AIRCRAFT REFUELLING OPERATIONS -modificado-.FH11 by dfgh4bnmu


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products to fuel their future.

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FACET INTERNATIONAL is the leading filtration and separation
company serving the world's petrochemical, refining, marine,
offshore and aviation industries. Facet International has earned
worldwide recognition with more than 50 years of experience in
the separation of solid-liquid, liquid-liquid, liquid-gas and solid-gas.
The aim of this brochure is to explain the
background, principles of operation and
importance of FACET Filter/Separator and Filter
Monitor units in aircraft refuelling operations.
It is not meant to be a servicing manual but simply to provide
users and students with a basic understanding of the
techniques and methods employed in removing water and
solid contaminants from aviation fuels.
The text is partly historical in that it refers to some methods
used to remove the undesirable water and solid contamination
from aviation fuels in the early years of flying. It also explains
the background to the gradual development of
filter/separators to improve their standards of performance
in line with the discovery of solutions to problems which, in
earlier years, appeared to be inexplicable.
Some of these discoveries have taken years of patient
research by scientists and engineers of many National,
University and private company establishments. We at
FACET are proud of our associations with these
establishments, and proud also of our contributions to the
interchange of scientific knowledge aimed towards the better
understanding, and solution, of the problems associated
with liquid filtration and separation.
water contamination
No one would dispute the fact that water is an essential               ingress of water is from the surrounding atmosphere during
of everyday life but there are times when it can be a                  the transfer of fuel to and from storage tanks. The amounts
nuisance, and other times when its presence can be                     of water taken in by the fuel will depend upon the humidity
potentially dangerous. Water in the fuel tanks of an aircraft          of the atmosphere while these operations are in progress;
comes into the latter category and has to be prevented at              the higher the humidity the higher the water moisture
all times. One of the reasons is easily understood by                  content of the surrounding air which makes more water
referring to Table 1 showing the approximate air                       available for the fuel to absorb.
temperatures at various altitudes.
                                                                       There are, in fact, two kinds of water contamination in fuel;
It will be seen at an altitude of 30,000 feet, which is average        one is the free water, the other the dissolved or soluble
for air travel these days, the ambient temperature is –45ºC            water. Fuel, like many other substances, absorbs water
and any water droplets in the fuel reserves of an aeroplane            from the surrounding atmosphere and, like sugar in a cup
flying at this altitude will obviously freeze. The ice crystals        of coffee; the water becomes completely dissolved in the
formed can easily obstruct or block the pipes, valves and              fuel. The fuel will sparkle with a brightness that will belie the
filters in the fuel feed lines to an engine and the resulting          presence of the dissolved water because it cannot be
loss of power can be an unpleasant, if not fatal, experience           detected except with the use of special instruments or tests.
for the people on board the aircraft.
                                                                       There is, however, a limit to the amount of water that can
Fortunately, the formation of ice in aircraft fuel                     be absorbed by the fuel. This limit is often referred to as
                                                                       a ‘saturation limit’, ‘maximum solubility level’ or similar
tanks is a rare occurrence these days. The                             term, and any further quantities of water which are added,
majority of commercial aircraft incorporate fuel                       or come into contact with the fuel after this point becomes
tanks served by heater systems, and although                           free water. It is free because the fuel cannot settle out to
                                                                       cause problems but free water, on the other hand,
such preventative measures are rarely included                         constitutes a major problem and has to be dealt with
in military aircraft the possibility of icing is avoided               continually to make sure that it is not passed into the
by the use of anti-icing additives in the fuel.                        aircraft fuel tanks.

Although the icing problems have all but completely disappeared        Another important factor is that the amount of water that
it should be appreciated that sudden intakes of water through          can be absorbed by the fuel is not constant, and will vary
the fuel system of an engine can be just as dangerous as ice           according to the fuel temperature. The higher the temperature
crystals. The power in an engine is generated by the continuous        the greater the fuel’s capacity to absorb water into solution.
burning of fuel and if this process is suddenly interrupted by         But a subsequent reduction in fuel temperature reduces its
a large globule of water the engine will, at best, misfire; at worst   capacity for water with the result that the fuel rejects the
a flame-out will occur which means that unless the engine can          excess which then becomes free water.
be restarted in flight it will stop running altogether. This may       Take, for example, a tank containing a million gallons of fuel
not be such a problem in multi-engined aircraft where only one         which is left to stand before being transferred to another
engine is affected, perhaps, but it would be positively disastrous     tank further along the pipe-line. If the fuel is already fully
in single-engined aeroplanes.                                          saturated with the maximum amount of water it can absorb
The imperative necessity of removing water from aircraft fuel          it will, at a day temperature of 20ºC, contain about 80 parts
should therefore be obvious; it is a need which has, in fact,          per million of dissolved water. If, during the night, the fuel
been recognised from the very first days of flying and has             temperature drops to 15ºC the saturation limit will be reduced
been satisfied, by one means or another, ever since.                   to approximately 50 parts per million, with the result that 30
                                                                       of the original 80 gallons of dissolved water will come out
Having considered the effects of water contamination in aviation       of solution and be converted into free water which gradually
fuels let us now examine the sources of this contamination             sinks to the bottom of the tank.
and how the problem can be dealt with.
Water in fuel is very rarely visible to the naked eye as isolated
                                                                            Altitude     Air Temperature
droplets. More often than not the only indication of water
                                                                         feet     metres      Deg. C
being present is a slightly misty appearance of the fuel
                                                                        5000       1500         +5
because the droplets are so tiny that they merely ‘haze’ the            10000      3000         -5
fuel; the higher the water content the cloudier the fuel.               15000      4500        -15
The fuel leaves the refinery in an almost ‘dry’ or water-free           20000      6000        -25
                                                                        25000      7600        -35
condition and, apart from the most obvious sources such                 30000      9000        -45               Table 1:
as water drainage into defective underground storage                    35000     10600        -55               Air Temperatures at
tanks, or the intake of rain through faulty vents, the main             40000     12000        -65               Various Altitudes (approx.)
As the temperature rises next day to 20ºC again the fuel
will rapidly re-absorb water from the atmosphere and, at
a much slower rate, from the accumulated layer at the
bottom of the tank until the fuel again becomes fully
saturated at 80 parts per million. As the day wears on
the air in contact with the surface of the fuel will, depending
on humidity levels, continue to deposit water moisture
into the fuel which, since it cannot be absorbed, will again
become free water floating around in the fuel in the form
of tiny globules.
Under ideal conditions any droplets of water suspended in
the fuel of an undisturbed tank will, given sufficient time,
separate out under gravity and settle to the bottom of the
tank. It is invariably a slow process which depends upon the
size of the water droplets and can take hours or possibly
days to complete. But the possibility of allowing a large tank
of fuel to be left undisturbed for a few days while the water
settles out is the ultimate in wishful thinking. Instead, the fuel
is almost continually on the move, being thrashed about by
large pumps and buffeted by bends, valves and obstructions
in the pipe-line from one pumping station or storage tank to
the next on its way from the oil refinery to the airport storage
tanks and finally into the aircraft fuel tanks.
In the meantime any water being picked up by the fuel at
various points along the route, such as the storage tank
referred to earlier, is being thoroughly mixed into the fuel
and disintegrating into smaller and smaller droplets which
become suspended in the fuel and will be progressively
less inclined to settle to the bottom of intermediate tanks.
The fuel being transferred from one storage tank to the
next is, in reality, a fuel/water emulsion (another term for
a liquid containing particles of another liquid held in
suspension, not necessarily dissolved in each other) and
this emulsion is continually subject to the absorption,
rejection and re-absorption of water according to the
temperature changes within the fuel mixture. Without some
means of removing the free water at the intermediate
storage areas the water content of the fuel will continue
to increase as it is transferred form tank to tank on its way
to the airport storage tanks and the refueller vehicle before
it is pumped into the aircraft fuel tanks.
It is to cater for this situation that FACET Filter/Separators
will be found at many of the storage areas and pumping
stations between refinery and airport. The purpose of
these units is, of course, to prevent the build-up of water
in the fuel system itself while a further unit in the refueller
vehicle provides the final conditioning of the fuel before
it passes into the aircraft tanks. In many modern refuellers
the final filter/separator has been superceded by the more
compact and highly efficient FACET Filter Monitor unit
which removes any remnants of free water by absorption
rather that separation.
solid contaminants
No mention has yet been made of the solid contaminants           3 milligrams per cubic foot the total amount of dust
in the fuel and it should be appreciated that an aircraft        drawn into the tank will be almost 50 grams.This amount
fuel laden with dirt is equally as undesirable as one            may not seem excessive at first sight but it has to be
containing free water. The fuel systems in turbo-jet             remembered that the transfer of fuel from one tank to
engines incorporate numerous control valves, pumps,              another may be an hourly occurrence at some airports,
filters and jets, and accumulations of dirt within these         and each transfer will draw in its quota of dust particles
components can develop into blockages which can                  which will be deposited in the fuel.
prevent the flow of fuel and eventually stop the engine
running. Another point to consider is that where the dirt        Considering the accumulation of solid
includes abrasive particles, such as metal and sand,             contaminants from corrosion, airborne dust,
these particles will cause wear or possible seizure of
the moving parts of precision-made pumps and valves
                                                                 internal abrasion of pipes, valves, sealing
which can have immediate or long-term consequences.              gaskets, etc. that can occur it is quite
                                                                 understandable that adequate filtration is
It is essential therefore that the fuel being passed into
the aircraft tanks contains minimal amounts of both free         an essential aspect of the conditioning of
water and solid contaminants. To this end the Facet units        the fuel before it reaches the aircraft.
fulfill a double role in that they combine both filtration
and water separation functions at the same time which
is why, more often than not, the units are referred to as
Filter/Separators rather than simply Water Separators.
In some respects the dirt content of the fuel is closely
related to the water content. Water causes corrosion of
pipe-lines and tanks which is transferred to the fuel in the
form of flakes of rust which, through pumping and collision,
disintegrate into smaller and smaller particles. Although
the majority of these particles will be invisible to the naked
eye they will, nevertheless, be large enough to cause
damage and wear to engine components and will certainly
need to be removed before they reach the aircraft.
There are other water-related contaminants in the form of
microbial growths which can also develop into severe
problems but these will be dealt with at a later stage so that
we can, for the moment, concentrate on the more readily
recognised solid materials which contaminate the fuel.
The most prevalent of the solid contaminants is the dust
in the surrounding atmosphere, and the concentration
of this dust will vary according to the particular
environment. Conditions at the Cairo and Karachi
airports, for example, will be rather worse than at London
or New York, and the probability of contamination of
fuel by airborne dust will be significantly higher at the
former airports.
The dust itself consists mostly of abrasive particles of
sand, etc., which are drawn into the storage tanks with
the air that replaces the volume of fuel removed from
the tank. To give an indication of the quantities of dust
involved let us consider the transfer of, say 100,000
gallons of fuel from one tank to another. As this quantity
of fuel is removed the exact same volume of air will be
drawn into the tank through vents, breathers, etc. In
approximate terms the amount of air will be 16,000
cubic feet and since, in reasonably clean atmospheric
conditions, the concentration of dust is in the region of
old and new methods
of filtration and separation
In early aircraft the engines were of the piston type,        The continuous flow capability of the FACET
running on gasolene-type fuels which are of much lower
density than the fuels required for modern turbo-jet
                                                              Filter/Separator unit’s means that they can be
engines. Water and solid contamination settled out quite      located at any point between oil refinery and
rapidly in these light fuels and accumulations were           aircraft fuel tanks, and the removal of water
removed simply by regular draining from the bottom of         and solid contamination can be carried out at
the storage tanks prior to refuelling. To make doubly
sure that water and dirt were not passed into the aircraft    any of the intermediate stages along the way.
tank refuelling was carried out with the aid of conical
                                                              The final link in the chain will be the refueller vehicle
shaped pieces of metal gauze and chamois leather
                                                              pumping the pre-conditioned fuel directly into the aircraft
placed in a funnel. The fuel was then poured, or fed via
                                                              tanks and it is essential that the fuel loaded into the
hand nozzle, through the funnel into the over-wing tanks.
                                                              aircraft at this final stage does not contain more than
The gauze removed the dirt and the chamois leather
                                                              the specified minimal amounts of water and solid
soaked in fuel repelled the water droplets which then
                                                              contaminants which, if exceeded, could cause problems
ran to the bottom of the cone, and the operator simply
                                                              in flight. To achieve this aim the refueller vehicle system
emptied the cones of any accumulated dirt and water
                                                              will include the ultimate fuel conditioning unit which can
from time to time while he was refuelling.
                                                              be a filter/separator or a filter/monitor unit utilising
Similar hand-filling operations are still used today for      absorbent-type elements.
small aircraft although the filtration/separation equipment
                                                              The Filter/Monitor unit is a comparatively recent
is a little more sophisticated. But when we talk of
                                                              development which, after successful laboratory and field
aircraft refuelling these days we tend to think in terms
                                                              trials, has proved to be a highly efficient system of dirt
of the Concorde, Boeing 747 and similar large aircraft
                                                              and water removal. The use of this type of unit is
which are required to take on quantities of 20,000 to
                                                              particularly beneficial in the final stages of fuel treatment
40,000 gallons in a matter of minutes so that they can
                                                              such as refuellers where they are not expected to be
meet scheduled take-off times.
                                                              presented with excessive amounts of dirt and water in
With modern fuels and their regular transfers in huge         the fuel stream. As in the coalescer cartridge of the
quantities the need is to remove solid and water              filter/separator the Monitor cartridge incorporates primary
contamination on a contiuous flow basis, and this need        filtration stages to remove solid contaminants but the
is catered for in the FACET Filter/Separator units. The       subsequent media layers will trap and retain any free
solid contaminants are removed by a primary filtration        water to prevent its progressing any further. Since the
stage while water removal, on the other hand, is              Monitor elements absorb the free water presented to
achieved by continuously uniting groups of the tiny           them there is no necessity for manual or either automatic
water globules suspended in the fuel into larger ones         water drainage from the vessel. The cartridges are simply
which will sink at a rapid rate to the bottom of the unit,    replaced when the pressure drop across the unit reaches
where the water accumulation can easily be drained            a pre-set level showing that the elements have absorbed
away. In the majority of FACET units the filtration and       their maximum allowable quantities of solids and water.
water separation stages are incorporated within the           O P E R AT I N G AT R E D U C E D F L O W R AT E S T O
same vessel or in separate vessels.                           OVERCOME THIS SITUATION IS NOT PERMITTED.
the evolution of
filter/separator design
The information presented thus far has                              The light gasolene fuels in use during this period were of
                                                                    much lower density than the water and separation was
explained the purpose of the FACET
                                                                    comparatively easy to accomplish. To achieve separation
Filter/Separator and why it is an essential part                    the vessels were of large diameter to reduce the velocity
of aircraft refuelling operations from production                   of the fuel through the ‘hay-pack’ which assisted
of the fuel at the refinery right through to                        coalescence, and long in length to allow sufficient distance
                                                                    for the large water droplets to sink to the sump rather
loading it into the aircraft. The next step is to                   than to be carried over into the outlet pipe. In the same
consider the way in which adequate levels of                        period the solid contaminants were also relatively easy to
filtration and separation have been achieved                        remove; the rust flakes coming off tanks and pipes were
over the years and the principles of operation                      of large size and, although they were broken up into
                                                                    smaller particles by collisions in the pipe system, they
employed in the process.                                            were still large enough by the time they had reached the
An early method of continuously removing free water                 filter/separator unit to make their removal easy by the
and solid contaminants is illustrated in Fig. 1 which               comparatively coarse structure of the hay-pack. In these
shows that as the fuel/water/dirt emulsion passes through           earlier days of undosed fuels there was also a tendency
the special pad of compressed fibrous material the solid            for the very small dust and other particles to stick to each
contaminants such as rust, etc., are trapped within the             other, to agglomerate into larger particles which, again,
pad. At the same time the tiny particles of water are               were fairly easily trapped by the hay-pack.
forced to merge or coalesce into larger droplets which,             As an alternative to the cumbersome hay-pack and its
when ejected form the pad sink under gravity to form                attendant transportation and servicing difficulties a replaceable
an accumulation of the removed water in the sump                    cartridge system was introduced in the early ‘fifties’. This
where it can easily be drained.                                     system, shown in Fig. 2, could be incorporated into existing
This comparatively simple method was, in fact, one of               hay-pack units by the simple addition of the support plate
the first used for continuous flow systems in the early             and the centre-bolt fixings for the cartridges, which were
‘forties’. It was known as the ‘hay-pack’ unit in which             easily replaced when necessary. These cartridges used a
the filter/coalescer pad consisted of a compressed,                 coarse-grade fibreglass which proved to be superior in
cylindrical pack of large quantities of slivers and fibres          performance to the hay-pack and these improvements,
of a specially selected timber having a high resin content          together with ease of maintenance, advanced their usage
to withstand immersion in fuel without suffering undue              quite remarkably during the late ‘fifties’ and early ‘sixties’.
shrinkage or compaction effects. More often than not                In this same period the demand for air travel was expanding
these packs were found to swell rather than shrink when             quite rapidly which led to the production of larger aircraft
immersed in fuel, and since they were designed to fit               in greater numbers to cope with the demand. This, in turn,
tightly into the vessel when they were installed their              led to increased volume requirements of fuel which resulted
removal, when choked with contaminant at the end of                 in the building or expansion of storage, loading and pumping
their useful life, was frequently a two-man job requiring           complexes between refineries and airports to accelerate
patient work digging out the material a handful at a time.          the transfer of fuel from its point of production to the aircraft.
These ‘hay-pack’ units were in use for many years and               The need for filter/separator units to condition the fuel grew
were found to be perfectly satisfactory in coping with              hand in hand with these expansions, which explains the
the water and solid contamination conditions at the time.           parallel growth of the replacement cartridge business.

Inlet Fuel                                Clean/Dry                 Inlet Fuel                                     Clean/Dry
+Water+Dirt                               Fuel Outlet               +Water+Dirt              Coalescer             Fuel Outlet
                        Fibrous Pad                                                          Cartridges

              Coalesced                                                            Coalesced
              Water Droplets                                                       Water Droplets
                                               Fig 1:                                                                   Fig 2:
                 Accumulated Water             Early Filtration /      Support        Accumulated Water                 Replaceable
    Frame            Water Drain               Separation              Frame               Water Drain                  Cartridge
                                               System                                                                   System
From 1960 onwards the commercial aircraft industry                   Inlet Fuel
began to change from piston engines to turbo jet                     +Water+Dirt

engines, and this change led to important repercussions                                                                                            Clean/Dry
                                                                                                                                                   Fuel Outlet
in filter/separator design. The fuel needed by the jet
engine was of a higher density than had been used in
piston engines and the convergence of the densities
of the fuel and water gave rise to a slight difficulty. It
was found that the smaller of the water droplets
emerging from the coalescer cartridge remained
suspended in the heavier fuel for a much longer time                                                            Water
and were reluctant to settle out into the sump within                                                                                     Fig 4:
                                                                                                                                          Filter Separator
the distance available in the filter/separator unit. The                                                                                  (Manway) Type
result was that a high proportion of the smaller droplets                                                                                 S/SLD
were being carried over into the outlet.
To overcome this problem a second, separator stage
was introduced. As shown in Fig. 3 the original
separator was of simple cylindrical construction
wrapped with a specially treated metal mesh screen.
This very fine mesh repelled the small water droplets
                                                                                        Separator                                            Clean/Dry
and retained them on the screen surface until they,                                     Cartridge                                            Fuel Outlet
too, grew into larger droplets which ultimately sank
under gravity to the sump.
The introduction of the separator stage opened the
door to fundamental changes in filter/separator design
in that it destroyed the original concept of having to
allow large distances for the water droplets to fall out
under gravity. By incorporating effective separators it                               Accumulated
                                                                                           Water                                          Fig 5:
was found that the water carry-over possibility could                                                          Cartridge
                                                                                                                                          Filter Separator
be completely prevented and the distance between                                                                                          (End Open) Type
coalescer and separator stages could, therefore, be                                                                                       S/SLD
made quite minimal.
The result was that the shape and size of filter/separator
units could be reduced to much smaller proportions
and their manufacture was then possible in both
horizontal and vertical, space-saving designs. Typical
examples of these developments in the various
configurations which are in current use are shown in
Figs. 4, 5 & 6.                                                                                                               Head Lift

Inlet Fuel                     Fuel           Clean/Dry
+Water+Dirt    Coalescer       +Small Water   Fuel Outlet                          Coalescer                               Separator
               Cartridges      Droplets                                            Cartridge                               Cartridge
                                                                           Water Droplets

                                                                              Accumulated                                   Coalesced
                                                                              Water                                         Water Droplets
                                                                      Inlet Fuel                                             Fuel Outlet
              Coalesced                                               +Water+Dirt
              Water Droplets                       Fig 3:
                                                   Filtration /                                                                           Fig 6:
   Support        Accumulated Water                Separation with                                                                        Vertical Type
   Frame             Water Drain                   Additional                                                                             Filter /
                                                   Separator               Water Drain                                                    Separator
the mechanisms of filtration/
The free water droplets and solid contaminants in fuel are,                           Inner Coalescer Wrap
                                                                                      Outer Coalescer Wrap
in the vast majority of cases these days, no more than a                              Outer Support Screen
haze of microscopic particles suspended in the fuel and                               Outer Cotton Sock

invisible to the naked eye. The measurement of these
particles is usually quoted in microns or, more correctly,
micrometres. As an indication of these measurements in
comparative terms the thickness of a human hair is
approximately 50 microns whilst the smallest particle visible
to the naked eye is about 40 microns.                                                                          Flow
The majority of water and solid matter particles in a
contaminated fuel are less than 30 microns, and it is the
function of the filter/coalescer cartridge of the                                                      Metal End Cap
filter/separator unit firstly to remove the solids and then
to merge the particles of water into larger droplets of five
to ten millimetres in diameter before they are ejected to
fall, under gravity, to the sump from where the water                                                                     Metal Centre Tube
                                                                                                                        Filter Media Element
accumulation can be drained.                                                                                        Element Support Screen
                                                                                                                                  Metal Shell
The construction of a typical filter/coalescer cartridge is
shown in Fig. 7. The fuel/water/solids emulsion first flows                   Fig 7: Typical Filter / Coalescer Cartridge Construction
through a pleated assembly of fine-grade filter media, the
pleated configuration being necessary to obtain the optimum
area consistent with maximum dirt-holding capacity and            Fig. 8 shows how subsequently colliding particles gradually
efficiency. Following the almost total removal of solid           build up until a comparatively large opening surrounded
contaminant by this first filtration stage the fuel/water         by fibres is eventually blocked.
emulsion then passes through two wraps of coalescing
                                                                  When the media of the primary filtration stage becomes partially
media, graduating from a very fine grade material to a
                                                                  blocked in this manner the resistance to flow will increase
coarse grade material to effect the gradual coalescence of
                                                                  rapidly, and it will become necessary to replace the cartridge.
water particles from their original microscopic size to visible
                                                                  Failure to do so will result in the total blockage of the media
droplet size. The coalescer batts are supported by a high-
                                                                  and the subsequent build up of pressure across the cartridge
strength mesh screen which is finally enclosed in the outer,
                                                                  could, ultimately, rupture the media causing a sudden explosion
specially treated cotton sock. The weave pattern of the
                                                                  of dislodged contaminant from the filtration stage to the coalescer
outer sock determines the ultimate size of water droplet
                                                                  stage of the cartridge. Under normal circumstances such a
that emerges from the coalescer stage. A perforated metal
                                                                  failure would be disastrous. In FACET designs however the
tube is necessary to provide adequate rigidity to the whole
                                                                  possibility of serious after-effects is off-set by the inclusion of
assembly after it is finally sealed within the metal end caps
                                                                  a high strength wrap of p.v.c. mesh around the coalescer batts.
using a high-quality fuel-resistant adhesive.
                                                                  If filtration media rupture does occur in a cartridge the subsequent
The initial mechanisms of both filtration and coalescence         compression of the coalescer batts against the high-strength
in fibrous materials such as filter paper and fibreglass wraps    wrap converts these fibreglass wraps into a highly efficient filter
are virtually identical in that they rely on the probability of   media which, in turn, imposes a high restriction to flow across
collisions of particles with the fibres within the media. In      the particular cartridge. The result is that the remaining cartridges
both cases the media consists of numerous layers of fibres        have to cope with a slightly higher share of the total flow; this
roughly perpendicular to the flow which forms a maze, and         condition will be indicated by an increased pressure drop
the liquid passing through this maze is forced to follow          across the coalescer stage thereby forewarning the operator
tortuous paths around the fibres. If the flow through a           of the need to replace the cartridges. In the meantime the
section of the media is visualised as hundreds of tiny            quality of the fuel flowing from the coalescer stage has suffered
streams or jets which are forced into many changes of             no deterioration although one of the cartridges may be operating
direction and cross-sectional shape as they twist and turn        under only a part-flow condition.
through the maze of fibres, it will be easier to appreciate
that the probability of solid or water particles colliding with
a fibre is an extremely high one.
After collision has occurred the mechanisms of filtration
and coalescence are quite different. In filtration the solid
particle becomes permanently attached to the fibre and
                                                                                                              Fig 8: Mechanisms of Filtration
The mechanism of coalescence is different from that of             FLOW DIRECTION

filtration because the particles of water do not
permanently attach themselves to fibres. Provided that
the fibres are of the correct size, density and material
a water particle which has become attached to a fibre
will gradually slide along the fibre in same direction as                                                         Fig 9:
                                                                                                                  Mechanisms of
the flow until, as shown in Fig. 9, it meets an obstruction                                                       Coalescence
such as an intersection of fibres. The water particle will
momentarily be held at this point until other particles
following the same or adjoining fibres merge in turn to            The separator cartridges are, like the coalescer cartridges,
form an increasingly larger droplet. As the droplet                cylindrical in shape but the media is a simple wrap or
increases in size the flow around it will be imposing              pleated element of special material. The optional materials
higher and higher forces at its surface (viscous drag)             available are a high strength siliconized paper, Teflon-
until, at some critical point, the droplet is torn away to         coated fine mesh, or a unique woven synthetic material
follow another fibre and collide with another intersection         which has the distinct advantages of being both permanent
and the process is repeated. By the time it arrives at             and repairable. Separator cartridges in the latter material
the final layer of the fibrous wrap the droplet will have          simply require thorough cleaning at each service period,
been enlarged by the merging of possibly hundreds of               and where accidental damage has occurred special easy-
original tiny particles, and when it is finally torn from the      repair kits are available. This system was developed to be
fibrous medium it is confronted by yet another                     particularly beneficial to service engineers working in remote
construction –the cotton sock wrapped around the                   areas where the ‘spares’ situation may be difficult.
outside of the cartridge. The majority of water droplets
                                                                   The pore or mesh size of the media used in separator
will, by this time, be too large to pass directly through
                                                                   cartridges is critical because of the need to ensure that
the small holes in the sock, and they will be held against
                                                                   the flow resistance of the media is not excessive. Without
its inside surface until further droplets collide and merge
                                                                   this precaution the pressure difference from the inlet to
to cause further enlargement. Eventually, the large droplet
                                                                   outlet sides of the media would tend to force the small
will be forced through the sock in the form of an elongated
                                                                   water droplets through the pores, and the whole purpose
tube of water which regains its spherical shape
                                                                   of the separators would be negated. Where permanent
immediately after ejection from the sock and sinks to
                                                                   separators are utilised the cartridges should be removed
the bottom of the filter/separator unit.
                                                                   at the same time as the coalescer elements and cleaned
Because of the random nature of the fibrous batt used in           according to the recommended method before being
the coalescer cartridge there will be differences in the           refitted for further service.
effectiveness of the media from one section of the cartridge
to another. Consequently, there will be slight inconsistencies
in the size of the water droplets emerging from one area             FLOW DIRECTION

compared with another. Similar inconsistencies can be
caused by excessive additive content, certain chemical
impurities or microbial growths in the fuel, and through
combinations of these possibilities there is likelihood that
some of the water droplets emitted by the coalescer cartridge
will be too small to fall to the sump within the short available
distance within the vessel. To prevent the carry-over of these
smaller droplets into the filter/separator outlet a bank of
separator cartridges is interposed between coalescer
cartridges and the outlet to act as a safety screen.                        SMALL
This screen is, in reality, a hydrophobic (water-repellent)
barrier which allows the passage of fuel but prevents the
penetration of water. The separation mechanism is illustrated
in Fig. 10 and is known as ‘pore catchment’. By combining
water resistance and a critical pore or mesh size the media
repels any suspended water droplets and, while they are
held against the surface of the media by the flow trying to                                                       SEPARATOR MESH
force them through the holes, further droplets following
behind will collide and merge with the initial droplets until
they are enlarged to such a size that they will fall, under                                                       Fig 10:
                                                                                                                  Pore Catchment
gravity, to the sump area below.
As indicated in previous chapters the 1960-1970 decade               Research in this field indicated a significant risk of explosion
was one of important developments in the field of                    and for this reason an anti-static compound was added
filter/separator design. The world-wide change to jet engines,       to the fuel to increase its electrical conductivity thus allowing
and the rapid expansion of air travel generated an                   electrical charges in the fuel to be more readily conducted
unprecedented demand for the heavier Kerosene-type fuels             to earth through pipework and tanks and to be dissipated.
which led to urgent re-thinking on the part of engineers and         This problem of dissipating electrical charge to earth when
scientists involved in the development of filter/separators.         refuelling through insulated rubber hoses still exists and
The difficulty of devising improved methods of separating            for that reason the earth bonding of refuelling equipment
water from the higher density fuel was an initial concern but        remains an essential precaution.
this proved to be just the tip of the iceberg compared with
                                                                     The approved anti-static additives (Stadis 450 is the most
the problems that were to follow in the same period.
                                                                     common) have been developed from a combination of chemicals
One of these problems arose from the fact that aircraft were         and, although their effective concentrations in fuel can be as
flying at higher altitudes, which accentuated the occurrence         low as 0.75 p.p.m., they make the filtration of solids and the
of icing in fuel systems. Icing can only occur when the fuel         separation of water from fuel more difficult. Anti-static additives
contains free water but this free water can be formed simply         have similar effects to anti-icing additives in that they disperse
by the temperature drop between the refuelling temperature           agglomerations into smaller particles with the consequent
at ground level and the operating temperature at high                necessity to provide sufficient area of fine-grade filter media.
altitudes. From the explanation of free and dissolved water
                                                                     Another fuel additive used currently is DCI-4A. Its function
in an earlier chapter it will be appreciated that fuel being
                                                                     is to deposit a protective coating on all steel surfaces in
loaded into an aircraft at say 20 deg,C ground level
                                                                     contact with the fuel to prevent corrosion. Like the F.S.I.I.
temperature can be completely devoid of free water yet it
                                                                     and anti-static additives this corrosion inhibitor is added to
can contain up to 80 parts per million of dissolved water,
                                                                     the fuel in extremely small concentrations and has the same
which is then available for conversion into free water as the
                                                                     tendency to disperse agglomerations of solid contaminant
fuel temperature drops with increasing aircraft altitude.
                                                                     to make filtration and water separation much more difficult.
The solution to the possible icing problem was to add a
                                                                     All three additives have proved to be excellent antidotes in
Fuel System Icing Inhibitor (F.S.I.I.) to the fuel which lowers
                                                                     curing the ills for which they are intended although it has been
the freezing point of free water in the fuel in the same way
                                                                     necessary to develop the media, and construction of coalescer
that Anti-freeze prevents icing of the cooling water system
                                                                     cartridges in particular, to cope with the after-effects of these
in a car engine. Adding this inhibitor in minute concentrations
                                                                     additional chemicals. One of the most important requirements
has proved to be an excellent protection against icing but
                                                                     for the effective coalescence of water particles is that the
is troublesome as far as filter/separators are concerned.
                                                                     fibres in the coalescing media should remain in a smooth,
Whereas plain, additive-free fuels will allow the agglomeration
                                                                     clean condition so that the water globules can become
of small particles of solid contaminants into larger ones
                                                                     attached and slide easily along their surface; if these fibres
which are easily retained in a coarse filter media the effect
                                                                     are contaminated with a build-up of solid particles the
of F.S.I.I. is to separate and disperse the agglomerations
                                                                     coalescing ability of the media will be seriously impaired.
into their original smaller, discrete particles which then require
                                                                     Furthermore where any of the three named additives are used
a maximum area of much finer media to provide the necessary
                                                                     their action in dispersing agglomerations of solid contaminants
removal and service life characteristics.
                                                                     into particles of less than 1 micron in size makes it extremely
It should be pointed out that the anti-icing additive is not         difficult to prevent these particles from passing through all but
always included in fuels for commercial aircraft these days          the ‘tightest’ of preliminary filtration media and contaminating
because these aircraft often include fuel heating systems.           the coalescing media. The use of such ‘tight’ media would,
However its usage in military applications is more common.           of course, create flow restriction and high pressure drop
                                                                     situations, and to avoid these problems modern filter/coalescer
Another problem arose from the increased demand for fuel
                                                                     cartridges have to be a realistic compromise based on the
both in greater amounts and faster turn-round of aircraft. This
                                                                     best possible filtration performance providing protection for
increase in demand meant faster refuelling at higher flow rates
                                                                     the coalescing media whilst, at the same time, maintaining
which increases the build-up of electro static charge in the fuel.
                                                                     the lowest possible flow resistance.
The additives referred to are generally known as ‘surfactants’   Where surfactants generate serious persistent problems
which is the technical term derived from the expression          one of the most direct solutions is to treat the fuel using
‘surface active agents’ meaning something which affects          FACET Fullers Earth cartridges. These units remove the
the interface between a solid and a liquid or between two        surfactants by a process known as Adsorption in which
liquids that do not mix.                                         the surfactants are adsorbed by sites in the clay used
                                                                 in the construction of the cartridges, leaving the fuel
In the process of separating water from fuel the interface
                                                                 clean and responsive to water separation treatment by
between the water droplets and the fuel is an extremely
                                                                 the coalescing process. A measure of the success of
important consideration, and anything that degrades
                                                                 this method is that these units are regularly used in Test
this interface will reduce the force keeping the liquids
                                                                 Laboratory Installations where surfactants are deliberately
apart. This force is known as the interfacial tension (I.F.T.)
                                                                 added to evaluate and improve coalescer design and
and is tangible quantity that can be measured by various
                                                                 have to be removed before they contaminate the large
special instruments. For really effective coalescence the
                                                                 quantities of fuel held in the storage area specifically
I.F.T. between the water and the fuel needs to be of the
                                                                 for test purposes.
order of 30 dynes/cm. and while the figure for pure
water and AVTUR for example, is over 40 dynes/cm.                As a matter of interest, indications of microbial growth
certain impurities in either the water or the fuel can           are usually recognised by a series of dark brown patches
reduce this figure to a critical level.                          which discolour the outer sock of the coalescer cartridges
                                                                 and the separator media although these signs are not
The additives mentioned above undoubtedly cause
                                                                 necessarily related to a significant loss of performance.
reductions in the I.F.T. between water and fuel. But,
provided that their concentrations in the fuel are not           Since the presence of water is one of the essential
excessive, they will have only minimal effects on the            requirements for the microbial growths to be sustained
separation process. There are, however, other impurities         and to expand, it is obvious that the regular draining of
which severely affect the I.F.T. and degrade coalescence.        water accumulations from filter/separators and storage
One of the most important of these is the growth of              tanks is an equally essential precaution against
microbes, particularly at the water/fuel interface in storage    contamination of this kind and should be regarded as
tank bottoms. Without delving too deeply into the field of       an extremely important ‘good housekeeping’ operation.
Microbiology it is perhaps sufficient to know that some
airborne microbes deposited in the fuel need both water
and the nutrients from the fuel to survive and multiply.
But, in multiplying, these microbes produce surfactants
which contaminate the water and reduce its I.F.T. with
the fuel to critically low limits which severely impairs
coalescence. Worse still, the microbial growths cling to
the water-wetted sections of the coalescer cartridge
media and, by continuously depositing surfactants onto
the fibres of the media and the outer sock, will reduce
the size of coalesced water droplets to such small
diameters that they will not separate out under gravity.
Consequently, the separator cartridges become grossly
over-worked and through a combination of overloading
and reduction in I.F.T. the separators can break down
and allow the minute water droplets to be carried over
into the filter/separator outlet.
FACET engineers have, for many years, been closely
involved in problems related to microbial growths and
have found that where such problems exist the need to
introduce reduced cartridge change periods may be
necessary. Where so-called ‘permanent’ separators are
used these will need to be maintained strictly in
accordance with the recommended cleaning instructions.
fuel filter-monitor units
The FACET ‘Fuel Gard’ Monitors incorporate water-                 This feature is particularly useful when the unit has to
absorbent type media. Instead of separating water on              cope with a random slug of water entrained in the fuel
a continuous flow basis and diverting the extracted               flow; rapid absorption will cause immediate shut-down
water to a sump reservoir for draining the Monitor creates        of elements rather than allow further transfer of water
a complete barrier to the passage of free or emulsified           to the outlet side of the unit. In normal operating
water. The critical element in this system is a unique            conditions the retention of solids and absorption of water
porous media which allow the free flow of fuel yet traps          will be a gradual process and by ensuring that cartridges
and retains any water droplets presented to it. In contrast       are replaced before the differential pressure across the
to the separator media of the Water Separator unit the            unit exceeds the specified limit the user can easily
Monitor element media does not reject the water droplet           prevent solids/water overloading, whilst at the same
on its surface –it absorbs the water through a chemical           time providing fail-safe operation.
reaction in which the water droplet and the media to
                                                                  A typical cross-section of the Monitor cartridge is
which it becomes attached merge together to form a
                                                                  shown in Fig. 12. The outer cotton sock is a protective
highly viscous substance which is held in place by a
                                                                  sleeve to guard against damage to the multi-layer outer
matrix of surrounding fibres. Differential pressure across
                                                                  wraps of filtration media which trap and retain solid
the element compresses the element against its support
                                                                  contaminants down to less than 1 micron particle size.
to form a small circular area which bars any further flow
                                                                  The subsequent stage is the thicker water absorption
through its section. As each water droplet in the fuel is
                                                                  element backed by fine pore media layers supported
absorbed and converted into an impenetrable disc, the
                                                                  by a mesh screen wrapped around a very strong centre
element area open to flow decreases with a consequent
                                                                  tube. The end caps are manufactured to provide
increase in pressure drop across the cartridge. Eventually,
                                                                  superior strength with excellent support for the O-ring
as the element absorbs more and more water it becomes
                                                                  on the mounting/adaptor end.
increasingly resistant to the fuel flow until, when it
becomes completely water-saturated, it shuts down                 A further important feature of the Monitor cartridge is that
against any flow.                                                 its media is completely unaffected by surfactants in the fuel.
To cope with the flow rates associated with aviation refuelling
operations the Monitor Unit will employ groups of cartridges
in tandem or in parallel. Each cartridge is of such robust
construction that it can withstand a differential pressure in
excess of 12 Bar (180 p.s.i.) without collapsing or shedding
its absorbed water and retained dirt.

          Protective Sleeve
          Filter Media
          Water Absorbtion Media

      Fig 11:
Typical Filter /
      Monitor                               Moulded C/Tube
    Cartridge                         Multi-Layer PVC Mesh
                                       Anti Extrusion Media
performance standards
Considering the stringency of the standards of performance             by the major oil companies and is now the most widely
currently expected of Filter/Separator units it is perhaps             used standard defining the performance requirements
surprising to reflect that for the first thirty or forty years of      and test methods to be used in qualifying filter/separators
aviation there were no specific requirements relating to the           for aviation fuel.
quality of fuel suitable for aircraft use. It has to be realised, of
                                                                       This A.P.I. Spec. represents a major achievement in
course, that until about 1939 the fuel capacity of the largest
                                                                       terms of the International cooperation of National
aircraft was no more than 700 gallons of a light gasolene
                                                                       Institutions, oil companies and filter/separator
which, because of its low density, was incapable of retaining
                                                                       manufacturers, and in deference to that achievement
water and solids in suspension for very long without discarding
                                                                       the salient points of the performance requirements are
them to the bottom of storage tanks. With a little attention to
                                                                       reproduced in Table 2.
good housekeeping in draining tanks regularly it was, therefore,
comparatively easy to ensure the transfer of clean, ‘dry’ fuel         It will be seen from the information in this table that
to the aircraft tanks by the use of simple gauzes and water-           filter/separator units are classed according to their
repellent materials. Adding to this the fact that flying altitudes     application and are required, under specific test
rarely reached the freezing zone meant that any possibility of         conditions, to meet the minimum standard of
icing depended solely upon climatic conditions rather than             performance laid down for units manufactured to suit
height attained in flight.                                             the particular category and type.
It was not until the introduction of the jet engine in military        All of the listed types can be qualified as Category C for
applications and its use of the heavier Kerosene-type fuels            commercial fuel, Category M for military or pre-blended
that water contamination became rather more difficult to               fuel, and Category M+100 for fuel with +100 additive
remove. As a result it was necessary to formulate minimum              present. Qualification to Category M+100 does not qualify
standard for the quality of fuel regarded as suitable for aircraft     the element for any other classification. Qualification to
use and the first standards; U.S. Military Spec. MIL F.8508            Category M also qualifies the elements to Category C.
was issued in 1953. This standard was accepted by most
                                                                       Testing for Type S automatically qualifies for Type S-LD
American oil companies until, about three years later, a new,
                                                                       and S-LW since the water and solids rates are lower for
far more stringent specification, MIL F.8505A was introduced.
                                                                       these classifications.
From 1956 onwards the formulation of filter/separator
performance specifications graduated to the rather chaotic             To obtain A.P.I. approval for any filter/separator unit the
situation where individual oil companies and military authorities      manufacturers are obliged to follow rigid requirements in
each had their own unique but different standards to cover             the conduct of the tests laid down in the overall specification.
the quality of fuel frequently being loaded into the same aircraft.    For this testing the system to be used is shown in Fig. 12
Plainly, it was a situation which could not continue and resulting     for single element tests and Fig. 13 for full scale tests.
from logical argument and development by the American                  These systems require accurate control in the metering
Petroleum Institute, with the cooperation of the filter/separator      and injection of both solids and water according to specified
industry, a common A.P.I.Spec. was issued in 1973. This                concentrations based on the test flow rate applicable to
specification, referred to as A.P.I. 1581, has been accepted           the particular unit on test.
  COMPONENT                   API/IP or IP spec Particulate     Dispersed    Bulk water                                    Typical location
                                               matter removal water removal   removal                                           applied
                                                               Intermediate                                                 Into and out of
  FWS Type S                  API/IP 1581/1582 High capacity     capacity   Low capacity                                    airport storage

  FWS Type S-LD               API/IP 1581/1582         Low capacity               Intermediate          Low capacity         Out of airport
                                                                                    capacity                                   storage
  FWS Type S-LW               API/IP 1581/1582         High capacity              Low capacity             None             Into-plane only

                                                                                                                               Into-plane                Table 2:
  Filter Monitor                    IP 1583            Low capacity               Low capacity          Blocks filter        (refueller and              A.P.I. Specification
                                                                                                                           hydrant servicer)             Requirements.

 Both systems are also essential for approval testing of                                     FACET International’s Plants worldwide are
 Fuel Filter Monitors with absorbent type elements to the
 Institute of Petroleum / Energy Institute Specifications.
                                                                                             well equipped to carry out all the required
                                                                                             tests to Petroleum Institute Specifications
 These notes on API / IP Specifications provide only a very
 brief insight into the exacting standards demanded of
                                                                                             and has, to date, been awarded numerous
 manufacturers supplying filter/separators and filter/monitors                               separate approvals to the requirements for
 to the aircraft refuelling industry under these regulations.                                both filter/separators and filter/monitors.
 There are many more detailed requirements relating to                                       By the use of these extensive test facilities
 product integrity and quality standards which are far too
 extensive to cover in this brochure.
                                                                                             the company is fully committed to the
                                                                                             continued development of product to
 The aim of such rigid specifications is, of course, to achieve
 standardisation of test methods to ensure that elements                                     maintain and expand its reputation as an
 or complete units which satisfy these performance                                           A.P.I. and I.P./E.I. approved organisation.
 requirements can obtain an approval which is truly regarded
 as a world-wide qualification.

   Fuel Tank Single                             Cleanup Filter/Separator                           Fuel Tank                         Cleanup Filter/Separator
    Pass Capacity                                                             Clay Treater                                                                              Clay

                                                                                                                            Vessel              Full Scale Vessel
                                     Vessel                                                              Heater
                                                Single-Element                                           Exchanger
                                                  Test Vessel                                            (optional)
          Heater                                                                                                                                                         Flowmeter
          (optional)                                                      Flowmeter

                                                                                                                      Centrifugal       Flow through piping
                                                Flow rate                                                             Pump              >1,0 m/sec (3,3 ft/sec)
Water Flowmeter                   Centrifugal   >1,0 m/sec (3,3 ft/sec)                                               (2 950 RPM)
                       Filtered   Pump                                                       Filtered      Water
                       Water      (2 950 RPM)                                                 Water        Flowmeter

                       Fig 12: Single-Element Test Facility                                                             Fig 13: Full-Scale Test Facility
filter/separator modifications
It becomes necessary from time to time for users to             Similar caution is necessary when the user
consider up-dating their filter/separator installations by      contemplates modifying an existing filter/separator to
modifying the vessels to accept cartridges of improved          accept filter/monitor cartridges of the absorbent
construction and materials which have gained approval           element type. The internal constructional changes for
to the latest standard of A.P.I. Spec. requirements. But        such a modification are quite extensive and since, in
some caution is essential before embarking on such              the majority of instances, the Monitor unit with
work. There is, unfortunately, a general impression that        comparable flow rate capacity will be substantially
an existing unit simply fitted with the latest type of A.P.I.   smaller it may be far more economical to consider a
approved cartridges qualifies the unit for A.P.I. approval.     complete change of vessels.
This is not the case. For a conversion to gain A.P.I.
                                                                Where such conversions are contemplated it is
acceptance requires an extremely thorough redesign
                                                                strongly recommended that FACET International
which may entail different cartridge spacing, modified
                                                                should be contacted before decisions are made
sealing surfaces, replacement metallic components to
                                                                regarding such proposals.
omit non-ferrous materials and a number of other
important changes. These requirements cannot be
satisfied merely by changing the type of cartridge. It has
to be realised that each filter/separator has a unique
geometry in the relationship of the position of cartridges
to each other and their proximity to the vessel walls.
Under A.P.I. regulations this geometry will be substantially
different from the original vessel design and a conversion,
to be acceptable, demands a great deal of complicated
calculation and expert modification.
precautionary notes
It was pointed out in the Introduction that
this brochure is not meant to be a servicing
manual but there are certain precautions in
the use and care of FACET Filter/Separators
and their cartridges which cannot be
emphasised too often.
It should be borne in mind that to meet the exacting
standards required of aircraft fuel quality the coalescer
and separator cartridges are individually assembled by
hand and are subjected to rigorous quality control
inspections throughout their manufacture. In
consequence, these cartridges are rather expensive
and they demand care in handling appropriate to their
value both in terms of cost and their effectiveness in
providing clean, water-free fuel for aircraft, to ensure
the safety of the millions of people traveling by air
throughout the world.
Engineers involved in replacing cartridges in FACET
Filter/Separators are therefore strongly advised to handle
the cartridges with the utmost care. Where there is the
slightest evidence of damage which could affect cartridge
performance all risk should be avoided by discarding
any elements of doubtful quality. This advice is particularly
relevant to the surfaces of separator cartridges, whether
they are replaceable or permanent types, and to the
outer socks of coalescer cartridges; the idea that these
socks are simply protective coverings for the internal
media is completely wrong. As explained in a previous
chapter the sock forms an important, integral part of
the coalescence process and tears or holes in these
socks should not be ignored.
The same kind of care is necessary when handling monitor
cartridges. Although the outer sock is correctly regarded
as a protective sleeve for these items it is essential to
ensure that any damage to the outer sock has not affected
the layers of filter media beneath.
Another vitally important precaution relates to the re-
commissioning of filter/separators after servicing. It is
essential that bringing an empty unit on line is done at
very low filling flow rates. If the unit is filled too quickly
the fuel vapours rushing out through the air eliminator
will be at such a high velocity that there is a serious
risk of build-up of a high electrostatic charge, with
consequent danger of explosion.
call on Facet for clear solutions

Keep your JET A1 in A1 condition with FACET filter/water separators, prefilters clay
filter vessels and cartridges.
Facet filter/water separators – updated to comply with the latest civil and military specification. FACET filter water
separators provide clean dry fuel in airport and refinery installations worldwide.
Facet water absorbent monitors – remove solids and water from aviation fuels in full compliance with the latest
Institute of Petroleum / Energy Institute specifications.
Facet prefilters – used upstream of a filter water separator the FACET prefilter will extend the life of coalescer and
separator cartridges and significantly reduce overall filtration and separation costs.
Facet clay filters – using a process of preferential ionic adsorption FACET clay filters will clean up your fuel. Use FACET
clay filters in multiproduct pipeline installations and wherever surfactant laden or discoloured fuel is a problem.
Facet cartridges – manufactured to the highest specifications FACET coalescer, separator, micro-filter clay and water
absorbent cartridges are readily available to keep your equipment on stream at all times.
Whatever your problem in filtration and separation call on Facet – with more than 50 years experience in the
industry we are both ready and able to help. -

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