TRANSIENT SIMULATION OF NATURAL GAS CITYGATES STATIONS

The demand of natural gas in the Brazilian energy market is increasing very fast over the few years and it was necessary to enhance the operational performance and safety of the gas distribution. The perfect operation of the natural gas citygate stations is essential to guarantee the delivery of natural gas for the end users like local distribution companies, thermoelectric power plants and large industrial customers within the contracted marketing conditions. These stations receive natural gas directly from high pressure pipelines and reduce the pressure using regulation valves that provoke a temperature reduction due the Joule-Thompson (JT) behavior, typical of natural gases. This temperature loss is compensated by forcing part of the gas flow through water/glycol bath heaters that use natural gas as fuel in the heating process. Usually the downstream gas temperature condition is controlled above a minimal set point while modifying the three-way valve position that regulates hot and cold streams flows. A numerical tool has been developed to simulate the dynamic process inside the natural gas citygate station, and proved to be a reliable tool to analyze the transient performance of the main equipments (filter, three way valve, heater, JT valve, relief valves) when submitted to abnormal conditions or changes in capacity. The methodology developed is able to handle a variety of citygate design.