Oily Water Separator MARPOL has a regulation under ANNEX I which limits the oil content in the bilge water that vessel can legitimately discharge into the sea. It is now a requirement for all vessels to have an oil discharge monitoring and control system along with an oil filtering equipment known as the Oily Water Separator (OWS). A ship engineer may work with 5-10 different makes of marine engines, but he/she is more likely to encounter many more types and makes of OWS in his/her career span. Even for PSC inspectors and surveyors, oily water separator (OWS) has always been a preferred choice of machinery on the ship for inspection. Hence, it is imperative to know and understand the basics of oil and water separator design and how an oil and water separator works. As the name indicates, the function of oily water separator is to separate maximum amount of oil particles from the water to be discharged overboard from engine room or cargo hold bilges, oil tanks and oil contaminated spaces. As per maritime regulations, the oil content in the water processed from the OWS must be less than 15 parts per million of oil. Oily Water Separator (OWS), MARPOL Annex - 1 : Every ocean going ship must have Oily water separator. MEPC (marine environment protection committee) suggests OWS for all ocean going ships. Function: 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. An Oil Water Separator is a piece of equipment used to separate oil and water mixtures into their separate component. There are many different types of oil-water separator. Each has different oil separation capability and are used in different industries. Oil water separators are designed and selected after consideration of oil separation performance parameters and life cycle cost considerations. ”Oil” can be taken to mean mineral, vegetable and animal oils, and the many different hydrocarbons. Oil water separators can be designed to treat a variety of contaminants in water including free floating oil, emulsified oil , dissolve oil and suspended solids. Not all oil separator types are capable of separating all contaminants. The most common performance parameters considered are Oil droplet size (in the feed of the separators) Oil density Water Viscosity (temperature) Discharge water quality desired Feed oil water flow (daily and peak hourly) An oily water separator (OWS) (marine) is a piece of equipment specific to the shipping or marine industry. It is used to separate oil and water mixtures into their separate components. This page deals exclusively oily water separators aboard marine vessels. They are found on board ships where they are used to separate oil from oily waste water such as bilge water before the waste water is discharged into the environment. These discharges of waste water must comply with the requirements laid out in Marpol 73/78 Bilge water – is a nearunavoidable product of shipboard operations. Oil leaks from running machinery , such as diesel generators, air compressors, and the main propulsion engine . Modern OWS have alarms and automatic closure devices which are activated when the oil storage capacity if the oil water separator has been reched. Purpose The primary purpose of a shipboard oily water separator (OWS) is to separate oil and other contaminants that could be harmful for the oceans. The International Maritime Organization (IMO) publishes regulations through the Marine Environment Protection Committee (MEPC). On July 18, 2003, the MEPC issued new regulations that each vessel built after this date had to follow. This document is known as MEPC 107(49)[ and it details revised guidelines and specifications for pollution prevention equipment for machinery space bilges of ships. Each OWS must be able to achieve clean bilge water under 15 ppm of type C oil or heavily emulsified oil, and any other contaminates that may be found. All oil content monitors (OCM) must be tamper-proof. Also whenever the OWS is being cleaned out the OCM must be active. An OWS must be able to clear out contaminants as well as oil. Some of these contaminating agents include lubricating oil, cleaning product, soot from combustion, fuel oil, rust, sewage, and several other things that can be harmful to the ocean environment. Gravity plate separator A gravity plate separator contains a series of plates through which the contaminated water flows. The objective of the design is to allow oil droplets in the water to coalesce on the underside of the plate eventually forming larger oil droplets which floats off the plates and accumulates at the top of the chamber. The oil accumulating at the top is then transferred with some en-trained water to a waste oil tank. This type of oily water separator is very common for many industrial applications as well as in ships but it has some flaws that decrease efficiency. Oil particles that are sixty micrometers in size or smaller do not get separated. Also the presence of chemicals and surfactants in the water greatly reduce oil droplet coalescence, impeding the separation effect The variety of oily wastes in bilge water can limit removal efficiency especially when very dense and highly viscous oils such as bunker oil are present. Plates must be replaced when fouled, which increases the costs of operation.[2 Centrifugal oily water separator A centrifugal water–oil separator, centrifugal oil– water separator or centrifugal liquid–liquid separator is a device designed to separate oil and water by centrifugation. It generally contains a cylindrical container that rotates inside a larger stationary container. The denser liquid, usually water, accumulates at the periphery of the rotating container and is collected from the side of the device, whereas the less dense liquid, usually oil, accumulates at the rotation axis and is collected from the center. Centrifugal oil–water separators are used for waste water processing and for cleanup of oil spills on sea or on lake. Centrifugal oil–water separators are also used for filtering diesel and lubricating oils by removing the waste particles and impurity from them. Hydrocyclone oily water separator An oil water separation hydrocyclone is a device designed to separate oil from water by the use of a strong vortex. These separators are passive (no moving parts) and resemble long tapered pipes. They typically contain an inlet section, long tapered section and a long outlet section. In operation the strong vortex is created when the oily water is injected tangentially into the inlet end of the separator. This creates a centrifugal force, that accelerates as it moves down the tapered cone. The centripetal and centrifugal forces separate the heavier water component to the outside of the vortex while the lighter oil droplets are forced to the centre. The separated oils are removed through an orifice at the inlet end of the cone and treated water is discharged through the opposite end. The centrifugal forces generated inside the vortex of the better de-oiling hydrocyclone separators are of the order of 1,000 times the force of gravity. This is why smaller emulsified oil droplets as low as 15 microns can be removed. Oil removal hydrocyclones, or de-oiling hydrocyclones, are very different in geometry, design and operation compared to the more common solid removal hydrocyclones. When correctly designed and operated oil removal hydrocyclones Hydrocyclones are very useful for removing both large oil droplets and smaller emulsified oil droplets in a broad range of applications across many industries. The technology has been successfully applied to treat oily water produced in the mining industry, meat processing, dairy manufacturing, petrochemical, oil refining, oil marketing and oil production operations. Principle: Separation force & Resistance force 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 Oily Water Separator 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 Effective separation: 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) Maintenance: 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.
