Helical Strakes: Lab Testing Thermowells & Sample Probes
Orbital Gas Systems, in partnership with Daily Thermetrics, conducted flow testing on May 1, 2017, at a respected North American independent metering research facility. This first-of-its-kind test proved the effects of vortex-induced-vibration on traditional and helical probe geometries in a typical high-pressure, high-velocity pipeline environment. This testing was commissioned to validate thousands of hours of research, whitepapers, CFD models and millions of hours of operational service over the past 12 years by tracking data and performance in a real-time, real-world, setting.
The probes were tested in a high-pressure recirculation test loop used to simulate flowing conditions in natural gas transmission pipelines using distribution-quality natural gas as the flowing medium. The testing was carried out at a range of velocities/flowrates within both a 6” and 12” pipeline at a pressure of ~950psi.
The testing validated the ASME PTC 19.3 code in terms of calculating both the natural frequency and the wake frequency for a sample probe or thermowell. Although the natural frequencies of the 2 types of thermowell differed slightly (as expected due to helix geometry), across the full range of velocities tested there was clear vibration in both the in-line and transverse directions for a traditional ASME thermowell that were not present at any velocity for the helical thermowell.
Given the trends in the data from the 12-inch helical strake thermowell, the strain gauges in the 6-inch helical strake thermowell would have been expected to measure strains at a cyclic frequency around 440 Hz. However, no frequency content in the region around 440Hz was measured (above the typical noise level of 0.01 με to 0.02 με across the spectrum). This is possibly due to turbulence created by the helix having insufficient energy to cause any deflection in the increased stiffness of the shorter thermowell (compared to the 12” line). This confirms that the helix is disrupting the vortices that would typically be shed and therefore, removing the cyclic strain on the thermowell/probe flange/shaft connection.
The strain due to the tip displacement was consistently 2 orders of magnitude greater for the ASME thermowell than in the helix thermowell ( 98% or greater reduction in strain). This crucial strain reduction proves that the dynamic wake frequency calculations according to ASME PTC 19.3 are now less relevant than the static bending and/or pressure/ temperature calculations for a helical straked sample probe or thermowell. The static loads are now the driving design parameter when calculating the length, diameter and wall thickness allowable.
Although often ignored, the ASME thermowell continues to vibrate, albeit at a lesser magnitude due to the wake frequency not matching the natural frequency, even when outside of the lock-in zone.