Question and answer about Oily water separator. For MEO Class 2 (Sheet-1)

With reference to oily water separator, give a reasoned opinion as to why:

(a)        efficiency can be improved by merely increasing the size of a unit,              (4m)

(b)        single stage gravity type units offer a distinct advantages over multi stage        counterparts,                                                                                                          (4m)

(c)          most units operate within the turbulence flow range.                                     (2m)

 Answer:-

(a)                The efficiency of an oily water separator can be improved by increasing in size because in this way more baffles, conical plates, pebble beds, sub divisions can be incorporated whereby more surface area is available for the oil globules adhere and coalesce.

Also the larger the size means the more passages of patterns the flow of mixture has to pass through before reaching the outlet. Thus the slight turbulence caused at the entrance of the separator is greatly reduced and finally moves into laminar type of flow, thus assisting in better separation of the oil globules from the water. Also the more passages created and the appreciable time required  before the fluid reaches the outlet, it gives sufficient time for the oil globules to detach itself from water, adhere coalesce and later collected and discharge.

(b)               Single stage gravity type units offer a distinct advantages over multi stage counterpart in thus they are more:

1)      compact in design. Does not occupy much space and give a reasonable efficiency of effluent discharge within the range of 50 – 500 ppm.

2)      It is less complex in design, easy to maintain and cheaper.

3)      However with new regulation being enforced whereby an effluent of 15 ppm is required, such single stage gravity type is no longer able to meet the need requirement. However by incorporating second, third stage separators, where coagulant coalesces are fitted, the 15 ppm effluent discharged is achieved.

4)      However newer design have appeared single stage gravity type of separations that are able to meet those stringent rules of 15ppm effluent, is introduced by Hamworthy models HS1 – HS55. The separation is done in the primary separation channel where natural separation takes place due to gravitational differential and later final separation taking place at the plate pack coalescers. Thus the newer single stage gravity type units are more compact, more efficient, more reliable and less complex.

(c)                Most units does operate with the turbulence flow range because most of the existing separators pumps oily water from the bilges or tank into separator, meaning that the delivery pump is placed before the separator. This causes in turbulence flow due to the action of the impeller, vanes, gears etc where breakdown of the oil globules takes place.

However, with the new design, it is possible to eliminate turbulence from occurring at the entry by placing the pump after the separator, thus the mixture is now drawn through the separator by means of float switches and non return valves. Thus the flow pattern is more laminar and better separation of oil from water is obtainable.    

(a)        Explain why the supply pump should be matched to its associated static oily water separator and be of the rotary positive displacement type.                              (4m)

(b)        State how the complexity of compartmental sub division in static oily water separator contributes to greater effectiveness in separation,                                         (3m)

(c)        If rate of separation is directly related to density differential, state how this relationship can be effectively employed in practice.                                      (3m)

Answer :-

(a)                Rotary positive displacement supply pump must match in separator because if the throughput is more, it will overload the separator whereby turbulence in flow will take place. This break down the oil globules into finer particles causing difficulty in separation. Therefore, the capability of the effluent discharged will be much affected, hence contaminant of oil with the effluent will take place.

With large throughput, insufficient time is given to allow the oil globules to separate effectively on the water, adhere and coalesce, but instead the high throughput washes away the adhering oil globules and carry it over to discharge.

(b)               internal subdivision of the OWS into a complexity of small compartments does contribute to the greater functional efficiency of the separator in that, with these subdivision. The different degrees of oil globules can be arrested at different divisions, whereby coarse separation, where large and thick oil globules are arrested at earlier stage and the more finer oil globules at the intermediate stages, the even more finer oil globules entered with the water can be made to coalesce at the final and finer stage of separations. Thus the quality of effluent discharge is of almost of clean water and thus subdivisioning of the oily water separator is an advantage.

(c)                The principle of separation for an oil globule in water as given by Stoke’s Law is as follows:

V = ( rw - ro ) d² g

          18 µ

            where v = rising velocity of oil globule

                   rw  = water density

                   ro  = oil density

            hence, we can say that the rate of separation is directly related to density differential.

In practice, this can be effectively employed by heating up the oily mixture prior sending it into the separator. As shown by the graph below,

           

Hence, the higher the temperature, the bigger is the density differential, the better the separation efficiency.

Also the higher the temperature allow for a lower viscosity of the oil globules in the oily mixture, enabling a better flow of the collected oily mixture which also increases the efficiency of the separator.

With reference to static oily water separators, state:

(a)        why in the separation of oil and water, static means are used for bilge and ballast water and dynamic means for lubricating oil and fuel,                             (4m)

(b)        how oil density and mixture temperature affects separation,                         (3m)

(c)        how and why maximum throughput of bilge or ballast water is restricted.   (3m)

Answer :-

(a)                Static means of separation is adopted for bilge and ballast water because:

The percentage of oil to water in the bilge and ballast water is very much smaller since the oil is finely dispersed in the mixture (that is bilge and ballast water). By using static separation, the finely dispersed oil is made to adhere to the provided attachments, made to coalesce, rises, collected and discharged and clean water is allowed to pass through and discharged as clean effluent. The dwi method of separation in static separation enables a greater volume of oil globules can be made to coalesce since discharging of bilge and ballast water out to sea does not being economic reduce employing a centrifuge to separate small amount of oil globules in the winter that is not to be reused for any purposes would increase in operational cost, power, high maintenance and spare parts.

While lubricating oil and fuel oil are consumed at an approximately fast rate for the efficient operation of the supporting machineries and main machinery of the ship. As such, continuous supplies of treated  or separated oil whether fuel or LO are needed. If static separation is done, this would mean great time is consumed to effectively separate the newer and solids from the oil. Also the volume it would be able to handle in a stipulated time is greatly reduced. This would put the adequate supply of treated lubricating oil and fuel oil in a critical situation for efficient running of a ship. Thereby dynamic type of separation is selected for lubricating oil and fuel oil since dynamic separation such as in a centrifuge, handles large volumes of oil, separation takes place in a much shorter time and thus a continuous supply of oil for efficient operation of ship machineries is provided.

(b)               Density and mixture temperatures does effect separation because of increasing the temperature of the mixture, the volume of the mixture will also increase accordingly.

That is volume is proportional to temperature.

            V µ T with mass fixed and p is constant.

But the density of the mixture, e = mass of mixture___  

                                                        Volume of mixture

Therefore it can be seen with an increase in volume due to an increase in temperature of the mixture, consequently the density of the mixture will be reduced. However, since the mixture is made up of oil and water of different density. Obviously, with an increase in mixture temperature, their respective density also will change.

Since molecular change and expansion of oil is faster than water therefore the change in density of oil is greater as compared to water at a change in mixture temperature. Due to great different in densities between oil and water, the oil being lower in will readily separate itself from the water when entranced at the coalescer, baffles and pebble beds. The water being higher in density will flow through the separator and leaving the lighter oil adhering to the surfaces of the separator attachment and thus contaminant of oil with discharged effluent is reduced. 

 

(c)                Maximum throughput of bilge or ballast water to the oily water separator is restricted because:

1)      maximum throughput would mean operating the oil separator at over capacity whereby the flow becomes turbulence, causing re-entrainment of the oil and consequently deterioration of the effluent capacity.

2)      The throughput of the mixture send to the separator much governed by the proper matching of the type and capacity of the pump to the separator capacity at the design stages. Also by having a higher level of overboard discharged valve for the separator, a slight back pressure is maintained in the separator, thereby restricting too large of throughput.

3)      Also by incorporating spring loaded valves at the separator discharge, an acceptable working back pressure is created. Thereby sudden increase in throughput is avoided.

By incorporating of a baffles to allow for more residence time of separation , thus creating more effective separation.