Answer SSEP Question 16

Q16. With reference to an oil/water separator:

A. (i) Sketch such a device;

(ii) Describe the passage of oil/water mixture from the delivery of pump to the outlets of separator.

B. State how oil density and temperature affect the case of separation of oil from water.

C. State how the movement of oil on board ship or its discharge and the discharge of oily-bilge ballast water overboard is recorded.
Answer: A. (i) Sketch such a device;

(ii) Describe the passage of oil/water mixture from the delivery of pump to the outlets of separator.
It consists of three vessels which are initially fitted with clean sea water. The 1st stage consists of a vertical cylindrical pressure vessel containing a number of inverted conical plates. The oily water is admitted to the 1st stage separator at the upper half and directed circumferentially round the shell in a horizontal plane to effect good distribution over the area of the vessel and then starts a downward path to the conical plates. The majority of large globule free oil separate out in the upper part of the separator. The smaller globules are carried by the water into the spaces between conical plates at a controlled velocity which causes the free oil droplets to coalesce and rise to the top and get trapped underneath an annular baffle and is then led up through the turbulent inlet area by risers to collect in the dome of the separator. The water leaves the conical plate pack via a central pipe to the 2nd stage of the separator.
Oil is discharged from the second stage by a piston operated oil valve which is motivated by a solenoid controlled pilot valve, which takes pressure from water discharge valve. The water discharge form the first stage enters the 2nd stage at the bottom and passes vertically through a coalesce bed on fitter. The coalesced-up oil is removed from the top of the vessel and the water discharged to the 3rd stage. The 3rd stage is similar to the 2nd stage, where further coalescing takes place, oil goes on top and is removed. The water discharge from this stage can be passed overboard after monitoring.
Automatic Control of Oil Discharge
It is effected, by means of vertical capacitance probe in the top of 1st stage vessel. The probe being normally salt water finding. When oil completely surrounds the probe, altering its capacitances, it energizes the solenoid of the pilot valve which in turn opens the oil valve. This causes a partial loss of pressure in the vessel and the water valve will close.
On completion of oil discharge the capacitance of the probe will again alter to de-energize the pilot valve solenoid and so close the oil valve, when the pressure will build up again and the water valve will open. An alarm probe is fitted to warn off an oil build up should the control system malfunction.
B. State how oil density and temperature affect the case of separation of oil from water.
The main principle of separation by which O.W.S functions is the gravity differential between oil and water. In oil mixtures, the oil exists as a collection of globules of various sizes. The force acting on such a globule, causing it to move in the water is proportional to the difference of weight between the oil particle and a particle of water of equal volume. This can be expressed as:
$\displaystyle \mathrm{F_s = \frac{\pi }{6}\times d^3\times (f_w- f_o)\times g }$
Where,
$\displaystyle \mathrm{F_s}$  = separation force
$\displaystyle \mathrm{F_w}$  = density of water
$\displaystyle \mathrm{r_o}$  = density of oil
$\displaystyle \mathrm{d}$  = diameter of oil globules
$\displaystyle \mathrm{g}$ = acceleration due to gravity
The resistance to the movement of the globule depends on its size and the viscosity of the fluids. For small particles moving under stream line flow condition, the relationship between these properties can be expressed by Stoke's law:
$\displaystyle \mathrm{F_r = 3\pi v\mu d }$
Where,
$\displaystyle \mathrm{F_r}$ = resistance to movement
$\displaystyle \mathrm{\mu}$ = Absolute viscosity of water
$\displaystyle \mathrm{V}$ = terminal velocity of particles
$\displaystyle \mathrm{D}$ = diameter of the particle
Where separation of an oil globule in water is taken place, $\displaystyle \mathrm{F_s=F_r}$ and above equations can be worked out to express terminal velocity of the globule in terms of viscosity, relative density and particle size.
$\displaystyle \mathrm{V= \frac{1}{18\mu} (f_w-f_o)\times d^2\times g }$ 
In general a high rate of separation is encouraged by a large size of oil globule, elevated temperature of the system (which increases the specific gravity differential of the oil and water and reduces the viscosity of the oil) and the use of sea Water. Turbulence and agitation should be avoided since it causes mixing the re-entrainment of the oil. Laminar or streamlined flow is beneficial.
C. State how the movement of oil on board ship or its discharge and the discharge of oily-bilge ballast water overboard is recorded.
Oil Record Book Part I shall be provided to every Oil tanker of 50 GT and above and every ship of 400 GT and above, other than oil tankers, to record relevant machinery space operation. For oil tankers, oil record book part II shall also be provided to record relevant cargo/ballast operations. Thus, all movements mentioned are recorded in relevant Oil Record Book