Gas Metering system in City Gas Distribution

City Gas companies are taking Natural Gas from Cross country terminals. These terminals are RT, IP, SV, RR or dedicated terminals. Gas pressure at these points are normally 90 Bars. In some CGS pressure is less may be 49 or 19 bars. In case Gas receives from marginal fields, pressure will be less than 19 bars.

Cross country company sign an agreement with City Gas company. This agreement called as hook up agreement. Agreement elaborated about delivered pressure range and maximum flow with many other points.

City Gate Station

City Gate Station (CGS) is a custody transfer point of Natural Gas from Cross country operator to City Gas company. Type of meter installed at delivered pressure is crucial for correct measurement of Natural Gas. Apart from Gas meter other parts like Pressure regulating skid, Gas Chromatograph, Flow computer, PT and TT are other integral part of a metering system.

City Gate company must have their check metering system identical or more efficient to check the Natural Gas quantity received on real time basis. Natural Gas should filter with appropriate size of filtration system. Dirt free gas is not affecting the accuracy of metering in due course of time.

To measure Natural Gas in City Gas company, Volumetric and Mass measurement meters are prevalent and popular for different applications at different pressure.

Ultrasonic meter (USM-A volumetric meter)

USM is suitable for high pressure and high flow application. USM is measuring Gas quantity with ultrasonic rays in multiple path system. This process is based on Doppler effect. Best part of USM is non-moving component used in meter. In due course of time with continuous use its errors are very nominal and within limit. USM requires Gas chromatograph data, online pressure and temperature data on real time basis. All data are process in Flow computer to achieve accurate volume flown.

USM requires straight length of pipeline in inlet as well outlet. Gas conditioning is required for accurate measurement.

Turbine Flow meter (TFM-A volumetric meter)

Natural Gas delivered at high pressure with constant flow or low variation in flow TFM is suitable. This is a mechanical meter with moving component. It has a small turbine to rotates with the flow of Gas. Mechanical movement of turbine recorded in index. It required online Gas composition data from Chromatograph, pressure and temperature data on real time basis to calculate the accurate volume of gas.

Turbine flow meters are suitable for small size CGS and industrial customers. TFM is suitable to capture accurate measurement of flow within range of 20% to 80%.

At the place of Flow Computer, Electronic Volumetric Corrector can be used. EVCs calculates corrected volume of Gas. At the time of installation of EVC, temperature and Gas composition data are fixed as fed. These are not accounted any variation in real time data. In absence of real time data, variation may be in negative or positive.   

Rotary Positive displacement meter (RPD-A volumetric meter)

RPD meters are mechanical meters with full moving component. RPD meters are suitable for medium flow at medium pressure. With the help of two rotors, at each turn, Gas is capture and measure a specific volume. Mechanical movement captured from Gas flow is transferred to counter/index. In some meters, it converts in electronic counter/index.

To calculate Gas flow, it requires Gas composition data, pressure and temperature. Electronic Volume Corrector calculates the gas volume with fixed input. Any variation in such input data affects the gas measurement. With variation in data measurement may be negative or positive.

RPD meters are suitable for medium pressure and medium flow with high accuracy. RPD meters are suitable for small industrial and all kind of commercial units.

Positive Displacement meter (Diaphragm-A Volumetric meter)

PD or Diaphragm meter is a mechanical meter. This is the most common type of gas meter for residential and small commercial installations. It can be used for low pressure and low flow.

PD meters are measuring Gas at standard conditions. For accurate measurement it needs a correction with a factor. With correction factor, corrected Gas quantity can be measured.

Due to its low cost, compact design and light weight, it is first choice for residential Gas measurement. For small commercial with low pressure requirement it is very useful. For final Gas measurement always use correct factor.

Mass Flow meter (MFM-A Mass meter)

Mass Flow Meter is highly accurate meter. MFM are popular to measure Compressed Natural Gas at High pressure (at 200 Bars).

MFM are measuring Natural Gas on Coriolis principle. Fluid flows in stainless steel tubes and generates vibrations. High accuracy sensitive Sensors are capturing the vibrations/oscillations with respect to the flow.  

Coriolis meters are compatible to handle a wide range of flow rates and have the unique ability to output mass flow. This gives the highest accuracy of flow measurement currently available for mass flow measurement.

At each CNG dispenser, MFM are used to measure high pressurized CNG to fill in Automobile.

Thermal meter (THM-A Volumetric meter)

Look wise Thermal meter are similar to Diaphragm meter but these are highly accurate Smart meters works on different principle. Thermal meters are measuring Natural Gas flow w.r.t temperature difference.

Thermal meters are electronically controlled mechanical meters without any moving component. These are suitable for commercial and residential application.

Essential component in Gas Metering

Gas Chromatography

Gas Chromatograph is a device to check Natural Gas composition, gas density and other parameters. It uses a reference gas and check Gas composition online and generated data for Volumetric meters. It is a complex device with accurate result it has a big advantage.    

Pressure & Temperature Transducer (PT & TT)

Natural Gas is a compressible fluid. Temperature and pressure are vital in measurement of quantity. Online PT and TT use to send data related to pressure and temperature to flow computer.

PT & TT are required to calibrate in an prescribe interval.  

Electronic Volume Corrector (EVC)

Electronic Volume Corrector is a small device which gathered data from mechanical meter’s uncorrected volume and Gas flow pressure with the help of a PT. EVC corrects the uncorrected volume data to corrected data based on the Gas composition data, temperature and other fixed parameters.

EVC corrects the uncorrected meter’s reading based on certain average fixed data fed into. It is low cost and easy solution compare to flow computer and gas chromatograph.

Flow computer

Flow computer is an electronic device which collects data from mechanical meter, pressure and temperature transducer, Gas chromatograph, meter’s calibration error curve and data at standards conditions. With the help of all data it processes and provide corrected volume.

General Measurement idea

1. Use suitable meter w.r.t pressure and flow
2. Consider Q-start, Q-Min and Q-Max in selection of meter
3. Meters are measuring on standard condition. Only MFM is measuring corrected measurement.
4. Collect the data of online gas pressure, temperature and Gas composition flowing from meter.
5. Correct the meter reading with the help of Flow computer or EVC.
6. Meters with inbuilt correction mechanism are giving corrected volume like Thermal meter.
7. Use SMART meters

Meter’s flow range

Selection of meters:

Above chart is for general calculation. This may vary based on pressure and Gas flow requirement of application.

Monetary loss in metering

Gas Meters are used to purchased and sell Gas. In wrong selection of meter there will a monetary loss in Gas purchasing and selling. Gas composition, pressure and temperature reading are also playing role in calculation of corrected gas quantity. In case of error, If loss is 1% then for a span of five years, monetary loss will be multiples of new meter cost. In a rough calculation, it can be seen.