By: Thomas Yoon

In our previous issue, we discuss about the properties of lubricating oil and what to look for when buying or replacing them.

Today, we want to find out as to when to replace the lubricating oil. If you have a large quantity of lubricating oil to change, it is going to burn a hole in your pocket. So most plant operators try to preserve the properties of the lubricating oil for as long as possible.

One of the most important functions of lubricating oil is to reduce the friction between the moving parts of machinery. But there are other features to look at.

When do you know that the oil needs to be changed? Below is a rough guide:

  1. Viscosity has changed by 10%
  2. Flash Point has dropped to 150 degree Celsius
  3. Water Content has reached 2%
  4. TBN, or Total Base Number has reduced by 20%
  5. Insoluble Content has increased to 5% of the oil

Due to the oxidation of the oil when exposed to heat and oxygen, the viscosity of the oil tend to reduce. With the reduction of viscosity, the film of oil between rubbing metal surfaces becomes more difficult to maintain. This results in metal to metal contact, micro seizures that leads to scuffing, abrasion and other damages.

In large diesel engines, fuel oil from dripping injectors or fuel pumps sometimes finds their way into the lubrication oil sump. This has the tendency to reduce the flash point of the lubricating oil. In addition to reducing the viscosity that is detrimental to lubrication, this contamination with fuel oil can be quite dangerous. If there is a hotspot in any of the rubbing parts, this can lead to a crankcase explosion.

Water can also find its way into the lubricating oil from leaks in the cooling water system o-rings or gaskets. In addition to reducing the lubricating properties of the oil, the presence of water in the oil can give rise to bacteria or fungal growth, which will quickly damage the oil properties as well as contributing to acid corrosion and oxidation of the oil, changing the chemical composition of the oil itself. However, if the water content is below 0.5%, it can still be removed by centrifugal purifiers.

The total base number is especially needed for the cylinder liner lubrication of engines that run on poor quality fuel with high sulphur content. The base additive is used to reduce the corrosive effects of the sulphuric acid fumes on the cylinder.

With large diesel engine installations, the lubricating oils are continuously filtered and purified to reduce the insoluble particles in the oil. Special strainers containing magnets are used to trap particles of carbon or iron particles. In large diesel engines, the carbon particles are byproducts of combustion while the iron particles comes from rubbing of gears, cams or other parts where wear down still occur.

The presence of these particles interferes with the lubrication of bearings, most of which contain soft white-metal coating. The particles can become embedded into the soft metal and cause abrasion of the metal parts.

The contents of this page are part of a page from my e-book “General Engineering Knowledge Notes” that will help candidates prepare for the Marine Certificate of Competency Examinations. This e-book is available for FREE downloading

Until next time…