1、 AGA Report No. 9 Measurement of Gas by Multipath Ultrasonic Meters Third Edition July 2017 Prepared by Transmission Measurement Committee Operations rather it is an indication that a measurement is more accurate when it offers less error or uncertainty. Confidence Level The probability, expressed a
2、s a percentage, that the true value lies within the stated uncertainty. For example: A proper uncertainty statement would read: “500 lb/h 1.0% at a 95% level of confidence.“ This means when sampled numerous times, it is expected that approximately 95 out of every 100 observations are between 495 lb/
3、h and 505 lb/h. Calibration The process of determining, under specified conditions, the relationship between the output (or response) of a device to the value of a traceable reference standard with documented uncertainties. The relationship may be expressed by a statement, calibration function, cali
4、bration diagram, calibration curve or calibration table. In some cases, it may consist of an additive or multiplicative correction of the indication with associated measurement uncertainties. Any adjustment to the device, if performed, following a calibration requires verification against the refere
5、nce standard. Error The result of a measurement minus the reference value of the measurand. Note: Since a true value cannot be determined, in practice, a conventional true (or reference) value is used. Error, Percent % error = (measured value reference value) / reference value x 100% Flow Meter Body
6、 The pressure-containing section of the meter where the gas velocity flow measurement is determined. Flow Meter, Multipath Ultrasonic Multipath ultrasonic meters have at least two independent pairs of measuring transducers (acoustic paths). Inside Pipe Diameter The inside diameter of a pipe, as dete
7、rmined from direct physical measurement or calculated from pipe schedule and wall thickness. Length, Settling The distance required between a flow disturbance and a flow conditioner that allows the flow conditioner to function properly. Maximum Error The allowable error limit within the specified op
8、erational range of the meter. Maximum Peak-to-Peak Error The difference between the largest and smallest error values within a specified flow-rate range. 4 Maximum Speed-of-Sound (SOS) Path Spread The maximum difference in speed-of-sound values between any two acoustic paths. Mean error The arithmet
9、ic mean of all the observed errors or data points for a given flow rate. Measurement Uncertainty A parameter, associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measured quantity. This dispersion includes all compone
10、nts of uncertainty, including those arising from systematic effects. The measurement uncertainty is typically expressed as a standard deviation (or a given multiple of it), defining the limits within which the true value of the measurement is expected to lie with a stated level of confidence. Meteri
11、ng Package A piping package that consists of a meter and adequate upstream and downstream piping, along with thermowell(s), sample probe, and any flow conditioning to ensure that there is no significant difference between the results indicated by the meter in the laboratory and those indicated in th
12、e final installation. No-Flow Cutoff A flow rate below which any indicated flow by the meter is considered to be invalid and indicated flow output is set to zero. (historically referred to as “low-flow cutoff”). Nominal Pipe Diameter (ND) Pipe diameter corresponding to Nominal Pipe Size. For example
13、, the ND of schedule 40 NPS 4 pipe is 4 inches, whereas the inside pipe diameter may be 4.026 inches. qi The flow rate through a meter under a specific set of test or operating conditions. qmax The maximum flow rate through a meter that can be measured within the specified performance requirements a
14、t a specific process condition. qmin The minimum flow rate through a meter that can be measured within the specified performance requirement at a specific process condition. qt The transition flow rate through a meter at which performance requirements may change. Reference Flow Meter A meter or meas
15、urement device of proven flow measurement uncertainty. Reference Gas A gas of known physical properties, e.g., nitrogen, that is used as a baseline for comparison. 5 Repeatability The closeness of agreement between the results of successive measurements of the same measurand carried out under the sa
16、me conditions of measurement. Notes: 1. These conditions are called repeatability conditions. 2. Repeatability conditions include: the same measurement procedure, the same observer, the same measuring instrument used under the same conditions, the same location, and repetition over a short period of
17、 time. 3. Repeatability may be expressed quantitatively in terms of the dispersion characteristics of the results. 4. A valid statement of repeatability requires specifications of the conditions of measurement (temperature, pressure, gas composition, etc.) that may affect the results. When a value o
18、f repeatability is given, a note shall be provided indicating the specific calculations used to compute the dispersion characteristics. Reproducibility The closeness of agreement between the results of measurements of the same measurand carried out under changed conditions of measurement. Notes: 1.
19、A valid statement of reproducibility requires specification of the conditions changed. 2. The changed conditions may include one or more of the following: Principle of measurement, method of measurement, observer, measuring instrument, reference standard, location, conditions of use, or time. 3. Rep
20、roducibility may be expressed quantitatively in terms of the dispersion characteristics of the results. 4. A valid statement of reproducibility requires specification of the changed conditions of measurement that may affect the results. When a value of reproducibility is given, a note shall be provi
21、ded indicating the specific calculations used to compute the dispersion characteristics. Resolution The smallest change in the measurand that can be observed. Roughness Average (Ra) The roughness average (Ra) used in this report is that given in ANSI B46.1, and is “the arithmetic average of the abso
22、lute values of the measured profile height deviation taken within the sampling length and measured from the graphical centerline” of the surface profile. Significant Change The difference in a value that can be shown, through statistical analysis, to be different from a previous value. 6 Speed-of-So
23、und (SOS) Deviation The difference, in percent, between the average speed of sound reported by a meter and the speed of sound of the gas being measured, as calculated per AGA Report No. 8, Part 1: DETAILED Equation of State or Part 2: GERG-2008 Equation of State. True Value The value determined with
24、 a perfect measurement process. The true value is always unknown because all measurement processes are imperfect to some degree. Velocity Sampling Interval The time interval between two successive gas velocity measurements by the full set of transducers or acoustic paths. Zero-flow Reading The maxim
25、um allowable flow velocity reading when the gas is assumed to be at rest, i.e. both the axial and non-axial velocity components are essentially zero. 7 3.0 Operating Conditions 3.1 Gas Quality The meter shall meet the performance requirements in Section 6 operating within the natural gas property ra
26、nges specified in AGA Report No. 4A, 2009 revision, Table 4.1. If any of the gas properties are outside of this range, the manufacturer should be consulted. The manufacturer should also be consulted if the operating conditions are at or near the critical density of the natural gas mixture. Deposits
27、due to normal gas pipeline conditions (e.g., condensates, glycol, amines, inhibitors, water or traces of oil mixed with mill-scale, dirt or sand) may affect the meters accuracy by reducing the meters cross-sectional area and changing the surface roughness, thus affecting the gas velocity profile. In
28、dependent of transducer mounting, deposits may also attenuate or obstruct the ultrasonic sound waves emitted from and received by the ultrasonic transducers or reflected by the internal wall of the meter. 3.2 Pressures Ultrasonic transducers used in USMs require a minimum gas density (a function of
29、pressure) to ensure acoustic coupling of the sound pulses to and from the gas. Therefore, the designer shall specify the expected minimum operating pressure as well as the maximum operating pressure. 3.3 Temperatures, Gas and Ambient As a minimum, the USM should operate over a flowing gas temperatur
30、e range of -4 F to 140 F (-20 C to 60 C). The designer shall specify the expected operating gas temperature range. The operating ambient air temperature range should be at a minimum -40 F to 140 F (-40 C to 60 C). This ambient temperature range applies to the flow meter body with and without gas flo
31、w, field-mounted electronics, ultrasonic transducers, cabling, etc. If the meter and the associated electronics are in direct sunlight, the temperature limits stated may not be adequate. The manufacturer shall state the flowing gas and ambient air temperature specifications for the multipath ultraso
32、nic meter, if they differ from the above. 3.4 Gas Flow Considerations The flow-rate limits that can be measured by a USM are determined by the actual velocity of the flowing gas. The designer should determine the expected gas flow rates and verify that these values are within the range specified by
33、the manufacturer. The designer should also consider the maximum velocity for piping and equipment safety (e.g., “API RP 14E Offshore Production Platform Piping Systems”, “API MPMS Chapter 14, Section 1 Collecting and Handling of Natural Gas Samples for Custody Transfer”, etc.). USMs have the inheren
34、t capability of measuring flow in either direction with equal accuracy; i.e., they are bi-directional. The designer shall specify if bi-directional measurement is required so that the manufacturer can properly configure the SPU parameters. The designer/operator is cautioned that operating ultrasonic
35、 meters at flow rates below qt may incur greater measurement uncertainty due to potential thermal gradients and non-ideal flow profiles. 8 3.5 Upstream Piping and Flow Profiles Upstream piping configurations (i.e., various combinations of upstream fittings, valves, regulators, and lengths of straigh
36、t pipe) may affect the gas velocity profile entering a USM to such an extent that significant measurement errors may result. The magnitude and sign of any error will be, in part, a function of the meters ability to correctly compensate for such conditions. Research results have shown that this effec
37、t is dependent on the meter design, as well as the type and severity of the flow profile distortion produced at the meter. Although a substantial amount of data is available on the effect of upstream piping, the full range of field piping installation configurations has not been studied in detail. M
38、eter station designers/operators may gain insight into expected meter performance for given upstream piping installation configurations by soliciting available test results from meter manufacturers, or by reviewing test data found in the open literature. To confirm meter performance characteristics
39、for a particular piping installation configuration, flow calibration of the metering package, with the same upstream piping configuration, may be required. 3.6 Acoustic Noise The presence of acoustic noise in a frequency range coincident with a USMs operating frequency may interfere with pulse detec
40、tion and, therefore, transit time measurement. If the USM cannot detect pulses, the transit times between transducers cant be measured and flow measurement ceases. Acoustic noise interference can also cause pulse “miss-detection” resulting in erroneous transit time measurements that translate into v
41、olumetric errors. Designers shall consider whether interfering acoustic noise is anticipated at a particular installation and take steps to prevent adverse effects on USM performance during the station design phase. Acoustic noise may be generated from numerous sources related to gas flow turbulence
42、: e.g., high gas velocities through piping and/or fittings, protruding probes, flow conditioners, pressure and regulating control valves, etc. Since USM manufacturers specify the operating frequencies of their transducers, the frequency range in which a particular meter might be affected by acoustic
43、 noise is known. Dynamic operating conditions (flow, pressure and temperature), and the variety of acoustic noise generators, make prediction of offending noise frequencies difficult. Consequently, decoupling a USMs operating frequency from piping system noise can be challenging. Manufacturers recog
44、nize the potential for operating problems, and most USMs have diagnostic outputs that indicate when acoustic noise impairs meter performance. The following strategies have been devised to estimate and/or limit a USMs susceptibility to noise interference: Enhanced signal processing to improve ultraso
45、nic pulse recognition and detection Signal filtering to narrow the bandwidth surveyed for better/faster pulse recognition Evaluation of USM response to acoustic noise prior to station installation Attenuation between noise source(s) and USM, if required, could include blind tees, other fittings, or
46、acoustic filters. The designer should be aware that close-coupling of pipe fittings, such as blind tee fittings, may distort velocity profiles. In general, noise sources upstream of USMs have a more adverse impact on meter performance than those installed downstream, although downstream installation
47、 of pressure reduction or other noise generating equipment does not guarantee interference will not occur. 9 When considering installation of a USM, particularly in the vicinity of pressure or flow regulators, the following factors should be assessed during the station design phase. The valves (i.e.
48、, noise source) installed position relative to the meter upstream or downstream, distance between meter and source, number and type of fittings between meter and source. Operating frequency of the meters ultrasonic transducers, the range of frequencies generated by the noise source, and any digital
49、signal processing features that can be implemented that do not impact the accuracy of the meter. Additional separation between the USM and the noise source. Signal processing to improve ultrasonic pulse recognition and detection. When installation of a USM near a potential noise source is anticipated, the designer should contact the manufacturer prior to finalizing the station design. Cooperation between designer and manufacturers during facilities design can avoid the need for potentially expensive remedial actions after the meter is placed in service.
