Fundamentals of Gas Pipeline Metering Stations _ Pipeline & Gas Journal

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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 (/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 fromhigh-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 systemconsists 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 systemand storage tanks.
Filter Separators
Natural gas filter units are installed at each station to remove any entrained liquids and solids
fromthe 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 upstreamof the meter. The filter separator is
normally a horizontal unit with a full-size, quick-opening closure and access platformfor
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
A control valve should be installed downstreamof the meter run to control both the flow
through the meter and the delivery pressure. This valve will primarily operate to limit the
station throughput in order to prevent the incoming gas volume fromexceeding the meter
capacity or the nominated volume but will also be equipped with a pressure override.
The control valve is generally controlled by a gas flow computer (GFC) based upon set points provided by the gas control
center. The control valve will normally operate in the fully open position to minimize pressure losses through the station and
should have a positioner, position indicator and position transmitter.
The GFC would also monitor and control the facilities as well as performcustody transfer quality measurement. The GFC
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