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July/August 2012

Standards

Tip Timing subcommittee generates first worldwide ballot

By Bryan Hayes

One of ISA’s newest committees, ISA107, Advanced Measurement Techniques for Gas Turbine Engines, will be voting on its first document this month, ISA-107.1-2012, Industry Standard File Format for Revolution-Based Tip Timing Data, which will benefit turbine engine manufacturers worldwide.

The subcommittee, ISA107.1, Tip Timing, focuses on an area of instrumentation that deals with the measurement of rotating components. The rotating components are mostly blades in turbine engines but can be compressor blades for wind tunnel plants or large gas turbine power generators.

Multiple tip timing sensors are installed on a rotating component to measure the time of arrival for each blade. Unique tip timing algorithms are used to convert each blade’s time of arrival to a blade tip deflection and frequency. If a detailed finite element model is available, then the tip timing blade tip deflection can convert to a stress field to see if the tip deflection is causing damage to the blade.

Tip timing measures all blades on a single rotating component, which is a major advantage over the typical instrumentation of applying a strain gage directly to the blade. Tip timing requires holes to be machined in the case to view the blades, but the rotating component is not affected.

By writing the new document, the subcommittee’s goal is to develop a standard for gas turbine instrumentation used to measure blade tip deflections during engine operation. The scope is to standardize the application of tip timing instrumentation, including the acquisition and data processing of tip timing data. The objective of the specification is to provide common tools and agreed-upon methods for testing different systems to the same standard.

Tip timing around the world

The major users of tip timing are the turbine engine manufacturers around the world. The advancement of tip timing in the last 10 to 15 years has made applying tip timing sensors to a commercial or military test program very commonplace. Also the ground-based power turbines are taking advantage of tip timing for long-term health monitoring and accelerating products to the market. With so many companies using tip timing across the world, the committee saw a need to develop a standard.

The standard will create tools and processes to apply tip timing internationally. One potential major benefit of a new standard is to gain Federal Aviation Authority (FAA) certification by using tip timing as the primary instrumentation to certify a rotating component on a turbine engine. Once standards from ISA107 are in place, U.S. Air Force turbine engine tests are expected to be required to adhere to these standards and best practices.

Other areas of interest are large plant compressors that provide the air for wind-tunnel test sections. Tip timing sensors are important to the industry because they can last during the rotating component’s lifetime, they are much cheaper to install, and they acquire data for every blade.

Strain gages have a short lifetime, require a large investment in wiring, require a slip ring or telemetry device, and ultimately are very expensive. Tip timing will not completely replace strain gages, but, for the most part, tip timing is used industry-wide to acquire the characteristics of rotating components around the world.

About the Author

Bryan Hayes (bryan.hayes@arnold.af.mil) is the ISA107.1 subcommittee chairman and a senior electrical engineer with Arnold Engineering and Development Center at Arnold Air Force Base, Tenn.