What is Gas Metering System?

Flow Measurement

What is Gas Metering System?

Gas Metering System

Gas Metering System

  • The gas metering system is defined as a supervision transfer, which occurs when fluid or gas is measured for sale from one party to another.
  • When transferring a material, accuracy is extremely essential for both the company, & recipient during material delivery.
  • A custody transfer meter (CTM) is a meter that is designed, installed, and operated to meet the requirements for custody transfer measurement.
  • Custody transfer necessitates an entire metering system, not just flow meters, that is designed and engineered for measuring the product gas application.
  • The custody transfer metering skid is a piping system supported by a structural base.
  • It houses all primary measurement instruments, such as ultrasonic flow meters, pressure transmitters, temperature transmitters, and MOVs, as well as straight lengths as specified by AGA 9(American Gas Association).
  • Flow profiler, piping, and structural design, fabrication, and testing adhere to project specifications as well as applicable codes and standards while keeping functional requirements in mind.
  • The analyzers analyze gas components and provide data to flow computers for energy calculation.
  • The analyzer house is designed to be located in the hazardous zone of Zonal Gr-HB T3 and includes HVAC units, fire and gas sensors, and a safety PLC.
  • To ensure representative samples, sample probes (installed on the metering skid) and an analyzer sample handling system are provided.
  • The metering cabinets are housed in a secure area of the control room.
  • The Human Machine Interface (HMI), also known as the metering station computer, makes it possible to keep track of all the information concerning the measured quantity and quality of the gas that is being exported.
  • This computer serves as the interface between all instrumentation and the operator. The operator can review the history, print the report, and diagnose the flowmeter, among other things.
  • It also serves as an interface between the gas metering system and other control systems at the plant, such as the PCS (Process Control System), ESD (Emergency Shutdown) system, and so on.
  • The flow computers, station computers, GC controllers, PLC, HMI, and Ethernet switches with DCS interface are all housed in the panel.
  • All flow and energy calculations are performed by flow computers, which collect data from skids and analyzer houses.
  • The station computers compile and generate reports based on the station values.
  • A Gas Metering System (also known as a CTM) typically includes the following components:

Ultrasonic Flow Meter

  • Two ultrasonic transducers installed at an angle to the gas flow act alternately as a transmitter and receiver.    
Ultrasonic Flowmeter
  • Signals transmitted through the gas accelerate in the direction of flow and decelerate in the opposite direction.
  • The difference in propagation (transit) times that result is used to calculate the mean gas velocity.
  • The cross-sectional area is used to calculate the volumetric flow rate.
  • Gas velocity is measured using multiple chordal paths to improve measurement accuracy.
  • The pressure, temperature, or gas composition has no effect on the uncorrected measurement.

Gas Chromatograph

Gas Chromatograph
  • Gas chromatographs (GC) are used throughout natural gas pipeline networks to analyze the flowing gas and calculate the physical properties used for flow calculations and custody transfer.
  • A gas chromatograph (GC) is an analytical instrument to calculate the concentration of the sample.    
Gas Sampling System
  • The sample solution injected into the instrument is carried by a gas stream into a separation tube known as the “column.” (As the so-called carrier gas, helium or nitrogen is used.)
  • Within the column, the various components are separated.
  • The detector counts the number of components that exit the column. A known concentration standard sample is injected into the instrument to measure a sample with an unknown concentration.
  • The quality of the gas is determined by its composition and characteristics.
  • The gas composition will be measured using a gas chromatograph and provided as a percentage composition. For example, for a specific measurement, CH4 (methane) composition is 98%, C2H6 (ethane) composition is 0.05%, and so on.
  • This composition data will be input into the flow computer, which will calculate the Gross Heating Value (GHV).    

Flow Computer

  • The Flow computer was created specifically for measuring hydrocarbon liquids and gases where versatility and accuracy are critical.    
Flow Computer
  • The flow computer enables the configuration of multi-stream, multi-station applications for the simultaneous metering of liquids and gases.
  • The flow computer’s primary function is to collect gas flow data from connectors at meter runs, meters, or transmitters and calculate volumetric flow rate and energy flow under reference conditions.
  • Flows are computed on an hourly, daily, and overall basis.
  • Flow computers generate real billing data, which is stored and processed.
  • The volume of gas consumed is calculated independently for each meter by the flow calculator.
  • Flow calculators are intended to calculate the flow of energy and gas volume while considering the signals from the respective meter run metering device, temperature, pressure, and the overall chemical analysis using gas chromatographs.

Function of Flow Calculators

  • Calculate the gas volume (m) and flow rate (m/h) at the meter’s operating pressure and temperature.
  • Apply the error correction equation for the respective meter (ultrasonic flow meters) to the meter run pressure and temperature to calculate the gas volume (m) and volume flow rate (m/h).
  • Under normal pressure and temperature conditions, compute the gas volume (Nm) and volume flow rate (m/h) (reference conditions).
  • Calculate the gas compressibility for the meter run temperature and pressure conditions, as well as the chromatograph composition.
  • Using the calculated Gross Calorific Value and volume of the gas, calculate the energy of the passing gas (MJ) and the energy flow rate (MJ/h).

Energy Contents of Gas

  • The calorific value (CV) of a standard volume of gas is generally measured in units of MJ/SCM.
  • The CV is a measure of the amount of heat generated by the combustion of a standard cubic meter of gas, and it is measured under contractual conditions (pressure and temperature)
  • CV is determined by gas composition, which can be measured directly with a calorimeter or calculated from gas compositional analysis measured with an online gas chromatograph.

Gas Measuring Concept

  • The flowmeter measures the actual volume of gas under actual conditions (pressure and temperature measured in Actual Cubic Meters) (ACM)
  • To calculate the amount of energy contained in this volume, it must first be converted to standard volume, which is measured in Standard Cubic Meters (SCM)

SCM = Standard Cubic Meter to standard temperature and pressure conditions 15 Degrees Celsius and 1 Bar.

Vstd = Vact * (Pact / Pstd) * (Tstd / Tact)

Where

  • Vstd = is the Volume at standard conditions
  • Vact = is the Volume at actual conditions
  • Pstd = is the Pressure at standard conditions
  • Pact = is the Pressure at actual conditions
  • Tstd = is the Temperature at standard conditions
  • Tact = is the Temperature at actual conditions

Since Natural Gas is not Ideal gas, compressibility (Z) adjustment must be considered 

E = Vstd *Z * CV

E is Energy Billing Quantity,  Z is being calculated using AGA report no.VIII

In the event that the gas flow meter is measuring mass flow

E = M / (Sp.G x ρ) x C.V

  • M is the mass of gas delivered.
  • Sp. G is the specific gravity (or relative density) of dry gas to dry air at standard conditions.
  • ρ is the density of dry air at standard conditions.

Gas Metering Station Components

Gas measuring staion Component
  • A metering station would typically have several metering “streams” to provide redundancy and cross-checking.
  • Each stream must include the following components for energy measurements.
  • The flow will be measured by the primary device, while the field instruments or analyzers will send the measured values to the gas flow computer.

The gas flow computer will perform the following functions

  • Determine the mass and volumetric flow rates.
  • Conformity of density and temperature – measured downstream of the primary device – to the contractual conditions upstream
  • If no density is measured, density is calculated from gas composition using AGA report no. VIII.
  • Determine the calorific value based on the gas composition.

Selection Consideration for Primary Element

  • Contractual requirements
  • Range ability and accuracy or uncertainty over the flow rate range performance
  • Cost
  • Size and weight
  • Maintenance and calibration requirements
  • Suitability for gas quality

List of Applicable Standards

Orifice Meter

  • ISO 5167-1
  • AGA Report No.3

Ultrasonic Meter

  • BS 7965
  • BS ISO/TR 12765
  • AGA Report No.9

Turbine Meters

  • ISO 9951
  • AGA Report No.7

Coriolis Meters

BS ISO 10790

Calculation of CV, Density, and Relative Density

ISO 6976

Gas Chromatograph

  • ISO 6974
  • ISO 6975
  • ASTM – D 1945

Compressibility Calculations

  • ISO 12213
  • AGA Report No.8

People also ask

What is the meaning of Gas Metering?

  • An instrument to record the quantity of gas passing through a particular outlet.
  • A gas metering system is a device that measures the quantity of any gas or the energy content of any gas, either directly or indirectly.

What is a Gas Metering System?

  • The gas metering system is defined as a Custody transfer, which occurs when fluid or gas is measured for sale from one party to another.
  • Accuracy is essential during material transfer between the company and the recipient.

Why do we need metering?

Metering and monitoring systems are critical components of good facility management. These systems provide valuable insight into facility and equipment performance and support better energy use and cost management, as well as improved system feedback and optimization.

What are the different types of gas metering?

There are four different types of gas metering.

  • Mass Flow Meters,
  • Velocity Flow Meters,
  • Differential Pressure,
  • PD Meters

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