What is Custody Transfer Metering system?

Custody Transfer Metering system is one of the crucial and important system as it deal with transfer/measurement(a very accurate one) within two companies.

Custody Transfer Metering system

Special care and precaution shall be taken care while design engineering the Custody Metering Package for it accuracy levels , response time and component measurement.

Because of this very stringent requirement very few flow meter qualify for the custody metering flow measurement for gasses. The most commonly suggested one are

  1. Ultrasonic Flow Meter: most widely used

  2. Orifice Type Flow Meter : suggested for line size less than 4”

  3. Coriolis Flow Meter : rarely suggested due to techno economic consideration and lower density limit is 4 Kg/3

Pressure Drop:

Generally Allowable pressure drop for the package: Minimum but shall not exceed 0.5 barg with the Ultrasonic flow meter, 1 barg for the orifice flow element. Its is generally VENDOR to provide detailed calculations with the bid to demonstrate that the pressure drop within the skid for all flow conditions are within the required limits.

Ultrasonic meters measure the speed of gas movement by measuring the speed at which sound travels in the gaseous medium within the pipe. American Gas Association Report No. 9 covers the proper usage and installation of these meters, and it specifies a standardised speed-of-sound calculation which predicts the speed of sound in a gas with a known pressure, temperature and gas composition.

Custody Metering is the most crucial and most sought after package which needs careful consideration of the specification and requirements.

Custody Metering is a small instrumentation complete package in itself. Generally the metering skid have following instruments/equipment as a part of the package

  • Flow Meter:-Flow measurement device

  • Flow Computer:- For Monitoring Controlling of Flow meter and report generation

  • Temperature and Pressure instrument :- For Temperature Pressure Compensation

  • Analyzer: Like Gas Chromatograph and Moisture and Hygrometer

  • MOV( Motor Operate Valves): For controlling the flow through meters.

  • Prover/ Calibrator: A standby meter which is used for calibration purpose.

  • Stand-by Meter: Meter which will be used for in case of main meter fails or in maintenance

  • Supervisory Computer in some cases which is used for monitoring and Supervision

Various Standard which need to be considered while preparing specification for Metering are

  • OIML standard

  • AGA requirments

  • Various requirment for anlayser

Special precaution and care shall be taken care while mentioning requirements for Testing and calibrations.

Selection of Flow meter generally depends upon the process conditions and accuracy requirements

The most common type used these days is Ultrasonic flow meter for gasses.