Start-up seal gas arrangement Findings: In the review of the compres- sor system set-up, it was identified that make-up hydrogen is employed as a back-up seal gas for the start-up phase, with procedural controls intended to shut this make-up hydro- gen line during regular operations. This make-up hydrogen, delivered at a substantially higher pressure than the compressor discharge, originates from a make-up gas compressor, which is generally a lubricating type recipro- cating machine. Analysis: Concerns were raised with the EPC contractor regarding the operational risks linked to the make-up hydrogen line. It was highlighted that if operators fail to close this line by mis- take, the make-up hydrogen, despite its very low dew point, contains lubri- cant oil, presenting a contamination risk to the DGS system. The make-up hydrogen’s movement at a speed of 60 m/s, typical for gas velocity within piping systems, signif- icantly increases the likelihood of the entrained lube oil being aerosolised. This high-speed gas flow can mechan - ically fragment the oil into extremely fine droplets, which may then be con - veyed as an aerosol through the DGS filter. The potential for these aerosol particles to be smaller than 1 micron, the size for which the seal filter is designed, poses a direct threat to sys- tem cleanliness and functionality. Mitigation: To address this issue, a decision was made to enhance the system’s control mechanisms. A pressure control valve (PCV) was introduced alongside the procedural controls. This PCV is set to activate at a pressure lower than the minimum discharge pressure of the compres- sor, ensuring the make-up hydrogen line is automatically sealed off when not needed. This additional layer of control significantly minimises the risk of operator error and the subsequent contamination by lubricant oil. Conclusion The extensive discussions and negoti- ations, while crucial, have underscored the significant potential costs in terms of both project schedule and budget.
This experience highlights the value of adopting a conservative approach in the FEED deliverables for compressor systems. It is evident that incorporat- ing specific considerations and rec - ommendations at this early stage can lead to substantial long-term benefits and cost savings. To this end, the fol- lowing measures are recommended for inclusion in FEED deliverables related to centrifugal compressors: • P&ID specifications: It should be explicitly noted that the seal gas line must be designed to minimise pockets and ensure a slope back to the tapping point from its highest point, with heat tracing and insulation to maintain the temperature well above the dew point. • Compressor datasheet enhance- ments: The datasheet should include a dew point curve as illustrated in Annex-B of API-692, providing clear guidelines for managing temperature and condensation risks. • External source dry gas use: When dry gas is sourced externally, the data- sheet and P&ID must highlight that it should not preferably be sourced from any lubricating type of compressor. If such gas is utilised, its use should be confined to start-up phases only, controlled through a pressure control valve to mitigate risks of contamina- tion during normal operation. Implementing these proactive meas- ures at the FEED stage could poten- tially result in significant cost savings for a DHT unit for each shutdown. Each shutdown event, triggered by liquid condensation in the dry gas seal, can lead to the tripping of the RG compressor and result in a production downtime of up to 72 hours for the DHT unit. Therefore, these recom- mendations not only aim to enhance operational reliability and efficiency but also significantly contribute to the overall economic performance of the project. Rajib Talukder is a Process Specialist in Aramco’s Global Manufacturing Excellence department, with more than 30 years of expe- rience in process engineering activities work- ing on numerous projects around the world. He is a chemical engineer who graduated from NIT Tiruchirappalli, India. Email: rajib.talukder@aramco.com
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