Fundamentals Of Gas Pipeline Metering Stations

By Saeid Mokhatab, Tehran Raymand Consulting Engineers, Iran, and Greg Lamberson International Construction Consulting, LLC, USA, Tulsa, OK | January 2009 Vol. 236 No. 1

This article looks at pipeline natural gas metering station design but does not address the equations and empirical data used to calculate gas flow rates and volumes for custody transfer.

Nor does it cover the different standards of particular flow measuring devices, such as AGA-3 for differential pressure measurement; AGA-7 for turbine meters and vortex meters; and AGA-8 which provides equations to compute compensation factors for measured rates. Pipeline gas metering stations are designed for simultaneous, continuous analysis of the quality and quantity of natural gas being transferred in a pipeline, as follows:

  • Upper calorific value, which is the latent energy content of a gas that is released during combustion. It is the major variable when defining the price.
  • Concentration of sulphur compounds. Hydrogen sulphide and mercaptans are partially present as natural compounds and sometimes are mixed with the gas together with other sulphur compounds as odorants.
  • Hydrocarbon dew point, which is a temperature at which higher hydrocarbons condense. Liquid phase is produced in the gas pipeline if the product temperature falls below the hydrocarbon dew point. The accumulation of liquid in the pipelines can lead to a plug flow and may destroy the compressors in the pumping station
  • Water dew point is the temperature at which water condenses out. Water, together with hydrocarbons, favors the generation of solids, in particular during the decompression of gas from high-pressure pipelines. The solids block gas fittings, and the water is corrosive.
  • The fiscal effects of natural gas volume and mass flow measurement and calculations.

The system consists of a multi-path ultrasonic flowmeter, process gas chromatograph and computer workstation installed, pre-wired and pre-piped in a special air-conditioned shelter with all auxiliary equipment and utilities.

Each gas metering station branches off of the pipeline and is used to reduce pressure and meter the gas to the various users. For the pressure reduction and metering stations, the main equipment includes filters, heaters, pressure reducers and regulators, and flow metering skids. In addition, each station is generally equipped with drains for collection and disposal, instrument gas system and storage tanks.

Filter Separators

Natural gas filter units are installed at each station to remove any entrained liquids and solids from the gas stream. The filters may comprise cyclonic elements to centrifuge particles and liquids to the sides of the enclosing pressure vessel. These particles and liquids will then drop down for collection in a sump, which can be drained periodically.

A station inlet filter-separator should be installed upstream of the meter. The filter separator is normally a horizontal unit with a full-size, quick-opening closure and access platform for element change out. The vessel should be equipped with level gauges, high liquid level switches and a differential pressure transmitter across the filter elements. The filter-separator sumps should have automatic drain valves.

A condensate tank is installed for atmospheric storage of any liquids removed by the filter separator. Most tanks installed for this purpose are double-walled and installed on a concrete pad. The tank should contain a level gauge and a high liquid level switch.

Flow Control