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How Often Do You Change Oil?

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

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

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 at

Until next time...

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About The Author Many years of working experience in Marine, Facilities, Construction has given the author material for writing e-books and articles related to engineering, and management. Subscribe to facworld ezine at mailto:facworld-subscribe@yahoogroups.comMore information at and
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