Oxidative Degradation (most common)

Oxidation happens when a fluid is exposed to air while at elevated temperatures. This is one of the most frequent ways a heat transfer fluid deteriorates, often leading to the accumulation of sludge inside the system. Although different fluid chemistries are impacted at varying thresholds, the typical petroleum-based fluids usually begin to oxidize at temperatures above 93°C (200°F). In fact, it’s widely recognized that for every 15°C rise beyond 93°C, the rate of oxidation approximately doubles.

Thermal Breakdown

Thermal degradation, also known as thermal cracking, occurs when excessive heat breaks the carbon–carbon bonds in the fluid’s molecules beyond the manufacturer’s recommended maximum bulk temperature. The process can end there, producing smaller molecules than those originally present, or the fragments can recombine, forming polymeric molecules that are larger than the original ones.

So what can be done?

Vulnerabilities to oxidation can usually be identified by locating any areas where the fluid is exposed to air. Once identified, measure the fluid’s average operating temperature in that location—commonly in the expansion tank or reservoir.

If the fluid temperature measures below 93°C, the system is generally protected from severe oxidation. If it exceeds 93°C, consider the following actions:

1. If there is no external expansion tank or reservoir, think about adding one at the system’s highest point. Placing a reservoir of “cooler” fluid at the point of air contact can greatly minimize oxidation.
2. If an external reservoir exists but operates at a high temperature, check its flow path. If fluid is circulating through it, adjust the plumbing so that the reservoir is connected as a “T” branch rather than part of the main loop.
3. If the expansion tank isn’t in the main loop yet still runs hot, consider relocating it farther from the system or applying a nitrogen blanket to shield the fluid from direct air exposure.

It’s important to note that not all fluids react the same way to oxidation. Since oxidation is a major cause of fluid failure, all Duratherm products are formulated with a robust additive package to help resist and slow down oxidation effects. Confirm with your fluid supplier that their product also contains protective additives, especially if your system is open to the atmosphere.

Thermal degradation takes place when a fluid surpasses its rated bulk or film temperature—often leading to molecular cracking. Besides verifying that your fluid is correctly specified for your equipment and operating range, watch out for these common contributors to thermal damage:

1. Startup and shutdown procedures. Many systems are brought online too quickly or shut down before the fluid has cooled sufficiently. In startup, especially for electrically heated systems, gradual heating is key. This reduces the chance of thermal damage, allows trapped moisture or vapors to be released safely, and prevents pump cavitation—or worse—sudden eruptions of hot vapor and liquid from vent points.
2. Shutdown practices. Just as critical, systems should not be turned off without giving the fluid time to cool while still circulating. If pumps stop too soon, fluid can remain in the heater/boiler and be exposed to temperatures far above its rated limit.
3. System modifications. Alterations to the original design may also trigger thermal degradation. Well-designed systems are balanced so that pumps, valves, watt densities, and load demands all work together efficiently within the intended parameters. As systems age or needs shift, adjustments may be necessary. If so, always consult the system manufacturer, engineers, and your fluid supplier to ensure changes won’t push the fluid beyond its limits or require compensations to remain within safe operating ranges.