Oily Water Separator Product

Overview

Bilge water—the oily, murky fluid that accumulates in a ship's engine room bilges—is a hazardous waste requiring treatment before discharge. Sources include:

• Engine crankcase oil seepage during operation.
• Hydraulic fluid leaks from main and auxiliary systems.
• Fuel oil carryover from fuel treatment systems.
• Coolant and lubrication spills.

International Maritime Organization (IMO) regulations (MARPOL Annex I) mandate that discharged bilge water contain <15 ppm oil by concentration. Violating this limit incurs fines ($20,000–1,000,000+) and vessel detention. Larger vessels must operate an approved Oily Water Separator and maintain records proving compliance.

Principle of Operation

The OWS operates on coalescence—the tendency of oil droplets to cluster and form larger, buoyant globules that separate from water. The process:

1. Gravity settling (Inlet Tank): Bilge water enters a baffled receiving tank. Oil droplets drift upward over 30 minutes; Settling Baffle plates slow water flow, increasing residence time. Free oil pooling on the surface is skimmed via the Oil Skimmer.

2. Coalescing filtration (Coalescer Stage (One Unit)): Water flows through a Plate Pack—parallel-plate pack with oleophobic (oil-attracting) polymer sheets spaced 3 mm apart. As water passes, dispersed oil droplets (microns in size) hit the plates, coalesce into larger droplets, and drift upward toward the Outlet Header. The outlet header separates clean water downward and coalesced oil upward.

3. Final polishing (Drain Filter): Treated water flows through a 10 micron cartridge, removing remaining suspended solids and tiny oil droplets.

4. Real-time monitoring (Oil Content Monitor): A continuous Sensor Probe (infrared spectroscopy) measures oil concentration in the discharge line. If concentration exceeds 15 ppm, the Shutoff Solenoid closes the discharge valve, diverting flow back to the inlet tank for re-treatment.

5. Controlled discharge: The Discharge Pump regulates outflow at 5 m³/hr, allowing adequate residence time and polishing.

Coalescer Design Rationale

The Plate Pack oleophobic material (polypropylene or polyamide) has a critical property: it repels water molecules but attracts oil molecules. When a 2 micron oil droplet suspended in water contacts an oleophobic sheet, the droplet wets the surface, merges with adjacent droplets, and grows. Larger droplets have higher buoyancy and drift faster toward the outlet header.

The 3 mm spacing between plates is a critical tolerance: too narrow (<1 mm) and the unit clogs prematurely; too wide (>5 mm) and oil droplets don't reach the coalescence zone before exiting as free water. Modern plates are self-supporting polypropylene with rib reinforcement, withstanding 5 bar pressure without buckling.

Two parallel coalescer stages increase treatment surface area (4 m² total) and improve oil removal from 100 ppm inlet to <1 ppm at the outlet—comfortably below the 15 ppm discharge limit.

Flow Equalization and Residence Time

A critical design feature is the Flow Equalizer (baffle chamber). If inlet tank water flows directly to the coalescers without stabilization, transient surges cause short-circuiting—some water bypasses the plates with residual oil. The equalizer acts as a buffer: incoming water is distributed evenly across a Flow Distribution Weir, and internal baffles ensure laminar flow. This guarantees 30–45 minutes residence time, essential for droplet coalescence.

Oil Concentration Monitoring

The Oil Content Monitor uses infrared light absorption. Oleophilic compounds (oils and hydrocarbons) absorb infrared light at 3 microns wavelength; water is largely transparent. A transmitter-detector pair measures light absorption, correlating to ppm oil concentration with <1 second response.

The sensor range is 0–100 ppm; measurement uncertainty is typically ±2 ppm. At 15 ppm threshold, operators have ~3 ppm margin before automatic shutdown triggers—a safety buffer accounting for sensor drift.

If discharge exceeds 15 ppm, the Shutoff Solenoid valve closes, and the Discharge Pump circulates treated water back to the inlet tank via the Collection Solenoid. The coalescer cartridge may be fouled (oil-saturated), requiring chemical cleaning or replacement.

Separated Oil Handling

Coalesced oil accumulates in the upper compartment of the Outlet Header and is diverted via the Oil Collection Valve back to the Inlet Tank (or a dedicated sludge tank). This oil is not discharged to sea; instead, it accumulates in shipboard tanks and is pumped ashore at port facilities (costing $500–2,000 per cubic meter for disposal). Proper oil collection prevents re-emulsification and improves overall system efficiency.

MARPOL Compliance and Recordkeeping

The Control Panel with Data Logger interface records every discharge event. IMO requires that vessels maintain an Oily Record Book documenting:

• Date, time, quantity discharged.
• Oil content (ppm) at discharge.
• Location (latitude/longitude, port).
• Operator signature.

Port state control inspectors board vessels and audit OWS logs. Gaps in records (e.g., 5 days of operation with no discharge log) imply illegal overboard dumping and trigger fines and detention.

Maintenance and Failure Modes

Coalescer fouling (most common): Oil-saturated plates lose oleophobic surface properties. Cleaning involves circulating hot seawater or mild chemical detergent through the plates. Replacement plates cost $5,000–10,000 per stage.

Sensor drift: Infrared sensors degrade over years. Annual calibration using certified reference oils (known ppm standards) verifies accuracy. Failure mode: sensor reads <15 ppm when actual is >25 ppm, enabling illegal discharge undetected.

Pump failure: Positive displacement gears jam if sand/scale ingests bilge. A strainer at the inlet tank outlet is mandatory to protect the pump.

Backpressure check valve stiction: Salt spray corrodes the Backpressure Check poppet, preventing oil return and allowing water to enter the sludge tank. Annual inspection and lubrication extend service life.

Operational Best Practices

1. Bilge source segregation: Route clean cooling water separately from oil-bearing engine bilges, reducing OWS inlet oil concentration from 5,000 ppm to 500 ppm. This extends coalescer cartridge life.

2. Preventive pump-down: Discharge bilge daily (even small volumes <0.5 m³) rather than waiting for accumulation. Small, frequent discharges are less likely to overload the coalescer.

3. Cartridge change planning: Monitor Filter Cartridge resistance (pressure rise). Replace before exceeding 2 bar backpressure to prevent rupture.

4. Sensor periodic checks: Annually calibrate the Oil Content Monitor using reference standards to catch drift.

Modern tanker and cruise ship fleets operate redundant OWS units (two skids, either capable of handling full bilge flow), ensuring continuous compliance even during maintenance.