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 provides definitions of gas measurement units
Industrial Metrology supports the 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 rooted 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 the Meter Convention, Signed in Paris in 1875. The Convention, modified slightly in 1921, remains the basis of all international agreements on units of measurement. Under the same convention, the BIPM (International Bureau of Weights and Measures) was incorporated in France to ensure the worldwide unification of physical measurement under the exclusive supervision of the 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 the Euro-Asian region
EURAMET is a collaborative alliance of national metrological organizations from member states of the European Union and the European Free Trade Association
GULFMET is an RMO established under the auspices of the 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 concern for finance, and policy 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 Standards

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 using 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

IEC Standards

(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
IEC 60751, Industrial platinum resistance thermometer sensors
IEC 60770-1, Transmitters for use in industrial-process control systems — Part 1: Methods for performance evaluation

AGA – Reports

The following reports from the American Gas Association apply to various types of meters used to measure gas flow quantities:

AGA Report 3 – Part:1 – General Equations and Uncertainty Guidelines
AGA Report 3 – Part:2 – Specifications and Installation Requirements
AGA Report 3 – Part:3 – Orifice metering of natural gas
AGA Report 3 – Part:4 – Background, Implementation Procedures and Subroutine Document
AGA Report 4a – Natural gas contract measurement and quality clauses
AGA Report 5 – Natural gas energy measurement
AGA Report 6 – Field proving of gas meters using transfer method
AGA Report 7 – Measurement of natural gas by turbine meters
AGA Report 8 –Part:1 – Thermodynamic properties of natural gas and related gases, detail and gross equations of state
AGA Report 8 – Part:2 – Thermodynamic properties of natural gas and related gases, germ-2008 equation of state
AGA Report 9 – Measurement of gas by multi-path ultrasonic meter
AGA Report 10 – Speed of Sound in Natural Gas and Other Related Hydrocarbon Gases
AGA Report 11 – Measurement of gas by Coriolis meter

API Standards

API MPMS: Chapter 1—Vocabulary provides definitions and terms used throughout the API Manual of Petroleum Measurement Standards (MPMS).
API MPMS: Chapter 14—Natural Gas Fluids Measurement standardizes practices for measuring, sampling, and testing natural gas fluids.
API MPMS: Chapter 14.1 – Collecting and Handling of Natural Gas Samples for Custody Transfer
API MPMS: Chapter 14.2 – Compressibility Factors of Natural Gas and Other Related Hydrocarbon Gases
API MPMS: Chapter 14.3.1 – Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids—Concentric Square-Edged Orifice Meters, (AGA Report No. 3, Part 1)
API MPMS: Chapter 14.3.2 – Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids—Concentric, Square-Edged Orifice Meters, (AGA Report No. 3, Part 2)
API MPMS: Chapter 14.3.3 – Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids—Concentric, Square-Edged Orifice Meters, (AGA Report No. 3, Part 3)
API MPMS: Chapter 14.3.4 – Orifice Metering of Natural Gas and Other Related Hydrocarbon Fluids—Concentric, Square-Edged Orifice Meters, (AGA Report No. 3, Part 4)
API MPMS: Chapter 14.4 – Converting Mass of Natural Gas Liquids and Vapors to Equivalent Liquid Volumes
API MPMS: Chapter 14.5 – Calculation of Gross Heating Value, Relative Density, Compressibility and Theoretical Hydrocarbon Liquid Content for Natural Gas Mixtures for Custody Transfer
API MPMS: Chapter 14.6 – Continuous Density Measurement
API MPMS: Chapter 14.7 – Mass Measurement of Natural Gas Liquids
API MPMS: Chapter 14.9 – Measurement of Natural Gas by Coriolis Meter (AGA Report No. 11)
API MPMS: Chapter 14.10 – Measurement of Flow to Flares
API MPMS: Chapter 15 – Guidelines for the Use of the International System of Units (SI) in the Petroleum and Allied Industries
API MPMS: Chapter 20 – Allocation Measurement of Oil and Natural Gas
API MPMS: Chapter 20.1 – Allocation Measurement
API MPMS: RP 85 – Use of Subsea Wet-Gas Flowmeters in Allocation Measurement Systems
API MPMS: Chapter 21 – Flow Measurement Using Electronic Metering Systems
API MPMS: Chapter 21.1 – Flow Measurement Using Electronic Metering Systems—Electronic Gas Measurement
API MPMS: Chapter 21.2-A1 – Addendum 1 to Flow Measurement Using Electronic Metering Systems, Inferred Mass
API MPMS: Chapter 22 – Testing Protocols
API MPMS: Chapter 22.1 – General Guidelines for Developing Testing Protocols for Devices Used in the Measurement of Hydrocarbon Fluids
API MPMS: Chapter 22.2 – Testing Protocols—Differential Pressure Flow Measurement Devices
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 accurately measure gas flow from one process system to another. 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 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). The 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
UV radiations
Insect and pest control

The primary element is a meter’s main flow-measuring component. Based on primary element classification, commercial meters are available in the following types.

a. Orifice Meter

An orifice meter generates a pressure differential when gas flows through a concentric (mostly used) orifice plate. The differential pressure signal and other data input from secondary and auxiliary elements are processed in the flow computer to calculate gas volume and energy content.

b.Turbine Meter

The 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

The rotary positive displacement meter passed the calibrated volume of gas in one rotation. The number of rotations thus provides the basis for the computation of gas flow through the meter at line conditions. The flow and energy content of the gas are corrected to standard measuring conditions through the 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 the meter, which get oscillations from a drive coil. Each tube oscillates in opposition to and at natural resonant frequency. Tube oscillations cause the generation of voltage sine-wave at its pickoff. Each tube generates a different voltage sine-wave, indicating the 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. The Coriolis meter generates an 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 the gas mass flow rate. The standard volume flow and energy rates are calculated by processing other data inputs from secondary and auxiliary elements.

e. Ultrasonic Flow Meter (Multi-Path)

The ultrasonic flow meter works on the principle of change in sound velocity in gas as the sound waves move IN or OPPOSITE to the direction of gas flow. Numerous sound wave transmitters and receivers are installed on the inner diameter of UFM (at the upstream end of the meter), with corresponding receivers and transmitters installed at the downstream end of the meter. Each pair of sound wave transmitters and receivers constitutes a single sound wave path. Multi-path meters have up to 12 pairs of transmitters and receivers. Difference (Delta-T) in the time the sound wave travels along the gas flow and opposite the gas flow is obtained. This delta-T is directly proportional to the gas flow rate through the meter. UFM generates an 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 the gas volume flow rate at line conditions. The standard volume flow and energy rates 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 lengths of tubes are required upstream and downstream of the meter to 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 a 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 the meter’s flow computer. All auxiliary elements should be rated as Class-1 Compatible and NEMA 4 with IP 4 ingress protection during normal operations.

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 the Control Room / Server

3 Fiscal Metering

When used to generate financial statements or transactions, commercial meters are termed fiscal meters. There can be various fiscal applications, including:

Fiscal metering that supports internal energy reconciliation and monitoring efficiency of operations (e.g. flare gas meter)
Fiscal Allocation Metering is used to ascertain gas flows from/to different sources/regions or parties. When two or more commercial entities are involved in the measurement process, these may be governed by contractual agreements.
Custody transfer metering systems are fiscal meters that 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 the metering system through the 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
A single party never carries out changes in meter configuration 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