Page 37 - Improving Machinery Reliability

Requirements Specification 9

Using a state-of-the-art balancing facility for “at-speed” evaluations has these
operational benefits:

Complete confidence that all rotor shaft deflections reach only minimal amplitudes
throughout the entire operating speed range
@ Improved rotor reliability
@ Full assurance in a smooth running rotor through its full-speed range
@ Increased bearing and seal lifetime
Extended operational life between scheduled maintenance turnarounds.
Needless to say, state-of-art “at-speed” balancing facilities can define rotor unbal-
ance response with utmost precision and can achieve balance qualities that we could
only dream of a few decades ago.
Returning to our data sheet topics, we note the entry “diaphragms.” Compressor
diaphragms are generally made of cast iron. With compressors becoming larger and
larger, obtaining sound cast-iron diaphragms is becoming progressively more diffi-
cult. Uneven cooling of very large diaphragm castings can set up intolerably high
residual stresses. A thorough experience check is needed.
The last arrow on data sheet page 3 points at the item “air run-in.” Here, the
owner’s representative will have to ask himself whether speed, driver horsepower,
discharge temperatures, and piping arrangement lend themselves to run-in on air.
Will it be necessary to run-in on helium? What questions need to be resolved for a
helium run? Cost? Availability? Leakage losses?
Data sheet page 4 (Figure 1-5) shows an arrow pointing at babbitt thickness. Cer-
tain babbitt types are stronger than others and permit higher loadings at the expense
of being less forgiving if dirt particles should enter the bearing. Conversely, the soft-
er babbitt may have less tolerance to high vibration or surge loading, but will pass
slightly larger dirt particles without undue risk. Consequently, the babbitt type
should be determined.
An up-to-date reliability professional may, at this point, explore the applicability
of, and vendor experience with, flexure pivotTM and magnetic bearings. Flexure
pivot bearings are produced by KMC, Inc., in West Greenwich, Rhode Island, and
Bearings Plus, Inc., in Houston, Texas. Their development was prompted by the fact
that with conventional tilting pad bearings, the high contact stresses between the
pads and bearing shell can cause brinelling at the pivot location. This pivot wear
increases the bearing clearance and reduces the bearing preload, thus altering the
operating characteristics and increasing the susceptibility to vibration problems.
Unloaded conventional tilting pads can also experience damage due to pad flutter.
Flexure pivot radial pads are integral to the bearing shell and therefore experience no
pivot wear. The relatively low rotational stiffness in the support webs is sufficient to
eliminate pad flutter in the unloaded pads.