Page 88 - An Introduction To Predictive Maintenance

78 An Introduction to Predictive Maintenance

Bearings

Turbines use both rolling-element and Babbitt bearings. Narrowbands should be estab-
lished to monitor both the normal rotational frequencies and failure modes of the
specific bearings used in each turbine.

Blade Pass
Turbine rotors consist of a series of vanes or blades mounted on individual wheels.
Each of the wheel units, which are referred to as a stage of compression, has a dif-
ferent number of blades. Narrowbands should be established to monitor the blade-pass
frequency of each wheel. Loss of a blade or flexing of blades or wheels is detected
by these narrowbands.

Mode Shape (Shaft Deflection)
Most turbines have relatively long bearing spans and highly flexible shafts. These
factors, coupled with variations in process flow conditions, make turbine rotors highly
susceptible to shaft deflection during normal operation. Typically, turbines operate in
either the second or third mode and should have narrowbands at the second (2X) and
third (3X) harmonics of shaft speed to monitor for mode shape.

Speed
All turbines are variable-speed drivers and operate near or above one of the rotor’s
critical speeds. Narrowbands should be established that track each of the critical
speeds defined for the turbine’s rotor. In most applications, steam turbines operate
above the first critical speed and in some cases above the second. A movable nar-
rowband window should be established to track the fundamental (1X), second (2X),
and third (3X) harmonics of actual shaft speed. The best method is to use orders analy-
sis and a tachometer to adjust the window location.

Normally, the critical speeds are determined by the mechanical design and should not
change; however, changes in the rotor configuration or a buildup of calcium or other
foreign materials on the rotor will affect them. The narrowbands should be wide
enough to permit some increase or decrease.

5.2 INTERMEDIATE DRIVES
Intermediate drives transmit power from the primary driver to a driven unit or units.
Included in this classification are chains, couplings, gearboxes, and V-belts.

5.2.1 Chains
In terms of its vibration characteristics, a chain-drive assembly is much like a gear
set. The meshing of the sprocket teeth and chain links generates a vibration profile
that is almost identical to that of a gear set. The major difference between these two