- Gas flow computer
Originally the gas flow computer was an electronic module that simply provided a dedicated gas flow computer function. Today "gas flow computer" is a misnomer since the gas flow computer function is a subfunction to an overall data acquisition and control program installed within PLCs and RTUs.
The "gas flow computer" senses a mixed "dry" gas stream flow rate plus gas temperature and pressure. The most common method of measuring gas flow is via differential pressure across an orifice plate inserted into a flow metering pipe. The basic concept is simple enough but there is a devil in the details.
As the differential pressure is not directly porportional to the gas flow rate a flow computer algorithm is required to convert the differential pressure reading into a flow rate (may include square root extraction to linearize the input). Since gas is compressible and affected by temperature, the gas temperature and pressure must also be monitored and compared to a specified standard temperature and pressure within the algorithm. This is referred to a volumetric flow measurement.
Next we need to calculate mass flow AGA3 based upon the specific gravity of the gas. Since the gas stream contains a mix of various hydrocarbon gases of different specific gravities, mole percents must be determined via from a laboratory gas sample analysis. Note also that the mixed gas stream will also contain some useless inert gases such as nitrogen and carbon dioxide. Therefore the gas flow computer also requires the entry of mole percents for each gas component.
Based on accurate mass flow calculations it becomes possible, based upon the energy content of each gas component, to calculate
energy flow measurement, i.e., AGA8 since each gas component contains different energy content. These values in joules (or calories or Btu's) are typically built into the gas flow computer algorithm. Therefore energy flow metering is our ultimate goal since this is where the true value is for the client. Also these mineral reserves are taxed based upon energy content. The inert gases such as nitrogen have no value.
Other input parameters include contract hour as well as location latitude and altitude above sea level, isentropic exponent and type of materials used in the metering device to optimize the accuracy of calculations. In summary the gas flow computer requires approximately 30 initial input parameters in conjunction with "near realtime" gas flow, pressure and temperature sensing.
In addition to providing volumetric, mass and energy flow data, the gas flow computer also provides date and time, instantaneous, hourly and daily data. To ensure the data is never lost, the gas flow computer typically stores these date/time stamped records in RAM for up to 35 days.
Flow metering accuracy is easily compromised if there are liquids in the gas stream. Therefore methods are implemented to remove liquids from the gas stream before measurement. However a newer
V-Conetechnology (the inverse of orifice plate technology) is being used to more accurately meter gas that contains some liquids.
American Gas Association
Landfill gas monitoring
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