MARPOL Annex – 1 : Every ocean going ship must have Oily water separator. MEPC (marine environment protection committee) suggests OWS for all ocean going ships. It separates oil from bilge water as per regulations and also used in de-ballasting or when cleaning oil tanks.
OWS & Centrifuges:
- Both are used for separating oil and water.
- In OWS, there is large quantity of water and less amounts of oil is removed.
- In Centrifuges, there is large amount of oil and less amounts of water is removed.
- In OWS, all components are static but in Centrifuges rotating components are available.
- Main principle of separation, is the density difference between oil and water.
- In oily water mixtures, oil exists as a collection of globules of various sizes.
- Force acting on these globules, causing it to move in the water is proportional to the difference between the weights of oil particle and water particles.
- Resistance to the movement of globule, can be given by stoke’s law.
- Now, separating force is equal to resistance to movement.
- Above equation shows, separation of oil depends on various factors.
Note : The entrance area in OWS is made large so that flow is slow and large slugs of oil can move to the surface quickly. Faster rate of separation is obtained with large size oil globules or slugs.
Separate, What & What not?
- Oil droplets from water
- HC & Vegetable oils
- Solid particles
- Not, dissolved contents
- It’s Simple.
- Stationary (static) parts.
- There are 2 separate compartments in it and connected in series operation.
- First compartment is for physical impurities and second for de-oiling.
- In first compartment baffle plates are fitted and in second, coalescer unit is fitted.
- OCM (oil content monitoring) is fitted to monitor the oil content.
- Control valves are fitted to collect oil.
- For releasing air, vent valve is fitted on separator unit.
Arrangement & Description:
- Working can be divided into 2 stages : Separation (1st Stage Filter) & Filteration (2nd Stage Filter) or Coalescer unit.
- In separation stage oil content is reduced in 100 ppm and after Coalescer unit oil content is reduced in 15 ppm (i.e desired ppm level).
- Now, the oily water (Bilge water) enters the separator in the upper half of the unit and is directed downwards to the Baffle plates.
- Large globules of oil separate out in the upper part and smaller globules are carried out by the water into the spaces between plates.
- Raising velocity of globules carries them upwards where they become trapped by the under surfaces of the plates and coalesce until the globules have sufficient rising velocity to travel along the plate surface and break away at the periphery.
- Particles are settled down in compartment when passes through baffles.
- Through control valve, oil is collected in sludge tank or any desired tank.
- Vent valve releases air whenever required.
- Now, from this separating unit oily water is sent to 2nd stage filter unit. First of all oil is filtered by dedicated filter, particles settle down in compartment.
- Coalescer is used for de-oiling finally. Coalescer is just a reverse of Emulsifier, separate emulsions into their components.
- Oil rises above the water and it is collected through control valve.
- Finally oily water mixture (very less oil) is controlled and monitored by OCM Unit (oil content monitoring). When mixture has less than or equal to 15 ppm oil content , then it is discharged through 3-way control valve.
- If oil content is more than 15 ppm then alarm is automatically activated and mixture is sent to Bilge holding tank again.
In Simple :
- 1st Stage– Oily mixture (Bilge water) enters into separator unit and passes through baffles – larger particles settle down – oil rises up due to less density – oil is collected through control valve – mixture is passed via a pipeline to Filter unit
- 2nd Stage– Oily mixture is filtered by filter and particles are separated – Coalescer breaks the oil-water emulsion in its components – now, oil rises up due to less density – oil is collected through control valves in desired tank – now ppm level is very less and it is monitored by OCM – OCM monitors the required ppm level and controls over it.
Stats & Data:
- Regulation = MARPOL 73/78, Annex 1
- 1st stage ppm level = 100 ppm
- 2nd stage ppm level = 15 ppm (more or less – OCM)
Note: OCM works on the principle of U-V Fluorescence.
- Large size of globules (oil) – easily rises up and separated
- Temperature (high) – density decreases and tends to expand
- Use of sea water – having more density compared to fresh water, easily separated
- Pumping system – effective pumping always helps (reduces turbulence)
- Very less maintenance required because of static parts
- That’s why it serves for long time
- But for effective running, we have to perform some maintenance also
- Oil absorbent material should be changed regularly
- Cleaning of chambers
- Checking of Oil Monitoring Discharge System (ODM) or (OCM)
- Checking of valve etc.