There are a few types of gas meter. All of these meters at the end have one goal, which is to get a reliable result of measurement that can be converted to amount of money. In the old days, measurement is only based on standard volume total. But the amount of energy will be different for different composition of gas. That’s why nowadays people use energy measurement as a standard for buying and selling gas. There are a few types of gas meter, we have mass meters, and we have volume meters. We classify the type of meter base on what the raw output of the meter is. Then from the raw data, with the help of some calculation standards, we will have parameters such as density, compressibility, calorific value, etc. These parameters will help us determine all other totalisers.
For a volumetric meter, there are two methods of getting gross volume flow rate. The first one is where the signal from the meter is in the form of frequency. And the second one is where the signal from the meter is in the form of velocity of gas. If the signal from the meter is in the form of frequency, then the meter has to have a k factor.
K factor is a value that determines how many pulses represents 1 volume-unit. For instance the k factor is 1000 pulses/m3. It means that 1 m3 will be represented by 1000 pulses. If the signal taken from the meter is vog which taken from serial connection, then to get the flow rate, the velocity has to be multiplied by the area of the internal diameter of the pipe. No meter is linear. That’s why they need to have wet calibration, which a meter being calibrated is compared to a more accurate meter on a few flow rate points. A value that corrects the deviation between the meter being calibrated and the reference meter is called by meter factor.
A meter factor value is applied in the meter being calibrated. On some meters such as usm, when wet calibration is done, meter factor is applied in the usm it self. In that case, there is no need to apply the meter factor in the flow computer for the gross volume flow rate calculation anymore. But it is a choice of a customer to disable the meter factor in the meter and apply it in flow computer. The only thing to be noticed is that meter factor must not be applied in both flow computer and meter. Another thing that will affect flow rate is that pipe expands with increase of temperature and pressure. Because of the expansion, volume of the gas that goes through will be different depending on the temperature and pressure. To compensate this, there is correction factor on the temperature (ctsm) and correction factor on the pressure (cpsm).
Standard Volume Flow Rate
Standard volume flow rate is a flow rate that is calculated at standard pressure and standard temperature. These standard pressure and temperature are standards defined by standard association such as AGA, ISO, etc or standard pressure and temperature agreed by buyer and seller.
Mass Flow rate
Mass flow rate is defined by how much mass goes through a meter in every certain time unit. Mass can be determined by knowing fluid density. The formula for mass volume is:
Energy Flow rate
Energy flow rate is defined by how much energy goes through a meter in every certain time unit. Energy can be determined by knowing Gross heating value which is defined by how much energy contain in certain standard volume unit. The formula for energy flow rate is:
The value 3600 is for volume/hour. It will be 86400 for volume/day. It varies depends on the flow rate unit.
Standard Volume Flow Rate
Standard volume flow rate is a flow rate that is calculated at standard pressure and standard temperature. These standard pressure and temperature are standards defined by standard association such as AGA, ISO, etc or standard pressure and temperature agreed by buyer and seller.
At most applications, these parameters such as compressibility and density are calculated. The calculation is defined in standards. AGA 8 is on of the most used standard to get the compressibility and density. Density also defined in ISO 6976 standard.
Mass flow rate is defined by how much mass goes through a meter in every certain time unit. Mass can be determined by knowing fluid density. The formula for mass volume is:
For mass meter such as orifice, mass flow rate is the raw output from the calculation of the orifice parameters. And by knowing the line and standard density, standard volume and gross volume will be determined.
Energy flow rate is defined by how much energy goes through a meter in every certain time unit. Energy can be determined by knowing Gross heating value which is defined by how much energy contain in certain standard volume unit. The formula for energy flow rate is:
Calculation for GHV is defined in standards such as ISO 6976 and GPA 2172.
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