In This Issue:

• Up Front: Unusual Leakage Problem Solved

• Book Bits: Depth Type Filter Elements

• Today's Tip: Smart Valve Labeling

• Q & A: How Clean Is Your Grease?

Up Front

Unusual Leakage Problem Solved

We have a 25 MW generator that was experiencing a significant oil leak from the generator bearing housing labyrinth-type seal. The leaked oil was also entering the generator rotor causing damage to its windings.

A review of the maintenance log book indicated that the oil leak was heaviest whenever the oil reservoir's vapor extraction fan was out of service for maintenance. We now temporarily line up a mobile vapor extraction fan under these circumstances, as we believe that when vapor is allowed to collect unchecked, the bearing housing is becoming pressurized.

So far we have seen a reduction in oil accumulation at the generator's base, indicating a reduction in leakage. (Submitted by Manohar Nayak, Maintenance Manager, Mangalore Refinery & Petrochemicals. Thanks Mangalore!)

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Book Bits

Depth Type Filter Elements

From "The Practical Handbook of Machinery Lubrication".

Depth type filters are those whose elements are constructed of one of two groups of materials. One group of materials is referred to as "absorbent" and these consist of inactive materials such as cotton waste, waste paper, wound paper, cloth, wood pulp, asbestos, etc. These filter elements depend upon the absorption of contaminants as the oil flows through the media in long tortuous paths.

These filters will generally remove contamination and particulate in the range of 1-10 micrometers and are most often mounted in a parallel or "side stream" circuit, permitting a flow of 10-15% of the total oil volume. Because of this feature, these filters usually do not contain a by-pass valve. (A plugged depth type filter with no by-pass valve incorporated, mistakenly mounted in a full flow application, will eventually restrict or stop the flow of oil, resulting in a catastrophic failure).

Many poorly constructed absorbent depth filters are susceptible to a condition called channeling. Channeling is a condition whereby the oil flow through the media creates a "channel" or locates a path of least resistance. Once channeling occurs, effective filtration ceases.

Depth type absorbent filters will not remove oil additives (unless the additive is a solid lubricant such as graphite and particle sizes are in the size range which may prevent them from moving through the filter).

More information about "The Practical Handbook of Machinery Lubrication".

Today's Tip

• A good way to make sure valves are put in the proper operating position after maintenance is to affix permanent tags on the valves indicating their normal operating position. Wording such as "This valve normally open for operation" is very helpful. A pre-startup checklist will add another layer of protection. (Submitted by Bill Keeter, ARMS Reliability Engineers)
  

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Q & A

How Clean Is Your Grease?

"We have worked through setting and improving our oil cleanliness, but our mill does not have a cleanliness standard for greases. Should we assume that the greases have the same concentration of solid contaminants as the oils that we receive from our supplier? Is this even relevant to our reliability initiative?"

Grease lubricated bearings generally do not have the same level of reliability as oil lubricated bearings. One reason for this is our ability to filter oil before and during use in lubrication systems or sumps.

This of course cannot be done with grease. There are no broadly accepted standards or limits on hard particle contaminants in greases. The user is subject to the quality imposed by the grease manufacturer.

There are a few military standards for grease contaminant limits. MIL-G-81322 is used to rate aircraft grease and MIL-G-81937 is used to rate (ultra-pure requirement) instrumentation grease. The G-81322 rating limit still allows the manufacturer to leave enough solid contaminants (thickener, dirt, manufacturing debris, etc) that would equate to the amount of solid debris in an oil bath at an ISO cleanliness level of 19/16.

Industrial grade products do not receive the special filtration treatment that these MIL Spec products receive. It is likely that solid particle contaminations are higher in most industrial grade products.

This begs the question: are you filling your grease guns with lubricants that, measured by your ISO contaminant control expectations, are certain to cause poor reliability? Something to think about.

Mike Johnson, Senior Technical Consultant, Noria Corporation

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