In This Issue:

• Up Front: Take Our Survey - Get Your Reward

• Book Bits: Additives for Heavier Loads

• Today's Tip: A Crude Crackle Test
  

• Q & A: Varying Viscosities
  

Up Front

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Book Bits

Additives for Heavier Loads

From the book "Practical Lubrication for Industrial Facilities":

There are different degrees of severity under which boundary lubrication conditions prevail. Some are only moderate, others extreme. Boundary conditions are met by a variety of special lubricants with properties corresponding to the severity of the particular application. These properties are derived from various additives contained in the oil, some singly, some in combination with other additives. Their effect is to increase the load- carrying ability of the oil.

Where loads are only mildly severe, an additive of the class known as oiliness agents or film-strength additives is applicable. Worm-gear and pneumatic-tool lubricants are often fortified with these types of agents. Where loads are moderately severe, antiwear agents or mild EP additives are used. These additives are particularly desirable in hydraulic oils and engine oils. For more heavily loaded parts, a more potent class of additives is required; these are called extreme pressure (EP) agents.

Click here for more information about "Practical Lubrication for Industrial Facilities".

Today's Tip

• In a refinery, you can use a hot steam turbine case for a quick crackle test to assess the presence of water in oil. This works just as well as a hot plate. (Tip submitted by Randy Hardin, Reliability Tech, Phillips Petroleum Co.)

Each tip published will earn the sender $25. Click here to submit your tip.

Q & A

Varying Viscosities

"We are using an onsite viscometer to measure viscosity at 40 °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 onsite test equipment. However, like all onsite 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 onsite measurements may not correlate directly with the lab data.

First, most onsite test equipment actually measures absolute viscosity, but calculates the kinematic viscosity by dividing absolute viscosity by density. Because the onsite 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 onsite instrument does not heat the oil to 40 °C, and most do not, you are likely determining the oil's viscosity at the temperature of the onsite lab (typically in the 20°C - 25°C range), and extrapolating, again using a software algorithm to determine the viscosity at 40°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), used properly, onsite viscometers make very valuable additions to any onsite test program. As a general rule, always baseline your new oils using your own onsite viscometer to determine nonconforming used oil viscosities quickly and simply.

Mark Barnes, Senior Technical Consultant, Noria Corporation

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