It is known that acoustic noise can interfere with Ultrasonic Meter performance!
Instromet offers a unique, free, service to customers who may question whether piping elements such as regulating valves might interfere with meter performance. By completing the questionnaire below, and returning to us for review, Instromet can determine whether meter performance is threatened by noise, and can make recommendations to avoid the negative impact of noise on meter performance. A more detailed description of potential noise effects is also included in the word document attached below.
Acoustic Noise
Acoustic noise may interfere with ultrasonic pulse detection, and therefore, transit time measurement. If the ultrasonic meter cannot make transit time measurements, gas flow measurements cannot be made. Additionally, acoustic noise interference that causes “mis-detection” of ultrasonic pulses can result in mis-measurement of transit times and hence volumetric measurement errors. Therefore users must consider whether interfering acoustic noise is likely to be present in a piping system, and if so, take steps to militate against its affects on UM operation to assure reliable and accurate gas flow measurement. It should be noted that acoustic noise that adversely affects meter operation is also ultrasonic, i.e. outside the audible range of human hearing. An audibly noisy piping system doesn’t necessarily mean there will be problems with successful UM operation, and conversely, audibly quiet piping systems don’t always assure successful meter function.
Acoustic noise may be generated in a piping system from numerous sources related to gas flow turbulence: high gas velocities through piping and/or fittings, protruding probes, flow conditioners or pressure and regulating control valves. Interference with ultrasonic pulse detection occurs when the frequency of the acoustic noise is coincident with the meter’s operating frequency, and is of sufficient amplitude to “drown out” the ultrasonic pulse. UM manufacturers specify the operating frequencies of their transducers, so the frequency range in which a particular meter might be affected by acoustic noise is known. Unfortunately variable operating conditions (flow, pressure and temperature) and the variety of, and signature frequencies for, acoustic noise generators, mean that nearly infinite combinations of frequencies and amplitudes might be generated that can interfere with ultrasonic pulse detection.
UM manufacturers recognize the potential for operating problems, and most UM’s have diagnostic capabilities that indicate whether acoustic noise impairs meter performance while operating. Strategies have also been devised by user’s and manufacturer’s to estimate and/or limit a UM’s susceptibility to noise interference:
 | enhanced signal processing to improve ultrasonic pulse recognition and detection |
| signal filtering to narrow the bandwidth surveyed for better/faster pulse recognition |
| installation of fittings, such as blind tees or filters, between the meter and noise source to isolate or reflect noise from the UM |
| development of specialized silencers that are installed in the piping between UM and noise sources to isolate the meter from the offending noise |
| evaluation of UM response to acoustic noise prior to station installation, based on estimated noise generation given operating conditions and piping design, to determine if a given station design will provide acceptable UM performance |
In general, noise sources upstream of UM’s have a more adverse impact on meter performance than those installed downstream (although installing the source downstream of the UM doesn’t guarantee it won’t generate interference with the meter). The greater the distance, and number of fittings (tees, elbows, etc), located between a meter and noise source, the greater the noise from the source is attenuated, and the consequent adverse impact on meter performance reduced.
When considering installation of a UM, particularly in the vicinity of pressure or flow regulation (the most common noise generators), the following factors should be weighed during the station design phase:
| the valve’s (i.e., noise source) installed position relative to the meter; upstream, downstream, distance between meter and source, number and type of fittings between meter and source |
| operating frequency of the meter, and the range of frequencies generated by the noise source (whisper trim type valves are of particular concern since they achieve operating “quietness” by using designs that generate noise in the ultrasonic range: outside of human hearing, but oftentimes coincident with UM operating frequency) |
| whether, due to the requirement to locate pressure/flow control near a UM, additional attenuation between noise source and UM is required |
| whether enhanced filtering of digital signal processing should be applied, and if so, whether it slows signal processing time beyond acceptable limits (limits prescribed for a linear measuring device in API COGFM Chapter 21.1) |
| the cost/benefit of pursuing one or more strategies to limit UM exposure to offending acoustic noise |
If a UM is to be installed near a potential noise source, it is recommended users contact manufacturers for recommendations specific to their products and the proposed installation prior to finalizing station design. Cooperation between users and manufactures during facilities design can avoid the need for potentially expensive remedial actions at a completed meter installation.