AND Logic in Bently Nevada Vibration System

One of the most important and confusing concepts in Bently Nevada is Normal AND vs True AND.

This is used in logic building.

Suppose there is one heavy rating machine in your plant and obviously it is expensive. We use special safety systems for these machines like vibration and temperature monitoring and protection systems so whenever there is an abnormality in vibration/temperature observed, then the machine should trip accordingly.

Bently Nevada Vibration System

Now assume the following logic:

High vibration in two vibration probes is used for machine trips. (The two probes must detect High vibration then only the machine will be tripped)

In the above case, what if 1 vibration probe goes to NOT OK? (NOT OK means the sensor probe failed or it is damaged). In this case, say only 1 probe detected it and as per logic, the machine should trip on high vibration of both probes, but the machine won’t trip because only 1 probe detected and the other probe is in a faulty state.

Now again think of another example where the process is more important and machines should not trip on spurious vibrations. Now here what if one vibration probe goes to NOT OK and the other detects high vibration?

Yes, it’s confusing!

Now here we have to understand the role of Normal AND and True AND.

Normal AND Logic

In Normal AND whenever a probe goes into the NOT OK state, it is taken out of logic, and logic is degraded.

Consider 2 out of 2 logic.

In the 2oo2 logic, if the Probe 1 detects high vibration and Probe 2 is NOT OK

Then logic becomes 1 out of 1 considering Probe 1 as input.

This type of configuration is used when the machine is more important than the process.

True AND Logic

In True AND whenever a probe goes into the NOT OK state, the logic remains the same.

Consider 2oo2 logic again.

In two-out-two logic, if Probe 1 detects high vibration and Probe 2 is NOT OK.

Then machine won’t trip even if Probe 1 detects high vibration.

This type of configuration is used when the process is more important than the machine.

This logic is driven through a relay which is normally de-energized (can be changed to normally energized also).

Normal AND Truth table

The below table shows the different possible scenarios using Normal AND logic in the Bently Nevada condition monitoring system.

Vibration Probe 1	Vibration Probe 2	Relay Status
Normal Vibration	Normal Vibration	De-energized
Normal Vibration	NOT OK	De-energized
NOT OK	Normal Vibration	De-energized
High Vibration	NOT OK	Energized
NOT OK	High Vibration	Energized
NOT OK	NOT OK	De-energized
High Vibration	High Vibration	Energized

True AND Truth table

The below table shows the different possible scenarios using True AND logic in the Bently Nevada condition monitoring system.

Vibration Probe 1	Vibration Probe 2	Relay Status
Normal Vibration	Normal Vibration	De-energized
Normal Vibration	NOT OK	De-energized
NOT OK	Normal Vibration	De-energized
High Vibration	NOT OK	De-energized
NOT OK	High Vibration	De-energized
NOT OK	NOT OK	De-energized
High Vibration	High Vibration	Energized

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