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Pump Integration Inc. personnel have successfully measured static and dynamic strains on a variety of rotating equipment, from steam turbines and gas turbines to jet engines and axial flow compressors, using a variety of signal transfer techniques including slip-rings and radio telemetry. We have also designed, built and calibrated torque transducers for performance grade measurements fitted to existing pump drive shafts that have been supplied by our clients. Hydro-tests, spin-pit tests, and operational tests on this type of equipment are common place support activities offered by PII.

STATIC STRAIN MEASUREMENTS

The application of thin foil static strain gages to key locations on pump and compressor casings for the purpose of pressure qualification or correlation to Finite Element Analysis predictions is common place in the rotating machinery industry. Three element strain rosettes are typically employed so that the principal stresses can be calculated from the strain measurements. The plot at the right shows the strain level of a hoop gage as a function of casing pressure. This value should be a straight line for acceptable strain ranges indicating that the strain level is in the linear response range.

Another common application for static strain measurement is the installation of static strain gages on impeller wheels for strain measure-ment during spin pit testing, to check the FEA predicted stress levels. An instrumented impeller is fitted to a hollow arbor and the leadwork routed to a slip-ring and then to Wheatstone bridge conditioning circuits. As the impeller is speeded up the strain levels are recorded and later plotted vs. impeller speed or speed squared. The plot on the right is typical for a linear strain vs. speed plot, with a parabolic response for strain with speed. 

DYNAMIC STRAIN MEASUREMENTS 

The application of high temperature foil dynamic strain gages on axial flow compressor blading or wire wound flame spray dynamic strain gages on gas turbine blading represents a significant effort due to the teardown and instrumented rotor stack required just to get to the test stand with an acceptable installation. The careful planning of the test program and attention to detail is required for its successful execution. With all this preparatory work completed the experimental investigator is then in a position to provide the client with some of the most valuable and sophisticated information that is supplied to industrial clients today. This type of information was once only available to the aerospace industry.

The picture to the right shows a high-frequency (~ 6,000 Hz) strain gage response to a gas turbine blade decelerating from 6,600 RPM to 6,000 RPM due to the coincidence of a blade resonance with an upstream vane passing excitation frequency. While FEA methods may predict this coincidence, only an actual measurement of the dynamic strain on the blade can rate the significance of that response to fatigue life and other considerations necessary for safe operation. Also the sharpness of that coincident response over the narrow speed range (see above) still must be measured rather than predicted through calculation. 

TRANSDUCER CONSTRUCTION 

Typical transducer applications include four active arm Bending bridges, Thrust bridges and Torque bridges. Their accuracy requirements can be investigative grade (+/- 5% accuracy) requiring no calibration other than calculations made from the structure geometry and material characteristics or performance grade (+/- 1% accuracy) requiring a physical traceable calibration prior to use. An example of a Torque bridge built for performance measurements to be taken at elevated temperatures is shown in the picture on the right.

High temperature epoxy cements were used to install the static strain gages and leadwork. Then the installation was thermally cycled to stability and an apparent strain curve was made for correction (if necessary) of the bridge output due to minor temperature errors. The completed installation was calibrated to an accuracy of +/- 1% Full Scale on a Tinus Olsen torque testing machine. A Wireless Data Corporation model WDC-2100 radio telemetry system (not shown in the photo) was used to power the bridge and to transmit the torque bridge signals.

Other transducer applications include the installation of bending, thrust, and torque bridges on the nozzles of a boiler feed pump to determine transient loading conditions from system piping during plant startup.

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