Water Intrusion Test
Integrity Testing
Introduction
Test description
Theoretical principles.
Among other things, a pressure
gradient that is dependent on
pore size is necessary to overcome negative capillary forces
(cohesive forces). This pressure is
generally called the "Water
Penetration Point" (WPP), the
pressure at which water is
pressed through a hydrophobic
membrane. The WPP is dependent on the hydrophobicity of the
filter material and the pore size
and is comparable to the bubble
point.
Conventional integrity testing of
membrane filters using the bubble point, diffusion or pressure
hold tests requires that the filter
membrane be completely wetted
with a suitable wetting agent.
Hydrophobic filter elements can
be tested by this method only if
the surface tension of the
wetting agent is less than the
critical surface tension of the
filter material being used. Otherwise, complete wetting does not
take place.
This is why different organic
solvents like ethanol or solvent|
water mixtures are used as practical wetting media. This aspect is
especially detrimental when the
system is used as a “sterile air
filter" for venting tanks, bioreactors and fermenters .
Definition Water Intrusion
Test (WIT).
The Water Intrusion Test (WIT)
is an in-situ integrity test for
hydrophobic filters. The WIT
measures the decay rate of
a pressure level imposed upon
a hydrophobic membrane
enveloped in water.
The relationship can be
illustrated as follows:
Integrity testing of filters for liquids is usually performed in situ.
However, the in situ integrity
testing of hydrophobic gas
filters is only possible to a limited
degree due to safety reasons (e.g.,
explosion protection when testing with organic solvents) and to
keep the product from solvent
contamination. Besides these
complicated individual tests must
be followed by a drying phase
before any filter elements can
be installed.
Additionally the filter sealing in
the housing still has to be tested
in situ. Since the filters should be
retested immediately aher in-line
steam sterilization, tests with a
solvent|water mixture are
impractical for the user.
Moreover, the drying procedure
must be validated since solvent
residues can contaminate the
product if drying has been
insufficient.
Dmax =
4 · σ· cosθ
·k
∆p
D max =
Diameter of the largest
pore
σ
θ
= Surface tension of the
liquid in dynes/cm
(water 72)
= Angle of contact
(greater than 90° in
hydrophobic filters)
∆ p = Available upstream
differential pressure
(bar |psi)
k
= Correction factor
(required since
membrane filter pores
are not cylindrical
capillaries)
Water Intrusion Characteristics at Different Test Pressures
350
Intrusion (ml/10 min)
300
The Water Intrusion Test (WIT)
was developed to overcome all of
these disadvantages. The WIT can
be used to run routine integrity
tests simply, easily and reliably.
250
200
150
100
Water Penetration
50
Water Intrusion
0
0
1
2
3
4
Test Pressure (bar)
5
Pore Size: 0.2 µm, Height: 10’’, Temperature: 20°C
6
Principle of Water Intrusion Test
The Water Intrusion Test (WIT)
was developed as an integrity
testing method to test hydrophobic sterilizing grade filters. These
filters are often used as inlet air
and off-gas filters for fermenters
bioreactors and for venting
sterilizers, freeze dryers autoclaves and tanks.
The WIT is employed in both the
qualification of filter elements
and as an inplace test procedure.
Origin of the pressure drop.
The pressure drop has two main
reasons:
1. The upstream volume is in fact
increasing due to remaining
compaction processes. Since
this compaction is an asymptotic process, it is never
absolutely completed. Therefore, a certain time has to be
determined after which most
of the compaction is already
done. That is the main reason
for the stabilization time of
10 Minutes.
The WIT - like the mercury
intrusion test - is based on the
capillary depression of non
2. Water molecules actually get
wetting liquids on the outer
transported through the memsurface of the membrane. To
brane. In case of an integer
withstand these capillary forces,
membrane, this process is
a certain pressure gradient,
mainly due to evaporation of
which is dependent on the pore
water molecules through the
size among other factors, is
pore structure. If the memnecessary. The test run is in a
brane is punctured, water is
similar manner to the Diffusion
mainly flowing through the
Test, although there is a major
punctured hole.
difference. In the Diffusion Test,
the diffusive gas flow through a
Water vanishes from the upwetted membrane is measured.
stream side. The effect is, that the
However, during the WIT a
upstream volume is increasing
hydrophobic filter installed in a
(keep in mind that water is not
filter housing is flooded with
subject to compaction). This
water on the upstream side.
amount of water (∆V) can be
The pressure drop detected by an
determined using the fundamenautomatic integrity tester indital law of Boyle and Mariott
rectly measures the volume of
(p1 · V1 = p2 · V2)
water intruding into the membrane matrix. At first the test
system has to be flooded with
water, pressurised at the defined
test pressure and then stabilised
for a defined time. After the
stabilisation time
is complete, the
water intrusion
compressed air |gas
will be measured.
 p1
 V1 · ∆p
∆V = V2 - V1 = V1 ·  p - 1 =
 2
 (p1 - ∆p)
with:
p1: Absolute pressure at the beginning of the pressure drop
measurement [mbar]
p2: Absolute pressure at the end of the pressure drop measurement
[mbar]
∆p: = p1 - p2
V1: Upstream Volume at the beginning of the measurement [ml] (= Vn)
V2: Upstream Volume at the end of the measurement [ml]
The increase in Volume per time is defined as:
∆V
Vn · ∆p
∆t = (p1 – ∆p) · ∆t
This value is defined – as one easily understands – as the “Water Flow
Value”. This value is basically linked to the Water Intrusion Value by a
simple mathematical correlation:
WIT =
∆V
∆t
·
p1 - ∆p
Vn · ∆p
(p1 - ∆p)
Vn · ∆p
=
=
1000 mbar (p1 - ∆p) · ∆t · 1000 mbar 1000 mbar · ∆t
The Water Intrusion is a value that has been defined in analogy to the
diffusion. For small pressure drops (< 5% of the absolute test pressure),
the following equation holds:
Diff. =
with:
Vn:
∆p:
1000 mbar:
∆t:
Vn · ∆p
1000 mbar · ∆t
Upstream Volume [ml]
Pressure Drop [mbar]
Reference Pressure
measurement duration [min]
Physically, air molecules pass through the wetted membrane. If one
collects all these molecules, they will fill a certain volume at 1000
mbar reference pressure at the ambient temperature as reference
temperature. This volume pre time is the Diffusion value.
The Water Intrusion is calculated using the same formula:
WIT =
Vn · ∆p
1000 mbar · ∆t
If one imagines the volume ∆V that has been added to the upstream
Volume due to the Water Flow to be filled with gas, it would use a
different volume at an ambient pressure of 1000 mbar. Again, the
required volume at 1000 mbar (∆V10OO) can be calculated with Boyle
Mariott's law to be:
∆V1000 = ∆V ·
Principle of WIT testing
(p1 - ∆p)
1000 mbar
Test parameters
Practical aspects Water
Intrusion Test.
• Allows inplace testing after
sterilization
• Avoids using wetting agents,
e.g. IPA/Water
- Therefore no contamination
- No removal problems
No alcohol residue or downstream contamination
• The hydrophobicity of the
filter stays hydrophobic during
the test
• High sensitivity of the test
• The hydrophobicity of the filter
is evaluated by the test
• The time is reduced, especially
at multiple systems, which
reduces the shut down periods
• Directly correlated to the ASTM
Bacteria Challenge Test.
System set-up.
The WIT can be used in a variety
of air filtration applications that
require minimal engineering
changes, however, some times
systems may have to be reconfigured. This is where the Sartorius
Technical Support team comes to
your assistance.
The diagram to the right shows
an example of a manual WIT system. The basic construction does
not differ significantly from that
used for the diffusion test. In
principle, WIT systems can be
designed as manual, semi-automatic and fully automatic units
to suit the applications, e.g.,
sterile venting systems for autoclaves, Iyophilizers, fermenters
and tanks.
Basic requirements for the
test are:
1. The surface tensions of the
water used must be > 72
dynes/cm.
2. Minimized temperature difference between the water and
the inlet air ± 1k.
3. Temperature fluctuations must
be avoided during the test.
4. Filter element must be completely hydrophobic.
5. Adequate effective filter area,
i.e., > 0.1 m2.
6. The integrity tester must have
sufficient accuracy, e.g.,
Sartocheck® unit.
Operating sequence:
1. Completely fill the upstream
side of the filter housing with
water.
2. Close all upstream valves.
3. Connect the integrity tester,
e.g., Sartocheck 3 unit.
4. Start the test: The unit performs the WIT automatically.
5. The test is completed and the
results are printed out.
6. Empty the filter housing
through an appropriate drain
or condensate valve.
7. Briefly vent dry using pressurized air with the inlet and
drain valves open.
8. Start operating the system.
Maximum permissible
intrusion rates
False failures and troubleshooting.
If the filter cartridge does not
pass the test, it might be that
partial hydrophilization of the
membrane has occurred. In this
case, the filter must be steamsterilized for at least 30 min. at
121°C or 134°C and then repeatedly blown dry with hot air.
Other false failures might be
attributable to water surface
tension and temperature fluctuations. All of these factors will
invariably cause elevated water
intrusion values (false failures).
Experience has shown that these
increased values indicate
“marginal“ failures and do not
lead to high water flow rates or
extreme pressure drops. An
isopropanol|water test will determine whether one of these
causes is the problem or whether
the filter cartridge is out of
specification. Fault free filters
will pass the solvent test because
this test cannot detect any
hydrophilization or reduced surface tension of the water used
for the initial WIT.
After the WIT, the filter
cartridges need to reach their
original air flow rate as quickly
as possible. In other words, they
must not become blocked with
water, e.g., residual water in the
fleeces. The time that the filter
cartridge needs to achieve 100%
of the original flow rate is called
the “blow-down" time.
Test pressure.
The high air pressures needed to
force water penetration of the
PTFE membranes used in the
Sartofluor filter cartridges result
in very high test pressures used
for the various pore size ratings.
This increases the tests reliability,
accuracy and precision.
Pore Size Water
WIT Test
Penetration Pressure
Pressure
bar psi
bar psi
0.20 µm
4.5 65
2.5 36
0.45 µm
2.8 41
1.5 22
These variables are dependent on
the introduction of water with a
surface tension of > 72 dynes/cm.
Stabilization and test time
The compressible upstream air
volume above the water column
and the actual water intrusion
level are both low. This illustrates
the importance of maintaining
sufficient stabilization and test
times. Enough time must be
available for the water column to
become saturated with air at the
test pressure. Similarly, the water
must be distributed over the
entire membrane area without
the presence of air bubbles and
the filter pleating must be fully
compacted. The following, values
have been established for the
stabilization and test times:
Stabilization time 1 13 min
Stabilization time 2 10 min
Test time
10 min
If these times are shortened the
intrusion values may be too high
due to insufficient distribution
of water across the membrane
surface or faulty compacting of
the membrane pleats.
As in the diffusion test (and in all
integrity tests), the maximum
permissible water intrusion rates
given by the filter manufacturer
for the WIT must follow the
ASTM bacteria challenge test
methodology (HIMA Document
No. 3, Vol. 4, April 1982).
Factors influencing the Water Intrusion Test
Hydrophilizing active agents.
As already mentioned, the use of
WIT requires that the filter
elements be com pletely dry and
hydrophobic. Surface active
agents like solvent vapors,
detergents or oils can lead to
partial membrane hydrophilization and thus to false negative
results.
Moreover, these agents might
also reduce the air flow rate if
parts of the membrane matrix are
blocked by these agents. During
normal operations, strict attention must be paid to these factors
and appropriate counter measures taken when necessary.
If partial hydrophilization occurs
the filter cartridge must be handled as described on previously
page.
Difference in temperature.
False failures may occur if the
temperature difference between
the test air and the water is too
great. If the water temperature
is much lower than the air temperature, great drops in pressure
and high intrusion rates can
result. If the water temperature is
much higher, the pressure drops
will be too low.
This situation can be remedied by
storing the water in a storage
tank in the room where the test
will be performed or by filling the
housing with water adjusted to
the right temperature.
Low surface tension of
the water.
This situation mostly arises when
the tanks being used are not
cleaned and rinsed thoroughly,
old ion exchange resins are used
for preparing the water, or the
water temperature is too high
(> 32°C|90° F). These parameters
can be avoided by taking preventive measures or adapting the
water temperature.
Fluctuations in ambient
temperature.
Such temperature fluctuations
affect the pressure drop
measured during the test. The
same applies to the diffusion
and pressure drop test. The possibilities for the test being affected
are minimized because the area
of the housing that comes into
contact with the test air is very
small compared to that in the
diffusion test.
Moreover, the great heat
capacity of water compensates
for any fluctuations. Integrity
tests are normally performed
in airconditioned rooms. This
source of error is therefore
encountered very rarely.
Inlet volume.
Generally with large-volume
systems, the precise volume can
be easily determined, whereas
the measured pressure drop may
merely amount to a few mbar.
The results may then border
on the accuracy limits of the
integrity tester and false results
could be obtained.
Similarly, when dealing with very
low net volumes, high pressure
drops can occur, leading to
inaccurate test results.
The net inlet volume must therefore be selected so that both
parameters relevant for intrusion
measurement – namely, volume
and pressure drop – can be determined as accurately as possible.
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