Explain why the incorporating of each of the following
features contributes to the greater functional efficiency of static oily water
separators:
(a) internal
subdivision into a complexity of small compartments, (2m)
(b) intricate
arrangement of internal obstructions in passage fluid flow, (2m)
(c) pebble beds, (3m)
(d) rotary
positive displacement pump matched to its separator. (3m)
Answer :-
(a)
Complexity of compartmental sub division permits
incorporation of baffles plates, coalescer and / or pebble beds. These can :
1)
direct and reduce the flow rate.
2)
Increases retention time for more efficient
separation.
3)
provided extended surfaces for the oil droplets
to merge into bigger droplets.
4)
A coalescer will allows small oil globules to
collect and forms larger globules to rise up faster, forming oil bulk.
(b)
by having intricate arrangement of internal
obstructions in the passages of fluid flow, the turbulence created at the entry
of the separator can be reduced, since the flow is made follow the pattern set
to the intricate obstruction the flow, thereby slowing down the fluid flow,
creating a more constant flow and thus creating a laminar pattern. Thus as
such, the oil globules can separate from the water and adheres to the
attachments before coalescing, rising and selected and being discharged.
(c)
Grills and baffles are normally placed on top of
the pebble bed. When oil particles flows through the perforation in the plate,
due to its viscosity and surface tension, begins to form globules which
increase in size until they have sufficient self buoyancy to flow upward at a
faster rate. They also gives following advantages :
1)
increase residence time
2)
increase surface area for oil droplets to
agglomerate and coalesce into larger droplets
3)
direct fluid flow direction.
(d)
Rotary positive displacement pump matched to its
separator because they are:
1)
self priming
2)
reduce possibility of mixing suction with air
3)
provides low fluid velocity which in turns
reduce effects of high turbulence, increase residence time giving better
separation efficiency and minimize chances of emulsification.
With reference to static oily water separators, comment upon
the validity of the following assertions:-
(a) efficiency
is directly related to throughput, (3m)
(b)
internal baffles make an appreciable
contribution to the process of separation,
(3m)
(c)
separation is only partially dependant upon
gravitational settling (4m)
Answer :-
(a)
With reference to static oily water separators,
the statement efficiency is directly related to throughput is valid because if
the throughput is increased, this would mean overloading the separator, thus
creating turbulence of flow, causing disintegration of the oil globules in the
mixture and hence affecting the quality
of the effluent discharge. Also the throughput does not allow the oil
globules sufficient time to coalesce and thus causing oil entrainment with the
effluent.
(b)
This statement is also valid because internal
baffles do make an appreciable contribution to the process of separation in
that causes the incoming fluid to follow the pattern set up by the baffles,
thereby reducing the sudden in rush of fluid velocity which is quite turbulence
in flow simmer down and gradually changes into a laminar type of flow pattern.
Thus when the flow is laminar in nature, appreciable amount of time is allowed
for the oil globules to be separated from the water.
A better separation of oil from the
water takes place. As the mixture flows through this baffles, more surface area
is available for the oil globules to adhere and later form into larger bubbles
and then rises to the oil action chamber.
(c)
Separation depends on:
1)
size of oil globules
2)
viscosity of oil fluid
3)
temperature of mixture
4)
density of oil and water
From Stoke’s law, the velocity of
the rising oil droplet in static separator is given by:
V
= ( Dw
– Do )
d² g
18 u
Hence, from the equation,
separation is only partially dependent upon gravitational settling (g). Thus,
this statement is also true.
With reference to oily water separators, explain why:
(a) internal
baffles are commonly fitted,
(b)
coagulators are sometimes incorporated,
(c)
rotary positive displacement pumps are
preferable to centrifugal pumps for supply purposes.
Answer:-
(a)
Grills and baffles are normally placed on top of
the pebble bed. When oil particles flows through the perforation in the plate,
due to its viscosity and surface tension, begins to form globules which
increase in size until they have sufficient self buoyancy to flow upward at a
faster rate. They also gives following advantages :
1)
increase residence time
2)
increase surface area for oil droplets to
agglomerate and coalesce into larger droplets
3)
direct fluid flow direction.
(b)
Coagulants are sometimes incorporated in some
oily water separators that only employed inverted conical plates to arrest the
oil globules with the quality of effluent discharge is from 50 to 150 ppm.
With the new regulations, i.e. the
maximum effluent discharge of less than 15 ppm only, these arrangement of
separation is no longer acceptable. Thus modification is made to the 1st
stage separation or by employing two or three stages of separation.
Since the first stage of separation
only manage to produce 100ppm effluent, the remaining oil globules are made to
pass through the 2nd stage separator where oil globules will adhere
to these coalescer, where the oil globules has a climbing effect with a speed
slower than the water. Thus, more oil globules merge and causes them to
increase in size and until they have sufficient buoyancy to float and collect
out at the oil collection zone. Since coalescers are very fine, it is capable
of forming oil globules up to 12mm in diameter and thus easily and thus easily
can trap even the finest oil particles and produce excellent quality effluent
up to 15 ppm or less.
(c) Rotary
positive displacement pumps are preferred over centrifugal pumps as they are:
1)
self priming.
2)
reduce possibility of mixing suction with air.
3)
Provides low fluid velocity which in turns
reduce effects of high turbulence, increase residence time giving better
separation efficiency and minimize chances of emulsification.
With reference to oily water separator:
(a)
outline the routine attention needed to maintain
satisfactory performance,
(b)
define the contribution of the test cocks
towards functional efficiency,
(c)
give two reasons why oil might be carried over with
the water.
Answer :-
(a) Routine
attention includes:
1)
ensure separator filled up with sea water before
start using and after finish using, to flush thoroughly through with sea water.
2)
Electric level sensing probes to be kept clean.
3)
Oil content monitor to be calibrated at regular
interval.
4)
Ensure automatic air release valves is working
properly.
5)
Double checking with test cocks to confirm /
ensure proper functioning of sensing probes.
6)
If pressure differential across separator is too
high, to carry out cleaning of internal parts. If backflush system is provided,
to make use of back flushing periodically.
7)
Ensure supply pump is working efficiently,
pump’s and valve’s glands, filters to maintain correctly.
8)
Use of steam heating during operation to
facilitate easier separation
9)
Checks on corrosion on internal parts which can
give rise to liquid being by passed.
(b)
Test cocks are provided for checking the water /
oil level interface. It provides the operator with an estimated idea as to the
position of the water / oil interface. Thus it provides a means of checking on
the oil discharge mechanism operating efficiency. It also serves as a means for
obtaining sample.
No. 1 test cock shows water and if
opening of no. 2 test cock shows oil, it indicates that the interface level is
below test cock no.1. At this point, to check the control mechanism is
operating the oil discharge valve to dump the oil to oily bilge tank. If not,
then, it is indicating of some operational problems with the control
system.
(c)
Reasons for oil being carried over:
1)
throughput too high, retention period for
separation insufficient, hence less oil being separated.
2)
Oil content level in water / oil mixture too
high / excessive, resulting in separator completely filled with oil.
3)
Air release valve faulty (jammed close) allowing
interface to be pushed down by air pressure particularly during start up.
4)
Temperature of mixture too low, affecting
separation rate, also less oil being separated.
5)
Defective oil level sensing probes and failure
to check oil level interface by the test cocks.
Identify with reasons which one of the following statements
relating to oily water separators is false:
(a)
test cocks are provided to locate oil / water
interface,
(b)
oil density and temperature relate directly to
ease of separation,
(c)
rate of separation is unrestricted,
(d)
maximum permissible contamination level
restricts overboard discharge rate.
Answer :-
(a)
The statement is true because in manual
operation type of oily separators, no automatic means are employed to detect
the interface between oil and water to operate oil discharge valve and clean
water discharge valve. The test cocks will guide when it is necessary to open
the oil discharge valve and close the clean water outlet valve and vice versa.
Also the test cocks acts as a safe guards in case the automatic control of the
oily water separator fails.
(b)
This statement is also true because in increase
in mixture temperature means a decrease in the density of oil in the mixture.
This allow oil globules to detached easily from the water when it gets
entrained at the coalescer, pebble beds, baffles etc. thus it helps in
separation of oil and water.
(c)
This statement is false because handling
capacity of oily water separators are rated by the volume of mixture of fluid
it can handle per hour. If the rate its handling capacity is large, then its
separation capability is greater, whereas if the handling capacity of the
separator is small, then its separation capability will also reduce. This is to
avoid overloading the separator whereby turbulence of flow can occur causing
emulsification of oil globules in the mixture and thus affecting the quality of
effluent to be discharge.
(d)
The statement is true because with the
enforcement of the maximum permissible contamination level of the effluent
leaving the separator being introduced, the indiscriminate discharging of
uncontrolled amount of effluent exceeding the level of contamination allowed
could have taken place and causes pollution problems. With such an enactment,
any effluent more than 15 ppm is not allowed to be discharge and any
indiscriminate rate of overboard discharge is prevented. Also with such
requirement, most of the present separators have automatically control oil and
clean water discharge valve being operated in relation to the quality of the
effluent to be discharge. Thus, there is a controlled rate to the overboard
discharge.
No comments:
Post a Comment