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Million-horsepower steam turbine problem solved by remote stress analysis
A power plant in Asia was experiencing high vibration and
cracking in the exhaust hood of a critical large low-pressure
steam turbine (Figure 1). The typical troubleshooting approach
Mechanical Solutions Inc. (MSI, www.mechsol.com)
uses in such situations is to travel to the site of the machinery,
collect the data, analyze it and recommend a fix. However, in
this case, such a trip might have jeopardized company personnel
in an area where insurgents and bird flu were threatening,
so a remote problem-solving approach was implemented.
Figure 1. Exhaust hood cracking produced a reliability and safety
concern.
Figure 2. Remote test data were applied to a CAD model (left)
and diagnosed by finite element analysis (FEA, right).
Data acquisition hardware and a large amount of instrumentation
was prepped and shipped to the jobsite, with
proper export licensing and paperwork filed consistent with
U.S. State Department and ITAR controls. The equipment
included a large number of accelerometers, pressure transducers,
strain gages and a modally tuned impact hammer.
An Internet Protocol (IP) telephone and camera also were
included to facilitate inexpensive verbal and visual real-time
communication between the expert engineers in the United
States and the plant personnel in Asia. The plant’s local
personnel installed the instrumentation under MSI’s careful
oversight from 12,000 miles away.
MSI’s troubleshooting process was carried out step by
step, with the local plant personnel acting as MSI’s hands
and legs at the site. Visual and voice communication
were continuous to avoid misunderstandings despite the
distance and language difference. MSI staff controlled a
40-channel FFT analyzer and a 16-channel strain gage
amplifier and recorder remotely over the broadband
Internet connection, available through a high-speed satellite
intermediary.
Remotely collected data were immediately compared
with theoretical results provided by finite element analysis
(FEA) performed at MSI’s U.S. offices (Figure 2). Assumptions
about boundary conditions, flange-to-flange
stiffness and dynamic loads were confirmed or adjusted
until the theoretical predictions matched the actual behavior.
The resulting analysis was calibrated with reality
in this manner, and then was used with confidence to
evaluate potential fixes.
The remote test data on the exterior of the machine, and on
the shaft within the bearing housings, were extrapolated from
the locations where data acquisition was feasible, to zones inside
the machine where it was impractical to locate data probes.
This is a form of noncontact stress-measurement system
(NSMS) that MSI pioneered in government research projects
during recent years. The approach determined behavior of the
complete (not just the exposed) turbine rotor, and revealed how
rotor motion coordinated with structural motion.
The combined test and analysis model identified a dangerous,
running-speed resonance of a natural frequency of the
exhaust hood that involved synchronous motion at both
16 APRIL 2009 www.PLANTSERVICES.com