International Pipeline Conference — Volume 2 ASM E 1996 IPC1996-1914 Transit Time Ultrasonic Flow Meters for Natural Gas Measurement E. LOY UPP AND KEVIN L. WARNER, PH.D.- Daniel Industries Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40214/1049/2506946/1049_1.pdf by guest on 01 October 2021 SUMMARY UFM : Transit-time ultrasonic flow meter. Type I meter : Multipath, spoolpiece style meter Transit-time ultrasonic flow meters for gas have intended for custody transfer accuracy. gained a larger acceptance within the natural gas Type II meter : Single path, spoolpiece style meter. industry in recent years, and are now an option for Type III meter : Single path, insertion (hot tapped) custody transfer metering in several countries. meter. Additionally, there are several varieties of less expensive transit-time ultrasonic flow meters which How Ultrasonic Meters Operate are excellent in check metering applications although limited in accuracy. The proper choice of ultrasonic The basic operation of UFM’s may be understood flow meter normally depends on the absolute without a full knowledge of the science of ultrasonics. accuracy required, with the multipath configuration The generation, propagation, and reception of offering the best accuracy. Ultrasonic flow meters ultrasonic waves and pulses would provide enough must be properly installed and the natural gas must material for a full text, however only the conceptual be of good quality to achieve an accurate part of the measurement needs to be understood here. measurement, as with any type of gas flow meter. As UFM's are velocity meters; they measure the flow experience grows within the measurement velocity of moving gas. Once velocity is determined, community, the use and applications for transit-time a flowrate is calculated from the product of the ultrasonic flow meters may expand greatly. velocity and the meter body cross sectional area. Therefore, a description of the UFM performance can ABSTRACT be separated into two distinct treatises, one on the cross sectional area measurement and the second on The advantages of ultrasonic flow meters are well the meter's ability to measure the velocity of the gas. know - no line obstructions, large turndown ratio, bi-directionality, high accuracy - and ultrasonic On the Cross Section Measurement meters are now widely used within the gas industry in a wide variety of applications. Ultrasonic meters are The meter's cross section must be constant and available in several different configurations ranging known throughout the length of the ultrasonic from high accuracy multipath spoolpiece meters to measurement, typically a few pipe diameters. Clearly single path hot-tapped meters. While all of these if the cross section is not constant, the velocity meters operate on the same basic transit-time through the meter body will vary for a fixed flow rate principle, there are significant differences in their and an accurate measurement would not be possible. operational aspects. This paper is intended to review For both type I and type II meters, the spoolpiece is the basic concepts of ultrasonic flow meters, to manufactured and is expected to be in excellent clarify the differences listed, and to explain how condition. The cross section will be constant ultrasonic meters should be specified. throughout the body and can be measured to acceptable tolerances by mechanical devices. While BODY this may appear to be an easy task, in reality the tolerances necessary for these measurements In order to simply the terminology used in this paper, necessitates the use of sophisticated measurement the following abbreviations are used: equipment. Type I I I meters present are more difficult Copyright © 1996 by ASME to characterize. Typically existing field pipe is not relate to the number of paths (amount of sampling) round and does not have a machined interior surface, and the configuration of those paths. and the cross section measurement must be obtained without intruding the pipe. Circumference and wall One of the simplest methods to measure an object’s thickness measurements (using an ultrasonic average velocity is to measure the time required for thickness gage) along the pipe can yield a cross the object to travel a known distance. In a UFM, the section value which is close to the correct value. The object is an ultrasonic pulse and the average velocity next difficulty is locating the probes on a true pipe of the pulse is measured along a path in two opposing diameter. If the ultrasonic measurement is not well directions. From those two time measurements, the centered, the flow measurement will suffer. average gas velocity along the path of the ultrasonic pulse can be determined. Unfortunately, this number Optimal location of the transducers and a alone is not adequate to determine the average Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40214/1049/2506946/1049_1.pdf by guest on 01 October 2021 measurement of the diameter in type III meters may velocity of the gas in the pipeline because it is an be verified within limits once the transducers are in average value only along the path, not the entire cross place by using the UFM itself. If the composition, section. pressure, and temperature of the gas in the pipe is known then the speed-of-sound of the gas can be In Type II and Type III meters, a flow-dependent calculated. The distance between probes can be correction factor is applied to the velocity reading. determined by multiplying the transit time by the This correction factor is necessary because the speed-of-sound, in either zero-flow or flowing average value along the line is quite different than the conditions. The fine tuning of ultrasonic systems is average value for the cross section. The average sometimes referred to as "optimization." value measured along the line will be higher than the average for the cross section because there is more The error in flow measurement is proportional to the area (actually volume) towards the outer edges of the error in the cross sectional area measurement. For meter than in the center, and the outer gas moves example, if the area measurement is 1% low, the slower than the center gas. The correction factor is flowrate measurement will be 1% low. Errors based on Reynolds number, and corrects the biased associated with unround pipes are more difficult to velocity measurement for the proper average. predict and may not be simple proportions. Partially Typical values for the correction factor are between blocked meter sections present a similar flow 0.92 and 0.98. measurement problem since the effective area is different than the one used in the calculation. The most accurate ultrasonic measurement is one in which multiple paths are used to measure the velocity On the Velocity Measurement of the gas in different regions of a tightly controlled meter body. The Type I meter is often referred to as Velocity meters are designed to determine the correct a custody transfer meter because it offers the average velocity of the gas flowing through a accuracy associated with conventional custody pipeline, not the velocity at the edge, center, or any transfer devices. The velocity measurement along other particular point in the pipe. An ideal velocity each path is determined the same as in Type II and meter would measure the velocity at an infinitely Type El meters, but the gas is measured within close-packed number of grid points, and then sum the various regions, so the average value for the meter velocities over the cross section of the measurement body can be determined with a much higher degree of plane to derive an average velocity. Actual meters confidence. Rather than assuming a symmetric must rely on a finite number of velocity readings and velocity profile, the multipath meter will detect a then infer an average value based on the set of non-symmetric velocity profile, and compute the readings taken. The total number of velocity correct average value for the cross section of the readings in a UFM is equal to the number of meter body. The correction factor associated with ultrasonic paths, and each velocity reading will be the Type II and Type III meters can be eliminated average value along the line connecting the two entirely if the path positions are chosen properly. transducers. For example, single-path meters measure one value of velocity, three-path meters The direction of gas flow in pipelines is not always measure three values of velocity, and six-path meters completely in the direction of the axis of the pipeline. measure six values of velocities. The main In other words, the gas may be moving within the differences in the quality of the average velocity pipeline in directions other than the intended delivery measurement between types I, II, and III meters direction of the pipeline. Common examples of non­ axial flow include swirling gas (rolling around the similar to understanding the accuracy statement of center axis of the pipe) and cross-flowing gas (gas any other type flow meter. A short review of basic moving along a chord of the cross section). In measurement principles is given here. conventional metering systems, the non-axial flow is eliminated by using straightening devices which Accuracy of a measurement device is simply the prepare the flow profile for measurement. By quotient of a measured quantity and the "true" value preparing the flow, response of the primary of the measured quantity. For example if a reference measurement device is closer to the theoretical or 1.000 pound block is weighed on a scale as 1.003 expected value. The practice of preparing the flow pounds, the scale is no more accurate than 0.3% at 1 may be extended to ultrasonic meters for best pound.