Power August 2023 - 14

O&M
Using ASME PTC 6 Testing
to Improve Steam Turbine
Performance
When a power plant experiences issues
with performance, the obvious indicators
are usually a loss in load capacity or
an increase in fuel consumption. However,
other issues exist that can be costly,
such as turbine cycle performance
problems, which are difficult to detect
because of existing instrumentation
limitations and insufficient plant monitoring
points. That's why it is important
to regularly test steam turbine cycles to
maintain a quality reference for future
performance changes.
Successful programs track thermal
performance test data through annual
capacity tests like the American Society
of Mechanical Engineers (ASME) Performance
Test Code (PTC) 6 for fossil and
nuclear-fueled utility-grade steam turbines/generators
(Figure 1). PTC 6 is the
international standard for steam turbine
acceptance testing, which provides a
consistent method for determining existing,
retrofitted, and new steam turbine
performance within the minimum
practical uncertainty.
Several steps can be followed to create
a performance plan for your plant
using the procedures of PTC 6. The plan
includes identifying corrections, test
planning, evaluation of the turbine, and
reporting. Doing so will identify maintenance
issues early to keep the plant from
potential performance losses.
Identify Corrections
The first step is to create the foundation
with a heat balance diagram (HBD).
Using a representative HBD of the turbine
generator cycle and the correction
curves included in the original equipment
manufacturer's thermal kit, the load and
heat rate can be corrected to design and
compared to their respective values on
the representative HBD. Changes to the
corrected load, or any correction component,
will indicate a deficiency or improvement
in cycle performance.
Corrections are broken into two
groups: Group 1 for variations in cycle
parameters and Group 2 for steam or
boundary conditions (see below). These
corrections are meant to calculate the
effect of off-design steam and cycle parameters
for comparison to the design.
Cycle heat balance software can be
used to develop additional or alternate
correction curves per the objectives of
the test. This can be especially useful if
the plant arrangement differs from the
design HBD.
Here are examples of Group 1 corrections:
■
Heater terminal temperature difference
(TTD).
■ Heater drain cooler approach (DCA).
■ Extraction line pressure drop (ELPD).
■ System water storage changes.
■ Condensate subcooling.
■ Makeup flow.
Here are examples of Group 2 corrections:
■
Throttle pressure.
■ Throttle temperature (or quality).
■ Reheat temperature.
■ Reheat system pressure drop.
■ Low-pressure (LP) exhaust pressure.
1. Testing of steam turbines and generators,
such as this equipment in a coal-fired power
plant, is important to identify potential operating
issues and maintain optimal performance.
Courtesy: GSE Engineering
14
The corrections are parametric components
whose product is applied to
the test load and tested heat rate, which
then gives the corrected load and heat
rate. The corrected load and heat rate
can then be compared to the HBD to determine
how the plant operates relative
to the design.
The corrections listed are the minimum
required for a PTC 6 alternative
test. LP heater corrections have minimal
impact on overall performance and are
typically neglected for the alternative
test, and therefore, not listed in this example.
Corrections with the most impact
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are throttle flow (or thermal power), LP
turbine exhaust pressure, reheater pressure
drop, and feedwater temperature.
Special attention to these measurements
is critical for quality test results.
Throttle pressure correction can be significant
for a control stage machine, or
effectively zero for a full-arc admission
design with a separate flow correction.
Heater
performance
corrections
are usually smaller, given that the test
parameters are close to the reference
and decrease in magnitude significantly
through the lower pressure zones. The
test plan can vary from the code with
the parties' agreement and should be
tailored to the specific goals of the plant.
In addition to load and heat rate, cycle
pressures can be corrected to the ratio
of test flow divided by reference flow to
compare the pressure-flow relationships
of the various turbine sections to design.
Over sequential yearly tests, flowcorrected
pressure can uncover issues
with deposits, cylinder leakages, or solid
particle erosion.
Records of corrected load, heat rate,
cycle pressures, and heater performance
parameters should be tabulated following
each test to track variations over
time. This data can be used to build a
trend that helps diagnose issues that
arise. Performance test tracking is also
useful for determining degradation over
time for maintenance or baseload dispatch
planning. It is often executed yearly,
or biyearly.
Test Planning and Methods
The testing method should be defined
before the turbine is installed. The plant
should start planning months ahead of a
performance test. Test preparations may
require outage work scope to be added.
Some timing limitations associated with
test planning include identifying test
points that need service, procuring test
instrumentation, inspecting basket tips
in the condenser for LP exhaust pressure,
and seasonal considerations for attaining
the desired condenser pressure.
If the reference condenser pressure
is not practical in normal operation,
the LP exhaust pressure correction
curve should be referenced and testing
planned around operating in the more
linear portion of the LP exhaust pressure
correction curve (Figure 2) to avoid the
choked region. Often, condenser basket
POWER | August 2023
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Power August 2023

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