Transcript What is it, why do they need it, Transient

XVII ENCONTRO USUÁRIOS
VITEK
Evaluation of Turbomachinery by means of
Transient Analysis
Lasse Hansen
Vitek Consultoria Ltda.
Fontes:
Deane Horn, Tomek Lech and Attila Kiss
Emerson Process Management
Why Machinery Health…
• Machinery failures are the single largest pain issue for process
plants
Operators lack feedback to know they are abusing machinery
Causes of Large Losses
Maintenance Spending
“68%
of reported equipment failures result
from improper installation and
start-up.”
Causes
of Plant Incidents
Natural
source:
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Source: Marsh & McLennan Protection
Consultants
Factory Mutual
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Multiplex system is perfect for…
Gear Mesh Fault
Sidebands
increase with
gear wear
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Many distinct peaks
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Multiplex system is perfect for…
Bearing wear shows up at specific peaks
related to the geometry of the bearing
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Rolling Element
Bearing Fault
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Supported by sleeve bearings, over
critical
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High Stakes of Turbomachinery
• Large, heavy, high speed,
expensive assets
– US$50M - US$100M cost for
turbomachinery
– Machines close to 65 meters long
with 100’s of thousands of Kg of
rotating mass
– Spinning 50 times per second,
24 x 7 x 365
– Blades nearing supersonic speeds
with wafer thin clearances
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High Stakes of Turbomachinery
• The consequences of faults
and failure are often dramatic
– Human safety at risk if machines
fly apart spreading debris over
100’s of meters
– Missed production schedules
and customer commitments
– US$100,000 per hour in
downtime typical
– Weeks or months to make
repairs
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Difficulties of Spectrum Analysis
If we have nonlinear system elements…
Misalignment
Looseness
Response = Force x System
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TurboMachinery – nonlinearities
The spectrum is less important,
than the transient behaviour…
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Performance
• Key Performance Indicators
•
•
•
•
Gas Turbines
Boilers/HRSGs
Compressors
Steam Turbines
Power Loss
Efficiency
Efficiency
Power Loss
Heat Rate
Tube Fouling
Blade Fouling
Condenser
Fouling
Fouling
Fuel Costs
Head Generation
Stage Efficiency
Fuel Costs
Steam Loss
Power
Consumption
Optimum Steam
Rates
Operating Cost
Operating Cost
Operating Cost
Operating Cost
Total Equipment Performance
Individual Section Performance
Impact of Components upon Performance
All Compared to Design & Expected
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Extending Process Control
• What would the
process control system
show?
Bearing Case
– Nothing until high
bearing temperature
• What would you do?
– After high bearing
temperature…
– Eventually schedule
maintenance to investigate
if it didn’t suddenly fail and
schedule maintenance for
you
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Extending Process Control
• What’s really going on
inside…
Bearing Case
Rotor
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Extending Process Control
1Y
1X
270
– Correlate movement and position
at other bearings
– But with this information, you
might know that this was an oil
instability
• Change oil temperature, and might
correct the problem
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1X
270
180
• What would you do?
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Shaft
Centerline
Orbit
• What would vibration
monitoring and process control
show together?
– Early on, rotor movement or
position changes
1Y
180
Y
Machine
X
Turbines Have Unique Challenges
case “hogs”
• Start-up
– Changes vs. RPM
– Thermal transitions of rotor and
stator
Temperature
effects on rotor
during start-up
overcome forces
of gravity
HP
• Rubs, rotor bow
• Generator
– Critical speed characteristics
• Production
– Changes vs. time
– Changes vs. load
First stage of rotor is 480C (900F)
• Coastdown
HP
– True mechanical assessment
• Stopped, turning gear
outlet
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inlet
Every time you start-up a turbine…
Rotor expands
more rapidly
than stator
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Rotor could
contact stator
Result:
Rub seals, seal leak
Rub, damage bearings
Bow shaft
Crack, or broken blades
…or healthy, optimized start-up
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Transient Monitoring, Introduction
• Transient analysis is usually synonymous with
start-up and coast down vibration monitoring of
extremely critical turbo machinery
• In this presentation
– Explain transient analysis
– The five operating modes of a turbine
– Explain the plot types and examples of how they are
used to evaluate machine health
• Practical tips throughout the presentation you can
use
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Transient Monitoring
• Instrumentation
• Plot Types
• Turbine Mode, Start-up
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Instrumentation
1
Y
2
2
4
X
X
3
1
3
Y
4
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What is Transient Monitoring
• Multi-channel
simultaneous data
acquisition
• All bearings
simultaneously
• View changes
– vs. rpm
– vs. time
– vs. process
parameters
Y
X
1
Y
2
X
X
1
X
1
3
4
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1
3
4
3
4
Y
2
• Monitoring for all turbine
operating modes
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Y
2
2
4
3
What is transient monitoring?
• Instrumentation
Tach
HP
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IP
LP
GEN
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EXC
Transient Analysis
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•
•
•
•
•
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•
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•
Long time waveform
Orbits
RPM vs. time
Shaft centerline
Bode plots
Polar or Nyquist plots
Cascade plots
Monitoring critical speeds
Monitoring critical resonances
Start-up, coast down and bump in the night
Transducer output vs. speed and/or time
Live and post processed data (extraction)
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Trend
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Waveform and spectrum
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Waveform and spectrum – runout
compensated
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Waterfall spectrum
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Waterfall spectrum – runout
compensated
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Cascade spectrum
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Cascade spectrum – runout
compensated
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Plot Types: Orbit
• AC, dynamic
motion or the
path of the shaft
centerline as
seen by the
transducers
• Shape can help
determine rubs,
oil instabilities,
imbalance
Y
2
X
3
1
4
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Unfiltered Orbit
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Unfiltered Orbit – runout compensated
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Filtered Orbit
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Filtered Orbit – runout compensated
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Plot Types: Shaft Centerline
• Average, DC position
of the shaft centerline
• Should rest at the
center and bottom of
the bearing at zero
RPM
• Rotor should rise in the
fluid film and load will
influence final position
• SCL can help
determine
misalignment, bow
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Y
2
X
3
1
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4
Plot Type: Shaft Centerline
• Shaft centerline forced near bearing clearance
• Misalignment forces shaft in opposing quadrants
of shaft centerline plot in adjacent bearings
Bearing 1
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Bearing 2
Shaft Centerline
Example
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Shaft Centerline
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Bode and Polar plots
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Bode and Polar plots – runout
compensated
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Live and Replay Mode
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Live Turbine Dashboard for Rapid Decisions
Overlay baseline
data on live plot
Multiple shaft
centerline plots
for bearing-tobearing
comparison
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Turbine Mode, Start-up
• Start-up
– Shaft position and
motion
– Case position and
motion
– System dampening
– Changes in resonance
speed
First stage of rotor is 480C (900F)
outlet
– Compare with baseline
• Changes vs. speed
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540C (1000F)
HP
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inlet
Turbine Mode - Start-up, Rotor Bow
• A rotor that is bent, or bowed, will produce 1X
vibration at slow roll speed
• It can be mathematically shown that rotor bow
“repairs itself” if you get through the first critical
resonance
• Remember, imbalance will not produce 1X at slow
roll
`
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Turbine Mode - Start-up, Rub
• Worn bearing, rotor moves away from bearing
then re-contacts the bearing
• Loose support structure
• Unlubricated contact (must often)
– Creates hot spots, local melting, welding, bowing
• Lubricated contact
• May occur once per several revolutions
• Full annular rub
– Forward precession is light
– Reverse precession, forces are large, can rapidly destroy
a machine, acts like a planetary gear
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+
Symptoms of rub
• Changes in 1X vibration
• Abnormal orbit shape
– Look for flattened orbit
– Look for 1/2X component in spectrum
• Subsynchronous or subharmonic
vibration
• Reverse precession components
• Harmonics in spectrum
• Thermal bow
• Changes in average shaft centerline
position
• Wear, damage, lose of efficiency
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Turbine Mode - Start-up,
Bode, Polar Plot
• What is a Bode and
Polar Plot?
• Used to analyze
systems to determine
critical resonance
frequency
• 1X vibration will peak
• 1X phase will shift
180˚
• Polar plot is 1X peak
and phase on same
plot
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Turbine Mode - Start-up,
Bode/Polar Plot
• Look for changes in critical resonance frequency
– Shifts in resonance means that dynamic stiffness has changed
(dampening or spring stiffness) in the system
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Turbine Mode - Start-up,
Bode/Polar Plot
• Look for changes in synchronous amplification factor
– Changes in SAF means changes in the dampening or
spring stiffness properties of the bearing.
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Turbine Mode - Start-up,
Bode/Polar Plot
• Look for 1X vibration below and above critical
– Certain anomalies will have vibration at 1X below critical,
others will only have 1X above, others will have 1X at both.
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Turbine Mode - Start-up,
Bode/Polar Plot
• Look for shaft centerline movement with rpm
compared to baseline.
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Plot Types: Cascade
• Stacked spectral plots
• Spectral plot vs RPM
• Used to view changes
in frequency
components vs. RPM
• Used to view non
changing frequencies
vs. RPM
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Bode/Nyquist
Amplitude
climbing
Amplitude should
have dropped as
it passed through
resonance
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Turbine Mode - Production State
• Production state monitoring
– Changing rotor motion, rotor position and process
conditions versus time
– Only one operating speed is examined
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Why Production State Monitoring…
•
•
•
•
•
•
•
•
•
Shaft cracks
Coupling degradation
Bearing condition and
lubrication changes
Alignment
Foundation, piping resonance
changes
Imbalance changes
Correlate process data and
events with vibration
Bump in the night capture and
replay
Predictive planning for next
outage
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Turbine Mode - Coast Down
• Coast down
– True representation of
mechanical condition
without the affects of the
process or temperature
transitions (unlike during
start-up)
hogs
HP
• Changes vs. speed
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Easily overcomes
forces of gravity
Turbine Mode - Coast Down
Prior to coast down
At coast down
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Why Coastdown Monitoring…
•
•
•
Critical speed comparison
Synchronous amplification factor
comparison
True representation of
mechanical condition without
influences from the process
–
•
Decision to start-up may be based
on this data
See inside the turbine, make
decisions fast and early
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Coast Down Data
• Use coast down data on Polar plot to determine
imbalance heavy spot
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Turbine Mode - Slow Roll
• Slow roll: Speed well below machine’s first critical
resonant frequency
• Normally where 1X amplitude and phase are not yet
changing in the Bode Plot
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Slow Roll
• Slow roll data useful for
–
–
–
–
Slow roll vectors
Slow roll waveforms
Shaft position
Mechanical and electrical runout
• Any 1X response at slow roll speed
must be due to something other
than imbalance
–
–
–
–
Runout
Bow
Severe misalignment
Coupling problem
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Eccentricity
•
•
•
•
•
Eccentricity is the amount of bow or sag in the rotor
After an extended shutdown, the shaft will bow if heated
unevenly
Prior to startup, the rotor is placed on turning gear and
slow-rolled, allowing the shaft to straighten to within
acceptable limits - the turbine is not brought up to speed
until eccentricity is within limits
Excessive eccentricity could cause rubs and damage to
the seals
Eccentricity measurement may also provide indication of a
bent shaft
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Turbine Mode – Stopped
• Stopped
– Adjacent machines, shaft centerline position measured,
electrical noise
– Note: Electrical noise will not change as RPM changes
Machine is stopped.
Make sure there is no vibration.
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Turbine Mode – Stopped
• Specify resting voltage
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Turbine Malfunctions and Symptoms
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•
•
•
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•
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•
•
Rub (seals)
Bow
Imbalance
Looseness
Misalignment
Couplings
Runout
Fluid instabilities
Cracked shaft
Etc.
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Loose parts or debris
• Part shifts position
– Rotor disk
– Thrust collar
– Broken turbine
blade
– Usually a step
change in the
amplitude vs phase
over time plot
• Looseness, 1X to
8X
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Misalignment Symptoms
• Some symptoms occur in pairs
• The overloaded bearing will have a high
temperature while the adjacent bearing will have
an abnormally low temperature
• Average eccentricity ratio can be high in one
bearing and low in adjacent bearing
• SCL position may be in one quadrant in one
bearing and in the opposite quadrant in adjacent
bearing
• These pair characteristics will also appear across
couplings
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Shaft Centerline
• Shaft centerline forced near bearing clearance
• Misalignment forces shaft in opposing quadrants
of shaft centerline plot in adjacent bearings
Bearing 1
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Bearing 2
Transient Extraction
Shaft position
not changing
with speed.
Movement may
be restricted
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2X is
high
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Coupling Problems
• Offset (parallel) misalignment can produce a
“cranking” effect and will produce 1X in one or
both machines
• An off center coupling bore or off center coupling
bolt circle will produce a cranking action
• Cranking-induced 1X vibration will continue at
slow roll
• If a gear coupling should lock-up, look for a
sudden change in both 1X and average SCL
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Runout -- Scratches
• Produce negative-going voltage spikes
• Positive going if there is material displacement, not
just material removal
• Spikes in an unfiltered orbit that usually point away
from the probe
• A single scratch will
produce a 1X component
and its harmonics in the
spectrum visible over all
speeds
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Modes of Instability:
Whirl and Whip
• The instability behavior is called whirl when it
tracks rotor speed
• Oil whirl is typically a little less than ½X vibration
• Oil whip locks to a particular frequency and phase
changes rapidly
• Oil whip is the most severe of the two phenomena
and should be avoided
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Misalignment induced fluid
instability
• If a cylindrical fluid-film bearing is underloaded
because of misalignment, the journal may operate
near the center of the bearing
• Bearing can become fully lubricated making rotor
system vulnerable to fluid instability
– Predominately forward, subsynchronous vibration from
0.3X to just below 0.5X
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Rules of Crack Detection
• If a rotor with a crack has a steady, unidirectional
radial load, then a strong 2x response may appear
when the rotor is turning at half of any balance
resonance speed
• 1x and 2x amplitude and phase are likely to
change as the crack propagates
• Start-up and shutdown 1x and 2x bode plot
comparisons should be examined
• 2X peak and phase should be trended during
steady state operation
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Unbalance
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Looseness
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Misalignment, bent or cracked shaft
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Sleeve Bearing Fault Types
•
Balance
•
Cracked Shaft
•
Loose Rotating Part
•
Oil Whirl
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Sleeve Bearing Fault Types
•
Oil Whip
•
Excessive Preloading
•
Rub
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Oil Whirl to Oil Whip
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Summary
• Turbomachinery requirements are unique
– Start-up, shutdown, production state, slow roll
• For critical turbo machines, a complete,
integrated solution is required
–
–
–
–
Vibration shutdown protection
Machinery predictive diagnostics
Performance real-time
Integration to process automation
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Events
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Events
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Events
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Events
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Events
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Events
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Unit 2 – Expensive Failures
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Unit 2 – Safety Concerns
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Unit 2 – Large Holes in the Roof
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