Oily Water Separator (OWS) — Working Principle, 15 ppm OCM & Siphon Breaker
How bilge water earns its overboard ticket in two stages plus a 15 ppm gate, and why a vented loop stops silent siphoning when the plant is stopped.
Key Principles at a Glance 5 points
- Bilge water passes two reductions: gravity settles the first stage to ~100 ppm, the coalescer polishes the second stage toward the 15 ppm gate.
- The OCM is a gatekeeper, not a meter: under 15 ppm the overboard valve opens; over 15 ppm the 3-way valve recirculates to bilge and sounds the alarm.
- A capacitance probe watches the collected oil cap and auto-drains it to the sludge tank — no manual sounding, no oil blanket creeping into the outlet.
- A stopped OWS below the waterline siphons oily water overboard silently — the vented loop air break is the only thing stopping it.
- The unit runs only en route with bridge informed, overboard valve locked under the Chief Engineer, and every run entered in the Oil Record Book.
1. The Two-Stage Chain — Gravity Roughs, Coalescer Polishes
Idea in one line: big drops settle by gravity, small drops get caught and grown by fibres, and a light-beam gatekeeper decides what may leave the ship.
The separator is two chambers in one casing. The bilge pump does not push oily water straight in — first the unit is primed with clean water until air vents out of the second stage, so separation starts against water, not an air pocket.
Stage one — gravity plus baffles. Oily water enters the first chamber; heavy water sinks, oil floats. Baffle plates give stray droplets a surface to stick to, and stuck droplets creep upward along the plates into the oil-collection space at the top. Outlet of this stage carries roughly 100 ppm — legal nowhere, but an easy load for stage two.
Stage two — the coalescer grows what gravity missed. Water dives down the central pipe and rises through fibre elements. Two element families exist: oleophilic fibres that grab oil and let water pass, and hydraulic (glass-wool) media that trap solids. Microscopic oil mist sticks to fibres, merges into bigger drops, and the grown drops are buoyant enough to float back up to the oil cap. Oleophilic elements run long; glass-wool elements are renewed on schedule.
Each stage carries its own relief valve. An over-pressured separator does not politely wait — it blows oily water straight overboard, which is exactly the pollution event the machine exists to prevent.
2. The 15 ppm Gate — the OCM Decides, the 3-Way Obeys
Idea in one line: nothing reaches the ship's side without passing a light beam that can slam the door shut.
The Oil Content Monitor sits at the outlet with an audio-visual 15 ppm alarm. It does not throttle or warn-and-continue — it switches a 3-way solenoid valve with two hard positions. Clean goes overboard; dirty goes back to the bilge for another pass:
Under 15 ppm — discharge
Overboard valve stays open, clean effluent flows to the ship's side. A sample cock just before the overboard valve lets the watchkeeper eyeball clarity as a second check on the instrument.
Over 15 ppm — recirculate
Alarm sounds in the engine room, the 3-way valve shuts the overboard leg and reopens the bilge-tank leg. Dirty water returns for another pass; nothing illegal leaves the ship.
The start drill that keeps the gate honest: the unit runs only while the ship is en route. Bridge is informed and logs time and position. The overboard manual valve lives locked with its key held by the Chief Engineer, and is opened in his presence. Control panel and OCM power on, separator primed with clean water till the second-stage vent runs clear, then the supply pump starts and the watch begins — OCM reading, bilge-tank and sludge-tank soundings, sample clarity, and an eye on the overboard discharge at the ship's side.
After the run: power off, overboard valve shut and locked, key back to the Chief. He enters the operation in the Oil Record Book, signed by operating officer, Chief Engineer and Master. A missing ORB entry turns a legal discharge into a detention.
3. The Brains — Capacitance Probe, Auto-Drain & Mountings
Idea in one line: oil and water conduct electricity differently, so a probe can feel the oil cap and drain it without human help.
A capacitance probe sits in the oil-collection space. When the oil layer reaches its level, the probe energises a solenoid pilot valve: control air drives a diaphragm piston valve on top of the first stage, and collected oil drains automatically to the oil/sludge tank. Level falls, probe drops out, valve shuts. No manual draining, no oil blanket creeping toward the outlet.
| Mounting | Job | When it speaks |
|---|---|---|
| Capacitance probe | Senses oil-layer level in the collection space | Energises the auto-drain solenoid at set level |
| Diaphragm piston drain valve | Air-driven oil drain on stage-one top | Opens on solenoid signal, shuts as level falls |
| Air vent + test cock | Priming vent; cock proves the sensor reads true | Vent runs water during prime; cock checked on routine |
| Relief valve (each stage) | Over-pressure protection | Lifts before the casing becomes the discharge path |
| Drain valve | Chamber draining for cleaning | Opened on maintenance routines |
Maintenance in five habits: scrub chamber walls where oil clings, prove the OCM reading, prove the probe through the test cock, renew the absorbing media on schedule, and exercise every regulating valve so none seizes half-open.
Fault logic to memorise: effluent stuck above 15 ppm with a clean OCM means the coalescer is spent or oil-fouled — renew elements. Repeated recirculation with falling throughput means the first stage is choked with sludge — open, clean, re-prime. Auto-drain cycling constantly means bypassing oil upstream or a leaking drain seat.
4. The Silent Polluter — Siphon Breaker / Vented Loop
Idea in one line: an inverted-U pipe can pump without a pump — and a stopped OWS below the waterline is exactly that pipe.
A siphon is an inverted-U leg that, once full of liquid, keeps flowing uphill-over-down with no pump driving it. The OWS overboard line forms that U whenever the outside sea head differs from the separator level. While running, pump pressure dominates and nobody notices. The danger starts at stop: with sea head higher outside, oily mixture keeps creeping overboard on its own — silent, unmonitored, illegal.
The fix is air at the crown. The siphon breaker (anti-siphon valve, vented loop) admits air into the top of the U. Air splits the liquid column so each leg drains back to its own level instead of pulling the other over. Check it the way it fails: a choked vent or a valve stuck shut looks identical to no breaker at all — vent clear, valve free, every routine.