All there is to know about gas metering stationsTemitayo Oketola | March 09, 2020
An enormous volume of natural gas is transported along the gas supply chain daily. Statistics from Statista show that over 3,800 billion cubic meters of natural gas were produced globally in 2018. From the separation of liquid condensate upstream to the distribution and usage of gaseous products downstream, it is imperative that standards exist for measurement and regulation.
Measurement and regulation form the basis for conducting any business. Companies looking to purchase natural gas from suppliers need to ensure that they are being supplied what they paid for, and at the specified requirements. Besides quality assurance, producers also need to quantify their gaseous output. Gas metering involves the analysis of the quantity and quality of natural gas transferred in pipelines. A gas metering station is made up of several components specifically designed to carry out this analysis. To do that, custody transfer, conditioning, metering and regulating operations must be performed.
Operations in gas metering stations
Custody transfer measurement describes the transaction where fluids are exchanged between two parties. Because of the high value of gas and the large volume being transferred, one of the stringent requirements of any metering station is that measurements are accurate since small errors in measurement can result in substantial losses in revenue. Both parties use flow meters, metering skids, control valves and a host of other sensors for custody transfer measurement.
Explored product feedstock from gas wells consists of natural gas and liquid condensates (water and liquid hydrocarbon). A mixture of gas, water and liquid hydrocarbons are routed through gas-gathering systems to separator vessels, separating natural gas from liquid condensates.
Even after the initial stages of production, it is not unusual to find minute fractions of liquid condensates along the lines downstream. These impurities hinder the performance of the gaseous products. Consequently, through the use of heaters and separators, a metering station must also condition natural gas to ensure that preset standards are met.
Measurement of gas volume and flow rate is essential in the gas metering station. Flow meters are primary devices in the metering station, and they rely on the principle of conservation of mass for taking measurements. Apart from being calibrated, these meters work together with flow conditioners to minimize the risk of errors.
Several other secondary devices, such as flow computers, temperature gauges and pressure gauges, are predominantly found in the station. They work in line with the flow meters and other auxiliary devices to provide data on the gas flow.
What happens when the gauges display an undesired gas pressure or temperature during metering? Gas metering stations are designed not only to measure the temperature, pressure and volume of gas, but also to restore the gas to a desirable state after deviation. Pressure regulation in metering stations is achieved by the combined effect of filters, heaters and condensate tanks.
The four primary operations in the gas metering station can only be successful when proper devices are used. From the smallest sensors to the largest heat exchangers, these components perform critical functions.
Metering station components
Gas heaters and heat exchangers
Remember how Gay-Lussac found a relationship between the temperature and pressure of a fixed mass of gas? Without going into the minute intricacies of his experiment, it is worth noting that when the volume of a given mass of gas is kept constant, the pressure and temperature of the gas are directly proportional.
Gases have a dew point temperature at which they condense and become liquid. Therefore, condensation occurs when a drop in pressure of the gas is accompanied by a temperature drop below the gas dew point temperature. Gas heaters and heat exchangers prevent this condensation by keeping gases within a specified temperature range. While gas-fired water bath heaters and electric immersion heaters serve the same heating function, piping insulation also plays a significant role in preventing condensation.
Valves and actuators
Without regulation devices, gases would flow downstream nonstop. Things could get real messy during an emergency. Valves are mechanical devices that control fluid flow by opening, obstructing or sealing passageways. There are many types of valves in use today, and they can be operated manually or automatically.
The slam shut valve prevents excessively low or high pressure from causing downstream damage in the station. They are usually automatically operated.
Relief valves are also used to prevent excessively high pressure downstream. It differs from the slam shut valve through its method of mitigation. They are designed with pressure valves that release gas from the pipeline to special manifolds.
Pressure switching valves are often used with pneumatic systems. They are used for keeping the pressure at outlet ports within the desired range.
Pressure gauges are simple devices used for measuring gas pressure. The Bourdon tube pressure gauges are the most common gauges found in metering stations.
There are quite a number of temperature gauges used in metering stations. Oftentimes, radiation pyrometers or thermistors are used. But most of the time, bimetallic devices, thermocouples and resistance temperature detectors (RTDs) are used for sensing the temperature of natural gas within the system. What matters most is selecting the right device, depending on the level of sensitivity needed. Maintenance and power requirements are other factors to also consider.
The liquid condensate obtained from the separator vessels is stored in a condensate tank. Though these tanks can be operated manually, they are equipped with safety valves to prevent overpressure.
Flow meters are generally used in stations to measure the volumetric flow of fluid. As requirements for flow meters vary along the gas supply chain, different types of flow meters with different operating principles are in existence today. The advantages of one over another are dependent on the requirements of the station. For instance, Coriolis flow meters are ideal for when high accuracy is needed, and less maintenance is done, while the orifice flow meters are used when budget is an issue. Thermal flow meters, turbine flow meters, ultrasonic flow meters and rotary displacement flow meters are also in use today.
Nevertheless, all flow meters fall under two categories: Energy additive flow meters and energy extractive flow meters.
Extractive meters function by using the energy from the flow stream, while additive meters take flow rate measurements by supplying energy into the flow stream.
Though some secondary devices still exist that function primarily to supply auxiliary devices with inputs, this article addresses the essential components necessary to understanding gas metering stations.