ELECTRIC ENERGY | SUMMER 2019 - 31

heating. Further, there are other concerns that
can occur at full speed low-load operation.
Some examples are windage heating, stall flutter vibration of last stage LP blades, last stage
trailing edge blade erosion, and increased differential expansion.
Opportunities for Pre-Heat: If an impending startup
is known in advance, an auxiliary boiler or adjacent
unit steam source can be used in most cases to supply steam to the glands while the unit is on turning
gear. A benefit of this approach is the glands are
located at the exhaust ends of high temperature
rotors where thermal mass and stresses are greatest. Since these locations will heat the slowest
during a start-up, significant benefit can be realized
in startup time margin and reduction.

RESULTS OF A RECENT OPTIMIZATION

FIGURE 5: Limiting Calculated Critical Crack Sizes at Bore Below Each Blade Stage

lower steam pressures, which can improve
superheat levels and lower the steam to metal
temperature mismatch.
However, steam temperatures must be well
above the rotor metal FATT or there will be little
heating benefit along with a risk of exposing the
rotor to high rotational stresses while it has poor
fracture properties.
Optimization of Hold Speeds and Times: A common and potentially concerning finding are cold
start procedures that do not include a low speed
heat soak. As previously mentioned, the objective is to hold the rotor at speeds well below
operating speed to minimize centrifugal stresses
until the rotor has good fracture toughness.
Ramping the unit to high speeds with a cold
rotor can have significant consequences and is
not recommended by TG Advisers. Adding or
extending a low speed hold can improve the
safety margin of the startup significantly, and
often allow for reductions on the later stages
of start up when the material exhibits good

fracture properties. Thermal stresses are controlled by maintaining steam to metal mismatch
at reasonable levels; a good target is less than
250°F but lower is better.
It is important to note hold speeds must not
be arbitrarily selected. In some cases, TGA has
found hold speeds have been modified because
of lateral vibration concerns. This can be a dangerous practice since holding at a non-compliant
speed can cause LP blading to vibrate at its natural
frequencies, which would not be known to an
operator. Blade vibration cannot be measured by
bearing lateral vibration probes. Turbine instruction manuals will include allowable hold speeds
or avoidance zones. Ensure these are adhered to
and are still current if a retrofit or a new LP blade
design was installed.
Reduction in Full Speed No- or Low-Load Holds:
It is generally accepted that high temperature rotor fracture toughness curves reach an
upper shelf, after which there is no appreciable
improvement in rotor properties with additional

Table 1 summarizes the reduction in cold
start-up time for a recently evaluated vintage
unit achieved by applying finite element and
fracture mechanics analyses and industry best
practices. In this particular case, the operator is
now able to dispatch the unit in one-third less
time than before the optimization.

CLOSING COMMENTS
We are in the midst of a challenging and
dynamic time in the industry, where flexibility is
a significant benefit. Utilizing sound engineering
practices, inspection results and modern fracture mechanics principals offers an opportunity
to optimize steam turbine startup times. In TG
Advisers' experience, cold start times on older
steam turbines commonly can be reduced by
25% to 50% while still adhering to critical OEM
design parameters and requirements. These
reductions provide significant startup cost fuel
savings and enhance an older steam unit's
ability to be dispatched in today's challenging
environment.

Table 1: 2019 Cold Start Optimization-Vintage Steam Turbine

Cold Start Time (Hours)
Pre Optimization

Optimized

Reduction

Percent Reduction

Pre Turbine Roll

7.2

4.4

-2.8

38%

Turbine Roll to Breaker Closed

5.4

4.2

-1.2

23%

Breaker Closed to Dispatch

3.5

2.2

-1.3

38%

16.1

10.8

-5.3

33%
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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
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