Oxidation: Leading Cause of Thermal Fluid Replacement Recommendations

Oxidation, which occurs when hot thermal fluid meets air and produces acids, is responsible for over 95 % of thermal fluid replacement recommendations. Acids formed by oxidation build up over time and polymerize, forming solid coke and sludge. They are also responsible for turning the fluid into “black yogurt” or worse when it is cold.

The reaction between hot fluid and air occurs in a tank (vented or open, or a reservoir in standalone temperature control units (TCUs)) that constantly remains hot, over 140 °F.

Indications of a hot expansion tank or reservoir include smoke or vapor from the tank vent, surface rust on the lower half of the tank (due to missing paint), glass stained black, a tank that is too hot to touch or which has visible heat waves emerging from it or fluid with a high acid number.

Expansion tank corrosion is another issue that is intensified by oxidation. In the vapor space, water in the tank continually vaporizes and condenses during operation, eventually causing rust, which then catalyzes the acid formation rate, increasing the acidity of the water and accelerating corrosion, eventually causing a leak to develop.

Preventing Oxidation

To prevent oxidation, a gas blanketing system must be installed to eliminate oxygen from the vapor space. As a cheap and readily available gas, nitrogen is the most favored for this task; natural gases and blends of low-weight hydrocarbons are used in gas processing installations where they can be used safely and are readily available.

The system can be installed on non-ASME tanks if a maximum 15 psig pressure relief valve is used.

Standalone TCUs

It is difficult to control oxidation in TCUs because their components are enclosed in a box made of sheet metal, which typically has minimal ventilation valves.

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The only way to keep acidity low and prevent carbon formation is to schedule replacements of the thermal units. A fluid with an antioxidant can be used to change the frequency of replacements but will not eliminate them.

Separate Expansion Tanks

The oxidation rate of the fluid doubles with every 18 °F (10 °C) increase in temperature, so it's crucial to keep the tank temperature as low as possible. However, heating during operation is inevitable: each time the burner or electric elements activate, a small volume of hot fluid is forced up through the expansion line into the tank.

This volume depends on the fluid's expansion coefficient, the total system volume, and the temperature rise with each cycle. Additionally, as the fluid cools, this cycling process draws in fresh air.

Maintaining the expansion tank at three-quarters full can reduce the overall tank temperature by more than 30 °F compared to keeping it only one-quarter full. This greater volume serves as a more effective heat sink during heater cycles and increases the tank walls' surface area, enhancing heat transfer from the fluid.

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Insulation

Insulation of an expansion line will prevent the cooling of the fluid as it flows into the tank. Insulation should be removed from this line.

Unless a manufacturer specifies insulating certain parts of a tank, the tank should never be insulated.

Valve Positions

Many heaters have two lines connected to the expansion tank: an expansion line which is close to the pump section, and a warmup or boil-out line. Usually, these lines are open during cold start-ups to remove entrained air and water.

If the expansion tank has two parallel lines (A) connected to its bottom, ensure that the second line (the one farthest from the pump) is closed off once the oil temperature exceeds 275 °F. Additionally, if there is a valve on the expansion line, it should be two-thirds closed to minimize thermal currents entering the tank.

This valve should never be completely closed during operation.

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If the second line runs from the heater outlet to the side or the top of the tank, it is vital to make sure the valve is closed before the outlet temperature exceeds 250 °F.

The tank may not feel hot because the flow rate through the typically small-diameter line (less than 2 inches) is not strong enough to heat the entire tank. However, oxidation will still occur. This happens because the fluid entering the tank is at the heater outlet temperature and mixes thoroughly with the air in the headspace.

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Tank Location

Expansion tanks do not have to be located on top of the heater; expansion lines that have long runs of bare pipe can be effective in cooling fluid before it reaches the tank.

Thermal Buffer Tank

Figure C shows an uninsulated vertical tank that is part of the expansion line. The upper end is piped to the main loop and the lower end connects to the expansion tank. The purpose is to trap the hotter, less dense fluid at the top of the mechanism. A well-designed tank will have ample residence time to cool the fluid before it reaches the tank.

Image Credit: Paratherm

Exposed Tanks

If a tank is located outside, it should be painted silver or a light color to reflect the sunlight or a sun shield should be installed.

This information has been sourced, reviewed and adapted from materials provided by Paratherm.

For more information on this source, please visit Paratherm.