Page 348 - Intro Predictive Maintenance

Establishing a Predictive Maintenance Program 339

nonrepetitive or spurious data can be introduced into the database. The microproces-
sor should be able to acquire multiple blocks of data, average the total, and store the
averaged value. Basically, this approach enables the data acquisition unit to automat-
ically reject any spurious data and provide reliable data for trending and analysis.
Systems that rely on a single block of data will severely limit the accuracy and repeata-
bility of acquired data. They will also limit the benefits that can be derived from the
program.

The microprocessor should also have electronic circuitry that automatically checks
each data set and block of data for accuracy and rejects any spurious data that may
occur. Auto-rejection circuitry is available in several of the commercially available
systems. Coupled with multiple block averaging, this auto-rejection circuitry ensures
maximum accuracy and repeatability of acquired data. A few of the microprocessor-
based systems require the user to input the maximum scale that is used to acquire data.
This will severely limit the accuracy of data.

Setting the scale too high will prevent acquisition of factual machine data, whereas
too low a setting will not capture any high-energy frequency components that may be
generated by the machine-train. Therefore, the microprocessor should have auto-
scaling capability to ensure accurate data. Vibration data can be distorted by high-
frequency components that fold over into the lower frequencies of a machine’s sig-
nature. Even though these aliased frequency components appear real, they do not exist
in the machine. Low-frequency components can also distort the midrange signature
of a machine in the same manner as high frequency. The microprocessor selected for
vibration should include a full range of anti-aliasing filters to prevent distortion of
machine signatures.

The features illustrated in the example also apply to nonvibration measurements. For
example, pressure readings require the averaging capability to prevent spurious read-
ings. Slight fluctuations in line or vessel pressure are normal in most plant systems.
Without the averaging capability, the microprocessor cannot acquire an accurate
reading of the true system pressure.

Alert and Alarm Limits. The microprocessor should include the ability to automati-
cally alert the user to changes in machine, equipment, or system condition. Most of
the predictive maintenance techniques rely on a change in the operating condition of
plant equipment to identify an incipient problem. Therefore, the system should be able
to analyze data and report any change in the monitoring parameters that were estab-
lished as part of the database development.

Predictive maintenance systems use two methods to detect a change in the operating
condition of plant equipment: static and dynamic. Static alert and alarm limits are pre-
selected thresholds that are downloaded into the microprocessor. If the measurement
parameters exceed the preset limit, an alarm is displayed. This type of monitoring does
not consider the rate of change or historical trends of a machine and therefore cannot
anticipate when the alarm will be reached.