Every engineer on the ship knows the importance of oily water separator (OWS) and must have heard stories of legal actions taken against seafarers who tried to fiddle or bypass the automation system of the OWS.

The most common cause of people tampering with oily water separator is the deterioration of the performance of the system.

Apart from the filter getting clogged due to continuous usage, there can be many other reasons for the lack of performance.

A few critical points, which are often ignored and less commonly known to engineers operating oily water separator on daily basis, are discussed in this article to ensure efficient performance of oily water separator (coalesce filter type) used onboard ships:

1. Oil In Bilge: The separator is suitable for separating a small quantity of oil in the bilge and not the other way round. If there is a mixture consisting of a small quantity of water in oil, better not to pass it through the OWS and directly transfer it to waste oil tank for sludge disposal to shore or for incineration.

2. Keep Viscosity in Limit: Highly viscous oil if supplied for oily water separation will clog the filter. It is important to keep the viscosity of the oil within 1000 mm²/s

3. Discharge Outside Separator: Never discharge or drain water-oil mixture from the separator abruptly out of OWS as it will lead to the separated oil adhere to coalescers, making it inoperative. Ensure to discharge the collected separated oil outside the separator and clean the internals of the OWS by supplying water.

4. Install Dust Filter: While using OWS for discharging bilge mixture containing dust and sand (a coarse-grained entity), it will be difficult for the coalescer filter to pass dust and sand due to their sizes. This will reduce the operating hours of the filter and in-turn the efficiency of the OWS. The ideal service life of the filter normally ranges from 1-2 year depending upon the manufacturing process, considering the daily operation of one hour. It is advisable to install a dust filter in the inlet line of the OWS to avoid this trouble.

5. Exchanging Probe for Fault Finding: In most of the 15ppm OWS models, the oil level detecting probe and transmitting converter in the 1^st and 2^nd stage chamber are identical. In the event of trouble, exchange these with each other, between the chambers, as this will help in finding out the source of trouble quickly.

6. Keep Check On Heating Device: If a heating device is provided, ensure it is ON when OWS is in operation and switched OFF before OWS is stopped. In case when the OWS is run for a long time, keep a track on the heater for overheating of coils. If the separator is overheated to a high extent, there may be some production of inflammable gases inside.

7. Protect Internal Coating: The OWS internals are applied with tar epoxy coating, which is inflammable. Avoid bringing fire near to it or perform welding over the surface or body as the heat generated will damage the coating, making OWS susceptible to corrosion

8. Check Water Level: Ensure that the separator is initially filled with seawater before the bilge mixture is supplied to it. This is to increase the life of filters and also to maintain the operational efficiency of the separator

9. Prevent Leakage: Ensure the means provided to avoid leaking or flowing out of water due to siphon effect. Failure to do so will allow the oil to flow in the secondary chamber highly affecting the treatment capability of the machine and clogging the 2^nd stage filters

10. Check Effluent: Frequent checks on effluent to be done to assess the performance of the separator. If the effluent is found contaminated, immediately stop the separator and take preventive actions

The operator of OWS should be thorough about the system, design, piping arrangement and of course the operating procedure of oily water separator. Out of all the machinery systems, OWS is kept at high priority due to stringent rules and regulations attached to it for ensuring environmental compliance. Hence it is important for the ship staff to maintain this machine in best of its shape.

Over to you..

Do you know any OWS maintenance tips that should be added to the list?

Let’s know in the comments below.

10 Oily Water Separator (OWS) Maintenance Tips Every Ship Engineer Must Know appeared first on Marine Insight - The Maritime Industry Guide

An oily water separator clears the bilge water of oily content to bring it inside the acceptable range to discharge it overboard. An oily water separator is a machinery for such importance that it is handled by only the 2nd or chief engineer. (However, the duty engineer might also be asked to operate under supervision)

Operating an Oily Water Separator

An oily water separator can only be operated when the ship is sailing and en route. According to MARPOL, the oil content of the effluent must be less than 15 ppm and the ship has in operation an oil discharge monitoring and control system and oily-water separating/filtering equipment.

In case of failure to follow any of the above mentioned rules, the ship will be fined and stopped, and the chief or 2nd engineer can even be imprisoned.

Because of such high risks, operating an oily water separator should be done with utmost precision to minimize the risks of marine pollution. Though a “How to Operate?” guide is always posted near the oily water separator, there are few points to be kept in mind and followed to prevent any mistake.

Operating Procedure

The following points are to be followed while operating OWS.

1) OWS overboard manual discharge valve is to be kept locked and keys are to be kept with the chief engineer. Open the lock and overboard valve. Open all the other valves of the system.

2) Open the desired bilge tank valve from which the oily water mixture is to be discharged from OWS.

3) Open air if the control valves are air operated.

4) Switch on the power supply of the control panel and OCM unit.

5) Fill the separator and filter unit with fresh or sea water to clean up and prime the system till the water comes out from vent of second stage.

6) Start the OWS supply pump which is a laminar flow pump and one that will supply the oily water mixture to OWS.

7) Observe the OCM for ppm value and keep checking sounding of bilge tank from where OWS is taking suction and of the OWS sludge tank.

8 ) A skin valve/sample valve is provided just before overboard valve and after the 3-way valve. Keep a check on the sample for any effluent and clarity.

9) Keep a watch on the ship side at the overboard discharge valve.

10) After the operation, Switch off the power and shut and lock the overboard valve. Keys to be handed over to the chief engineer.

11) Entry to be made by chief engineer in the Oil Record Book (ORB) with signature of operating officer, chief engineer and the master.

If you liked this article, you may also like to read- What is oil spill at sea & Fighting oil spill on ship

How to Operate an Oily Water Separator (OWS) on Ship? appeared first on Marine Insight - The Maritime Industry Guide

Optimal performance of the Oily Water Separator is dependent on the knowledge of certain factors such as design, operation, bilge management and maintenance etc. For more information, read what factors affect oily water separator on ships?

In this article we shall discuss the various operational factors influencing performance of the Oily Water Separator.

Operational Factors

Correct operational knowledge is required for all machinery on ships to run properly and the Oily Water Separator is no exception. Marine engineers work with different types of Oily Water Separators through their sea career and each type has its own unique features.

Equipment specific knowledge is essential for the correct operation of oily water separator and can be learnt from the operational manual on board. However a generic knowledge is also required as the basic working principle is essentially the same.

An idea borrowed from existing shore technology, the Oily Water Separators for ships are designed to work properly under ideal conditions. However ships do not have ideal conditions, and therefore ship’s staff should consider the following operational factors which affect the performance of  Oily Water Separator on ships.

1. Avoid Emulsions: Emulsions are formed when the inter-facial tension between two liquids is reduced by certain means sufficiently to allow droplets of one liquid to disperse in another. Mechanical agitation, shearing forces, solvents, chemicals, surfactants and the presence of particulate matter can all reduce inter-facial tension and result in formation of emulsion.

2. Avoid Chemical emulsions:  The chemical emulsion is formed by the addition of some chemicals in the water. These chemicals act as surfactants and they hold the oil drops together in emulsified state.  The surfactants may be the detergents used for cleaning, alkaline chemicals used for boiler cleaning and conditioning etc.

3. Avoid secondary dispersion:  Mechanical emulsions are of primary and secondary types. The primary emulsions are larger drops of oil dispersed in water and are generally separated within 24 hours. The secondary emulsions are fine droplets of oil that are thermodynamically stable and do not separate. The secondary dispersion is caused by turbulent conditions.

4. Avoid suspended solids: Suspended solids cause stabilization of emulsion and cause problems in separation of the oil from the bilge water. Suspended solids can be mud, boiler soot and cargo residues sucked from the blowers. The suspended solids get coated with oil and stabilize the emulsion. Neutrally buoyant solids that neither rise nor fall are most troublesome as it is difficult to remove them. They also generate the high ppm alarm.

5. Avoid Turbulence: OWS needs laminar flow to operate optimally as per their design. Avoid using OWS in times of heavy rolling and keep all line valves fully open to avoid generating turbulence. Rolling motion, retrofitting on old pipelines and inadequate opening of the suction line valves can lead to turbulent flow inside the OWS with resulting fall in OWS capacity due to formation of emulsion. Sometimes due to turbulence some of the oil droplets become less than 8 microns and are affected by the random motion of the water particles. This random motion is called as Brownanian motion and it nullifies the forces of buoyancy and the oil drops fail to rise.

6. Avoid Particulate Matter: Fine particulate matter like soot, rust, microbial contamination of bilge water etc. also act as emulsifying agent. Although most of the soot of the boiler washing settles down in the bilge holding tank, fine soot particles (1μm or less) will give the bilge water a blackish appearance. These particulate matter will not only fool the ppm meter into activating the high ppm alarm but will also physically act as emulsifying agent

7. Optimal use of Chemicals: Sometimes it is required to use special chemicals called emulsion breakers to separate the oil from the water and release free oil. If emulsion breaker chemical is used, care should be taken that it is used as per the instructions given by the manufacturer emulsion does’t go worse. Sometimes putting more than the recommended amount can worsen the problem.

8. Restrict Drainage of Chemicals: Lot many chemicals are used in engine room for special purposes such as water conditioning, corrosion inhibition, rust removal, cleaning, degreasing etc. However care should be taken to collect these chemicals and disposing them properly. Allowing all kind of chemicals to run free into the bilges is not good housekeeping. More over the pH of water above 10 and below 4 can cause chemical emulsification.

9. Detergent Disposal: Detergents are used for mopping and soap washing of bulkheads and such areas. Generally these are the same detergents we use at home or ashore. These detergents act as surfactants and cause emulsion of oil in water. Disposing mop water separately or using quick break detergents would help significantly towards better separation.

10. Avoid prolonged storage: Prolonged storage of the bilge water causes modification in the nature of free oil. Normally oil water mixture when allowed to stand for some time (say 24 hours) separates into a layer of oil on top of water called as free oil. This free oil is easy to remove but long retention of the bilge water can cause modification in the properties of free oil due to oxidation and microbial action. If this modification occurs then it is difficult to remove the oil.

11. Do Proper Filtration: If there is large amount of solid particles, floating media, jute etc. in the bilge water, it should be properly removed using strainers to avoid fouling of the filter media.

12. Collect leakages: Ensure that minimum of oil reaches the bilge wells and if the oil quantity is more in a mixture (of oil and water) put it in separated oil tank.  Always remember that the Oily Water Separator is not a purifier.

13. Heat the influent:  Heating the influent reduces the viscosity of the continuous media causing better separation.

14. Segregation of Wastes:  Do not mix sludge and bilges. Even a bit of sludge can contaminate large amount of bilge water. In some ports even discharge of treated sewage is not permitted due to local regulation and therefore in absence of dedicated sewage holding tank, treated sewage is put in bilge holding tank. This should be avoided as it would be impossible to run the Oily Water Separator thereafter without cleaning the tank.

15. Fill up OWS prior use: Prior to operating the OWS and allowing the bilge water to enter always ensure that it is filled up with clean water and all air pockets have been removed. This is important as air pockets can confuse the capacitive sensors and can make automation go haywire.

16. Back Flush: Back flushing of the OWS should be done as per the recommended frequency given by the manufacturer if there is a provision for doing so as increases the life of the filter media.

17. Clean Sensors: Frequent cleaning of the electronic interface sensors would ensure the correct operation, proper oil removal and sharp cutting off ensuring less discharge of water to separated oil tank.

18. Remove Accumulated Oil: Apart from the automated oil removal any other accumulated oil should be removed from the OWS chambers regularly.

19. Proper Operating Procedure: Make sure that the operating procedure of OWS is followed in a proper step-by-step and systematic procedure. Learn more about OWS operating procedure here. 

20. Proper OWS Maintenance:  Needless to say proper maintenance of the OWS as per the instructions of the manufacturer would keep it ship shape.

20 Important Factors For Oily Water Separator Operation On Ships appeared first on Marine Insight - The Maritime Industry Guide

As a seafarer on a merchant ship or as a ship manager/owner of a vessel, one of the biggest nightmares is that of any kind of oil pollution accident on a ship.

A ship produces oil and water mixture on a daily basis which needs to be separated from each other before discharging the dirty water out of the ship using equipment such as an oily water separator.

MARPOL has a regulation under ANNEX I, which limits the oil content in the bilge water that a 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 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, an oil 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.

Related Reading: How to operate an oily water separator? 

As the name indicates, the function of an oil-water separator is to separate the maximum amount of oil particles from the water to be discharged overboard from the 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.

Related Reading: What is Oil Discharge Monitoring and Control System?

Regulation for Oily Water Separator:

As per Annex 1 of MARPOL under regulation four specified under paragraphs 2, 3, and 6, any direct discharge of oil or oily water mixture into the sea shall be prohibited. The regulation further explains how an oily water mixture can be treated on board and can be discharged out at sea:

Oil Water Discharge Regulation-

For a ship with 400 GT and above, discharge of oil mixture can be done under the following conditions:

1.    The ship is en route;

2.    The oily mixture is processed through an oil-water separator filter meeting the requirements of regulation 14 of this Annex;

3.   After passing the oil-water separator system, the oil content of the effluent without dilution does not exceed 15 parts per million;

4.    The oily mixture does not originate from cargo pump-room bilges on oil tankers

5.    In an oil tanker ship, the oil-water mixture is not mixed with oil cargo residues

When the ship is plying in the Antarctic area, any discharge into the sea of oil or oily mixtures from any vessel shall be prohibited.

Related Reading: What factors affect separation in oily water separator? 

Important Oily Water Separator (OWS) requirement:

1. As per the MEPC 107(49), the bilge alarm or an Oil Content Monitor (OCM), which provides for internal recording of alarm conditions, must be certified by an authorized organization
2. The OCM provided with the oily water separator must be tamper-proof
3. The OCM must activate and sound an alarm whenever freshwater is used for cleaning or zeroing purposes
4. Separator capable of achieving 15 ppm on type C emulsion.
5. The oily water separator must pass through the minimum discharge limit of 15ppm.
6. Sensors and alarms must be installed on the equipment where it can’t be monitored and maintained all the time.

The OWS must only be operated by officers who are familiar with the equipment and who are directly supervised by the Chief Engineer, who bears sole responsibility for its maintenance and operation. When the equipment is not in use, the Chief Engineer is responsible for ensuring that a procedure is in place to lock it out to prevent unauthorized operation with keys in possession of the Chief Engineer. 

It should be emphasized that sailing from a port without a functional OWS is unlawful and that appropriate spares for the unit must be carried onboard. The Critical Equipment section contains information on MARPOL equipment.

Construction and Working of Oily Water Separator (OWS)

OWS consists of mainly three segments:

1. Separator unit

• This unit consists of catch plates which are inside a coarse separating compartment and an oil collecting chamber.
• Here the oil has a density which is lower than that of the water, which makes the former rise into the oil collecting compartment and the rest of the non-flowing oil mixture settles down into a fine settling compartment after passing between the catch plates.
• After a period of time, more oil will separate and collect in the oil collecting chamber. The oil content of water which passes through this unit is around 100 parts per million oil.
• A control valve (pneumatic or electronic) releases the separated oil into the designated OWS sludge tank.
• The heater may be incorporated into this unit for smooth flow and separation of oil and water.
• A heater may be incorporated in this unit either in the middle or sometimes in the bottom part of the unit (depending upon the area of operation and capacity of the separator equipment) for smooth flow and separation of oil and water.
• The first stage helps in removing some physical impurities to achieve fine filtration in the later stage.

Related Reading: How Good Bilge Management Practice Help Improve OWS Performance?

2. The Filter unit

• This is a separate unit whose input comes from the discharge of the first unit.
• This unit consists of three stages – filter stage, coalescer stage and collecting chamber.
• The impurities and particles are separated by the filter and are settled at the bottom for removal.
• In the second stage, the coalescer induces a coalescence process in which oil droplets are joined to increase their size by breaking down the surface tension between oil droplets in the mixture.
• These large oil molecules rise above the mixture in the collecting chamber and are removed when required.
• The output from this unit should be less than 15 ppm to fulfil legal discharge criteria.
• If the oil content in water is more than 15 ppm, then maintenance work such as filter cleaning or renewal of filters is to be done as required.

A freshwater inlet connection is also provided to the filter unit to clean and flush the filter. This is usually done before and after the operation of an oil separator unit.

Related Reading: 20 Important Factors For Oily Water Separator Operation On Ships

3. Oil Content Monitor and Control Unit

• This unit functions together in two parts – monitoring and controlling.
• The ppm of oil is continuously monitored by Oil Content Monitor (OCM); if the ppm is high, it will give an alarm and feed data to the control unit.
• The control unit continuously monitors the output signal of OCM, and if an alarm arises, it will not allow the oily water to go overboard by operating a 3-way solenoid valve.
• There usually are three solenoid valves commanded by the control unit. These are located in the first unit oil collecting chamber, second unit oil collecting chamber and one on the discharge side of the oily water separator, which is a 3-way valve.
• The 3-way valve inlet is from the OWS discharge, where one outlet is overboard and the second outlet is to the OWS sludge tank.
• When OCM gives an alarm, the 3-way valve discharges the oily mixture in the sludge tank.

A small pipe connection of fresh water can be provided to the OCM unit for flushing. Whenever this line is in use, an alarm is sounded and recorded in the OCM log, ensuring a record to check the discharge valve was shut during this period.

As in most shipping companies, the OWS is meant to be operated only by the chief engineer, and the training levels on OWS systems for other crew members are found to be very low. The ship operators should ensure onboard guidance and training are included in the training schedule of the ship.

Factors Affecting Separation Of Oil From Bilge Water

Several factors affect the separation of oil from bilge water, such as :

• Density: As water has more density than that oil, it tends to rise
• Adequate Density Difference: Seawater has more density than that freshwater. Thus, by using seawater, we can increase the rate of separation.
• The viscosity of Continues Fluid: we know a less dense fluid with less viscosity offers better conditions for oil to move towards the surface than a dense fluid with more viscosity. Here the viscosity of fresh water is less than that of seawater.
• Temperature: Temperature plays a vital role. It is a significant factor that affects both density and viscosity. When the temperature is low, among fluid particles restricting separation, viscosity is higher.
• Size of particles: The separation of oil from water is directly proportional to the size of oil particles.

Dismantling Procedure for cleaning oil water separator

1. First stop the O.W.S Bilge pump
2. Top the steam flow to heating coils
3. Shut the main overboard valve
4. Open vent for both separation and filtration chamber
5. Slowly open the drain valve of each section at the bottom
6. Oil must be drained out
7. All electric and pipe connections must be removed
8. Nut and bolts of the top cover of the separation chamber must be opened.
9. Baffle plates should be taken out and cleaned with a brush and oil.
10. Now open all the nuts and bolts of the top cover of the separation chambers.
11. The condition of the coalescer filters must be inspected.
12. If necessary, put back a new filter.
13. After everything is over, assemble the whole system and fill the O.W.S with fresh, clean water to check for any leakage.

What causes an Oily Water Separator to malfunction?

1. The oil-to-water gravity gradient is the basis upon which the Oily Water Separator works.

Due to the general difference in weight between an oil particle and a particle of water of similar volume, the force exerting on an oil globule to travel in water is proportional to the difference in weight.

A globule’s resistance to mobility is determined by its size and the viscosity of the fluids in which it is contained.

Thus, a high rate of separation is often favoured by

• the large size of the globule.
• Higher system temperature (which affects both the specific gravity difference of the oil and water and the viscosity of the water) and the usage of saltwater.

2. Pumping Consideration

An important consideration is to minimize any disintegration of large-sized oil droplets in the feed oil for a separator, which may be greatly impacted by the type and rating of the pump used in the process.

Many bilge pumps are centrifugal, and they are often employed as supply pumps for the separator.

Thus, when the supply is turned on, it creates tiny oil droplets that scatter throughout the water, posing a major threat to the separation’s performance.

A positive displacement pump, such as a slow-running double vane, screw, reciprocating, or gear pump, allows the separator to work considerably better since it does not generate a significant number of tiny droplets. Pumping after the separator may result in a discharge with a concentration of less than 15 ppm without the need for 2nd Stage filters.

As a result of the preceding two arguments, it is clear that even if the separator is properly maintained and operated, the following causes might result in the separator malfunctioning:

1. When the separator has an abnormally high throughput.
2. Breakdown of wall globules due to extreme rolling and pitching of the ship
3. The pump type and rating is not matching, causing too much turbulence
4. Due to the high pumping rate, there is turbulence. 

Numerous traditional oily water separators cannot split stable emulsions and remove colloidal particles suspended in the water phase. This often leads to equipment failure due to insufficient oil separation below 15 parts per million (ppm) or blockage from excessive solids. An effluent with an oil content of not more than 15 parts of oil per million parts of water by volume may be produced via a combination of a bilge separator, a coalescer, or other techniques.

Recirculation facilities should be installed after and next to the stopping device’s overboard outlet to permit testing of the 15 ppm bilge separator system, including the 15 ppm bilge alarm and the automated stopping mechanism, with the overboard discharge locked.

The influence of bilge water containing cleaning chemicals is one of the primary reasons for oily water filtration system failure. Cleaning fluids based on detergents may form chemically stabilized oil emulsions that cannot be separated on board ships just by gravity. Avoiding the use of surfactant-based cleaning chemicals is the greatest strategy to optimize the effectiveness of oily bilge water separation equipment.

Maintenance of Oily Water Separator

To remain in excellent operating condition, an oily water separator must be properly and routinely maintained. When these devices are not drained and cleaned consistently, oils and other debris clog them and render them inoperable. This may result in oil levels in the discharged water exceeding the effluent limitations mentioned.

Oily water separator models that are elevated above the ground are considerably simpler to clean since everything is easily accessible. This is because there are no restricted places, and plates may be removed, cleaned, and changed. All forms of above-ground structures may be maintained from the ground. Additionally, solid waste may be conveniently evacuated via a big hole at ground level.

When employing a below-ground oil-water separator, it is essential to adhere to a tight maintenance plan. It is more probable that you will overlook cleaning the unit since it is buried in the ground. These are the oil-water separator maintenance methods to follow.

After a month of usage, do the following checks and cleanings on the inlet area:

• Turn off the unit’s supply of water.
• Open the cover of the unit.
• If there is any oil present, remove it and dispose of it in accordance with corporate and legal regulations.
• Water should be drained from the separator.
• Take a measurement of the depth of the residual solids and record it. In the future, this will serve as a foundation for planning maintenance and cleaning tasks.
• Get rid of the solids if required.

The media plates will need to be cleaned once all of this has been completed. They may be cleaned either inside or outside the unit. Insert a low-pressure hose in between the plate gaps on the plate packs to clean them without dismantling them. Remove any debris drained from the plates by draining it down the hopper outlet.

When cleaning the plates outside of the oil-water separator, use a low-pressure hose to rinse them off, taking particular care not to get any of the discharge on the ground where it might contaminate groundwater. Getting rid of the accumulated sludge and oil is the only thing that’s required.

To make sure that the tank is safe, check it for damage and fix the internal coating if it needs to be done. Simply re-insert the plate packs into the separator in the same order as they were removed in order to restart the machine. Make sure that they are firmly attached to the device so that they don’t fly away when it restarts. Depending on the unit’s use and the quantity of oil and other debris collected, maintenance should be performed on a regular basis by a skilled specialist.

References

Introduction to Marine Engineering– By D.A Taylor

Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used in the article, have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendations on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight. 

Oily Water Separator: Construction and Working appeared first on Marine Insight - The Maritime Industry Guide

Oily Water Separators are one of the most important yet neglected equipment on board ships. Revered and feared alike, marine engineers are often afraid to use it, and want to keep it unused and working so that they can show it to the Port State Control Officer (PSCO) and stay on the safer side.

Based on simple working principles and designed to be reliable and less labor intensive, oil water separators are however troublesome and demanding to engineers due to later’s lack of knowledge, proper training, operational negligence and sometimes apathy.  However proper care, maintenance, correct trouble shooting and efficient starting/stopping can keep it working properly and make life easier and safer. With stricter control and PSC inspections combined with stringent penalties including criminal action against the operating crew, it ‘s in the interest of the marine engineers to keep the OWS working properly.

A lot has been said and written about the inadequacy of some of the market equipment and especially those that have been retrofitted in old ships in seminars and forums. However, the seagoing marine engineers do not have access to these seminars and their voices are often unheard.  In the absence of a sympathetic ear, marine engineers just endeavor to keep the equipment running during their “contract time” and abstain from complaining  for the fear of victimizing.

Factors affecting separation in OWS

The oil water separator (OWS) is designed to work properly under ideal conditions; however the ship is not one and there are various sources of contamination in the bilge water. The bilge water is a mixture of various grades of oil in water, along with suspended solids, rust, chemicals, detergents, soot, paint chips and cargo dust etc.

In view of such a complex scenario it is necessary to have the knowledge of the various parameters and factors that influence the satisfactory operation of OWS. The satisfactory operation of the oily water separator (OWS) is dependent on the knowledge of the following:

• Design Factors: These include the basic working principle of the OWS and the enhancements made for marine context.
• Operational factors: Operational Knowledge and correct operational procedures that are generic and not equipment specific.
• Bilge management: Efficient Bilge management and knowing what goes into your bilges and how it affects it.
• Care and Maintenance: The general care and maintenance required to keep the OWS running smoothly.
• Equipment Specific Knowledge: There are numerous varieties of OWS in the market and each has their own peculiarities and problems. Knowledge of these will be beneficial.
• Crew Attitude and Aptitude:  No matter what kind equipment is fitted on board it needs correct operation and some training to run it optimally.
• Good House Keeping: Last but not the least, good housekeeping is essential to a cleaner engine room and to reduced production of oily water to be treated.

In these series of articles we shall endeavor to discuss the various ways to optimize the performance of OWS such as the design factors, operational limitations, efficient bilge management, training requirements and some good working practices.

Design Factors

Oily Water Separator is used to remove oil from the bilge water prior to discharging it overboard. It works on theprinciple of Stokes law and basically separates the two components utilizing their difference in specific gravity. The OWS for marine use are optimized to make them smaller in size due to space constraints. Additional components are fitted to help with the separation process but the knowledge of the basic design factors is beneficial.

The rate of rise as per stroke’s law is as follows

The Stoke’s Law generally states that the velocity or rise and hence the separation rate is directly proportional to the difference in density of the oil and the continuous fluid and the size of the droplets of oil. It is also states that rate of rise is inversely proportional to the viscosity of the surrounding fluid. We infer from the famous law the following:

• Density of Oil: Light oil is having higher rate of rise than heavier oil and therefore easier to separate. This information is useful to adjust and lower the flow rate when heavier oil contamination is suspected.
• Density of Continuous Fluid: Rate of rise will be higher when continuous fluid is sea water instead of fresh water or condensate. This information is useful and we can discharge the condensate drains into dedicated clean drain tanks instead of bilges.
• Viscosity of Continuous Fluid:  As rate of rise is inversely proportional to the viscosity of the continuous fluid the OWS performance is better when continuous fluid is fresh water. As this deduction is contrary to the above one it is a compromise between the two but it does not concern us much as we don’t really have much control on what goes to the bilges as per the original design.
• Size of Oil Droplets: The larger the diameter of the oil droplets the better is the rate of separation. This information is very helpful and we can assist the OWS by avoiding small drops of oil by mechanical agitation and emulsification.
• Temperature:  This is another factor which is important as it affects the density and viscosity directly.
  • Low temperature of the continuous fluid hinders the separation of oil due to additional viscous drag in view of the increased viscosity of continuous fluid in cold temperature. Oil separation is better in warmer temperatures and slightly increasing the temperature of the bilge water would give better separation. This information is important as we can increase separation rate by warming the bilge water in the holding tanks or heating incoming fluid by the steam coils fitted in some of the OWS.
  •  In high temperature the formation of emulsion by mechanical agitation is more than in lower temperature. This information is important as we should not heat the bilge holding tank when ship is rolling excessively or where we suspect mechanical agitation. 

What Factors Affect Separation in Oily Water Separator on Ships? appeared first on Marine Insight - The Maritime Industry Guide