Oxidative Stability - The Key to Oil Life

 17th Dec 2014

One of the key performance metrics in evaluating lubricating oil performance is the oil drain interval (ODI). This is the period of time an oil lasts in a given application before it needs to be replaced. Increasing the ODI for a particular application can deliver many financial and sustainability-related benefits.

For instance, with fewer oil changes, companies can minimise oil consumption and maintenance-
related labour costs, as well as enhance safety by reducing the frequency at which employees must interact with machinery.

Improved ODIs can also yield reduced equipment downtime, resulting in a substantial boost to productivity, particularly in industries where an oil change might require several days of downtime. Finally, longer ODIs can significantly reduce the volume of waste oil produced on site, meaning greater environmental performance and further cost savings.

One of the most important factors when determining how long an oil will last in a particular application is oxidative stability.

Oxidation, a chemical reaction between the hydrocarbon lubricant and oxygen in the air, is the most common cause of lubricating oil degradation. The process causes an increase in oil viscosity, increased acidity, filter plugging, corrosion and the formation of varnish, sludge and rust,, among other issues.

While its effects can be mitigated, oxidation is inevitable; over time, all oils will oxidise. The oxidation reaction is accelerated by high temperatures, presence of wear metals- such as copper- and contamination from water and air.

Further, the oxidation process is self-propagating, meaning that once the process is initiated, the oil will quickly start to degrade and lead to operational issues, unless steps are taken to halt the reaction, such as changing the oil.

The image below shows an operational servo valve from a gas turbine hydraulic system compared to a clean, unused valve.

Screen Shot 2014-11-03 at 8.58.45 AM

The varnish coating that is evident on the used valve can lead to valve sticking and, ultimately, cause a turbine trip, leading to substantial downtime and productivity loss. Such a shutdown would have a significant impact on the bottom line.

To combat such issues, lubricant formulators add additives known as anti-oxidants to base oils. There are different anti-oxidant chemical types, but all are designed to break up the propagation stage of the oxidation reaction, slowing the oxidation process (anti-oxidants are themselves depleted over time, reducing the oil’s oxidation resistance). Operators can also help mitigate oxidation by taking steps to lower operational temperatures, remove wear metals and ensuring old, or oxidised, oil is completely removed from a system before a new oil charge is filled.

However, the most effective strategy to combat oxidation and maximize lubricant service life is selecting the proper lubricant, particularly a lubricant that utilizes synthetic base oils.

The type and quality of the base oil used in formulating a lubricant affects the rate of oxidation, and some synthetic base oils have inherently higher oxidative stability than their mineral counterparts. For example, oxidative stability is a key metric in the formulation of ExxonMobil’s balanced formulation products. In fact, our Mobil SHC range of synthetic oils and greases can last up to six times longer in application than an equivalent mineral oil, largely due to their enhanced oxidation stability of the synthetic, polyalphaolefin (PAO) base oil.*

By choosing a high performance, synthetic product, operators can help maximise the ODI of their equipment and generate a range of financial and sustainability-related benefits.

*Mobil SHC 600 Series

Written by; Conor Wilkinson, Industrial Field Marketing Advisor for the Nordics, UK & Ireland

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