TURBINE RETROFITS BY THIRD PARTIES ARE THE RAGE, BUT ROTOR MEASUREMENT IS A TRICKY PROPOSITION
Power
plant operators continually face the need to cut fuel costs while also
reducing emissions. A popular way to achieve this is to retrofit their
turbines with components supplied by non-OEM providers - often at a
lower cost than the original turbine supplier.
This is a trend not only in gas turbines but also in steam turbines,
many of which arc custom-made. "For instance, the major steam turbine
manufacturers are moving in a new direction, reverse engineering each
others' turbine rotors," says Peter Tavis, a Quality Control Inspector
for Edison ESI (Westminster, CA), a wholly owned subsidiary of utility
Southern California Edison (SCE) that performs measurements of
turbomachinery components.
The problem with such retrofits is that only the original
manufacturer has the actual design specifications for the turbines, and
they closely guard this proprietary data. Consequently, accurate
measurements must be taken in the field and then loaded into an
engineering software solution to create 3D models of the turbine
components so that new components can be designed and manufactured.
Before deciding on which firm to hire when doing a repair or
upgrade, turbine operators should understand the various types of
measurement technologies used to retrofit or remanufacturc components
for both gas and steam turbines, as well as how to address problems
that arise when taking measurements. This will help them to make
informed decisions on which firm to hire for a repair or upgrade.
types of measurements
The story of Edison ESI's growth shows the increasing need for these
measurements in today's retrofit market. Started as a division of SCE
to measure the utility's equipment, Edison ESI has been going out in
the field since the last ten years to measure turbines and shells for
retrofits built by Alstom, Siemens, Mitsubishi and Toshiba. "Most of
our jobs have been in the U.S., but we are seeing a growing demand
overseas where environmental regulations are much stricter (a driver
for these retrofits)," says Tavis. "We have measured turbines in
Australia, Japan, China, England, and South Africa."
There is no single method of conducting measurements which is ideal
for all applications. There is one, however, which can be eliminated:
manual. "Most of the issues we sec in our primary market space, - in
situ geometry recovery for turbine retrofits - is that most engineers
will have a team of people capturing 2D data using traditional hand
tools (rulers, tapes, calipers, plumb-bobs) and then will go back to
their offices to assemble the data into a drawing," says Tavis. "The
problem with this is that the data, captured by different people, is
often rife with human error."
To eliminate these problems requires the use of automated tools that
operate from a fixed reference point, and the loading of the data into
Computer Aided Design (CAD) software to generate a 3D image. These
tools include CoordinateMeasuring Machines (CMMs), laser trackers,
photogrammetric devices, and surveying equipment.
CMMs are mechanical devices that move a probe to different points on
an object to get a measurement and feed it back to a CAD software. Some
of the more popular CMMs are manufactured by Faro Technologies, Inc.
(Lake Mary, FL; www.faro.com). These devices have articulated arms
(Figure I ) that can be moved to various points around the component to
take measurements. They have a probe that makes physical contact with
the location being measured, or a laser takes remote measurements
without contact.
There are portable versions for setting up at a remote site, as well
as permanently mounted units for use in the shop. Faro arms are limited
by the reach of the arm. However, the arms have the advantage of being
able to move around to different positions and measure points that arc
not within the line of site, say users. "For single-point measurement
of a large, but nottoo-precise object, an articulated arm system could
be a viable option," says Robert Waddcll, President of Applied
Precision, Inc. (Mississauga, Ontario, Canada), a provider of optical
3D surface digitizing, measurement and inspection solutions. For more
precision, he recommends a white-light or laser-based system.
Nevertheless, Faro arms are said to be useful for taking
measurements within about 0.005-inch accuracy. ESI has CMMs in its shop
(a Mitutoyo BN 1020 and a DEA Gantry) with accuracies of up to 0.0002
inches. It also has a portable, tripod-mounted Faro arm for use in the
field that offers 0.005 inches of accuracy over its 12 feet arm length.
Tavis says that he has used the Faro arm in the field to measure the
entire turbine train, rcpositioning the ami as needed. With it, he was
able to obtain 0.050 inches or better accuracy for the entire distance
of 60 foot. "Of the thousands of measurements that arc necessary, the
Faro Ami quickly produces a CAD model that is extremely accurate and
eliminates most of the human error of tedious hand measurements," he
says.
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