How Long Should Oil Be Filtered?

Today's Tip: How Long Should Oil Be Filtered?

When filtering oil it is a good rule of thumb to run your filtration unit until the total flow through the filters equals the volume of seven times the reservoir capacity. Because of differences in oil viscosities and the bypass valve used to keep the filter pressure in the acceptable range you may not be able to use rated pump flow rates to determine how long it will take to reach this goal. We had to add flow meters on our filtration skid so we would know how much oil had actually gone through the filters. Now we are sure that we are getting our minimum level of acceptable filtration. (Submitted by Bill Jacobyansky, Maintenance Manager, Guardian Industries. Thanks Bill!)

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From the "Rolling Bearings Handbook and Troubleshooting Guide".

When equipment is to be stored, the bearings and the housings should be coated with a good rust preventive and totally filled with grease. The entire housing should then be wrapped with close-conforming plastic sheeting. This will protect bearings and steel cages for four years, and bearings with bronze cages for three years. When the equipment is ready to be put in service, all of the rust preventive and the storage grease must be removed from the bearing housing. The bearing can them be lubricated with grease or oil, depending on the application.

Q & A: Viscosity Results Can Vary

"We are using an on-site viscometer to measure viscosity at 40 degrees C. At the same time, our lube supplier is testing samples from the same system regularly. However, our viscosity numbers are often up to 10 percent different from the lube suppliers. What are we doing wrong?"

Because the viscosity of an oil is probably its single most important property, it makes sense to measure viscosity frequently, using on-site test equipment. However, like with all on-site equipment, it’s important to understand how these instruments work and their relative strengths and weaknesses.

When it comes to viscosity, there are two determinable parameters, absolute and kinematic viscosity. Kinematic viscosity measures the resistance of an oil to flow and shear under gravity, such as oil flowing through a funnel. Absolute viscosity, on the other hand, determines an oil’s internal resistance to flow and shear. To visualize absolute viscosity, imagine the force needed to stir an oil using a metal rod.

The viscosity reported by your lube supplier and oil analysis lab is likely the kinematic viscosity, as determined by ASTM D445. There are two main reasons why your on-site measurements may not correlate directly with the lab data.

First, most on-site test equipment actually measures absolute viscosity, but calculates the kinematic viscosity by dividing absolute viscosity by density. Because the on-site viscometers don’t actually measure density but rather estimate it from the oil’s spec sheets, an error can occur when translating absolute viscosity into kinematic viscosity. The measure of the absolute viscosity is correct, but because the density of the oil is only estimated, the conversion to kinematic viscosity can become overstated. Contamination and oxidation, among other things, can cause a rise in the density of used oil.

Second, if your on-site instrument does not heat the oil to 40 degrees C, and most do not, you are likely determining the oil's viscosity at the temperature of the on-site lab (typically in the 20 degrees C to 25 degrees C range), and extrapolating, again using a software algorithm to determine the viscosity at 40 degrees C. This extrapolation can also introduce errors into the reported measurement due to changes in viscosity index of the used oil.

Despite these limitations (which are actually not negatives), when used properly, on-site viscometers make very valuable additions to any on-site test program. As a general rule, always baseline your new oils using your own on-site viscometer to determine nonconforming used oil viscosities quickly and simply.

Mark Barnes, Noria Corporation