While proper design and maintenance of dry gas seals is a well-understood topic, dry gas seal failures are still relatively common. These failures can result in frequent repairs and costly downtime. Although several case studies of individual failures are available, relatively few large-scale dry gas seal failure studies exist. Based on a review of existing literature, very little has been published on failure statistics aimed at improving seal reliability. As a part of an industry-funded dry gas seal reliability project, a failure database has been populated with information provided by both end users and original equipment manufacturers. The database includes details on dry gas seal and separation seal configuration, seal gas supply, operating history, conditions at time of failure, and failure symptoms, including any results from failure analyses performed by the survey respondent. In total, eight companies contributed 194 failures. Of these, 144 cases had root causes provided. From this database of failures, statistical analysis is used to determine common reasons behind dry gas seal failures in gas compression service. Failure trends are identified based on data collected, and corresponding recommendations are provided for improving dry gas seal reliability.

Supercritical carbon dioxide (sCO2) power cycles require high compressor efficiency at both the design-point and over a wide operating range. Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part-load operating conditions. For sCO2 cycles operating with compressor inlets near the critical point, large variations in fluid properties are possible with small changes in temperature or pressure. This leads to particular challenges for air-cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events. Design and off-design operating requirements for a wide-range compressor impeller are presented where the impeller is implemented on an integrally-geared compressor-expander (IGC) concept for a high temperature sCO2 recompression cycle. In order to satisfy the range and efficiency requirements of the cycle, a novel compressor stage design incorporating a semi-open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range. The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range. These advanced impeller design features are enabled through the use of direct metal laser sintering (DMLS) manufacturing. The resulting design increases the range from 45% to 73% relative to a conventional closed impeller design while maintaining high design point efficiency.

Supercritical carbon dioxide (sCO2) power cycles require high compressor efficiency at both the design point and over a wide operating range. Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part-load operating conditions. For sCO2 cycles operating with compressor inlets near the critical point, large variations in fluid properties are possible with small changes in temperature or pressure. This leads to particular challenges for air-cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events. Design and off-design operating requirements for a wide-range compressor impeller are presented where the impeller is implemented on an integrally geared compressor–expander concept for a high temperature sCO2 recompression cycle. In order to satisfy the range and efficiency requirements of the cycle, a novel compressor stage design incorporating a semi-open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range. The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range. These advanced impeller design features are enabled through the use of direct metal laser sintering (DMLS) manufacturing. The resulting design increases the range from 45% to 73% relative to a conventional closed impeller design while maintaining high design point efficiency.

Supercritical carbon dioxide (sCO2)-based cycles have been investigated for pumped heat energy storage (PHES) with the potential for high round-trip efficiencies. For example, PHES-sCO2 cycles with hot-side temperatures of 550°C or higher could achieve round-trip efficiencies greater than 70%. The energy storage cycle and equipment also synergize well with other systems incorporating thermal storage and/or sCO2 power blocks, e.g., concentrating solar power. These sCO2 cycles are closed Brayton cycles whose efficiency and system cost and complexity are sensitive to leakage and makeup/recompression requirements for long-term application. Therefore, incorporating hermetically-sealed machinery is an attractive option for minimizing system leakage and improving system cost and performance. Bearings that enable hermetic machines include sCO2 process-lubricated bearings and magnetic bearings. Ongoing developments in sCO2-lubricated bearings are addressing the well-known limitations that have challenged their use in megawatt-scale machinery (load capacity, damping), yet magnetic bearings have decades of performance in commercial applications at that scale and are worthy of consideration. This paper discusses a proposed sCO2-based PHES system application, and a cycle model establishes nominal conditions that define CO2 environment pressures and temperatures that magnetic bearings would have to operate in. A sensitivity study of the cycle’s round-trip efficiency is presented to see the impact of improved compressor and turbine efficiencies, which would result from expected windage loss and seal leakage reduction from a hermetic machinery configuration compared to one using conventional oil-film bearings. The result is approximately two points of round-trip efficiency for each point of isentropic efficiency from all machines. In the nominal cycle, the highest process temperatures exist for the charge mode compressor and discharge mode turbine, which would require magnetic bearings capable of operating up to 410°C. This exceeds the capabilities of typical commercial magnetic bearings (200°C), though it is within temperature ranges demonstrated for high-temperature magnetic bearings operating in low-pressure air (550°C). However, high-pressure sCO2 presents unique challenges that require further development. The paper discusses how these technical issues can be addressed to advance magnetic bearings for sCO2 applications.

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