Multipath transit-time clamp-on ultrasonic flow meters (UFM) have been employed in the gas industries for many years. Since their inception, technological advancements have been made regarding configurations, electronics and sensor design. Today UFMs have proven to be reliable, versatile and capable tools for meeting the demands of the gas markets – but it is important to understand that different UFM technologies offer different advantages, and that these advantages ultimately allow one solution to outshine the others.
In order to make the best choice and avoid challenges that can lead to lost time and profits, it is important to consider several key factors when selecting a UFM for a gas application. Among these factors are governmental and industry standards, benefits of intrusive vs. non-intrusive ultrasonic flow technology, installation methods, ease of use, long-term costs, flexibility, repeatability and accuracy.
The standard most important to the midstream gas pipeline industry is AGA-9 from the American Gas Association: “Measurement of Gas by Multipath Ultrasonic Meters, Second Addition, April 2007.” This report sets the operational requirements that the gas industry expects from ultrasonic flow meters used for gas pipeline flow measurement and includes supporting details about the mechanical and electrical components needed to support this type of measurement. AGA-9 also addresses circumstances when multipath UFMs (installations with more than one pair of transducers on a pipeline wall) will be required to improve performance in more challenging or turbulent flow profiles.
Intrusive vs. Non-Intrusive Ultrasonic Meters
Intrusive (or insertion) ultrasonic flow meters inject an ultrasonic beam across the pipe in a configuration referred to as chordal. Multiple chordal beams are used to gather data for velocity averaging and flow profile information. One of the advantages of the intrusive UFM is that the chordal design provides an inherently unique perspective of the gas flow profile. Each chordal beam measures a section of gas and when all the beams are combined, a picture of the gas flow velocity profile can be developed with a high degree of precision.
Although almost all intrusive-type UFMs are commonly termed “wetted,” there is a barrier between the actual sensor and the gas medium to allow for removal and replacement of the sensor. Typically, intrusive UFMs come as a flanged spool section ready to be inserted into a pipe. One difficulty is that the sensor cavities, or wells, that enable use of the chordal design can become agglomerated or clogged with particulates and non-gaseous contaminates. Higher pressures can also challenge intrusive UFMs because of the flange rating limitations and the potential for leakage through the sensor ports.
Non-intrusive UFMs, also known as clamp-on ultrasonic flow meters, feature sensors mounted externally to the pipe wall. The sensors inject an ultrasonic signal into the pipe, which uses the pipe wall as a carrier or sound waveguide into the flow stream. Clamp-on UFMs offer flexibility in mounting the sensors virtually anywhere on the pipeline with little effort and without significant piping configuration concerns. This improves the operator’s ability to measure the flow of gas through a pipeline since the pipe wall is never altered, which alleviates concerns about pipeline pressures.
Direct Mount vs. Reflect Mount
There are two basic mounting methods for non-intrusive clamp-on UFMs: direct mount and reflect mount. Direct-mounted sensors can be physically located on the opposite sides of a pipe, offset axially at a predetermined spacing based on the pipe size and the type of gas being measured. This principle allows for direct transmission between sensors across the flow path of a gas stream.
In reflect mount, the sensors are placed on the same side of a pipe, spaced axially apart. This mounting method increases accuracy because the sound beam traverses the gas medium twice. Essentially, one signal path of a reflect-mount sensor pair can produce similar averaging and accuracy as two direct-mount sensor pairs. Because of the advancement in technology, reflect mounted sensors have become the gas industry’s preferred choice when using clamp-on technology.
Narrow vs. WideBeam Clamp-on Technology
Narrow beam, or shear beam, sensors generate a high-energy acoustic signal into the pipe wall that effectively forces the signal through the pipe and into the gas. Typically, these types of sensors use a narrow range of frequency around 1Mhz. The higher voltage is required in order to pierce the pipe and enter the flow stream, transmit the signal across the gas media, and then exit the pipe to be received by the opposing sensor. Such high energy usually produces a high level of noise that is injected into the pipe wall. This noise is ultimately present on the receiving sensor; therefore, powerful software filtering must be applied to reduce the excessive signal-to-noise ratio. Such narrow beam sensors are also sensitive to anomalies in axial gas flow that can lead to poor performance or failure of the receive signals across the inside pipe diameter.
WideBeam or Lamb wave technology is designed to utilize the resonant frequency of the pipe wall. It assists and transmits the sound signal into the flowing media, with the wall of the pipe acting as a waveguide that injects the signal into the flowing gas. This method allows for a low transmit voltage and produces a focused, coherent signal that covers a large axial area of the inside pipe diameter. Because this technology makes use of a wide beam, it is also much more resilient to anomalies in axial gas flow. It takes a very high concentration of particulate matter across the flow stream to cause any disturbance to the flow measurement, transmitting via a Lamb wave. This ability to overcome and accurately measure gas flow even when there is contamination, less-than-perfect pipe geometries or process noise means has led WideBeam to become the default choice for much of the developing gas flow piping infrastructure.
Calibration and Installation Accuracy
Every ultrasonic flow meter is judged by the reliability and accuracy demonstrated after it has been calibrated. AGA-9 requires that UFMs have an error of 0.7% without any adjustment on 12-inch or larger pipes. For a 10-inch or smaller pipe, the error should be ±1.0% for low to maximum flow ranges. The maximum repeatability error should be ±0.20%. Thus, the “out-of-the-box” performance rating of a UFM is what determines the suitability of the sensor and ultimately the end user’s satisfaction.
Several factors can affect a meter's accuracy and repeatability. For example, when temperature varies internally in a pipeline, the result is thermal stratification, or horizontal layers of differing densities produced by changes in temperature. To properly address such changes, a clamp-on UFM is paired with an internal or external temperature monitor. In addition, if flow is turbulent, AGA-9 recommends the use of flow conditioners or to install the UFMs in a long, straight pipe run when possible to laminarize the flow. While the meter will measure gas flow even when the profile is less then laminar, a good straight run will improve the flow profile and, thus, the accuracy. The pipeline surface must also be considered. Ensuring that the proper permeant or temporary acoustic couplant is used between the transducers and the pipe will result in strong sonic contact, thereby reducing or eliminating the number of times maintenance will be required over the life of the measurement system.
Application Story: Evaluating the Right Flow Meter Fit
A natural gas company in the western United States decided to optimize operation by adding gas ultrasonic flow meters to its pipelines. These pipes encompass several compressor stations and receipt meter stations where the gas volumes entering the pipeline are measured. What was missing was an effective pipeline monitoring system between the primary measuring points. Rather than spending time and money on troubleshooting the entire system, the company decided to future-proof it instead by installing clamp-on UFMs.
The main reason for this decision was that the sensors can be clamped on to the outside of the pipes, eliminating the need to stop the flow and to cut the pipes, which can result in lost revenue and additional installation costs. The dedicated meters were strategically located at the inlet, compressor, delivery and outlet stations, feeding the control center with constantly updated and accurate flow data. By monitoring the day-to-day, real-time data, the operators get an exact picture of the pipeline performance. This enables quick and effortless detection of any measurement discrepancies or other issues.
This solution offered a significant improvement over the company’s existing system, which was using orifice plates and positive displacement transmitters coupled with a flow computer. The gas company now has flow technology that ensures easy and cost-efficient installation, accurate operation, and optimized performance; it also enables fast detection and troubleshooting of any system inconsistencies.
As the gas pipeline industry continues to expand, it is important that end users examine all the factors discussed in order to determine the right flow measurement fit for their application needs. With the regulations already defined for the industry, it is simply a matter of finding the best possible option among the available flow meters. The good news is that non-intrusive clamp-on ultrasonic flow meters have evolved to become custom-made and cost-effective as well as highly reliable for use in the gas industries pipelines.
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