agua en sistemas de lubricación

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.



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.



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



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.

agua en sistemas de lubricación

Documents