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Routine fluid analysis is a critical part of any preventive maintenance program.
Not only do the results indicate what condition the fluid
is in, they also provide an early warning of system or equipment
problems that can eventually shut down the process.
How Often To Test The Fluid
Compared to other process heating systems such as direct
fire, electric resistance or steam, thermal fluid systems
require very little care or supervision for a number of years
after start-up. But this "set it and forget it" feature can
backfire if it results in surprises like reduced throughput
or an unscheduled outage. At that point, it's too late to
test the fluid to see if it has "gone bad". Thermal fluids
very seldom "go bad" without help—95% of fluid degradation
is caused by equipment malfunctions, poor design and/or operating
errors. Most of these problems can be identified early on
if the fluid is tested within the first 6 to 12 months after
start-up and at most once a year after that. Early identification
and correction of these problems can prevent surprises.
Taking the Sample
Samples should be taken directly into the sample container
with the pump running and the fluid temperature at a minimum
of l80°F (82°C). Samples taken cold or from a stagnant loop
or from the expansion tank will not be representative of the
entire fluid charge. If the system cannot be cooled below
270°F (135°C), install a two-foot length of metal tubing on
the sample point to cool the sample; this will prevent the
glass sample container from breaking. Some good locations
to take the sample include the blow-down valve mounted on
a Y-strainer or any low-point drain or pressure gauge tap
near the pump or heater inlet. Purge the sample line with
at least one full jar of fluid.
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Inspection
Many fluid problems can be detected by appearance and smell.
Fine black sediment in the bottom of the jar usually indicates
solids are accumulating. Liquid contaminants (such as water
or hydraulic fluid) can show up as a separate layer in the
bottom of the sample. Contaminants that are soluble in the
fluid (such as aromatic- based “synthetic” fluids) will affect
the sample odor.
Laboratory Testing
Because thermal fluids operate in closed- loop systems (no
continuous exposure to air), they require different tests
and testing frequency than lubricating oils or hydraulic fluid
which operate in open systems (continuous exposure to air).
Thermal fluids usually only need to be tested once a year
since there are no additive packages that need to be monitored
and controlled or water contamination to worry about (water
in a thermal—fluid system makes itself known pretty quickly).
And since thermal—fluid pump clearances are not critical,
metals analysis and particle counts are meaningless. In decreasing
order of importance, the three tests that are run on the sample
are as follows:
Acid Number (ASTM D-664) measures the amount
of acid present in the fluid which in turn is an indication
of the amount of oxidation that has occurred. The higher the
number the more oxidation has occurred. Because thermal fluids
are blended from multiple components each with its own boiling
point, the fluid will boil over a range of temperatures which
is called the Distillation Range (ASTM D-2887).
The difference in temperature between the new fluid baseline
and the test results are averaged and shown as the “Low Boilers—%
change” or “High Boilers—% change”. The higher the number,
the more degradation has occurred.
Viscosity (ASTM D-445) measures how easily
the fluid flows. The data is compared to new fluid and the
results are shown as a “% change” from new. A positive % indicates
that the viscosity has increased (become thicker) while a
negative % indicates that the fluid is thinner than new.
Analyzing the Results
Comparing the used fluid to new
fluid is usually sufficient to
determine whether the fluid has
degraded enough to require change
out. However, as noted above, 95%
of fluid degradation is caused by
external forces. Unless the
equipment or process conditions
that are causing accelerated
degradation are corrected, fluid
change out could become an annual
routine. Properly analyzed and
interpreted, the test results can
provide the information necessary
to identify and correct these
undetected problems. Even more
valuable are the trends that are
evident when samples have been
taken at routine intervals.
Knowledge of the equipment is also
necessary to properly interpret the
results and identify the source of
the degradation.
Questions? We'd like to hear from you. Call toll-free,
800-222-3611 -- or fax or e-mail.
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