Hotel Service Unit 18 Oil Separators Q5 Answer

  1. Describe, with the aid of a sketch, a single stage static Oily Water Separator designed to meet MARPOL Annex I requirements.(10)

Construction & flow path

  1. Bilge holding tank → strainer → positive-displacement bilge pump
    • PD pump gives steady, controllable flow and good suction on dirty bilge mixes.
    • Suction strainer protects the pump/OWS from debris.
  2. Flow-control/relief arrangement
    • Manual or automatic throttling valve (and a small relief/bypass) ensures the OWS design flow rate is not exceeded (performance is sensitive to flow).
  3. Static OWS pressure vessel (single stage, gravity/coalescing)
    • Inlet diffuser spreads flow and reduces turbulence.
    • Coalescer pack (oleophilic media or cartridges) causes fine oil droplets to agglomerate into larger drops.
    • Inclined plate/baffle settling zone increases rise path and residence time; oil rises, water falls to the clean-water draw-off.
    • Oil-rise weir & skim outlet sends separated oil to an oil drain/slop tank.
    • Air release/vent allows entrained air/gas to escape and prevents air locking.
    • Low draw-off outlet for the clean water (below the oil/water interface).
    • Sample cock provided at the outlet.
  4. 15-ppm oil content monitor (OCM) + automatic stopping device
    • Monitors discharge; if > 15 ppm, it alarms and drives a 3-way valve to recirculate back to the bilge/holding tank (or shuts discharge pump).
    • If ≤ 15 ppm, the 3-way valve routes to overboard. The overboard line is higher than the OWS outlet to prevent back-flooding.
  5. Ancillaries
    • Pressure gauge(s), thermometer(s), drain and vent valves, and a vacuum breaker/air release on the vessel shell.

Operating principle (single stage / “static”)

  • Separation is by gravity enhanced with coalescence. The coalescer media and plate pack enlarge droplet size and create quiescent, laminar paths so oil rises to the top and is skimmed off, while cleaned water is withdrawn from a low outlet. No centrifuge is used—hence “static”.

MARPOL Annex I compliance points

  • Discharge to sea only when OCM reads ≤ 15 ppm and ship is where discharge is permitted.
  • Automatic stopping/recirculating device interlocked with the OCM.
  • Flow to OWS within rated capacity (set by the flow-control).
  • Oil/slops led to a dedicated oil drain/holding tank.