PURPOSE: • Gas custody transfer metering system is a complex realizing a gas custody metering station consisting of: • Measuring pipelines transferring the measured media (gas); • Orifice plate; • Measuring instrument; • Monitored media parameters (pressure differential, temperature, gas composition, density); • Handling technique of measurement results (manual and automatic calculating devices); • Complex of hardware tools providing data processing, storage, presentation and transmission; • Interconnecting lines and auxiliary devices to prepare the measured media for measurements (filters, separators, valves and etc.)
Gas flow measuring procedures used in Gas custody transfer metering system: Gas flow and its quantity are mainly measured by means of a method of variable pressure drop using standard differential devices (for example orifice plate), multivariable sensors and controllers for data processing on gas industry sites. In these later days, ultrasonic flowmeters, turbine and rotary meters are well known. The main document regulating the procedure of gas consumption accounting is ISO 5167-1:2003 “Measurement of fluid flow by means of differential devices”. • Variable pressure drop method (differential pressure measurement before and after differential device installed in gas flow); • Tachometric flow measurement method (operating principle of the method is based on dependence of speed of rotation of transducer installed in pipeline); •Meters assisted method (the meters have been used before flow meters entered in use and shaft of flow transducers of meters via reduction gear is connected with counting mechanism that allows measuring the quantity of passed substance).
Note: Comparative analysis of gas quantity and flow measuring devices allows confirming so that variable pressure drop method is the effective method of gas quantity and flow measurement in gas industry.
Standard composition of Gas custody transfer metering system includes: • Primary gas quantity measuring device; • Flow transducer for gas quantity measurement; • Sensing line; • Gas grade preparation devices (in accordance with requirements placed to equipment including in Gas custody transfer metering system); • Gas grade analyzers (for metering skids installed in gas production places, on boundaries of trunk pipelines after the places of possible gas mixing from different gas fields, underground gas storage facilities and from third-party vendors); • Complex of automation equipment including data processing, storage and communication; • Complex of automation system providing the control over gas pollution and fire availability.
Support technologies can be included in Gas custody transfer metering system composition: • filters and separators for gas flow purification; • gas flow transducing apparatus (elimination of flux distortion effect on metrological characteristics of flow transducers); • safety locking devices; • condensate collection system; • gas flow pulsation suppressing device (straightening vane).