FACET LIQUID FILTER/SEPARATORS & MONITORS
IN AIRCRAFT REFUELLING OPERATIONS
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leading brands, extensive distribution network and technical expertise to serve customers worldwide.
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customers receive the best filtration and on-time delivery directly to each business location to protect
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CLARCOR offers the broadest array of filtration products, technologies and services to meet current
and future Aviation Industry requirements. Our customers, worldwide, depend on CLARCOR filtration
products to fuel their future.
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FACET LIQUID FILTER/SEPARATORS & MONITORS
IN AIRCRAFT REFUELLING OPERATIONS
CLARCOR is positioned to meet your Total Filtration and
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.
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 fuels 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.
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
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 units 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
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
Water Droplets Water Droplets
Fig 1: Fig 2:
Accumulated Water Early Filtration / Support Accumulated Water Replaceable
Frame Water Drain Separation Frame Water Drain Cartridge
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
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:
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
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
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 Water Droplets
Inlet Fuel Fuel Outlet
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
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:
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:
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.
Water Absorbtion Media
Typical Filter /
Monitor Moulded C/Tube
Cartridge Multi-Layer PVC Mesh
Anti Extrusion Media
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.
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
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 Internationals 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
Test Vessel (optional)
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
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.
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.
www.facetinternational.com - www.clarcor.com