Heat is the Enemy of Oils – General Operating Notes
The oil is not in danger as long as it works at a low or moderate temperature.
As the oil temperature goes up, degrading factors get activated, which at low temperatures do not carry the hazard of a significant acceleration of the oil degradation process - copper and oxygen, or, to be more general, non-ferrous metals and air.
The cause (that cannot be eliminated) why the heating oil quality deteriorates is the oil heating boiler. Therefore, to avoid the intensified oil degradation, the system pump should have a sufficient capacity to enable full turbulent flow through the boiler at a speed of 2-3 m/s.
The oil performance duration depends on the heating system design and the conditions in which it is operated. In a well-designed system where oil is not thermally overloaded, oil lasts for many years.
In heating systems with a temperature above 150OC, elements made of copper and its alloys should not be used and the oil must not be able to get mixed with the air. Getting continuously mixed with fresh air at a high temperature is the primary reason for the degradation of the heating oil. The system must not contain the “free outflow to an open vessel” type elements. The system should keep a pressure higher than the atmospheric pressure on the suction side of the pumps to prevent air from being sucked by pump and valve chokes.
After filling the system with oil, deaerate it well.
Once a year, check the oil acid value and oil viscosity at 100oC. If the acid value increases beyond 1.8 mgKOH/g and viscosity increases by 50%, the oil qualifies to be replaced.
In this case, it is not possible to avoid direct contact with elements heated up to a temperature exceeding the oil resistance level. Adequate measures can be taken to prevent premature oil degradation. Mineral oils undergo two kinds of aging processes under the impact of high temperatures:
1. Cracking, that is, breakdown of hydrocarbon particles under the impact of heat. Large particles fall apart into smaller ones. Some of them change into the gaseous state, some unstable particles polymerise to form non-soluble sediments. At a temperature up to 320OC, the cracking process speed is very low.
2. Oxidation or reaction of hydrocarbons from the oil with oxygen in the air. This reaction takes place at a very slow rate at the ambient temperature but, as the temperature goes up, the reaction rate increases rapidly.
The oxidation process results in acid products being formed in the oil; the products form non-soluble sediments while being oxidised further.
The oil performance duration depends on the quenching tub loop system design and the operation of the temperature stabilisation system. In a well-designed system which is not overloaded, oil lasts for many years.
The quenching oil consumption is accelerated by the lack of circulation, unstable temperature, insufficient tub capacity, lack of filtration or separation of sludge.
Viscosity, flash point and content of non-soluble sludge must be regularly controlled.