Rapid, Wide-Field Measurements of Complex Transient Shell Vibrations

The mapping and evaluation of complex vibrational fields is often highly desirable in the pressure vessel and piping industry. It is also tedious and expensive using conventional technology such as strain gage and accelerometer arrays. This paper describes field and laboratory measurements made with a portable pulsed laser system that instantaneously captures displacement data over areas up to 2 m², with submicron sensitivity. The results indicate that pulsed holographic or electronic speckle interferometry facilitates the evaluation of nonstationary vibrational fields with significant advantages over conventional techniques. Pulsed interferometry is an effective tool for rapidly determining locations of worst-case dynamic displacements and strains. Initial field measurements at a natural gas pumping station provided an exciting glimpse at both the measurement capability of the pulsed interferometry system and never before seen dynamic responses of turbo-compressor discharge piping. The piping immediate to the compressor discharge nozzle as well as a recycle pipe was investigated at a range of operating conditions. Several characteristic patterns were observed in the instantaneous operating deflection shapes. Most notable were spiral waves progressing both clockwise and counterclockwise relative to the axial flow direction. A “shock,” sudden drop in deformation, presumably caused by instantaneous back pressure, was also captured during an extensive statistical survey. Subsequently, laboratory measurements were made on a pressure vessel built to ASME Code requirements, with various internal fluid and pressure conditions. During shaker excitation, dynamic strains logged from gage rosettes were compared to captured displacements and mode shapes. Interestingly, the ratio of circumferential to axial dynamic strains was found to depend on the operating deflection shape of the vessel. Long, thin antinodes resulted in strain ratios expected for static loading, but short antinodes typical of higher frequency responses were accompanied by significantly increased axial strains. The authors intend to continue investigating the usefulness of pulsed interferometry measurements for the oil and gas industry. It is considered important to further correlate the interferometry measurements with traditional modal analysis and strain measurement techniques. Follow-up efforts will also attempt to quantify the relationship between wide-area vibrations and noise emanation from piping systems. An additional goal is to increase the efficiency of noise abatement solutions using insight obtained from wide-field vibration measurements.