Page 232 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 203

Where these values are exceeded, the user should experimentally verify the effect of
adding tip weights on blade static natural frequency. In one such test, identical
weights were clamped to the tips of two blades, one having an unweighted f,,, of 7.48
cps, and the other having an f,,, of 7.12 cps. The addition of this weight lowered the
f,, values of the two blade to 6.52 cps and 6.32 cps, respectively. This test proved
that the permanent bonding of equal weights into the blade tips could be considered
a viable fix for these blades, and would shift fnd into the “safe” range.
Blade stress investigations would follow. These experimental tests would require
that strain gauges be bonded to the most highly stressed portions of blade spar and
hub arms. Wires would have to connect with telemetry instrumentation located in the
center of the fan. This would allow the recording of alternating stresses of fan com-
pclnents exposed to vibration frequencies and vibration amplitudes of blades fitted
with weighted tips.

Reliability Reviews in Uprate Situations
In principle, uprate situations require at least the same diligent review of a manu-
facturer’s design as would the original review of rotating machinery being built from
the ground up. In addition to the thermodynamic and rotor-dynamics analyses, much
emphasis must be devoted to strength-of-materials criteria.
However, well-structured stress reviews can be rewarding, and have resulted in
very significant cost savings to process plants. The actual example of a large
mechanical-drive steam turbine in an overseas installation illustrates how one such
uprate review task was approached.

Overview

Steam turbines and centrifugal compressors are generally provided with standard-
ized shaft dimensions at their respective coupling ends. While this standardization
approach will, of course, result in stress levels within manufacturer’s allowable lim-
its for initially rated conditions of the equipment, the equipment owner may find the
maximum safe (or allowable) stress levels exceeded at some proposed future uprate
conditions.
At first glance, then, the equipment uprate would appear to require time-consuming
and often costly shaft replacements. However, closer examination of how the equip-
ment vendor arrived at his maximum allowable stress levels may show that such shaft
replacements can often be avoided without undue risk if the coupling selection is opti-
mized. This conclusion is based on the fact that gear-type couplings have the potential
of inducing in a shaft both torsional stresses and bending stresses, whereas diaphragm
couplings tend to primarily induce torsional stresses and insignificant bending stress-
es at best. Bending moments caused by couplings transmitting torque while mis-
aligned can be quite high and possibly contribute to bearing distress, seal wear, shaft-
fatigue stresses, shaft lateral vibrations, deflections, and whirl.