In This Issue:

• Up Front: Oil Level Monitor Failure Costs $700,000

• Book Bits: How Erosive Wear Works

• Today's Tip: Oil Sampling Made Easy

• Q & A: A Little Dirt Can Go A Long Way
  

Up Front

Oil Level Monitor Failure Costs $700,000

We recently had a bearing failure on one of our waste gas fans. There were temperature and vibration monitoring devices on the bearing. There was also a low oil protection circuit in the system.

But this system was not fail-safe as it uses a magnet that floats to induce current in a coil when the oil level drops. This current closes a switch, which gives a low oil level alarm and eventually trips out the fan motor. The cables to the switch were damaged and when a leak developed in the system and the oil level in the tank dropped, the protective circuit did not work as planned. The switch remained open and we lost the bearing on the fan. Why? Because our temperature sensor was not pressed against the bearing and picked up the temperature increase too late.

The level switch was a normally open switch. So if the oil level is correct, no induction takes place in the coil and the switch remains open. Also, if there is a break in the circuit the switch will stay open even if the oil runs low as no current flows in the circuit. So it still says the system is healthy even when you have problems in the system.

To correct this problem, we modified all protective circuits to be fail-safe. Now the detection has constant current flow and this flow is disrupted either by low oil level or by any damage to the circuit. We estimate that the inclusive cost of this type of failure is $700,000 U.S.

Submitted by Thembelani Ngulube, Maintenance Superintendent, Mamatwan Mine. Thanks Thembelani!)

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Book Bits

How Erosive Wear Works

From "Understanding How Components Fail":

Erosive wear occurs when particles in a fluid or other carrier slide and roll at relatively high velocity against a surface. Each particle contacting the surface cuts a tiny particle from the surface. Individually, each particle removed is insignificant, but a large number of particles removed over a long period of time can produce staggering degrees of erosion. The classic example is the Grand Canyon of the Colorado River. Whenever the dirt particles carried by the river current come in contact with the relatively soft rock of the riverbed, small amounts of the rock are removed. Over millions of years this giant chasm has been cut through the rock by the erosive action of dirt particles in the river which flows rapidly in some places but is more placid in others.

Click here for more information about "Understanding How Components Fail".

Today's Tip

• If you collect a lot of oil samples from tight areas or hard to reach locations, obtain a battery-operated medical suction unit. Your sample bottle hand pump can be easily converted so the suction unit will draw the sample directly into the bottle as the hand pump did. Just install a new bottle and turn on the suction unit. You now can use both hands, and all your concentration to get the tubing into the oil in the sump. You can get these units from any medical supply store or a private ambulance service may sell a good used one. Mine is a DeVillbus unit. (Tip submitted by Bill Wallace, PdM, NB Power. Thanks Bill!)

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Q & A

A Little Dirt Can Go A Long Way

"You usually can't see the dirt in a fluid, whether its cleanliness is as dirty as an ISO Code of 23/21/18 or as clean as an ISO 14/12/9. Is there really that much difference between the two extremes?"

Yes! Because of the multipass nature of dirt in a fluid, at ISO 23/21/18, a 50-gpm oil pump will circulate almost 7000 pounds of abrasive dirt to your components each year. Conversely, at ISO 14/12/9, the same pump will deliver just 14 pounds of dirt to the components per year. All else held equal, the pump in the clean system will last 15 times longer!

The dirt you can't see with the naked eye is even more dangerous than the dirt you can see. The small stuff is often clearance-sized, meaning that it is just the right size to impede the blood cell-sized machine surface clearances and cause abrasion and surface fatigue. Likewise, the small particles remain entrained in the oil for a long period of time, even with just minimal agitation.

Adam Davis, Technical Consultant, Noria Corporation