Heat transfer fluids are often taken for granted, despite their importance. Incomplete knowledge about their selection and maintenance often results in premature fluid degradation. The implications of a poorly selected or an improperly managed heat transfer fluid can be far-reaching when one considers associated equipment downtime, high maintenance costs, and safety risks. This article will review solutions to common heat transfer fluid problems, such as oxidation, thermal cracking, and fluid contamination.
Before appropriate solutions can be offered, it is important to adequately diagnose the root cause(s) of thermal fluid degradation. One of the best ways to do this is through a heat transfer fluid laboratory analysis. Several parameters that tell us about the general health and operational status of a heat transfer fluid sample are measured, including the fluid’s flash point, viscosity, and total acid number (TAN), amongst others. Plant managers and operators are beginning to recognize the importance of periodic fluid analysis in the preventive maintenance of heat transfer fluids.
Common Heat Transfer Fluid Problems
Oxidation — Oxidative degradation occurs when oxygen (in the air) reacts with a heat transfer fluid. This is a two-stage, free-radical chemical reaction that eventually leads to the formation of weak acids and insoluble solids. As a result, the viscosity of the fluid increases. Because of the consequent decrease in turbulence, the fluid’s thermal efficiency is reduced. Once beyond a certain viscosity band, pumpability can become an issue. A fully oxidized fluid is discolored and contains sludge. Another common indicator of oxidative degradation is an increase in the TAN, especially at elevated temperatures. A drastic increase in the TAN value is therefore a measure of oxidation and can be used to compare the oxidative stability of different fluids. However, this analysis is only possible with a history of TAN results and the ability to compare the current result to past trends.
The choice of a fully inhibited heat transfer fluid is the first protection against oxidative degradation. It is advisable to choose a fluid that inhibits the primary and secondary stages of oxidation. Many heat transfer fluids contain no antioxidation additives. In addition, lubricating oils, hydraulic fluids, and turbine oils are not designed to transport heat, and, as such, they often lead to significant oxidation problems when wrongly deployed as thermal fluids. Caldera Heat Transfer Fluids, manufactured by Dubois, contain a primary and secondary antioxidation package that allows for an extended fluid life.
Another effective method of inhibiting fluid oxidation is to blanket the expansion tank with an inert gas, such as nitrogen. The purpose of inert gas blanketing is to maintain an oxygen-free atmosphere in the expansion tank, and one of positive pressure to prevent air entry. A regulated supply of inert gas with a backpressure regulator on the vent outlet line is necessary to obtain this protection. A pressure relief valve also is required to protect the expansion tank from overpressure due to regulator failure, fire, and other causes. Only a static pad of pressure is needed inside the expansion tank to minimize inert gas usage. Maintaining a positive pressure slightly over atmospheric barometric pressure is all that is necessary to prevent air and moisture from entering the tank. A manual vent valve also should be installed for routine fluid maintenance.
In open bath systems, oxidative degradation is more drastic because of the exposure to air. It is very important to select a fluid that mitigates oxidation. Caldera 7 Heat Transfer Fluid is a silicone-based heat thermal fluid that eliminates this problem of oxidation. Caldera 6 Heat Transfer Fluid is a polyalkylene glycol-based heat transfer fluid that is highly resistant to oxidation.
Thermal Cracking — Thermal cracking or degradation occurs when a thermal fluid is heated above the maximum bulk temperature, which causes the formation of components with lower molecular weights than the original molecule. Thermal cracking causes a reduction in viscosity, flash point, fire point, and auto ignition temperature. It is not uncommon to discover carbon varnish on the inside of heat transfer surfaces. To mitigate thermal cracking, it is important to understand ways in which fluid overheating can occur, including incorrect fluid selection, burner flame impingement, improper startup and shutdown procedures, and a low-flow regime.
Choosing a fluid with the right thermal properties helps to ensure fluid longevity. Caldera Heat Transfer Fluids are made from high-quality base stocks that maintain their thermal stability over an extended temperature range. Furthermore, many plant operators inadvertently crack fluids when they start up and shut down heat transfer systems. A drastic rise in temperature in conjunction with a low velocity flow induces thermal degradation. Always start the pump before turning on the heater. This ensures fluid circulation and good mixing prior to heating. It also reduces the residence time of the fluid on the heated surfaces, allowing for a steady rise in temperature and helping to prevent fluid cracking. Start the burner on the low fire setting, circulate heat transfer fluid at full flow, and slowly raise the temperature. Once there is circulation through the heater, the operator should increase the temperature of the bulk fluid by 20°-25°F (11°-14°C) increments until the fluid reaches a viscosity of 10 centipoise (cP). Once the fluid reaches 220°F and is pumping smoothly without cavitation, follow the manufacturer’s recommendation for a full fire heat up.
Other recommendations include the proper alignment of the burner flame. A faulty flame disperser may cause the coils adjacent to the flame to absorb more heat energy than optimally designed. Hence, a heat transfer fluid can be cracked if the temperature at the tube surface exceeds the maximum bulk temperature. It is also sometimes helpful to check for plugged y-strainers and malfunctioning or improperly set bypass valves. This may free up some of the flow that may have been obstructed in the system.
Accordingly, during a shutdown of the system, heat must be deliberately decreased in the reverse of the startup procedures. Keep the thermal fluid circulating until the system temperature drops below the 200°F or 93°C temperature gauge mark, which indicates that the residual heat is removed.
Contamination — Contamination occurs in a heat transfer system when a different fluid (other than the heat transfer fluid) is introduced into the system, potentially causing wide variations in the physical and chemical properties of the original thermal fluid. Common fluid contaminants include water and degraded heat transfer fluid. It is important to ensure that used fluids are completely flushed from the system before charging a new system fill. Heat transfer equipment with water as an operational fluid must be inspected regularly for leaks. Other good housekeeping practices include the proper labeling of full and partially full storage drums. These drums may be placed sideways if they are to be stored outside to prevent the accumulation of water/moisture on the top of the container.
Preventive Maintenance — Knowing when the conditions outlined in this article are occurring and preventing them from persisting is the best step in proactively maintaining the thermal fluid, which decreases the preventive maintenance required by the plant. Utilizing simple yet efficient regular checks of key parameters and capturing the data in an easy-to-use, real-time reporting system, such as DuBois Analytics, is the best way to monitor the health of the thermal fluid system while not burdening maintenance staff.
A review of solutions to the most common heat transfer fluid problems will significantly reduce the cost of operating a thermal fluid system and improve the uptime and efficiency of heat transfer. The importance of adequate maintenance procedures and logging of critical operational data cannot be overemphasized. This has significant impacts on fluid and equipment longevity, minimal downtime, and personnel safety.