1、This is a sample of format to be used in other Technical Notes Engineering Technical Note Prepared by the AGA Operating Section Distribution Measurement Committee 400 N. Capitol St., N.W., 4thFloor Washington, DC 20001 U.S.A. Phone: 202-824-7000 Fax: 202-824-7082 Web site: www.aga.org The Theory and
2、 Operations of Meter Shop Sonic Nozzle Proving Systems for the Natural Gas Industry Copyright 2003 American Gas Association All Rights Reserved Catalog No. XQ0308 March 2003 This is a sample of format to be used in other Technical Notes TABLE OF CONTENTS TABLE OF CONTENTS ii DISCLAIMERS AND COPYRIGH
3、Tiv ACKNOWLEDGMENTSv ABSTRACT .vi 1. SONIC NOZZLE THEORY 1 1.1. Definition of a Sonic Nozzle Element1 1.2. Sonic Nozzle Development and the Need for Its Use 1 1.3. Sonic Nozzle Design 2 1.4. The Toroidal Sonic Nozzle Design 2 1.5. Nozzle Flow Calculations 4 2. APPLICATIONS OF SONIC FLOW NOZZLES6 2.1
4、. Calibration Methods of Sonic Flow Nozzles6 2.2. Bell Prover Calibration of Sonic Flow Nozzles .6 2.3. Various Applications for the Calibrated Sonic Nozzle 9 2.4. Sonic Nozzle Proving of Gas Meters Positive Pressure Versus Atmospheric Pressure Proving .9 2.5. The Sonic Nozzle As a Gas Meter Proving
5、 Device .10 3. THE SONIC NOZZLE PROVER FOR TESTING GAS METERS 12 3.1. The Sonic Nozzle Array.13 3.2. Measurement Instrumentation14 3.3. Prover Flow Path and Vacuum Source.15 3.4. Overall Prover Construction.16 3.5. Meter Volume Measurement17 3.6. Meter Handling and Testing Features 19 3.7. The Prove
6、r Control System 19 3.8. Human Interface with the Prover .20 3.9. Safety Features .20 4. GENERAL OPERATION OF THE SONIC NOZZLE PROVER .21 4.1. Gas Meter Proving and Testing Operations .21 4.2. Saving of Meter Test Data23 4.3. Diagnostics.23 5. MAINTENANCE AND CALIBRATION OF THE SONIC NOZZLE PROVER 2
7、5 5.1. Suggested Maintenance and Service Schedule.25 5.2. Calibration of Sensors and Instrumentation .26 iiThis is a sample of format to be used in other Technical Notes 6. RECERTIFICATION OF SONIC NOZZLE PROVERS.28 6.1. Certification of the Master Bell Prover 28 6.2. The Bell Interface Procedure28
8、6.3. Acceptance of the Bell Interface Method.30 7. COMMON TROUBLESHOOTING CONCERNS31 8. SUMMARY34 9. REFERENCES 35 iiiThis is a sample of format to be used in other Technical Notes DISCLAIMERS AND COPYRIGHT The AGA and the AGA Distribution Measurement Committee disclaim liability for any personal in
9、jury, property or other damages of any nature whatsoever, whether special, indirect, consequential or compensatory, directly or indirectly resulting from the publication, use of, or reliance on this document or whether based on information contained in or omitted from this document. All warranties,
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13、sers own discretion and at its own risk. The user should seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Permission is granted to republish material herein in laws or ordinances, and in regulations, administrative orders, or simi
14、lar documents issued by public authorities. Those desiring permission for other publications should consult the Operating and Engineering Section, American Gas Association, 400 North Capitol Street, NW, 4thFloor, Washington, DC 20001, U.S.A. Copyright 2003 American Gas Association, All Rights Reserv
15、ed ivThis is a sample of format to be used in other Technical Notes ACKNOWLEDGMENTS This technical note represents work by the members of the Distribution Measurement Committee Sonic Nozzle Prover Task Group, chaired by Terry Camden, Vectren Energy. The collaboration of Gregory Germ American Meter C
16、ompany, Donald Jones Actaris US Gas, Paul Werner Heath Consultants, Harry Deutsch Measurement Systems, Phil Whittemore Dresser Measurement, and Ken Meates Halliburton, is appreciated. In addition, the following list includes those who provided information for the technical note, reviewed the drafts,
17、 offered comments and helped in the writing of this document. Their contributions are acknowledged with thanks. Though every attempt was made to include in the list everyone who contributed, we sincerely regret if any omissions have occurred. Last Name First Name Organization Cavey Lee BGE Fraser La
18、rry Measurement Canada (Retired) Higgins Alan Piedmont Natural Gas Johnson Ron KeySpan Energy Kopidlansky Russ Wisconsin Public Service Leary Brian Pacific Gas its a design that has become the international standard for critical flow measurement devices. Figure 1.1 shows the modern toroidal sonic no
19、zzle design. Please note that the majority of the nozzle dimensions are in terms of the nozzle throat diameter (d). Figure 1.1: THE TOROIDAL NOZZLE DESIGN 2This is a sample of format to be used in other Technical Notes For a given throat diameter, d, the inlet plane of the nozzle element perpendicul
20、ar to the axis of the symmetry of the throat diameter is equal to 2.5 times the throat diameter, or 2.5*d, with an allowable uncertainty of +/- 0.1d. The converging part of the sonic nozzle converges from the inlet plane to the nozzle throat diameter with a radius of curvature equal to 2 times the t
21、hroat diameter, or 2.0*d, with an allowable uncertainty of +/- 0.2d. The inlet convergent part of the nozzle actually extends through the throat and is tangent to the nozzles recovery cone, or divergent section. The nozzle divergent section, or recovery cone, forms a frustum of a cone with a half-an
22、gle between 2.5 and 6 degrees or a full cone angle between 5 and 12 degrees. The length of the recovery cone must be at least one times the throat diameter. Modern designs permit a recovery cone length between 7 times to 10 times the throat diameter, or between 7*d to 10*d, with a divergent angle be
23、tween 7 or 8 degrees. Smith and Matz originally designed their critical flow nozzle to have a recovery cone angle equal to 12-degrees. Further testing of sonic nozzle designs by the natural gas industry found that a more shallow recovery angle equal to 7 or 8 degrees actually provided better pressur
24、e recovery, meaning that the shallow nozzle design was more practical than the steeper 12-degree cone. Figure 1.2 shows the ASME/ANSI standard nozzle design, from the MFC-7M publication. Please note that a specification has been chosen for the nozzles internal surface roughness. A surface roughness
25、no greater than 0.000015d is required in the convergent and nozzle throat section (the entire radius of curvature), and a surface roughness no greater than 0.001d is required in the nozzle recovery cone. The minimal surface roughness specification is required to guarantee that the fluid flow through
26、 the nozzle remains streamlined and attached to the internal wall, minimizing the boundry layer thickness. Figure 1.2: ASME/ANSI NOZZLE STANDARD 3This is a sample of format to be used in other Technical Notes The specified nozzle geometry outlined by Smith and Matz was developed to facilitate a more
27、 accurate method for calculating the nozzles discharge coefficient. As stated earlier in this section, the discharge coefficient is required for computing deviations from ideal flow conditions, or deviations from one-dimensional isentropic flow. Distortions from ideal flow conditions are created fro
28、m a finite boundary layer thickness between the fluid profile and the internal nozzle wall and the slight change in flow direction as the fluid (or gas) travels in through the nozzle inlet plane and through the convergent section. The Smith and Matz design has been thoroughly tested by the aerospace
29、 and natural gas industry to ensure that the value of the nozzle discharge coefficient is minimized. 1.5. Nozzle Flow Calculations The standard sonic nozzle flow equation is expressed in terms of mass flow rate and is defined as follows: QM= Cd* At* C* PP* (RM TP)1/2Where QM= mass flow rate (lb./sec
30、.) Cd= nozzle discharge coefficient (dimensionless) At= nozzle throat area (in.2) C = nozzle critical flow factor, which is = (k)1/2 (2 / (k+1)(k+1) / (2(k-1) where k = ratio of specific heats for the fluid or gas PP= nozzle inlet plenum pressure, absolute (psia) RM= gas constant = 48.03 / (Molecula
31、r Weight) TP= nozzle inlet plenum temperature, degrees Rankine (oR) Since the need for sonic nozzles in the natural gas industry is mostly for volume measurement and analysis, the calculation of volumetric flow rate of gas or air through a nozzle is more convenient than that of mass flow rate. The v
32、olumetric flow rate through the nozzle can be expressed in terms of the upstream stagnation pressure and temperature. The volumetric flow rate is defined as QV= Cd* At* C* ZP* (RVTP)1/2Where QV= volume flow rate (cubic feet per second) at an upstream pressure and temperature ZP= compressibility fact
33、or upstream of the nozzle RV= gas constant = 2.398 / (Molecular Weight) 4This is a sample of format to be used in other Technical Notes The terms in the above equation are fixed for a given nozzle and gas composition. The sensor measurements required to calculate the nozzle volumetric flow are upstr
34、eam pressure and temperature. Future discussions will mention the need to measure relative humidity when air is used as the flow medium (to correct for changes to the specific gravity of air due to water vapor content). With the theory of volumetric flow through a sonic nozzle element defined, the t
35、ask at hand is to take this theory and apply the math and technology toward the natural gas industry to measure the performance and accuracy of gas metering equipment. 5This is a sample of format to be used in other Technical Notes 2. APPLICATIONS OF SONIC FLOW NOZZLES As stated in the previous sect
36、ion, the sonic flow nozzle element restricts upstream volumetric flow by achieving local sonic velocity at the nozzle throat. The local sonic velocity at the nozzle throat is defined as critical flow. Critical flow is maintained through the nozzle element, as long as a minimum of 10% of the absolute
37、 upstream inlet pressure is maintained. Therefore, a given sonic nozzle element, of a given size and throat diameter, will produce a given volumetric flow rate. The sonic flow nozzle can be used as a metering device for air or natural gas. The use of a sonic nozzle within a flow regime will restrict
38、 the rate of flow of gas or air, as long as the minimum differential across the nozzle is maintained. Similarly, the sonic nozzle can be used as a gas meter proving device measuring the accuracy and performance of a gas meter by passing a defined volume of air or gas through the gas meter and sonic
39、nozzle over a specified period of time. Volume measured by the gas meter can be compared with actual volume passed through the meter and controlled by the sonic nozzle element to determine the accuracy of the gas meter. 2.1. Calibration Methods of Sonic Flow Nozzles There are two ways in which the v
40、olumetric flow rate through a sonic nozzle element can be determined: Physical measurement of the nozzle throat diameter Calibration using a volume standard Physical measurement is the easiest method to determine the nozzle flow rate. Once the nozzle throat diameter is measured, its cross-sectional
41、area is known, and the nozzle volumetric flow rate can be calculated based on the relationship described in section 1.5. Physical measurement of nozzle throat diameters, however, is not always practical. For example, domestic and light commercial gas meters are commonly tested at flow rates of less
42、than one hundred cubic feet per hour. For a sonic nozzle to restrict flow below 100 cfh, its nozzle diameter must be machined to an orifice size of less that 0.1 inches. As a nozzle throat diameter becomes smaller, the effect of the boundry layer thickness can be interpreted as the nozzle discharge
43、coefficient. The larger the nozzle throat diameter, the less effect the nozzle discharge coefficient has on the actual flow traveling through the throat. Therefore, larger nozzle throats (greater than one inch) can be measured to determine the volumetric flow rate of the sonic nozzle element at spec
44、ific conditions (with minimal uncertainty). The majority of sonic nozzle elements used in distribution measurement, however, have nozzle throat diameters much smaller that one-inch, and therefore calibrated nozzle volumetric flow rate must be determined using a volume standard. 2.2. Bell Prover Cali
45、bration of Sonic Flow Nozzles Sonic flow nozzles can be calibrated against a known volume standard. In the natural gas industry, the most common and practical volume standard is the bell prover. 6This is a sample of format to be used in other Technical Notes Sonic flow nozzles can be calibrated in t
46、wo different ways using a bell-type prover: under positive pressure or at atmospheric pressure. Figure 2.1 shows a sonic nozzle calibration using a bell-type prover under positive pressure. Compressed air is regulated upstream of the nozzle element using a precise flow regulator. A dead-weight teste
47、r loaded to the required nozzle inlet pressure ensures that the regulator controls Figure 2.1: POSITIVE PRESSURE BELL CALIBRATION OF NOZZLES the compressed air pressure. Temperature and pressure measurements are made at the nozzle inlet plane in accordance with the ASME/ANSI MFC-7M standard (see ref
48、erence). Once the compressed air has traveled through the nozzle element, the air fills the bell prover. Again, pressure and temperature measurements are made at the bell prover. An electronic timer device (such as an optical pick-up) “gates” the amount of bell volume passed by recording bell prover
49、 scale increments. The calibration of the sonic nozzle element is normally standardized to a base condition, so that the element can be inserted into various field applications with varying pressure and temperature ranges. When a sonic flow nozzle is calibrated under positive pressure, the standard conditions are normally 60oF, 14.696 psia, and dry air (0% relative humidity). Figure 2.2 shows a sonic nozzle calibration using a bell-type prover at an atmospheric condition. A bell prover filled with air under nominal bell prover pressure (1.5 inches
