Give a reasoned opinion as to accuracy of the following assertions:
(a)
oily water separator efficiency can be improved
by merely increasing the size of the unit. (5m)
(b)
single stage gravity type separators offer a distinct advantage over
multi stage counterparts. (5m)
Answer :-
(a)
Efficiency can certainly be improved by
increasing the size of the unit for the same amount of effluent discharge. This
can be affected by:
1)
subdivision into more chambers
2)
provision of more coalescing surface
3)
with a larger size, residence time is increased.
However, efficiency can also be
increased without increasing the size, by:
1)
use of rotary displacement pump to match the
separator
2)
reduce turbulence by design of piping and less
usage of valves
3)
placing a coalescer filter on the discharge
side.
(b)
Single stage gravity type have a distinct
advantages due to:
1)
lower cost
2)
less maintenance
3)
compact size
4)
reasonable efficiency
However, with regulations now
demanding effluent to be less than 15 ppm of oil, such result may not be
possible with a single stage operation, hence a second stage filter coalesce is
provided to remove remaining oil content and bring it down to less than 15 ppm.
(a)
Describe briefly with the aid of a sketch, the
working principle of the oil water interface detection in an oily water
separator. State why does some oily water separators employ a single detector
probe while some employs two or three detector probes. (5m)
(b)
Sketch a
simplified line diagram of a typical shipboard engine room arrangement for oily
water separation, monitoring and control.
Describe briefly an oil content
monitor meter of the light (ultra violet) absorption type commonly employed
onboard ships. (5m)
Answer :-
(a)
Detection of the oil water interface is achieved
by a variation in charge in a capacity which is accomplished by the alteration
in dielectric strength. One plate of the capacity is a probe, possibly made of
stainless steel, the other is the separator shell and both are connected to an
AC bridge which is supplied with high frequency low AC voltage.
As the oil water interface moves
down covering the probe in oil, the
dielectric strength (relative permeability) alters, the unbalance due to
capacitance change will function and activate the solenoid valve to open on the
separator and at the same time the waster discharge valve to close.
When the separator is empty, that
is filled with air, the two probes indicator lamps will out, oil discharge lamp
lighted on and the alarm buzzer will sound and the pump trip off.A third probe is sometimes fitted
lower than in the clean water space and acts as an emergency cut out should the
upper probes fail or excessive priming
over occurs, separators employing two probes for detection.
For separators employing a single
probe for detection of the oil water interface, the second probe used in such
case functions as an emergency probe.
(b)
One type of oily water monitoring system
functioning on the principle of ultra violet absorption and follows ‘Beers Law’
which states that the absorptivity of a substance is proportional to the
concentration of that substance.
Here, a sample of the effluent
discharge is passed through a vertical cylinder and receives energy from a
ultra violet lamp, this energy emitted due to fluorescence is monitored by a
photo electric cell which produces a signal dependent on the amount of oil
present in the sample. In case of excessive oil contamination, the signal
operates the diverting valves and the oil is discharge to the oily bilge tank
instead of going overboard.
This method is affected by presence
of impurities in the water sample and hence requires calibrations often.
(a)
Explain why in some instances automatic air
discharge valves are mounted on the shell of static oily water separators.
(b)
Give reasons why each separators should be
matched to it’s own supply pump.
Answer :-
(a)
Automatically operated air discharge valves are
provided to:
1)
keep the oil water interface as high as
possible, giving more space for separation.
2)
Air has similar capacitance as that as oil,
tendency of having false alarm from the oil level sensor when capacitance probe
is used.
3)
If the oil level comes down on the separator
shell due to air, it will sticks to the sides.
4)
Air space subjected to corrosion due to oxygen
in air along with moisture present.
5)
To vent the separator during starting up.
(b)
The pump capacity to be matched to the separator
so as to:
1)
avoid excessive pressure build up.
2)
Avoid excessive turbulence.
3)
Avoid oil carry over.
4)
Deal with bilges in order to be able to pump out
in a reasonable time.
Avoid possibility of
emulsification 
No comments:
Post a Comment