In the gas producing plant (onshore or offshore), the gas metering system is the final stage of the production step before it send to the end user of the gas product. It plays a very important role as a custody transfer meter. Every measurement is required as practical as possible to have a very accurate measurement or a very minimum error. This issue (measurement accuracy or error) is very important since it will determine the quantity of the gas product that will be paid by the customer. Larger the error means less money that will be paid by the customer in correlation with the amount that they received. Since it plays very important role and almost appear in every gas producing plant, it will be good for us to know the basic principle regarding this gas metering system. So what is the basic principle to measure the gas that will be exported?
First of all, there will always be a contract between the plant business units with the customer. This contract stated the quantity and quality of the gas that they will buy. The quantity of the gas will be measured by an ultrasonic flow meter. The used of the ultrasonic flow meter is the most used flow measurement device right now, but for alternative we can also use an orifice with differential pressure transmitter as a measurement device. Usually, it requires a 2 x 100% flow meter stream with active and standby stream.. This ultrasonic flow meter installation & configuration shall conform to AGA 9 standard & codes.
Since the flow of a gas is very affected by pressure and temperature, the pressure and temperature transmitter will also install in the downstream of the ultrasonic flow meter to compensate the flow reading. All row measurement data will fed to flow computer to determine the actual gas flow in relation with pressure and temperature variation.
The quality of the gas is related to composition and characteristic of the gas. The composition of the gas will be measured by gas chromatograph and presented in form of % composition. For example for particular measurement, CH4 (methane) composition is 98%, C2 (ethane) composition is 0.05%, etc. This composition data will be fed to the flow computer to calculate the Gross Heating Value (GHV). The use of this gas chromatograph and also the calculation of GHV shall conform to GPA-2172 and GPA-2261 standard & codes.
The characteristic of the gas will be measured by a moisture and dew point analyzer. The need of measuring moisture is related with the water content in the gas. The water vapor in the natural gas mixture doesn’t burn. Thus the excess of the water content in the gas will reduce the value of the gas. That’s why there was some gas dehydration treatment in the gas processing plant to reduce the water content as low as possible. The quality of this gas dehydration treatment will be measured again by moisture analyzer before the gas is being exported.
The need of dew point analyzer is related with the condensation of the gas product. The dew point analyzer will determine at what temperature the gas will start to condense at given operating pressure. This parameter is very important such as for gas turbine fuel. There were no allowable liquid hydrocarbons forming inside the burner. If the liquid hydrocarbon forming inside the burner, there will be a less efficient turbine, high NOx emission, and can caused several damage. The dew point analyzer will inform the end user so that they can establish a pre-heat stage before they use the gas to prevent the condensation forming. The installation and consideration of sampling system of this analyzer (dew point and moisture) shall refer to API RP 555.
All the data regarding the measured quantity and quality of the gas being exported can be monitored in the Human Machine Interface (HMI) called metering station computer. This computer is the interface between all instrument equipment and the operator. The operator can check the history, print the report, diagnose the flowmeter etc. It also the interface between the gas metering system with other control system available at the plant such as PCS (Process Control System), ESD (Emergency Shutdown) system etc.
As a conclusion, we can conclude that the gas metering system assembly consists of the following sub-skid:
- Metering skid, consist of ultrasonic flow meter, pressure transmitter, temperature transmitter, pressure gauge, temperature gauge etc.
- Analyzer shelter, consist of gas chromatograph, dew point analyzer, and moisture analyzer.
- Gas metering panel, consist of flow computer and station computer (usually gas metering panel are located inside the control room).
Please take note that the mentioned sub-skid is only for simplify our understanding. For detail arrangement, we should refer to any particular project requirement.