Page 158 - Improving Machinery Reliability

130 Improving Machinery Reliability

17. Discharge pressure and brake horsepower versus ICFM curves at rated condi-
tions for each section or casing on multiple section or casing units in addition
to composite curves at 80%, 90%, loo%, and 105% of rated speed
For variable molecular weight (MW) gases, curves also shall be furnished at
maximum and minimum MW. For air compressors, curves also shall be fur-
nished at three additional specified inlet temperatures.
18. “Pressure above suction pressure behind the balance drum” versus “unit load-
ing of the thrust shoes,” both in pounds per square inch (bar), using rated con-
ditions as the curve basis
The curve shall extend from a pressure equal to suction pressure behind the
drum to a pressure corresponding to at least 500 pounds per square inch (35
ata) unit loading on the thrust shoes. Balance drum OD, effective balance
drum area, and expected and maximum recommended allowable pressure
behind the balance drum shall be shown on the curve sheet.
Will balance drum labyrinth wear cause overloading of the thrust bearing?
What happens when fouling (polymerization) occurs in the balance line? Is the
design safe for a wide range of suction pressures?
19. Speed versus starting torque curve
Will the motor be designed to safely start the compressor? Even more
important for gas turbine drivers!
20. Vibration analysis data, including:
a. Number of vanes-each impeller
b. Number of vanes-each guide vane
c. Number of teeth-gear-type couplings
These are required for machine signature “real time” on line diagnostic or
spectrum analysis. They will allow identification of relevant frequencies, and
possibly be useful in determining which component has undergone deteriora-
tion. Refer also to the illustrative example in Chapter 1, “How to Deal with the
Typical API Data Sheet.”
21. Lateral critical analysis, including:
a. Method used
b. Graphical display of bearing and support stiffness and its effect on critical
speeds
c. Graphical display of rotor response to unbalance
d. Graphical display of overhung moment and its effect on critical speed
Reviews will identify whether there is risk of operating too close to critical
speed, or whether rotor is likely to vibrate at the slightest sign of unbalance. If
gear couplings are used, the effective (instantaneous) overhung moment may
change as a function of tooth loading or tooth friction. The probability of
encountering critical speed problems as a function of gear-coupling deteriora-
tion can be investigated by examining graphical displays of effective overhung
moment versus critical speed.
22. Torsional critical speed analysis for all motor and gear units, including:
a. Method used
b. Graphical display of mass-elastic system
c. Tabulation identifying the mass-moment torsional stiffness for each compo-
nent in the mass-elastic system