ELECTRIC ENERGY | SUMMER 2019 - 29

critical. These inspections define the location and
size of detected flaws. If there are no reportable
indications, a flaw equal to the minimum detectable flaw size in the location of peak stresses
should be assumed. No bore rotors have lower
centerline stresses. These rotors should be periodically ultrasonically inspected from the periphery.
In all cases, findings should be assessed using a
fracture mechanics approach.
Casing Condition: Casing cracking is common
due to often large steam to metal temperature
mismatches during startups. It is important to
assess the depth of cracking relative to the wall or
ligament thickness, as additional cycling will result
in further crack propagation providing the stress
field at the crack tip has not significantly reduced.
Reducing steam to metal temperature mismatches during startup is the most effective way
to reduce future cracking and propagation rates.
Stationary and Rotating Blading Condition: Solid
particle erosion rates in early rows of the HP and
IP turbine will increase with additional cycling.
On/off cycling causes exfoliation from the boiler,
which carries over to the turbine. Depending
on condition, stationary and rotating blading
refurbishment or replacement may be required
in advance of additional cycling.
Review of Rotor Properties: Rotors are supplied with Fracture Appearance Transition
Temperatures (FATT) and Charpy Impact data that
can be correlated to fracture toughness curves.
Critical crack sizes can then be calculated from
knowledge of the stresses, fracture toughness,
and existing crack characteristics. Finding or
obtaining this material data for a specific vintage
rotor can be nearly an impossible task. However,
opportunities may exist for material testing during outages to quantify missing material properties. In the absence of known material properties,
conservative industry data can be applied from
similar vintage forgings, although this can significantly hamper the optimization process.

build clearances are restored to prior successful
levels and rotor design float is maintained. For
units inhibited by differential expansion, alarm
levels should first be confirmed as being suitable
based on build clearances. If appropriately set,
operational sensitivity testing can be completed
to determine the impact of different parameters,
such as steam temperatures, on the differential
expansion of the unit.
Vibration: Vibration can result for a multitude of
reasons. One reason, particularly for older units, is
a thermally sensitive rotor bow that develops with
time due to asymmetric material properties. This
bow acts as an unbalance force and can produce
high vibration as the rotor passes through critical speeds. The characteristics of bows will also
change with temperature. If vibration is of a high
enough magnitude, it can have other detrimental
impacts such as babbitt fatigue damage and
radial rubbing of seals throughout the blade path.
Unit Metal Temperatures: If properly placed and
calibrated, casing thermocouples can offer insight
into the effectiveness and heating rates during
startups. Although rotor metal temperatures will
differ, review of this data can provide direction
on adequacy of the existing start up processes.
Operational Transient Faults: With increased
cycling, the probability of operational events
such as water induction, overspeed, and loss of

lube oil can increase. It is highly recommended
procedures and protections against these events
are understood and managed in advance of any
cycling program.

THE OPTIMIZATION PROCESS
Fatigue vs. Fracture Mechanics Approaches
Assuming there are no condition and operational limitations that significantly lessen the
ability of a unit to cycle, an optimization study
can be completed to evaluate the reduction of
unit start time. Existing startup procedures and
online stress monitors are designed to maximize
the number of cycles before rotor bore crack
initiation. The lower the bore stress levels during
transients, the more cycles that can be expended
before crack initiation occurs. Bore stresses cannot
be directly measured so these models indirectly
estimate stress levels based on metal temperatures and rotational speed. The most effective
means to limit the stress levels in this scenario is
to balance turbine speed and the rate of change
of steam temperatures. This is achieved through
extended start-up times.
Thirty to forty years ago, fracture mechanics
principals began to be routinely applied to steam
turbine rotor design and life assessments. Rather
than assuming a crack-free rotor and calculating
an initiation time, a fracture mechanics approach

REVIEW OF UNIT OPERATIONAL DATA
Differential Expansion: Adequate clearance is
required between rotating and stationary parts
and seals to ensure there is no contact during all
modes of operation, including cold starts and
full load trips. Rotors and casings heat differently,
and therefore grow at dissimilar rates (leading
to differential expansion). Rotor length is also
affected by speed. It is critical during outages that

FIGURE 1: Example of Steam Path Damage during Cycling from Insufficient Clearances
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ELECTRIC ENERGY | SUMMER 2019

Table of Contents for the Digital Edition of ELECTRIC ENERGY | SUMMER 2019

RMEL Board of Directors
Former NFL Star and Cancer Survivor Merril Hoge’s “Find A Way” Journey Sparks Intention at RMEL’s Spring Conference
Austin’s Experience Instituting a 5G Wireless Program
APS’ Fossil Unit Monitoring Tool Improves Efficiency, Generates Savings
Charging a Path Towards Battery Storage
Xcel Energy’s Unmanned Aircraft Systems Future
28 Steam Turbine Cycling—Operator Considerations, Best Practices and Options for Optimization
Maximize on the New Energy Paradigm at RMEL’s 116th Fall Convention
2019 Calendar of Events
Member Listings
Foundation Board of Directors
Advertiser’s Index
ELECTRIC ENERGY | SUMMER 2019 - Intro
ELECTRIC ENERGY | SUMMER 2019 - cover1
ELECTRIC ENERGY | SUMMER 2019 - cover2
ELECTRIC ENERGY | SUMMER 2019 - 3
ELECTRIC ENERGY | SUMMER 2019 - 4
ELECTRIC ENERGY | SUMMER 2019 - 5
ELECTRIC ENERGY | SUMMER 2019 - RMEL Board of Directors
ELECTRIC ENERGY | SUMMER 2019 - 7
ELECTRIC ENERGY | SUMMER 2019 - Former NFL Star and Cancer Survivor Merril Hoge’s “Find A Way” Journey Sparks Intention at RMEL’s Spring Conference
ELECTRIC ENERGY | SUMMER 2019 - 9
ELECTRIC ENERGY | SUMMER 2019 - 10
ELECTRIC ENERGY | SUMMER 2019 - 11
ELECTRIC ENERGY | SUMMER 2019 - Austin’s Experience Instituting a 5G Wireless Program
ELECTRIC ENERGY | SUMMER 2019 - 13
ELECTRIC ENERGY | SUMMER 2019 - 14
ELECTRIC ENERGY | SUMMER 2019 - 15
ELECTRIC ENERGY | SUMMER 2019 - 16
ELECTRIC ENERGY | SUMMER 2019 - 17
ELECTRIC ENERGY | SUMMER 2019 - APS’ Fossil Unit Monitoring Tool Improves Efficiency, Generates Savings
ELECTRIC ENERGY | SUMMER 2019 - 19
ELECTRIC ENERGY | SUMMER 2019 - Charging a Path Towards Battery Storage
ELECTRIC ENERGY | SUMMER 2019 - 21
ELECTRIC ENERGY | SUMMER 2019 - 22
ELECTRIC ENERGY | SUMMER 2019 - 23
ELECTRIC ENERGY | SUMMER 2019 - Xcel Energy’s Unmanned Aircraft Systems Future
ELECTRIC ENERGY | SUMMER 2019 - 25
ELECTRIC ENERGY | SUMMER 2019 - 26
ELECTRIC ENERGY | SUMMER 2019 - 27
ELECTRIC ENERGY | SUMMER 2019 - 28 Steam Turbine Cycling—Operator Considerations, Best Practices and Options for Optimization
ELECTRIC ENERGY | SUMMER 2019 - 29
ELECTRIC ENERGY | SUMMER 2019 - 30
ELECTRIC ENERGY | SUMMER 2019 - 31
ELECTRIC ENERGY | SUMMER 2019 - Maximize on the New Energy Paradigm at RMEL’s 116th Fall Convention
ELECTRIC ENERGY | SUMMER 2019 - 33
ELECTRIC ENERGY | SUMMER 2019 - 34
ELECTRIC ENERGY | SUMMER 2019 - 2019 Calendar of Events
ELECTRIC ENERGY | SUMMER 2019 - Member Listings
ELECTRIC ENERGY | SUMMER 2019 - 37
ELECTRIC ENERGY | SUMMER 2019 - Advertiser’s Index
ELECTRIC ENERGY | SUMMER 2019 - cover3
ELECTRIC ENERGY | SUMMER 2019 - cover4
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