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Establishing a Predictive Maintenance Program 345

limited data collector memory will severely hamper the program’s effectiveness.
Effective programs limit routine monitoring to a maximum of 800 lines of resolution.
This resolution will provide enough definition to detect incipient problems without
the negatives associated with higher resolutions.

To determine the impact of resolution, calculate the display capabilities of your
system. For example, a vibration signature with a maximum frequency (F max) of
1,000Hz taken with an instrument capable of 400 lines of resolution would result in
a display in which each line will be equal to 2.5Hz or 150 rotations per minute (rpm).
Any frequencies that fall between 2.5 and 5.0 (i.e., the next displayed line) would be
lost.

15.5.4 Defining Alert and Alarm Limits
The methods of establishing and using alert and alarm limits vary depending on the
particular vibration-monitoring system that you select. These systems usually use
either static or dynamic limits to monitor, trend, and alarm measured vibration. We
will not attempt to define the different dynamic methods of monitoring vibration sever-
ity in this book. We will, however, provide a guideline for the maximum limits that
should be considered acceptable for most plant mechanical equipment.

The systems that use dynamic alert and alarm limits base their logic (correctly in my
opinion) on the concept that the rate of change of vibration amplitude is more impor-
tant than the actual level. Any change in the vibration amplitude is a direct indication
that a corresponding change in the machine’s mechanical condition has occurred;
however, there should be a maximum acceptable limit (i.e., absolute fault).

The accepted severity limit for casing vibration is 0.628 inches per second, ips-Peak
(velocity). This unfiltered broadband value normally represents a bandwidth between
10 and 10,000Hz. This value can be used to establish the absolute fault or maximum
vibration amplitude for broadband measurement on most plant machinery. The
exception would be machines with running speeds below 1,200rpm or above
3,600rpm.

Narrowband limits (i.e., discrete bandwidth within the broadband) can be established
using the following guideline: Normally, 60 to 70 percent of the total vibration energy
will occur at the true running speed of the machine. Therefore, the absolute fault limit
for a narrowband established to monitor the true running speed would be 0.42
ips-Peak. This value can also be used for any narrowbands established to monitor
frequencies below the true running speed.

Absolute fault limits for narrowbands established to monitor frequencies above
running speed could be ratioed using the 0.42ips-Peak limit established for the true
running speed. For example, the absolute fault limit for a narrowband created to
monitor the blade-passing frequency of a fan with 10 blades would be set at 0.042 or
0.42 divided by 10. Narrowband designed to monitor high-speed components (i.e.,