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While driving your car, the car sputters to a stop and the fuel gauge reads empty.

These are fairly strong indicators of a problem. Each indication is separate and individual, but taken together, you are pretty sure you have run out of gas.

What you’ve just done is perform correlation analysis. It’s defined as the use of more than one related analysis and/or technology to confirm a condition. It is the second analysis type covered in this Coach series.

You usually find correlation analysis used in conjunction with predictive maintenance technology. It’s ideal for those applications. There, one technology can indicate a problem area. The second or third is used to further identify and isolate the problem for subsequent repair.

For instance, vibration, oil wear particle, thermal, and filtration/debris technologies can effectively be used together to analyze equipment health of a gearbox. New CMMS functionality can now perform this type of analysis across technologies. 

You can use predictive correlation analysis with the heat exchanger example we talked about in the last issue (visit Coach Oas' archives on this site to refresh your memory).

Say we are monitoring its performance by tracking the temperature differential (out minus in) across the heat exchanger. Our monitoring shows that the temperature differential is changing, but it doesn’t tell us why.

The reason can be hard to identify because heat load, cooling water flow, fouling and total heat transfer all play a part and are related to one another. Together, they determine the temperature difference across the heat exchanger.

For example, fouling of the tubes’ surfaces increases the tubes’ thermal resistance to heat transfer. That, in turn, limits the amount of heat removed by the water passing through the tubes, which should cause the temperature of the cooling water to decrease. However, blocking of the tubes reduces cooling water flow rate and, subsequently, the amount of water available to remove heat from the hotter fluid.

With the reduced flow rate, the coolant stays in contact with the tubes for a longer period of time, collecting more heat. Therefore, the coolant outlet temperature will rise. All these factors create the need for a second verification or correlation of the problem’s cause.

The verification that fouling is the problem is done through the use of flow analysis.

To better understand the condition of the heat exchanger, utilize the correlation between the flow and thermal analysis. When the heat exchanger started to foul, the flow decreased and the temperature differential of the cooling water increased. Both the decrease in flow and the increase in temperature differential are indications that the heat exchanger is fouling.

Either analysis could stand on its own, but taken together, they are more forceful and credible. Together, they provide a strong picture of what is really happening and will happen to the heat exchanger in the future.

The real advantage to you from this type of analysis comes from the ability to identify and correct the real problem when the second technology or analysis confirms the results of the first.

Arne Oas is the senior maintenance consultant at Management Resources Group. If you have a maintenance management software question, contact Coach Oas at , or e-mail .

This article appeared in the December 2001/January 2002 issue of MRO Today magazine. Copyright, 2002.

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