By: John S. Stahley
After investigating the operating history of the plant, it was discovered that the process infrequently required full production, and it was rarely necessary to operate all of the compressors simultaneously.
A gas processing plant in Asia included several gas turbine-driven compressor trains, operating in parallel to each other. All of the compressors were equipped with tandem dry gas seals, each using compressor discharge gas as the source of seal gas. The plant operator had experienced multiple gas seal failures. After investigating the operating history of the plant, it was discovered that the process infrequently required full production, and it was rarely necessary to operate all of the compressors simultaneously. However, rather than shut down a compressor when reduced plant output was required, the operators would reduce the operating speed of one or more of the compressors to an idle speed, reducing the compressor’s throughput.
During operation in this “hot standby” mode, the compressor operated at a very low speed and, therefore, produced a very low pressure rise across the compressor. Because the seal gas was taken from the compressor discharge, the reduced discharge pressure resulted in insufficient seal gas supply pressure. In the absence of a positive flow of seal gas, the process gas was able to flow directly into the gas seals, resulting in contamination of the primary seal and eventual seal failure.
Corrective actions
There were a few possible solutions to this problem:
- Discontinue the practice of hot standby operation and shut the compressor down when it was not needed.
- Provide an alternate source of seal gas during the hot standby mode of operation.
- Increase the idle speed of the hot standby operating mode to a level that provided enough gas pressure rise across the compressor to assure a positive flow of seal gas.
The compressor operator chose the last option, as it was determined to be the most easily implemented and provided the least disruption to plant operations. The operator now runs the compressors at sufficient speed to assure a pressure differential across the compressor, and thus a positive flow of seal gas to the seals; this has eliminated the gas seal problems.
Lessons learned
This case study again demonstrates that the operation of the compressor can directly influence gas seal reliability. When using the compressor discharge as the source of seal gas supply, operation of the compressor at transient conditions can increase the opportunity for gas seal contamination. If operation at reduced speeds or other transient conditions is necessary, the operator must assure that sufficient pressure rise exists across the compressor to allow for a positive flow of seal gas. Alternatively, another source of seal gas must be made available during this type of operation. Some options include gas turbine fuel gas, plant nitrogen, or process gas from other compressors within the plant. If no alternative seal gas sources exist, a seal gas booster can be used to increase the pressure of seal gas taken from the compressor discharge.
About the Author
John S. Stahley has over 32 years of professional experience including a diversified background in the areas of general management, operations management, project management, and services with a primary focus on delivering outstanding results in an engineered to order, complex projects environment. His current responsibilities include the management of installation and commissioning activities for turbomachinery. His education includes an MBA in Management from St. Bonaventure University, and a BS in Industrial Engineering from Rochester Institute of Technology. Stahley has authored and presented several technical papers on the subject of dry gas seals.
Note: The views, thoughts, and opinions expressed in the content above belong solely to the author and do not necessarily reflect the opinions and beliefs of Refining Community or its parent company, CRU Group.
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