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Title: Impact Of Pipe Vibrations On Vortex Flowmeters Under Operating Conditions
Author: E. Van Bokhorst, M.C.A.M. Peters, C.H.L. Limpens
Source: 1999 International Symposium on Fluid Flow Measurement
Year Published: 1999
Abstract: The TNO Institute of Applied Physics experimentally investigated the impact of vibrations on the output of five different make 3 inch industrial vortex flowmeters in an air flow circuit under atmospheric operating conditions. To our knowledge the impact of pipe-wall vibrations on the accuracy of vortex flowmeters under flowing conditions has not been investigated in detail so far. Various installation effects, such as swirl, a-symmetry of the flow profile and pulsations are investigated and reported amongst others by Mottram 1. We have investigated the impact of flow pulsations on the accuracy of the five different vortex flowmeters in an earlier study as reported at the Flomeko 1998 2. In the investigation we performed pipe-wall vibrations were imposed in three directions over a wide range of frequencies and vibration amplitudes. It was shown that under these conditions vibrations can cause large systematic errors in flowmeter reading depending on make flowmeter, vibration frequency and amplitude. These errors are caused by the sensor, picking up not only the dynamic forces from the vortex shedding, but also the wall vibration. Subsequently, the sensor cannot distinguish between the vortex signal and the pipe-wall vibration. The reading of the vortex meter at zero or low flow rates is primarily determined by the vibration frequency. This effect was observed already at low vibration acceleration amplitudes in the order of 0.25 g peak. This is, for the frequency range above 20 Hz, below the acceptable level of 20 mm/s peak, regarded as acceptable for pipe systems in a design stage from dynamic stress point of view. This vibration criterion differs from those used in vibration tests intended to check the structural integrity of the meter according to IEC 68-2-6. Concluding, it can be stated that one should be careful with the use of vortex flowmeters when pipe-wall vibrations are present near the metering section with frequencies within or nearby the operating range of the flowmeter, even when the amplitudes of pipe vibrations are small. It should be noticed that the tests are performed for air at atmospheric conditions, the sensitivity for pipewall vibrations will decrease if the medium density increases. The different measures, chosen by manufacturers to decrease the sensitivity of vortex flowmeters for vibrations are so far not fully succesfull and further improvements on construction, electronic filtering, sensor type and sensor location should be investigated.