Upload
servidor01
View
222
Download
0
Embed Size (px)
Citation preview
8/7/2019 Agua en sistemas de lubricación
http://slidepdf.com/reader/full/agua-en-sistemas-de-lubricacion 1/4
/ APPLICATION NOTE
Avoiding free water in a lubrication system
Free water in lubrication oil can
cause major failures to large
machinery, especially in applications
where water is constantly present
as in ship propulsion systems and
hydro power turbines.
Free water prevents oil from forming
a uniform lubricating layer on
metal surfaces deteriorating both
the lubrication performance and
the ability of the oil to protect the
machine. Equipment is damaged
by corrosion, cavitation, micro
pitting, and spot heating. Free
water also ruins polar additives
of oil. In offshore applications,
free water tends to be even more
destructive due to the presence of
salt water creating a more corrosive
environment for the metals it is in
contact with.
In cases of sudden leaks, a free water
layer can form quickly, and if only
periodic oil sampling is performed,
water is most likely detected too
late resulting in damage. On-line
monitoring enables the machinery
operator to make corrective actions
before any failure occurs.
Formation of free waterbased on oil's watersolubility
Just like air, every fluid (e.g.
lubricating oils, hydraulic f luids)
has the ability to hold water in the
dissolved state below the saturation
point. Once the saturation point of
that fluid has been reached, any
additional water that enters the fluid
will separate out into “free water”
which can be seen as a distinct layer
– usually below the oil.
Oils typically have very limited
water solubility. The saturation
point of oil is affected not only by
the base oil type, additives and
anti-oxidants, but also by the fluid’s
age, temperature and the chemical
reactions that take place over the life
of the fluid. It is also very typical that
different commercial oils used for the
same application, vary significantly
with respect to their ability to hold
dissolved water (Figure 1). A safe
moisture level indicated in parts
per million (ppm) for one oil may be
above saturation in another.
Figure 1 Water solubility of different commercial lubrication oils (2001).Tests run at 31 °C.
8/7/2019 Agua en sistemas de lubricación
http://slidepdf.com/reader/full/agua-en-sistemas-de-lubricacion 2/4
Effects of oil temperatureon the solubility
In lubrication systems, the oil is
typically rather warm e.g.
+40…+60 °C. In situations whentemperature decreases considerable,
like when engines are turned off,
there is a risk of free water formation,
because the oil cannot anymore hold
the same amount of water. While the
absolute water content (ppm) has
remained unchanged, the saturation
point has been reached and the risk
of, for example, corrosion becomes
likely (see Figure 2).
In case free water has formed during
a shutdown, the oil should be driedfirst or the system has to be started
carefully, running only the lubrication
system until normal operating
temperature is reached and the water
dissolves back into the oil. This
phenomenon can be monitored much
more easily with water activity
(Aw, see p. 3).
One should also consider that the hot
lubrication oil in the reservoir can
easily absorb more water if it is in
contact with ambient air. This kind of
long term "leak" will increase water
contamination in oil slowly.
Effects of oil ageingon the solubility
The used oil's ability to hold water
is remarkably higher than a new
oil's. This is caused by the chemical
reactions that take place over the
life of the fluid changing the water
solubility. This is a very important
point to consider when oil is changedor when new oil is added to existing
oil in the lubrication system.
Figure 3 demonstrates how the oil
ageing affects the water solubility
(saturation level 2000 vs 6000 ppm).
Figure 2 The effect of temperature on free water formation. The risk increases
when engines are switch off and temperature decreases. When the water content reaches the saturation point free water starts to form.
Figure 3 The effect of oil ageing on its water solubility (water content in ppmversus water activity reading). The test was at 40-47 °C with a new lubricationoil and the same oil after 30 000 hours in service.
8/7/2019 Agua en sistemas de lubricación
http://slidepdf.com/reader/full/agua-en-sistemas-de-lubricacion 3/4
Benefits of continuouswater activity (Aw)measurement versus ppmmeasurement
Traditionally water contamination
has been measured by regularly
scheduled oil samples following
a maintenance program using
Karl Fisher titration. The output
of the titration is absolute water
content in ppm (parts per million).
Absolute water content cannot tell
the operator whether water is in
dissolved or free form i.e. safe or
unsafe level. When measuring the
absolute water content of the oil,
the operator has to know the safetylimits for each specific oil type he
uses. He also has to consider the
ageing effect and what the safe
moisture operating levels are for
aged and possibly contaminated oils.
Water activity (aw) is a measurement
which indicates moisture (water
content) in oil based on a scale from
0…1. (0 being completely dry, 1 being
completely saturated).
Water activity (aw) does not need
any temperature, oil contamination
or ageing compensation, because it
always indicates the true margin to
saturation i.e. free water formation in
real time.
Sensor technology whichenables continuous Awmeasurement
Since 1995 the Vaisala HUMICAP®
sensor has been used to monitorwater contamination in oils. The
sensor, launched in 1975, uses
a capacitive thin film polymer
technology in which water molecules
in the oil absorbs into/desorbs out
of the polymer thereby changing its
dielectric properties. The sensor
output is proportional to the relative
(water) saturation of the oil.
Due to the porous upper electrodeand very fine micro structure of the
polymer, larger molecules like oil,
its additives, oxidation products or
fine metal particles cannot penetrate
into the sensitive area of the sensor,
and thus do not affect the sensor
performance. However, oil ageing
and additives will change the water
solubility of oil (Figure 3) and thus
affect its relative saturation as well.
As the output is always proportional
to the true saturation of the oil, the
sensor does not need any oil specific
calibration, regardless of which
lubrication oil or hydraulic fluid is
being used.
Figure 4 The structure of Humicap® thin film polymer sensor invented by Vaisala.1. water molecule permeable upper electrode 2. water sensitive polymer layer 3. bottom electrode 4. sensor substrate 5. connection pins
1
2
3
4
5
H ² O
8/7/2019 Agua en sistemas de lubricación
http://slidepdf.com/reader/full/agua-en-sistemas-de-lubricacion 4/4
Re. B211029EN-A ©Vaisala 2010This material is subject to copyright protection, with all
copyrights retained by Vaisala and its individual partners. All
rights reserved. Any logos and/or product names are trademarks
o Vaisala or its individual partners. The reproduction, transer,
distribution or storage o inormation contained in this brochure
in any orm without the prior wr itten consent o Vaisala is strictly
prohibited. All specifcations — technical included — are subject
to change without notice.
For more inormation, visit
www.vaisala.com or contactus at [email protected]
Figure 5 Stability graph of Humicap® sensor exposed to Skydroll, a harsh hydraulicfluid. The errors observed during reference calibration at 75%RH are shown in Y-axisvalues.
Checklist: what to look
at when choosing the
sensor
▪ Can the sensor tolerate
and remain stable in theapplication conditions - both
in long and short term?
▪ Is the sensor responsive in low
moisture levels?
▪ Does it have any cross-
sensitivity to other solid or
liquid materials present in
oil like additives, ne metal
particles, etc.?
▪ Is the measurement sensitive
to fow rate, fow direction or
installation direction?
▪ Does the sensor have ast
response time even in
situations involving rapid
water spikes? Ater the
ailure situation is over, the
sensor should continue to
measure without the need or
recalibration or any corrective
actions.
▪ What are the maintenance
and service capabilities? Can
the sensor be replaced easily,
without shutting down themachinery?
The polymer is highly stable and
resistant to chemicals present in
oils resulting in exceptional long
term stability (Figure 5). When
calibration is needed the sensor can
be calibrated and adjusted on site
and on-line with a portable hand-
held reference. Another option is to
switch out the transmitter, which
can easily be detached and replaced
even from pressurized oil lines.