Abstract: Associated gases from an oil treatment plant are transported to a gas processing plant after pressure boosting. The terminal gas-liquid separator can only be used to treat liquids produced in pipeline during normal operation, but not receive pigging slug. In order to make separator work properly during pigging operations, pipeline pressure is reduced and gas flowrate is increased to blow the liquid out of the pipeline to reduce the volume of fluids accumulated in the pipeline. However, due to improper operation pressures and gas volumes, the separator fails to work properly before and after pigging operations. By using OLGA, the dynamic process that volumes of fluids accumulated in the pipeline and volumes of feeding liquids in the separator change with pipeline pressures and flow rates of gases at inlet is simulated to highlight the optimal pipeline purging program. As first step in the program, operation pressure of the pipeline is reduced; then, the inlet gas flowrate is increased once the volume of liquids accumulated in pipeline become stable. Pigging operation is initialized when the volume of fluids is lower than the processing capacity of the separator. The proposed program can ensure proper functioning of separator at the receiving terminal before and after pigging operations.