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Bewildering Oil Analysis Results
Ted Naman, Technical Specialist-Industrial Lubricants
& Greases, ConocoPhillips
The silicon count in used turbine oil is generally accepted
as indicative of presence of sand and dust particles less
than 10 microns in size, but not necessarily dirt particles
that are greater than 10 microns, such as airborn chemicals
and and salt.
ISO 4406:1999 Cleanliness Standard reports particles
in three-digit codes (e.g. 20/16/13), with the first set
of digits reported value at 4.6 microns, second digits
reporting at 6.4 microns, and third set of digits at 13.6
microns levels. Depending on the which lab is used, the
results can vary significantly depending on instrument
calibration.
Most likely, the higher than normal particle count is
from coal dust which is common in coal-fired steam turbine
plants. Even though seals and breathers are in good condition,
they are not 100% tight. Coal dust can be ingested in
the turbine oil via seals, breathers and ventilation systems.
Steam leaks from the boiler feed pumps and other sources
can also ingest contaminants in the oil, which cannot
be detected by silicon analysis.
A quick check for coal dust contamination is to take
a sample in a clear glass bottle and visually inspect
it for discoloration with black particles. Filtering the
oil to 2 microns usually removes the coal dust particles.
Another symptom for presence of coal dust is periodic
oil foaming, where coal dust tends to compete with the
defoamant for oil surface tension.
Steps that can be taken to determine the root cause of
the elevated particle counts are:
1. Minimize coal dust in the plant by installing air
filters in the ventilation system and shields over breathers
and seals.
2. Repair steam leaks as soon as they develop
3. Install 2-5 microns air breathers on the turbine oil
reservoirs
4. Inspect turbine shaft seals on a regular basis
5. Filter the turbine oil using a filter cart, and install
high efficiency filters in the turbine oil system
6. Take oil samples from the same location every time
7. Use the same lab for oil analysis and particle count.
Specify ISO 4406:1999 Cleanliness Standard which reports
particle count in three digits.
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