Varnish – The Cholesterol in Hydraulic Systems

What is Varnish?

Efficient Plant Magazine describes Varnish as the cholesterol in your machine’s hydraulic systems. Simply put- Oil degradation begins at one part of the machine, slowly builds and transforms from dissolved form into insoluble products, and then carries on to other mechanical components of the system as the fluid/grease circulates through the system. Varnish formation is akin to arteries getting clogged and fat getting deposited in organs of your body.

What causes Varnish?

    1. Oxidation:
Typically Varnish tends to start forming when the oil is in static or inactive mode within hydraulic systems. Similar to cholesterol forming in human bodies that are not warmed up enough or not moving enough! Like most chemical reactions, oxidation rates increase with increasing temperatures. As a thumb rule, once oil temperatures reach/exceed the 55-60 degree range, oxidation rate increases by a factor of two for every 10 C increase in temperature. Once oxidation starts occurring, it becomes an autocatalytic reaction where byproducts such as aldehydes, ketones and peroxides react with metallic compounds within the base oils and the metal tubes/mechanical parts in contact with the oil to form bigger molecules of these byproducts.

    2. Thermal Deterioration:
As the name suggests, this results from heat: overheating. Most Oils in circulating systems have a temperature limit that cannot be breached. This is roughly in the range of 180-200C. However, often oil temperature exceeds this ‘red zone’ through being in contact with a superheated metallic surface or by a phenomenon known as “adiabatic compression”. Adiabatic Compression occurs when air bubbles in an oil transition from low pressure to high pressure zones typically in hydraulic systems. These air bubbles “trap” a high amount of heat after compressing rapidly in a short period of time, resulting in a sudden rise of the overall oil temperature; leading to thermal deterioration.

    3. Electric Discharge:
This is a form of thermal deterioration described above, and occurs when the electric potential between the oil and mechanical surfaces results in localized thermal degradation due to electric sparks. These sparks are estimated to be between 10,000°C and 20,000°C and results in dielectric breakdown of the oil flowing through the system. This is due to the oil being nonconductive, which effectively self-insulates the charged fluid zones from grounded surfaces. Once these charges build up in the working fluid zones, including reservoirs, the subsequent static discharging, similar to lightning strikes through the fluid, may cause localized thermal-oxidative oil degradation.

    4. Particulate Contamination:
The most obvious and common form of oil degradation: metallic discharge from components and tubes through which the oil flows, fly-by pieces of filters, mixing of oils; water contamination during oil restoration or replacements/top ups and aeration caused by improperly sealed storage tanks causes a ready recipe for oxidation.

In the follow up blog, I shall talk about how varnish in oil can be tackled, minimized and some of the analysis today in the market around varnish measurement.