Up Front

Source of Leak is a Mystery
While working onboard a cargo ship, the crew encountered a challenge in the engine room. We were confused as to why the main propulsion engine leaked lube oil from the crankcase doors and relief valve seals at will.

There were times when no leaks were visible. Other times, from out of the blue, oil would seep from even the tiniest cracks in the casing.

Eventually we discovered that one of the engine crew members was leaving the engine room entrance door open. This amazingly caused the leak!

We made it a practice to keep our engine spaces under positive air pressure to ensure the turbochargers were never starved of air. This provided higher pressure outside the engine casing than inside. Leaving the door open caused a severe drop in engine room air pressure.

Additionally, the buildup of crankcase pressure increased through partial leakage past the piston rings (blowby). This resulted in the lube oil on the inside walls of the casing being forced out of the narrow openings.

This problem helped us to identify the leaks in the casing and take corrective actions quickly! (Submitted by Sujit Mukherjee. Thanks Sujit!)

Book Bits

Wear Debris Reflects Condition of the System From
"Wear Debris Analysis":

The whole concept of Wear Debris Analysis (WDA) is based on the idea that debris released from a system conveys the condition of that system. Take a simple example: if a hose is flexed and no debris is generated from it, then the hose is probably in excellent condition and likely to last a long time. However, if bits of hose are coming off the surface (either within the hose or on the outside), then the likelihood is that fracture will occur soon.

Today's Tip

When rolling element bearings are lubricated from oil held in a sump, attention to the oil sump level indicator will not only bring savings in terms of extended bearing life, but also will lead to reduced bearing temperatures due to churning and metal-to-metal contact resulting from possible oil starvation. As a general rule, the lowest rolling element should be half covered with the oil, when the bearing is stationary. This may seem like a simple rule to observe but it is suprising how often it is completely overlooked! (Submitted by Derek Peasley, FAG Bearings. Thanks Derek!)

Q & A
Filter Quality and Oil Analysis
"How does the quality of a filter impact oil analysis results (contamination and ferrous wear)? Is it correct to say that using filters with different beta ratios in same equipment will result in different analysis results? Is there any correlation in setting alarm levels and filter beta ratio?" - Erwin Sadhoe, Alcoa

The quality of the filter is directly proportional to the cleanliness and wear rate of the system. The higher the beta for the same micron pore size will result in a faster cleanup rate; thus, it is better able to control ingression of solids in the system. The main system ingression sources typically are seals, breathers and oil top-ups, and wear debris. When catalytic wear metals are removed from the oil, the lubricant will experience longer oxidative life.

Therefore, expect to see better cleanliness results on the hard particle measurements, as well as lower levels of wear debris in the system, assuming that the oil is in good condition and there are no other problems like water ingress or misalignment or imbalance.

In terms of setting alarm levels, the first step is to set the alarm levels to achieve a reasonable and realistic gain in lubricant and machine life. Using any of several methods, this can be done to optimize your requirements based on financial, safety, environmental and reliability goals.

On that basis, the appropriate filter can be selected. Most filter manufacturers have methods to determine the required element size and pore rating to not only meet the cleanliness target, but also to match the lubricant viscosities at operating temperature that would affect the flow rate. Most companies have established a beta ratio of 200 for their quoted pore size, although recent changes mean that they must also quote a beta value for a number of size ranges to establish performance. (See this article for more information.)

Over the life of the machine, a better-quality filter (generally more expensive) will prove to be more cost-effective than a cheaper filter owing to the relationship between filter performance and the system wear rates. To add value to that, many high-performance filters offer an associated improved dirt- holding capacity.

Martin Williamson, Senior Technical Consultant, Noria UK Ltd.

Book Bits

From the book "Machinery Failure Analysis and Troubleshooting":
A rather large number of factors influences lubricating oil degradation and, consequently, pump bearing life. If your centrifugal pumps are equipped with rolling element bearings, there is little doubt that medium viscosity turbine oils (ISO Grade 68) will perform better than the lighter oils originally specified by many pump manufacturers. But, by far, the most frequent cause of lube-oil-related failure incidents is water and dirt contamination. With only 20 ppm water in pure mineral oil, bearing surface and rolling element fatigue life is reduced by an incredible 48 percent. Although the fatigue life reduction is less pronounced with inhibited lubricants, there are always compelling reasons to exclude dirt and water from pump bearing housings. Lip seals are a poor choice for centrifugal pump installations demanding high reliability. Face seals represent superior, "hermetic" sealing and should be given serious consideration.

Today's Tip

Viscosity is not the only issue associated with high pressure drop across filters on start-up. It might also have to do with the fact that the oil does not have a high enough cloud point. For indoor equipment where filter elements are involved, this can be much more important than the pour point.

The cloud point is the point when you can first see wax coming out of solution. If you can see the wax then you know that the filter will also see it. In most cases the wax goes back into solution as the oil warms up. There is generally not a good correlation between cloud point and pour point because pour point depressants (an additive to lower the pour point) enter into the picture.

(Tip submitted by Ken Brown, Utility Service Associates)

Q & A

"We change many of our lubricants according to a schedule, ranging from six months to two years, depending on the lubricant and application. Because we use a scheduled oil change criteria (instead of condition-based oil change) we are uncertain as to whether the time interval we have chosen is optimum. How do we know if we should extend or shorten the scheduled interval?"

In many cases the original equipment builder can suggest helpful guidelines in this area. There may be useful technical support information available from your lubricant supplier as well. However, because a lubricant's service life has a lot to do with the stressing environment it is exposed to, the best way to tweak your oil change interval is to make actual assessments of its condition and remaining useful life. Unlike routine oil analysis, the type of tests you might want to select could be more similar to what you see on your lubricant's spec sheet.

Begin by talking to your lubricant supplier about which performance properties would need to be evaluated. These could include oxidation stability, rust/corrosion protection, air release and foam stability, demulsibility, antiwear protection, VI (Viscosity Index), silt particle concentration and other essential performance properties. Expect the cost of these tests to run several hundred dollars. However, you only have to run tests on samples from a few representative machines.

Obtain the sample just prior to the currently scheduled oil change. Send this sample along with a sample of the new lubricant (for baselining purposes) to a lab that can perform ASTM performance tests. You may need to obtain several hundred milliliters of fluid of each sample.

Once the tests are complete and an assessment made on remaining useful life a decision can then be made on whether the oil change interval can be shortened or lengthened. A reasonable safety margin needs to be included in the decision. Once changes in the drain interval are made monitor the oil carefully.