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. Without further information, one may not measurement, but is not always necessary. infer the scale to be 0.3% accurate at any other

Conservative users of UFM’s may choose to include weight. What would the scale read for a 2.000 pound Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40214/1049/2506946/1049_1.pdf by guest on 01 October 2021 conditioners or straighteners to their UFM meter reference block? For a simple offset, the scale would tubes to insure the performance characteristics of the read 2.003 pounds, but for a linear type correction meter. Another option used in some Type I meters is the scale would read 2.006 pounds. With only one to measure non-axial flow components and then data point, a calibration is meaningless and similarly apply correction factors to compensate for these if an accuracy is stated without stating the range, the patterns. This can be a difficult approach since the accuracy statement is meaningless. behavior of unconditioned gas is complicated and often unpredictable. Type I meters can also be Unfortunately world standard organizations do not designed with path configurations which are arranged own a "standard cubic foot” of gas, and so we are to "cancel" specific non-axial effects. The most often forced to speak relatively about the accuracy of accurate method of measurement is to eliminate non- volume measurement devices. It is similarly difficult axial flow, verify the condition of the flow using the and costly to establish “absolute” flow rates. In order UFM, and then measuring the flow rate with a well to calibrate a 12" ultrasonic meter up to 70 ft/sec (21 designed path configuration. m/sec), a flow range from zero to 8 MMSCFH at 500 psi is needed, with increased resolution at the near­ In summary, all transit time UFM's measure gas zero end. Establishing this flow calibration curve is velocities by measuring the time required for challenging and at this time no facility in North ultrasonic pulses to travel known distances, in two America is capable of achieving it. The best data on directions. Single-path meters apply correction ultrasonic flow meters sheds much light on the factors to the measured velocity to compensate for characteristics of these devices. They typically have differences in the velocity distribution across the very flat calibration curves which fall within 0.5% of pipe. Multipath meters use multiple pairs of the reference meter over the range from about 5% of transducers to sample the velocity at different regions full scale to beyond 100% full scale. Below about within the pipeline, and thus measure a truer average. 5%, the resolution factor or zero-offset factors And finally, some multipath meters have paths becomes larger and the absolute accuracy is worse. specifically designed to measure non-axial Meter size is also a factor with smaller meters being components of flow, while others use non-planar path less accurate at low flow velocities than larger configurations to achieve a compensated balance. meters. For example, a 20 nanosecond transit-time difference offset represents a velocity offset of about Understanding Specifications - Accuracy 0.006 ft/sec in a 36" iheter, or a 0.012 ft/sec velocity Statem ents offset in an 18" meter. This is why the turndown ratio of larger meters is higher than for smaller All of the factors mentioned above ultimately meters. The best way to establish the accuracy of influence' overall meter accuracy. Like all ultrasonic meters is to examine a collection of technologies, the ultrasonic meter has certain calibration curves under various conditions, and to advantages over other flow measurement methods, then draw conclusions based on all of the data but meter accuracy will depend on proper obedience available. to the basic rules of good metering practice. Under the best of conditions, uncalibrated ultrasonic flow Understanding Specifications - Turndown Ratio meters have demonstrated accuracy to within 0.5% of reference measurements over a very wide range of Turndown ratio is defined as the ratio between the measurement, sometimes better. Understanding the highest flowrate measurable and the lowest flowrate accuracy statement of ultrasonic flow meters is measurable. High turndown ratios are most easily created by lowering the minimum value to smaller velocity resolution will be about 0.01 ft/sec in a 12" value. For example, if a meter has a maximum meter. This represents about 1% of the total flow flowrate of 100 units and a minimum flowrate of 5 rate at 1 ft/sec flow velocity. units, the turndown will be 20:1. If the minimum can be extended to 0.1 unit, the turndown ratio is now a Flow Calibration and "Dry” Calibration whopping 1000:1. In UFM's the lower limit is somewhat arbitrary because the meters are very The process of calibrating a flow meter is the most sensitive to and can measure small velocities, but direct method to verify the meter's accuracy. In the with decreased accuracy. As discussed previously, case of an uncharacterized meter, such as a turbine the transit-time difference offset becomes more meter, the calibration process will establish the significant at low velocity and the nature of the meter's calibration curve. Flow calibration normally measurement problem changes. The turndown ratio refers to the process of correlating the output of a Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40214/1049/2506946/1049_1.pdf by guest on 01 October 2021 should always be accompanied by an accuracy metering device with a reference flowing condition. statement, or the user may be mislead into a false In other words, a stated amount of flow is put through understanding of the meter’s performance at low the meter and the output of the meter is recorded. flowrates. Other meters may by-pass the flow calibration requirement. Orifice meters are rarely flow Repeatability calibrated because the uncertainly of the measurement can be specified by controlling the The repeatability of UFM's is excellent because the dimensions of the orifice plate along with the proper instruments are not subject to change. The placement of the plate within the carrier. This electronics timing circuitry is known to be stable over method of inferred calibration is desirable because it a term of years, and since there are no moving parts, greatly reduces the cost of a meter while preserving frictional forces do not effect the calibration. Tests at the integrity of the measurement by placing the calibrated flow facilities have verified UFM's to be burden on the manufacturer and installer. repeatable down to the level of the laboratories, typically 0.1 or 0.2%. "Diy" calibration is the UFM equivalent of measuring the dimensions and specifying the position of an Short-term UFM repeatability can be influenced by orifice plate within an orifice meter. If the the method used for data handling. UFMs measure dimensions of the paths and the cross section of a transit times to determine velocities, but normally a UFM can be specified, and if the characteristics of set or ’batch'' of transit times are measured before a the electronics can be adequately characterized, then calculation of velocity is made. Typically batch sizes the calibration of the UFM can be properly inferred. range from 4 to 32 and update times are anywhere After zero-flow verification of the dry calibration is from 0.1 second to S seconds depending on the performed, a UFM is deemed to be "dry calibrated." application. If the batch size is increased, the The dry calibration can be proven at a flow velocity measurement will be more stable and the laboratory by running the meter against a reference repeatability of the meter will improve. meter. The tolerance which is acceptable for the dry calibration is normally referred to as the Resolution "performance envelope" of the meter. If a dry calibrated meter performs within the performance Resolution of an UFM is really the resolution of the envelope, the meter has verified the dry calibration internal clock, typically operating at about 100 Mhz, procedure. Current multipath meters are typically or with a period of 10 nanoseconds. Translating the within 0.5% of the laboratory reference over a range clock's resolution into a velocity or flowrate of about 25:1 by the dry calibration method. As resolution depends on meter size (as discussed in the confidence in the method increases it is likely less accuracy section) and gas properties (the speed of and less people will require proving of the calibration sound). For example, in a small meter 10 curve. Meter size will influence the performance nanoseconds represents a larger velocity increment envelope, mainly due to the transit time difference than it does in a larger meter, and the velocity offset discussed earlier. resolution of the larger meter is better. Additionally, if the speed of sound decreases, then resulting transit Application of Ultrasonic Flow Meters times will be larger and resolution of the Ultrasonic flow meters are electronic devices, and measurement will be better. For a 10 nanosecond their safety certification requires the meter to pass clock period, and a speed of sound of 1400 ft/sec, the both electrical and mechanical tests. The transducers in UFM's are "wetted" (in direct contact with the REFERENCES pipeline gas). Transducers are electromechanical devices which convert between electrical and Dane, H.J., Ultrasonic Measurement of Unsteady Gas mechanical (ultrasonic) energy. Depending on both Flow.. Proceedings : American Gas Association, Las country and operator, the inside of the pipeline may Vegas, May, 1995. require specific safety classifications. Some pipeline operators consider the inside of the pipeline to be a Freund, W.R. and Warner, K.L., Performance "Division 1" area, or "Zone 0" area, and therefore the Characteristics of Transit Time Ultrasonic Flow transducers must pass division one specifications Meters.. Proceedings: Third International Fluid Flow (either through intrinsic safety or explosion proof Symposium, San Antonio, March, 1995. ratings). Other operators do not consider the inside

of the pipeline to be a hazardous area and are less Sakariassen, Reidar, Whv We Use Ultrasonic Gas Downloaded from http://asmedigitalcollection.asme.org/IPC/proceedings-pdf/IPC1996/40214/1049/2506946/1049_1.pdf by guest on 01 October 2021 stringent with their safety requirements. Flow Meters.. Proceedings : North Sea Flow Measurement Workshop, 1995. The safety classification effects the design of the ultrasonic meter by limiting the amount of energy Upp, E.L., Fluid Flow Measurement. Houston : Gulf which can be generated at the transducer when Publishing Company, 1993. transmitting. If a user does not consider the inside of the pipe to be Division 1, then a non-intrinsically safe UFM can be used, producing higher output voltages and corresponding stronger ultrasonic signals. This would make signal processing of the signal easier and the installation requirements in the presence of noise would be simplified.

Trade-ofTs in Ultrasonic Flow Meters

Ultrasonic flow meters have been under development for almost 15 years and have been proposed for applications from check metering to custody transfer. With such a wide range of applications, clearly some users are willing to sacrifice some meter advantages in exchange for others. For example, offshore users are often willing to sacrifice the accuracy gained from straight upstream pipe diameters to lessen the weight and space requirements. If a turndown of 500:1 is essential to a user, it can only be achieved by accepting less accuracy at low flow rates. If absolute accuracy is desired, a user should be aware of the most accurate measurement range for the specific size meter. Literature associated with ultrasonic meters often reflects specific applications and may not apply to all users or applications. Ultrasonic meters are versatile and rugged meters, and their continually increasing application is almost certain.

THANKS

The authors would like to express their thanks and appreciation to William R. Freund, Jr. and Klaus Zanker for their diligence in advancing the art of ultrasonic gas flow meters for custody transfer.