The gas metering system is defined as a Custody transfer where the fluid or gas is being measured for sale from one party to another.
During custody transfer, accuracy is of great importance to both the company delivering the material and the eventual recipient, when transferring a material.
CTM is defined as a meter designed, installed, and operated to meet the requirements for custody transfer measurement.
Custody transfer requires an entire metering system that is designed and engineered for measuring the product gas application, not just flow meters.
Gas Metering System
The custody transfer metering skid is an integrated piping system supported on a structural base. It accommodates all primary measurement instruments including the Ultrasonic flow meters, pressure transmitters, temperature transmitters and the MOV’s, straight length as per AGA 9. Flow profiler, piping and structural design, fabrication and testing are in line with project specifications and applicable codes and standards keeping in view the functional requirements.
The analyzers perform analysis of gas components, and provide input to flow computers for Energy computation. Analyzer house is designed to be located in the hazardous area of Zonal Gr-HB T3 with HVAC units, Fire and gas sensors with safety PLC is provided within the enclosure. Sample probes (installed on the metering skid) and analyzer sample handling system is provided ensuring representative samples.
The metering cabinets are located in the control room in a safe area. The panel houses the flow computers, station computers, GC controllers, PLC, HMI and Ethernet switches with interface to DCS. All flow and energy calculations are done by the flow computers which collect data of various signals from skids and analyzer house. The station computers compile the station values and generate reports.
Components of a gas metering system (also called CTM) typically include:
- Ultrasonic Flow meter
- Gas Chromatographs
- Flow computers
- Multiple meters/meter runs
- Supporting automation
- Analyzers (Moisture, Dew Point, CO2, H2S)
- Pressure and Temperature Transmitters
Ultrasonic Flow Meter
Two ultrasonic transducers, which are installed at an angle to the gas flow, operate alternately as a transmitter and receiver. The signals transmitted through the gas accelerate in the direction of flow and decelerate against the direction of flow.
The resulting difference in propagation (transit) times is used to determine the mean gas velocity. The cross-sectional area is then used to compute the volumetric flow rate.
To increase measurement accuracy, gas velocity is measured with multiple chordal paths. The uncorrected measurement is not affected by the pressure, temperature or gas composition.
Gas chromatograph (GC) are used all over natural gas pipeline networks, providing an analysis of the flowing gas and calculating the physical properties used for the flow calculations and for custody transfer.
A gas chromatograph (GC) is an analytical instrument that measures the content of various components in a sample. The sample solution injected into the instrument enters a gas stream which transports the sample into a separation tube known as the “column.” (Helium or nitrogen is used as the so-called carrier gas.) The various components are separated inside the column.
The detector measures the quantity of the components that exit the column. To measure a sample with an unknown concentration, a standard sample with known concentration is injected into the instrument. The standard sample peak retention time (appearance time) and area are compared to the test sample to calculate the concentration of various gas components.
The Flow computer designed specifically for hydrocarbon liquid and gas measurement where versatility and accuracy are high priorities. The flow computer allows multi-stream, multi-station applications to be configured for simultaneous metering of both liquids and gases.
The Principle role of flow computer is to collect gas flow data from the connectors at meter runs, meters or transmitters, and calculate the volumetric flow rate under reference conditions, as well as energy flow. Flows are calculated based on hourly, daily and overall total. All data generated by flow computers are real billing data which are stored and processed.
The volume of the gas consumed is calculated separately for each meter run by the respective flow calculator. Flow calculators are designed to calculate the flow of energy and gas volume, considering the signals from the respective meter run metering device, the temperature, pressure and the overall chemical analysis using the gas chromatographs.
More specifically flow calculators do:
- Calculate the gas volume (m) and volume flow rate (m/h) at meter run pressure and temperature.
- Calculate the gas volume (m) and volume flow rate (m/h) following the application of the error correction equation for the respective meter (ultrasonic flow meters) to the meter run pressure and temperature.
- Calculate the gas volume (Νm) and volume flow rate (Νm/h) under normal pressure and temperature conditions (reference conditions).
- Calculate gas compressibility for meter run temperature and pressure conditions and the composition detected by the chromatograph.
- Calculate the energy of the passing gas (MJ) and the energy flow rate (MJ/h) on the basis of the calculated Gross Calorific Value of the gas and its volume.