Page 278 - Intro Predictive Maintenance

Operating Dynamics Analysis 269

Many predictive maintenance programs are limited to simple trending of vibration,
infrared, or lubricating oil data. The perception that a radical change in the relative
values indicates a corresponding change in equipment condition is valid; however,
this logic does not go far enough. The predictive analyst must understand the true
meaning of a change in one or more of these relative values. If a compressor’s vibra-
tion level doubles, what does the change really mean? It may mean that serious
mechanical damage has occurred, but it could simply mean that the compressor’s load
was reduced.

A machine or process system is much like the human body. It generates a variety of
signals, like a heartbeat, that define its physical condition. In a traditional predictive
maintenance program, the analyst evaluates one or a few of these signals as part of
his or her determination of condition. For example, the analyst may examine the vibra-
tion profile or heartbeat of the machine. Although this approach has some merit, it
cannot provide a complete understanding of the machine or the system’s true operat-
ing condition.

When a doctor evaluates a patient, he or she uses all of the body’s signals to diagnose
an illness. Instead of relying on the patient’s heartbeat, the doctor also uses a variety
of blood tests, temperature, urine composition, brainwave patterns, and a variety of
other measurements of the body’s condition. In other words, the doctor uses all of the
measurable indices of the patient’s condition. These data are then compared to the
benchmark or normal profile for the human body.

Operating dynamics is much like the physician’s approach. It uses all of the indices
that quantify the operating condition of a machine-train or process system and eval-
uates them using a design benchmark that defines normal for the system.

13.1.3 Influence of Process Variables
In many cases, the vibration-monitoring program isolates each machine-train or a
component of a machine-train and ignores its system. This approach results in two
major limitations: it ignores (1) the efficiency or effectiveness of the machine-train
and (2) the influence of variations in the process.

When the diagnostic logic is limited to common failure modes, such as imbalance,
misalignment, and so on, the benefits derived from vibration analyses are severely
restricted. Diagnostic logic should include the total operating effectiveness and effi-
ciency of each machine-train as a part of its total system. For example, a centrifugal
pump is installed as part of a larger system. Its function is to reliably deliver, with the
lowest operating costs, a specific volume of liquid and a specific pressure to the larger
system. Few programs consider this fundamental requirement of the pump. Instead,
their total focus is on the mechanical condition of the pump and its driver.

The second limitation to many vibration programs is that the analyst ignores the
influence of the system on a machine-train’s vibration profile. All machine-trains are