POWER April 2021 - 16

FOCUS ON O&M
predictive approach allows savings to be
realized by avoiding unnecessary periodic
maintenance and assuring component
integrity, thereby avoiding episodic maintenance
due to in-service failures that
often result in forced outages.
This service is available for all types of
steam and gas turbine valve actuators.
Differing approaches can be used to test
everything from mechanical hydraulic
units to the latest digitally controlled systems.
Two instances of this practice were
performed by MD&A Control Systems at
two Xcel Energy-Colorado facilities: Unit
1 at the coal-fired Pawnee Generating
Station and Unit 1 at the natural gas-fired
Fort St. Vrain Generating Station.
" We wanted to test and diagnose our
hydraulic actuators for our control valves,
main stop valves, and combined reheat
valves prior to our planned overhaul, "
said Leif Bothun, a principal operational
support consultant at Xcel Energy. " Our
normal three-year valve outage interval
had not normally covered removal of our
actuators and all 10 were in the plan to be
removed. Our valves operated smoothly,
but that isn't a good indication of the
health of the actuators. By plotting the
position versus the servo coil voltage,
we were able to see if an actuator was
in need of attention. Knowing which of
the 10 actuators needed attention could
have significant cost and time savings for
the outage. "
No regulatory maintenance practices
or intervals have been specified in
the U.S. for non-nuclear steam turbine
valves and actuators. Typically, original
equipment manufacturers (OEMs), nonOEMs,
and other industry organizations
recommend minimum maintenance intervals
for steam turbine valve inspection
and repairs every three to five years.
Some in the industry say these inspections
and repairs should occur, at
a minimum, every 25,000 effective operating
hours (EOH). Typically, customers'
hydraulic actuators have not been
looked at on any defined interval and
are often not addressed until there is an
issue. Adopting a data-driven approach
to condition analysis will produce the
following benefits when compared to
strictly periodic maintenance:
■ Understanding of, and insight into,
the asset's current operating performance.
■
Awareness of life expectancy predictions
for individual system components.
■
The time and confidence to plan and
16
5. This graph shows the valve motion data for a main stop valve using MD&A's steam turbine
valve actuator diagnostics service. Courtesy: MD&A Control Systems
optimize downtime.
■ The ability to focus on and manage
costly repairs before they are required.
MD&A Control Systems began its
steam turbine valve actuator diagnostics
service by recording valve motion
data from each of the customer's control
valves, main stop valves, and combined
reheat valves. Each of these valves had
servo modulated position control capabilities,
and were opened and closed at
a rate of 3% per second. Data was gathered
for servo coil voltage and demodulated
position feedback voltage.
Individual valve hysteresis test results
for each valve were then graphed.
There were no concerns of mechanical
problems with the hydraulic cylinders on
the control valves and intercept valves.
As shown in the graph (Figure 5), on
main stop valve 2 at Pawnee, the MD&A
controls engineer observed that the
measured servo coil voltage ramp rate
showed anomalies. The slope changes
indicate the actuator cylinder is either
leaking or sticking.
Recommendations for the customer
included further investigation of main
stop valve 2 (Figure 6) within the next
year. The hysteresis test should be performed
again to confirm the anomaly is
repeatable.
" Overall, I think it was and is a valuable
way to do some testing to prevent
unnecessary maintenance, " Bothun said.
" These anomalies showed the actuator
cylinder was likely leaking by internally
given the smooth movement of the
valve but an increase in servo coil voltage
to overcome the increase in fluid
flow through that section of the cylinder.
I am glad they pinpointed which main
stop valve had issue and recommended
that we should inspect that actuator during
our outage. We were able to reduce
our planned maintenance costs by not
having to remove all of the hydraulic cylinders
and reduce the likelihood of unplanned
downtime on the unit. MD&A
was quick and responsive to our needs. "
The cost savings associated with this
6. A control system engineer recommended
further investigation of main stop valve 2 at
the Pawnee station based on information
from a hysteresis test. Courtesy: MD&A Control
Systems
www.powermag.com
type of predictive analysis can be substantial.
Maintenance on a high-pressure
positioning actuator typically ranges from
$20,000 to $30,000, including removal
and re-installation costs. If the actuators
on an average-size turbine are found to
be in good condition as a result of the
analysis, savings could be well above
$200,000. ■
-Mike Broggi is a general manager
for MD&A Control Systems.
POWER | April 2021

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POWER April 2021

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