Acoustic Effects in Metering Stations; Impacts on Performance of Flow Metering Equipment
H.J. (Henk) Riezebobs
Abstract:
Acoustics plays a central role in the current state-of-the-art gas metering equipment. In the last decade the benefits of ultrasonic techniques in fiscal metering of high gas volumes has become apparent. These metering technique expresses lots of advantageous features, like high accuracy, large rangability, low pressure drop and self-diagnostic capabilities. Recently in addition to flow metering applications the acoustic techniques have also been applied in energy flow or Wobbe metering devices. An example of this is the Ensonic, a fast energy meter, which uses a correlative technique based upon measurements of the velocity of sound (VOS) at two different pressures and the CO2 percentage. This technique yields a fast, low cost device, which is with respect to uncertainty (< 0,3 %) and reproducibility (< 0,1 %) fully comparable to the highly accurate field GC which are used for custody transfer purposes. Because of its very fast measuring cycle of <~ 5 seconds this technique is excellently suited for gas quality control purposes. Application of these techniques raises the question whether acoustic effects always present in piping systems can influence the reliability and accuracy of the measurements. Acoustic effects are present in any piping systems, either through sources like compressors or valves, but also through possible flow induced pulsations. All flow measurement devices in some way are affected by their presence. Orifice and venturi flow meters, based on measurement of differential pressure across a restriction experience a square-root error and/ or gauge line errors due to pulsations. Turbine meters experience so-called rotor-slip errors when operating in dynamic acoustical conditions. Moreover, practical experience at Gasunie shows that sometimes turbine meters are rotating under the influence of acoustic waves even when they are positioned in a standby metering run with closed valves! Some examples are shown in the paper. How about the devices based upon the current state-of-the-art ultrasonic measurement techniques? Theoretically some aliasing errors can occur when the acoustical periods are close to the sampling time interval. In practice we have noticed some other effects. A serious drawback of the ultrasonic measurement technique is its sensitivity to noise of pressure reducing elements. It is somewhat ironical, that especially low-noise valves have more disturbing ability than regular valves. This ability originates in the fact, that low-noise techniques focus on the audible noise region and this sometimes implies a shift of the noise levels to the more ultrasonic part of the spectrum, where they can influence the signal-to-noise ratio of the US metering equipment. Besides disturbing valves also certain flow straighteners can produce acoustic effects. These effects have been studied at Gasunie Research in bi-directional metering run tests at our flow facility in Westerbork. In such a setup always one of the two flow straighteners experiences an opposite flow direction. Such a flow straightener will produce whistling sounds at high flow velocities, which at a certain levels will affect the US metering performance. As long as metering devices remain sensitive for all types of acoustic influences it is strongly advised to have an acoustic eye already in the design stage of metering or calibration facilities. Acoustic control is crucial in obtaining state-of-the-art quality and low measurement inaccuracy.
Keywords:
Natural gas metering, Acoustic design, Acoustic techniques, Flow and Energy Metering, Pulsations, Ultrasonic Noise, Metering Accuracy and Reliability
