PDF Actual Sample: GB/T 18604-2014
| Standard ID | GB/T 18604-2014 (GB/T18604-2014) |
| Description (Translated English) | Measurement of natural gas flow by gas ultrasonic flow meters |
| Sector / Industry | National Standard (Recommended) |
| Classification of Chinese Standard | E98 |
| Classification of International Standard | 75.180.30 |
| Word Count Estimation | 44,456 |
| Date of Issue | 2014/2/19 |
| Date of Implementation | 2014/6/1 |
| Older Standard (superseded by this standard) | GB/T 18604-2001 |
| Quoted Standard | GB 3836.1; GB 3836.2; GB 3836.4; GB/T 4208; GB/T 11062-1998; GB/T 13610; GB/T 17747.1; GB/T 17747.2; GB/T 17747.3; GB/T 21446-2008; SY/T 0599-2006; JJG 1030-2007; ISO 5167-1-2003; AGA REPORT NO.10 |
| Drafting Organization | National oil and gas flow metering station, sub-station in Chengdu |
| Administrative Organization | National Standardization Technical Committee of Petroleum and Natural Gas |
| Regulation (derived from) | 2014 National Standards Bulletin No. 2 |
| Proposing organization | National Oil and Gas Standardization Technical Committee (SAC/TC 355) |
| Issuing agency(ies) | General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China, Standardization Administration of the People's Republic of China |
| Summary | This Standard specifies performance requirements for measuring ultrasonic gas flowmeter, flowmeter body requirements, installation, and maintenance, on-site verification testing requirements, as well as the flow calculation method and the measurement unce |
ENGLISH: GB/T 18604-2014 (Translated) GB/T 18604-2014
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 75.180.30
E 98
Replacing GB/T 18604-2001
Measurement of natural gas flow by gas ultrasonic
flow meters
用气体超声流量计测量天然气流量
ISSUED ON: FEBRUARY 19, 2014
IMPLEMENTED ON: JUNE 01, 2014
Issued by: General Administration of Quality Supervision, Inspection and
Quarantine;
Standardization Administration of the People's Republic of
China.
Table of Contents
Foreword ... 4
1 Scope ... 6
2 Normative references ... 6
3 Measurements, terms and definitions ... 7
4 Measurement principle ... 10
4.1 Fundamental principle ... 10
4.2 Influencing factors of accuracy measurement ... 11
5 Working conditions ... 11
5.1 Natural gas quality ... 11
5.2 Pressure ... 12
5.3 Temperature ... 12
5.4 Flow range and flow direction ... 12
5.5 Velocity distribution ... 12
6 Measurement performance requirements ... 13
6.1 Measurement performance requirements for multi-path gas ultrasonic flow meter ... 13
6.2 Measurement performance requirements for single-path gas ultrasonic flow meter ... 16
6.3 Influence of working conditions on measurement performance ... 16
7 Flow meter requirements ... 16
7.1 Composition and basic provisions ... 16
7.2 Meter body ... 17
7.3 Ultrasonic transducer ... 19
7.4 Electronic components ... 20
7.5 Flow computer ... 22
8 Installation requirements and maintenance ... 24
8.1 Installation influencing factors ... 24
8.2 Pipe configuration ... 25
8.3 Maintenance ... 27
9 Testing requirements for on-site verification ... 28
9.1 Testing contents and steps ... 28
9.2 Testing report ... 28
10 Flow calculation method and estimation of measurement uncertainty ... 29
10.1 Flow calculation under standard reference conditions ... 29
10.2 Determination of measured flow value under standard reference conditions... 31
10.3 Flow calculation under working conditions ... 31
10.4 Estimation of flow measurement uncertainty ... 31
Annex A (informative) Fundamental principles ... 35
Annex B (normative) Flow calibration of flow meter components ... 46
Annex C (normative) Exit-factory testing requirements ... 53
Annex D (informative) Documents available ... 56
Annex E (informative) Generation and prevention measures of acoustic noise
... 59
Annex F (informative) Performance verification tests of flow meter and flow
conditioner ... 65
Annex G (informative) Monitoring and guarantee of on-site measurement
performance of flow meter ... 67
Foreword
This Standard was drafted in accordance with the rules given in GB/T 1.1-2009.
This Standard replaces GB/T 18604-2001 “Measurement of natural gas flow by
ultrasonic flow meter”. Compared with GB/T 18604-2001, in addition to editorial
modifications, main technical changes as follows:
- added three terms: maximum error shift with one path failed, speed of
sound (SOS) deviation and metering package (see 3.2.17, 3.2.18 and
3.2.19);
- modified influencing factors of measurement accuracy; divided the
influencing factors into two categories: internal factors and external factors
(see Clause 4);
- further specified the range of working temperature as medium temperature
and ambient temperature (see 5.3);
- improved the requirements for zero-flow reading of multi-path gas ultrasonic
flow meter; added two technical requirements of sound velocity deviation
and maximum sound velocity difference; modified the maximum peak-to-
peak error requirement above the demarcation flow (see 6.1);
- added that according to the ambient conditions and working conditions,
taking necessary thermal insulation, anti-freeze measures, and related
requirements for acoustic noise and pulsation to flow meter components
(including upstream and downstream straight pipe sections, flow meters
and flow conditioners, temperature tapping holes and sampling holes)
(see 8.1.1, 8.1.4, 8.1.5 and Annex E);
- modified relevant requirements for pipe installation upstream straight pipe
section and flow conditioner installation position, temperature
measurement hole and sampling hole insertion depth, flow conditioner
(see 8.2.2, 8.2.5 and 8.2.7 and Annex F);
- added the requirements for on-site measurement performance of ultrasonic
flow meters in routine maintenance (see 8.3.1 and Annex G);
- added the requirements that theoretical sound velocity is calculated
according to the method provided by AGA Report No.10 “Speed of sound
in natural gas and other related hydrocarbon gases” and other methods
that are same as the calculation (see 9.1.3);
- added the calculation method and uncertainty estimation for mass flow and
energy flow (see Clause 10);
- modified flow calibration to flow calibration of metering package; modified
stability requirements for temperature and pressure, test traffic point (see
B.2.2 and B.3.3);
- deleted the original Annex E “Upper and lower pipe length requirements”;
- added technical requirements of “acoustic noise generation and prevention
measures” (see Annex E);
- added technical requirements of “flow meter and flow conditioner
performance verification test” (see Annex F);
- added technical requirements of “monitoring and guarantee of flow meter
on-site measurement performance” (see Annex G).
This Standard uses redrafting method to modify and adopt AGA Report No.10
“Speed of sound in natural gas and other related hydrocarbon gases”.
This Standard was proposed by and shall be under the jurisdiction of National
Technical Committee on Petroleum Gas of Standardization Administration of
China (SAC/TC 355).
The drafting organizations of this Standard: National Oil and Gas Large Flow
Metering Station Chengdu Sub-Station, PetroChina Southwest Oil and Gas
Field Branch, PetroChina Group Engineering Design Co., Ltd. Southwest
Branch.
Main drafters of this Standard: Duan Jiqin, He Min, Wen Dailong, Ren Gui,
Huang He, Liu Yongming, Chen Huiyu, Wang Qiang, Chen Qi, Ni Rui.
Measurement of natural gas flow by gas ultrasonic
flow meters
1 Scope
This Standard specifies measurement performance requirements of gas
ultrasonic flow meters, meter body requirements, installation and maintenance,
field verification test requirements, and flow calculation methods and
measurement uncertainty estimates.
This Standard is applicable to gas ultrasonic flow meters of plug-in transit-time
difference method (hereinafter referred to as the flow meter), which are
generally used for natural gas flow measurement in gathering devices, gas
pipelines, storage facilities, gas distribution systems and customer metering
systems. The use of external clamp-on gas ultrasonic flow meter can refer to
this Standard.
2 Normative references
The following referenced documents are indispensable for the application of
this document. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any
amendments) applies.
GB 3836.1, Explosive atmospheres - Part 1: Equipment - General
requirements
GB 3836.2, Explosive atmospheres - Part 2: Equipment protection by
flameproof enclosures “d”
GB 3836.4, Explosive atmospheres - Part 4: Equipment protection by
intrinsic safety “i”
GB/T 4208, Degrees of protection provided by enclosure (IP code) (IEC
60529)
GB/T 11062-1998, Natural gas - Calculation of calorific values, density,
relative density and Wobbe index from composition
GB/T 13610, Analysis of natural gas by gas chromatography
GB/T 17747 (all parts), Natural gas - Calculation of compression factor
GB/T 21446-2008, Measurement of natural gas flow by means of standard
orifice meter
SY/T 0599-2006, Metallic material requirements - Resistance to sulfide
stress cracking and stress corrosion cracking for gas surface equipment
JJG 1030-2007, Verification Regulation of Ultrasonic Flowmeters
ISO 5167-1:2003, Measurement of fluid flow by means of pressure
differential devices inserted in circular cross-section conduits running full -
Part 1: General principles and requirements
AGA Report No.10, Speed of sound in natural gas and other related
hydrocarbon gases
3 Measurements, terms and definitions
3.1 Measurements
See Table 1 for the measurements, names and symbols of unit used in this
Standard.
Table 1 -- Measurements and names as well as symbols of unit
Symbol of
measurement Name of measurement
Measurement
dimension Symbol of unit
D Inner diameter of flow meter L m
kc Velocity distribution correction 1
L Path length L m
P Static pressure ML-1T-2 Pa
qv Volumetric flowrate L3T-1 m3/s
qm Mass flowrate MT-1 Kg/s
qe Energy flowrate L3T-1 J/s
qt Transition flowrate L3T-1 m3/h
Qn
Volume accumulation over a period
of time under standard reference
conditions
L3 M3
T Airflow thermodynamic temperature K
t Time T s
V Mean axial fluid velocity LT-1 m/s
Average flow velocity along acoustic
path LT
-1 m/s
X Axial distance L m
Z Compression factor 1
φ Path angle 1 rad
NOTE 1: In the measurement dimension, symbol L refers to length, symbol T refers to time, symbol
M refers to mass and symbol refers to thermodynamic temperature.
NOTE 2: The symbols not listed in the table are explained in the text.
3.2 Terms and definitions
The following terms and definitions apply to this document.
3.2.1 transit-time difference method
A gas flow measurement method that within the same stroke in the flowing gas,
using the transit-time difference of two ultrasonic signals of downstream and
upstream transits to determine the average flow velocity along acoustic path.
3.2.2 ultrasonic transducer
A component that converts acoustic energy to electrical signal and vice versa.
Generally, it is installed in a pair and the pair work simultaneously.
3.2.3 signal processing unit
A part of flow meter, consisting of electronic components and microprocessor
system.
3.2.4 meter body
A pipe section where the gas to be tested passes, where components such as
ultrasonic transducer and pressure measuring connector are installed, that is
manufactured in a special way to comply with relevant regulations in any aspect.
3.2.5 acoustic path
The actual path of the ultrasound signal between a pair of transmitting and
receiving ultrasound transducers.
3.2.6 path length; L
The distance between the end faces of a pair of ultrasonic transducers (see
Figure 1).
3.2.7 axial distance; X
The projection length of path length on the parallel line of pipe axis (see Figure
1).
Figure 1 -- Simplified geometric relationship diagram of plug-in gas
ultrasonic flow measurement
3.2.8 path angle; φ
The angle between acoustic path and pipe axis (see Figure 1).
3.2.9 average flow velocity along acoustic path;
Gas flow velocity in a plane that is determined by acoustic path and flow
direction.
3.2.10 mean axial fluid velocity; V
Ratio of flowrate to measured cross-sectional area.
3.2.11 velocity distribution correction; kc
Ratio of mean axial fluid velocity to average flow velocity along acoustic path.
3.2.12 velocity sampling interval
Time interval between two adjacent gas flow measurements by a pair of
ultrasonic transducers or acoustic paths.
3.2.13 zero-flow reading
The reading of the maximum permissible flow rate when gas is in a state of rest.
3.2.14 transition flow rate; qt
The flow value between the maximum flow and the minimum flow. It divides the
flow range into two zones with different tolerances, that is, “high zone” and “low
zone” (see Figure 2).
3.2.15 maximum peak-to-peak error
The difference between the upper limit maximum error point and the lower limit
maximum error point (see Figure 2).
3.2.16 flow calibration factor
Flow meter coefficient that to conduct flow calibration on the flow meter and
correct the test results according to a certain correction method, hereinafter
referred to as the calibration factor.
3.2.17 maximum error shift with one path failed
At the same flow, the maximum difference BETWEEN the measurement error
when all acoustic paths work AND the measurement error when one acoustic
path fails.
3.2.18 speed of sound (SOS) deviation
The maximum relative deviation between average sound velocity obtained by
the measurement of flow meter and theoretical sound velocity in the gas.
3.2.19 metering package
The component that consists of a flow meter, supporting upstream and
downstream straight pipe sections, temperature measuring hole, pressure
obtaining hole as well as flow conditioner.
4 Measurement principle
4.1 Fundamental principle
The gas ultrasonic flow meter of transit-time difference method is a velocity flow
meter that measures high frequency sound pulse transit-time to obtain gas flow.
The transit time is measured by the acoustic pulses that are transmitted and
received between pairs of transducers outside the pipe or within the pipe. The
acoustic pulse transits along the diagonal direction (see Figure 1). Acoustic
pulses transmitted downstream are accelerated by airflow while acoustic pulses
transmitted in reverse flow shall be decelerated. The transit-time difference is
related to the mean axial fluid velocity. Use numerical calculation technique to
calculate the mean axial fluid velocity and the flow that pass through the gas
ultrasonic flow meter under working conditions. The flow meter that only has
one acoustic path is called single-path gas ultrasonic flow meter. The flow meter
that has two or more acoustic paths is called multi-path gas ultrasonic flow
meter. When the ultrasonic transducer is in direct contact with the gas, it is
called plug-in. When the ultrasonic transducer is not in direct contact with the
gas, it is called external clamp-on.
See Annex A for more details.
4.2 Influencing factors of accuracy measurement
4.2.1 Internal factors include:
a) Geometric size of meter body as well as accuracy and stability of
ultrasonic transducer’s position parameters;
b) Quality and accuracy of ultrasonic transducer and electronic components
used for transit-time measurement (including electronic clock stability);
c) Sampling period and integral calculation method used for transit-time
testing and average flow rate calculation;
d) Calibration (including compensation for electronic component and
ultrasonic transducer signal lag).
4.2.2 External factors include:
a) Airflow velocity distribution;
b) Temperature gradient;
c) Airflow pulsation;
d) Acoustic and electromagnetic noise;
e) Solid and liquid depositions;
f) Geometric size changes over time.
5 Working conditions
5.1 Natural gas quality
The natural gas components measured by the flow meter are generally within
the scope specified in GB/T 17747 (all parts). The relative density of the natural
gas is 0.55~0.80.
In one of the following cases, it shall consult the manufacturer for the material
of the flow meter, the type of the ultrasonic transducer as well as whether the
measuring accuracy of the flow meter meets the requirements:
a) CO2 content exceeds 10%;
b) Work under conditions that are close to the critical density of natural gas
mixture;
c) Total sulfur content exceeds 460mg/m3, including mercaptans, hydrogen
sulfide and elemental sulfur.
Under normal gas delivery conditions, the attachments (such as condensate or
oil residue with processing impurities, ash and sand, etc.) inside the meter body
shall reduce the flow area of the flow meter, which shall affect the measurement
accuracy. Meanwhile, the attachments shall also obstruct or attenuate the
ultrasonic transducer to emit and receive ultrasonic signals OR affect the
reflection of ultrasonic signals on the inner wall of the meter body. Therefore, it
shall regularly check and clean the flow meter.
5.2 Pressure
The ultrasonic transducer has certain requirements for gas minimum density (it
is the function of pressure). The minimum working pressure shall ensure that
the acoustic pulse can transit normally in natural gas.
5.3 Temperature
The manufacturer shall, according to the actual working condition requirements
of the user, provide a flow meter that meets temperature range requirements.
The working medium temperature of the flow meter is -20°C~60°C; the working
environment temperature range is -40°C~60°C.
5.4 Flow range and flow direction
The flow measurement range of the flow meter is determined by the actual flow
rate of gas. The typical flow rate of the measured natural gas is generally 0.3m/s
~ 30m/s. The user shall verify that the measured gas flow rate is within the flow
range specified by the manufacturer. The corresponding measurement
accuracy shall be in accordance with the provisions of Clause 6.
The flow meter has the ability to measure in both directions. The bidirectional
measurements have the same accuracy. The user shall indicate if bidirectional
measurement is required, so that the manufacturer can properly configure the
signal processing unit parameters.
5.5 Velocity distribution
Under ideal conditions, the natural gas flow that goes into the flow meter shall
be in symmetrically and fully-developed turbulent velocity distribution. The
configuration of upstream pipe (i.e., various upstream pipe fitting, pressure
regulator and length of straight section) shall influence the velocity profile of the
gas that goes into the flow meter, so as to affect measurement accuracy. The
size of the influence and whether the influence is positive or negative are related
to the compensation capability of the flow meter to a certain extent.
6 Measurement performance requirements
This clause specifies that the flow meter shall meet a set of minimum
measurement performance requirements. Before the adjustment of flow
calibration factor, the flow meter shall meet the requirements for those
performances, so as to ensure that the problems and defects of the flow meter
are not covered due to the adjustment of flow calibration factor.
The user shall, according to the provisions of Clause 7 and Annex B, require
inspection and flow calibration to the flow meter. It shall also follow the
installation requirements in Clause 8 to ensure that the measurement accuracy
of the flow meter is improved based on meeting the minimum performance
requirements.
For the flow meter of each size, the manufacturer shall specify the flow
threshold, that is, minimum flowrate qmin, transition flowrate qt and maximum
flowrate qmax. No matter whether it has been subject to flow calibration, within
the flow range specified by the manufacturer, the flow meter shall meet the
measurement performance requirements of this clause.
6.1 Measurement performance requirements for multi-path gas
ultrasonic flow meter
6.1.1 General
Before any adjustment of flow calibration factor, the general measurement
performance of all multi-path gas ultrasonic flow meters shall be able to meet
the following requirements:
a) Repeatability:
0.2%, qt ≤ q ≤ qmax;
0.4%, qmin ≤ q < qt;
NOTE: q is the flow to be measured, same as follows.
b) Resolution: 0.001m/s;
c) Velocity sampling interval: ≤1s;
d) Zero-flow reading: <6mm/s for each acoustic path;
e) Speed of sound (SOS) deviation: ±0.2%;
f) Maximum speed of sound (SOS) difference between each acoustic path:
0.5m/s.
6.1.2 Accuracy of large-diameter flow meter
Before any adjustment of flow calibration factor, the multi-path gas ultrasonic
flow meter of which the diameter is equal to or greater than 300mm shall meet
the following requirements for measurement accuracy (see Figure 2):
a) Maximum error:
±0.7%, qt ≤ q ≤ qmax;
±1.4%, qmin ≤ q < qt;
b) Maximum peak-to-peak error:
0.7%, qt ≤ q ≤ qmax;
1.4%, qmin ≤ q < qt.
6.1.3 Accuracy of small-diameter flow meter
Before any adjustment of flow calibration factor, the multi-path gas ultrasonic
flow meter of which the diameter is less than 300mm shall meet the following
requirements for measurement accuracy (see Figure 2):
a) Maximum error:
±1.0%, qt ≤ q ≤ qmax;
±1.4%, qmin ≤ q < qt;
b) Maximum peak-to-peak error:
1.0%, qt ≤ q ≤ qmax;
1.4%, qmin ≤ q < qt.
NOTE: When the path length is short, it is difficult to measure the transit-time of acoustic wave in
turbulent gas. Therefore, the requirements for the small-diameter flow meter are low.
Figure 2 -- Summary of measurement performance requirements for multi-path gas ultrasonic flow meter
Zero-flow reading <6mm/s (for each acoustic path)
Uncorrected
calibration curve of
flow meter
Repeatability
Large-diameter flow meter (≥12’’)
Maximum peak-to-peak error=0.7% (qj≥qt) Small-diameter flow meter (<12’’)
Maximum peak-to-peak error=1.0% (qj≥qt)
Maximum peak-to-peak error=1.4% (qj
Repeatability
Extended
error limit
Error limit of small-
diameter flow meter
Error limit of large-
diameter flow meter
Error limit of large-
diameter flow meter
Error limit of small-
diameter flow meter
Extended
error limit
Flow / qi
6.2 Measurement performance requirements for single-path gas
ultrasonic flow meter
The measurement performance requirements for single-path gas ultrasonic
flow meter may be lower than the measurement performance requirements for
multi-path gas ultrasonic flow meter. The specific indicators are provided by the
manufacturer.
6.3 Influence of working conditions on measurement performance
Under the working conditions specified in Clause 5, the flow meter shall meet
the measurement performance requirements specified in 6.1 and 6.2 without
any manual adjustment. If the manual input of physical parameters is required
to determine the physical parameters (such as density and viscosity) under
natural gas flow conditions, the manufacturer shall give the sensitivity degree
that the flow meter is affected by those parameters, so that when the working
conditions are changed, the user can determine whether the influence brought
by those changes is acceptable.
7 Flow meter requirements
7.1 Composition and basic provisions
7.1.1 Composition
The flow meter mainly consists of the following two parts:
a) meter body, ultrasonic transducer and its mounting parts;
b) signal processing unit (SPU) that is composed of electronic components
and microprocessor system. It receives ultrasonic transducer signals and
has functions such as processing measurement signals and displaying,
outputting and recording measurement results. The electrical signal
processing and conversing part located in the field is installed in the
converter.
7.1.2 Basic provisions
Use the materials that are suitable for working conditions of the flow meter to
design and manufacture the meter body and all other parts, including pressure
members and external electronic components, in accordance with the process
requirements for the metering system where they are. If the user has special
requirements, it shall comply with the appropriate specifications or regulations
applicable to each specific installation condition specified by the user.
Before the flow meter exits the factory, the manufacturer shall conduct exit-
factory testing and provide the exit-factory testing report to the user. See Annex
C for the exit-factory testing requirements. There shall be corresponding
documents, see Annex D.
7.2 Meter body
7.2.1 Maximum working pressure
The maximum designed working pressure of the flow meter shall be the
minimum among the maximum working pressures of the following components:
meter body, flange, ultrasonic transducer components and their mounting
connectors.
The connecting flange of the meter body shall comply with general industrial
standards, national or international standards.
7.2.2 Corrosion resistance requirements
All components of the flow meter that are in contact with the medium shall be
made of the materials that are suitable for natural gas. The materials used for
the flow mater that contains corrosive mediums such as H2S and CO2 shall
comply with the provisions of SY/T 0599.
All external parts of the flow meter shall be made of corrosion resistant materials.
Or use corrosion resistant coating that is suitable to use in a typical atmospheric
environment of the natural gas industry.
7.2.3 Ability of adapting to environment
The shell, nameplate of the flow meter and each component shall comply with
the provisions of GB/T 4208, at least meeting the level requirements of IP65.
7.2.4 Length and nominal diameter
The manufacturer shall give the standard length of the meter body of each
pressure level and nominal diameter. In order to match existing pipes, the user
can specify different lengths and nominal diameters.
7.2.5 Ultrasonic transducer port
Natural gas may contain impurities (such as condensate or dust). The designed
ultrasonic transducer port shall minimize the possibility of liquids or solids
staying on them. According to user’s requirements, a valve may be provided so
that the ultrasonic transducer can be replaced with internal pressure in the
meter body.
7.2.6 Pressure obtaining hole
There shall be at least one pressure obtaining hole on the meter body to
measure the static pressure. The nominal diameter of each pressure obtaining
hole shall be within 4mm~10mm. If the wall thickness of meter body is less than
20mm, the nominal diameter of the pressure obtaining hole shall be 4mm. And
from the inner wall of meter body, at least within the length 2.5 times the
diameter of pressure obtaining hole, it is cylindrical; and the axis of the pressure
obtaining hole shall be perpendicular to the measuring tube axis. The edges of
pressure obtaining hole of inner wall of meter body shall be right angle, without
burrs and curls.
Each pressure obtaining hole shall have internal thread that can be equipped
with isolation valve and swing space where it can directly mount the isolation
valve on the pressure obtaining hole. The specification of the internal thread is
better to be 1/4" NPTF or 1/2" NPTF. The pressure obtaining hole shall be set
at the top, left or right of meter body. When necessary, the pressure obtaining
hole can be added, so as to provide the flexibility to install pressure transmitter
to the user, be conducive to maintenance and discharge the condensate in the
pressure transmitter pressure tube back into the meter body.
7.2.7 Flow meter markings
A nameplate that contains the following information shall be set on the flow
meter:
a) Manufacturer’s name, flow meter model, serial number and date of
manufacture;
b) Nominal pressure and total mass;
c) Nominal diameter and inner diameter;
d) Highest and lowest storage temperature;
e) Working pressure and temperature range;
f) Maximum and minimum hourly flow under working state;
g) Positive direction of gas flow;
h) Explosion-proof level.
For easy identification, each ultrasonic transducer port shall be marked with a
permanent unique mark. If a stamp mark on the meter body, it shall use low
stress stamping form, i.e., round bottom print.
7.2.8 Appearance quality requirements
The appearance quality requirements include:
a) The appearance of the flow meter shall be neat and beautiful. The surface
shall be well treated. There shall be no burrs, nicks, cracks, rust, mildew
and peeling;
b) All words and symbols shall be clear;
c) The sealing surface shall be smooth and not damaged.
7.2.9 Other requirements
The flow meter shall be designed that when it is placed on a smooth surface
with a slope of 10%, it shall not roll, so as to prevent that, during installation or
maintenance period, when it is placed on the ground temporarily, it shall
damage the exerted ultrasonic transducer and signal processing unit.
The flow meter shall be also designed as that it shall be easily moved during
transportation and installation period. It shall be also designed to have space
for hanging holes and slings.
The user can request the manufacturer to provide the locating pin on the
upstream and downstream flanges of the flow meter, so as to ensure the correct
positioning of the flow meter’s on-site installation.
7.3 Ultrasonic transducer
7.3.1 Technical requirements
The manufacturer shall give general technical indicators of ultrasonic
transducer, such as key dimensions, maximum permissible working pressure,
range of working pressure, range of working temperature and gas composition
limits.
The manufacturer shall designate the minimum working pressure according to
the model of ultrasonic transducer, the dimensions of ultrasonic transducer and
expected working conditions. This minimum working pressure shall be marked
on the flow meter, so as to remind the on-site personnel that when the pipe
pressure is less than this pressure, the flow meter may not record the flow.
7.3.2 Rate of pressure change
When the pressure of the flow meter suddenly drops, the gas remaining inside
the ultrasonic transducer may expand and cause damage. Therefore, the
manufacturer shall provide clear instructions for the installation, startup,
maintenance, buck and boost rates of the flow meter during operation. The
pressure drop rate of the flow meter shall not exceed 0.5MPa/min.
7.3.3 Replacement, dismantling and reinstallation
When the ultrasonic transducer is replaced, dismantled or reinstalled, it shall
not obviously change the performance of the flow meter. That is, after replacing
the ultrasonic transducer and making corresponding adjustment of signal
processing unit software constant, the measurement performance of the flow
meter shall also meet the requirements of Clause 6. The manufacturer shall
specify the procedures for the replacement of the ultrasonic transducer and the
mechanical, electrical and other tests and adjustments required.
7.3.4 Testing
The manufacturer shall test each ultrasonic transducer or each pair of ultrasonic
transducers. The testing results shall be recorded and stored as a part of quality
assurance system of ultrasonic flow meter in a form of document. Each
ultrasonic transducer shall be marked with a permanent serial number. The
manufacturer shall provide the technical parameters required by 7.3.1. If the
signal processing unit requires particular characteristic parameters of the
ultrasonic transducer, it shall provide testing documents of each ultrasonic
transducer or each pair of ultrasonic transducers, including specialized
calibration test data, calibration methods used, and characteristic parameters.
7.4 Electronic components
7.4.1 General requirements
The electronic components, that is, power supply, microprocessor, signal
processing component, and ultrasonic transducer excitation circuit, can be
assembled in one or more boxes and installed on or near the flow meter. They
are unified as signal processing unit (SPU). The manufacturer shall give a mark
of SPU’s uniqueness. The testing of electronic components shall meet relevant
requirements in Annex A “Type evaluation” of JJG 1030-2007.
Remote units such as power section and working interface can be installed in
non-hazardous zone. Use shielded cable to connect it with the signal
processing unit.
The signal processing unit shall work within the required range of indicators for
flow meter’s measurement performance specified in Clause 6 and the
environmental conditions specified in Clause 5. And when the entire signal
processing unit or any field replacement module is replaced, it shall not cause
the obvious change of flow meter’s measurement performance (see 7.3.3). The
manufacturer shall remind the user that whether it shall obviously influence the
measurement performance of flow meter when the entire SPU or its module is
replaced.
The signal processing unit shall have watchdog timer function, so as to ensure
that the signal processing unit is re-started in the event of a program failure lock.
The flow meter power supply is generally 50Hz, 220V AC power or 12V~24V
DC power or battery.
7.4.2 Technical requirements for output signal
The signal processing unit shall at least have the following output signals:
a) Frequency signal that represents the volumetric flow under working
conditions;
b) Serial communication data interface, such as RS-232, RS-485 or
equivalent interface.
The flow meter shall also have a 4mA~20mA analog signal for the volumetric
flow under working conditions. The flow signal shall be adjusted to 120% of
maximum flow qmax.
It shall set small flow cutting function, that is, when the flow is less than a
minimum value, set its output as 0 (but not applicable when serial
communication data is output).
When it exceeds the maximum flow of the flow meter, the manufacturer shall
provide optional flow outputs to the user. These flow outputs can be zero,
maximum flow or user-determined flow.
For bidirectional flow applications, it shall provide two independent flow outputs
and one flow direction state output as well as serial communication data value.
Use corresponding flow computer and flow state output signal to calculate the
cumulative calculation of flow, respectively.
All output signals shall be isolated from ground and have the necessary
overvoltage protection.
7.4.3 Electrical safety requirements
All electronic components of the flow meter shall be analyzed, tested and
verified by a qualified laboratory. Then stick a label on each flow meter. The
explosion-proof level of the electrical equipment and instruments of the flow
meter shall comply with the provisions of GB 3836·1. Explosion-proof electrical
equipment and instruments shall comply with the provisions of GB 3836.2.
Intrinsically safe circuits and electrical equipment shall comply with the
provisions of GB 3836.4. Other explosion-proof electrical equipment shall also
comply with the provisions of the corresponding special standards. The user
can specify the explosion-proof rating that the flow meter shall meet, so as to
adapt to more secure installation requirements.
Cable jackets, rubber, plastic and other exposed parts shall be resistant to UV
light, grease and flame retardant.
7.4.4 Replacement of components
After ultrasonic transducer, cables, electronic components and software are
replaced or re-installed, the manufacturer shall provide a set of reliable working
procedures and enough data to the user, so as to ensure that after any
component is replaced or re-installed, the measurement performance of the
flow meter shall still meet the requirements of Clause 6 and the functions after
replacement are not lower than the standard functions before replacement.
When these components are replaced but the flow meter is not re-calibrated, it
may cause additional measurement uncertainty. Before the components are
replaced, it shall store a set of reference data (see 7.5.5). After the components
are replaced, the user shall compare the velocity ratio, sound velocity ratio
between each path with the reference data, so as to ensure that the
measurement performance of the flow meter still meet the requirements of
Clause 6.
7.5 Flow computer
7.5.1 Hardware
The computer code that is used for flow meter control and operation shall be
stored in non-volatile memory. All flow calculation constants and manually-input
parameters shall also be stored in non-volatile memory.
The manufacturer shall store all records of hardware modifications, including
modification serial number, date of modification, applicable flow meter model,
board modification, and description of hardware changes.
The checker visually checks hardware module, display or data communication
port to obtain hardware modification number, modification date, serial number,
and number of checks.
The manufacturer may provide improved hardware at any time, so as to change
the performance of the flow meter or add more features. The manufacturer shall
inform the user whether the modification of the hardware affects the accuracy
of the flowmeter calibrated by the real flow.
7.5.2 Configuration and maintenance software
The flow meter shall have the ability to perform local and remote configuration
of the signal processing unit and monitor the operation of the flow meter. This
software shall at least display and record the following data: instantaneous flow,
axial average flow rate, average sound velocity, sound velocity along each path,
and gas flow rate, the quality of the acoustic signal received by each ultrasonic
transducer. The manufacturer can use partial embedded software of the flow
meter to provide these software functions.
7.5.3 Configuration and inspection functions
In order to check and inspect, when the flow meter works, it shall be able to
check and determine all flow calculation constants and parameters on the flow
computer.
The checker or inspector can view and print the flow measurement
configuration parameters of the signal processing unit.
It shall take measures to ensure that the parameters that affect the performance
of the flow meter are not accidentally or unnoticeably changed. The measures
include sealed switch or jumper, cured programmable read-only memory chip
or password set in signal processing unit. The inspector shall be able to verify
all algorithms, constants and configuration parameters used by any specific
flow meter, so as to ensure that the flow meter meets or exceeds the
performance of the original flow meter for flow testing or achieves or better than
the performance of the specific flow meter after the most recent flow calibration
and its calibration factors are changed.
The user shall establish the basic materials of the flow meter, store the
relationship between data such as acoustic path transit time, acoustic path’s
automatic gain control, sound velocity along each path, average sound velocity,
axial average flow rate, and uncorrected volume of the flow meter during exit-
factory testing, flow calibration and initial installation, so as to determine if the
flow meter under different conditions is working properly, and decide if it needs
to calibrate the flow meter after the electronic components or hardware is
replaced.
7.5.4 Alarm
It shall provide the outputs of the following alarm states in the form of fail-safe
relay contacts or passive contacts isolated from ground:
a) Output failure: when the flow indicated under the condition of the pipe is
invalid;
b) Fault state: when any of several monitoring parameters exceeds the
normal operating range for a set period of time;
c) Partial failure: when one or multiple paths are not available.
7.5.5 Diagnostic measurement
The manufacturer shall at least provide the following diagnostic measurements
through RS-232, RS-485 or equivalent serial communication port:
a) Automatic gain control level for each acoustic path;
b) Signal to noise ratio per acoustic path;
c) Average axial flow rate through the flow meter;
d) Flow rate per acoustic path (or equivalent to evaluating flow rate
distribution);
e) Sound velocity along each acoustic path;
f) Average sound velocity;
g) Average time interval;
h) Percentage of pulses received per acoustic path;
i) State and measurement effect indication;
j) Alarm, fault indication and corresponding records.
7.5.6 Others
The flow computer shall have the function of theoretical sound velocity
calculation under working conditions. The theoretical sound velocity calculation
method shall be the sound velocity calculation method provided in AGA Report
No. 10 or other methods that are the same as the calculation results. See GB/T
21446 Annex H “Basic technical requirements for natural gas flow computer
system” for other technical requirements.
8 Installation requirements and maintenance
8.1 Installation influencing factors
8.1.1 Temperature
The ambient temperature of the installed flow meter shall comply with the
provisions of 5.3. Meanwhile, it shall, according to the specific environment and
working conditions of installation point, take necessary insulation, antifreeze
and other protective measures (such as rain protection, sun protection) to the
components of the flow meter.
8.1.2 Vibration
The installation of the flow meter shall avoid a vibration environment as far as
possible. It shall especially avoid an environment that can cause resonances in
components such as signal processing unit and ultrasonic transducer.
8.1.3 Electrical noise
When installing the flow meter and its associated connecting wires, it shall avoid
environments that may have strong electromagnetic or electronic interference;
otherwise it shall consult the manufacturer and take the necessary protective
measures.
8.1.4 Acoustic noise
The installation of the flow meter shall prevent the adverse effects of acoustic
noise on the measurement performance. In a pressure regulating metering
station, the flow meter shall normally be installed upstream of the regulating
valve. See Annex E for other technical requirements.
8.1.5 Pulsation
It shall consider any possible flow pulsation near the flow meter. And take
appropriate measures to minimize additional measurement uncertainty caused
by pulsation.
8.2 Pipe configuration
8.2.1 Flow direction
If the flow meter has a two-way measurement function and is also ready to be
used for such a measurement occasion, then during the design and installation,
both ends of the flow meter shall be considered upstream, that is, the
downstream pipe configuration and related technical requirements shall be
consistent with the upstream, in accordance with the provisions of 8.2.2~8.2.9.
8.2.2 Pipe installation
In order to ensure that, within the full range of the flow meter, the on-site
measurement performance of the flow meter can meet the requirements of
Clause 6, and the additional measurement error caused by installation
conditions does not exceed ±0.3%, the manufacturer shall, according to the
expected installation conditions of the flow meter provided by the user,
recommend the lengths of the upstream, downstream straight pipe sections of
the flow meter as well as whether it has flow conditioner. The user can request
the manufacturer to provide relevant report of the test that is conducted
according to the provisions of Annex F.
When the manufacturer does not provide the requirements for the lengths of
the upstream, downstream straight pipe sections of the flow meter and the
installation of the flow conditioner, or when the user is unable to provide the
expected installation conditions, in the absence of the flow conditioner, the
upstream of the flow meter at least requires 50D straight pipe section; in the
existence of the flow conditioner, the upstream of the flow meter at least
requires 30D straight pipe section and the flow conditioner shall be installed at
the upstream 10D of the flow meter. The length of the downstream straight pipe
section of the flow meter shall at least be 5D.
8.2.3 Intrusion and alignment
The inner diameter of the flow meter, the connecting flange and its immediate
upstream and downstream straight pipe sections shall have the same inner
diameter. Its deviation shall be within 1% of the pipe diameter and does not
exceed 3mm. When the flow meter and its adjacent straight pipe section are
assembled, it shall be strictly aligned and ensure its internal circulation path
smooth and straight. There shall be no obstacles in the connecting part, such
as steps and protruding gaskets.
8.2.4 Internal surface
For the straight pipe section that matches the flow meter, there shall be no rust
and other mechanical damages on its inner wall. Before assembly, the rust
preventive oil or sandstone dust and other appendages in the flow meter and
its connecting pipe shall be removed. During the use, it shall also keep the
medium circulation path clean and smooth at any time.
8.2.5 Temperature measurement hole and sampling hole
If the flow meter is only for unidirectional flow measurement, the temperature
measurement hole and sampling hole shall be placed 2D~5D between the
downstream of the flow meter and the flange end face. If the flow meter is used
for bidirectional flow measurement, the temperature measurement hole and
sampling hole shall be placed 3D~5D between the downstream of the flow
meter and the flange end face. Multiple temperature measurement holes shall
not be arranged in a straight line. The manufacturer or supplier shall provide
the best temperature measurement hole related to flow meter’s acoustic path
arrangement to the user.
The manufacturer shall recommend the installation position of the temperature
measurement hole relative to the acoustic path. Generally speaking, the axis of
the temperature measurement hole is perpendicular to the axis of the pipe. The
installation of the temperature measurement hole shall ensure that the heat
transfer of the pipe, the accessory components of the temperature
measurement jacket and the thermal radiation of the sun do not affect the
measurement of the gas temperature. The insertion depth of the thermometer
and the sampler shall be 1/3D. For large-mouth flow meter (DN300 and above),
the insertion depth shall not exceed 125mm. It shall pay attention to avoid the
resonance of the thermostat caused by the high velocity airflow.
When the ambient temperature and the gas temperature are very different, it is
advisable to install a thermal insulation layer at the 1D downstream of the
farthest temperature measurement hole on the upstream pipe of the flow meter
to the downstream pipe and install a sunshade on the flow meter.
8.2.6 Flow conditioner
Whether to install a flow conditioner and which form of the flow conditioner to
install shall depend mainly on two factors: the type of flow meter selected
(single-path or multi-path) and the severity that the upstream velocity profile of
the flow meter is disturbed. The ultrasonic flow meter shall be installed with a
flow conditioner plate. The flow conditioner plate shall meet the relevant
requirements of ISO 5167-1:2003 Annex C “Flow conditioners and flow
straighteners”. See Annex F for its performance testing requirements.
8.2.7 Installation position of flow meter
The flow meter shall be installed horizontally. It shall consult the manufacturer
about other installation methods. In design and installation, it shall leave
sufficient maintenance space.
8.2.8 Gas filtration
In the case of dirty gas quality, a good gas filter can be installed at upstream of
the flow meter. The structure and size of the filter shall ensure that it shall
generate pressure loss as low as possible and flow state change at the
maximum flow. During the use, it shall monitor the differential pressure of the
filter and perform regular discharge and cleaning, so as to ensure that the filter
works under a good state.
8.3 Maintenance
8.3.1 General operation and maintenance
The manufacturer shall be responsible to conduct basic operations and
maintenance skills training to field staff. Daily management is mainly based on
the information fed back from the self-diagnostic system of the ultrasonic flow
meter for targeted inspection and maintenance, so as to ensure the on-site
measurement performance of the flow meter. See Annex G for relevant
technical requirements.
8.3.2 Regular check
It shall, according to the suggestion of the manufacturer, regularly perform the
periodical check of the flow meter, for example, whether the signal processing
unit and the timing system are working properly, whether the acoustic path is
fault-free, whether the zero-flow measurement is accurate, whether the surface
of the ultrasonic transducer is deposited with dirt, and whether the signal gain
is significantly changed. It shall also, based on the actual situation, check
whether there are deposits in adjacent pipes. Discharge sewage according to
the check results.
9 Testing requirements for on-site verification
The manufacturer shall provide written documents for on-site verification testing
of the flow meter to the user, so that when it requires to conduct on-site
verification testing to the gas ultrasonic flow meter, it shall follow the following
requirements.
9.1 Testing contents and steps
9.1.1 Appearance check
In appearance check, it shall carefully check whether the flow meter cavity and
the ultrasonic transducer tip have dirt deposits, wear or other damage that may
affect the performance of the flow meter.
9.1.2 Zero-flow testing
In the absence of a flowing medium, check whether the flow meter reading is
zero or within the allowable range specified by the flow meter itself.
9.1.3 Sound velocity testing
During the on-site verification testing, when necessary, it can conduct the sound
velocity testing. First, test the actual sound velocity under some working
conditions. Then, according to the method provided by AGA Report No.10 or
other methods that have same calculation results, calculate the theoretical
sound velocity under the same working conditions. The difference between the
two shall be in accordance with the provisions of 6.1.1.
9.2 Testing report
According to the testing of 9.1, check and analysis results, it shall give a testing
report that contains flow meter name, model specification, manufacturer,
commissioning date, on-site conditions (temperament, flow, pressure,
temperature and installation method), test organization (personnel), testing
content and method, testing results, abnormal situation analysis and
recommendations measures.
10 Flow calculation method and estimation of
measurement uncertainty
10.1 Flow calculation under standard reference conditions
The standard reference conditions used by this Standard are: For volumetric
measurement, the pressure is 101.325kPa (absolute pressure) and the
temperature is 20°C; For energy measurement, the pressure is 101.325kPa
(absolute pressure), the temperature is 20°C, dry basis. Other reference
conditions specified in the contract can be also used.
The flow meter is designed and manufactured by ultrasonic propagation
principle and digital integration technique. The flow calculated according to
formula (A.18) refers to the natural gas flow under working conditions. The flow
under standard reference conditions shall be calculated according to on-line
measured airflow static pressure, temperature and gas state equation.
10.1.1 Instantaneous volumetric flow calculation under standard
reference conditions
The instantaneous volumetric flow calculation under standard reference
conditions is calculated according to formula (1):
Where,
qn - Instantaneous volumetric flow calculation under standard reference
conditions, in cubic meters per hour (m3/h);
qf - Instantaneous volumetric flow calculation under working conditions, in cubic
meters per hour (m3/h);
pn - Absolute pressure under standard reference conditions, in megapascals
(MPa);
pf - Absolute static pressure under working conditions, in megapascals (MPa);
Tn - Thermodynamic temperature under standard reference conditions, in
Kelvins (K);
Tf - Thermodynamic temperature under working conditions, in Kelvins (K);
Zn - Compression factor under standard reference conditions, calculated
according to GB/T 17747;
Zf - Compression factor under working conditions, calculated according to GB/T
17747.
10.1.2 Cumulative volume calculation under standard reference
conditions
The cumulative volume calculation under standard reference conditions is
calculated according to formula (2):
Where,
Qn - Cumulative volume under standard reference conditions within the time of
t0~t, in cubic meters (m3);
t0 - Initial time for cumulative flow credit, in seconds (s);
t - End time for cumulative flow credit, in seconds (s);
dt - Time increment.
10.1.3 Mass flow calculation
The mass flow calculation is calculated according to formula (3):
Where,
qm - Mass flow, in kilograms per hour (kg/h);
ρn - Gas density under standard reference conditions, in kilograms per cubic
meter (kg/m3).
10.1.4 Energy flow calculation
The energy flow is calculated according to formula (4):
Where,
qe - Energy flow, in Joules per hour (J/h);
- Gas calorific value under standard reference conditions, in Joules per
cubic meter (J/m3).
10.2 Determination of measured flow value under standard
reference conditions
The manufacturer has used the flow meter and relevant flow computer to make
a flow measuring system. Its output function is complete and flexible, and the
user can choose according to needs.
10.2.1 Output is flow under contract reference conditions
When the output is the flow under contract reference conditions, view whether
the contract reference conditions are the same as the standard reference
conditions. If they are same, then the output indicated value shall be the flow
value under standard reference conditions. If they are different, calculate the
flow under standard reference conditions according to formula (1) and formula
(2). The calculated value shall be the measured flow value under standard
reference conditions.
10.2.2 Output is flow under working conditions
When the output is the flow under working conditions, it shall calculate the flow
under standard reference conditions according to formula (1) and formula (2).
The calculated value shall be the measure flow value under standard reference
conditions.
10.3 Flow calculation under working conditions
During type selection of flow meter, it shall calculate the flow under standard
reference conditions qn according to formula (1) as the flow under working
conditions qf according to formula (1). Reasonably select the specification of
the flow meter.
10.4 Estimation of flow measurement uncertainty
10.4.1 Estimation of flow measurement uncertainty without flow
calibration under standard reference conditions
According to formula (2), it may use formula (5) and formula (6) to estimate the
extended uncertainty of flow measurement without flow calibration under
standard reference conditions:
Where,
Uqn - Extended uncertainty of flow measurement under standard reference
conditions;
uqn - Standard uncertainty of flow measurement under standard reference
conditions;
uqf - Standard uncertainty of flow measurement under operation conditions,
determined by accuracy level of flow meter;
upf - Standard uncertainty of absolute static pressure measurement under
operation conditions, estimated according to the static pressure measuring
instrument performance used and formula (9);
uTf - Standard uncertainty of thermodynamic temperature measurement under
operation conditions, estimated according to the temperature measurement
instrument performance used and formula (9);
uZf - Standard uncertainty of compression factor measurement under operation
conditions; if the compression factor calculation method uses GB/T 17747, its
extended uncertainty shall take 0.1% (k=2); if it uses AGA NX-19:1997, it shall
take 0.5% (k=2);
uZn - Standard uncertainty of compression factor measurement under standard
reference conditions, related to the analysis method of natural gas composition
and standard gas, calculated according to the provisions of GB/T 13610;
uan - Uncertainty of additional flow measurement caused by installation, taking
1/3 of the maximum permissible error of flow meter.
10.4.2 Estimation of uncertainty of flow measurement under standard
reference conditions with flow calibration
It may use formula (7), formula (8) to estimate the extended uncertainty of flow
measurement under standard reference conditions with flow calibration:
Where,
us - Standard uncertainty of flow measurement of standard device used by
calibration;
um - Standard uncertainty of calibration data; may be similar to the flow meter
repeatability of calibration processing.
10.4.3 Estimation of uncertainty of mass flow measurement
It may use formula (9) to estimate the synthetic uncertainty of mass flow
measurement under standard reference conditions:
The extended uncertainty can refer to formula (6) to calculate.
Where,
uρn - Uncertainty of density calculation under standard reference conditions,
calculated according to the provisions of GB/T 13610.
10.4.4 Estimation of uncertainty of energy flow measurement
According to formula (6), it can use formula (10) to estimate the synthetic
uncertainty of energy flow measurement under standard reference conditions:
The extended uncertainty can refer to formula (6) to calculate.
Where,
- Uncertainty of calorific value calculation under standard reference
conditions, calculated according to GB/T 11062-1998; its extended uncertainty
can take 0.05%(k=2).
10.4.5 Estimation of uncertainty of absolute static pressure or
thermodynamic temperature measurement
The uncertainty of absolute static pressure or thermodynamic temperature
measurement is estimated according to formula (11):
Where,
uY - Standard uncertainty of absolute static pressure measurement or
thermodynamic temperature measurement;
ξY - Accuracy level of static pressure measuring instrument or temperature
measuring instrument;
YK - Scale upper limit of static pressure measuring instrument or temperature
measuring instrument;
Yi - Predetermined static pressure measurement value or predetermined
temperature measurement value.
Annex A
(informative)
Fundamental principles
A.1 Overview
The gas flow by gas ultrasonic flow meter is a flow measuring instrument that
consists of meter body, electronic components and microprocessor system,
ultrasonic transducer. The ultrasonic transducer is usually installed along with
the tube wall and directly in contact with the gas and shall withstand the gas
pressure. The ultrasonic pulse emitted by an ultrasonic transducer is received
by another ultrasonic transducer and vice versa. As shown in Figure 1, it is a
simplified geometric relationship between two ultrasonic transducers, Tx1 and
Tx2. The angle between the acoustic path and the axis of the tube is φ. The tube
diameter is D. The path length is L. The axial distance is X. In some flow meters,
it uses reflection channel. At this time, the acoustic pulse is reflected one or
more times on the tube wall.
Ultrasonic pulses pass through the pipe like a ferry crossing the river. If the gas
does not flow, the sound waves shall travel in both directions at the same speed.
When the gas flow rate in the pipe is not zero, the pulse that transmits
downstream along the airflow direction shall speed up but the counter-
transmitting pulses shall slow down. Therefore, the time of downstream
transmitting tD shall be shortened compared to the absence of airflow and the
time of upstream transmitting tU shall increase. Both transmitting times are
measured by electronic components. According to these two transmitting times,
it can calculate the measured flow rate according to formula (A.1):
Where,
- Average flow rate of gas along the acoustic path, in meter per second
(m/s);
L - Path length, in meters (m);
X - Axial distance, in meters (m);
tU - Time of sound pulse upstream transmitting, in seconds (s);
tD - Time of sound pulse downstream transmitting, in seconds (s).
It can use formula (A.2) to calculate the speed of sound:
Where,
C - Transmitting speed of sound waves in the airflow, in meters per second
(m/s).
A.2 Speed of sound in natural gas
The flow meter emits the acoustic pulse signal into natural gas stream from two
directions of upstream and downstream. The difference between the upstream
transmitting time of the acoustic wave and the downstream transmitting time is
the transmitting time difference. And it eliminates the effects of the speed of
sound. From formula (1), it can clearly see that when it uses flow meter to
measure the flow, it can measure the air velocity without knowing the speed of
sound. The formula (A.2) shows that by dividing the path length by the
transmitting time, the flow meter is able to measure the speed of sound.
Compare the measured sound velocity value with the theoretical calculation
value to determine whether the flow meter works normally.
However, for the flow meter user, it is important to understand the influence of
changes in gas properties on the speed of sound. The sound velocity of natural
gas is related to pressure, temperature, true relative density and composition.
The changes are shown in Figure A.1 and Figure A.2.
The three natural gas mixtures in the figure are GRI reference natural gas in
the report of the Southwest Research Gas Research Institute (acronym as
SwRI GRI) GRI-93/0181. Refer to Table A.1 for the components and
characteristics.
Table A.1 -- Components and characteristics of GRI reference natural
gas mixtures
Figure A.1 -- Speed of sound of “Gulf Coast” natural gas (Gr=0.58)
Parameter
Sound speed / (m/s)
True relative density Gr
High calorific value /
(MJ/m3)
Molar
fraction
/%
Gulf Coast GRI
reference natural
gas mixture
Amarillo GRI
reference natural
gas mixture
Ekofisk GRI
reference natural
gas mixture
Air
Methane
Nitrogen
Carbon dioxide
Ethane
Propane
Isobutane
N-butane
Isopentan
N-pentane
Hexane
Sp
ee
d o
f s
ou
nd
(m
/s)
Pressure / MPa
Figure A.2 -- Sound of speed of several natural gases and air at 16°C
A.3 Ultrasonic flow measurement theory
A.3.1 Flow rate in pipe
It can use a three-dimensional velocity vector v to describe the flow rate. The
flow rate is usually related to the spatial position x and time t: v=v(x,t). For a
steady non-vortex flow in a long straight tube of radius R, the only non-zero
time-averaged velocity component is along the axis direction and is only a
function of the radial position r. For a fully developed turbulent velocity
distribution, it can be represented by a semi-empirical power function:
Where, n is a function of the pipe Reynolds number ReD and the pipe roughness.
For a smooth tube, it can be expressed as a Prandtl equation:
If the Reynolds number is known, n can be calculated. Then use this n value to
calculate the speed distribution v (r).
This is actually a description of the steady flow state. Figure A.3 is the velocity
distribution curve calculated according to the above formula. And it has been
Sp
ee
d o
f s
ou
nd
(m
/s)
Pressure / MPa
gas
gas
gas
air
normalized according to the maximum flow velocity vmax at the center of the
pipe. The values of the three Reynolds numbers are ReD=105 (n=7.455),
ReD=106 (n=9.266), and ReD=107 (n=11.109).
Figure A.3 -- Turbulent velocity distribution of smooth tube
For the fully developed turbulence, the instantaneous flow rate is a composite
function of time and space. According to the derivation of Hinze (1975), v=v(x,t)
can be decomposed into:
Where, u represents the instantaneous average flow rate (generally a function
of time) and w represents the zero-mean turbulent velocity pulsation value.
These turbulent velocity pulsations always occur in stable turbulence, so they
can be regarded as random processes.