Alliance Pipeline is a 2,300-mile line delivering rich gas from North East British Columbia, Canada and the Williston Basin in North Dakota to the Chicago market. The success of this pipeline system is dependent on low maintenance costs, high reliability, minimum fuel usage, application of best practices, direct access to manufacturers’ technology, and strategic alliances and partnerships to resolve technical issues.
All of these factors are critical for the pipeline to meet owners’ expectations and profitability. Reduced gas volumes due to poor compressor reliability result in losses for all involved.
New Gas Stream Causes Seal Failures
The pipeline was successfully operated for more than 12 years until a new major gas stream with compositional differences in the NGL was brought online. Dry gas seals are more susceptible to liquid contamination and failures when heavier hydrocarbons are present in the process or transported gas. As a result of the new gas stream composition, dry gas seal failures increased from an historic zero to one per year to 14 failures in 2011.
Two failure modes were identified: Liquid contamination from the seal gas supply during normal operation and contamination from the process (compressor casing) shortly after start-up.
As a key strategic partner with PDGS seals installed in the pipeline’s 30 compressor stations across Canada and the US, EagleBurgmann worked with Alliance Pipeline to identify causes for the increased seal failures, to develop solutions and then implement the best technology to solve the problem.
The Role of a Reliable Seal Gas Supply
Seal gas supply is taken from the discharge line in pipeline compressors, which is at a higher pressure than the internal casing pressure. During start up conditions, the discharge line is at the same pressure as the internal casing pressure, so no clean seal gas is provided to the dry gas seal. This allows contamination to enter the seals, reducing their reliability and leading to premature dry gas seal failures.
With the presence of heavy hydrocarbons, the gas dew point also becomes an issue. When the discharge gas is at normal operating temperatures, the components in the gas are gases. As the pressure and/or temperature of the gas is lowered, some of these components will precipitate into liquids.
The discharge gas passing through lines to the seal gas panel also lowers the gas temperature, as the ambient temperature cools these lines. The gas pressure is regulated down to the sealing pressure, which is typically just above suction pressure.
This results in additional temperature and pressure reduction causing components in the gas to turn to liquid. Depending on the types of filtration in a seal gas system, limitations can be present for the amount of liquids they can handle. This was the case for the Alliance filters.
The next step in dealing with the heavy hydrocarbons is the seal itself. The pressure drops from the sealing pressure down to almost atmospheric pressure as it passes across the seal faces. As the pressure drops across the seal faces, the components in the gas can to turn to liquid, which sooner or later results in seal failures.
The first important component was to ensure seal gas was being provided to the dry gas seals at all times. With this being a pipeline application, no other gas sources are available, so incorporating a booster into the system was required to provide pressure/flow for reliable delivery of seal gas.
Available booster designs were either air driven piston boosters or electric driven RoTechBoosters. Air-driven boosters require a considerable amount of instrument air to support the air drive for achieving the necessary flow requirements. Continuous failures and high maintenance have been consistent issues for the industry with air driven boosters.
RoTechBoosters consume less energy and require fewer utilities to operate, as electrical power, compressed air and signals to and from the PLC are all that is required. A small amount of compressed air is used to operate an air actuated control valve and the signal to and from the PLC initiates the operation of the booster and provides alarms.
To manage the dew point of the seal gas, a two-stage coalescing filter system and electric heater were incorporated into the seal gas supply system. These two components manage liquids in the seal gas and maintain a seal gas temperature to ensure that all components in the seal gas remain gases following the 2 stage filter system.
For trouble-free installation onsite, the RoTechBoosters came as ready-to-install skids. Models 225H-120 and 225L-120 with 11 kW and 15 kW motors were utilized. Depending on the application, the power consumption is about 60 percent of the motor power. Runtime varies for each job – from 24/7 down to one time per year.
With the incorporation of these solutions, the seal failures from process contaminants and liquids in the seal gas have been completely eliminated.
Eliminating the need for seal removal, seal repairs, unit outages, and lost production due to seal failures increases profits and reduce costs. The minimum investment for the proven reliability of a RoTechBooster and seal gas conditioning quickly proves it worth with the reduction in seal failures.
For more information on RoTechBooster, contact us at firstname.lastname@example.org.