Gas Metering Stations Using Orifice Flow Meter

[From:来自网络] [Author:admin] [Date:12-02-22] [Hit:]

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. There are several flow measurement methods that can be used as a custody flow meter such as turbine flow meter, orifice flow meter, ultrasonic flow meter, coriolis flow meter, etc. This article will discuss the gas metering system that uses an orifice plate as its flow element.

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 orifice flow meter. Usually, it requires a 2 x 100% flow meter stream with active and standby stream. This orifice flow meter sizing, installation, & configuration shall conform to ISO 5167 and or AGA 3 standard & codes.

Since the flow of a gas through the orifice is very affected by pressure, temperature, and density of the fluid (due to its dimension that has temperature dependence etc), the pressure, temperature, and density transmitter will be installed along with orifice flow meter to calculate the actual flow reading in actual flow condition. All row measurement data will be fed to flow computer to determine the actual gas flow in relation with pressure, temperature, and density variation. The density of the fluid itself might be calculated from the gas chromatograph component measurement. Thus if we already use a gas chromatograph then we don’t need to use a density transmitter and vice versa. In general application, the orifice gas metering station can achieve up to 1% of mass flow rate uncertainty by using a complete package above.

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 of 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 density and 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. As alternative, when the GHV isn’t a concern, we can use only a density meter (densitometer) as a device to determine the actual fluid density.

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 dehydrator treatment in the gas processing plant to reduce the water content as low as possible. The quality of this gas dehydrator 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 transmitter 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 orifice gas metering system assembly consists of the following sub-skid:

1.Metering skid, consist of orifice flow meter, pressure transmitter, temperature transmitter, density meter (if needed), pressure gauge, temperature gauge etc.

2.Analyzer shelter, consist of gas chromatograph, dew point analyzer, and moisture analyzer.

3.Gas metering panel, consist of flow computer and station computer (usually gas metering panel are located inside the control room).

Please take a note that the mentioned sub-skid is only for simplify our understanding. For detail arrangement, we should refer to any particular project requirement. But in typical, the system will be more or less similar to the arrangement above. See below P&ID example to get more clear understanding.

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