Page 227 - Intro Predictive Maintenance

218 An Introduction to Predictive Maintenance

Inclusion of process parameters in a predictive maintenance program can be accom-
plished in two ways: manual or microprocessor-based systems. Both methods nor-
mally require installing instrumentation to measure the parameters that indicate the
actual operating condition of plant systems. Even though most plants have installed
pressure gauges, thermometers, and other instruments that should provide the infor-
mation required for this type of program, many of them are no longer functioning.
Therefore, including process parameters in your program will require an initial capital
cost to install calibrated instrumentation.

Data from the installed instrumentation can be periodically recorded using either
manual logging or with a microprocessor-based data logger. If the latter method is
selected, many of the vibration-based microprocessor systems can also provide the
means of acquiring process data. This should be considered when selecting the vibra-
tion-monitoring system that will be used in your program. In addition, some of the
microprocessor-based predictive maintenance systems can calculate unknown process
variables. For example, they can calculate the pump efficiency used in the example.
This ability to calculate unknowns based on measured variables will enhance a total-
plant predictive maintenance program without increasing the manual effort required.
In addition, some of these systems include nonintrusive transducers that can measure
temperatures, flows, and other process data without the necessity of installing per-
manent instrumentation. This technique further reduces the initial cost of including
process parameters in your program.

10.1 PUMPS
This section provides a general overview of the process parameters or failure
modes that should be a part of a viable inspection program. Design, installation,
and operation are the dominant factors that affect a pump’s mode of failure.
This section identifies common failures for centrifugal and positive-displacement
pumps.

10.1.1 Centrifugal Pumps
Centrifugal pumps are especially sensitive to: (1) variations in liquid condition
(i.e., viscosity, specific gravity, and temperature); (2) suction variations, such as
pressure and availability of a continuous volume of fluid; and (3) variations in
demand. Table 10–1 lists common failure modes for centrifugal pumps and their
causes.

Mechanical failures may occur for several reasons. Some are induced by cavitation,
hydraulic instability, or other system-related problems. Others are the direct result of
improper maintenance. Maintenance-related problems include improper lubrication,
misalignment, imbalance, seal leakage, and a variety of others that periodically affect
machine reliability.