CFD Study on the Effectiveness of Purging and Loading Procedures for Gas Compressor Units Between Isolation Valves

Abstract Effective purging and loading procedures during start-up of compressor stations on gas transmission lines are critical in ensuring safe operation. Most gas compressor units have a single small-diameter line (typically NPS 2 or 3) across the unit suction valve for purging/loading and another small diameter line upstream of the unit discharge valve for venting. Due to either the depressurization rate limit on the compressor unit or the desired vent time window, the vent line is typically equipped with a restriction orifice (RO) or a control valve to achieve the desired depressurization rate. Consequently, when gas at mainline pressure is introduced through the small-diameter line to purge the unit, the purge flow rate is also limited by the RO or the control valve, which affects the degree of pressurization within the unit casing and piping. There is no readily available method to determine the purge velocity and system pressure that would minimize the risk of having flammable mixture inside the unit while avoiding an unnecessarily lengthy purge process that could delay unit start-up. CFD simulations are performed to analyze how the flammable zone would evolve during the purge process for different station layouts including detail compressor casing internal geometry, different piping lengths and vent line RO sizes. The time it takes for a system to reach above the rich flammability limit, the size of the flammable zones and the potential for gas-air stratification at different purge pressures inside each system will be analyzed. Furthermore, scenarios where ignition sources are present during purging are also simulated. This is particularly important once loading begins as the vent line will be closed. These results can guide operation in setting acceptable purge times and pressures.