Chemical Engineering August 2023 - 9

Facts At Your Fingertips
In-Service Heat-Transfer-Fluid Testing
Department Editor: Scott Jenkins
R
egular heat-transfer-fluid monitoring
can prevent fluid problems
that lead to equipment
failure, costly downtime and repairs.
Parameters to watch include viscosity,
acid neutralization number, moisture,
solid or residue content, thermal
degradation and flash point. This onepage
reference provides information
on how to examine these parameters.
Representative
industry
standards
are cited with the understanding that
comparable BS (British Standard) or
DIN (German Institute of Standardization)
standards could also be used.
Viscosity
Viscosity value is unique to each fluid
and is typically measured at 100°F
or 40°C using the industry standard
method ASTM D-445. The test can
be run in a temperature-controlled
bath with calibrated glass capillary
viscosity tubes using elapsed time
of fluid flow through the tube. Or, the
test can be carried out using a Stabinger
viscometer, where the kinematic
viscosity is calculated by dividing
the measured dynamic viscosity
by its density. With use, fluid viscosity
can shift based upon changes from
degradation or contamination.
Acid number
New organic fluids typically have acid
numbers of near zero milligrams of
KOH per gram of sample. The analysis
can be done by potentiometric
titration using ASTM D-664. The
acid number is found by dissolving
some fluid in a mixture of toluene and
isopropyl alcohol containing a small
amount of water, and then titrating
potentiometrically with alcoholic potassium
hydroxide to an endpoint
pH of 11. An alternative method is to
use a color indicator and make the
determination calorimetrically. The
acid number value can increase with
acidic contamination or from heated
fluid oxidation.
Moisture
Analysis for moisture concentration
in organic fluids can be done by
volumetric Karl-Fischer titration using
ASTM E-203. Water content is
found by titrating the fluid with
standard Karl-Fischer reagent
to an electrometric endpoint.
The method applies to fluids
with moisture content in the
range of about 50-1,000 parts
per million (ppm). A second
method is coulometric KarlFischer
titration per ASTM
D4928 in a range of 0.005 to
0.02 mass percent. Moisture
in high-temperature heattransfer-fluid
service must be
kept low to avoid short-term
operational and long-term corrosion
concerns.
Solids or residue content
Measurements of solids content
can be done by gravimetric
separation and quantification
of solids (mass) per unit volume
of fluid filtered. A second method
involves bulb distillation of a fluid
sample under extreme vacuum with
temperature increasing to 250°C until
only non-volatile residue remains. The
mass percent of non-volatile residue
of fluid is reported. A third technique,
known as Conradson Carbon Residue
and described in ASTM D189, is
sometimes performed to quantify carbon
residue in mass percent remaining
after evaporation and pyrolysis of
an oil sample. This quantity can be an
indicator of coking potential of an oil.
Thermal degradation
With measurement by simulated
distillation gas chromatography using
ASTM D-7213, the oil boiling
range can be compared with that
of unstressed (that is, unused) fluid
to quantify the percentage of lowboiling
and high-boiling degradation
products present. Critical to accuracy
is knowing the boiling range of the
same fluid in its unstressed condition.
Flash point
Flash point temperatures are measured
using ASTM D-92 (Cleveland
Open-cup; COC) or ASTM D-93
(Closed-cup; CC) techniques, reported
in Fahrenheit or Celsius. CompariCHEMICAL
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AUGUST 2023
FIGURE 1. Analysis of heat-transfer-fluid parameters, such as
acid number, viscosity and solids content, can help identify
changes to fluids that are in operation, and determine if action
should be taken
son to the flash point temperature of
the unstressed fluid standard can determine
the change in fluid condition.
Knowing what the fluid is like in an
unstressed condition can help determine
what should be done to improve
fluid condition. From this analysis,
engineers familiar with the fluid's operating
environment can assess what
caused the changes and decide on
what should be done. Since expertise
at a general-service oil-testing laboratory
can be more limited, a more detailed
understanding of the test results
and related action-thresholds, as they
apply to specific fluids, is available
from the manufacturer (Figure 1). For
more on interpreting results, see Ref.
1. For coverage beyond the above
tests, see Refs. 2 and 3.
n
Editor's note: Content for this column was authored by Conrad
Gamble, Eastman heat transfer fluids
References
1. Gamble, C. and Schopf, M., Optimizing heat transfer fluid performance:
How to avoid costly consequences, Eastman Chemical
Co., white paper, www.eastman.com, 2015.
2. ASTM International, ASTM Standard D5372-20: Evaluation of
Hydrocarbon Heat Transfer Fluids, ASTM International, www.
astm.org.
3. Deutsches Institut für Normung e.V. (DIN) DIN 51529 - Testing
of mineral oils and related products: Testing and evaluation of
used heat transfer fluids, www.din.de.
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Chemical Engineering August 2023

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