Dynamics of Purging, Loading and Venting of Compressor Casing and Piping Between Unit Isolation Valves in Natural Gas Compressor Stations

Abstract Purging, loading and venting are unsteady processes required prior to the start-up (in the case of purging and loading) or shut-down (in the case of venting) of compressor units on gas transmission systems. Compressor stations have valves on the suction and discharge of the unit for isolation purposes and incorporate small diameter lines (a bypass line on the unit suction isolation valve and a blowdown line upstream of the unit discharge isolation valve) to facilitate the purging, loading and venting of the total volume between the valves, including the compressor casing. Flow rates through these small diameter lines are typically limited by a control valve and/or a restricting orifice on the line. Correctly designing for such flow rates is necessary to achieve design objectives such as purging efficiently, effectively and safely or venting at the maximum permissible depressurization rate established by compressor original equipment manufacturers (OEMs). This paper describes a dynamic model used to calculate the purging, loading and venting of compressor units. The model accounts for the dynamics and thermodynamics in two parts: one for the total volume (defined as the volume between the unit’s suction and discharge valve, including the compressor casing), and one for the purge, load and vent lines. The total volume model balances the mass and energy flow into and out of the total volume to calculate the instantaneous gas pressure in the compressor casing. The purge, load and vent line model identifies the choke location along each line and calculates the instantaneous mass flow rate, taking into account each line’s control-valve open position. The paper compares the model results to plant data from compressor stations and offers insights into the relationship between parameters such as purge time, purge pressure, load time and depressurization rate.