The performance of the thermal oil system does not depend only on the boiler body. Chemical structure of thermal oil, operating temperature, pump flow rate, expansion tank and system contact with air; It directly affects efficiency, maintenance interval and safety.
Wrong oil selection or low flow rate operation may cause the oil to overheat and break down on the boiler tube surface. For this reason, “oil with high maximum temperature” alone is not the correct selection criterion.
Which temperature should be considered when choosing thermal oil?
Three temperatures must be distinguished:
- Average fluid temperature: It is the oil temperature measured in the system.
- Maximum batch temperature: It is the allowable continuous operation limit of the oil in the main flow.
- Film temperature: It is the temperature of the thin layer of oil in contact with the heating surface and may be higher than the main flow.
Exceeding the film temperature limit accelerates thermal cracking. Boiler design, heat flux and oil flow should therefore be evaluated together.
Difference between mineral and synthetic thermal oil
Mineral-based oils can offer economical solutions in many medium-high temperature applications. Synthetic fluids can provide a wider temperature range or special low-temperature properties; but cost, sealing compatibility and disposal conditions are different.
In the selection, the manufacturer’s technical data sheet, safety data sheet, maximum film/stack temperatures, viscosity curve, flash point and compatibility with the system materials should be examined. Oils of different chemistries should not be mixed without manufacturer approval.
Why does thermal oil break down?
Thermal cracking
Fat can break down into smaller molecules at extreme surface temperatures, even in the absence of oxygen. Low boiling point ingredients may increase the tendency for evaporation and cavitation; heavy components can form carbon and sediment.
Oxidation
thermal oil oxidizes when it comes into contact with air. Acidity and viscosity may increase and sludge may form. The temperature and air contact of the expansion tank affect this process.
Contamination
Water, process fluid, cleaning chemicals or assembly residue may enter the system. Water at high temperatures can cause sudden evaporation, pressure fluctuation and pump problems.
How to monitor the life of the oil?
Calendar year alone is not enough. The sample should be taken by the appropriate method from a representative and safe point while the system is running. According to application in the laboratory:
- Viscosity,
- Acid number,
- Flash point,
- Low and high boiling components,
- Carbon residue,
- Amount of water,
- Insoluble matter
can be examined. The result should not be evaluated as a single measurement, but as a trend with previous analyses.
Task of the expansion tank
When oil heats up, its volume increases. The expansion tank covers this volume change, helps the required static height in pump suction and ensures the separation of gases. The volume, connection height and minimum/maximum level of the tank must be calculated according to the total system oil volume and temperature range.
Unnecessary heating of the tank by oil circulation in the expansion line may accelerate oxidation. In systems where an inert gas blanket is used, pressure control and safety equipment are designed separately.
Circulation pump and minimum flow
The circulation of oil at a sufficient speed in the thermal oil boiler is essential for cooling the heating surface. If the flow rate decreases, the film temperature may increase rapidly. thermal oil viscosity, system pressure loss, NPSH conditions, seal and redundancy should be taken into consideration when selecting a pump.
The flow key should not be just an indicator; It must be connected to the safety chain that stops the burner safely below the minimum flow rate. When the pump stops, the fuel must be cut off and the residual heat in the boiler must be managed.
Filling and initial commissioning
New installation may contain welding slag, moisture and cleaning residues. The system must be cleaned and dried using the appropriate method. Oil should be filled in a controlled manner from the low point, air should be evacuated and the first heating should be done gradually.
If there is a possibility of water in the system, a safe removal procedure is applied by slowly increasing the temperature. Rapid heating may cause sudden evaporation of water, pressure surge and overflow from the expansion tank.
Leakage and fire safety
Even if the flash point of thermal oil is high, the fine oil mist sprayed onto the hot surface can ignite. Flanges, valves, pump seals and flexible connections should be checked regularly. Oil absorbed into the insulation may accumulate unseen and create a fire load.
Basic precautions:
- Appropriate insulation and leakage monitoring opportunity on hot surfaces,
- Safe drainage around the pump and flange,
- Keeping oil leaks away from burning and hot chimney surfaces,
- Flow, temperature and level safety interlocks,
- Emergency stop and fuel cut-off device,
- Detection and extinguishing system appropriate to the facility risk,
- Trained operator and written emergency procedure.
How should the system be handled during oil change?
Adding new oil does not always solve the problems of spoiled oil. System cleaning may be required if heavy degradation products and sediment are present. The oil producer, boiler manufacturer and expert maintenance team must decide on filtration, partial renewal, cleaning or full replacement according to the analysis results.
Waste thermal oil must be managed through licensed channels in accordance with applicable environmental legislation.
Frequently asked questions
How many years does it take to replace thermal oil?
There is no fixed duration. Operating temperature, air contact, commissioning and maintenance conditions are different; Laboratory analysis and trend monitoring should be taken as basis.
Can different brands of thermal oils be mixed?
It should not be mixed without verifying chemical compatibility. The mixture may affect viscosity and performance as well as warranty conditions.
How to reduce the risk of fire in a thermal oil boiler?
Correct flow and temperature control, prevention of leaks, insulation management, safe fuel cut-off and appropriate fire system should be applied together.
Why does the expansion tank overflow?
The cause may be incorrect fill level, water entering the system, gas/air pockets, overheating or incorrect tank volume. The system should be stopped safely and expert inspection should be performed.
Technical resources
- Technical and safety data sheets of relevant thermal fluid manufacturers.
- Design, occupational safety and environmental legislation applicable to thermal oil systems.