Compressor-Expander Speed Control for Critical Zone Avoidance
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Solution Overview
Problem
Compressor-expander sets in gas refrigeration cycles for LNG production often operate near critical speed zones, which can lead to unstable conditions and surge, disrupting the process and potentially causing damage, and there is a need for a control system to avoid extended operation in these zones while maintaining process stability.
Innovation Solution
The implementation of a control method that uses adjustable nozzles and recycle valves to manage the operational points of compressors, avoiding surge by increasing flow rate and adjusting rotational speed, and incorporating a feed-forward signal to anticipate and prevent compressor trips, thereby maintaining process stability and avoiding critical speed zones.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If the compressor-expander set operates near critical speed zones, then the process can maintain continuous operation, but unstable conditions and surge occur disrupting the process
Solution Approach 1:
The control system anticipates entry into critical speed zones and takes preliminary action by adjusting the recycle valve position before the compressor actually enters the unstable operating region. This feed-forward control prevents surge from occurring in the first place, maintaining both continuous operation and process stability.
Solution Approach 2:
The control system continuously monitors compressor operating parameters and uses feedback control to adjust the recycle valve position, maintaining stable operation by responding to deviations from desired operating conditions and preventing surge.
2Reliability
If the control system avoids critical speed zones by adjusting operational parameters, then surge is prevented, but additional control complexity is introduced
Solution Approach 1:
The recycle valve serves as an intermediary control element that mediates between the compressor and the process stream. By adjusting the recycle valve position, the control system can prevent surge and avoid critical speed zones without requiring complex modifications to the compressor itself or the overall process configuration.
3Reliability
If adjustable nozzles and recycle valves are used to manage operational points, then surge is avoided by increasing flow rate, but energy consumption increases
Solution Approach 1:
The control system applies partial action by using the recycle valve to maintain just enough flow rate to prevent surge, rather than continuously maintaining maximum flow. The recycle valve is adjusted to provide only the necessary excess flow to keep the compressor operating point safely away from the surge line, minimizing unnecessary energy consumption.
Data Source
AI summary
A control method and apparatus for critical rotational speed avoidance in a compressor-expander set in a gas refrigeration system. By varying an opening of an antisurge or recycle valve, a shaft power used by the compressor in the compressor-expander set may be varied, thereby varying the rotational speed of the compressor-expander set to move it away from its critical speed zone. Additionally, a feedforward signal may be provided by a compressor-expander set control system to cause an antisurge valve for a recycle compressor to open upon a trip or shutdown of one compressor-expander set.