Page 159 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 131

d. Graphical display of exciting sources (revolutions per minute of any gear in
the train, etc.)
e. Graphical display off torsional critical speeds and deflections (mode-shape
diagrams)
Torsional critical speeds coinciding with the running speeds of rotating ele-
ments in a turbine-gear-compressor or motor-gear-compressor train can cause
oscillatory forces of such magnitude as to drastically shorten component life.
The data listed are required to determine the probability of speed coincidence,
and should coincidence exist, they will allow calculation of resulting stresses.
The purchaser may opt to duplicate the manufacturer’s torsional analysis with
in-house or outside resources. Alternatively, he may arrange for a field test of
actual torsional stresses.
23. Transient torsional analysis for all synchronous motor-driven units
Transient, momentary torsional stresses on synchronous motors can be
extremely severe and have been responsible for a number of catastrophic fail-
ures. The vendor should submit his analysis for review by the purchaser or his
consultants.
24. Allowable flange loading (not to be exceeded by piping forces and moments)
These fforces and moments can be readily calculated by computers, and vir-
tually all contractors now employ this analysis tool. Correctly used, it will
ensure that equipment flange loadings remain within acceptable limits not
only under all foreseeable operating conditions but also while spare equipment
connected to the same piping system is temporarily removed for maintenance.
25. An alignment diagram, including recommended limits during operation
Cold-alignment offset calculations are to be reviewed for accuracy and
appropriateness of manufacturer’s assumptions. These data are then used for
initial cold alignments (via reverse indicator readings).
26. Weld procedure
These procedures are commonly reviewed by the purchaser’s metallurgy
specialists. Improper procedures have been responsible for commissioning
delays and serious failures. A review of weld procedures can encompass pip-
ing, vessels, machinery casings, and even fan-blade spares.
27. Hydrostatic test logs
Together with weld procedures, hydrostatic test logs should become part of
the inspection record system of modern process plants.
28. Mechanical run test logs, including:
a. Oil flows and temperatures
b. Vibration
c. Bearing-metal temperatures
d. Actual critical speeds
These test logs should provide verification for all predicted values. If audits
and reviews have been properly conducted, the mechanical run tests will, at
best, uncover vendor quality-control errors. Deep-seated design errors should
not surface at this stage in the job execution. The mechanical run test can pro-
vide typical target values for comparison with initial field operation of major
machinery. These logs should be retained for future reference.