Tuesday, October 19, 2021

Gas Turbine Inspections Take Flight

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EPRI investigates how technology used to inspect jet engines can benefit utilities

A self-described “turbine guy,” Clinton Lafferty knows the problems that can potentially occur if gas turbine blades in a power plant fail. “It can be catastrophic,” said Lafferty, senior program manager for gas turbines at the Tennessee Valley Authority (TVA). In extraordinarily rare circumstances, blades can come loose and damage surrounding equipment and even injure plant personnel. “There is a lot on the line for keeping turbines running reliably for TVA and other utilities. If a turbine failure leads to an outage, it’s not just the costs of replacing the parts. It can also be lost generation revenue, and in the worst cases, it can be a safety issue.”

A big part of Lafferty’s job is looking for ways to lower the costs and improve the speed and effectiveness of inspecting gas turbine blades that are in service or about to be installed. That’s why Lafferty and his colleagues at TVA were eager to participate in an EPRI-led initiative to evaluate the effectiveness of a nondestructive evaluation (NDE) technology called process compensated resonance testing (PCRT) that is new to the utility sector.

PCRT was originally developed by researchers at the Los Alamos National Laboratory to analyze the effects of manufacturing variations on metal components. Albuquerque, New Mexico-based Vibrant Corporation, later commercialized the technology, successfully applying it to the inspection of jet aircraft engines. Like gas turbines in power plants, jet engines have turbine blades that operate in an environment of extreme temperature and pressure. The technology’s approval by the Federal Aviation Administration and a Delta Airlines paper on its effectiveness caught the attention of EPRI researchers.

“Delta told the story of applying PCRT to an underperforming engine with an old design and a history of blade failure—and turning this engine into one of the most reliable units in the fleet,” said Lem Hunter, president, and CEO of Vibrant. “EPRI called the author of the paper and said, we’d like to come and get an introduction to the technology and see it in action on the Delta floor. That led EPRI to get in touch with us.”

For EPRI, the main question was whether PCRT could effectively inspect industrial gas turbine blades, which are much larger than jet engine blades. EPRI, Vibrant, and utilities like TVA have now tested more than 12,000 turbine blades using PCRT. The results so far have convinced gas turbine experts like TVA’s Lafferty that PCRT has an important role in a gas turbine’s inspection regimen, which includes other NDE technologies such as eddy current probes, X-rays, CT scans, and visual inspections by utility personnel.

“If we were to do a full series of NDE tests on a set of blades, it might take anywhere from a few days to several months, depending on which tests we were doing. PCRT takes between one and five minutes for us to identify potential problems in need of further inspection,” said Lafferty. “We’re now looking into how we can integrate PCRT into our inspection process.”

How PCRT Works

Consider that bells make different sounds depending on their shape and material. For example, a bell made of bronze sounds different than a bell of the exact dimensions made of aluminum because each material has a different elasticity. While a particular bell may make the same sound for years, it will emit a very different sound if it cracks. In other words, each bell has a sound signature—a set of sound waves with measurable frequencies—that can reveal a lot about its properties and health.

Like bells, turbine blade components resonate at different frequencies depending on their geometry, mass, materials, and defects. PCRT involves recording and analyzing the resonant frequencies of these components. If a frequency is far out of line with what is considered normal, that can indicate a problem.

Here’s how PCRT works: A technician uses a piezoelectric transducer to feed energy into turbine blades, causing them to resonate at specific frequencies. With the use of algorithms and advanced pattern recognition software, these resonant frequencies are compared with a database of frequency data to identify potential abnormalities. Based on this analysis, turbine blades with standard resonant frequencies earn a passing grade, while those with irregular frequencies fail.

To enable this kind of analysis, EPRI has compiled a database of model-specific blade frequency characteristics of more than 12,000 turbine blades—a number that continues to grow as Vibrant, EPRI, and utility members test more turbine blades. When coupled with this database, PCRT can be used to assess a turbine blade’s current health relative to what would be expected if it were aging normally.

turbine engine

“We are vibrating parts to identify common patterns of resonant frequencies when the parts are performing properly,” said Hunter. “When there are disruptions to the patterns in those resonant frequencies, that can be due to material property changes and damage, such as fatigue.”

“Vibrant owns the technology and the proprietary algorithms, and its work led to the development of ASTM standards for PCRT,” said Nicholas Smith, a technical leader in EPRI’s gas turbine turbomachinery design and maintenance group. “EPRI provides knowledge and understanding of gas turbine components and is working with Vibrant to do the testing and assemble this database.”

Each Inspection Technology Has a Role

PCRT adds to the menu of tools utilities can use to assess turbine blades quickly and inexpensively. “This is a very inexpensive, rapid test, taking just one to five minutes per part. You can inspect all your components in a short amount of time and isolate and identify ones that may be suspect,” said EPRI’s Smith.

X-rays, CT scans, eddy currents, visual inspections by turbine experts, and other techniques each have strengths and limitations. For example, X-rays are extremely powerful in identifying structural defects but can expose workers to radiation, and excessive use can raise health concerns. In addition, X-ray inspections are expensive and require components to be shipped to and from an inspection site, which can take days. Perhaps no inspection is as effective as the close visual examination of a turbine blade by an expert. But that is not a scalable solution, given the high volume of inspections necessary and the attention to detail required to pinpoint problems. For example, a General Electric 7HA gas turbine has 300 blades that require inspection.

PCRT can complement these techniques. “There’s almost never just one inspection that can be done on a part that gives you all of the information that you need,” said Hunter. “If we really want the highest reliability for turbines, we want to employ all of the technologies at the right time, at the right place, and in the right way.”

PCRT’s role in inspections might be to diagnose issues that require additional inspection quickly. “If you are looking for a specific type of defect in a specific area of a component, like an indication of a crack, a fluorescent penetrant inspection is the way to go,” said Smith. “That is the most detailed inspection for that type of defect. But what PCRT can do is perform an inspection of the whole component and point you towards potential problem areas where you could use additional inspection techniques.”

The addition of PCRT to a utility’s inspection toolbox can also provide more assurance about manufacturers’ and maintenance companies’ quality of components and services. “We’re putting tools in the hands of the plant operators to help them determine whether they’re getting the quality that they paid for,” said Hunter.

For its part, EPRI will continue to expand the database of turbine blade characteristics and partner with utilities interested in PCRT. “Our goal is to bring the industrial gas turbine world up to the same speed as the aerospace world,” said Smith. “We want to be able to inspect components on a pass-fail basis. We’re confident this can help utilities perform inspections even more quickly and cheaply than ever before.”

EPRI Technical Expert:

Nicholas Smith