Page 160 - Intro Predictive Maintenance

Vibration Monitoring and Analysis 151

Steady-state analysis techniques are based on acquiring vibration data when the
machine or process system is operating at a fixed speed and specific operating para-
meters. For example, a variable-speed machine-train is evaluated at constant speed
rather than over its speed range.

Steady-state analysis can be compared to a still photograph of the vibration profile
generated by a machine or process system. Snapshots of the vibration profile are
acquired by the vibration analyzer and stored for analysis. The snapshots can be used
to evaluate the relative operating condition of simple machine-trains, but they do not
provide a true picture of the dynamics of either the machine or its vibration profile.

Steady-state analysis totally ignores variations in the vibration level or vibration gen-
erated by transient events such as impacts and changes in speed or process parame-
ters. Instruments used to obtain the profiles contain electronic circuitry, which are
specifically designed to eliminate transient data.

In the normal acquisition process, the analyzer acquires multiple blocks of data. As
part of the process, the microprocessor compares each data block as it is acquired. If
a block contains a transient that is not included in subsequent blocks, the block con-
taining the event is discarded and replaced with a transient-free block. As a result,
steady-state analysis does not detect random events that may have a direct, negative
effect on equipment reliability.

Dynamic
While steady-state data provides a snapshot of the machine, dynamic or real-time data
provide a motion picture. This approach provides a better picture of the dynamics of
both the machine-train and its vibration profile. Data acquired using steady-state
methods would suggest that vibration profiles and amplitudes are constant, but this is
not true. All dynamic forces, including running speed, vary constantly in all machine-
trains. When real-time data acquisition methods are used, these variations are captured
and displayed for analysis.

Single-Channel
Most microprocessor-based vibration-monitoring programs rely on single-channel
vibration data format. Single-channel data acquisition and analysis techniques are
acceptable for routine monitoring of simple, rotating machinery; however, it is impor-
tant that single-channel analysis be augmented with multichannel and dynamic analy-
sis. Total reliance on single-channel techniques severely limits the accuracy of analysis
and the effectiveness of a predictive maintenance or reliability improvement program.

With the single-channel method, data are acquired in series or one channel at a time.
Normally, a series of data points is established for each machine-train and data are
acquired from each point in a measurement route. Although this approach is more than
adequate for routine monitoring of relatively simple machines, it is based on the