Heat transfer fluids that are not monitored and maintained can have adverse effects on operating equipment and manufacturing processes. In extreme cases, improperly maintained heat transfer fluids can be a safety risk. Periodic analysis of heat transfer fluids provides the owner with the information needed to make informed decisions regarding fluid maintenance.
Case in Point. I was called to a plant that uses oil-heated embossing rolls as an integral part of the process. The plant's reject rate was up and production speed was down due to an increase in the temperature difference (T) across the rolls. This phenomenon was noticed on all the rolls in the plant. In the course of the normal barrage of troubleshooting questions, I asked, “When was the fluid last sampled and tested, and may I review those results?” Silence. The fluid, which was a high performance fluid from a well-known manufacturer, had never been tested in the seven-year life of the system. The owner assumed that fluid testing was necessary because he used a “good” fluid and because the system had been specified and designed by a reputable engineering firm. The system had a closed expansion tank with a nitrogen blanket system, and the owner assumed (there's that word again!) that the fluid was adequately protected.
The owner had failed to take into account that, when the embossing rolls were changed (a regular occurrence), the replacement rolls were installed with an appreciable amount of air inside. This air introduced a small amount of oxygen into the system each time the rolls were changed, and over time, the fluid became highly oxidized. One property of oxidized fluid is an increase in viscosity, and in this case, the increased viscosity was sufficient to affect pump performance and reduce turbulence in the rolls, both of which contributed to the adverse changes in system performance.
The fluid was changed, piping was modified and procedures were put in place to mitigate further fluid oxidation. One of the procedures was semi-annual fluid testing.
Types of Fluid DegradationFor purposes of this article, observations about fluid degradation will be limited to petroleum-based and synthetic aromatic hydrocarbon fluids. This will cover most users and will avoid this article becoming a treatise on organic chemistry. Silicone-based and glycol-based fluids also suffer degradation, but not in the same manner as the more commonly used hydrocarbon fluids.
Materials that are selected as heat transfer fluids are generally chosen because they have good intrinsic properties for heat transfer and they are relatively stable at elevated temperatures. Notice the term “relatively”. All hydrocarbons will break down or degrade in some manner at elevated temperatures, but heat transfer fluids degrade less than other organic compounds. The fact that slow degradation occurs is one reason fluids should be regularly sampled and tested, as test data will allow the owner to determine the best time to supplement or replace fluid before it can cause problems. Fluids generally degrade via two paths: oxidation and cracking. An additional contributor to poor system performance is water, which also can find its way into heat transfer systems and can cause problems if not properly managed.
Fluids that are oxidized often exhibit increases in viscosity and can carry free, insoluble solids. Oxidized fluids can have a negative impact on heat transfer by leaving solids in heat transfer equipment, coating heat transfer surfaces and clogging up passages. Fluids with high solid loading also can reduce pump mechanical seal life. As a fluid gains viscosity, its intrinsic heat transfer properties change, namely as a lowering of the heat transfer coefficient, which can cause subtle and hard-to-diagnose problems in the system. Oxidized fluids, by their acidic nature, can potentially corrode carbon steel components if not replaced.
Oxidation is often seen in small, cabinet-style fluid heaters and in larger systems that have expansion tanks that are vented to atmosphere. But, as seen in the case in point noted at the outset, oxygen can find its way into the system by other paths as well.
Cracking is most prominent in systems that operate the heat transfer fluid at or very near the fluid's maximum rated bulk temperature for extended periods. Cracking also is seen in systems where the fluid's maximum recommended film temperature is exceeded. This can be caused by the heater not having been matched with the heat transfer fluid or in heaters that have very high heat transfer rates. Electric heaters with high watt densities or heaters where the fluid is allowed to be throttled can accelerate fluid cracking. Fired heaters with relatively small radiant heat transfer sections also can exacerbate cracking.
Fluids that are cracked will often exhibit lower viscosities, high vapor pressure and depressed flashpoints. While the low viscosity does not pose a direct threat to the process, the high vapor pressure can be problematic. High vapor pressure can contribute to pump cavitation and, in severe instances, cause vaporization in the heater. Depressed flashpoints are generally accompanied by depressed firepoints and depressed autoignition temperatures, which can be a safety concern.
Oxidation and cracking are generally not exclusive of each other and can occur simultaneously.
The startup procedures for new systems and systems with new fluid charges should include a slow heatup while circulating through the expansion tank with open vent to safely remove any water prior to being increased to operating temperatures.
What Tests Should Be Run?While there are many tests and test methods to monitor fluid condition and performance, the following tests are most common in the industry and will provide useful information for evaluating the fluid (table 1).
Water. The system should always be sampled and tested for water content after new fluid is added, after the system has been down for a period of time, or if a heat exchanger leak is suspected. Many plants have laboratory apparatus for checking water content, and this test can occasionally be run locally.
Viscosity. Checking viscosity is a quick test that often can be done at the plant site. Changes in viscosity -- an increase or decrease -- can be an indicator of degradation.
Total Acid Number (TAN). The TAN test is a titration that determines how much of a standard base solution the fluid can neutralize. It is basically a measure of the number of carboxyl end groups (CEGs) that have been generated through oxidation. In systems that are subject to oxidative degradation, this is an extremely useful and informative test to track the rate and degree of oxidation.
Percent Solids. The presence of solids is an indication of oxidative degradation, but it also can indicate cracking, depending on what chemical species are formed when a particular fluid degrades. Solids can be hard particulates or they can be soft and sticky. They can plate out on surfaces and impede heat transfer or collect in places that can impede flow. Solids also can contribute to reduced mechanical seal life.
Flashpoint. Flashpoint is the lowest temperature where a substance will burn if presented with a source of ignition. A reduced flashpoint value is an indication of cracking, where the fluid degrades into low molecular weight molecules, but it also can indicate fluid contamination. Even though, in practice, most fluids are operated above their flashpoints, it is considered good practice to track this property and take steps to keep the flashpoint close to the manufacturer's specification.
If unsure of methods or procedures, the owner should consult with the fluid supplier or other experienced resources for assistance with sampling methods that will yield representative samples. The same resources also can be of assistance in interpreting the test results reported back.
Sampling and testing do not take a great deal of time, nor do they cost much money. Regular fluid testing can result in longer fluid life, more consistent system operation, decreased system maintenance and lower operating costs. Finally, the knowledge gained from regular testing is invaluable in charting the condition of the fluid and is a great troubleshooting tool when problems do occur. PH