LESSON 6
• PURIFICATION(净化) OF
FUEL OILS
•Fuels vary considerably in
viscosity, quality and chemical
composition. They also contain
impurities from producing,
transferring and storage.
•All heavy fuels contain
insoluble(不能溶解的)
impurities such as mineral salts,
asphalt(沥青), foreign matter
and some liquids not
miscible(易混合的).
•These impurities can to a great
degree(extend) be removed by
centrifugal(离心的) and
filtration(过滤) treatments.
•Failure of any cleaning
equipment could cause very
serious troubles to the
mechanical parts of the fuel
injection equipment,
•therefore standby duplicate
cleaning equipment must be
provided and so arranged that
continuous operation can be
maintained should the working
unit fail.
•The most common method of
cleaning fuel is by centrifuging
and there are various makes of
purifiers(分水机) and
clarifiers(分杂机) in use
•such as Alfalaval, Titan,
Sharpies, etc., the basic
principles of operation and
design are similar.
Dirty oil
water
Separation disc
Clean oil
gravity disc
Cylindrical
interface
bowl
•The separation discs(分离盘)
in purifiers and clarifiers are
conical in shape and are made
of stainless steel,
•the outer periphery(外围) is
perforated(打孔) by a series of
holes through which the fuel
passes, the distance between
each disc is approximately 0.5
mm and the thickness is about
1 mm.
•When the fuel is pumped into an
operating centrifuge the
centrifugal force generated
causes the heavier fractions(部分)
such as sludge and water to be
forced along the underside of the
discs to the outer periphery.
•A cylindrical interface(圆柱分
界面) is formed between the
heavier fractions (water) and
the lighter fractions (oil). The
lighter fraction is forced
inward and up to the clean oil
outlet.
•The impurities and sludge can
be collected in the bowl(分离筒),
sludge space, or in some designs
discharged at regular intervals to
a sludge tank.
•The best results are obtained
when the interface zone is close
to the periphery of the bowl
and it must be outside the disc
stack(堆叠) areas.
•The specific gravity of the fuel
determines the size of the
gravity rings(比重环) to be
fitted and it is important to
refer to the maker’s instruction
manual for verification.
•A higher throughput(处理能力)
and more efficient process can
be achieved if the fuel is heated
to reduce its viscosity.
•Heaters are, therefore,
installed in the system.
Recommended temperatures
for high viscosity fuels are
between 80 to 95℃ depending
on the viscosity of the fuel.
•If the purifying of relatively
clean fuel is needed, a
centrifuge fitted with a clarifier
bowl should be used.
•There is only one outlet for
clean oil, no gravity discs(rings)
are fitted for the separation of
water.
•Consequently, the maximum
cleaning efficiency is achieved,
as the oil feed will at all time be
outside the disc stack zone.
•Experience has shown
excellent results from a twostage centrifuge treatment—
purifying followed by clarifying.
•The fuel is pumped into the
purifier fitted with gravity rings
and with separate discharges for
oil, water and sludge.
•The relatively clean oil is then
pumped into a clarifier which
extracts(分离出) any small
amount of water or impurities
still left in the oil.
•Unacceptably large quantities of
impurities are left in the oil after
the purifying process if the oil is
insufficiently heated, or the
throughput is too high; or the
wrong type of gravity disc is fitted.
•Improvements in oil, oil purifying
techniques and centrifuges have led
to the development of single phase
purifiers which have in themselves
been aided by improved
temperature control, heaters, flood
alarms and other ancillary
equipment.
•A further development has been
the self-cleaning purifier which
operates unattended(无人照管
[监视]的, ) for long periods and
discharges its own sludge.
•The self cleaning unit can be
stopped and started at any time
without dismantling(拆卸) for
manual cleaning which is a
dirty and time consuming job.
•If the unit is not self-cleaning,
then a strict maintenance
routine must be followed.
•Poorly maintained units are a
continuous source of trouble and
allowing a machine to run beyond
its recommended time can result
in a choked bowl, especially if very
dirty fuel is being purified.
•The efficiency of centrifugal
unit decreases rapidly when the
accumulation of impurities
reaches a certain point.
•For self-cleaning purifiers used
in unmanned machinery spaces
controls are arranged that if a
centrifuge fails the standby unit
will automatically comes into
operation.
•The purifiers are fitted with
electric timers for controlling the
period between bowl emptying
operations and sequential
regulating devices control the
various' emptying operations;
•The units must incorporate an
audio-visual alarm system
which come into operation
when any running condition is
out of order.
•When an alarm condition prevails
with the unit, it should be
arranged to shut down or the fuel
oil recycling valve should open and
the oil flow should be passed to the
outer circle thus preventing the
loss of oil.
•In order to control the
interface zone of the bowl a
constant pressure control valve
is fitted at the clean oil outlet.
•By applying a high back pressure
the interface moves outward and
with a low back pressure the
opposite results.
•It is important to note that when
an excessive amount of water is
present in the fuel the temperature
for centrifuging must be kept
below the boiling point of water,
otherwise foaming(起泡) and,
agitation(搅拌) of the fuel takes
place.
•Under these circumstances it
will be necessary to purify at a
very low rate to achieve a
reasonable efficiency.
READING MATERIAL
• A. OIL TREATMENT
•Both fuel oils and lubricating
oils require treatment before
passing to the engine.
•This will involve storage and
heating to allow separation of
water present, coarse and fine
filtering to remove solid
particles and also centrifuging.
•The centrifugal separator is
used to separate two liquids,
for example oil and water, or a
liquid and solids as in
contaminated oil.
•Separation is speeded up by the
use of centrifuge and can be
arranged as a continuous process.
Where a centrifuge is arranged to
separate two liquids, it is known as
a 'purifier'.
•Where a centrifuge is arranged to
separate impurities and small
amounts of water from oil it is
known as a ‘clarifier’.
•The separation of impurities and
water from fuel oil is essential for
good combustion. The removal of
contaminating impurities from
lubricating oil will reduce engine
wear and possible breakdowns.
Centrifuging
•A centrifuge consists of an
electric motor drive to a
vertical shaft on the top of
which is mounted the bowl
assembly.
•An outer framework(机壳)
surrounds the assembly and
carries the various feed and
discharge connections.
•The bowl can be a solid assembly
which retains the separated sludge
and operates non-continuously, or
the bowl can be arranged so that
the upper and lower parts
separate and the sludge can be
discharged while the centrifuge
operates continuously.
•The dirty oil is admitted into the
center of the bowl, passes up
through a stack of discs and out
through the top.
The Purifying Process
•The centrifugal separation of
two liquids such as oil and
water, results in the formation
of a cylindrical interface
between the two.
•The position of this interface
within the centrifuge is very
important for correct operation.
•The setting or positioning of
the interface is achieved by the
use of dam ring(阻水环) or
gravity disc at the outlet of the
centrifuge.
•Various diameter rings are
available for each machine
when different densities of oil
are used.
The Clarifying Process
•Cleaning oil that contains little
or no water is achieved in a
clarifier bowl where the
impurities and water collect at
the bowl periphery.
•A clarifier bowl has only one
outlet. No gravity disc is
necessary since no interface is
formed.
•The bowl therefore operates at
maximum separating efficiency
since the oil is subjected to the
maximum centrifugal force.
The Bowl Discs
•Purifier and clarifier bowls
each contain a stack of conical
discs. The discs may number
up to 150 and are separated
from one another by a small
gap.
•Separation of impurities and
water from the oil takes place
between these discs. A series of
aligned(排列的) holes near the
outside edge permits entry of
the dirty oil.
•The action of centrifugal force
causes the lighter components
(the clean oil) to flow, inwards
and the water and impurities
flow outwards.
•The water and impurities form
a sludge which moves outwards
along the undersides of the
discs to the periphery of the
bowl.
Selection of Gravity Disc
•The interface between the
liquid seal (water) and the oil
should be positioned as close as
possible to the bowl periphery.
•However, the interface must
not be located so far from the
bowl center that the oil will
pass the outer edge of the top
disc, breaking the liquid seal
and discharging with the water.
•Factors influencing the
interface position are:
•Oil viscosity and density. A
high oil density will position the
interface closer to the bowl
periphery than will a low
density.
•Throughput and back
pressure. As a rule the
interface will be located closer
to the bowl periphery at a high
throughput than at a low one.
•The same effect is produced by
a high back pressure, and a low
one respectively, in the clean oil
outlet.
•Gravity disc. The location of
the interface is adjusted by
altering the outlet for the water,
i.e. exchanging the gravity disc.
•Changing to a gravity disc
with larger hole diameter will
move the interface towards the
bowl periphery,
•whereas a disc with smaller
hole diameter will position the
interface closer to the bowl
center.
•The nomogram(诺莫图) is an
aid to select a tentative(暂定的)
gravity disc when the density of
the oil at a given temperature is
known.
•The hole diameter of the disc
to be tried first appears directly
from the nomogram.
•However, in practical operation the
best result is obtained by using the
gravity disc with largest hole
diameter that will not cause a break
in the liquid seal in the bowl or an
emulsification(乳化)in the water
outlet.
Discharge of Sludge
•Modern centrifuge designs
enable continuous operating,
over a considerable period of
time.
•This is achieved by an ejection
process which is timed to
discharge the sludge at regular
intervals.
•The sludge deposits build up
on the bowl periphery as
separation-continues, and the
ejection process is timed to
clear these deposits before they
begin to affect the separation
process.
•To start the ejection process the
oil feed to the centrifuge is first
shut off and the oil remaining in
the bowl removed by admitting
flushing water(置换水).
•Water is then fed into the
hydraulic system in the bottom
of the bowl to open a number of
spring-load valves.
•This 'operating' water causes
the sliding bowl bottom to
move downwards and open
discharge ports in the bowl
periphery.
•The sludge is discharged
through these ports by
centrifugal force. Closing
‘operating’ water is now feed in
to raise sliding bowl up again
and close the discharge ports.
•Water is fed into the bowl to
remake the liquid seal, the oil
feed reopened, and separation
continues.
Maintenance
•The bowl and the disc stack
will require periodical cleaning
whether or not an ejection
process is in operation.
•Care should be taken in
stripping(拆卸) down the bowl,
using only the special tools
provided and noting that some
left-hand threads are used.
•The centrifuge is a perfectly
balanced piece of equipment,
rotating at high speeds; all
parts should therefore be
handled and treated with care.
• B. OPERATION OF AN
OIL SEPARATOR
Before Starting
•The bowl should be well
cleaned and assembled
according to the instruction.
Check particularly: The brake
is released.
•The collecting covers (frame hood
respectively) are clamped(夹紧)
with the hinged(装铰链) bolts. The
oil level in worm gear (蜗轮)
housing is somewhat above the
middle of the gauge glass.
•The operating liquid tank is
full and the control valves are
closed (operating liquid feed
turned off; bowl open).
•Note: The oil level must never
be allowed to sink below the
lower edge of the gauge glass. If
the glass is provided with
corrugation(沟纹) they should
be vertical.
•Keep the gauge glass clean,
otherwise a line, which could be
mistaken for the oil level, will
in time build upon the inside of
the glass.
•If the machine has been idle
(for instance during a night),
screw out the drain screw some
turns and drain off any water.
Starting the Motor
•If the process liquid is to be
preheated, circulate it through
the preheating until suitable
temperature is reached.
•Shortly after starting it may
occur that the bowl begins to
vibrate more than normal.
•The cause is generally lack of
balance due to bad cleaning of
the bowl. Stop the machine and
clean the bowl if the vibrations
are very heavy.
•Heat is always generated in
the clutch(离合器) coupling
during the running-up period.
•This will be noticeable, especially
when the pads(垫, 衬垫) on the
friction block are new, through
smoke and smell of burning. Like
the sliding sound, this is quite
normal and has no importance.
•During acceleration the power
consumption is higher than in
normal operation.
•The acceleration time may
vary somewhat depending, for
instance, on the condition of the
friction pads in the clutch
coupling(离合器).
Bowl Closing
•After attaining the right speed,
the bowl should be closed.
•Before starting the closing
operation, the number of
revolutions of the speed
indicator must be checked
against the speed table in the
instruction book.
•The acceleration time can vary
somewhat depending, i.e. on the
wear of the friction pads.
•Close the bowl by opening the
corresponding control valve. Wait
till the bowl has closed. Then fill it.
Filling
•The filling procedure differs
for purification, clarification
and concentration(浓缩分离),
i.e. for machines provided with
purifier, clarifier and
concentrator bowl respectively.
Clarification (clarifier bowl)
•Set the flow regulator to
wanted throughput and open
the process liquid feed valve.
Purification (purifier bowl)
•Supply liquid, usually water,
to form the liquid seal. This
liquid should preferably have
the same temperature as the
process liquid and must be
supplied quickly.
•Shut off the feed of sealing
liquid when this begins flowing
out and becomes visible in the
sight glass.
Set the Flow Regulator to
Desired Throughput
•Slowly open the process liquid
feed valve. A certain quantity of
sealing liquid will now escape,
until equilibrium is reached.
•If the valve is opened too
quickly, the liquid seal could be
forced away to the effect that
light liquid phase is discharged
in the wrong way,
•i.e. through the outlet for the
heavy liquid phase. When this
occurs with the filling
procedure has to be repeated.
•Adjust to suitable back
pressure in the conduit for light
phase (For machine with
equipment for interface
disposition see Selection of
gravity disc)
Concentration (concentrator
bowl)
•The liquid seal builds up
automatically. Adjust to wanted
throughput-see Purification
above.
Running
•Check particularly: that
throughput and working
temperature are constant; that
oil does not leak from the worm
gear housing (oil level at
operative height);
•that, in purification, light
phase is not escaping together
with the heavy one, thereby
indicating that the bowl is
clogged or the liquid seal is
broken.
•If so, a sludge discharge must
be carried out immediately and
henceforth the interval between
discharges be reduced.
•If sludge had packed(塞满)
between the bowl discs it may
be necessary to stop the
machine for manual cleaning of
the bowl.