Save time, reduce maintenance costs, decrease down time and increase profit all from a few simple tests.  Sound good?  Then read on

As machined surfaces are not perfect, the separation of all disparities, and hence the prevention of metallic contact, scuffing, micro-welding and wear of sliding surfaces, is dependent upon the lubricants viscosity.

Viscosity is defined as the measure of a fluid’s resistance to flow and is expressed in two ways, kinematic or dynamic (also known as absolute) viscosity. The more common expression is kinematic viscosity, which is measured by the time taken for a fixed volume of oil to flow through a capillary tube.

The usual measurement unit for viscosity is the centiStoke (cSt); one cSt = mm2/s

Dynamic (absolute) viscosity is usually measured by a rotating viscometer and is commonly expressed in centiPoise (cP). 1 cP = 1 mPa.s

An oils viscosity can be affected by a number of external factors, the most direct of which is changes in temperature during machinery operation.  A lubricants viscosity will decrease with an increase in temperature and will increase with a temperature reduction. 

The rate at which temperature related increases and decreases occur is dependent upon the oils Viscosity Index (VI).  The VI is an arbitrary scale used to measure a fluid’s change in viscosity with changes in temperature. An oil with a high VI experiences smaller decreases in viscosity as the working temperature increases than an oil with a lower VI.

Comparison of Oil Viscosities

The figures in the table below show the viscosity temperature relationship of a range of commonly used ISO and SAE grades.

 ISO 32* ISO 46* SAE 30 SAE 40 ISO 220*
Vis. @ 40oC 28.8 - 35.2 41.4 – 50.6 80 - 130 120 - 185 198 - 242
Vis. @ 100oC 3.9 - 4.5 6.2 – 7.0 10.1 – 13.6 12.2 – 16.6 17.5 - 20
*Approximate viscosity, mm2/s for 95 VI mineral oil.

It is notable that there is a relationship within the ISO naming standard i.e. the average viscosity at 40oC of an ISO 32 oil is 32mm2/s.  This is not the case with the SAE classification used for engine oils. As a general rule, an SAE 30 grade would be equivalent to an ISO 100 and an SAE 40 to that of an ISO 150, so far as viscosity is concerned.

Viscosity Changes – Identifying the Culprits

Viscosity Decreases - Ingress of distillate fuel

The ingress of distillate fuel is one of the most common causes for a decrease in viscosity.  A decrease in viscosity is significant as there will come a point when the lubricant is no longer sufficient to maintain a fluid film. This ingress tends to occur due to a mechanical fault within the machine e.g. leaking fuel pumps, injectors and seals on a diesel engine.  Decreases can also be caused by topping up with an incorrect oil grade.
It is not uncommon to find contamination levels of up to 30% distillate fuel within the sump of auxiliary generators. There are also several other important safety issues related to this. This level of contamination by a typical viscosity marine gas oil could easily reduce an SAE 30 grade viscosity                            to 30 mm2/s. A reduction in viscosity to 30 mm2/s @ 40oC is enough to render the oil film thickness inadequate and to begin the onset of major machinery damage. 

Shear Thinning

Shear thinning of a lubricant can also give a rapid drop in the viscosity. Shear thinning is the term used to describe the degradation of the Viscosity Improving polymers used in most high VI oils. 

The use of such polymers is common practice in, for example, hydraulic oils developed for deck equipment on internationally trading vessels, as the reduced tendency for viscosity changes in such oils makes them suitable for both temperate and tropical climates. 

However oils containing these viscosity improving polymers must be correctly matched to the machinery, as certain onboard hydraulics systems will reduce a VI improved oil by as much as 2 ISO viscosity grades in a matter of several hour’s operation.  This is due to these systems being made up of many different components, each of which will subject the lubricant (and polymer) to a variety of extreme conditions.

Viscosity Increases - Contamination

Increases in viscosity are generally caused by contamination by soot and other insoluble by-products of combustion, oil oxidation and the ingress of residual fuel oil. 

If a regular machinery condition monitoring programme and maintenance schedule are adhered to, these problems are greatly reduced.  However in some high powered heavily loaded modern engines, the total insolubles entering the oil are far greater than those seen under normal operating conditions. Under these circumstances the oil life will be considerably reduced, but with regular oil testing, correct intervals of maintenance and good housekeeping, this can be minimised.

In the system oil of a crosshead diesel engine, a viscosity increase may be due to residual fuel contamination or perhaps more likely the leakage of cylinder oil drains into the sump via the stuffing box.  This situation can normally be identified by the trend of increased Base Number (from the higher BN cylinder lubricant) that accompanies the viscosity increase.

Product mixing

An admixture of products can and frequently does occur during delivery of lubricants to a vessel or during transfers onboard.  Mixing of different oils can raise or lower the viscosity of to a level that renders the product unusable.  It is extremely important that care is taken to check that the product delivered is the product that was ordered and that it is then loaded into the correct tank.  Onboard transfer of product should be carried out by personnel with a good knowledge of all lubricant storage, transfer and supply systems.

Viscosity Measurement
Knowledge of fuel oil viscosity is important for verifying the correct grade of fuel is delivered, for calculating combustion performance, and for ensuring the correct adjustment of fuel handling and injection systems. Viscosity in fuels gives no indication of the fuels quality but is essential information when ordering and bunkering, particularly in older vessels as the ships fuel treatment plant may be limited in its ability to heat the fuel and hence reduce its viscosity to a level suitable for the engine injection rail.

To ensure that fuel and lube oil viscosities are measured correctly, specific instrumentation should be used. The Fuel and Lube Oil Viscometer from Kittiwake is one example of such equipment. It has been designed specifically for use within the harsh marine environment and is suitable to be used with oil from a wide variety of applications including diesel engines, gas turbines, gear boxes, hydraulics and fuel oils.
Viscosity testing solutions from Kittiwake vary in sophistication from the basic go – no go style type through to the most advanced, laboratory standard heated viscometers. An overview of each type can be seen in the table:
Viscometer Type Application Area Technical Accuracy Test Time
Viscostick Lubes Only Low (Go/No Go) Slow*
Viscotube Lubes & Heavy Fuel Oils (heated) Medium (using free software) Medium*
Heated Viscometer Fuel & Lubes High (Laboratory Standard) Fast
*Oil temperature must be allowed to stabilise before any testing commences.  Test time is Viscosity dependant – usually 30 seconds to 3 minutes.
Kittiwake Heated / Unheated Viscometers
Viscosity is measured in a viscometer and is determined by the time taken for a fixed quantity of oil to flow through an orifice or capillary under defined laboratory conditions. Because the head of oil is controlled in a viscometer, an oil of lower density will take longer to flow through the orifice than that of a higher density oil with the same absolute viscosity. This is why the kinematic viscosity is used as the common measure and reported number, as this is the absolute viscosity divided by the density.
Viscotube
The Kittiwake Viscotube is a device suitable for quick and simple testing of hydraulic oils, lubrication systems and fuel oils.  The Viscotube is light, easy to use and offers a high degree of accuracy for an unlimited number of tests, while its price ensures an effective return on investment after only a few tests.
The standard unit of measure for oil is in centistokes at 40C and with the Viscotube, the samples can be tested on-site for later input into the viscosity calculation software that is provided with the set. Weighing only 0.5 kg, the Viscotube can be easily carried to the sampling points throughout the plant, site or ship.
Viscostick
The ECON Viscosity test from Kittiwake provides a basic indication of viscosity change. Typically it will detect 5-10% distillate fuel dilution of an SAE 30 to 40 engine oil.  With a go / no-go result the stick is a quick analysis between old and new oils.
With the Viscometer and Viscotube devices, viscosity readings are provided directly to the user in centistokes and the results can be interpreted against information provided, allowing the engineer to make an informed technical decision regarding the condition of the fuel or lubricating oil.
Onsite Monitoring

Essentially, frequent and consistent checking of oil viscosity will enable the engineers to reduce wear, improve machinery operating performance and minimise both expenditure and risk.  Making routine onboard viscosity analysis a part of a wider oil condition monitoring programme promotes a greater knowledge among the crew of oil and machinery condition onboard, which in turn should add value to the companies operations strategy.   Contact Kittiwake for more onsite and online oil testing solutions.

 
ENDS
For further information, please contact:
Paul Froome - Marketing Manager
Kittiwake Developments Limited
Tel: 01903 731470, Fax: 01903 731480
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.
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