Minimize GHG emissions (methane) from compressor stations transporting natural gas through pipelines
Special Focus—Valves, Pumps, Turbomachinery and Compression
S. L. CHAKRAVORTY, Consultant, Noida, India
Gas compressors are the heart of natural gas transportation over long distances through cross-country pipelines, and centrifugal compressors are the preferred choice for this application. In petroleum refining operations, especially for secondary processing units, recycle gas compressors are used to circulate hydrogen (H2)-rich gas through catalysts/conversion reactors to produce finished products.
Compressor stations for transporting natural gas can be the source of considerable greenhouse gas (GHG) emissions (methane emissions) if not controlled. Oil and gas companies along the natural gas supply chain are under pressure to reduce GHG emissions. This article details the various sources of such emissions and the options available to control and mitigate them.
Process hazard analyses (PHAs). PHAs for compressor operation—especially for startup and shutdown activities—are needed to answer basic process safety questions: What can go wrong? What safeguards are needed? Are the provided safeguards working? With the objective to control GHG emissions, the most relevant questions must also be answered, such as the effects on the environment.
“What if?” analysis. To answer these questions, the following example uses a “What if?” methodology for a fixed-speed centrifugal compressor (motor- or gas turbine-driven). The process flow diagram (PFD) indicating the compressor’s typical control system, including the anti-surge control, is shown in FIG. 1.
When conducting a what-if analysis, it is essential to address the most important process safety information—i.e., the settle out pressure (SOP) of the gas compressor and its importance to ensure operating integrity.
Compressor SOP and its importance. In the case of an unplanned event—such as an emergency shutdown or power failure—the compressor trips first, followed by the anti-surge valve opening (refer to FIG. 1). Thereby, the gas content in the suction side and discharge side mix with each other and are equalized out. This equilibrium pressure throughout the compressor loop is called SOP. FIG. 2 shows a typical time vs. compressor discharge, inlet and SOP while the suction pressure at the settled-out condition reaches higher than normal suction pressure of the machine.
The primary reasons to calculate and know the SOP is to define the design pressure of equipment, piping and instruments located on the suction side of a compressor loop to avoid unnecessary flaring during compressor startup or shutdown. SOP is also necessary to establish the minimum dry gas sealing pressure in case of compressor pressurized shutdown.
During a pressurized shutdown, the isolated section remains pressurized, thus the compressor gets locked as the gas grips tightly to the rotor and impellers, preventing them from rotating.
SOP is used not only to set design parameters for piping and equipment on the suction side of the compressor, but it also plays an important role in startup due to higher SOPs that correspond to higher torque requirements. For example, if a compressor’s driver is unable to provide enough torque to restart after settling out, the system must be depressurized prior to the restart. In addition, as per API 521, the minimum design pressure of the separator drum/scrubber should be calculated as 1.05 times the SOP. This provides an adequate differential between the operating pressure and the set pressure of the pressure relief device for a compressor shutdown contingency.
The what-if analysis on the compressor PFD is detailed in TABLE 1.
Emissions from compressor stations. Oil and gas companies along the natural gas supply chain are under pressure to reduce GHG emissions. Compressor stations that help transport natural gas use a fixed-speed electrical motor/turbine to drive the compressor. This equipment can be a significant source of methane emissions from the following sources if not controlled and prevented:
- Fugitive emissions from compressor seal
- Volatile organic compound (VOC) emissions due to venting from the compressor system prior to carrying out maintenance
- ESD of the compressor and evacuation through depressurization in case of loss of containment
- Depressurization during compressor restart, as the compressor goes to SOP.
Conducting environmental hazards, along with a PHA. While conducting the PHA, the environmental hazards from compressor operations must be analyzed to derive safeguards for such hazards.
Possible options to minimize methane emissions. TABLE 2 provides emissions reduction options that should be scrutinized, depending on the design of the gas compression system and configuration.
Green H2 transportation through pipelines. Green H2 is being used to reduce GHG emissions and carbon footprint. Compression systems are needed to transport green H2 from production facilities to end users. Green H2 can also be vented to the atmosphere at compression stations, so it is therefore important to implement emissions prevention options for compressor stations during the design stage.
Takeaway. If not prevented, compressor stations can be the source of significant GHG emissions (methane). The options for emissions reduction detailed in this article will help control, mitigate and prevent GHG emissions in operations, as well as reduce the amount of natural gas that escapes to the atmosphere. GP&LNG
ABOUT THE AUTHOR
Satyalal Chakravorty is the former Executive Director of Indian Oil Corp. Ltd. He has also worked in the Oil Industry Safety Directorate (OISD), a body formulating safety standards for India’s oil and gas sector. Chakravorty now serves as an Independent Consultant based in Noida, India. His specialities are in process safety and safety auditing. He has published papers and presented at national and international forums on process safety management, asset integrity, HAZOP, reliability, LOPA, SIL and accident prevention.
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