Mechanism of oil oxidation

We will now investigate if oil deterioration is inevitable.

What is oil deterioration?

An analysis of "deterioration" oil proves that when oil is contaminated, its physical characteristics are not changed. Only a tiny percentage of oil molecules chemically react in the oxidation process & produce a resinous matter which is generally referred to as "sludge". (Mud in water indicates that water is contaminated, it does not however alter the basic molecular structure of water.)

Oil is said to have deteriorated when a few dozen PPM of resinous matter exists in oil. A relatively high acid value is measured in such oil and resinous matter of light brown colour is deposited on a 0.8 micron membrane patch, when oil is checked by membrane analysis method.

Resinous matter is soluble in oil at temperatures higher than 60 degrees C.
and remains insoluble at temperatures lower than 60 degrees C.
Resinous matter is also measured as benzene insoluble.

This resinous matter or sludge passes through filters when its concentration is small but clogs filters when its concentration increases.

This is Deteriorated oil.

What is the cause of deterioration?

Sludge in oil denotes deterioration. Therefore understanding the causes of sludge formation, will help us to understand the causes of deterioration. The process of sludge formation, starts with oxidation.

Oxidation of hydraulic oil requires:

# Oxygen # Heat # Catalyst

15% of mechanical energy is converted to heat energy, which is why hydraulic systems require oil coolers.

10% of air is always dissolved in oil. Therefore both heat & oxygen are always present in oil. Fine material particles from wear of pumps & water act as catalysts to oxidize oil & produce sludge.

Deterioration of oil therefore starts with solid particles introduced in stage 1, causing wear of pumps and moving parts. These worn metal particles, along with moisture, act catalysts to oxidise oil & produce sludge.

Function of Catalysts

Test	Catalyst Metal	Water	Time (Hrs)	Acid Value
1	nil	nil	3,500	0.17
2	nil	exist	3,500	0.90
3	iron	nil	6,500	0.65
4	iron	exist	400	8.10
5	copper	nil	3,000	0.89
6	copper	exist	100	11.20

Note:

1. Time: hours for for reaching a certain level of acid value.
2. Test oil is turbine oil with viscosity of 32cst/100 degree F (37 degree C)
3. Tested oil temperature is 200 degree F (93 degree C)

We have, so far explained - how oil gets contaminated.

Oil oxidation is one of the biggest cause of lubrication system malfunction, since "sludge" is a by product of the oil oxidation process.
Centrifuges promote oil oxidation.

Sludge is sticky in nature and is easily deposited on metallic surfaces of valves, oil coolers & pipes leading to :

Malfunctioning or sticky valves;

Inefficient heat transfer due to sludge deposition on cooler tubes/plates. Increased machine working temperatures & oxidation of oil, and decreases in viscosity.

Oil can not be maintained with mechanical filtration, at the level necessary to prevent downtime and costly oil changes.