Regular sampling and analysis of fuel, lubricating and hydraulic oils will enable maintenance engineers to assess the condition and performance of their machinery oils over time. This allows critical adjustments to be made to ensure maximum operational performance and efficiency. One of the key indicators for oil condition is viscosity, the management and control of which is essential to meet the daily operational requirements of the vessel.
The principle objective of lubrication is to produce a state known as fluid film, or hydrodynamic lubrication. This is the ideal situation created when moving parts to be lubricated are completely separated by a film of lubricant. Viscosity is the most important characteristic of an oil. It is the viscosity that determines the oil's resistance to flow and the load-carrying capability of any bearing contact.
Due to surface roughness (or asperities), machined surfaces are far from perfect, and the separation of the surfaces is entirely dependent on the lubricating oil and its viscosity. The correct film thickness prevents metallic contact, scuffing, micro-welding and wear of sliding surfaces, and is dependent upon the lubricant's viscosity.
Viscosity is defined as the measure of a fluid's resistance to flow and is measured 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 at a known temperature.
The usual measurement unit for viscosity is the centiStoke (cSt); one cSt equals one mm2/s.
Dynamic (absolute) viscosity is usually measured by a rotating spindle viscometer and is commonly expressed in centiPoise (cP); one cP equals one mPa.s
An oil's viscosity can be affected by a number of external factors, the most direct of which are changes in temperature during machinery operation. A lubricant's viscosity will decrease with an increase in temperature and will rise when the temperature drops.
The rate at which temperature-related increases and decreases occur depends upon the oil's 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, because the working temperature increases more than an oil with a lower VI.
Table 1 shows the viscosity temperature relationship of a range of commonly used ISO and SAE grades.
Note that there is a relationship within the ISO naming standard, that is, the average viscosity of an ISO 32 oil at 40°C is 32 mm2/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 equivalent to that of an ISO 150, as far as viscosity is concerned.
Changes in Viscosity Viscosity Decreases - Ingress of Distillate FuelThe ingress of distillate fuel is one of the most common causes for a decrease in viscosity. A decrease in viscosity is significant because 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, such as 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 percent conventional distillate diesel fuel within the sump of auxiliary generators in marine service. 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 of about 100 cSt to 30 mm2/s at 40°C. This would be thinner than a typical 10W diesel engine oil. A reduction in viscosity to 30 mm2/s at 40°C is enough to render the oil film thickness inadequate and create major machinery damage.
Shear ThinningShear thinning of a lubricant can also cause a rapid drop in the viscosity. Shear thinning is the term used to describe the degradation of the VI polymers used in most multigrade, high VI oils.
The use of such polymers is common practice in, for example, hydraulic oils developed for ship deck equipment on international trading vessels and any other outdoor hydraulic applications in cooler climates, 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 two ISO viscosity grades in a matter of several hours of operation. This is because these systems are made up of many different components, each of which subjects the lubricant (and polymer) to a variety of extreme shearing (cutting) conditions.
Viscosity Increases - ContaminationIncreases in viscosity are generally caused by contamination by soot and other insoluble by-products of combustion, oil oxidation and the ingress of heavy residual fuel oil.
If a regular machinery condition- monitoring program 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; however with regular oil testing, correct intervals of maintenance and good housekeeping, this can be minimized.
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 MixingAn adverse mixture of products frequently occurs during delivery of lubricants to a vessel or during transfers onboard a ship. Mixing of different oils can increase or decrease the viscosity of to a level that renders the product unusable. It is extremely important to carefully check that the product delivered is the product that was ordered, and that it is 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 in marine applications 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 fuel's quality but is essential information when ordering and bunkering, particularly in older vessels because the ship's fuel treatment plant may be limited in its ability to heat the fuel and therefore 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 viscosity testing equipment from Kittiwake are examples of such equipment. Designed for use within the harsh marine or industrial environment, they are suitable to be used with oil from a wide variety of applications including diesel engines, gas turbines, gear boxes, hydraulics and fuel oils.
These viscosity testing solutions vary in sophistication from the basic go/no-go type to the most advanced, laboratory standard heated viscometers. An overview of each type of equipment is listed in Table 2.
Table 2. Viscosity Testing Equipment
Heated and Unheated Viscometers
Viscosity is measured with a viscometer and is determined by the time it takes a fixed quantity of oil to flow through an orifice or capillary under defined laboratory conditions. Because the volume (and thus head pressure) of oil is controlled in a viscometer, an oil of lower density will take longer to flow through the orifice than a higher density oil with the same absolute viscosity. This is why the kinematic viscosity is used as the common measure and reported number, because this is the absolute viscosity divided by the density.
The Viscotube (Figure 1) is a device for quick and simple testing of hydraulic oils, lubrication systems and fuel oils. The Viscotube is light, easy to use and offers an acceptable, or medium, 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 40°C and with the Viscotube, the samples can be tested on-site for later input into the viscosity calculation software 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 provides a basic indication of viscosity change. With a go/no-go result, the Viscostick (Figure 2) provides a quick analysis for used and new oils.
The Viscometer (Figure 3) and Viscotube provide viscosity readings directly to the user in centistokes, allowing the engineer to interpret the results to make an informed technical decision regarding the condition of the fuel or lubricating oil.
On-site MonitoringFrequent and consistent checking of oil viscosity enables engineers and other personnel to reduce wear, improve machinery operating performance and minimize both expenditure and risk. Making routine onboard viscosity analysis part of a wider oil condition-monitoring program promotes a greater knowledge among the crew of oil and machinery condition onboard, which in turn, adds value to the company's operations strategy.
