Articles about thermal fluid systems often start with a variation of the statement that “thermal fluid systems typically require little ongoing maintenance for the first few years of operation” and then go on to extol the various advantages of indirect thermal-fluid process heating over competitive heating methods, such as direct heat, steam and so on. The corollary to that statement, however, is that by the time there is a problem, the operating personnel that were trained on the system have moved on, been excessed or promoted. As a result, when things do go wrong, the guessing begins. And, unless there is an obvious cause like a geyser from the expansion-tank vent or a pump that sounds like it’s moving ball bearings, someone will likely blame the thermal fluid for the problem. There are several problems that seem to occur with some frequency. This article reviews a number of real examples and describes how the symptoms can be misinterpreted. The suspected fluid properties and the testing procedures necessary to determine which of the fluid properties (if any) is responsible for the problem are examined. Finally, recommended corrective actions are proposed.
WHY HOT-ROLL CALENDERING OPERATION WAS EXPERIENCING REPEATED ROTATING JOINT SEAL FAILURES?
A hot-roll calendering operation was
experiencing repeated rotating jointseal failures. The seal faces were
being scored severely enough from the
inside out to create fluid leakage. Two
of the oldest seals were experiencing the greatest number of failures. While
the fluid tests showed no significant
change in the fluid condition, there
was visible residue of a previous brand
of insoluble fluid. The most compelling
evidence of what the problem might be
was that particles were collecting on
the sidestream filter elements. To prevent further problems from what was
suspected to be carbon (from degraded
heat-transfer fluid), the user began
to evaluate a system flush and fluid
The majority of carbon particles
produced by fluid degradation are
the result of fluid oxidation (as determined in fluid analysis by the total
acid number). These acids are formed
when hot fluid is exposed to air in the
expansion tank. They are thermally
unstable (compared to the fluid itself) and thereby degrade into carbon
at relatively low temperatures (375
to 400°F) once the concentration has
reached an acid number of 0.3 or so.
If the expansion tank continues to run
hot, the acid number will either stabilize or continue to increase. If the
cause of the oxidation has been corrected, the acid number will decrease
as the acids are consumed.
The carbon that is formed is similar to “soot” in appearance and will
remain suspended in the fluid, which
causes the fluid to appear to be black.
The particles will drop out of suspension in stagnant fluid and form sediment (sludge). However, individually
these particles are extremely fine
(<0.5 micron) and as such are incapable of damaging rotating seals because
they pass between the rotating faces.
However, they will clump on 25-micron filter elements, which can be misleading during troubleshooting. In this
case, the solution required analysis of
the filter. The particles were analyzed
and found to contain over 90% iron.
This information was transmitted to
the user, who then shelved the plan to
flush the system and replace the fluid.
Instead, he concentrated on improving
his filtration system to eliminate the
Author Jim Oetinger is the director of technology at Paratherm Corp. (4 Portland Rd., West Conshohocken, PA 19428; Phone: 800-222-3611, Fax: 610-941-9191; Website: www. paratherm.com). He has over 30 years experience in the chemical and plastics indus- tries. He has been involved with a wide range of products and processes including pig- ments, refrigerants, consumer plastic recycling, polymer compounding, process instrumentation and spray dried polymers. In addition, Oetinger has over 20 years experience in sales, marketing, and technical support of thermal fluids. He has authored articles on thermal fluid and system troubleshooting for this and other publications. A member of the Delaware Valley Chapter of the AIChE, he holds a B.S.Ch.E. from Clarkson University and a Masters of Management degree from Northwestern University. Oetinger and his family reside in a suburb of Philadelphia, Pa.
content and image courtesy:www.paratherm.com