Page 112 - Improving Machinery Reliability

84 Improving Machinery Reliability

Of course, rotordynamics design audits may be equally valuable for existing
equipment with inherent design defects. These defects may manifest themselves in a
number of ways; they include sensitivity to unbalance and sensitivity to misalign-
ment, and range all the way to frequent, unexplained downtime. Moreover, retroac-
tive rotordynamic design audits represent an excellent means of determining the
merits of component upgrading in existing turbomachinery.
On new equipment, the decision to perform a rotordynamic design audit is gener-
ally based on the type of machine, the manufacturer’s experience with similar sizes,
speeds, etc., and the assessment of the benefits versus the cost of the analysis. If it
could be assumed that nothing would go wrong, then the audit would not be needed.
However, statistics show that design and manufacturing problems do occur that
result in considerable delay to projects. Cook’ indicates that over half of the major
projects in the 1974-1984 time frame encountered a critical speed design problem
and/or high vibration near rated speed. This study indicated that the delay time to
correct design equipment error could be as high as 100 weeks. For some perfor-
mance-related problems, up to four years were needed to correct the difficulties.
Exxon Chemical Company statistics for the late 1970s and early 1980s indicated
that approximately 22% of the unscheduled downtime events for major turbocom-
pressors in process plants were caused by the rotor/shaft systems.2 Considering all
the unscheduled downtime causes which could be vibration-related, the percentage
would be greater than 50%. A study by an insurance company found that failures
expected each year were about one out of every 186 for steam turbines, and one out
of every 26 for gas turbines.
Data such as this and the author’s experience in troubleshooting vibration and fail-
ure problems indicate that design audits can help prevent many of the problems
causing unscheduled downtime, project delays, and/or failures by identifying poten-
tial problem areas before manufacture.
Another reason for performing an independent audit is the fact that the system
may consist of used equipment. In order to avoid any contractual liabilities, the man-
ufacturer may not want to perform the rotordynamic calculations on the new system
or the changes that are being made.
The following are major types of rotordynamic design audits that can be per-
formed, and they are discussed in the following sections.

1. Lateral Critical Speed Analyses
Critical speed map
Undamped natural frequencies
Undamped mode shapes
Bearing and seal stiffnesses and damping
Rotor response to unbalance
Pedestal and foundation effects on response
Stability
2. Torsional Critical Speed Analyses
Natural frequencies
Mode shapes