging too often, not discharging, or losing oil to the water side — rather than by the component suspected.
Identity evidence — the separator-specific fields
For any separator RFQ, buyers should send:
- Separator make and model — from the nameplate (e.g. Alfa Laval MOPX 210, Westfalia OSC 15-01-067, Mitsubishi SJ-2000).
- Serial number — critical because the same nominal model can have several build revisions with different internals.
- Application — fuel oil (FO), lubricating oil (LO), diesel oil (DO) or another documented centrifugal-separation duty. Fuel-oil and lubricating-oil units can look similar externally while using different internals, and on a vessel with several machines in a row the plates are the only reliable way to tell which is which. Note the position tag as well as the model, since a spare bought for one and fitted to its neighbour is a common way to lose a bowl.
- Oil specification and operating condition — heavy fuel oil, marine diesel oil or marine gas oil, together with density, temperature and throughput where available. These values affect gravity-disc selection and bowl setup. Where the vessel has changed fuel — and most have — the disc that was correct for the old density is not necessarily correct now, so this is worth restating rather than carrying forward from the last order.
- Service context — is this a routine service kit, a mid-life overhaul, or an unplanned repair? Kit contents differ, and so does what else needs to be in the same order. An unplanned repair usually carries a cause with it, and naming the cause — a vibration, a discharge that would not close, oil in the water line — often changes which parts belong on the list.
- Photo of the nameplate — always. Serial and model are the two identifiers OEMs cross-reference.
See the general RFQ evidence guide for the full picture, or what to send when you don't have a part number.
Gravity disc RFQs — a recurring ambiguity
Gravity discs are sized by inner diameter, and the correct size depends on the density and temperature of the throughput. Sending "gravity disc" as an RFQ line without the inner diameter, the fuel/oil type and the operating temperature is not actionable. If you do not have the current disc measured, note that in the RFQ — Vessel Core will structure the return question so you can measure and reply without extra rounds. If the machine turns out to be one of the ALCAP-type units, that is the answer rather than a delay: there is no disc to size, and the line becomes a transducer, a discharge valve or a control item instead.
OEM route vs verified alternative
For bowl and drive components, use the OEM route or another option supported by the required dimensional, material, balancing and traceability records. For gasket kits and O-rings, a documented alternative may be considered when material, dimensions, warranty and maintenance requirements allow it. Sensors and control modules must be matched to the exact hardware and software revision. That pairing is the reason a control item is quoted against the machine rather than against a part number found online: the same sensor designation has been supplied against more than one control revision, and the two are not interchangeable in service.
Vessel Core surfaces both routes at RFQ time when both are legitimate for the buyer's case. See the RFQ case notes for the pattern.
What we do not claim before an RFQ is reviewed
Vessel Core does not quote separator-part prices, lead times or supply route before the specific separator, application and identification data are confirmed. Pricing for a gravity disc without its inner diameter, or for a bowl component without the exact model and serial number, should be treated as preliminary. On separators the serial number does more work than on almost any other engine-room machine, because the external appearance stayed stable across revisions whose internals did not.
How to send the RFQ
Use the RFQ form. Attach photos of the separator nameplate and, where relevant, a photo of the existing gravity disc (a caliper reading of the inner diameter is even better). If you don't know the model, the no-part-number path explains what to send instead.
