ELECTRIC ENERGY | SUMMER 2019 - 30

Resulting Critical Crack Sizes: Limiting critical
crack sizes can then be calculated throughout
the start up from knowledge of the temperature
and fracture toughness relationship, stresses and
existing flaws in the rotor. The calculated critical crack sizes are compared to known flaws in
the rotor, to ensure substantial safety margin.
Figure 5 shows calculated critical crack sizes at
the bore beneath each stage of the IP rotor. As
can be seen, the limiting portion on this start
up (smallest calculated critical crack size) occurs
within 1.5 hours of turbine roll. It is important to
be conservative with the introduction of stresses
during cold starts, particularly while the rotor is at
or below the FATT. However, from a rotor integrity
perspective, there is little technical benefit to
prolonging start up processes once good fracture
properties have been achieved.

TARGET AREAS FOR OPTIMIZATION
FIGURE 2: High Temperature Rotor Fracture Toughness as a Function of Temperature

assumes a flaw exists that may propagate based
on a combination of cyclic stress cycles and material fracture toughness. In this case, rotor life is
dictated by a safety factor on number of cycles
for a crack to reach a critical size where rapid
fracture would occur. Optimizing startup and load
ramp times involves avoiding, to best advantage,
combinations of high stress and low toughness.
High temperature rotors are more susceptible
to failure when cold. New HP and IP rotors have
Fracture Appearance Transition Temperatures
(FATT) values of around 200°F. With service
exposure, the FATT may degrade due to embrittlement in select temperature zones to above
300°F. Industry material test data for high temperature rotors shows the fracture toughness can
improve with increased temperatures by a factor
of approximately 3 to 5 during a cold start-
this relationship is shown in Figure 2. For these
reasons, high temperature rotor prewarming is
necessary for safe operation.
It should be noted-low pressure turbine and
generator rotors have superior fracture properties
even at room temperature and do not limit the
unit start-up rate. However, they may limit the
ability to cycle the unit based on the size and
locations of existing flaws.

stresses are a function of both temperature and
speed, rotor temperature is a critically important
parameter. An optimization of start-up and load
ramp times must include a transient heat transfer
analysis. A snapshot of a temperature distribution
for an IP-LP rotor from such an analysis is shown
in Figure 3. This model utilizes unit operational
data to calculate the heating throughout start up.
Rotor Stress Model: Similar to the thermal analysis, stress distributions are calculated along the
rotor throughout the start up as well. Figure 4
shows a snapshot of stress distributions on the
same IP-LP rotor, which illustrates that rotor body
stresses are greatest at the bore surface.

If there are no major condition or operational
red flags and the baseline analysis is favorable,
then optimization can be considered. The following are common areas that should be considered
in this process. As a first step, benchmarking of
current start up conditions and turbine roll parameters is essential. Ensure critical OEM requirements
are met, such as steam to metal temperature
differentials and superheat requirements.
Turbine Roll Conditions: There are a number of
permissives that must be met prior to rolling
a steam turbine off gear. Some examples are
steam line warming, steam superheat requirements and rotor eccentricity. Note rolling at
optimal steam pressures and temperatures can
greatly improve cycle time. Consider rolling at

FIGURE 3: Thermal Finite Element Analysis Model of IP-LP Rotor During a Cold Start

ANALYTICAL APPROACH
TO OPTIMIZATION:
Rotor Temperature Model: Since fracture toughness is a strong function of temperature, and
30 ELECTRIC ENERGY | SUMMER 2019

FIGURE 4: Stress Distribution of IP-LP Rotor During a Cold Start



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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