1 Commercial Gas Metering Systems: INTRODUCTION & BACKGROUND
Accurate energy measurement is a complicated and essential function in trade markets – gas being a compressible fluid; its measurement is more intricate. Metrology provides solutions to all gas measurement problems under three basic categories:
Fundamental metrology providing definitions of gas measurement units
Industrial Metrology supporting application of gas measurement systems in business and industry
Legal metrology covers the regulations and statutory requirements for gas measurement equipment, methods and systems. OIML (International Organization of Legal Metrology) deals with such matters. OIML works through National Metrology Institutes (NMIs), or Designated Institutes established in individual countries for implementation.
Metrology is the scientific study of measurement having its roots in the political motivation to standardize units of measurement under CGPM (General Conference on Weights and Measures). CGPM is an intergovernmental treaty organization created under a diplomatic treaty called Meter Convention Signed in Paris in 1875. The Convention, modified slightly in 1921, remains the basis of all international agreement on units of measurement. Under the same convention BIPM (International Bureau of Weights and Measures) was incorporated in France to ensure worldwide unification of physical measurement under the exclusive supervision of CIPM (International Committee for Weights and Measures). The CIPM Mutual Recognition Arrangement (CIPM MRA) is the framework through which NMIs demonstrate the international equivalence of their measurement standards and the calibration and measurement certificates they issue. The outcomes of this arrangement are the internationally recognized (peer-reviewed and approved) Calibration and Measurement Capabilities (CMCs) of the participating institutes.
CIPM has recognized various RMOs (Regional Metrology Organizations as associations of NMIs) for implementing CIPM MRA as follows:
AFRIMET is Inter-Africa Metrology Systems acting as RMO for African countries
APMP is a grouping of national metrology institutes (NMIs) from the Asia-Pacific region
COOMET is a joint forum of metrologists of Euro-Asian region
EURAMET is a collaborative alliance of national metrological organizations from member states of the European Union and of the European Free Trade Association
GULFMET is an RMO established under auspices of GCC Standardization Organization
SIM is an Inter-American Metrology System that promotes and supports an integrated measurement infrastructure in the Americas
Geographical Distribution of Regional Metrology Organizations
International Standards Applicable to Commercial Gas Metering
Fiscal metering implies concerning to finance and policy as such is taken to mean the best accuracy in terms of international best practices. Gas Fiscal metering and Custody Transfer applications are covered under legal metrology and International Standards, as stated in this section.
ISO 5970:2008 – Natural gas – Measurement of properties – Volumetric properties: density, pressure, temperature and compression factor
ISO 2186, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary and secondary elements
ISO 5167-1, measurement of fluid flow by means of pressure differential devices inserted in circular cross-section conduits running full — Part 1: General principals and requirements
ISO 6976, Natural gas — Calculation of calorific values, density, relative density and Wobbe index from composition
ISO 10715, Natural gas — Sampling guidelines
ISO 12213-1, Natural gas — Calculation of compression factor — Part 1: Introduction and guidelines
(International Electrotechnical Commission for electronic instrumentation)
IEC 60079-0, Explosive atmospheres — Part 0: Equipment — General requirements
IEC 60079-1, Explosive atmospheres — Part 1: Equipment protection by flameproof enclosures “d”
IEC 60079-11, Explosive atmospheres — Part 11: Equipment protection by intrinsic safety “i”’
IEC 60079-14, Explosive atmospheres — Part 14: Electrical installations design, selection and erection
IEC/TR 60079-15, Electrical apparatus for explosive gas atmospheres — Part 15: Construction, test and marking of type of protection ‘n’ electrical apparatus
IEC 60381-1, Analogue signals for process control systems — Part 1: Direct current signals
IEC 60381-2, Analogue signals for process control systems — Part 2: Direct voltage signals
API MPMS: Chapter 22.3 – Testing Protocol for Flare Gas Metering
API MPMS: Chapter 22.6 – Testing Protocol for Gas Chromatographs
API MPMS: TR 2571 – Fuel Gas Measurement
API MPMS: TR 2575 – Measurement of Thermally Cracked Gas
API MPMS: RP 551 – Process Measurement Instrumentation
2 COMMERCIAL GAS METERING SYSTEMS
Commercial gas meters are required to measure gas flow from one process system to another accurately. A complete gas measurement, recording and archiving system consists of Primary, Secondary and Auxiliary elements.
Primary and Secondary elements are usually mounted on a Metering Skid fabricated as per site requirements under ASME B 31.3. Auxiliary elements (except Transmitters and Calibration gas cylinders) are installed in a purposely built climate-controlled room under ISO Vibration Criteria. Such a climate-controlled room is designed and fabricated for equipment and operators’ safe operations under guidelines provided in ISO 11064 (also refer to ISO 10418:2019-Chapter 6). Following are important control parameters in the design of a climate-controlled room:
Climate conditions and dust
Illumination & Visibility
Physiological and psychological effects (human performance)
Noise and vibration
Accidental release of hazardous chemicals/gases
Venting and cabling requirements
Insect and pest control
The primary element is the main flow measuring component of a meter. By classification of primary elements, commercial meters are available in the following types.
a. Orifice Meter
Orifice meter generates pressure differential when gas flow passes through a concentric (mostly used) orifice plate. The differential pressure signal, along with other data input from secondary and auxiliary elements, is processed in the flow computer to calculate gas volume as well as energy content.
Turbine meter generates pulses when gas flow passes through the rotating turbine. These pulses, along with other data input from secondary and auxiliary elements, are processed in the flow computer to calculate gas volume as well as energy content.
c. Rotary Positive Displacement Meter
Rotary positive displacement meter passed calibrated volume of gas in one rotation. The number of rotations thus provide the basis for computation of gas flow through the meter at line conditions. Flow and energy content of gas is corrected to standard measuring conditions through processing of data input from secondary and auxiliary elements.
d. Coriolis Meter
Coriolis meter works on the principles of motion mechanics. Gas is split as it enters different sensor tubes of meter which get oscillations from a drive coil. Each tube oscillates in opposition to and at natural resonant frequency. Tube oscillations cause generation of voltage sine-wave at its pickoff. Each tube generates a different voltage sine-wave, indicating motion of one tube relative to the other. The difference in the period of each sine wave (Delta-T) is directly proportional to the mass flow rate through the meter. Coriolis meter generates inferential pulse train (electronically generated from a processor) which provides time-tagged gas flow snapshots. This data is processed in the meter processor to calculate mass flow-rate of gas. Standard volume flow rate and energy rate are calculated by processing other data inputs from secondary and auxiliary elements.
e. Ultrasonic Flow Meter (Multi-Path)
Ultrasonic flow meter works on the principle of change in sound velocity in gas as the sound waves move IN the Direction or OPPOSITE to the Direction of gas flow. Numerous sound wave transmitters and receivers are installed on the inner dia of UFM (at upstream end of the meter) with corresponding receivers and transmitters installed at the downstream end of the meter. Each pair of sound wave transmitter and receiver constitutes a single path of a sound wave. Multi-path meters have up to 12 pairs of transmitters and receivers. Difference (Delta-T) in the time taken by the sound wave to travel-along-the-gas-flow and travel-opposite-the-gas-flow is obtained. This delta-T is directly proportional to the gas flow rate through the meter. UFM generates inferential pulse train (electronically generated from a processor) which provides time-tagged gas flow snapshots. This data is processed in the meter processor to calculate volume flow-rate of gas at line conditions. Standard volume flow rate and energy rate are calculated by processing other data inputs from secondary and auxiliary elements.
Secondary elements are the components of a meter that provide additional information about flowing gas conditions in the pipeline, to correct the measured flow rate to standard conditions. Following are the various types of secondary elements used in gas meters:
f. Flow conditioner
Used to create laminar flow for improved meter accuracy
g. Meter Tubes (for use with Orifice, Turbine and Ultrasonic meters)
Necessary length of tubes are required upstream and downstream of the meterto ensure laminar gas flow as it reaches the primary element.
h. Pressure Tap with Pressure Sensor
For sensing line pressure and differential pressures
i. Temperature well and Temperature Sensor
For sensing gas flowing temperature
j. Line Sample Tap with Chromatograph Sample Collector / Stinger & Hose
For continuous sampling of gas for online analysis by a chromatograph
k. Line Sample Tap with Water Analyzer Sample Collector / Stinger & Hose
For continuous sampling of gas for online analysis by water analyzer
l. Line Sample Tap with H2S / TS Analyzer Sample Collector / Stinger & Hose
For continuous sampling of gas for online analysis by H2S / TS Analyzer
Auxiliary elements are the components of gas measurement, recording and archiving systems which are necessarily required to classify a meter as fit for “Fiscal Metering Applications”. For smaller sized fiscal meters (used for commercial or domestic gas flows) data input requirements from various gas analyzers are key-punched into meter’s flow computer. During normal operations, all auxiliary elements are recommended to be rated as Class-1 Compatible and NEMA 4 with IP 4 ingress protection.
Pressure, Temperature and Differential Pressure Transmitter
Online Gas Chromatograph
Online Water Content Analyzer
Online H2S / TS Analyzer
Calibration Gas Cylinders and Connecting hoses with Chromatograph / H2S / TS Analyzer
Flow Computer and / or Totalizer
RTU for Communication with Control Room / Server
3 FISCAL METERING
Commercial meters are termed as Fiscal Meters when they are used for generating financial statements or transactions. There can be various fiscal applications, including:
Fiscal metering that supports in internal energy reconciliation and monitoring efficiency of operations (e.g. flare gas meter)
Fiscal Allocation Metering for ascertaining gas flows from/to different sources/regions or parties. These may be governed by contractual agreements when two or more commercial entities are involved in the measurement process.
Custody Transfer metering systems are fiscal meters which determine the quantity of gas sold or purchased by one commercial entity from another. Such metering systems are always governed under long-term contractual agreements for gas supplies.
All types of fiscal gas metering systems should be able to perform the following functions:
Flow computer communication with primary and secondary meter elements
Flow computer communication with gas quality analyzers
Auto-calibrations, error indications, tempering indications and diagnostics
Manual flow proving / calibration set-up and termination, including proving report
Control sampling for third party verification of gas quality
Indicating fixed value parameters
Computation of totals (hourly, daily, weekly, etc.) and average values
Access to metering system through operator interface
Batch handling and reporting
Scheduling flows between individual metering lines
Indicating fixed values
Printing standard and user-defined reports
4 CUSTODY TRANSFER APPLICATIONS
Fiscal metering systems that are used for Custody Transfer applications for gas service have the following additional considerations:
Complete gas metering, recording and archiving system is configured, calibrated and validated in the presence of all parties before commissioning of operations
Changes in meter configuration are never carried out by a single party without prior consent or presence of other parties to the transaction (under contractual arrangement)
Mandatory joint meter calibration at a contractually determined frequency
Availability of primary stand-by elements of the main meter (for maintaining flow measurement while the main meter is erratic or under calibration)
Complete redundancy of secondary and auxiliary elements with hot-standby functionality
Mandatory record-keeping usually for not less than five years