Page 280 - Intro Predictive Maintenance

Operating Dynamics Analysis 271

To be effective, predictive analysts must have a thorough knowledge of machine/
system design and machine dynamics. This knowledge provides the minimum base
required to effectively use predictive maintenance technologies. Typically, a graduate
mechanical engineer can master this basic knowledge of machine design, machine
dynamics, and proper use of predictive tools in about 13 weeks of classroom training.
Nonengineers, with good mechanical aptitude, will need 26 or more weeks of formal
training.

13.1.5 Understanding Machine Dynamics
It Starts with the Design
Every machine or process system is designed to perform a specific function or range
of functions. To use operating dynamics analysis, one must first fully understand how
machines and process systems perform their work. This understanding must start with
a thorough design review that identifies the criteria that were used to design a machine
and its installed system. In addition, the analyst must also understand the inherent
weaknesses and potential failure modes of these systems. For example, consider the
centrifugal pump.

Centrifugal pumps are highly susceptible to variations in process parameters, such as
suction pressure, specific gravity of the pumped liquid, back-pressure induced by
control valves, and changes in demand volume. Therefore, the dominant reasons for
centrifugal pump failures are usually process related.

Several factors dominate pump performance and reliability: internal configuration,
suction condition, total dynamic pressure or head, hydraulic curve, brake horsepower,
installation, and operating methods. These factors must be understood and used to
evaluate any centrifugal pump-related problem or event.

All centrifugal pumps are not alike. Variations in the internal configuration occur in
the impeller type and orientation. These variations have a direct impact on a pump’s
stability, useful life, and performance characteristics.

There are a variety of impeller types used in centrifugal pumps. They range from
simple radial-flow, open designs to complex variable-pitch, high-volume enclosed
designs. Each of these types is designed to perform a specific function and should be
selected with care. In relatively small, general-purpose pumps, the impellers are nor-
mally designed to provide radial flow, and the choices are limited to either enclosed
or open design.

Enclosed impellers are cast with the vanes fully encased between two disks. This type
of impeller is generally used for clean, solid-free liquids. It has a much higher effi-
ciency than the open design. Open impellers have only one disk, and the opposite side
of the vanes is open to the liquid. Because of its lower efficiency, this design is limited
to applications where slurries or solids are an integral part of the liquid.