Session: 01-01: Safety Management

Paper Number: 134055

134055 - CFD Study on the Effectiveness of Purging and Loading Procedures for Gas Compressor Units Between Isolation Valves 

Abstract: 

Effective purge and load procedures during start-up of compressor stations on gas transmission lines are critical in ensuring

Effective purge and load procedures during start-up of compressor stations on gas transmission lines are critical in ensuring safe operation.  Most of TC Energy’s compressor units have a single small-diameter line (typically NPS 2 or 3) downstream of the unit suction valve for purging/loading and another small-diameter line upstream of the unit discharge valve for venting.  Due to the depressurization rate limit on the compressor unit, the vent line is typically equipped with a restricted orifice (RO) to control the vent flow rate in order to avoid potential seal damage (O-ring) requiring the replacement of the dry gas seal.  Consequently, when natural gas at mainline pressure is introduced through the small-diameter line to purge the unit, the purge flow rate is also limited resulting in high pressure within the unit casing and piping.  There is no readily available way 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.  1D model simulations can provide pressure profiles during purging/loading; however, the risk of deflagration/detonation can only be assessed if the mixing patterns inside the entire domain between the unit isolation valves can be simulated.

 

Computational Fluid Dynamics (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, the size of the flammable zones and the potential for methane-air stratification at different purge pressures inside each system will be analyzed.  Furthermore, scenarios where ignition sources are present and resulting in deflagration or detonation are also simulated.  This is particularly important once loading begins as the vent line will be closed.    These results can guide operation on setting acceptable purge times and pressures. 

Presenting Author: Teresa Leung NOVA Chemicals Corporation

Presenting Author Biography: Dr. Teresa Leung is a senior research engineer at the Center of Applied Research, NOVA Chemicals in Alberta, Canada. She has over 10 years of experience in developing and applying advanced computational methods for various pipeline and process simulations. She holds a B.Sc. and Ph.D. in Mechanical Engineering from the University of Calgary. Her current research interests include turbulent and multiphase flow modeling for industrial-scale problems, fast-transient dynamic modeling and physics-based optimization.

Authors:

Teresa Leung NOVA Chemicals Corporation
Kamal K. Botros NOVA Chemicals Corporation
Colin Hartloper NOVA Chemicals Corporation
Jason Lu TC Energy
Greg Szuch TC Energy